I've seen a couple of launch escape systems firing and the rockets main engines are on, isn't it more safe if you turn off the engines? Edit: i agree that the engines turning off might be not so gentle and that if you immediately stop the fuel that it might explode but what about slowly reducing the amount of propellant gradually then when it hits the least amount of thrust turning it off? And on the launch escape system test by the new shepherd a rocket which it's engines are meant to be turned off and on again but when the test happens the engine is still firing, in my opinion if there's a problem detected in the rocket the most important priority is to get the crew as far away from the big container of chemicals that can explode with great force as fast as possible
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6$\begingroup$ A guess: launch escape systems are used when things are going badly wrong; being unable to turn off the booster engines is part of what can go wrong, so you test the LES with the worst-case scenario. $\endgroup$– Russell BorogoveCommented Jan 18, 2019 at 19:32
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1$\begingroup$ In addition, the circuitry which turns the engines off when something goes wrong can itself malfunction and turn the engines off when it should not have. $\endgroup$– void_ptrCommented Jan 18, 2019 at 20:36
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$\begingroup$ If the first stage begins to disintegrate and will explode soon you can't stop that by turning off the engines. Getting away as fast and soon as possible is top priority for the launch escape system. Turning off the main engines as fast as possible may cause an explosion. The huge mass of propellants rushing through the pipes from the tanks to the engines can't be stoped immediately without destroying the pipes. $\endgroup$– UweCommented Jan 18, 2019 at 23:21
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$\begingroup$ @Uwe by this logic you could never shut down an engine in flight, yet we know that Saturns did this routinely. $\endgroup$– Organic MarbleCommented Jan 18, 2019 at 23:32
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2$\begingroup$ A well designed engine has a graceful shutdown. $\endgroup$– Organic MarbleCommented Jan 18, 2019 at 23:49
1 Answer
Great idea! The Apollo designers agreed. The Emergency Detection System / Abort Sequencer would indeed shut down the engines when the appropriate abort was called. Shutdown of the engines was inhibited for the first 40 seconds after liftoff so that the vehicle wouldn't fall back onto the pad.
The subsystem is activated automatically by the emergency detection system in the first 100 seconds or manually by the astronauts at any time from the pad to jettison altitude. ...............................
After receiving an abort signal, the booster is cut off (after 40 seconds of flight), the CM-SM separation charges fired, and the launch escape motor ignited. The launch escape motor lifts the CM and the pitch control motor (used only at low altitudes) directs the flight path off to the side. ...............................
The emergency detection system operates from the time of umbilical separation until 100 seconds after liftoff. It is designed to detect emergency conditions of the launch vehicle, display the information to the astronauts, and, if the system is on automatic, start an abort. Under certain conditions (excessive vehicle rates or two booster engines out), the system initiates an abort signal. This signal resets the event timer, activates the launch escape subsystem, and (after 30 to 40 seconds of flight) cuts off the launch vehicle engines. A "lockout" system prevents the emergency detection system from operating before liftoff.
source (pp. 137-139)
source (highlighting mine)
