What If a Space Shuttle Orbiter Had to Ditch? (1975)

Final approach: the inner and outer wing flaps and body flap are visible at the aft end of the descending Orbiter. Image credit: NASA.

High on any pilot's list of things not to do with an aircraft is to ditch — that is, to make an emergency landing in water. Planes that nimbly slip through air become about as graceful as a brick when they touch an ocean swell. Anyone who has flubbed a dive and belly-flopped knows how painfully hard water can be.

Aircraft ditching behavior became of great interest in the United States during the Second World War, when B-24 Liberator bombers damaged by enemy fighters and flak — or simply lost and low on fuel — ended otherwise successful bombing runs over Nazi Germany by ditching in the English Channel or North Sea. Even in calm seas, the B-24 did not fare well. More often than not the plane broke apart and sank within minutes.

The Arsenal of Democracy: Workers assemble B-24 Liberator aircraft in the Consolidated Vultee plant in Fort Worth, Texas. More than 18,000 B-24s flew a wide range of missions in all Second World War operational theaters. Image credit: Wikipedia.

Because of this, on 20 September 1944, four months past the beginning of the D-Day invasion of Europe, a B-24 Liberator with two brave airmen on board intentionally ditched in the calm waters of Virginia's James River. Close by in a small boat was a team of engineers from the National Advisory Committee for Aeronautics (NACA) Langley Aeronautical Laboratory in nearby Hampton, Virginia. As the 44,100-pound, four-engine, straight-wing aircraft skimmed the water's surface at 97 miles per hour with its landing gear up, instruments inside the fuselage collected data on motion and deceleration.

The B-24's belly touched water at a point just behind the wing trailing edges. The plane began to skip, its nose rising and dipping to the left; then the water seemed to grab the bomber hard. Inside, the crew felt deceleration equal to 2.6 times the force of Earth's gravity. It threw them forward against their safety harnesses. Propellers still whirling, the plane nosed down and pieces flew out of an enormous cloud of spray that momentarily hid it from view. Rescuers moved in quickly; meanwhile, both pilots climbed atop the aircraft.

The men were in good shape, but their plane, a veteran of several European bombing missions, would never fly again. Even under the relatively benign conditions of the test, its fuselage had cracked, nearly breaking in two. The crack acted as a hinge, so the plane floated, rapidly filling with water, with both its tail and its nose in the air. The right inboard motor was gone, sheared away upon contact with the water.

When the NACA engineers lowered themselves inside to recover their instruments, they were in for a shock. Almost every piece of equipment bolted to the interior of the B-24 had torn loose and been flung forward, forming a nearly impassable heap just behind the cockpit. They found their instruments, secure in water-tight containers; meanwhile, a U.S. Navy salvage boat with a crane moved in fast to hoist the plane out of the water before it joined its missing engine 30 feet down on the muddy bottom of the James River. Smaller boats collected floating pieces.

On the salvage boat's deck, the plane looked even worse than it had in the river. A large dent marked where its belly first touched the water at a descent rate of 1.8 feet per second. Though they had been reinforced for the ditching test, the bomb bay doors had been pressed inward. The bomber's thin skin was rumpled over large areas; where it wasn't creased and puckered, it was ripped.

The 20 September 1944 ditching test became a pivotal event in aerospace history. It brought home to engineers as never before the powerful forces that ditching brought to bear on aircraft. It led NACA Langley to study ditching behavior in many types of aircraft. Because ditching full-size planes was both costly and dangerous, the lab developed techniques for testing scale-model airplanes in a water trough in what became known as the Langley Impacting Structures Facility (LISF). Above all, their experiments showed that ditching was actually crashing.

Fast forward 30 years to 1975. The era of scale-model experiments was gradually drawing to a close — computer models of complex phenomena, though still crude and costly, had made their debut in aviation and other fields. When NASA had formed in 1958, Langley had become one of its research centers. With the splashdown of the Apollo-Soyuz Test Project Apollo Command Module in the Pacific Ocean in July 1975, the first era of U.S. space capsules was over. The era of wings in Earth orbit was about the begin.

The Space Shuttle stack as envisioned in 1975 with major components indicated. Image credit: NASA.

NASA and its contractors envisioned several plausible scenarios which might lead a Space Shuttle Orbiter to ditch. The Orbiter was a glider, so it could not try again if it missed its runway at Kennedy Space Center (KSC), Florida. If the Orbiter crew realized the problem in time, they might ditch in the Banana or Indian Rivers or in the Atlantic Ocean off Cape Canaveral.

In the event that two of the Orbiter's three Space Shuttle Main Engines (SSMEs) failed early in its ascent to space, the Orbiter would need to return to KSC; however, depending on when the engines failed, it might not have enough altitude and energy to turn around, line up with the Shuttle runway, and stretch out its descent. In that case, it would probably fall short of the coast.

In October 1975, William Thomas, a former Langley engineer who had taken a job with Grumman Aerospace Corporation in Bethpage, New York, published results of 67 tests of a 1/20-scale model of the Space Shuttle Orbiter. The tests took place in the LISF starting in 1974. The actual Orbiter was planned to be 37.2 meters (122 feet) long, so the tests saw a 1.86-meter (6.1-foot) fiberglass and balsa-wood Orbiter launched into a broad trough of water using a ceiling-mounted "catapult" device. The trough could simulate smooth seas or seas with swells and waves.

The model included a replaceable balsa insert designed to give some sense of the belly damage a ditching Orbiter could expect. Removable weights enabled Thomas to simulate either 32,000-pound or 65,000-pound cargoes in its cargo bay. With the lighter cargo, the full-scale Orbiter would weigh 85,464 kilograms (188,247 pounds); with the heavier, 103,200 kilograms (227,313 pounds). The corresponding model weights were 10.68 kilograms (23.53 pounds) and 12.9 kilograms (28.41 pounds). Small lead weights permitted tests at intermediate Orbiter weights and allowed Thomas to trim the model so that it would, for example, dip one wing as it approached the water.

The model also included adjustable flaps and landing gear which could be installed to simulate gear-down ditching or left off to simulate ditching with landing gear doors closed. Flaps and landing gear were designed to break away at scale stresses — for example, on a full-sized Orbiter, the main landing gear would fail under a load of 356,270 pounds. The 1/20-scale landing gear would break if subjected to a torque of 6.41 pounds.

In the first of the 67 tests, the model Orbiter's nose was pitched up 16°, its aft-mounted body flap — located below the SSME engine bells — was tilted down 11.7°, and its wing flaps (inner and outer) were tilted up 4°. The water in the LISF trough was calm.

The 1/20-scale Orbiter, with a simulated mass of 93,000 kilograms and a simulated speed of 53.5 meters per second (just 120 miles per hour — slow for a Shuttle landing), contacted the water with its landing gear up. It skipped, then sank deeper on the next contact. The model decelerated very rapidly — it stopped four fuselage lengths from where it first touched the water. For the full-size Orbiter, this would have amounted to about 160 meters (490 feet). It was, Thomas commented, a "very stable run."

Beginning with Test 4, the model Orbiter was fitted with instruments for recording normal (fore-aft) and longitudinal (left-right) deceleration. These revealed that even a perfect ditching would likely harm the Orbiter crew. Apart from the instrumentation, the Test 4 Orbiter model was configured exactly like the Test 1 model. It touched calm water with its nose pitched up 12° moving at a scale speed of 72 meters per second (161 miles per hour).

Had it been a full-scale Orbiter carrying a crew, they would, after the initial skip, have been thrown forward against their straps with a force equal to 8.3 times the pull of Earth's gravity. As the Orbiter swerved to a stop, they would have felt a longitudinal jolt of 4.5 gravities. Test 4 was, it would turn out, only a little more arduous than average.

For Test 5, conditions were virtually identical to those of Test 4. Landing speed was slightly higher at 75.1 meters per second (168 miles per hour). Yet had the model been a full-scale Orbiter, it would have subjected its crew to 19.4 gravities of deceleration when it made its second contact with the water. The model's inner wing flaps broke free; this hinted that structural damage to the full-scale Orbiter was likely.

The first test of an Orbiter model with a simulated 65,000-pound payload (Test 17) saw a scale deceleration of nearly 11 gravities. In all, 14 of the 67 tests subjected the model Orbiter to greater than 10 gravities of scale deceleration.

Tests 9 and 62 were without question the most dramatic. In both, the model Orbiter stalled following release and hit the water tail first.

Other tests saw the 1/20-scale Orbiter model skip along simulated 2.1-meter (seven-foot) wave crests, plow through waves with a series of sharp jolts, dive under the water and bob to the surface, and lose both its inner and outer wing flaps. Tests with landing gear down never went well; the gear always broke away. In a full-scale Orbiter, tearing away the landing gear would likely have permitted water to enter through the damaged wheel wells.

Thomas's interpretation of the results of the 67 tests was notable: "a fairly smooth runout is expected but considerable fuselage tearing and leaking or flooding will occur." He was confident, however, that a full-scale Orbiter would remain afloat if its wings, which contained hollow spaces, remained intact. The more damage the wings suffered, the faster the Orbiter would sink.

The LISF 1/20-scale Orbiter tests were, of course, simplistic. They did not reflect the fact that Space Shuttle Orbiters were not designed to withstand the deceleration loads most of the model tests indicated. The average ditching deceleration ranged between five and eight gravities; this would have been sufficient to cause significant structural damage. In other words, in almost all cases, the Orbiter would have snapped apart.

Moments before the end: the plume from the Solid Rocket Booster leak that doomed Challenger is clearly visible. Image credit: NASA.

