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Say a fault triggers the launch escape system and the crew vehicle fires away.

What trajectory would the LES controller follow?

  1. Keep minimal angle of attack to minimize aerodynamic loads (e.g., in the max-q region)?
  2. Hold the current attitude whatever it might be when the LES is triggered until it shuts down?
  3. Pitch down or yaw left/right to minimize the chance of collision with the rocket trailing behind?

Honestly not sure what the risk of colliding with the rocket would be. I imagine it would be small if it is malfunctioning, especially since it seems the main engines are programmed to shut down in emergencies?

But still, the LES controller would need some attitude program to follow, even if it's just holding attitude, which would not be assured if separation somehow imparted a large rotation to the crew vehicle.

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  • $\begingroup$ I would have thought option 2 $\endgroup$
    – Slarty
    May 21 at 21:35
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    $\begingroup$ Ha ha. That was also my first guess. But now I'm second-guessing because it seems just as easy to aim a few degree off the rocket's path just in case it's still firing? But then again, if something's so wrong that you need to LES away, then... even if the rocket is still firing... what are the odds it would still be on course? Yeah, I guess #2 still makes more sense. $\endgroup$
    – user39728
    May 21 at 23:00
  • $\begingroup$ #3, most emphatically. Hitting the air at 3km/s is a bit owch. Being rammed by a busy-exploding rocket moving at mach 10 is.... almost guaranteed lethal. $\endgroup$
    – PcMan
    May 22 at 11:29
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In the Apollo system the LES was comprised of three solid rocket motors. In an emergency two were used. These were solid motors so once started there was no LES control; only initiation. They did what they were designed to do and could not be adjusted..

The LES main motor had 4 nozzles on the aft closure. Two nozzles were identical throat size. Two nozzles were asymmetrical in throat size. The asymmetry of throats developed an off-centerline thrust vector of 2.5 degrees off centerline. Aerodynamic studies showed this offset angle to be adequate to take the command module clear of the vehicle.

A small motor called the pitch control motor provided a short thrust burst to keep the separated command module/LES oriented vertically.

The third motor was called the tower jettison motor and was used to remove the tower assembly from the command module in a normal launch and also in a launch abort.

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  • $\begingroup$ Interesting! So the Apollo LES was designed to steer the crew vehicle away from the rocket. But the LES didn't have differential throttling to control attitude? You mention a pitch control motor, so they still had attitude control, it seems? You must have meant something else by "vertical", right? After all, the vehicle could be at 30 deg to horizontal when the LES fires, and you wouldn't want to correct for a 60 deg error... Beside, the 2.5 deg offset from centerline suggests you wanted the vehicle to move at an angle to the rocket? Does "vertical" = 2.5 deg to the original flight path? $\endgroup$
    – user39728
    May 22 at 1:07
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    $\begingroup$ I speak like this all the time it seems? Question marks seem useful for emphasis I guess? Or maybe my keyboard broke or I can't find the full stop? Either way, interesting info? $\endgroup$
    – user39728
    May 22 at 1:08
  • $\begingroup$ The pitch control motor was just mounted at an angle to the main motor. There was no active control. Pull the trigger and hang on. $\endgroup$ May 22 at 2:05
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    $\begingroup$ Note that all these motors were solids, with predetermined thrust and burn time. The pitch motor just ensured that the CM wound up on a very different trajectory than the booster; what that trajectory actually was was a secondary concern. $\endgroup$ May 22 at 3:50
  • $\begingroup$ Ah, so I see. I imagine on a modern spacecraft like the crew dragon, you could do the same using differential throttling (e.g. firing the rear thrusters more than the fronts to pitch the vehicle away from the path of the rocket)? $\endgroup$
    – user39728
    May 22 at 4:33
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The exact strategy for LES guidance no doubt varies quite a bit. However, the top priority is going to be getting distance between the aborting launcher and the crewed spacecraft, and then keeping that distance. If the launcher is going straight up, or nearly so, when the LES is activated, it's a good idea to not accelerate straight ahead, because after your LES runs out of fuel, the launcher might still hit you. Keeping a modest angle of attack for the departing crewed pod is an important secondary goal.

