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On the Sci-Fi Stack Exchange there was a question about implementing self-destruct on spaceships and comparing that to scuttling on naval vessels. One person in a comment said, "They do implement a self-destruct on real spacecraft. If a launch is off of intended trajectory and is a potential danger they will order the rocket to self destruct."

At first I was suspicious of this statement because it seems to me that causing parts of a rocket to go careening off in random directions is no way at all going to increase the safety of the situation. Then I found this question here on this Stack Exchange, where the one and only answer verifies that most nations do implement self-destruct mechanisms and includes a recap on the situation with the Challenger disaster.

But now I am still wondering: why is it safer to have many small parts of the rocket come down separately than a fewer number of very large pieces? It's still the same mass overall and might just as well do more damage since it will surely fall over a larger area. This same reasoning has been used to disqualify the idea of blowing up an incoming asteroid as stated in this article which quotes Apollo astronaut Rusty Schweickart:

Another problem I can see is blowing up a large piece of rock only to create many smaller (but just as deadly) pieces of rock, doesn’t really extinguish the destructive power of an asteroid on collision course, in fact, it might increase it.

So what makes a rocket different? Are there any examples where the self-destruction of a vehicle was undoubtedly safer than the alternative?

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I also assume that their self-destruct sequence isn't just a "blow up the rocket!" command, but a deliberate sequence they've pre-planned for. Meaning, it's not some random explosions blowing the rocket up... instead, the sequence is thoroughly thought through where and how the rocket should self destruct, and where the explosives will be placed for self-destruct, in order to minimize the falling debris. – BruceWayne Feb 12 at 22:01

Self destructs do not result in smaller deadly pieces hitting people!

What the self destruct is designed to do is prevent powered chunks of rocket or payload getting to somewhere dangerous. If you look at any destruct videos, you will see that the explosion takes out any form of propulsion. Imagine leaving the craft to travel unguided until propellant ran out - you could hit a major city, or a nuclear reactor, or something equally as critical.

Yes, there may still be big or smaller pieces of the craft (generally much smaller, but not always) but they will come down within the safe exclusion zone near the platform or downrange.

If you end up with burning propellant and hot shards of metal over the deserted area around the launch platform, it's not a fatal problem (if we exclude manned launches...) as all spectators, civilians etc are well away from the area.

Oh, and it's entirely different to the blowing an asteroid into small chunks of asteroid concept. There we have a potential target of a hemisphere of the Earth.

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Well asteroid sizes can vary dramatically. It will be far more likely to have an asteroid that will endanger a city or a number of cities than one that will endanger half the planet. – Octopus Feb 12 at 0:19
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But that is still entirely irrelevant to this scenario. – Rory Alsop Feb 12 at 0:34
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One extra factor: that's the only way to shut SRBs down. – SF. Feb 12 at 8:39
    
Civilians, spectators etc. are hopefully also not in the downrange travel corridor of the launch. Manned launches often (not always -- see the STS) have crew escape systems such that even in the event of a catastrophic failure of the booster early during liftoff the crew can be saved. Later during liftoff the spacecraft usually has enough velocity to either land reasonably normally (like the STS's RTLS, TAL and AOA abort modes) or press to (possibly lower than intended) orbit (like the STS' ATO abort mode, which technically might not count as an abort). – Michael Kjörling Feb 12 at 12:42
    
That is also all assuming the rocket will be already moving downrange when the failure occurs. It seems like Long March 3 simply flew in an entirely different direction than "downrange". – SF. Feb 12 at 12:46

As for whether it's implemented on real spacecraft: This amateur video of the Challenger accident captures the destruction of the SRBs (about 1:45 in).

Three factors as to why:

When you push the button you convert one big rocket into a bunch of little pieces. Little pieces fall much more slowly.

When you blow the rocket you know where the debris is going to fall--and that's almost certainly an area that's already been cleared for the launch. Rather than on inhabited area like the Chinese Long March 3 catastrophe

When you blow the rocket you burn up the fuel. That's a whole bunch of mass that doesn't come back down at all and the energy in that fuel is up there with small atomic weapons. That's an awful lot of destruction you prevented. (Admittedly, the fuel isn't as damaging as a nuke of the same power but it can still be devastating.)

