The Little Engine that Could--and Did

COMPARED with the mighty Saturn 5, which generated 7,500,000 Ibs. of thrust in its first stage alone, the little engine seemed puny indeed. But the importance of the Apollo spacecraft's 20,500-lb.-thrust Service Propulsion System (SPS) engine was far out of proportion to its 31-ft. length. The engine's faultless operation made the difference not only between a relatively simple moon shot and last week's sophisticated mission, but also between life and death for the astronauts.

Had the SPS engine failed to ignite, or burned too briefly during the attempt to place Apollo into lunar orbit, the spacecraft would have looped around the back of the moon and headed directly back toward earth. If the engine had cut off during one crucial 30-second interval of the scheduled burn, Apollo would have been left in an unstable orbit and crashed into the surface of the moon. And, if the astronauts had not succeeded in restarting the engine after orbiting the moon, they would have been left stranded in space without hope of rescue. This point was not lost on Astronaut Borman. Shortly before launch, he said of the SPS engine: "It simply has to work at that point."

Despite its vital importance, the SPS engine was the only major system aboard the spacecraft designed without another complete system to back it. Like other systems, the engine had duplicate parts made to take over if its tanks, valves or propellant lines failed. But space and weight limitations had forced the manufacturer, Aerojet-General Corp., to include only a single combustion chamber, fuel injector and nozzle extension skirt (see illustration). The failure of any of these parts could have meant disaster.

To ensure reliability, engineers tried to make the engine as simple as possible. Devoid of any frills, the SPS engine consists basically of a combustion chamber and propellant (fuel and oxidizer) tanks. When valves to these tanks are opened, fuel flows from one tank and oxidizer to the other to meet in the combustion chamber, pushed by pressurized helium that takes the place of potentially troublesome pumps. Because the propellants are hypergolic--they react chemically and ignite on contact--no ignition system is needed. And to avoid including a complicated throttle, the engine burns only at its fully rated thrust of 20,500 Ibs. or not at all.

In reliability tests that extended over a period of five years, Aerojet and NASA technicians fired SPS engines some 3,200 times without a malfunction before qualifying them for manned flight. Although the total firing time on the Apollo 8 mission was scheduled to take no more than seven or eight minutes, the combustion chamber was designed to operate for 121 minutes. During tests, it actually held up for more than 30 minutes without burning out.

During the flight of Apollo 6, the SPS engine took over from an S-4B stage that failed to restart, and by itself propelled the unmanned spacecraft to an altitude of more than 13,000 miles. On Apollo 7, its first manned flight, it was started eight times. Thus, when Borman, Lovell and Anders embarked on their mission, they had a pretty good idea that their little engine could perform its tasks flawlessly.

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