Descent propulsion system

Most of the LM’s descent stage was taken up with the descent propulsion system (DPS). In the parlance of Apollo, the DPS was always pronounced dips’. The designers had come up with a simple cruciform structure for the descent stage which held a propellant tank in each of the four box-shaped bays around a central space where the engine was mounted.

This engine was remarkable for its time because it could be throttled, i. e. its thrust was variable, which was a major technical achievement. The basic idea of the throttle mechanism was to alter the area of the injector plate in use at any one time – similar

to an adjustable shower head. At its theoretical full power, the DPS could generate a thrust of 47 kilonewtons, about half of one engine on a Boeing 727 airliner. In operation, it could be throttled smoothly between 10 and 65 per cent full thrust or run at a steady 92.5 per cent. This ability to be throttled was essential to enable the computer to optimise the vehicle’s descent to the surface and to hover in the final moments before touchdown.

As in the SPS engine, propellants were forced into the combustion chamber by pressure alone. There were no pumps to fail. This pressure was provided by two helium tanks, one of which stored the gas at ambient temperatures; the other tank stored it as supercritical helium (SHe), a strange phase of the gas brought about by a combination of very high pressure and extremely cold temperatures. By using SHe, more of the gas could be crammed into a much smaller tank, thereby eliminating over 100 kilograms of weight from the vehicle. Care had to be taken, however, to carefully handle the heat in the overall system. In steady operation, the warmth in the fuel would be used to heat the SHc via a heat exchanger. But at engine start, before the fuel got flowing, there was a possibility of it being frozen by the SHe. This was the main reason for using the ambient Lank, which would not freeze the fuel while it provided the pre-start pressurisation. First, three explosively operated valves were opened by command from the crew7 to release the ambient helium into the tanks. This was part of the preparatory checklist. Later, once the engine was actually running, another explosively operated valve would automatically open to release SHc into the propellant tanks.