A little more than a decade later (28 January 1986), the Challenger accident made obvious the Space Shuttle Orbiter's fracture lines. The Orbiter Challenger did not explode; rather, the brown External Tank (ET) upon which it rode and from which it drew liquid hydrogen/liquid oxygen propellants for its SSMEs was destroyed by a malfunctioning Solid Rocket Booster (severe wind shear might also have played a role).

The fuel and oxidizer the ET contained came together and ignited, producing an explosion, but it was aerodynamic forces that tore the Orbiter apart. Basically, as its ET disintegrated, Challenger's nose stopped pointing in the right direction. This subjected it to drag and deceleration.

The crew compartment broke free, trailing behind it a comet's tail of cables. Some of the crew inside remained conscious long enough to take prescribed — though futile — emergency measures. The compartment remained mostly intact until it hit the water about two minutes later.

The Payload Bay disintegrated, but Challenger's main payload, a Tracking and Data Relay Satellite with an attached Inertial Upper Stage, remained more or less intact as it flew free of the fireball and fell toward the Atlantic. Challenger's wings separated, partly disintegrating. Each, however, remained recognizable in images and video captured from the ground. The aft compartment containing the SSMEs also emerged from the fireball mostly intact.

After Challenger, the Space Shuttle Program came under intense scrutiny. The Report to the President by the Presidential Commission on the Space Shuttle Challenger Accident cited Thomas's 1975 report when it stated that the probability was high that a Shuttle Orbiter would break up and sink after a ditching. Even if the Orbiter remained intact, the Commission report continued, cargoes mounted in the Payload Bay would break loose from their supports, slide forward, and smash into the back of the crew cabin. Though Thomas's report did not in fact say these things, they were logical conclusions based on the results of the 67 1/20-scale model Orbiter test runs.

Sources

YouTube - "Ditching of a B-24 Airplane into the James River" (https://youtu.be/WjadMxpXprk — uploaded by Jeff Quitney — accessed 16 January 2016).

YouTube - "Space Shuttle Orbiter Ditching Investigation of a 1/20-Scale Model" (https://www.youtube.com/watch?v=dlG-HcZIDl0 — uploaded by Jeff Quitney — accessed 16 January 2016).

Ditching Investigations of Dynamic Models and Effects of Design Parameters on Ditching Characteristics, Report 1347, L. Fisher and E. Hoffman, Langley Aeronautical Laboratory, National Advisory Committee for Aeronautics, 1958.

Ditching Investigation of a 1/20-Scale Model of the Space Shuttle Orbiter, NASA Contractor Report 2593, W. Thomas, NASA, October 1975.

Report to the President by the Presidential Commission on the Space Shuttle Challenger Accident, Presidential Commission on the Space Shuttle Challenger Accident, Volume I, pp. 182-183, June 1986.

"A Plane Crash in 1944 Is Saving Lives Today," Peter Frost, Daily Press, 22 February 2009 (http://articles.dailypress.com/2009-02-22/news/0902210116_1_b-24-successful-emergency-landing-hudson-river - accessed 16 January 2016).

More Information

What If an Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

What If a Lunar Module Ran Low on Fuel and Aborted Its Landing? (1966)

Where to Launch and Land the Space Shuttle? (1971-1972)

What Shuttle Should Have Been: NASA's October 1977 Space Shuttle Flight Manifest

One Space Shuttle, Two Cargo Volumes: Martin Marietta's Aft Cargo Carrier (1982)

North American Aviation's 1965 Plan to Rescue Apollo Astronauts Stranded in Lunar Orbit

Apollo 15 Command and Service Module Endeavour in lunar orbit. The drum-shaped portion is the Service Module and the conical portion is the Command Module. Note the Service Propulsion System rocket engine bell at upper left and the extended probe docking unit at lower right. Image credit: NASA.

North American Aviation (NAA) became the prime contractor for the Apollo Command and Service Module (CSM) spacecraft on 28 November 1961. In July of the following year, the company received the unwelcome news that its spacecraft would not land on the Moon. NASA had favored the Lunar-Orbit Rendezvous (LOR) mode for carrying out Apollo landings over Direct-Ascent or Earth-Orbit Rendezvous, both of which would have seen the CSM reach the lunar surface.

LOR made the CSM a lunar orbiter and spawned a new spacecraft: the Lunar Excursion Module (LEM) lander. The LEM, later redesignated the Lunar Module (LM - pronounced "lem"), would transport two astronauts from the CSM in lunar orbit to a landing site on the Moon's surface and back again. The LEM comprised a descent stage with landing legs and a throttleable rocket engine and an ascent stage with a pressurized crew cabin, flight controls, a rocket engine, and a concave drogue docking unit on its roof.

LOR meant that NASA needed to develop the technologies and techniques of rendezvous and docking in lunar orbit. The LEM ascent stage would use the descent stage as a launch pad and climb to a low lunar orbit. The CSM would then move in, extend the active probe docking unit on its nose, and dock with the passive drogue on the LEM.

After the LEM crew transferred back to the CSM, the ascent stage would be cast off. The CSM would subsequently ignite its large Service Propulsion System (SPS) main engine to escape lunar orbit and begin the fall back to Earth.

This image of the Apollo 16 Lunar Module Orion shows clearly the separation plane between the descent and ascent stages. The former has legs, a ladder, and is covered with black paint and gold-colored multilayer blankets for thermal control; the latter is silver and black and has four attitude-control thruster quads (two are readily visible), a crew hatch (square with rounded corners), and a pair of triangular windows. Image credit: NASA. 

In December 1965, NAA's engineers briefed the NASA Headquarters Office of Manned Space Flight (OMSF) and Bellcomm, the space agency's Apollo planning contractor, on results of a preliminary feasibility study of a one-person CSM mission to rescue Apollo astronauts stranded in lunar orbit. The NAA engineers did not describe specific lunar-orbit rescue scenarios, though the CSM modifications they outlined offer clues about the types of rescue missions they envisioned.

The most important piece of rescue hardware they proposed was a special docking adapter ring installed on the rescue CSM's nose. Either an active probe or an active drogue could be mounted on the ring, so the rescue CSM could dock with either a LEM or a CSM. The lone rescue CSM astronaut could reconfigure the docking unit during the flight from the Earth to the Moon; this would permit adaptation to changing circumstances in lunar orbit.

NAA anticipated that a lunar-orbit rescue might require spacewalks, so provided the rescue CSM pilot with a tether and a life-support umbilical extension, a cold gas-propelled hand-held maneuvering device, and a protective "meteoroid garment" of the type Apollo astronauts would wear over their suits on the lunar surface. In addition, the rescue CSM would carry an Expandable Structures Space Rescue System (ESSRS) device. ESSRS was an inflatable "pole" meant to serve as a handrail for astronauts spacewalking between two spacecraft.

Other rescue CSM modifications would include new crew couches to accommodate up to four astronauts, a fourth umbilical so that all could link their suits to the rescue CSM's life support system, added breathing oxygen, a dish-shaped LEM docking radar antenna on an extendable boom, and new rendezvous and docking computer software. Modifications and additions would add a total of 445 pounds to the rescue CSM's weight. Removal of science equipment and other systems not required to rescue and return to Earth a crew stranded in lunar orbit would, however, reduce the rescue CSM's weight by 415 pounds, for a net weight gain of only 30 pounds.

Rescue CSMs would be advanced Block II spacecraft of the type earmarked for Apollo lunar missions. In late 1965, NAA expected to build a total of six Block I and Block II CSMs per year beginning in late 1966. Block I CSMs would be used in Apollo testing and Apollo Extension System (AES) Earth-orbital missions. AES, a proposed program intended to apply Apollo hardware to new missions, became a predecessor to the Apollo Applications Program, which subsequently evolved into the Earth-orbital Skylab Program. In the event, only Block II CSMs carried astronauts; work on Block I CSMs ceased following the deadly AS-204 (Apollo 1) fire of 27 January 1967.

NAA offered two plans for providing rescue CSMs for the Apollo Program. The first, Rescue Vehicle Program "A," would see CSM-110 and CSM-113 converted into rescue CSMs; that is, diverted from lunar exploration missions. They would be flight-ready in early 1969 and mid-1969, respectively. Starting in mid-1970, one of the lunar CSMs NAA produced annually would be built as a rescue CSM; the first of these would be designated CSM-119.

Rescue Vehicle Program "B" would see NAA produce nine CSMs per year. The company's representatives told NASA that this approach would guarantee "non-interference with basic Apollo or AES." The first rescue CSM of Program "B," designated CSM R-1, would be ready for flight at the end of 1968, between AES CSM-109 and lunar CSM-110. Program "B" rescue CSMs R-2, R-3, and R-4 would be completed in mid-1969, early 1970, and late 1970, respectively.

NAA assumed that during every Apollo lunar mission a rescue CSM would stand by atop a three-stage Saturn V rocket on one of the two Launch Complex (LC) 39 pads at Kennedy Space Center (KSC), Florida. The lunar mission would launch from the other LC 39 pad.

The rescue CSM Saturn V would be outwardly nearly identical to the lunar mission Saturn V. The rescue rocket would, however, carry no LEM in the tapered Spacecraft Launch Adapter shroud that would link the aft end of the rescue CSM to the ring-shaped Instrument Unit atop the Saturn V S-IVB third stage. In addition, the Boost Protective Cover that protected the conical Command Module during the first part of ascent would need to be modified slightly to make room for the special docking ring.

On the launch pad, the Saturn V rocket bearing the rescue CSM would have appeared nearly identical to one bearing a lunar landing mission CSM Saturn V. The Boost Protective Cover, visible near the top of the image, would have had a slightly more bulbous nose. Internally, the most significant difference would have been the lack of a Lunar Module within the segmented Spacecraft Launch Adapter, the white tapered housing linking the bottom of the CSM to the ring-shaped Instrument Unit on top of the Saturn V S-IVB third stage. Image credit: NASA. 

