|238,000 miles from home - Earth as viewed by the Apollo 8 astronauts in lunar orbit, Christmas Eve 1968. Image credit: NASA|
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-prime 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-prime 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-prime 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 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 built up 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 LiOH canisters 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.
If NASA elected to include 10 additional LiOH canisters on CSMs bound for the Moon, and if immediately after TEI failure the astronauts powered down the CSM so that its three fuel cells remained just barely operational, then endurance might be stretched to about two weeks, the Bellcomm team estimated.
The 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; it was used also for cooling in 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 two or 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.
On 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.
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
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)