Explosive bolts are often used for stage and payload separation. If only one of the many explosive bolts used fails to break into two pieces, the payload of the rocket may be lost. Are there more than one detonator used to initiate the explosive charges of these bolts, together with separate electric circuits for the control? Very little weight will be added by several initiators and their connections, but the probability of a failure should be much lower.

  • $\begingroup$ The extra wires would weigh a lot more than the extra detonators. $\endgroup$ Jun 4 '17 at 4:48
  • $\begingroup$ I recall reading about devices such as the NASA standard detonator. Such pyrotechnic devices and their associated triggering equipment are rather elaborate and complex in order to ensure that they are pretty much 100% guaranteed not to go off when they are not supposed to, but are pretty much 100% guaranteed to fire fully and completely when they are supposed to. In other words, a lot of design went in to ensuring that these devices were about as fail-proof as anything could be made. $\endgroup$
    – Anthony X
    Jul 1 '17 at 1:16
  • $\begingroup$ @Anthony X thanks for the link to the Wikipedia page. There is another link to this PDF osti.gov/scitech/servlets/purl/71636 Even high speed X-ray serial photos were made to study the ignition of the detonators. The time interval between two X-ray flashs were 1 microsecond and below. $\endgroup$
    – Uwe
    Jul 1 '17 at 12:01
  • $\begingroup$ Perhaps of tangential interest; the British TV program(me) Top Gear lost it's orbiter (They built a shuttle in a garage using a Reliant Robin Car as the orbiter) when one of three explosive bolts failed to detonate. You can watch it here: youtube.com/watch?v=pJdrlWR-yFM $\endgroup$
    – JohnHunt
    Jun 18 at 18:23

Sometimes they can do their job a little too well, or blow out some stuff when they're not supposed to, but I've never heard of one failing. They are normally used in a redundant pair fired close to simultaneously where either one will do the trick. The electronics firing them are also redundant. If your pyro fails to do its job, I'll be willing to bet 20:1 odds it was because either a) there was no power, or b) the software or controller made a boo boo and didn't try to fire it.

  • 1
    $\begingroup$ If a redundant pair of detonators is used and one fails but the other ignites the pyro, is it possible to find out afterwards that one the redundant pair did fail? $\endgroup$
    – Uwe
    Jul 1 '17 at 18:13
  • $\begingroup$ Usually not.... $\endgroup$
    – Mark Adler
    Jul 2 '17 at 1:17
  • 2
    $\begingroup$ By way of an Organic Marble comment on another Q, it appears that one of a redundant pair on a shuttle launch pad hold-down failed on STS-112. It's easier to notice that on a pyro that stays on the ground, presumably. $\endgroup$ Jun 18 at 4:05

I found a document about that theme:
Apollo Spacecraft & Saturn V Launch Vehicle Pyrotechnics / Explosive Devices

Some cites:

  • More than 210 pyrotechnic devices per Apollo Mission.
  • All devices required high reliability and safety
  • Most devices were classified as either crew safety critical or mission critical.
  • When complete system redundancy was not possible, redundant cartridges or single cartridges with dual initiators were used.
  • Typically, two separate and electrically independent systems operated in parallel and provided complete redundancy in the firing circuitry.
  • No failures of any pyrotechnic device were ever detected during any of the Apollo missions.
  • $\begingroup$ @Jan Doggen thanks for editing, looks much better now. $\endgroup$
    – Uwe
    Jun 30 '17 at 19:11
  • $\begingroup$ Seems to me I read somewhere that during the Shuttle program, there was a pyrotechnic failure - the frangible nut of one of the SRB hold down posts failed to ignite. It says here: en.wikipedia.org/wiki/Frangible_nut that such a failure would not hinder launch, but still, a failure of any such device anywhere could suggest the possibility of an eventual failure of another such device somewhere more critical. $\endgroup$
    – Anthony X
    Feb 25 '18 at 23:32
  • $\begingroup$ @AnthonyX you likely read about a problem that happened with the pyros on an SRB holddown nut. Each nut had 2 independent pyro systems that were supposed to fire simo. Either system was adequate to do the job. One time, one system in one nut didn't fire. It was traced to a bad connector saver in the circuit. $\endgroup$ Jun 18 at 3:40

I know of at least one NASA pyrotechnic failure, although it was arguably not really the fault of the pyros themselves. It occurred on the Skylab 1 launch in 1973, the then-unmanned space station itself on a modified Saturn V. The interstage ring between the first and second stage failed to detach from the second stage because the linear shaped charge (LSC) used to cut them apart did not completely fire.

There were detonators at both ends of the LSC. When the first one fired, the detonation propagated partway around the ring until it was stopped by a section of LSC that had been damaged by impact from the micrometeoroid shield that had come off Skylab 63 seconds after launch. (The impact caused a section of the explosive to burn, not detonate.) Although the second firing signal was sent 100 ms after the first, the ring had separated just far enough to pull the electrical connectors apart so the firing signal did not reach the second detonator. (I assume that the second firing signal was delayed to minimize shock forces on the structure.)

The adapter ring, which weighed about 5 tonnes, remained attached to the launcher all the way into orbit. This caused serious thermal problems around the engines, but they tolerated it. And fortunately there was enough mass margin that the launch succeeded; had this been an Apollo lunar mission the smaller mass margin would have forced an abort.

This is a good example of how failures can quickly snowball in unanticipated ways, outsmarting design engineers even when they include redundancy.

See "Saturn V Launch Vehicle Flight Evaluation Report SA-513 Skylab 1", MPR-SAT-FE-73-4. I found it online.


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