Probe and drogue spacecraft sex

The docking system was one of the many engineering wonders of the Apollo programme and one whose importance is perhaps underplayed. The docking system was an ingenious, compact, pneumatic and mechanical arrangement that managed to elegantly fulfil an enormous range of tasks: it self-centred the two spacecraft as they contacted; it absorbed the shock of that contact to achieve a soft-dock; and, finally, it pulled the two craft together to achieve hard dock and straightened their axes as it did so. It was then collapsible and removable from either the LM or CM side and could be stowed when not required. Years later, Apollo 15 commander Dave Scott who was also the first CMP to use the Apollo docking system on Apollo 9, wrote about how crucial it was. "It is really quite important to the whole scheme of the Apollo concept - a very complex apparatus and one of the few single-point failures in the entire system. But at the end of the day, it was probably one of the more brilliant mechanical devices of the programme."

Like so many plug/socket combinations in the electrical and electronics world, it was a sexed affair with the CSM carrying the male docking probe and the LM providing the receptacle into which the probe was inserted. The movie Apollo 13 played An engineering model of the probe and drogue up the suggestive nature of this docking mechanism. arrangement when the character of

Jack Swigert, who would be the CMP on the Apollo 13 mission and was a bachelor with a reputation as a ladies' man, was shown using a glass and bottle to demonstrate the Apollo docking system to a beautiful young woman. Although the male/female nature of the system limited its scope for later androgynous spacecraft designs by virtue of its inherent inflexibility, the probe and drogue arrangement was well suited to the needs of Apollo.

On the CM side, an articulated tip was mounted on the end of a retractable rod. Around this rod was a tripod of arms with shock absorbers. The whole probe assembly was mounted within the docking ring which itself was bolted to the apex of the command module. This ring carried 12 automatic latches around its circumference that engaged with the LM's docking ring when the probe brought the two together on retraction.

The LM's side of the affair was much simpler. The drogue was really just an inverted cone with a hole in its centre. The cone's purpose was to gently shepherd the tip of the probe into the carefully sized hole. Three small spring-loaded capture latches built into the probe's tip then caught the edge of the hole. This was the 'soft-dock' condition. When commanded, one of four small bottles of nitrogen gas inside the probe energised the retraction mechanism to pull the spacecraft together and as it

Apollo Docking Mechanism

did so, three pitch arms ensured that both spacecraft were aligned by contact with the drogue's cone.

When the CSM was manoeuvred towards the LM by using the docking target and the COAS, it helped to ensure that they were both aligned in pitch, yaw and roll. Pitch and yaw errors were removed by the shock absorbers in the pitch arms as the retraction took place. Any roll errors were usually small because the COAS and docking target were displaced well away from the centre-line of the docking system. At a later stage of the mission, the LM's guidance system would be given an approximate alignment by the appropriate numbers from the command module being read across and adjusted for the difference in attitude between the two spacecraft. This calculation meant taking account of any angular misalignment between the two vehicles. The docking tunnel included marks that allowed the docking index angle to be read off.


As Stu Roosa was the command module pilot on the Apollo 14 mission, it was his job to guide the CSM Kitty Hawk towards the LM Antares which was still parked on top of the S-IVB.

Lunar module pilot Edgar Mitchell gave the television viewers a running commentary of their view of the approaching LM. "Okay, I'll chat for a minute,'' he began. "The S-IVB is surrounded here by typically thousands, or millions of particles that came out when we separated. They look like little winking stars, floating around in a very random pattern. The sunlight is shining very strongly off the top of the lunar module as we drift into it. Stu's doing an excellent job of sliding in here very slowly. As you can see, our approach speed is a few tenths of a foot per second, probably. And the LM is starting to get very large in our field of view; starting to cover the window. And the LM and the S-IVB are bore-sighted right out our x axis.'' The approach to the captive LM was carried out in such a slow, careful manner that it hardly appeared to move on the TV. A viewer's attention had to be taken away for a moment to realise, when looking back, that they had actually edged closer to the LM.

Mitchell continued as the final distance was closed. "We can see all of the orange, yellow thermal protection around the LM. The colours stand out very nicely. And Houston; we're about to dock. We're probably a foot or 18 inches to 2 feet out now.''

The probe contacted the drogue and scraped its way down to the central hole. As it nestled into the apex of the drogue, he called out, "And we docked.''

But they had not docked. The capture latches had failed to engage with the drogue and Roosa found that his spacecraft was gently rebounding away from the LM. He immediately tried again.

A minute later, he radioed his failure to achieve soft-dock. "Okay, Houston. We hit it twice. Sure looks like we're closing fast enough. I'm going to back out here and try it again.''

Docking Drogue

Over the next minute, Roosa made a third attempt, giving the CSM an extra push home by continuing to fire the RCS thrusters after the probe settled home. Again the latches failed to work.

''Man, we'd better back off here and think about this one, Houston. We're unable to get a capture,'' Roosa concluded.

For the next 5 minutes, Capcom in mission control had Roosa check the spacecraft's configuration but everything appeared to be just as it should. For a fourth time, he manoeuvred the spacecraft towards the LM.

''Okay, Houston. I hit it pretty good and held 4 seconds on contact and we did not latch.''

The crew could see the scratches on the drogue's surface where the probe had been guided into the hole but there was no obvious reason why the two ships were not holding. As PropeUant bemg vented fr°m Ap°n° they waited for the controllers and engineers 14 s S-IVB-in Houston to assess the situation, they were treated to a spectacular display of countless liquid droplets shimmering before them as the S-IVB began a planned dump of some of its propellants. Mitchell enthused about the sight as they drifted among the particles of propellant all around them.

''There's lightning - the whole sky. Of course, it's the source of another ten million particles floating out in front of us.''

