02 August 2015

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. As they cast off their CSM's drum-shaped SM prior to reentry, they saw that it bore scorch marks from the lightning strikes it endured during launch.

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. 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. 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.

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 began 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, so that it would be 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)

7 comments:

  1. A Saturn V failure is rarely mentioned in history or nostalgia, is it?

    If we made Saturn V today, how would it be different? Lighter materials, improved reliability, more power, different fuels? Is SLS Block-II a worthy comparison? Seems like it proposes to lift 10% more mass with way more power?

    Pretty amazing it was as reliable as it was. Curious how close we ever got to the "NOVA" variant.

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    1. Nostalgia tends to focus on missions, not on all the hard work that went into missions, which is one thing that I despise about it. NASA and its contractors performed many failure analyses. As you might expect, these were not widely publicized unless they were put to use. I plan a series of "Failure Was an Option" posts interspersed with posts on entirely different subjects. The next will focus on the Saturn IB.

      I can't address how we might build the Saturn V differently today, except in one area. During the Space Exploration Initiative (1989-1993), some folks proposed reviving the F1 engine. One issue they noted was the need to replace the asbestos used in the original design because of health concerns. I'm sure we'd use very different electronics, for example.

      BTW, my grandpa worked as an engineer designing tractors and farm equipment. He's alive and 102 years old, BTW, so when I speak in the past tense, I am talking about his career, not his existence. He didn't graduate high school, started on the assembly line, and got himself noticed. By the time he retired, he was a designing engineer.

      I took him to see the Saturn V laid out on the ground at JSC when I worked there, and by looking at the plumbing and pumps he could describe for me how it worked. What I took away from that was that it included a fair number of established, unexciting engineering practices.

      BTW, my dad was not too different from that when it came to radio. I come from a techy family, though I got the writing & research bug from somewhere. I expect if I'd not gone for history & writing I'd have gone for cartography.

      dsfp

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  2. A few points to make about the Launch Escape System: while it did possess three motors, these were not all used during any single abort mode. For example, the primary "Launch Escape Motor" provided the main impetus for any abort (incorporating a single motor with four nozzles). In a Mode 1A abort, the "Pitch Control Motor" would also be used to pull veer the Command Module away from the launch pad. Finally, the jettison sequence for the LES used a specific "Tower Jettison Motor"; it did not use the main abort motor(s) as the article suggests.

    One other interesting point regarding Skylab 1: the failure of the interstage to separate became critical not so much because of the additional weight, but because of heat. With the five J-2 engines contained within the interstage, the heat build up came seconds away from causing the thrust bulkhead on the S-II to fail catastrophically. It was a very close call.

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  3. Carl:

    Thanks for pointing out these details. I read about the Skylab heat problem, but figured I'd save it for another day since it would need some explaining. As for the LES, I obviously need to look into this more closely. I relied on knowledge (or whatever) I picked up years ago - probably some news article. Never a good idea. I'll check my sources and correct the abort system information.

    dsfp

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  4. Carl:

    I made the corrections. Thank you once again for pointing out my errors.

    dsfp

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  5. David, just wanted to let you know how much I enjoy the site - I've been catching up on all of your posts. I had entertained doing a blog similar to this for some time, then discovered someone beat me to it! Which is good - you do the topic far more justice than I could have.

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  6. Michael:

    Thank you for your kind words. I think you should do a space history blog - there can never be enough of them!

    I've not been devoting enough time to the blog in the past few months, but that will soon change. I've been undertaking a project to re-org my files, something I should have done a couple of years ago. I have a long list of new posts to write. I hope that you will enjoy them.

    dsfp

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