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.
Starting from a distance of nearly 3 kilometres, the commander executed 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 his checklists included suggested approach speeds, the commander used his piloting instincts to achieve the actual braking thrust at each. As explained by John Young, when talking about his approach on Apollo 16, there were wider issues that dictated the approach speed. If the LM thrusters stopped working, could the CSM finish the job? The LM was light and its thrusters were very effective, but not so 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 would always keep the braking within something that the command and service module could do. This means that, contrary to the braking gates that we 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 command and service module 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 - in order 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 rapidly the landscape below was passing, thereby letting the mechanics of the panoramic camera compensate for the 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, controllers noticed that the skin at the rear of the ascent stage, the part that faced the camera, appeared to have been disrupted at launch. This unpressurised part of the LM housed much of its electronic systems, including the electric control assemblies (ECAs), which were part of the spacecraft's electrical supply system. They asked Ken Mattingly in the CSM Casper 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
Orion, Apollo 16's LM, with disrupted thermal blankets over its rear.
for the rendezvous stage of this mission, 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.
The LM approached the CSM with its windows facing its quarry. When originally envisaged, the LM was to have had two docking ports, the second being at the forward hatch. But in the drive to cut weight from the spacecraft, the heavy docking collar was dropped and replaced by a simple square hatch through which a suited crewman and his back pack could crawl on his way to the lunar surface. The remaining docking port was at the top of the ascent stage and there was a small window above the commander's head to enable him to view through the roof of the LM. Having lined up in front of the CSM, the commander had to pitch down until the docking apparatus of both spacecraft faced each other, essentially lining up their x axes. The LM was then rotated 60 degrees to line up the docking aids between the two spacecraft. From here, the CMP took over, bringing the spacecraft together and docking. The commander could carry out the docking, but to do so would have meant craning his head backwards uncomfortably. It was much easier for the commander to hold the LM steady while the CMP, who was seated comfortably looking through a rendezvous window, brought the CSM up to the small spacecraft.
When Armstrong was manoeuvring Eagle for docking, he decided to change the procedure, but soon wished he hadn't. He realised that if he were to pitch down at that point, the Sun would come beaming straight into his eyes. Therefore, he chose to
line up 60 degrees further around before he pitched down, thereby avoiding the Sun. Having done so, Collins asked him to rotate a little further to get the docking aids properly aligned.
''We complied and promptly manoeuvred the vehicle directly in the gimbal lock,'' related Armstrong after the flight. Having gone into gimbal lock, the PGNS was no longer able to hold the ascent stage stable. ''I wasn't aware of it because I was looking out the top window. No doubt, we were firmly ensconced in gimbal lock. We had all the lights on.'' However, they had a backup system. ''We just put it in AGS and completed the docking in AGS. This was just a goof on our part. We never should have arrived at the conclusion from any series of manoeuvres. However, that's how it happened. It wasn't significant in this case, but it certainly is never a desirable thing to do.''
Unfortunately, holding the LM stable using the AGS had a side effect that caught Collins unaware once he had soft docked and moved to retract the docking probe for a hard dock. As the probe began to pull the light ascent stage towards the much heavier CSM, the AGS detected a change in attitude and furiously tried to compensate for it by firing the LM's thrusters. Collins didn't realise this and began his own attempt to correct the motions of the spacecraft relative to one another, but as the two spacecraft were flexibly joined at the capture latches, their motions were somewhat complex. ''That was a funny one,'' he remarked directly afterwards. ''I thought things were pretty steady. I went to 'retract' there, and that's when all hell broke loose. [The LM was] jerking around quite a bit during the retract cycle.'' In 8 seconds - the time it took the probe to retract - the problem disappeared as the two spacecraft became one.
Apollo ll's problems had worried engineers who looked at the dynamics of two vehicles joined by a flexible probe. Dick Gordon and Pete Conrad had no such worries bringing CSM Yankee Clipper and LM Intrepid together on Apollo 12. ''It was a simple, easy task to perform,'' explained Gordon post-flight. "It could have been done in darkness as well as daylight with just as much ease. I don't think that the vehicles moved hardly at all at contact. There was certainly no noticeable motion, anyway.''
Conrad concurred: "We came right in and stopped. I pitched over and did the yaw manoeuvre after Dick did his roll. We did the docking just the way we stated before. Dick came in and docked; I maintained attitude hold - tight dead band. As soon as he got his top latches barber poled, we went to Free. Neither spacecraft so much as moved a muscle, and we got a complete, good lock. He straightened out attitude with his translations thrusters and went to hard dock, and it pulled us right in there without either spacecraft deviating; bango! We had 12 latches.''
When Eugene Cernan brought Apollo 17's ascent stage, Challenger, to rendezvous with the CSM America he thought the spacecraft was a fine sight to see.
"Okay. I've got you right out the overhead, Ron,'' called Cernan to his CMP Ron Evans. He had just pitched over to face the LM's drogue at the CSM's docking probe and the mirror-like surface of the command module looked resplendent in the sunshine. "Now I'm going to yaw.''