Over an hour passed as possibilities were weighed, engineering minutiae were discussed and two large spacecraft flew in exquisite formation a distance greater than the diameter of Earth. Then Capcom made another call. ''Okay, we'd like to essentially try the docking again with the normal procedures rather than going to more drastic alternate procedures. Make your closing rate on this not fast, not slow, just a normal closing rate.''

''Okay,'' replied Roosa. ''We'll try it. I thought that's what I had the first time, but we'll give it a go.'' Ed Mitchell picked up on the commentary again. ''Okay, Houston, we're starting to close on it now.'' ''14, Houston. Roger.'' ''About 4 feet on it, Houston.'' ''Roger, Ed.''

''Here it comes,'' continued Mitchell as the probe homed in on its quarry for the fifth time, resulting in another disappointed call. ''No latch.''

''No latch, Houston,'' echoed Stu Roosa. His commander, Alan Shepard voiced what was on his mind. ''I'm sure you're thinking about the possibility of going hard suit and bringing the probe inside to look at, as we are.''

Mission control was thinking of this possibility, where the crewmen would seal themselves in their suits, depressurise the cabin and remove the probe to allow it to be inspected. But first they had one more suggestion that would avoid this cumbersome procedure. The backup commander Eugene Cernan, who had been working with the people analysing the situation, took the Capcom position.

"Okay. We got one more idea down here, before doing any hard suit work. We're thinking of attempting to dock actually without the aid of the probe.''

Their idea was to use the probe only as a way of aligning the docking rings of the two spacecraft. In a normal docking, once the probe had latched onto the hole in the drogue, it was pneumatically retracted, pulling the docking rings of the two spacecraft together and allowing the twelve strong docking latches to engage. Cernan's suggestion was for Roosa to manoeuvre the probe back to the centre of the drogue for a sixth time, then, while he continued to use the thrusters to push forward, Shepard would retract the probe. The hope was that the alignment of the docking rings would be maintained as they came together, so that the docking latches would be activated. However, having had one unexplained malfunction in the system, mission control had serious doubts about whether the probe would retract as commanded. Once again, Mitchell picked up the commentary for the TV viewers. "Okay, Houston. We're about 12 to 15 feet away.'' "Roger, Ed,'' replied Capcom. "We got a very good picture.'' Again the probe was guided to the hole at the centre of the drogue. Once it seemed to have settled in, Shepard retracted the probe. "We got some, Houston. I believe...'' Shepard was cut off as the loud bang-bang-bang of the engaging docking latches rippled through the cabin. Stu Roosa triumphantly exclaimed, "We got a hard dock, Houston.''

"Outstanding," came the relieved reply from Houston. "Super job, Stu.'' After the crew had folded up the docking equipment and brought it out of the tunnel for inspection, they could find no fault in its mechanism and it was used successfully for the crucial docking in lunar orbit after Shepard and Mitchell returned from their exploration of the lunar surface. Usually, the probe was discarded along with the LM at the Moon, but Apollo 14's was returned to Earth where no fault could be found. The engineers could only surmise that some unknown foreign debris had temporarily jammed the mechanism.

During a normal docking, after the capture latches had gripped around the edge of the drogue's hole, the crew would have waited a while to allow any swinging motion of the two spacecraft to damp down. The probe tip was gimballed to facilitate this rotation and it included springs to make it self-centre. Once everything had settled, the probe would be retracted as normal, bringing the docking rings together and engaging the twelve docking latches around the circumference of the tunnel.

Al Worden on Apollo 15 found that the capture latches appeared not to engage when he brought the CSM Endeavour up to the drogue in the LM Falcon. He then made sure of a positive engagement by a little extra forward push on the thrusters. This extra thruster firing, combined with the rotation given to the CSM by the probe being shepherded towards the centre of the drogue, contributed to a misalignment of the two vehicles. The two docking rings did not meet face on when the probe was retracted, putting some stress on the tunnel's structure.

On Apollo 16, Mattingly tried to ensure that the two craft were better aligned before retracting the probe. However, having engaged the capture latches, he found the spacecraft remarkably difficult to manoeuvre. ''Whatever gas we used during TD&E, we used after I hit in trying to get it re-centred.'' Mattingly was trying to make sure that the long axes of the two craft were aligned before he pulled them together with the probe retraction. ''They [management] busted the [Apollo] 15 guys about forcing it in. I tried to centre it up, and that is a pretty expensive operation. It's very inefficient when you have your nose hooked to something you're trying to push. I was using the translation controller and I was really surprised. Either the friction on the probe head or something is a lot more than I expected. It was very ineffective."

With all the thruster activity he was generating around the two spacecraft, Mattingly became aware of an unexpected noise from the RCS thrusters. ''I didn't hear any RCS sounds when I got off the S-IVB. I didn't hear any sounds during the turnaround; and, I didn't hear anything on closure until I got in real close. I would swear - I know it's not possible - but I'd swear I could hear the jets impinge on the LM before we docked.'' This was a surprise to him. Sound cannot travel in a vacuum, yet he seemed to be hearing the gas from the thrusters washing over the LM. He thought that perhaps the exhaust was forming a temporary local atmosphere around the spacecraft.

He continued, ''And you could certainly see it. Maybe I was visually seeing the skin of the LM kind of flutter and I knew that should make a noise. I heard the same noises every time we fired the engines after that. I don't know if there could be enough local atmosphere or whether you can get a reflected shock that you could hear. I don't know how it is, but, I know I could hear reflections off the LM before we docked.'' The mission's commander John Young supported his pilot. ''I think that is possible, Ken, with the gas going out and coming back and bouncing off your vehicle. There are a lot of particles in there.''

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  • saradas hornblower
    How does apollo docking probe work?
    8 years ago

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