BRAKING AND STATION-KEEPING

The final major manoeuvre of the rendezvous was braking. Since the TPI burn, the LM had been coasting on an intercept trajectory that was essentially part of an orbit. The apolune of that orbit was a kilometre or more higher than the altitude of the CSM and. without braking, the LM would have sailed by in front of its target. When at a distance of about three kilometres, the commander began a series of manoeuvres to reduce the closing speed of the two spacecraft. Each was pre-planned to occur at ever narrowing ranges to the CSM. and although the checklists gave suggested approach speeds, the commander used his piloting instincts to achieve the actual braking thrust at each. As explained by John Young, w’hen he talked about his approach on Apollo 16. the approach speed was determined by other factors. If the LM thrusters stopped working, could the CSM finish the job? Young was aw’are of the difference between the light LM with its effective thrusters and the heavy CSM, which was still loaded with propellant for the burn home.

”As opposed to the usual Kamikaze brake that I usually make, we kept it very conservative. We decided that we w’ould ahvays keep the braking within something that the [CSM] could do. This means that, contrary to the braking gates that w;e use in the LM. you sort of have to lead them. In other words, at the range that you want to be at, you almost have to be at the braking velocity to give the [other spacecraft] a fighting chance in case it has to do it. I never had any doubt that we would do it all ourselves because that machine was working so beautifully. We just closed in and it was so good I wanted to do it again. It was really slick.”

Buzz Aldrin was struck by the responsiveness of the lightweight LM. “Each time you hit the thrust controller,” he explained, “the vehicle behaved as if somebody hit it with a sledge hammer, and you just moved. There is no doubt about the fact that the thrusters were firing. It was sporty; there’s no doubt about it.”

“It’s a very light, dancing vehicle,” agreed Armstrong.

On completion of a successful rendezvous, it was normal for the two vehicles to spend some time station-keeping – that is, floating next to each other – to give each a chance to inspect the other. For example, on Apollo 15, Scott and Irwin were asked to look at the SIM bay in the side of Endeavour’s service module. While Worden had been operating the cameras and instruments mounted in the bay, mission control had noticed that the output from a sensor was not as expected. It was designed to measure how fast the landscape below passed by, to enable a mechanism in the panoramic camera to compensate for image motion. Scott was being asked if he could see any obstruction in front of the sensor, which he could not. The problem lay in its optical design.

On Apollo 16, as Orion lifted off the Moon in front of the rover’s television camera, flight controllers noticed that the launch appeared to disrupt the skin at the

rear of the ascent stage, the part that faced the camera. This unpressurised section housed much of its electronics, including the electric control assemblies (ECAs), which were part of the spacecraft’s electrical supply system. Ken Mattingly in the CSM Casper was asked to describe what he saw as Young made Orion perform a pirouette in front of his camera.

"Okay, on back side, it looks like some of the thermal blanket around the ECAs on the back end there is pretty badly chewed up,” radioed Mattingly. "A couple of panels are torn off. And some of the stripping in between, it looks like it was struck by something, but it looks like all the Mylar blankets underneath are still intact.”

Mission control were keen to know the depth of the damage. Jim Irwin, Capcom for the rendezvous, enquired further. "Ken, can you observe whether it’s possible for sunlight to directly impinge on portions of the spacecraft equipment?”

"No, sir,” replied Mattingly. "It’s not possible from the back; I can’t tell about the bottom; but, on the back side, the Mylar blankets are still intact – it’s only that outer covering that’s broken.” Orion’s damage did not prove to be a problem for the rest of its short life.