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@LocalFluff history.nasa.gov/rogersrep/v1ch9.htm search for "Range Safety". That's a NASA document that says the boosters were destructed. – Loren Pechtel Feb 12 at 1:38
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@HowardMiller The solid rocket boosters were not destroyed by the explosion of the main tank. They zipped around in the sky without any guidance system until the destruct command was sent. – Loren Pechtel Feb 12 at 1:39
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@LocalFluff believe it or not the STS did have a destruct system. Early in the program the ET and SRBs all had explosive charges. At some point after the ejection seats were removed from the orbiter, the ET had its destruct system removed. But the SRBs had them until the end of the program. The crew was also trained to perform a "Manual MECO" (shut off the engines) if the launch trajectory was going to impact inhabited areas (a call would be made to them....) They got a briefing from the Range Safety Officers before each launch....the crew would show the RSOs pictures of their kids... – Organic Marble Feb 12 at 2:22
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Little pieces certainly fall more slowly in the presence of atmosphere. – Russell Borogove Feb 12 at 2:28
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@Octopus You're forgetting that all launches to date where we care about something on the ground involved atmosphere. You wouldn't get that benefit if you destructed a rocket lifting off from the moon. – Loren Pechtel Feb 12 at 2:37

Self destruct is used in order to burn all the fuel in the air, preventing it from landing on the ground. The debris is not so dangerous in comparison. Most of a rocket is fuel, like most of the weight of a soda tin can is soda. Also to make sure that the event takes place over an evacuated area in a prepared launch range (although the Chinese don't seem to give that part of it high priority).

That spectacular 2011 Proton launch failure was caused by all the gyroscopes having been installed upside down, so the test of them before launch showed all correct because they were all equally wrong in agreement. Self destruct in that case would've been counter productive since the altitude was too low. The fuel would just have been spread out even more than by a crashing rocket. They shut down one of the six engines in the first stage in order to make it steer away from the launch pad and crash somewhere safe. Proton uses very toxic hypergolic fuel which adds to the problem of not self destructing. The landing ground had to be decontaminated.

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"all the gyroscopes having been installed upside down" - There you go, rocket scientists make five-year-old mistakes too. – immibis Feb 12 at 5:02
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@immibis That wouldn't be a rocket scientist, it would be a rocket engineer. Big difference. – Michael Kjörling Feb 12 at 12:50
    
@MichaelKjörling Or a drunk underpaid Russian worker who got his job because his wife's brother knew someone. Big difference again. – LocalFluff Feb 12 at 13:35
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@MichaelKjörling It wouldn't be an engineer, it would be a line technician. Engineers don't get to touch flight hardware :-( (source: am an engineer) – Tristan Feb 12 at 15:33
    
@Tristan Well, the take home from all of this is that a rocket scientist isn't going to install the gyroscopes upside-down leading to a launch failure. Good to know for the rocket scientist wanna-bes around here. – Michael Kjörling Feb 12 at 15:39

If you check for example the Jason 3 satellite launch webcast starting around 23 minutes, you will see this:

Jason 3 launch

(SpaceX webcast)

The white path shows the past trajectory of the rocket and the blue is a projection of ballistic path in case of total engine cutoff. The projection is always updated as can be seen in the video. For each launch there is a trajectory which the rocket should follow and then a safe flight launch corridor (not showed in there) where ships and airplanes etc. are forbidden during the launch. If there is any problem, the Range Safety Officer monitors the projection and in case it would get outside of the specified zone, they terminate the flight (autodestruct or just thrust termination) so the rocket or its remains have no chance of getting outside the safe zone.

That way the destruction of the rocket ensures that it does not just fly randomly still being powered by the powerful engines but it instead goes down where it is expected to not do any harm.

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In most scenarios in which a rocket flight needs to be deliberately terminated, the rocket is nearly full of fuel and oxidizer. The range-safety self-destruct system opens the propellant tanks rapidly, allowing the propellant to mix and burn while the rocket is still up in the air.

In this video of a Proton rocket guidance failure, it's clear by 0:15 seconds into the flight, we are not going to space today. The flight should have been terminated at that point,but apparently Proton doesn't have a self-destruct. The rocket starts to come apart, presumably due to aerodynamic stresses, around 0:25, and the body of the rocket ruptures and burns. Even so, there's a huge fireball when it hits the ground. If it could have been terminated earlier, the ground fireball would have been smaller, and closer to the launch pad (i.e. closer to the center of the area cleared of people for safety); if it hadn't exploded in the air, it would have been a bigger fireball, possibly travelling along the ground, and likely much further from the pad.

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This proton rocket is the same one linked to (maybe a different launch) in the 'this question here' link from the OP, where the answer says that Russia is the only nation that does not use self-destruct mechanisms. – Octopus Feb 12 at 0:28
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"The flight should have been terminated at that point,but apparently Proton doesn't have a self-destruct." IIRC, that's because Russia launches them in a giant wasteland. The US launches them about a minute away (by rocket, at least) from Disneyworld. – ceejayoz Feb 12 at 15:37

The purpose of self destruct is not necessarily to make it safer in all scenarios. It's to make it more predictable. As Rory mentioned, one of the major purposes of self destruct is to make sure there are no "powered chunks" flying in unpredictable directions. Predictability is the goal.

The self destruct is designed to be a floor on how bad things can get. We can run simulated scenarios as to how the pieces of a self-destructed vehicle behave in hundreds of scenarios, and make sure they all work out. Trying to do that with unpredictable hardware that was designed to be capable of reaching space is much harder. If it is decided that that unpredictability is unacceptable, and the predictable fallout of a self-destruct is acceptable, the button is pressed.

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