To an adjustable shower head At full Jk power the DPS could generate a

thrust of 47 kilonewtons, about half of one engine on a Boeing 737 airliner. 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 needed to enable the computer to optimise the vehicle's descent to the surface and to hover in the final phase.

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, because if the heat in the system was not handled properly, the extreme cold of the SHe could freeze the fuel that was used to warm it - and this was the main reason for using the ambient tank, which provided initial pressurisation until the engine ignited. First, three explosively operated valves were opened by command from the crew 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 SHe into the propellant tanks.

Much of the checkout of the DPS was done by mission control because, owing to telemetry, the flight controllers had access to more data than was available in the LM cockpit. Therefore, they preferred to use the LM's steerable antenna to carry all the required high-bit-rate data.

This could not be done on Apollo 16, however. The steerable antenna was the LM's equivalent of the CSM's high-gain antenna. Mounted on the right side of the LM's roof, its dish could be moved under manual or automatic control to aim at Earth. When Charlie Duke tried to move

Apollo 16's LM Orion and its faulty steerable dish antenna.

Orion's steerable antenna, he discovered it would only steer in one axis. ''Well, we're not gonna have TV from the LM, unless we get that high-gain up,'' he said glumly to John Young as they battled to get their spacecraft checked for the descent. Up to this point in their checkout everything had gone smoothly, but the failure of the mechanism to steer their antenna was the first of a series of problems that would threaten the surface mission. As they continued their checkout as well as they could using low-gain antennae, a failure in the RCS pressurisation system threatened to burst its safety devices, prompting Young to opine, ''This is the worst jam I was ever in.''

One consequence of the loss of the steerable antenna was that mission control could no longer access the computer's memory directly. It therefore fell upon Duke to copy down two lists of numbers, 179 digits long in total, which represented an updated state vector and the REFSMMAT they were to use for the landing. Capcom Jim Irwin read them all for Duke to copy down, who then read them back as a check. Young and Duke then began laboriously entering them into the computer's memory, checking constantly to ensure that a mistake was not made; or if it was, that they corrected it. They eventually managed to largely overcome many of the problems associated with the loss of the steerable antenna by using the extra sensitivity of the 64-metre dish at Goldstone in California. Also, by optimising the LM's attitude, the less capable omnidirectional antennae were operated through a favourable lobe in their reception pattern. Spacecraft communications were normally handled by 26-metre antennae.

During the final far-side pass before the landing, CMP Ken Mattingly prepared for his circularisation burn by testing the systems associated with his SPS engine.

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