Checkout nightmares

Much of the checkout of the DBS was the responsibility of mission control because telemetry gave the flight controllers access to far more data than was available in the LM cockpit. Therefore, they preferred to use the LM’s steerable antenna to carry the required high-bit-rate data.

Not 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 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 out for the descent. IJp to this point in the checkout everything had gone smoothly, but the failure of the mechanism to steer the antenna was the First of a series of problems that sprang out of nowhere and which threatened to jeopardise the surface mission. They continued the checkout as best as they could using their low-gain antennae, but when a failure in the RCS pressurisation system threatened to burst its safety devices. Young opined, “’fhis is the worst jam I was ever in.” He didn’t know the half of it.

One consequence of the loss of the steerable antenna was that mission control could no longer directly access the computer’s memory. Duke had therefore to copy down two lists of numbers, 179 digits long in total, which represented an updated state vector and the RLFSMMAT for the landing. Capeom Jim Irwin read them up,

and Duke read them back as a check. Young and Duke then laboriously entered them into the correct addresses in the computer’s memory, checking to ensure that they did not make a mistake. Spacecraft communications were normally handled by 26-metre antennae, but they managed to overcome many of the problems caused by 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.

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.