How do you reliably blow up a rocket that was built not to explode?

Question

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?

Answer 1

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...

Answer 2

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:

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!

Answer 3

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.