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There were some famous accidents where rockets launched and just went up into flames. Many had something to do with leaking fuel in some sort.

I want to focus on aerodynamic stress however, like when a rocket deviates from its path or has a wrong angle of attack, what causes it to be destroyed? Purely drag force?

As an example: What mechanics were at work when an Ariane 5 was destroyed on June 4, 1996 due to an integer overflow? It just burst into flames!

Edit: Well, it seems like I may have been a little inaccurate. As many of you pointed out, fuel leaks are only a consequence of catastrophic failure. Additionally, most rockets explode only because they were told to do so to prevent damage to the vicinities.

What I actually meant was not what causes the explosion but why does the rocket break apart when not following the planned trajectory.

I thought it was because of the immense drag force when flying perpendicular to the velocity vector with high speed. In any case, thank you for your answers and comments, they gave me a much deeper insight in rocketry accidents.

JustAGuy
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    That Ariane tried to fly sideways because its guidance system shut itself down due to a math error. Rockets are usually long flimsy columns full of propellants; they don't fly sideways well at all. I think you are mischaracterizing most failures as due to leaking fuel; that's like listing the cause of death as "heart failure". When a rocket's structure is torn apart, the fuel tends to leak out. – Organic Marble Sep 13 '18 at 12:55
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    I have good knowledge of the CRS-3/OrB-3 Antares failure. It had nothing to do with leaking fuel, but it didn't have anything to do with aerodynamics either. Would this be a useful answer to your question, or are you focused purely on aerodynamics? – Bear Sep 13 '18 at 12:56
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    Yep, most of failures end with leaking fuel+oxidizer and a big explosion. They usually start with something else that leads up to these. – SF. Sep 13 '18 at 13:17
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    In many cases, the cause of the explosion is the range safety officer doing the equivalent of pressing the BRB (big red button). The BRB is a fiction; it's actually two switches, both with plastic covers that need to be opened to protect against inadvertently toggling the switch. Every launch vehicle is outfitted with a flight termination system, a radio-activated explosive device. One switch enables sending a signal to the FTS. The other switch sends the signal, but only if the enable switch has already been thrown. Once enabled and activated, bye-bye, launch vehicle. – David Hammen Sep 13 '18 at 14:28
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    The range safety officer in the previously mentioned CRS-3/OrB-3 Antares failure was a bit slow on the draw. The signal was sent (and the launch vehicle did blow up), but the signal was sent too late to protect the launchpad. – David Hammen Sep 13 '18 at 14:30
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    I don't remember any rocket crash caused by fuel leak... – Heopps Sep 13 '18 at 14:56
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    @Heopps - If one considers all of the fuel leaking in a tiny fraction of a second a "fuel leak", then there have been many rocket crashes caused by fuel leaks. The intent of the flight termination system is to make the vehicle undergo immediate Rapid Unscheduled Disassembly on command. – David Hammen Sep 13 '18 at 15:13
  • I disagree that the range safety officer was too slow on the draw. On watching the video, blowing the rocket sooner wouldn't have been any safer. If it weren't for the radicalness of that tumble giving a good chance of pointing at the ground I'd have said it was safer to let it fly to AP even if unguided. – Joshua Sep 13 '18 at 15:48
  • @David Hammen - yes, usually "fuel leak" is a result of RUD, not a cause :) – Heopps Sep 13 '18 at 16:13
  • Obligatory XKCD (the part which is most apropos is at the bottom of the image) – Cort Ammon Sep 13 '18 at 19:51
  • When your rocket insists to go somewhere not the orbit you both agreed to you can be pissed up enough to explode the rogue guy before it decides to crash in some place full of people, like the place where you are for instance. – jean Sep 13 '18 at 20:17
  • @DavidHammen on the Antares launch, how would an earlier blowup have protected the launch pad? Usually, earlier = closer. – Hobbes Sep 13 '18 at 20:43
  • @Hobbes In this instance it seems that the rocket's trajectory was still vertical enough at the time of the failure that when propulsion failed, it fell back into the vicinity of the pad and then was detonated near the ground. Blowing it up at its peak may have caused less damage; I don't know enough to say definitively. – Chris Hayes Sep 14 '18 at 00:13
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    @Hobbes (and also Joshua): The CRS-3/OrB-3 Antares failure resulted from the launch vehicle losing propulsive power. It fell back toward the launchpad after this failure occurred. An automated fight termination system would have caused the vehicle to self-destruct shortly after detecting that failure. Waiting for clear confirmation resulted in the vehicle self-destruct occurring shortly before it would have hit the launchpad. An explosion higher up would almost surely have caused much less damage to the launchpad. – David Hammen Sep 14 '18 at 13:31
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    Very closely related: How can a rocket's self-destruct make for a safer situation than being out-of-control?, How are self-destructs implemented in rocketry? and Do all launches include self-destruct mechanisms? – David Hammen Sep 14 '18 at 13:35
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    I don't think any of these is a duplicate. The OP appears to think that vehicles blow up because of fuel leaks or aerodynamic problems. Modern launch vehicles typically explode for one of two reasons: (1) the vehicle detected a problem and made itself explode, or (2) key personnel watching the launch detected a problem and commanded the vehicle to explode. – David Hammen Sep 14 '18 at 13:40
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    I can't think of any rocket destroyed by a fuel leak. Usually it's either an engine not doing it's job (falls back), the guidance system not doing it's job (goes someplace it shouldn't, either destroying it outright or causing the RSO to push the button), or something broke and the rocket came apart (SpaceX CRS-7, Challenger.) – Loren Pechtel Sep 15 '18 at 04:38
  • @DavidHammen: Even so, the damage that actually occurred was still much less severe than if it'd fallen back onto the pad intact. – Vikki Jul 05 '19 at 22:37