The rescue CSM and Saturn V would stand by on the launch pad until the Apollo lunar landing mission CSM safely departed lunar orbit and began its fall back to Earth, then would be rolled back to KSC's cavernous Vertical Assembly Building for storage until the next Apollo lunar mission. A single rescue CSM could be prepared for flight three times and and mothballed twice; this meant that it could stand by during three lunar missions, then would need to be replaced.

NAA did not explain what would be done with unused rescue CSMs; presumably they would be scrapped, though perhaps some systems could be salvaged for use in other CSMs. Neither did the company explain what would happen to unused rescue Saturn V rockets.

The company assumed that in most cases the rescue CSM would launch immediately after NASA learned that a crew had become stranded in lunar orbit. Because it would not wait, in most cases it would not be able to rely on Earth launch geometry to help it to match orbits and carry out a rendezvous with the stranded spacecraft.

NAA determined that launching the rescue CSM immediately could create complications. It might, for example, increase the rescue mission's duration. NAA calculated that the time needed to reach a spacecraft stranded in lunar orbit and return to Earth could in fact exceed the Block II CSM's anticipated 240-hour (10-day) operational lifetime by up to 52 hours in the worst case. NAA recommended that NASA delay the rescue CSM's launch until launch geometry could ensure that its mission duration would not exceed 10 days.

When the rescue CSM reached the Moon's vicinity, it would ignite its SPS main engine to place itself into an elliptical "catch up" lunar orbit. At apolune (lunar orbit high point), the pilot could ignite the SPS again to line up the rescue CSM's orbital plane with that of the stranded CSM. At perilune (lunar orbit low point), the pilot would fire the SPS a third time to lower the rescue CSM's apolune, circularizing its orbit and placing it near the stranded spacecraft.

NAA estimated that Rescue Vehicle Program "A" would add a total of $86 million to the cost of the Apollo Program per year. An 18-month program of development and testing would cost $50 million, $6 million would pay for modifications to two Apollo lunar CSMs, and four new rescue CSMs would cost $38 million each. The company provided no cost estimate for its Rescue Vehicle Program "B."

The NAA engineers did not discuss how astronauts stranded in lunar orbit might eke out their limited supplies of consumables — for example, breathing oxygen — while they awaited rescue. This would be particularly worrisome in the case of a LEM stranded in lunar orbit by a catastrophic CSM failure, for at the time of the NAA study the LEM was expected to keep two astronauts alive for at most one or two days. Neither did they assess the risks of a one-person CSM mission to lunar orbit, nor the technical problems of running two lunar missions simultaneously.

Perhaps because of these difficulties, NASA chose not to make preparations for astronaut rescue in lunar orbit. This did not stop Bellcomm from considering the problems of lunar orbit survival three years later, in December 1968, shortly after the Apollo 8 CSM became the first piloted spacecraft to return from lunar orbit (see link under "More Information" below).

Source

4-Man Apollo Rescue Mission, AS65-36, M. W. Jack Bell, et al., North American Aviation, November 1965; presentation at NASA Headquarters, 13 December 1965.

More Information

What If a Crew Became Stranded On Board the Skylab Space Station (1972)

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

Space Race: The Notorious 1962 Proposal to Launch an Astronaut on a One-Way Trip to the Moon

If an Apollo Lunar Module Crashed on the Moon, Could NASA Investigate the Cause? (1967)

All alone in the gray: the Apollo 17 Lunar Module Challenger photographed by its crew from a distance of about two miles. Image credit: NASA.

The early piloted Apollo missions were a rapid series of test flights. Apollo 7 (11-22 October 1968), the first manned Apollo, saw a Command and Service Module (CSM) spacecraft and its three-man crew put through their paces in low-Earth orbit. Apollo 8 (21-27 December 1968), originally planned as a test of the CSM and the Lunar Module (LM) in high-Earth orbit, might have been postponed because the LM was not yet ready; instead, Apollo 7's success and the perceived threat to American prestige of a Soviet manned circumlunar mission induced NASA managers to make it a lunar-orbital CSM test and a trial run for the Apollo tracking and communications network.

Apollo 9 tested the CSM, LM, and the Apollo space suit in low-Earth orbit (3-13 March 1969). Apollo 10 (18-26 May 1969) tested the CSM and LM in lunar orbit and rehearsed the Apollo lunar descent procedure down to an altitude of 50,000 feet.

Apollo 11 (16-24 July 1969), the first lunar landing attempt, was also a test flight, though it is seldom seen that way today. In an effort to make that first landing as easy as possible, engineers directed the Apollo 11 LM Eagle to the northern Sea of Tranquility, one of the flattest stretches of lunar equatorial terrain scientists could find. It was, however, also a U.S. victory in the Cold War with the Soviet Union and the first time humans had explored an alien world first-hand. Scientists and engineers fought a running battle over the degree to which scientific exploration should play a role in Apollo 11, and President Richard Nixon telephoned moonwalkers Neil Armstrong and Edwin "Buzz" Aldrin to read a celebratory speech as they stood next to the U.S. flag.

Eagle landed downrange of its planned landing site. Its overworked computer might have flown it into boulder-filled West Crater had it not been for the quick thinking of former X-15 rocket plane test-pilot Armstrong. Apollo 12 (14-24 November 1969) thus became a test of the Apollo system's ability to make a pinpoint landing. The ability to reach a predetermined spot on the moon was important to scientists planning Apollo geologic traverses. It also helped to ensure safety. The Apollo 12 LM Intrepid landed on the Ocean of Storms, another flat plain, just 600 feet from its target, the derelict Surveyor 3 lander, which had preceded it to the site on 20 April 1967.

Any Apollo mission might have failed catastrophically far from Earth, a point driven home by the explosion on board the CSM Odyssey during Apollo 13 (11-17 April 1970). Hollywood scriptwriters notwithstanding, failure was an option during Apollo missions. Apollo pushed the limits of 1960s technology to do extraordinary things.

The Apollo Program had, in fact, claimed lives before the first Apollo spacecraft left Earth: the AS-204 (Apollo 1) fire killed Gus Grissom, Ed White, and Roger Chaffee during a launch pad training exercise on 27 January 1967, barely a month before their planned launch. Because the Apollo 1 fire occurred on the ground, engineers could take apart the AS-204 CSM piece by piece to try to trace the fire's cause. Even so, they never conclusively identified its ignition source.

A December 1964 report by R. Moore of the Project RAND think-tank anticipated that accidents taking place on the moon would be even more difficult to analyze. Moore proposed that NASA continue the Ranger lunar probe series to enable photography of lunar crash sites. The last four Rangers each carried a battery of six television cameras intended to return images to Earth as the spacecraft plummeted toward destructive impact.

If, for example, Eagle had crashed in West Crater, then NASA would have dispatched a Ranger to image the site. Ranger seemed well suited to aiding accident investigators: Ranger 7, which struck the Ocean of Storms on 31 July 1964, had imaged features as small as 18 inches wide in its final seconds before impact.

Ranger 7, 8, and 9 were designed for close-up photography of the lunar surface. Image credit: NASA.

NASA did not act on Moore's proposal, but the concept of Apollo accident site investigations was not forgotten (or, just as likely, it was discovered again). In November 1967, C. Byrne and W. Piotrowski of Bellcomm, NASA's Washington, DC-based Apollo planning contractor, wrote a memorandum in which they looked at whether a Command Module Pilot (CMP) whose moonwalking colleagues had suffered a fatal mishap on the moon might assist investigators by photographing the accident site from the CSM in lunar orbit.

They began by acknowledging that telemetry could provide valuable accident data: they added, however, that "certain types of failure can be imagined which would not permit enough data to be transmitted to support a diagnosis." In those cases, they wrote, observation from lunar orbit might be the only way to collect data that could guide engineers in their efforts to redesign the Apollo system to avoid similar accidents.

Byrne and Piotrowski then looked at the image resolution necessary to make useful observations of an accident site on the moon. To locate and identify an intact LM, which measured a little more than 20 feet tall, images showing details as small as 10 feet across would be needed. Eight-foot resolution would be needed to determine the status of the LM's 12-foot-tall ascent stage; for example, if it had lifted off from the descent stage and then crashed on the surface. Four-foot resolution would suffice to determine whether the LM had tipped over.

The ability to resolve features as small as a yard across would enable engineers to assess landing site roughness and slope. Two-foot resolution would, they estimated, be adequate to discern astronaut bodies on the surface. One-foot resolution would reveal whether the LM landing gear had failed, "hazardous sinkage" had occurred, the LM ascent stage crew cabin lay open to vacuum, or an explosion in the LM had scattered "litter" around the landing site.

Byrne and Piotrowski then took stock of the cameras and telescopes expected to be on board the CSM during a normal lunar mission and their performance if the CSM were orbiting 80 nautical miles (n mi), 40 n mi, or 10 n mi above the accident site. They suggested that CSM propellants budgeted for rescue of astronauts on board an LM ascent stage that attained only a low orbit could be used to lower the CSM's altitude for accident site observations.

The CSM's scanning telescope would, despite its name, not magnify objects, so would be of "no value" as a diagnostic tool, Byrne and Piotrowski judged. The sextant, on the other hand, could magnify objects 28 times. The Bellcomm engineers found that the sextant would offer 8.6-foot resolution at an orbital altitude of 80 n mi, 4.3-foot resolution at 40 n mi, and 1.1-foot resolution at 10 n mi. (Apollo CMPs did in fact use the sextant to spot LMs — or at least the shadows they cast — on the lunar surface.)

