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Many manned space exploration vehicles have some way to abort, to end the mission as quickly as possible and get back to relative safety.

I'm wondering if the Apollo command module had an abort, a way to get back to Earth while enroute to the Moon.

If so, why wasn't it used on Apollo 13?

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According to a Wikipedia article Apollo missions had multiple abort modes:

  • Pad abort - If an emergency occurred during the last few minutes before launch, the launch escape system could pull the command module away from the rocket stack.
  • Mode I - Would have been used during Stage I burn and would have used the lauch escape system to pull the command module out of the way. It was subdivided into IA, IB, and IC, depending on altitude: Below 3000 meters, IA would have been used. In it the command module would be moved out of the flight path of the rocket stack and pulled away by the launch escape system. IB would have been used between 3000 meters and 30.5 km. In it, canards would be used to turn the command module into a "bottom forward" flight so that the parachutes could deploy properly. IC was similar to IB, but would use motors to orient the CM. When the rocket/spacecraft stack was too high for the launch escape system to be useful, that system was jettisoned.
  • Mode II - In the event of a problem early in the second stage burn, the command and service modules would separate from the rest of the stack, using the service module's rocket. Once clear, the command module could separate from the service module and splash down.
  • Mode III (aka Contingency Orbit Insertion) - Later in the stage 2 burn, orbit was achievable, so the 3rd stage and service module engines would be used to get into orbit.
  • S-IVB to orbit - Similar to Mode III, but only the SIV-B stage is needed to get into orbit.
  • Mode IV - This was for a problem in the SIV-B stage and uses just the service module to achieve orbit.
  • Direct Abort Trajectory - For problems that might occur during the cruise to the moon, the service module would be used to turn the spacecraft around and direct it back to Earth.

Obviously, Apollo 13 was well past the point where the launch aborts could be used. The problem with using Direct Abort Trajectory was that the condition of the service module was unknown and it would have required jettisoning the LM. Since the ship and crew were almost into the zone of lunar influence, they re-established a free return trajectory to use the moon's gravity to return them to Earth. You can read more at Wikipedia's write up about the successful return of the crew.

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    $\begingroup$ After the final 3rd stage burn that established the vehicle on trajectory to the moon, wouldn't it have been too fuel-costly to "turn around"? Wasn't that burn essentially a commit to at least a lunar flyby i.e. "free return" was the only option no matter how soon "post burn"? $\endgroup$ – Anthony X Jun 4 '14 at 3:09
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    $\begingroup$ A fuel-efficient translunar trajectory is a Hohmann transfer: en.wikipedia.org/wiki/Hohmann_transfer_orbit . As you can see on that page, it's a half-ellipse with its perigee at LEO and apogee at lunar orbit. If you were to slow down by some amount from a point midway along the elliptical trajectory, you'd fall into a different ellipse -- one with a lower apogee, that misses the moon, and which returns to Earth earlier. That's attractive if: you know the SPS engine is good, and you're in a hurry, and you don't need the LM. A13 didn't know the condition of the SPS, and needed the LM. $\endgroup$ – Russell Borogove Oct 23 '14 at 16:04
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    $\begingroup$ Which is a long-winded way of saying "they don't exactly turn around" -- it's more like slow down, turn sideways and let gravity do the rest. $\endgroup$ – Russell Borogove Oct 23 '14 at 16:06
  • $\begingroup$ @MichaelKjörling: Yes, although this is almost two years older. $\endgroup$ – GreenMatt Jan 11 '16 at 1:10

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