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 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 later.

Though its target was the moon, Apollo 8 included no Lunar Module (LM). The manned lunar lander had suffered production delays. NASA's planned mission sequence for manned Apollo missions had begun with a low-Earth orbit test of the Command and Service Module (CSM) on Apollo 7 (Oct. 11-22, 1968). This was to have been followed by a low-Earth orbit test of the CSM and LM, then a CSM/LM test flight in higher Earth orbit. On the next 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.

Crew of Apollo 8, the sole CSM-only lunar mission: William Anders (left), James Lovell, and Frank Borman. Image: NASA.

Pushing off the next Apollo flight, the D mission, until the LM was ready would have placed in jeopardy the goal of landing a man on the moon 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 made public on 12 November 1968, three weeks after Apollo 7 accomplished a successful C mission.

Eleven hours after launch, the Apollo 8 crew carried out a course correction, firing 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 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.

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 swiveled to help steer the CSM.

Apollo Service Module cutaway. The Service Propulsion System engine bell is located at its aft end (right in this image). Image: NASA. Apollo Service Module cutaway. Colors are not true in this vintage NASA artwork.

The Apollo 8 SPS performed almost perfectly during the 21 December course correction burn and during a second burn 61 hours after launch designed to help ensure that the Apollo 8 CSM would enter the orbit about the moon planned for it. Three hours later, Apollo 8 was given 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 this 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 contained 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 had traded a saturated LiOH canister for a fresh one every 12 hours, thus expending 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.

Apollo 13 CSM Odyssey lithium hydroxide canister with duct-tape modifications to permit use in LM Aquarius. Image: NASA. Good-quality images of CSM lithium hydroxide canisters are hard to find. Apollo 13 astronauts modified this canister with duct tape and a plastic bag to permit its use in the LM Aquarius. Image: NASA.

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 had shown 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 the Bellcomm team estimated that endurance might be stretched to about two weeks. 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 could be 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 vital CSM systems - for example, the RCS quads, which the crew would use to conserve cooling water by maneuvering the spacecraft so that the ECS radiator stayed in shadow - and the problem of the LiOH canisters would remain. "The feasibility of extending survival time to as much as three weeks cannot be confirmed at this time," they wrote.

Nearing Earth, the Apollo 13 crew discarded the crippled CSM Odyssey's Service Module and photographed it to aid accident investigators. Image: NASA.

The Bellcomm study was mainly of academic interest, since 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. On 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. They employed its 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 speed their return to Earth.

On Apollo 16 (16-27 April 1972), as the CSM Casper orbited the moon, it suffered a malfunction in the system used to swivel the 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. Going ahead 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 by a day in the hope that, if the SPS misbehaved, 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.

Apollo 16 Command and Service Module Casper in lunar orbit as viewed from Lunar Module Orion. Image: NASA. Nearly nose-on view of the Apollo 16 CSM Casper in lunar orbit as seen from the LM Orion. Image: NASA.

Reference:

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