The sextant was, however, designed to superimpose a pair of star images, could not be used to photograph objects, and, with a field of view only 1.8° wide, would require a highly skilled operator to spot an LM at all. This would be the case especially at lower altitudes, when the CSM would be moving fastest relative to the surface. Byrne and Piotrowski estimated that an astronaut searching the surface with the sextant at an altitude of 10 n mi would at most have 10 seconds in which to find and observe an accident site.

Apollo 12 Command Module Pilot Richard Gordon trains with cameras and lenses in a Command Module simulator before his November 1969 flight to the moon. Image credit: NASA.

Byrne and Piotrowski wrote that NASA planned to include among the Apollo CSM experiment equipment a Swedish-built Hasselblad 500EL camera with 80-millimeter (mm) f/2.8 and 250-mm f/5.6 lenses. Used with S0-243 film and the 250-mm lens, the Hasselblad 500EL could in theory take photos of the lunar surface with a resolution of 13 feet at 80 n mi of altitude, 6.5 feet at 40 n mi, and 1.6 feet at 10 n mi.

Other constraints would, however, conspire to reduce camera performance. In particular, there was the problem of image motion compensation. Experience gained through Earth photography during the Gemini V mission (21-29 August 1965) showed that astronaut movements were jerky, not smooth, when tracking and photographing targets. Jerky tracking would cause image "smear," reducing resolution.

Byrne and Piotrowski recommended that the CMP mount the Hasselblad 500EL securely in a new-design clamp or bracket at either the CSM hatch window or one of the side windows after he located the LM site. He would then fire the CSM's Reaction Control System thrusters to roll the spacecraft and keep the surface target in his camera's field of view as the CSM passed over it. This form of image motion compensation was unlikely to be perfect; for one thing, roll rate would be affected by factors beyond the CMP's control, such as the distribution and movement of liquid propellants in the CSM's tanks.

As with the sextant, time-over-target would pose a constraint. The Bellcomm engineers assumed that the CMP would need at least 30 seconds to locate the LM on the moon, 15 seconds to prepare the camera and roll the CSM, and 15 seconds for photography.

For a CSM at an altitude of 80 n mi, an LM on the lunar surface would remain in view for two minutes and 24 seconds. This was ample for photography, but at that altitude resolution would be inadequate — no better than 10 feet. At 40 n mi of altitude, the CMP could keep the LM in view for 90 seconds. At 30 n mi, he would have about 60 seconds — the minimum necessary — to find and photograph his target. Byrne and Pietrowski thus selected 40 n mi as the optimum altitude for accident site photography.

The Bellcomm engineers looked at adding a special cartridge of high-contrast film and a 500-mm f/8 lens for the Hasselblad 500EL, and at replacing the Hasselblad 500EL with the Zeiss Contarex Special 35-mm camera and 200-mm f/4 and 300-mm f/4 lenses. These had already reached space on board Gemini V. They noted that both cameras would yield a resolution of about one yard at an altitude of 40 n mi with a secure mounting bracket and adequate image motion compensation. In the end, they favored the Hasselblad 500EL with 500-mm f/8 lens and high-contrast film because it would be about eight pounds lighter than the Zeiss camera.

Byrne and Piotrowski noted that the camera system and techniques they proposed would have uses other than accident site investigation. They might, for example, be used to photograph the landing site after a successful LM landing. This would, among other things, enable scientists to precisely locate the post-deployment position of the Advanced Lunar Scientific Experiment Package, a suite of instruments the moonwalkers would deploy some distance away from the LM. Images of the landing site might also assist geologists in understanding the context of the samples the moonwalking astronauts would return to Earth.

Sources

"A Suggestion for Extension of the NASA Ranger Project in Support of Manned Space Flight," Memorandum RM-4353-NASA, R. C. Moore, The RAND Corporation, December 1964.

"Diagnostic Observation of Lunar Surface Accidents – Case 340," C. Byrne & W. Piotrowski, Bellcomm, Inc., 7 November 1967.

More Information

What If Apollo Astronauts Could Not Ride the Saturn V Rocket? (1965)

What if an Apollo Lunar Module Ran Low on Fuel and Aborted Its Moon Landing? (1966)

"Assuming That Everything Goes Perfectly Well in the Apollo Program. . ." (1967)

What If a Crew Became Stranded On Board the Skylab Space Station? (1972)

Image credit: NASA.

On 28 July 1973, the Skylab 3 crew of Alan Bean, Jack Lousma, and Owen Garriott lifted off from Launch Pad 39B at Kennedy Space Center, Florida, bound for the Skylab Orbital Workshop in low-Earth orbit. Despite their mission's numerical designation, they were the second crew to visit Skylab; in a move guaranteed to generate confusion for decades to come, NASA had designated as Skylab 1 the unmanned Workshop launched on 14 May 1973, and had dubbed the first crew to visit it Skylab 2.

The Skylab 3 Apollo Command and Service Module (CSM) separated from the S-IVB second stage of its Saturn IB launch vehicle and began maneuvering to catch up with Skylab. During final approach to the Workshop, one of the four steering thruster quads on the CSM began to leak nitrogen tetroxide oxidizer from its forward-firing engine. The crew dutifully shut off the quad and used the three quads remaining to complete docking without further incident.

On 2 August, a second thruster quad began to leak, raising fears that tainted nitrogen tetroxide might have damaged both quads. If this were the case, then the Skylab 3 CSM's remaining two quads and Service Propulsion System (SPS) main engine might also have been compromised; though the individual quads and the SPS had independent plumbing, all contained oxidizer from the same batch. If the leaks continued and spread, moreover, nitrogen tetroxide might contaminate the inside of the CSM's drum-shaped Service Module, potentially damaging other spacecraft systems.

The leaks did not catch NASA off guard. As was common in the 1960s and early 1970s, NASA had considered potential Apollo and Skylab failures - however unlikely - and had planned ahead. Within hours of the second leak, The U.S. civilian space agency put into motion a variant of a plan Kenneth Kleinknecht, Skylab Program Manager, and Lawrence Williams, Apollo Spacecraft Program Office, had described less than a year earlier at the Fifth Annual Space Rescue Symposium in Vienna, Austria.

In their paper, Kleinknecht and Williams explained that Skylab would provide the first true opportunity for space rescue in the U.S. space program. One-seat Mercury and two-seat Gemini spacecraft had been too small and limited in capability to serve as rescue spacecraft. Apollo lunar CSMs were much more capable; even so, they each carried only a little more breathing oxygen, fuel cell reactants, and food than were needed to support a three-man crew for the duration of a lunar mission (about 10 days). If an Apollo CSM had become stranded in lunar orbit — by an SPS failure, say — then its crew would have perished long before NASA could have attempted a rescue.

The Skylab Orbital Workshop. The red arrow points to the Multiple Docking Adapter's radial port. Image credit: NASA.

If astronauts needed to evacuate Skylab, they could board their CSM docked at Skylab's front port, undock from the Workshop, and splash down in the ocean in less than a day. If, on the other hand, a crew's CSM became unusable while they lived and worked on board Skylab, then the astronauts could await rescue.

Stranded astronauts were unlikely to run out of supplies. Kleinknecht and Williams noted that the Orbital Workshop would be launched with enough oxygen, food, water, and other supplies on board to support three men for eight months. At the time they presented their paper, NASA planned three three-man Skylab visits lasting 28, 56, and 56 days — that is, a total of a little less than five months.

NASA, meanwhile, would prepare and launch a rescue CSM with a crew of two. Skylab, Kleinknecht and Williams explained, had a second, radial docking port on its Multiple Docking Adapter. The rescue CSM would dock at the radial port to pick up the stranded crew.

They proposed that the CSM intended for the next Skylab crew should become the rescue CSM. This would presumably reduce by one the number of long-duration Skylab missions that could be flown. A fourth CSM, which would serve as the backup CSM throughout the Skylab program, would serve as the rescue CSM for Skylab 4, the third and final planned Skylab crew.

Image credit: NASA.

Kleinknecht and Williams estimated that stripping out the rescue CSM's aft bulkhead lockers to make room for a "rescue kit" would require about a day. The rescue kit would include a pair of special astronaut couches, connectors and hoses for linking two additional space-suited astronauts to the rescue CSM's life support and communications systems, and an experiment-return pallet for bringing home a select few of the stranded crew's science results. The rescue CSM's two-man crew would recline in the left and right CSM couches; the three rescued Skylab crewmen would return to Earth in the center couch and in the two special couches mounted below the others in place of the lockers.

The rescue CSM would bring along a special Apollo probe-and-drogue docking unit that would enable astronauts inside Skylab to manually undock and cast off the crippled CSM. This would clear the Workshop's front port for any future CSM dockings. Kleinknecht and Williams did not explain what would happen to the unmanned CSM after it was discarded.

Though the time needed to install the rescue kit was minimal, the time needed to refurbish Pad 39B and prepare the rescue CSM and Saturn IB rocket for launch would depend upon when NASA declared that a rescue was necessary. After each Skylab Saturn IB launch, ground crews would need about 48 days to refurbish Pad 39B and prepare the next Skylab CSM and Saturn IB.

If a rescue were judged to be necessary at the beginning of the 28-day first manned Skylab mission (Skylab 2), then the mission would be extended by 20 days, making the total duration about 48 days. If a rescue were declared to be necessary late in Skylab 2 — say at the time of planned return to Earth — then preparations for the next Skylab CSM launch would be farther along, but would have started later. The rescue CSM and Saturn IB would thus need 28 days before they could lift off, bringing the total Skylab 2 mission duration to about 56 days, or double the duration planned at launch.

