Many rocket systems are equipped with self-destructs to prevent an out-of-control vehicle from wandering too far from its intended path and becoming a hazard. Presumably, the objective is to kill the rocket engine's thrust (and disable any of the upper stages) so that the vehicle or its debris falls within a designated zone. The question is: what is the intended action of the self-destruct devices? Is it intended only to terminate thrust, or is there also design intent to release and flare off propellant and/or fragment the vehicle (or are those just side-effect)? Given that there are size and weight constraints, any added piece of equipment aboard a rocket can only do so much... how is it ensured that the devices will do their job reliably, when and only when commanded? How does the type of rocket (solid fuel vs liquid fuel) affect the design and implementation of the destruct system?
3$\begingroup$ There is already a lot of high explosive on board of rockets which could be utilized in a self-destruct. (Though normally we call it fuel). $\endgroup$– HennesNov 1, 2014 at 16:07
$\begingroup$ @Hennes Plenty of fuel and oxidizer to make a big boom, but hard to imagine that any such would occur until after the vehicle has broken up sufficiently for the propellants to come into contact on a large scale. $\endgroup$– Anthony XNov 1, 2014 at 18:18
1$\begingroup$ @AnthonyX Blow open the fuel tanks and they will come into contact on a large scale. $\endgroup$– Loren PechtelNov 1, 2014 at 19:05
$\begingroup$ It's not as straight-forward with SRBs or other solid stages that you wouldn't want them to deviate off course and effectively each of them becoming a stray ballistic missile. ;) $\endgroup$– TildalWaveNov 1, 2014 at 19:30
2$\begingroup$ I assumed that we could ignite a SRB at many places at once. That would break up up and burn it down in a short time. $\endgroup$– HennesNov 1, 2014 at 19:56
Flight termination systems serve two purposes:
- Termination of thrust
- Dispersal of propellant
Termination of thrust can be accomplished any number of ways. For liquid fueled engines, it may be sufficient to simply stop the engine. For solid motors, once the fuel grain is started, it cannot be stopped, so it needs to be rendered nonpropulsive. This can be accomplished by either unzipping the casing using a linear shaped charge or puncturing the forward dome of the motor.
For propellant dispersal, the requirements depend on the type of propellant. For hazardous propellants like hydrazine and nitrogen tetroxide, the FTS is usually required to actually burn off the propellant to the greatest extent possible. For more chemically benign fuels like LH2 and LOX, simple dispersal is typically sufficient.
Often, the same mechanism is used for both thrust termination and propellant dispersal.
Flight termination systems are required to be redundant to ensure success. To prevent inadvertent initiation, safe-and-arm devices that physically isolate the destruct package from its initiator are used. To prevent interference or malicious third-party initiation of a destruct package, destruct signals are encrypted.
The current US standard for flight termination systems is defined in AFSPCMAN 91-710 (though some of the juicier bits are not publicly available).
1$\begingroup$ Follow-up questions: how does dispersal of propellant contribute to range safety? How do self-destructs affect forensic investigations of mechanical failures? For example: the recent Antares failure and attribution of cause to a failed turbopump. $\endgroup$ Nov 8, 2014 at 14:31
1$\begingroup$ Range safety is about making sure an errant launch vehicle does not harm people or property on the ground. For cryogenic propellants, dispersal of propellants literally means setting them free to evaporate and rejoin the atmosphere. While there will likely be a deflagration, it is not necessary, as they are basically harmless once dispersed. For hazardous propellants like MMH, UDMH, Hydrazine, and NTO, the goal is to try to burn them all off so that none of it reaches the ground and dissolves anybody's lungs. Destructs are typically as minimal as possible and do only localized damage. $\endgroup$– TristanNov 10, 2014 at 16:25
$\begingroup$ The STS SRBs had a linear shaped charge running up the side of the casing. When they were commanded to destruct, the charge split the casing and the sudden drop in chamber pressure presumably extinguished the burn. For the forensics in this case, if the burn hole in the case had been located under the charge...who knows. Finding the piece with the hole was, I think, important, but not critical to the investigation, since photos of the leak existed. upload.wikimedia.org/wikipedia/commons/6/6c/… $\endgroup$ Feb 13, 2016 at 0:07