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Launch abort rockets use powerful rockets. For a pad abort, they must have enough thrust to lift the capsule to an altitude where parachutes can function.

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The Apollo launch escape rocket had a 155,000 lb thrust solid fuel rocket.

The CST-100 Starliner uses four 40,000lb thrust hypergolic engines. Here is a relevant Wikipedia article about launch escape system.

The pad abort of Soyuz T-10-1 produced an acceleration of 14 to 17g, propelling the capsule to an altitude of 2000m

The unused system on Apollo flights was jettisoned after second stage ignition. After the booster had carried the escape rocket to separation altitude, it seems a waste of free thrust to jettison it without burning the rocket.

Why are unused launch abort rockets jettisoned rather than used to augment thrust?

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    $\begingroup$ The TLDR version for any question like this will almost always be “hurts delta-v”. If it actually helped with delta-v, it would be done. If it looked like it might help it would be investigated and then used if it proved to help. $\endgroup$
    – jmoreno
    Jan 22, 2022 at 3:24
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    $\begingroup$ Going to abuse the hell out of the release clamps that still need to work and weren't designed to pull on the rest of the rocket. $\endgroup$
    – Mazura
    Jan 23, 2022 at 10:57

3 Answers 3

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The thrust of the Launch Escape System (LES) might look significant, but the total impulse is not (that is, they burn for such a short time, they can't actually impart much delta-V, especially not while the spacecraft is still attached to its mostly-fueled carrier rocket). The longer the LES remains attached to the spacecraft, the more impulse the carrier rocket wastes accelerating the mass of the LES toward orbital velocity.

Now, yes, there's probably an optimal point - perhaps after first stage separation, so the carrier rocket mass is lower - when firing the LES with it still attached to the spacecraft, and then jettisoning the LES (perhaps with the last of its thrust), would produce some benefit. However, this is probably less safe than just jettisoning it cleanly. The LES is an emergency system; it's as safe as it can be, of course, but its design mandates instant, powerful, and reliable operation; those values all trade off, to some extent, against safe operation. If the LES renders the spacecraft un-spaceworthy, that's acceptable for its intended use - that spacecraft is having a bad time and will not go to space today anyhow - but would obviously be unacceptable as part of the normal launch process.

For that matter, the interface between the spacecraft and upper stage of the carrier rocket might not be designed to tolerate tension at all (after all, in normal operation, it will only experience compression or free-fall), and the LES is, by its design, trying to accelerate the spacecraft more than the booster is accelerating it, so there will be tension[*] (in theory, any scenario that triggers the LES should also trigger MECO, but you can't assume the main engines are under control in that kind of scenario either, so the LES has to be able to pull the spacecraft away even under full burn of the booster).

Given those risks, it seems decidedly not worth trying to get a few extra m/s (when you factor in the mass of the carrier rocket) by firing the LES as part of a standard ascent process.

[*] As @TooTea points out, quite a lot of tension, actually. If the LES generates 14g of acceleration, while the carrier rocket is generating at most 4g, that's going to be a massive and abrupt 10g of tension shock hitting the payload interface. Definitely not something you want to have to account for in normal operation! Even if it's possible to do safely without impairing the LES in abort scenarios, any mass you add to that interface to beef it up for that comes directly out of your payload mass budget. All for a few hundred thousand (at best) lb*s of impulse, lost in the noise of what the carrier rockets provides.

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    $\begingroup$ +1 There actually has to be a whole load of tension on the spacecraft-carrier interface, since the whole purpose of a LES is to quickly pull the spacecraft far from a booster about to explode. That requires a huge acceleration (10g+ with respect to the already accelerating stack, going by the numbers in the question). The interface would have to be built to reliably handle those hundreds of kN in tension when things are going well, as well as reliably not interfere with that same tension when you actually need the LES to perform its primary function. $\endgroup$
    – TooTea
    Jan 21, 2022 at 14:25
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    $\begingroup$ OP's own photo of the escape system firing shows the crew capsule virtually engulfed in the rocket plume. That may be acceptable when it's your last resort to save the crew, but probably not desirable as a course of routine operation. $\endgroup$
    – Seth R
    Jan 21, 2022 at 17:23
  • $\begingroup$ And yes, maybe it would be possible to build the LES in such a way that it has two operation modes, one with the 14g of acceleration needed for emergency and one with lower acceleration for "normal use", it would likely make it much heavier and more complex, therefore negating any benefits it could provide. $\endgroup$
    – vsz
    Jan 23, 2022 at 12:31
  • $\begingroup$ @vsz the LES was built with a solid fuel rocket system, which is about the simplest, most reliable type of rocket you can have. But a solid fuel rocket only has 2 modes: off, and full-throttle-until-out-of-fuel. Any other system would indeed be more complex and prone to malfunction, which is not something you want in an emergency system. $\endgroup$
    – Seth R
    Jan 23, 2022 at 20:18
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The thrust of the Apollo launch escape rocket was negligibly small when compared to the thrust of the Saturn V first and second stages. The burn time of the launch escape system was negligibly small, too.