Activation of the Skylab rescue capability early in the Skylab 3 or Skylab 4 mission might permit a rescue before the return time planned when the stranded crew left Earth, Kleinknecht and Williams found. A failure near the planned conclusion of Skylab 3 or Skylab 4 would see a rescue CSM launched as little as 10 days after the rescue plan was activated.

Skylab rescue crewmen Vance Brand (left) and Don Lind. Though he never flew to Skylab, Brand would reach space as part of the Apollo-Soyuz Test Project mission in July 1975 and as Commander of Space Shuttle missions STS-5 (November 1982), STS-41-B (February 1984), and STS-35 (December 1990). Lind would reach space as a Mission Specialist on Shuttle mission STS-51-B (April-May 1985). Image credit: NASA.

The 2 August 1973 failure of the second Skylab 3 CSM thruster quad unleashed a storm of activity. NASA prepared the backup Skylab CSM, not the Skylab 4 CSM, as its rescue vehicle, and tapped Skylab 3 backup crewmen Vance Brand and Don Lind to pilot it.

NASA had made other changes to Kleinknecht and Williams' rescue plan. The special probe-and-drogue docking unit for casting off the malfunctioning CSM had become a concave drogue unit that would be installed over the front port. It was launched with Skylab, not in the rescue CSM. After they installed it, the stranded astronauts would "trigger" the drogue to manually release their balky CSM. The rescue CSM would then dock at the front port, not the radial port.

Almost as soon as NASA activated the rescue plan, laboratory analysis on Earth showed that the batch from which the nitrogen tetroxide in the Skylab 3 CSM's propulsion systems had been taken was not tainted. As unlikely as it might seem, the two thruster quad malfunctions lacked a common cause.

Working in the CSM simulator in Houston, astronaut Brand demonstrated that the Skylab 3 crew could maneuver their spacecraft adequately even if they lost a third thruster quad. That is, if they were left with only one functioning quad when time came for them to return home, they could still safely deorbit their CSM.

Though rescue preparations continued as a precaution, by 10 August NASA managers had cleared the Skylab 3 crew for the full duration of their planned 59-day mission on board the Workshop. On 25 September 1973, Bean, Lousma, and Garriott returned to Earth as originally planned, in the CSM that had launched them to Skylab.

Sources

"Skylab Rescue Capability," Kenneth S. Kleinknecht and Lawrence G. Williams; paper presented at the Fifth Annual Space Rescue Symposium Organized by the Space Rescue Studies Committee of the International Academy of Astronautics, 23rd Congress of the International Astronautical Federation, Vienna, Austria, 9-12 October 1972.

Skylab News Reference, NASA Office of Public Affairs, March 1973, pp. IV-6 - IV-8.

"Skylab: Outpost on the Frontier of Space," T. Canby, National Geographic, October 1974, p. 460.

More Information

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

What If an Apollo Lunar Module Ran Low on Fuel and Aborted Its Moon Landing? (1966)

What If An Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

What If Apollo Astronauts Could Not Ride the Saturn V Rocket? (1965)

What If Apollo Astronauts Could Not Ride the Saturn V Rocket? (1965)

At the time a NASA Marshall Space Flight Center artist created this graphic, the first Saturn V test flight was 17 months in the future. The smaller rocket, labeled "Apollo Saturn I," was subsequently renamed the Saturn IB. The first piloted Apollo flight, Apollo 1, was scheduled for launch on a Saturn IB rocket in early 1967, about six months after this graphic was made. Image credit: NASA.

George Mueller left private industry to become NASA's new Associate Administrator for Manned Space Flight in September 1963. He immediately asked John Disher and Adelbert Tischler, two veteran NASA engineers not directly involved in Apollo, for an independent assessment of the Moon program. On 28 September, they told Mueller that it could not achieve President Kennedy's goal of a man on the Moon by 1970. They estimated that NASA might be able to carry out its first piloted Moon landing in late 1971.

Mueller took drastic action. When he joined NASA, the Apollo flight-test plan was based on the philosophy of incremental testing, which meant that untried rocket stages would launch only dummy stages and dummy spacecraft. On 29 October 1963, Mueller informed his senior managers that Apollo test flights would henceforth use complete systems. Mueller's directive meant that, when the Saturn V S-IC first stage flew for the first time, it would be as part of a complete 363-foot-tall three-stage Saturn V. The new "all-up" approach would, it was hoped, slash the number of test flights needed before the Saturn V could launch astronauts to the Moon.

George Mueller. Image credit: NASA.

All-up Saturn V testing, today hailed as a visionary and heroic step, made many Apollo engineers nervous. The Saturn V was the largest rocket ever developed. It had engines of unprecedented scale and power: the F-1 engines in the 33-foot-diameter S-IC first stage, which burned RP-1 kerosene fuel and liquid oxygen, remain today the largest ever flown. The J-2 engines in the top two stages, the 33-foot-diameter S-II second stage and the 22-foot-diameter S-IVB stage, gulped down temperamental liquid hydrogen and liquid oxygen propellants. Cautious engineers could see many opportunities for trouble, and they were aware that problems they could not foresee might be the most difficult to solve. Many believed that NASA should have in place backup plans in case the Saturn V suffered development delays.

Eighteen months after Mueller's announcement, E. Harris and J. Brom, engineers with The RAND Corporation think tank, proposed one such back-up plan. Their brief report, originally classified "Secret," looked at how NASA might accomplish a piloted Moon landing by 1970 if the Saturn V could not be certified as safe enough to launch astronauts.

Harris and Brom's backup plan would see the Apollo Saturn V lift off without astronauts on board. It would expend its S-IC first stage and S-II second stage in turn, then its S-IVB third stage would place itself plus lunar mission Apollo Command and Service Module (CSM) and Lunar Module (LM) spacecraft into parking orbit about the Earth. Because it would carry no crew, the lunar mission CSM would need no Launch Escape System (LES) tower on its nose.

Three Apollo astronauts would reach Earth orbit separately in a ferry CSM launched atop a two-stage Saturn IB rocket. The ferry CSM would carry a special drogue docking unit on its nose for linking up with the waiting lunar mission CSM's nose-mounted probe docking unit. The special drogue, the only new system required for RAND's backup plan, would need about one year and "perhaps several million dollars" to develop.

The top of the Apollo 13 Lunar Module Aquarius. The red arrow points to the concave drogue docking unit. Image credit: NASA.

The astronauts would dock with and transfer to the lunar mission CSM in Earth orbit, then would cast off the ferry CSM. The remainder of their mission would occur as in NASA's Apollo plan. The astronauts would restart the S-IVB stage to perform Trans-Lunar Injection (that is, to leave Earth orbit for the Moon). After S-IVB stage shutdown, they would detach the lunar mission CSM from the Spacecraft LM Adapter (SLA) shroud that linked it to the top of the S-IVB stage. The SLA, made up of four segments, would peel back and separate, revealing the LM. The CSM would then dock with the drogue docking unit on top of the LM and pull the Moon lander free of the spent S-IVB stage.

The RAND engineers declined to recommend whether the Saturn V or the Saturn IB should be launched first. They noted that liquid hydrogen fuel in the Saturn V S-IVB stage would boil and escape at a rate of 700 pounds per hour; the stage would thus need to be restarted within 4.5 hours of reaching parking orbit if it was to retain enough propellants for Trans-Lunar Injection. They noted that deletion of the 2900-pound LES would make the lunar mission Saturn V that much lighter, so its S-IVB stage could be loaded with an extra 2900 pounds of liquid hydrogen; that is, enough to permit it to loiter in low-Earth orbit for nearly 10 hours. Extending the loiter time further would demand a complex and costly S-IVB stage redesign.

Launching the crew first would avoid the S-IVB stage loiter-time constraint. Harris and Brom noted that, though the Apollo lunar mission was scheduled to last only from seven to 10 days, NASA planned a 14-day Earth-orbital Gemini mission by the end of 1965 to certify that astronauts could withstand long space flights. (That mission, Gemini 7, flew in December 1965. Astronauts Frank Borman and James Lovell returned to Earth after 14 days in good health and high spirits.)

Assuming that the Gemini flight confirmed that humans could endure 14 days in weightlessness, then the ferry CSM crew could in theory wait for from four to seven days for the unmanned Saturn V to join them in Earth orbit. Harris and Brom recommended that, in the event that launch of the Saturn V was delayed so that the astronauts waiting in orbit could not accomplish a lunar mission and return to Earth within 14 days of reaching space, then they should carry out an unspecified backup Earth-orbital mission in the ferry CSM so that their flight would not be wasted.

NASA officials did not take up the Harris and Brom proposal, though for a time in 1968 they might have wished that they had. The first Saturn V test flight, Apollo 4, lifted off without a crew on 9 November 1967. In keeping with Mueller's 1963 directive, it included complete S-IC, S-II, and S-IVB stages, plus a CSM with LES. Because LM development had hit snags, a dummy LM rode inside its SLA. The eight-hour Earth-orbital mission was an unqualified success.

Troubled flight: Apollo 6 Saturn V test, 4 April 1968. Image credit: NASA.

Apollo 6, was, however, another story. On 4 April 1968, two minutes into its automated flight, the second Saturn V to fly began to shake back and forth along its long axis. Dubbed "pogo" by engineers, the violent oscillations tore pieces off the SLA and damaged one of the S-II's five J-2 engines. Following S-II ignition, the engine under-performed and shut down prematurely, then a control logic flaw caused a healthy S-II engine to shut down. The remaining three S-II engines burned for a minute longer than planned to compensate for the two failed engines. The S-IVB's single J-2 engine then burned for 30 seconds longer than planned to reach a lopsided Earth orbit. Two orbits later, the engine refused to restart despite repeated radioed commands from flight controllers.

