Docking probe

[Luke Pemberton]

How did the docking probe create such a good vacuum seal between the LM and CM? I understand the drogue and probe mechanism. I'm curious to how a sealed joint was created.

Having worked with Ultra High Vacuum rigs, I know the pitfalls of creating vacuum seals using copper o-rings and flanges - not a very easy job despite working in a very controlled an non hostile environment.

[ka9q]

The probe and drogue were used first to "soft dock", putting the CSM and LM into rough alignment. Then the probe was retracted by pneumatic pressure from a gas (N2?) cylinder, pulling the two together until their flanges met.  Then a set of spring-loaded latches (on the end of the CSM tunnel, I think) fired to hold the two spacecraft together tightly; this was a hard dock.

At this point there was a gas-tight seal between the two spacecraft. A vent was opened to allow oxygen into the space between the CSM and LM hatches. Then the space was isolated again and the pressure monitored to check for leaks to space. If there were none, the CSM hatch could be opened and the probe and drogue removed and stowed. An umbilical cable was then connected to carry CSM power to the LM to save the latter's batteries. (This cable's function was reversed during Apollo 13 to carry LM power back to charge the CM's re-entry batteries.)

Docking was done twice: shortly after TLI, and after the LM returned from the lunar surface. The LM jettison was done more definitively, by firing shaped charges that cut the CSM tunnel just above the hatch. The probe, drogue and latches were all left attached to the LM to eliminate their weight from the CSM. In several pictures of jettisoned LMs you can see the latches sticking out of the top of the LM.

[VQ]

According to http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070025192.pdf, the seals themselves were silicone rubber. That source is a contemporary analysis of the museum examples, so they did not inspect the LM portions of the assembly that did not make it back to earth. The authors seem to think there were four seals total, two concentric rings at each end of the docking assembly mating with a knife edge.

Luke, from a seal performance standpoint, high vacuum applications are actually more demanding than the Apollo hatch seals. The maximum leakage specification for the CM was 4.8 lbm/day, a rate that would make a high-performance vacuum system useless. Since the goal was to keep most of the air inside rather than keep the extreme vacuum outside pristine the leakage rate was less critical and offgassing from the polymer seals was irrelevant.

[ka9q]

That source is a contemporary analysis of the museum examples, so they did not inspect the LM portions of the assembly that did not make it back to earth.

There are several unflown LMs in museums that could be inspected, though I don't know how complete they were. LM-2 (on display at the Smithsonian Air and Space Museum in DC) was never meant to fly manned or even to dock with a CSM, so it might not have the seals later flown. But LM-9 (on display at KSC) would have flown on Apollo 15 before it was redone as the first J mission, so it should be very similar to the LMs that flew on Apollos 11-14.

I think there's one more LM that would have flown on Apollo 18, but I don't know how complete it was before the program ended.

[Luke Pemberton]

Thanks for the replies and links.

[bknight]

Thread resurrection:  I remember from a thread sometime in the last year or so that Jay had indicated the docking probe failures on A14 were caused by a small amount of debris preventing the normal docking procedure.  Today I had a YT guy inform me that Alan had wrote in his book, which I have not read, that it was human error of the latch mechanism at launch was the culprit.  I hope you are monitoring, Jay, and could give me your feed back or anyone  else

[ka9q]

"Human error" takes in a lot of territory. But it would have to be the kind of error that keeps the latch from working at first, but then allows it to work properly for the rest of the mission. A piece of debris would certainly qualify.

[onebigmonkey]

Having just skimmed that section, the dirt theory was proposed on the ground as a possible reason for the failure, and that repeated and more insistent attempts might dislodge it. There was no concrete evidence that this was the case, but certainly the crew didn't feel that they were in error.

[bknight]

Well I believe the human part were the assembly crews on the ground not in the capsule.  The latches were in the wrong position at liftoff.