  • first stage 7,891,000 lbf (35,100 kN) sea level, burn time 168 seconds;
  • second stage 1,155,800 lbf (5,141 kN), burn time 360 seconds;
  • third stage 232,250 lbf (1,033.1 kN) vacuum, burn time 165 + 335 seconds (2 burns);
  • launch escape rocket 155,000 lb thrust, burn time only 4 seconds.

So the first stage had 50 times the thrust and 42 times the burn time of the launch escape system. The second stage 7.5 times of thrust and 90 times burn time.

The angle between those exhaust jets of the escape rocket was necessary but inefficient.

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    $\begingroup$ By your figures, firing the LES would add .15% to the impulse of the second stage. As CB Hacking pointed out, this would likely be cancelled by additional weight of the coupler structure. Hardly worth it. $\endgroup$
    – Woody
    Jan 22, 2022 at 5:50
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The weight and complexity of using the launch escape system negate any advantages.

  • Using the LE rocket as it was designed, i.e. to give a brief, powerful burst would have meant strengthening the couplings between the capsule and the launcher, and strengthening the launcher to be able to withstand that force. Adding strength means adding weight, and it could very well mean more weight to use the LE rocket than it would be worth
  • Redesigning the launch escape rocket to produce longer, less powerful burst would mean less weight added to the launcher, but would require additional complexity to the LE rocket so it could have a low-thrust mode as well as a burst mode. Complexity adds weight and risk - the launcher absolutely, positively has to work when needed

Add to that the actual usable thrust of the LE rocket is negligible, it's not that much to begin with, but a good deal of it will still be needed to thrust it clear of the rocket.

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    $\begingroup$ The LE motor of the apollo had 4 nozzles in the aft closure. Two of these nozzle had off-sized throats to generate a thrust vector angle of about 2.5 deg to pull the command module away from the main vehicle after it was separated. Starting that motor while still attached to the whole vehicle would produce a tipping force that would not be good for the vehicle control system.. $\endgroup$
    – tckosvic
    Jan 21, 2022 at 18:54
  • $\begingroup$ Good point @tckosvic, that offset also would steer the escape rocket clear when jettisoned. I'm going to work on the assumption that the slight angle of the thrust would be gimballed out. $\endgroup$
    – GdD
    Jan 21, 2022 at 21:43
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    $\begingroup$ @tckosvic --- the LES had separate main motor and pitch control motor. In a pad abort or Mode 1A abort, both would fire. Above 3000m (Mode 1B abort) the pitch control motor does not fire. The pitch control motor can be seen firing in the photo above. $\endgroup$
    – Woody
    Jan 22, 2022 at 5:43
  • $\begingroup$ Yes, I am aware of the engines of the Apollo LES. As a youth I worked as an engineer at Lockheed Propulsion Co (now defunct, of course) who manufactured all three of these motors. Went to many test firings. $\endgroup$
    – tckosvic
    Jan 23, 2022 at 3:11
  • $\begingroup$ In a non-abort situation the main LES motor (large thrust one) is never fired. The intermediate sized motor called the tower jettison (TJ) motor upon firing woud take the main LES motor and associated structure away from the path of the main vehicle. The TJ motor had two nozzles with different sized throats to generate an approx 4 deg offset thrust vector angle. The main LES motor would only be fired in a non-abort situation upon a failure of the TJ motor to ignite. $\endgroup$
    – tckosvic
    Jan 23, 2022 at 14:30

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