The pogo oscillations might have injured astronauts, had any been on board the Apollo 6 CSM; the S-IVB failure would certainly have scrubbed their flight to the Moon. Post-flight analysis showed, however, that the pogo and engine failures had relatively simple fixes. After intense internal debate, NASA announced on 12 November 1968 that the third Saturn V would launch Apollo 8 astronauts Frank Borman, James Lovell, and William Anders to the Moon. The giant rocket performed flawlessly, placing the Apollo 8 CSM on course for lunar orbit on 21 December 1968.

Sources

"Apollo Launch-Vehicle Man-Rating: Some Considerations and an Alternative Contingency Plan (U)," Memorandum RM-4489-NASA, E. D. Harris and J. R. Brom, The RAND Corporation, May 1965.

The Apollo Spacecraft: A Chronology, Volume II, NASA SP-4009,  Mary Louise Morse & Jean Kernahan Bays, NASA Scientific and Technical Information Office, 1973, pp. 104-106.

Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles, NASA SP-4206, Roger Bilstein, NASA, 1980, pp. 347-363.

Apollo: The Race to the Moon, Charles Murray & Catherine Bly Cox, Simon & Schuster, 1989, pp. 153-162.

More Information

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

What If an Apollo Lunar Module Ran Low on Fuel and Aborted Its Moon Landing? (1966)

What If An Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

238,000 miles from home — Earth as viewed by the Apollo 8 astronauts in lunar orbit, Christmas Eve 1968. Image credit: NASA.

The three-man crew of Apollo 8 — Commander Frank Borman, Command Module Pilot James Lovell, and Lunar Module Pilot William Anders — was the first to leave Earth on a giant Saturn V rocket. They departed Cape Kennedy, Florida, on 21 December 1968, and left Earth orbit for the Moon about two and a half hours after launch.

Though its target was the Moon, the Apollo 8 mission included no Lunar Module (LM). The piloted lunar lander had suffered production delays, which was understandable given that no one had previously built a vehicle for landing humans on another world.

NASA's planned mission sequence for piloted Apollo missions had begun with a low-Earth orbit (LEO) test of the Command and Service Module (CSM) during Apollo 7 (11-22 October 1968). This was to have been followed immediately by an LEO test of the CSM and LM, then a CSM/LM test flight in higher Earth orbit. During the fourth mission in the sequence, astronauts would test the CSM and LM in lunar orbit, then the first Apollo lunar landing attempt would take place. NASA designated these five increasingly ambitious missions C, D, E, F, and G.

Putting off the next Apollo flight — the D mission — until the LM was ready might have placed in jeopardy attainment of Apollo's goal of landing a man on the Moon ahead of the Soviet Union and before the end of the 1960s. Because of this, in late summer 1968, NASA began to look at a modified mission sequence.

The C' mission, which would see the Apollo 8 CSM orbit the Moon without an LM, was revealed to the public on 12 November 1968, three weeks after Apollo 7 successfully accomplished the C mission. Apollo 8 would test many CSM elements of the lunar landing mission and the world-wide system of radio dishes and transceivers NASA had created for Apollo lunar mission communications and tracking.

The C' mission had been the subject of intense debate at the highest levels of NASA, for it meant traveling to the Moon without the backup life support and propulsion systems the LM could provide. Intelligence reports that indicated that the Soviet Union might launch a man around the Moon during December 1968 gave C' supporters added credibility. The Soviet mission might steal Apollo's thunder; though it would merely swing around the Moon and fall back to Earth, it would enable the Soviets to claim that they had launched a man to the Moon first.

Eleven hours after launch, the Apollo 8 crew carried out a course correction. This required that they ignite the CSM's Service Propulsion System (SPS) main engine for the first time. Had the SPS not functioned as planned, the crew could have adjusted their course using the CSM's cluster of four Reaction Control System (RCS) thruster quads. The CSM would then have swung around the Moon without entering orbit and fallen back to Earth.

Partial cutaway of Apollo CSM spacecraft. Image credit: NASA.

The 20,500-pound-thrust SPS, an AJ-10-137 rocket engine manufactured by Aerojet, was located at the aft end of the CSM. Other AJ-10 variants had propelled Vanguard, Atlas-Able, and Thor-Able launch vehicles.

The SPS burned hydrazine/UDMH fuel and nitrogen tetroxide oxidizer. Chemically inert helium gas pushed the propellants into the engine's ignition chamber. Hydrazine/UDMH and nitrogen tetroxide are hypergolic propellants; that is, they ignite on contact with each other. The resulting hot gas then vented through a large engine bell, which was designed to swivel to help steer the CSM.

The Apollo 8 SPS performed almost perfectly during the 21 December course correction burn and during a second burn 61 hours after launch. Three hours later, Mission Control in Houston gave Apollo 8 a "go" to enter lunar orbit. The spacecraft passed behind the Moon, out of radio contact with Earth, and the crew ignited the SPS for the third time. It burned for a little more than four minutes, slowing the Apollo 8 CSM enough for the Moon's gravity to capture it into orbit.

The Apollo 8 CSM orbited the Moon 10 times over the next 20 hours. Then, on 25 December 1968, about 89 hours after launch, the crew ignited the SPS behind the Moon to begin the journey home to Earth. The rocket motor performed flawlessly during the critically important burn, which NASA dubbed Trans-Earth Injection (TEI).

Two and a half days later, on 27 December, the CSM split into two parts. The Service Module (SM), which included the SPS, separated from the Command Module (CM), which held the crew. The former burned up in Earth's atmosphere as planned, while the latter, protected by a heat shield, maneuvered in the upper atmosphere to reduce heating and deceleration, deployed parachutes, and splashed safely into the Pacific Ocean.

Four days after Apollo 8's triumphant return, A. Haron and R. Raymond, engineers with Bellcomm, NASA's Washington, DC-based planning contractor, completed a brief study of what might have happened had the SPS not ignited for the TEI burn. Specifically, they looked at how long a crew might survive in lunar orbit following a TEI failure.

Haron and Raymond found that the "first constraint" on the crew's endurance would be depletion of the CSM's supply of lithium hydroxide (LiOH) canisters. The square canisters were used in pairs to remove carbon dioxide exhaled by the crew from the CSM's pure oxygen atmosphere. During Apollo 8, the crew traded a saturated LiOH canister for a new one every 12 hours, so expended two per day.

The Bellcomm engineers calculated that, at that rate, the crew would use up the last of the 16 LiOH canisters launched on board the CSM 96 hours after TEI failure. They would then grow drowsy and become unconscious as carbon dioxide accumulated in the crew cabin. Had TEI failed on Apollo 8, Borman, Lovell, and Anders would probably have suffocated on 29 December.

Haron and Raymond noted, however, that LiOH canisters might be changed less often without harming the crew. They cited a November 1968 Manned Spacecraft Center study that showed that a LiOH canister could absorb carbon dioxide for up to 37 hours. If a stranded Apollo CSM crew began to ration its LiOH canisters immediately after TEI failure, they would be able to stretch their survival time to 148 hours. In that case, the Apollo 8 crew would have survived until New Year's Eve – the day Haron and Raymond completed their study.

By that point, limitations on crew survival other than carbon dioxide buildup would begin to come into play. The CSM fuel cells, manufactured by Allis Chalmers, operated by combining liquid hydrogen and liquid oxygen reactants to produce electricity and water. Electricity from the fuel cells powered the CSM through most of the mission. The crew drank the water, which also was used for cooling the CSM's Environmental Control System (ECS) and electronics. Excess water was dumped overboard.

Haron and Raymond looked briefly at the possibility of switching off two fuel cells to conserve reactants. If this were done, then the remaining fuel cell might operate for up to three weeks after TEI failure. However, a single fuel cell would probably not produce enough electricity to operate all CSM systems vital to the crew's continued survival, some of which were not immediately obvious.

As an example, Bellcomm cited the RCS quads. The astronauts would need to use them to maneuver the CSM to keep its ECS radiators in shadow to conserve cooling water. In addition, the LiOH canister shortage would remain. "The feasibility of extending survival time to as much as three weeks cannot be confirmed at this time," Haron and Raymond wrote.

The Bellcomm study was mainly of academic interest; a crew stranded in orbit around the Moon, 238,000 miles from Earth, could not have been rescued even if they did survive for three weeks. NASA did not have the ability to maintain a rescue Saturn V rocket and CSM on standby.

The space agency would have cause to recall the brief Bellcomm study twice during subsequent Apollo missions. During Apollo 13 (11-17 April 1970), an oxygen tank exploded in the CSM Odyssey, badly damaging its SM.

Because the explosion happened while the mission was en route to the Moon, its crew, commanded by Apollo 8 astronaut James Lovell, was able to use the LM Aquarius as a lifeboat. The astronauts employed the LM descent engine in place of the SPS. The docked spacecraft flew behind the Moon, where the crew fired the descent engine to adjust their course and accelerate toward Earth.

During Apollo 16 (16-27 April 1972), as the CSM Casper orbited the Moon, it suffered a malfunction in the system meant to swivel its SPS engine bell. The LM Orion, which had already undocked in preparation for landing, stood by in lunar orbit until the SPS problem was understood, then landed several hours behind schedule.

Had it been judged necessary, NASA could have scrubbed the Apollo 16 landing. Orion would then have redocked with Casper. The astronauts could have used Orion's descent engine and (if necessary) Casper's RCS quads to perform TEI.

Proceeding with the landing eliminated that option; the descent engine used most of its propellants to land on the Moon, then was left behind on the surface with the rest of the LM descent stage. The LM ascent stage, with its smaller engine, returned to lunar orbit with virtually dry tanks. This left only the SPS available for TEI.

