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The docking tunnel of the Apollo CSM is cut by pyrotechnics when the LM is jettisoned. (See Fig. 2.13-11 in Apollo Operation Handbook, Fig. 19 in Apollo Spacecraft Pyrotechnics) After this, docking with LM is no longer possible. What is the rationale for such design? Since the docking system is already in place, does the additional pyrotechnics provide any advantage over a reusable design?

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    $\begingroup$ No, the docking tunnel too is cut by pyrotechnics when the crew departs for Earth. See Apollo Operation Handbook. $\endgroup$ – Mys_721tx Jun 25 '20 at 7:36
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    $\begingroup$ Cool document @Mys_721tx! I think it would be worth adding that doc link into the question. I have no evidence for this but I suspect the answer is the pyrotechnics are there to ensure separation. If you have an empty LM and the mechanism doesn't release there's nobody to troubleshoot on the other side. $\endgroup$ – GdD Jun 25 '20 at 8:21
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    $\begingroup$ to remove unnecessary hardware before reentry $\endgroup$ – user20636 Jun 25 '20 at 8:54
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    $\begingroup$ Good Question. In general explosive fuses have been used very often in space exploration especially in the early phase. They are simple the fail-safties connectors. even if a connection had a mechanical seperation you put a explosive redundant system aside. It is logical for me, such a emergency system is installed to assure seperation of the LM. BUT why is it used here as the "regular way?" ...good question. $\endgroup$ – CallMeTom Jun 25 '20 at 9:31
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    $\begingroup$ This is a great question. $\endgroup$ – Organic Marble Jun 25 '20 at 15:17
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Source: Apollo Experience Report: Spacecraft Pyrotechnic Systems

Definitions:

  • LES = launch escape system (carries the CM away during an abort)
  • CM = command module (the main crew compartment)
  • SM = service module (uncrewed compartment with engines, equipment, and fluids)
  • LM = lunar module (lands on the moon)
  • SLA = spacecraft lunar-module adapter (conical part between the Saturn booster and the SM; carries the LM)
  • SBASI = single-bridgewire Apollo standard initiator (a tiny electrically-fired explosive detonator, i.e. a squib)
  • MDF = mild detonating fuse (a slightly more powerful explosive detonator)

  1. After launch of a normal flight, the launch escape system (LES) would be separated from the CM by explosive nuts on the LES legs:

    In a nominal flight, the LES is jettisoned after the ignition of the second-stage booster (fig. 4). The separation is accomplished by simultaneously igniting the tower-jettison motor and the frangible nuts in the base of each tower leg (fig. 5).

    p. 8

    However, during an abort, the separation occurred instead at the docking ring:

    However, if an emergency should occur, the CM would be separated from the launch vehicle immediately, and the pitch-control motor would ignite simultaneously with the launch-escape motor to provide lateral translation and to assure safety of separation. Eleven seconds after LES abort initiation, the canards would deploy to stabilize the system. Three seconds later, the docking ring would separate from the CM to be jettisoned with the LES. The jettison of the apex cover and the deployment of landing parachutes would complete the abort sequence.

    ibid

  2. The Apollo spacecraft had a lot of systems that operated under high pressure. The pressure tanks that supplied such systems were kept sealed until they were needed, at which time they were opened by explosive squibs.

    The piston which retracted the docking probe was such a system. It was armed by an explosive squib:

    Docking probe retraction. - During docking of the CM to the LM, the docking probe (which provided for CM/LM coupling) was retracted by means of SBASI units in the probe retraction system. The SBASI, when fired, punctured a gas bottle to release gas that drove a piston to retract the probe.

    p. 10

  3. There were four guillotines, none of which had to do with docking between the CM and LM:

    • the umbilical between the SM and the SLA
    • the umbilical between the LM and the SLA
    • the connections between the LM descent and ascent stages
    • the umbilical between the SM and the CM

    All four of these are outside the cabin, out of reach of the astronauts in the cabin. Thus, they were cut by pyrotechnics.

    In contrast, the CM-LM umbilical was inside the docking tunnel, easily accessible to the astronauts. They were able to manually connect and disconnect this umbilical.

  4. Separation of the CM and the LM for descent to the moon was done mechanically, without pyrotechnics. However, the final separation which discarded the LM was done by the same pyrotechnics that disconnected the docking ring during an abort (see #1 above):

    Docking ring separation. - The LM docking ring separation system was part of the CM. The system contained MDF charges that were used to cut the ring and to permit final separation of the LM from the CM. During a normal mission, the docking ring was separated from the CM and remained with the LM. In the event of a launch emergency requiring LES abort, the docking ring would have been jettisoned with the launch-escape tower.

    p. 10

I haven't found an explanation why the final separation was done this way. However, it would have been a redundant way to separate the two spacecraft, and would have shed some weight.

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    $\begingroup$ Interesting answer. Consider adding some acronymology, MDF and SBASI especially aren't that common and/or have more than one meaning. The last paragraph almost admits that it is 'not an answer' to what was asked but the rest is good info. $\endgroup$ – Organic Marble Jun 25 '20 at 11:37

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