NASA, and I'd imagine most space agencies around the world, demands abort systems of its commercial partners capable of destroying a rocket should it venture too close to populated areas. Note that abort system can also refer to launch escape systems, but to keep this question narrow in scope I'm only referring to the ability to blow up a misbehaving rocket.

The engineering here is fascinating, as rockets are basically bombs that have been carefully coaxed into not exploding, but there are circumstances in which you want to ensure reliable and immediate detonation despite having gone to great difficulty to design the rocket to... not do that.

On the technical side, you've got your fuel and oxidizer in two separate tanks, generally cryogenically cooled, often but not always at very high pressure. Given the danger of unburned rocket propellant being sprayed down on an inhabited area, one of the goals of this detonation is to rapidly consume both fuel and oxidizer. The other goal is to separate the rocket into pieces capable of causing less damage than being hit by the rocket as a whole. It seems that the three steps of combining all the fuel and oxidizer, burning them both all at once, and splitting up the rocket, all in one precisely timed explosion of a thing that has been built not to explode, is an engineering marvel in itself, one of the many involved in building rockets.

How do they do it?

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    $\begingroup$ @OrganicMarble Not really. While the title of the question implies it might, the wording of the question and the answer concern purpose and theory, not technical details of how it's designed and functions in actual rockets, which is the only focus of my question $\endgroup$ Apr 18 at 17:16
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    $\begingroup$ You're overthinking it, the goal is not to consume all the fuel and oxidizer, just to make it go boom, which really isn't very hard. $\endgroup$
    – GdD
    Apr 18 at 18:09
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    $\begingroup$ Add MOAR BOOSTAS $\endgroup$ Apr 18 at 22:16
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    $\begingroup$ Or y'know, pull an SN11 $\endgroup$ Apr 18 at 22:16
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    $\begingroup$ It seems that the three steps of combining all the fuel and oxidizer, burning them both all at once, and splitting up the rocket, all in one precisely timed explosion of a thing that has been built not to explode, is an engineering marvel in itself Why? Breaking stuff is easy. The hard engineering part is the "making it not explode" bit. To make it explode, all you have to do is compromise (ie : break) one of the hundreds of engineering controls you put in place to prevent it from exploding and the rest happens all by itself. $\endgroup$
    – J...
    Apr 19 at 17:53

Well, I can refer you to the Range Safety Wikipedia entry:

Two switches were used, ARM and DESTRUCT. The ARM switch shut down propulsion for liquid propelled vehicles, and the DESTRUCT ignited the primacord surrounding the fuel tanks. In the case of manned flight, the vehicle would be allowed to fly to apogee before the DESTRUCT was transmitted. This would allow the astronauts the maximum amount of time for their self-ejection.

The primary action performed by RSO charges is rupturing the propellant tanks down the middle to spill out their contents. In the case of boosters with cryogenic propellants, the RSO system is designed to rupture the tanks in such a way as to minimize propellant mixing, which would result in an extremely violent explosion, specifically by having the charges split the sides of the tanks open like a zipper, which spills out the propellants and minimizes mixing. On boosters with hypergolic propellants, the opposite happens—mixing is encouraged as these propellants burn on contact rather than mix and then explode. In addition, the toxicity of hypergolic propellant means that it is desirable to have them burn up as fast as possible. The RSO system used on these boosters works by rupturing the common tank bulkhead so the oxidizer and fuel immediately contact and burn.

See these guys (Ensign-Bickford) for the parts of flight termination systems that go boom...

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    $\begingroup$ Re the first paragraph: it's worth also noting the section in the linked Wikipedia article on autonomous flight termination, which describes how newer rockets will activate their FTS without human intervention. While it seems likely that the mechanism for triggering the explosion remains the same, not having to rely on a ground-to-rocket radio link simplifies the process somewhat. $\endgroup$ Apr 20 at 15:45

Thrust termination is the goal of Range Safety in the event of an errant rocket. Rendering the boosters inert is considered secondary, when possible.

The Range Safety Officers don't rely on the current positions of the vehicle to decide to terminate, but instead are looking at the IIP (Integrated Impact Point) on a map. The IIP is the point where the rocket is expected to land on a purely ballistic trajectory, absent thrust. If the Range Safety Officer or MFCO (Mission Flight Control Officer) sees the IIP cross a preliminary standoff line, they will ARM the thrust termination system. If the IIP crosses a final standoff line, the MFCO will hit the DESTRUCT button. If thrust is properly terminated, the rocket debris will land in an area around the IIP rather than on a populated area. The standoff lines are pre-computed based on expected estimated human casualties for each mission.

Thrust termination varies dependent on the rocket type, but every mission contractor flying on US ranges has to provide a tested thrust termination system meeting range safety requirements. For obvious reasons, most termination systems are not widely available to the public. For some systems, like Solid Rocket Boosters, the only way to reliably terminate thrust is to open the boosters along the side like a tin can and spill the solid propellant.

The United States put our major missile ranges on the coasts (Vandenburg and the Cape) so we wouldn't have to launch over populated areas. China isn't always quite as careful:
enter image description here

But we had many hard lessons to learn in the U.S. as well. A buddy of mine was working as the MFCO at Vandenburg when they lost a rocket off the pad, which landed in the ROCC parking lot and destroyed his car. That was pre-Uber/Lyft, in the late 1990s if I remember correctly. There were a lot of arguments about "Human in the Loop" for range safety back then. Unfortunately, I have no modern knowledge to share!

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    $\begingroup$ "For obvious reasons, most termination systems are not widely available to the public." it's not obvious to me what you mean by this, just that they're typically explosives? $\endgroup$
    – llama
    Apr 19 at 16:09
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    $\begingroup$ @llama Launch providers don't publish this information since it could potentially be exploited by terrorists or bad actors. $\endgroup$ Apr 19 at 16:13
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    $\begingroup$ I think @Connor Garcia means the exact signaling process for initiating the LTS. Linear shaped charges themselves are not a secret... These guys sell flight termination explosives as well as other things that go boom. $\endgroup$
    – BobT
    Apr 20 at 13:57
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    $\begingroup$ @Vikki-formerlySean He never told me that part of the story, but it sure sounds like an unusual insurance claim! $\endgroup$ Apr 21 at 2:59
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    $\begingroup$ @Vikki-formerlySean: I read about another case where a rocket failure rained debris on a parking lot. Someone described having called their car insurance, and as proof of their claim they told the person on the phone to "turn on the TV news" or something like that. And yeah, that worked, the insurance people said "oh, yeah, we'll get right on that for you". This might have been NASA, or possibly I'm even mixing it up with Mythbusters (testing something at a bomb range, not a rocket). $\endgroup$ Apr 21 at 3:28

I was a missile tech on a SSBN in the '70s and worked on the Poseidon C3 SLBM. This missile had six "Thrust Termination Ports" arranged around the periphery of the second stage motor dome. Each TTP was a 10" diameter fiberglass tube angled outward from the motor dome to the side of the missile with det-cord at each end.

When commanded by the guidance computer, the tops/bottoms of the tubes were severed causing the second stage thrust to end immediately. Simultaneously, the equipment section/post-boost vehicle would separate and continue on its journey, carrying up to 14 MIRVs to their release point.

Sorry for the long post, but that's how the Navy shut down a solid rocket motor on command.

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    $\begingroup$ Early shuttle SRB designs had thrust termination ports. Interesting to hear about a system that actually used them, +1 $\endgroup$ Apr 22 at 1:54

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