For Apollo/Saturn in particular, there were three different LES abort modes. First was one-Alpha; this used a sideways-pointed "pitch control" rocket motor to turn the escaping command module out of the path of the booster. Above 3km altitude, mode one-Bravo was used; at this point the rocket was pitched over sufficiently that the command module and launcher would wind up on significantly different trajectories without the pitch motor -- the launcher and spacecraft would be accelerating at different rates in one direction, while gravity was pulling them in a different direction. For one-Alpha and one-Bravo, canards at the top of the LES assembly would deploy after the motor ran out, causing the CM to rotate into a bottom-end-first attitude, which was required in order to deploy the parachutes safely. Above 30km, the air was too thin for the canards, so the CM's RCS thrusters would be used instead to orient the capsule for parachute deployment.

Note that the broad, blunt base of the Apollo command module provided excellent passive aerodynamic stabilization during the LES burn, acting like the tail of a badminton shuttlecock to keep the CM-LES assembly pointing generally into the airstream in spite of the pitch motor trying to tilt it. The Soyuz payload shroud uses a set of grid fins in the same way for passive stabilization of their LES.

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  • $\begingroup$ Thanks, Russel! This is so helpful. I'm not sure I understand how the CM and the rocket wind up in different trajectories in mode one-bravo, though. The gravitation acceleration vector would be the same on both CM and rocket, so it seems its impact would be the same also? But drag would have been wildly different between the two---much larger on the body of the rocket, which, as long as it was aerodynamically stable, would have tended to rotate down much more readily than the CM to track its velocity vector (itself rotating down due to gravity). So maybe gravity+drag combined explain this? $\endgroup$
    – user39728
    May 22 at 6:32
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    $\begingroup$ Say for sake of argument that the stack is going purely vertical, with the Saturn accelerating straight up at 1g or so when the LES fires. If the CM+LES flies straight up until the motors burn out, it will begin to fall back and meet the Saturn as it comes back down. Now tilt everything over by some angle. The different accelerations of the CM+LES and the booster will give them an increasing horizontal separation, which gravity won’t affect. In practice the trajectory of the booster may be unpredictable (depending on why we’re aborting), so it’s not quite as simple as that, of course. $\endgroup$ May 22 at 6:48
  • $\begingroup$ Oh I see. Thanks for explaining like I’m five :D $\endgroup$
    – user39728
    May 22 at 7:42
  • $\begingroup$ So, did the command module have two different aerodynamically stable modes? Point first (for abort) and base first (for reentry)? $\endgroup$ May 22 at 13:34
  • $\begingroup$ The command module plus LES was stable point-first (the LES being heavy, moving the center of mass upward); the command module alone, I believe, was only stable base-first. $\endgroup$ May 22 at 13:45
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The pitch control motor was oriented normal to the centerline axis of the vehicle. Thus it provided side thrust normal to the axis. It fired for only 0.5 seconds.

The LES would have been deployed long before the vehicle got to 30 deg angle to horizontal. There would be no recovering from that situation.

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    $\begingroup$ I think user# meant 30 degrees to the horizontal due to the programmed booster pitchover, not 60 degrees AoA in an abort situation. For Apollo, once the booster was tilted over far enough, the pitch motor simply wasn't used; the trajectories of booster and CM would diverge without it. en.wikipedia.org/wiki/Apollo_abort_modes#Details $\endgroup$ May 22 at 4:00
  • $\begingroup$ Yep! Thanks for clarifying it for me! And thanks for the useful info and link! $\endgroup$
    – user39728
    May 22 at 4:26
  • $\begingroup$ So was the pitch motor there to rotate the crew vehicle away from the path of the rocket? If it fired for only 0.5s, it wouldn’t have been available for active attitude control, so I want to say yes, that motor was there to rotate the vehicle? $\endgroup$
    – user39728
    May 22 at 4:30
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    $\begingroup$ See my answer for more details on the Apollo LES abort. $\endgroup$ May 22 at 6:20

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