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Another tragedy that could occur during Apollo 8 Flight Director Clifford Charlesworth calls the mission's "longest hour." If, after completion of Apollo's tenth lunar revolution, the SPS engine fails to ignite or burns for too short a time, the astronauts would be stranded in orbit without any chance of rescue; they could live only until their oxygen supply was gone. To minimize the possibility of SPS failure, NASA has made nearly all of the engine's components redundant. If one part were to fail, a duplicate would be on hand to take over.
But SPS has only one combustion chamber, one propellant injector and one nozzle. All these must operate reliably to avoid disaster.
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There is one final, crucial phase or Apollo 8's mission: reentry. As it plunges back to earth, traveling some 7,000 m.p.h. faster than a returning earth-orbit mission, Apollo will have to re-enter the atmosphere at an angle no greater than 7.4° nor less than 5.4°. Reentry at too steep an angle would cause too sudden a deceleration. The force on Apollo and its occupants could then exceed 20 g's, and friction with the atmosphere would heat the spacecraft far above its design limits. Says Lieut. General Samuel Phillips, Apollo program director: "There would be a structural breakup and loss of the spacecraft and the crew."
An equally dreadful fate would befall Apollo if it hit the atmosphere at too shallow an angle. Like a flat stone skipping on water, it would bounce off the atmosphere and sail into a large elliptical orbit around the earth. Having shed Apollo's service module before reentry, the astronauts would have insufficient oxygen and electrical power to survive the several hours it might take to return to the atmosphere and land. In Phillips' laconic words, "It's a crew-loss kind of situation."
Other, less perilous problems could turn Apollo 8 from a space spectacular into a humdrum engineering flight. Allowing for such contingencies as the failure of a backup system, an inadvertent early cutoff of the S-4B rocket while it is blasting Apollo toward the moon or unusually intense radiation from the sun, NASA has devised a number of alternative flight plans. Thus, Apollo 8 might merely remain in earth orbit, duplicating Apollo 7's eleven-day flight. It could also loop out as far as 25,000 miles from the earth and then descend into a low earth orbit for several days—or it could re-enter the atmosphere after traveling as far as 69,000 miles into space. Or, just as the Russians will probably do, the spacecraft could simply make a circumlunar flight, loop around behind the moon, and return directly to earth.
Although they freely acknowledge the numerous possibilities of failure, NASA officials nonetheless exude confidence in Apollo 8 and its crew. They expect the mission to go all the way. In the thorough investigation that preceded the decision to send the spacecraft into lunar orbit, says Manned Space Flight ""Director George Mueller, "we found no incipient problems. The odds for complete success of Apollo 8 are as good as they were for Apollo 7."
Man in the Moon