4 Answers4

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I want to focus on aerodynamic stress however, like when a rocket deviates from its path or has a wrong angle of attack, what causes it to be destroyed?

In many cases, it's not aerodynamic stress. Many launch vehicle explosions result because they are commanded to do so.

Every launch vehicle launched from the U.S., including the solid rocket boosters on the Space Shuttle, is equipped with a flight termination system (FTS). This includes an explosive device intentionally placed so that it will cause near-instant destruction of the vehicle if activated. For example, just outside the oxidizer tank works quite nicely on liquid propulsion vehicles.

The FTS also includes a mechanism that triggers this explosive device. Until recently, the triggering mechanism was a radio receiver that received an encrypted signal sent by the Range Safety Officer. The US Department of Defense and NASA have been working on an automated flight termination system, with the triggering mechanism sent instead by a dedicated computer on the launch vehicle.

The intent is to destroy a wayward vehicle before it causes damage to the launchpad or further downrange. This doesn't always work. The Range Safety Officer for the RS-3/OrB-3 Antares launch was a bit slow on the draw. The signal was sent when the rocket was obviously going awry, and the rocket did explode, but the signal was sent too late to prevent the rocket from damaging the launchpad.

In the case of the initial launch of the Ariane 5, that explosion was a result of an internally generated self-destruct command, followed shortly by a range safety officer command. The Ariane 5 was, and still is, designed to blow up when things go awry.

Brian Tompsett - 汤莱恩
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David Hammen
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    For reference also look up some videos of russian launch failures. They don't believe in self destruct. – xyious Sep 13 '18 at 15:37
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    @xyious any videos in particular you can link us? Sounds spectacular. – Magic Octopus Urn Sep 13 '18 at 16:07
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    @MagicOctopusUrn xyious seems to be quoting EverydayAstronaut. – leftaroundabout Sep 13 '18 at 16:09
  • You said that they're "working on" an automated system, but the Ariane 5 was destroyed by an "internally generated" command... could you expand on that? – fooot Sep 13 '18 at 16:16
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    @fooot Ariane 5 is a European rocket and launches from French Guiana, so not affected by the US's timetables. – hobbs Sep 13 '18 at 20:20
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    Perhaps an even better object lesson in how bad things can get if an out-of-control rocket doesn't explode quickly enough would be the Intelsat 708 launch in 1996, which crashed on a Chinese village and killed several (at least six, possibly many more) people there. It's not on the video that @leftaroundabout linked to because that one only includes incidents with no loss of life; I was going to write that there's also no video of it, but apparently there is. – Ilmari Karonen Sep 13 '18 at 20:40
  • It might be less of a necessity if you have lots of uninhabited space around your launch complex. – Joey Sep 14 '18 at 07:02
  • Well, you know what they say, RUD and FTS will ruin any day... – E.P. Sep 14 '18 at 09:57
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    @IlmariKaronen - Another example was the 1947 launch of a V-2 from White Sands, New Mexico. Instead of going north, it veered south and shortly thereafter crashed a mile and a half south of Jaurez, Mexico, and very close to an ammunition dump where Mexican mining companies stored powder and dynamite. That it's not a good idea to spark international incidents with wayward rockets is something the US learned very early in its development of space exploration technologies. – David Hammen Sep 14 '18 at 14:33
  • @xyious - that claim needs sources. Soyuz has a self-destruct system. The N-1 too. And that's just the first 2 launchers I looked into. – Hobbes Sep 15 '18 at 17:41
  • Orb-3 lost thrust when it was still directly above the launch pad and rising vertically. The launch pad would've been damaged no matter when it was blown up. – Vikki Jul 05 '19 at 22:39
  • @Hobbes: They have a system to shut off the engines, but not to destroy the rocket (it just falls downrange and blows a hole in the steppe). – Vikki Jul 06 '19 at 00:43
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This Proton M launch in 2013 was caused by a serious problem with the guidance system (some parts were connected back to front apparently). If you watch the video you can see that the vehicle reaches an "impossible" angle (for a rocket) and then the top is torn away due to air resistance before the tanks start rupturing causing a fire (hypergolic propellants) just before it hits the ground. I don't know if the Russians have FTS or not but if they do then it didn't work in this case. Perhaps the rocket "thought" everything was fine as the guidance system was totally confused.