As a precaution, NASA moved up Apollo 16's TEI burn by a day in the hope that, should the SPS misbehave, the crew and engineers on Earth would have adequate time to find a solution and ensure a safe, if delayed, return to Earth. As it turned out, the Apollo 16 SPS performed a flawless TEI burn.

Sources

NASA News Press Kit, Project: Apollo 8, 15 December 1968.

"Consumables Affecting Extended CSM Lifetime in Lunar Orbit," Case 320, A. Haron and R. Raymond, Bellcomm, Inc., 31 December 1968.

Apollo 8: "A Most Fantastic Voyage," Lt. Gen. Sam C. Phillips, National Geographic, May 1969, pp. 593-631.

Apollo 13: "Houston, We've Had a Problem," NASA EP-76, 1970.

NASA Mission Report: Apollo 13, A Successful Failure, 20 May 1970.

How Apollo Flew to the Moon, W. David Woods, Springer Praxis, 2008, pp. 236-238.

More Information

What If an Apollo Lunar Module Ran Low on Fuel and Aborted its Moon Landing? (1966)

What If an Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

"Assuming That Everything Goes Perfectly Well in the Apollo Program. . ." (1967)

What If an Apollo Lunar Module Ran Low on Fuel and Aborted its Moon Landing? (1966)

"The Eagle has wings!" The Apollo 11 Lunar Module Eagle shortly after separating from Apollo 11 Command and Service Module Columbia in lunar orbit, 20 July 1969. Image credit: NASA.

At 3:08 p.m. U.S. Eastern Daylight Time (EDT) on 20 July 1969, out of contact with Earth over the Farside hemisphere of the Moon, the computer that guided the Apollo 11 Lunar Module (LM) Eagle opened valves in its descent propulsion system, causing nitrogen tetroxide oxidizer and aerozine 50 fuel to come together in its Descent Stage rocket engine. The propellants were hypergolic, meaning that they ignited on contact with each other.

The descent engine fired for a little more than 12 minutes. At the beginning of the burn, Eagle, Apollo 11 Commander Neil Armstrong, and Lunar Module Pilot Edwin Aldrin were in a 54-by-66-nautical-mile lunar orbit. At the end of the burn, the 16.5-ton, 23-foot-tall lunar lander and its occupants were in an elliptical orbit with an apoapsis (low point) 50,000 feet above the Moon's Earth-facing Nearside hemisphere.

Apollo 11's target landing site was known officially as Site 2. Selected because it was flat and equatorial, Site 2 was a 10-mile-long east-west-trending ellipse on the Moon's Sea of Tranquility centered at 0° 42' 50" north latitude, 23° 42' 28" east longitude. Eagle descended to 50,000 feet about 260 nautical miles and 12 minutes of flight time east of Site 2, at which time its computer ignited its descent engine again to begin braking and final descent.

As the LM dropped below 7000 feet, its computer fired attitude control thrusters to tip it slowly upright so that it pointed its descent engine and footpads at the Moon. This maneuver also aimed Eagle's twin triangular windows forward so Armstrong and Aldrin could see Site 2 up close for the first time.

The astronauts immediately realized that they had a problem. They should have been above the eastern edge of the Site 2 ellipse, about five miles from their target landing point at the center of the ellipse. In fact, they had already flown past the center of their target ellipse and were descending toward its northwestern edge.

Apollo 11's flight plan called for Armstrong to let the computer do the flying until Eagle was about 500 feet above the Moon and 2000 feet east of the target touchdown point. He would then take manual control and lower Eagle almost vertically to the surface. He quickly realized, however, that Eagle's computer was steering it toward a boulder-strewn impact crater the size of an American football field. This was later identified as West Crater.

His heart rate leaping from 77 to 156 beats per minute, Armstrong assumed manual control early. Gripping his hand controller, he leveled Eagle's descent, then scooted the LM almost horizontally across the black lunar sky at an altitude of several hundred feet.

While Aldrin read off descent and translation rates, the LM computer flashed erroneous alarms and Capcom Charles Duke in Houston warned that Eagle was running low on propellants. Armstrong flew past West Crater and an adjacent smaller crater, then lowered to a safe touchdown just inside the Site 2 ellipse. At 4:18 p.m. EDT, he radioed his immortal words to hundreds of millions of people: "Houston, Tranquility Base here — the Eagle has landed."

The Apollo 11 Lunar Module Eagle on the Moon at Tranquility Base. Note lunar dirt stirred up by astronaut activities on the surface. Image credit: NASA.

Armstrong and Aldrin landed at 0° 41' 15" north, 23° 26' east, roughly four miles west and about three-quarters of a mile north of their planned touchdown point. Mission Control estimated that Eagle's Descent Stage tanks contained only enough propellants for about 25 seconds of flight when the descent engine was shut off at Tranquility Base. After the flight, more detailed analysis yielded an estimate of 45 seconds, demonstrating that the system for measuring available propellants in real time left much to be desired.

Mission rules called for an abort if propellants for fewer than 20 seconds of flight remained in the descent stage propellant tanks. What if, as Armstrong anxiously sought a safe place to land, flight controllers on Earth had mistakenly estimated an even slimmer propellant margin? They might then have done as mission rules dictated and called on Armstrong to abort the Apollo 11 lunar landing.

In June 1966, Charles Teixeira, with the Engineering and Development Directorate at the Manned Spacecraft Center in Houston, completed an Apollo Program Working Paper on the hazards of a landing abort during the 45-second period spanning from 65 to 20 seconds before planned touchdown. He assumed that the LM would be no more than 338 feet above the Moon 65 seconds before planned touchdown and about 100 feet high 20 seconds before planned touchdown.

As soon as an abort was initiated, the LM's Descent Stage engine would shut down. Nearly simultaneously, four explosive bolts linking the descent stage with the Ascent Stage would fire. A fifth pyrotechnic device would drive a guillotine that would cut the wiring umbilical linking the two stages. The Ascent Stage engine would then ignite to propel the astronauts toward lunar orbit. The abandoned Descent Stage, meanwhile, would fall to the lunar surface.

From abort initiation to Ascent Stage ignition, the abort procedure — which, apart from occurring at altitude, would duplicate the normal LM Ascent Stage launch procedure — would last from two to four seconds. During that time, the Ascent Stage would follow the same path as the Descent Stage; that is, it would fall toward the Moon.

Teixeira assumed that, following an abort during the 45-second period from 65 seconds to 20 seconds before planned touchdown, the four-legged Descent Stage would strike the Moon with enough force to rupture its propellant tanks. An abort within 20 seconds of planned touchdown — when the Descent Stage was at or below 100 feet — would leave the tanks intact.

If the tanks ruptured, either of two things might occur. The nitrogen tetroxide and aerozine 50 they spilled might boil and evaporate rapidly in the lunar vacuum. Evaporation would cool, then freeze, the propellants, so they would remain safely separated. Alternately, the propellants would come together. This might occur, Teixeira wrote, if after impact enough of the Descent Stage structure remained intact around the ruptured tanks to contain the propellants as they boiled.

Propellant mixing would cause an explosion that would drive gases and fragments of the Descent Stage outward at several thousand feet per second. Teixeira estimated that gases and debris would envelope the LM Ascent Stage less than one-tenth of a second after the explosion.

The extent of the damage this might cause would depend mainly on how long the abort procedure lasted; that is, on how quickly the ascent engine could ignite. The faster the ascent engine ignited, the farther away the astronauts would be when the Descent Stage impacted and exploded.

For a two-second abort procedure, gas pressure from the explosion would damage the Ascent Stage if the abort began between 32.6 and 20 seconds before planned touchdown. If the two-second abort began between 44 and 20 seconds before planned touchdown, then the Ascent Stage stood a greater than 20% chance of being struck by a Descent Stage fragment.

For a four-second abort procedure, gas pressure from the explosion would damage the Ascent Stage if the abort began between 53.7 and 20 seconds before planned touchdown. The Ascent Stage stood a greater than 20% chance of being struck by a Descent Stage fragment if the four-second abort began between 65 and 20 seconds before planned touchdown; that is, throughout the period Teixeira considered.

After the Landing: The Ascent Stage of the Apollo 11 Lunar Module Eagle as viewed from the Apollo 11 Command and Service Module Columbia during rendezvous in lunar orbit. Image credit: NASA.

Teixeira called the "critical time spans" during which damage would be likely to occur "rather short." He acknowledged that the risk of a Descent Stage explosion during a near-surface abort might not be great enough to justify "elaborate remedial action" — for example, a major redesign of the LM Descent Stage.

He recommended, however, that a Descent Stage propellant dump "at as high a rate as safely possible" become a part of the standard LM landing abort procedure. After due consideration, NASA elected not to follow his advice. Had Armstrong and Aldrin been forced to abort the Apollo 11 landing while above 100 feet of altitude, Teixeira's recommendation might have come back to haunt the U.S. civilian space agency.

Sources

Hazards Associated with a LEM Abort Near the Lunar Surface, NASA Program Apollo Working Paper No. 1203, NASA Manned Spacecraft Center, 24 June 1966.

Apollo 11 Mission Report, NASA SP-238, Mission Evaluation Team, NASA Manned Spacecraft Center, 1971.

Chariots for Apollo: A History of Manned Lunar Spacecraft, NASA SP-4205, The NASA History Series, C. Brooks, J. Grimwood, and L. Swenson, NASA, 1979, pp. 343-344.

More Information

What If an Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

What If an Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

Abort Mode One-Alpha. Image credit: NASA.

