Dead band

The next stage of the LM checks required the crew to think about the concept of the dead band, which is another of those curious terms in spaceflight where a simple concept lay behind opaque jargon.

Apollo was one of the first applications of a digital Пу-by-wirc system whereby control of a vehicle was placed in the hands of a computer. In the Apollo guidance computer, programmers included a scries of algorithms that would fire the RCS jets as necessary to bring the spacecraft to a desired attitude with respect to the IMU platform, and hold it there. These algorithms were called the digital autopilot (DAP). However, the gimbals around the platform were able to measure angular errors in the spacecraft’s attitude to an accuracy of hundredths of a degree, and to have constantly corrected the slightest drift to such a Light tolerance would have made the spacecraft seesaw backwards and forwards as the jets incessantly fought to maintain the ideal attitude, wasting propellant in the process. Instead, a range of attitude error around the ideal was deemed acceptable and the thrusters did not fire within this band; they were said to be ’dead’. This error band, the dead band, could be set to be cither a half or five degrees from ideal, depending on how accurately the spacecraft had to be pointed. A narrow er dead band used more RCS fuel, because the thrusters tended to fire more often when the spacecraft drifted beyond the permissible deviation.

While still docked, the commander gave the LM’s RCS system a checkout, first by using the computer to Lest that the hand controls were producing the commands expected of them, the so-called ‘cold-fire’ checks; and then by firing all 16 thrusters for short periods in a ‘hot-fire’ test. Prior to carrying out these tests, he had to ensure that his crcwmatc in the command module had the CSM’s digital autopilot set for a wide, five-degree dead band. That is, although overall the thruster firings – fore aft, left/right. etc. should be neutral, they would briefly rotate the entire CSM/LM stack by a few degrees, and it would have been a waste of propellant if the thrusters on the service module had to battle to restore attitude.

The commander also ensured that telemetry from the LM was being sent to mission control at a high bit rate. This maximised the number of engineering parameters that could be received while the health of the RCS was checked.

Almost like a third eye on the forehead of the LM’s face, another dish sprouted from the ungainly LM cabin, preferring a direction that faced forward. This was the antenna for the rendezvous radar, one of the subsystems that allowed the ascent stage to seek, find and follow the CSM as it chased the mothership around the Moon during the rendezvous. The radar operated in conjunction with a transponder on board the CSM so that the LM’s computer could tell how far apart they w ere, and in which direction. As the commander put it through a self-test routine, the final steps towards undocking were completed.

Breaking the link

Perhaps il was a bitter sweet moment for the command module pilot as he watched his crewmates leave for the Moon. There would probably be some relief that the mission had reached this point, and increasingly looked like it was going to be successful. At the same time, there might have been a deep longing for an opportunity to take a ride to the surface knowing that there were only a few kilometres between him and a moonwalk. But for all the CMPs, there was terror in the knowledge that it required only one or two of many possible failures to occur, and he might have to light his SPS engine and return to Earth alone, as a marked man, having left his crewmates on the Moon.

Alan Bean found Dick Gordon’s reaction to sending his crewmates away on Apollo 12 remarkably sanguine. Gordon and Conrad had flown together on Gemini 11, although their friendship went back to Patuxent River Naval Air Station where they were both test pilots and good buddies. Although Gordon’s friendship with the likeable and super-competent Conrad had helped to seal his place on Gemini, the experience subsequently gained prevented him from taking a ride to the Moon’s surface. Deke Slayton, who decided Apollo’s crewing arrangements, generally gave command to the most experienced in a crew, and that was Conrad. At the Lime, he preferred the astronaut in charge of the command module Lo have had experience of rendezvous in ease the CSM had to rescue an ailing LM. Thus Gordon got to fly the CSM solo. That left Bean, the rookie and the third member of this friendly crew-, with a ride to the moondust. Twenty-three years later. Bean completed a painting that imagined Gordon being down on the surface with his two buddies. In his notes on that work, Bean stated. "Dick was the more experienced astronaut, yet I got the prize assignment. In the three years of training preceding our mission, he never once said. Tt’s not fair, I wish I could w? alk on the Moon too.’ 1 do not have his unwavering discipline or strength of character.”