"Okay, yaw her around,'' replied Evans as Cernan began to line up the two craft to get the docking aids aligned.
"Okay, here we go.'' Cernan was enjoying the responsive spacecraft he had under his fingertips. "What a super flying machine!''
"Still looks kind of tinny to me,'' mocked Evans.
"Command module looks just as good as the day they put it on the pad,'' said Cernan.
"And, you know, so does Challenger, by gosh,'' said Evans. "You're missing some of the pieces.'' The last time he had seen Challenger, it had a descent stage attached, one that still sits quietly on the Moon.
"Yes, one big piece we left behind.''
When Evans tried docking with Challenger, he found the lightness of the LM took
a bit of getting used to. His approach speed was only 2 or 3 millimetres per second. ''Coming in nice and slow; no problems,'' he informed his commander.
''Okay, you're looking good, babe,'' said Cernan encouragingly. ''I got you on my COAS right up in the middle of the window. Looking good. Must be a couple of feet away.''
''Stand by,'' warned Evans as the point where 16.3 tonnes of CSM would impact 2.3 tonnes of LM ascent stage whose tanks were nearly empty. However, Evans had brought the CSM in too slowly and the capture latches failed to engage on the inside lip of the drogue.
''Okay. Might have been a little bit slow. Stand by.'' Evans went to have another go but this time, he would drive the probe home by firing his thrusters to give the CSM a positive push in the plus-x direction.
''You got it! Capture!'' Cernan had heard the three latches at the tip of the probe engage.
''Barber pole,'' called Evans, as he saw the indicators on his instrument panel change to show that they had latched. ''Capture, go Free.''
Evans's call for Cernan to 'go Free' meant that he wanted the LM to stop trying to hold its attitude. The lightweight spacecraft was at the end of the probe and any motion it had would be damped out by the probe's articulated tip.
''Crazy thing,'' muttered Evans as he waited for the CSM and LM to line up on the end of the probe.
Cernan wondered what the problem was: ''Say again?''
''I get the right...'' Evans laughed at the jittery LM. ''And then it goes around the other way. I think you're bouncing around up there, too, you know.''
''I know it. I'm just swinging free,'' replied Cernan.
''You're bouncing around more on the probe,'' said Evans. ''See, I'm not moving at all.''
To try to stabilise the situation, Evans suggested that Cernan allow the LM to hold its attitude again. Thruster jets began firing to pull the small spacecraft onto the attitude it had been programmed to keep. As it did so, it also applied a small torque to the CSM through the probe.
''Okay. I'm stable now,'' called Cernan once the LM's motions had damped out.
''Okay, when you're happy, I'll go Free.'' Cernan once more stopped the LM from controlling its attitude. ''Looking good now.''
''Looking good, yes,'' confirmed Evans. ''See that's what we needed. Okay. Why don't you go to Free, and we'll go to retract?''
''Okay, retract. Here you come.'' The struts of the probe mechanism began to fold, bringing the two halves of the docking tunnel together. ''Bang! I got two barber poles.''
''Okay,'' laughed Evans. He had got the sense of the talkback indicators the wrong way round. ''Two greys, I mean.''
Cernan shared the humour. ''That's better. Sounded good in here.'' ''Yes, sounded good in here,'' confirmed Evans. ''Okay, Houston,'' announced Cernan. ''We're hard docked.'' After the flight, Evans discussed the differences between this docking and his previous one when the LM was still attached to the S-IVB. ''One of the noticeable differences between this docking and the docking with the S-IVB is the fact that the ascent stage did dance a lot more than the S-IVB did. The S-IVB is steady as a rock. The LM dead band would change attitude, and you'd try to follow it.''
In view of the difficulties that Apollo 14 had experienced when Stu Roosa had tried to dock with the LM Antares while it was still on the S-IVB, his docking attempt in lunar orbit was approached with some apprehension by mission control. Before the lunar landing, Bruce McCandless at the Capcom console gave the crew a change to the procedures. ''With respect to docking, again we anticipate normal operation. However, we'd like to add to the normal procedures, a LM plus-x thrust of 10 seconds, four-jet RCS, to facilitate or to give us just a little more of a warm feeling on the docking.''
By having the LM thrusting towards the CSM at the same time as Roosa was docking seemed to be a manoeuvre neither Shepard nor Roosa was happy with. ''We mutually agreed that it would be better to give it one go at least using the normal technique with no thrusting,'' explained Shepard after the flight. ''That we did, and it worked fine.''
''We really didn't see any advantage to that LM thrusting,'' added Roosa. ''I didn't like that idea of the LM coming on with thrust. We didn't see where we had anything to lose by trying the normal docking method. If it didn't capture, then we'd try it.'' In the event, Roosa's docking went smoothly, without any hint of the troubles that had beset their earlier docking.
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