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?
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).