No member of the Saturn rocket family ever killed an astronaut. Two Saturn rocket designs were rated as safe enough to launch humans into space: the two-stage Saturn IB, which flew nine times between February 1966 and July 1975, and the giant Saturn V, which flew 12 times with three stages between November 1967 and December 1972, and once with two stages in May 1973. The 200-foot-tall Saturn IB flew five times with astronauts on board (Apollo 7, Skylab missions 2, 3, and 4, and the Apollo-Soyuz Test Project), while the 363-foot-tall Saturn V launched astronauts 10 times (Apollo missions 8 through 17).

Although man-rated, Saturn V rockets experienced four close calls. The first occurred on 4 April 1968, during the unmanned Apollo 6 test flight, when instability in the rocket’s fiery exhaust produced violent fore-and-aft shaking known as "pogo." Two of the five J-2 engines in the rocket’s S-II second stage shut down and pieces broke away from the streamlined shroud linking the Apollo Command and Service Module (CSM) to its S-IVB third stage. The CSM comprised the conical Command Module (CM), which carried the crew, and the Service Module (SM) which included electricity-generating fuel cells and the CSM's main engine, the Service Propulsion System (SPS). The Apollo 6 S-IVB's single J-2 engine under-performed, placing the stage and CSM into a lopsided orbit, then refused to restart.

Had the Apollo 6 CSM carried astronauts, pogo might have injured them; even if they had reached orbit unscathed, the S-IVB engine failure would have scrubbed their moon mission. As it was, flight controllers separated the unmanned CSM from the crippled S-IVB stage and used its SPS as a backup engine for completing the mission's Earth-atmosphere reentry test.

Apollo 12 experienced an even more perilous ascent. Following launch in a rainstorm on 14 November 1969, lightning struck its Saturn V 36.5 seconds and 52 seconds after liftoff. The lightning strikes knocked the Apollo 12 CSM Yankee Clipper's three electricity-generating fuel cells offline, along with its Apollo Guidance Computer and most other electrical systems.

The Saturn V's IBM-built Instrument Unit — its ring-shaped electronic brain, located atop its S-IVB third stage — soldiered on without a hiccup, however, safely guiding the giant rocket into Earth parking orbit. The Apollo 12 crew of Charles Conrad, Alan Bean, and Richard Gordon carried out a successful lunar landing mission and returned to Earth on 24 November. During the mission, Conrad reported seeing dark discoloration on the umbilical housing linking the CM and SM, but it remains uncertain whether this was a scorch mark left by lightning since discoloration has been noted on at least one other CSM umbilical housing (Apollo 15).

NASA would rename the "Uprated Saturn I" (right) depicted in this 1966 illustration the Saturn IB. Image credit: NASA.

Image credit: NASA.

The third Saturn V close call saw the unexpected return of pogo. During ascent to orbit on 11 April 1970, the middle engine of the Apollo 13 S-II stage began to rapidly oscillate fore and aft, then shut down two minutes early. The four remaining J-2 engines burned for longer than planned to compensate. Apollo 13 astronauts Jim Lovell, Fred Haise, and Jack Swigert subsequently left Earth orbit for the moon, but an oxygen tank explosion in their CSM, the Odyssey, scrubbed their moon landing. They used their Lunar Module (LM) moon lander, the Aquarius, as a lifeboat and returned safely to Earth on 17 April.

The final Saturn V to fly, intended originally for Apollo 20 but launched unmanned with the Skylab Orbital Workshop (OWS) on top in place of an S-IVB stage and the Apollo CSM and LM spacecraft, survived a close call on 14 May 1973. A design flaw caused Skylab's meteoroid shield to tear loose 63 seconds into the flight. As the disintegrating shield tumbled down the length of the accelerating rocket, it tore at least one hole in the interstage adapter that linked the OWS to the S-II second stage and apparently damaged the system for separating the ring-shaped interstage adapter that linked the S-II with the S-IC first stage. This meant that the 18-foot-long adapter did not separate from the S-II three minutes and 11 seconds into the flight as planned. The S-II stage had excess capacity, however, so dutifully hauled its unplanned five-ton cargo into Earth orbit.

Apollo 12 might easily have ended in a Launch Escape System (LES) abort. The image at the top of this post shows the LES in action during Pad Abort Test-2 on 29 June 1965. The LES was a 33-foot-tall tower containing three solid-fueled rocket motors. The largest was the Launch Escape Motor, which had four exhaust nozzles. The tower stood atop the Boost Protective Cover (BPC), a conical shell that covered the CM.

There were four successive abort modes during Saturn V ascent to Earth orbit. As the Saturn V climbed toward space, the aerodynamic environment around it changed - the air grew thinner, the rocket moved faster, and increasingly it tilted so that it flew parallel to Earth's surface. As the environment changed, the abort modes changed to compensate.

Abort Mode One was in effect on the launch pad, during S-IC first-stage operation, and during the 30 seconds following S-IC separation, by which time the Saturn V would have reached an altitude of about 56 miles. Had it occurred, the Apollo 12 abort would have taken place during the first part of Abort Mode One. Known as Abort Mode One-Alpha, it took effect 45 minutes before scheduled launch and continued until about 42 seconds after liftoff, by which time the rocket would have climbed nearly vertically to an altitude of 3000 meters (9800 feet).

In the event of a catastrophic Saturn V failure while Abort Mode One-Alpha was in effect, the 155,000-pound-thrust Launch Escape Motor would have pulled the BPC and CM free of the SM, which would have remained mounted on the doomed rocket. Meanwhile, the small side-mounted solid-propellant rocket motor near the LES's nose, the Pitch Control Motor, would have ignited to push the LES-BPC-CM combination eastward, toward the Atlantic and well clear of the Saturn V. The CM would then have dropped free of the BPC and deployed its three large parachutes to descend gently into the Atlantic within sight of Kennedy Space Center.

The Apollo 8 Saturn V rocket — the first Saturn V to carry a precious human cargo — stands on Launch Pad 39A at Kennedy Space Center, Florida. A Saturn V explosion before or during liftoff would have destroyed most of the structures visible in this image. Image credit: NASA.

27 April 1972: The Apollo 16 CM descends to a splashdown in the Pacific Ocean after an 11-day voyage to the moon. A CM descending into the Atlantic after an LES abort would have appeared very similar. Image credit: NASA.

In August 1965, R. High and R. Fletcher, engineers at NASA's Manned Spacecraft Center in Houston, Texas, calculated the characteristics of Saturn IB and Saturn V launch pad explosions to aid LES development. Of particular concern, they explained, was the damage an explosion fireball's heat might do to the CM's nylon main parachutes. In their report they did not, however, reach specific conclusions about parachute heat damage.

High and Fletcher found that calculating the characteristics of launch pad failures was not an exact science, in large part because there were so many variables to be taken into account, and also because no rocket as large as the Saturn V had ever exploded. They explained that "many of the [fireball] parameters may defy an accurate theoretical treatment."

For their analysis, they assumed that all propellants in the exploding rocket would contribute to forming a fireball. This would occur, they explained, because "large overpressures from detonations and the intense heat from both detonations and burning would cause failure of any propellant tanks not initially involved." If a Saturn V exploded on the pad at launch, 5,492,000 pounds of RP-1 refined kerosene, liquid oxygen (LOX), and liquid hydrogen would contribute to its fireball. For a Saturn IB pad explosion, 1,110,000 pounds of RP-1, LOX, and liquid hydrogen would fuel its fireball.

High and Fletcher wrote that the fireball from a Saturn rocket launch pad failure would expand in a "nearly fixed location." For the Saturn V, the fireball would expand to a diameter of 1408 feet. The Saturn IB fireball would expand to 844 feet. The fireballs would thus completely engulf the Saturn launch pads. For both rockets, fireball surface temperature would attain 2500° Fahrenheit, and heat would be felt up to a mile from the launch pad.

A fireball would begin to rise when it reached its maximum diameter. Fireball ascent would commence about 20 seconds after a Saturn V launch pad explosion and about 10 seconds after a Saturn IB explosion, High and Fletcher calculated. The Saturn V fireball would reach an altitude of about 300 feet in 15 seconds, while the Saturn IB fireball would climb 300 feet in 11 seconds. The Saturn V fireball would persist at its maximum diameter for 34 seconds, while the Saturn IB fireball would last for 20 seconds. The fireball would then begin to cool and dissipate.

Though they assumed for their calculations that all propellants in an exploding Saturn rocket would contribute to its fireball, High and Fletcher wrote that some would likely be "spilled on the ground, creating residual pools which [would] burn for relatively long periods of time." This was, they judged, especially likely if a launch pad failure began with the rupture of the fuel tank in the Saturn V's S-IC first stage. The ruptured tank would spill RP-1 onto the pad, then the oxidizer tank located above it would rupture and mix liquid oxygen with the burning fuel, triggering an explosion. They added that "the residual fire and extreme heat of the fireball [would] prevent approach to the ground area enveloped by the fireball for an unknown period."

Sources

Estimation of Fireball from Saturn Vehicles Following Failure on Launch Pad, NASA Program Apollo Working Paper No. 1181, R. High and R. Fletcher, NASA Manned Spacecraft Center, Houston, Texas, 3 August 1965.

Skylab 1 Investigation Report, Hearing Before the Subcommittee on Manned Space Flight of the Committee on Science and Astronautics, U.S. House of Representatives, Ninety-Third Congress, First Session, 1 August 1973, U.S. Government Printing Office, 1973.

Apollo Experience Report - Launch Escape Propulsion Subsystem, NASA Technical Note D-7083, N. Townsend, NASA, March 1973, pp. 1-7.

Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions, W. David Compton, NASA SP-4214, 1989, pp. 177-178.

How Apollo Flew to the Moon, W. David Woods, Springer-Praxis, 2008, pp. 69-73.

More Information

A Forgotten Rocket: The Saturn IB

"Assuming Everything Goes Perfectly Well in the Apollo Program. . ." (1967)