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Does the Falcon 9 first stage use an explosive FTS?

An FTS can operate in many different ways. The key goal is to terminate thrust, break up large chunks of burny stuff, and avoid killing people.

A solid booster, you can just blow the cap off the front, and now you have terminated thrust (It blows out both ends, no net thrust). But you might also want to split it down its length so as to reduce the size of burning chunks of propellant.

A liquid fuel tank, you can run explosives down the length of it, to unzip the tank when needed.

A liquid fueled engine, if you trust the engines, you can kill the turbopumps, no more thrust. Or maybe you can quench the ignition, and run the turbo pumps to empty the tank.

There was some discussion around the Falcon 9R-Dev1 loss that the FTS system was not explosive, rather it just ran the tanks dry, killing thrust.

What does the production Falcon 9 first stage use?

Ultimate reason for asking is a short lived video was available that showed the failed landing on the CRS-6 mission, as the stage touched down, tipped over, it started to burn, then something clearly went kaboom, and things exploded outward. I was wondering if that was the FTS going off.

Watch the end of this video to see the explosions.

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  • $\begingroup$ I doubt the explosion of the CRS-6 first stage was due to the flight termination system. (But good luck getting confirmation on that.) The flight termination system is the responsibility of the range safety officer (RSO). The RSO's job is done when the vehicle clears the space where a rapid unplanned disassembly would otherwise have severe consequences on human life or property (on land). The RSO doesn't care about a rapid unplanned disassembly that makes someone tweet "we falcon punched the barge." $\endgroup$ – David Hammen Apr 16 '15 at 3:43
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    $\begingroup$ @DavidHammen CRS-6 explosion was definitely not due to FTS, call is made at T+7m16s to safe. $\endgroup$ – ReactingToAngularVues Apr 16 '15 at 8:12
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Yes, Falcon 9 does include an active FTS system, that is in control of the range safety officer. Falcon 9 specifically uses a detcord that runs down the length of the tank. When activated, it zips the stage open, causing the LOX & RP-1 to ignite instantaneously in midair. This is the best approach, the quicker you can combust all the reactants, the better.

Not everyone knows it, but the most famous use of Falcon 9's FTS is on F9R-Dev1 Flight 13, a.k.a., the last flight of Grasshopper's successor at McGregor, Texas:

F9R's final moments.

Watch the video here. What occurred was a sensor malfunction in F9R's engine(s) - this caused the rocket to stray outside of its predicted flight path, and for its Instantaneous Impact Point (IIP), the place it would land if on a parabolic trajectory at that moment, to be outside of SpaceX's predefined safe zone. The rocket was automatically terminated by the FTS.

Falcon 9v1.1 is a bit different. It doesn't have an automatic termination system, it's manually controlled, through C-band radio:

The Rocket has a Flight Termination System consisting of two strings of transmitters, receivers and safe and arm devices. The FTS works with C-Band Communications and can be used to terminate the flight in case of any anomalies.

It has never been called upon operationally, and would not have helped CRS-5 or CRS-6. Listen in to the CRS-6 webcast. At T+7:16, the controller reads out "Stage 1 FTS has been safed". This occurs after the reentry burn and prior to the landing burn.

My understanding is that the reasoning for this is that F9 has so little fuel left at this point it is not necessary to have the FTS armed, plus having a landed rocket with an active FTS system is more risky than just safing it.

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  • $\begingroup$ My question though was, after the stage tipped, crashed, burned, did the detcord ignite due to the fire? I am not suggesting that they fired the FTS. $\endgroup$ – geoffc Apr 16 '15 at 12:20
  • $\begingroup$ In short: no. Detcord is specifically designed to be difficult to ignite accidentally, generally containing materials closely-related to plastic explosives. The propellant tanks used in rockets are very good at handling vertical loads, but not horizontal. So it's likely that the impact from falling over caused the tanks to crack open, spraying the pressurized RP-1 and O2 into the air which rapidly mixed and ignited. $\endgroup$ – rspeed Feb 13 '18 at 22:39

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