Bit like the whale from hitchhikers guide to the Galaxy "what's this thing coming toward me very fast? So big and flat and round, it needs a big wide sounding name like 'Ow', 'Ownge', 'Round', 'Ground'! That's it! Ground! Ha! I wonder if it'll be friends with me? Hello, Ground!"

Slarty
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    Russians don't believe in FTS, instead they try putting their rockets in the middle of no-man's land, where they can't hurt anyone. Also, like in many authoritarian-ish societies, the value of human life is not necessarily the same as in, say, Central Europe. Makes for spectacular pictures, though! – Jörg W Mittag Sep 14 '18 at 01:17
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    Some of the angular velocity sensors were installed upside-down. Naturally this inverted the information sent to the control system causing the rocket to amplify deviations from its trajectory rather than to correct for them. – J... Sep 14 '18 at 12:13
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    @JörgWMittag - that claim needs sources. Soyuz has a self-destruct system. The N-1 too. And that's just the first 2 launchers I looked into. – Hobbes Sep 15 '18 at 17:42
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    IIRC Proton has a FTS but it just shuts off the engines. Spaceflight 101 confirms the presence but not how it works. http://www.spaceflight101.net/proton-m-briz-m.html – Organic Marble Sep 16 '18 at 00:57
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During the Space Shuttle Challenger accident the actual orbiter was destroyed due to aerodynamic loads. It is often described as an "explosion" but most of what was visible was actually just venting hydrogen and oxygen. The Rogers Commission Report says:

19 Chapter III: The Accident Flight of the Space Shuttle Challenger on Mission 51L began at 11:38 a.m. Eastern Standard Time on January 28, 1986. It ended 73 seconds later in an explosive burn of hydrogen and oxygen propellants that destroyed the External Tank and exposed the Orbiter to severe aerodynamic loads that caused complete structural breakup.

And

All fractures and material failures examined on the Orbiter, with the exception of the main engines, were the result of overload forces, and they exhibited no evidence of internal burn damage or exposure to explosive forces. This indicated that the destruction of the Orbiter occurred predominantly from aerodynamic and inertial forces that exceeded design limits.

Adam
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Rockets don't explode as a consequence of failure. As discussed in other answers, the visible explosions are usually the deflagration of released propellants subsequent to either a commanded self-destruct, or a structural break-up. In the first case, the self-destruct is designed to open up the propellant tanks in order to disperse their contents so that the whole thing doesn't fall to the ground together; it seems almost inevitable that dispersed fuel and oxidizer will meet somewhere hot, or maybe there are hypergolics in the mix; either way, there is combustion and big smoke and flame. In the second case, if some structural part fails, the consequence is usually that some other structure gets overloaded and fails too, leading to a progression of failures inevitably involving the propellant tanks, and again the result is a big deflagration.

For an interesting twist on this, look at this youtube. This was a test flight of the Apollo launch escape tower using a Little Joe rocket. The objective was to test the system at high altitude, replicating the velocity/altitude conditions of a real Apollo launch. What actually happens is that by about the 17:13 mark in the video, the Little Joe booster has malfunctioned and begun an out-of-control roll. The roll gets so severe that at about the 17:35 mark, it has begun to undergo progressive structural failure. What's interesting about this, is that the failure prematurely triggers the escape system (it was deemed a successful test despite the malfunction); you don't really see an explosion of the failed stage, just bits and pieces flying off and a few big puffs of what appears to be smoke or perhaps aerosolized propellants.

Anthony X
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  • Definitely smoke; the Little Joe II was a solid-fuelled rocket. Also, that video link is broken. – Vikki Jul 06 '19 at 00:44