SPS: the engine that just had to work

The SPS engine was mounted at the rear of the central tunnel, below the helium tanks. From the point of view of crew safety, this was the one major engine in the Apollo system that had to be capable of being regularly restarted. Failure to operate at its first major firing to enter lunar orbit could be tolerated, because the spacecraft would loop behind the Moon and return to Earth’s vicinity and it would be possible for other engines to restore a trajectory home. However, a spacecraft already in lunar orbit was completely dependent on the successful restarting of that engine to bring the crew home.

With maximum reliability in mind, engineers endowed the engine with two of just about everything: two control systems, two sets of plumbing, two ways to pressurise the propellant tanks. In fact, except for the injector plate and combustion chamber and bell, the SPS engine was really two engines in one, with the crew having entirely separate control of either. Even pumps were banished from its systems in order to avoid having to rely on the moving parts that they contained. Instead, helium gas provided the pressure needed to push the propellants through the piping and into the combustion chamber with sufficient force. This helium was stored at extremely high pressure and passed through regulators which reduced its pressure to that required by the large propellant tanks. Reliability was further assured by the use of a hypergolic fuel and oxidiser combination. This meant that these substances – hydrazine as a fuel and nitrogen teiroxide as an oxidiser – merely had to mix to spontaneously ignite, unlike the engines on the Saturn V’s three stages which required igniters to begin their conflagration. On the Apollo stack, every engine within the two spacecraft used these propellants. To start the SPS engine, one had only to open valves and permit these exotic propellants to spray into the combustion chamber through hundreds of holes across the face of the injector. As they met. they burned fiercely to generate an almost invisible but powerful flame.

For the SPS to operate without pumps, the pressure with which the propellants were injected had to be sufficient to overcome the combustion pressure in the chamber where they met. which was 690 kPa (100 psi). To enable the propellant to reach the injector the tanks were pressurised at 760 kPa (110 psi). The engine valves could be operated manually by the crew via two prominent switches on their main console, or the computer could send a signal to achieve an automatic burn based on preloadcd information and the results of its computations. In fact, this engine was so intensively designed to ignite in response to a wide range of command sources, that on occasions when faults afflicted its control systems, the problem became one of ensuring it would not fire inadvertently.

Soon after the crew of Apollo 15 had extracted their LM and were about to settle into their long coast to the Moon, its commander David Scott reported to

mission control: "Okay; that all went fairly nominally, and the only different thing wc’vc noticed is the SPS Thrust light is now on. And wc don’t know when it came on; somewhere in the process here.” This light was mounted right in front of the left-seat occupant on Panel 1 of the main display console, an indication of its importance. It was there to indicate that the SPS engine valves were open and that the engine ought therefore to be firing. The light was energised, although the engine clearly was not.

‘■Roger. I understand the SPS Thrust light is on.’’ checked Gordon Fullerton, the Capcom in Houston.

”And all the switches are off," added Scott, ominously. He knew that one way to operate the engine was to throw one of two large, prominent, guarded switches in the middle of that panel. They allowed the crew to manually start the engine if automatic control was unavailable. Scott wanted to make clear to mission control that they had not been thrown and that there was obviously an electrical fault.

After an initial analysis, flight controllers asked the crow to open all the circuit breakers that could energise the engine’s propellant valves. They realised that if electricity had reached that light, then it could also reach the valves merely by arming the control system, which would cause the engine to fire immediately rather than on command. Apollo 15 was the first mission to have TLI not establish a frce-rcLurn trajectory in order to reach its northerly landing site. If the SPS were declared unusable, a large manoeuvre by the LM would be required to get home. Further careful diagnosis suggested that there was an electrical short in the vicinity of one of the two manual switches for the SPS. which Scott confirmed the next day by slowly manipulating the suspected switch to see how it made the light came on.

This fault was no show-stopper, as long as it was understood. Mission control drew up revised procedures for the rest of the mission and the crew operated the SPS using the redundant system whenever possible in order to prevent an inadvertent firing. After the flight, when the spacecraft’s anomalies were analysed, a tiny sliver of wire was found to have been trapped inside one of the two manual switches for the SPS during manufacture.

This was an occasion where the duplication of nearly every part of the engine allowed the faulty system to be isolated. The two separate halves of the engine’s control and plumbing were termed the A and В banks. The Apollo 15 crew could start and end all their burns accurately using the good В bank, and bring the A bank in and out manually only during long burns when it was desirable for the engine to keep running should either bank fail. When both banks operated, slightly more propellant reached the injector and the engine achieved a slightly higher thrust.