The history of spaceflight is littered with exists because of exploded nuts and bolts (and upon further reflection, a whole lot of other items!)

"Exploding bolts" is really a generic term. This answer to Would a “pyrotechnics” tag be useful? mentions Wikipedia's Frangible nut and Pyrotechnic fastener and while exploding bolts proper are one kind of said fasteners, there are all kinds of electromechanical devices that convert an electrical pulse into an irreversible mechanical state change.

We most recently heard about the pyrotechnics used to cut various tubes, and both electrical and mechanical cables allowing the Perseverance rover to drop the last few feet from where the sky crane held it and on to the surface of Mars.

Question: Which explode in spaceflight more often, nuts or bolts? Is it possible to ballpark a ratio? Does one number completely overwhelm the other?

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    $\begingroup$ Never even occurred to me that the nut could be, well, "frangible" is the word that the Wikipedia article uses. $\endgroup$
    – Greg
    Mar 20, 2021 at 20:59
  • $\begingroup$ @Greg frangible appears in 13 other posts here as well. $\endgroup$
    – uhoh
    Mar 21, 2021 at 1:10
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    $\begingroup$ Must they be 'in space'? Lots of the shuttle ones were on the pad and at SRB sep. $\endgroup$ Mar 21, 2021 at 20:11
  • 1
    $\begingroup$ @OrganicMarble I've implemented the space → spaceflight upgrade $\endgroup$
    – uhoh
    Mar 21, 2021 at 22:42
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    $\begingroup$ @Greg Eh, first time encountering this term? I tend to use it quite a bit in contexts like 'The Soyuz emergency landing procedure utilizes the fact astronauts bodies are frangible to a degree without threat of life" and "In Starship SN10's landing the performance of frangible bell nozzles was unsatisfactory. " $\endgroup$
    – SF.
    Jun 18, 2021 at 12:35

2 Answers 2


For Apollo: more nuts than bolts, but vastly more non-threaded connectors

Table I of Apollo Experience Report: Spacecraft Pyrotechnic Systems, NASA Tech Note D-7141, lists all of the pyrotechnics above the Saturn booster (e.g. the CSM, LM, escape tower, etc.). Of the more than 210 pyrotechnic devices, 8 were nuts and 4 were bolts:

  • 4 frangible nuts held the launch escape tower atop the command module. These were blown as part of the tower jettison.
  • 4 explosive nuts and 4 explosive bolts held the lunar module ascent and descent stages together. Yes, both sides of the connection were explosive. They really wanted to make sure that those two stages separated!

Explosives that connected things, or that cut connections, but were not threaded and therefore neither a nut nor a bolt:

  • Parachute disconnects.
  • Explosive charges that cut the docking ring, at the end of a launch abort or when discarding the LM at the moon.
  • Many detonators that separated the CSM from the SLA (where the LM sits), and joining the panels of the SLA together.
  • Explosive links in the tension ties connecting the CM and SM, and in the tiedown straps between the LM and the SLA.
  • The LM legs were retracted by straps while it was held in the SLA. An explosive-driven blade cut the straps, deploying the LM legs.
  • Lots of guillotines and umbilical disconnects.

The remaining explosive devices:

  • Cartridges to pop open the canard vanes of the launch escape tower in certain abort modes.
  • Igniters for various solid rocket engines on the launch escape tower, to thrust away the SLA panels, and to jettison the forward heat shield (to uncover the parachutes).
  • Parachute mortars.
  • Various valves which were remotely opened by small explosive charges.

Apollo pyrotechnics


It's nuts to the shuttle, 16 to 9

Launchpad to stack connections

4 bolts with frangible nuts held each Solid Rocket Booster (SRB) to the pad.

Each solid rocket booster has four hold-down posts that fit into corresponding support posts on the mobile launcher platform. Hold-down bolts hold the SRB and launcher platform posts together. Each bolt has a nut at each end, but only the top nut is frangible. The top nut contains two NASA standard detonators, which are ignited at solid rocket motor ignition commands.

enter image description here

Total - 8 nuts

Picture of the nuts

enter image description here

SRB to External Tank (ET) connections

Each SRB had a forward attach with one explosive bolt and an aft attach with three explosive bolts.

The forward attachment point consists of a ball (SRB) and socket (ET) held together by one bolt. The bolt contains one NSD pressure cartridge at each end. The forward attachment point also carries the range safety system cross-strap wiring connecting each SRB RSS and the ET RSS with each other.

The aft attachment points consist of three separate struts: upper, diagonal and lower. Each strut contains one bolt with an NSD pressure cartridge at each end. The upper strut also carries the umbilical interface between its SRB and the external tank and on to the orbiter.

enter image description here

Total - 8 bolts

ET to Orbiter

The ET was structurally attached to the Orbiter by a forward attach with one explosive bolt and two aft attaches with one frangible nut each.

The forward structural attachment consists of a shear bolt unit mounted in a spherical bearing. The bolt separates at a break area when two pressure cartridges are initiated. The pressure from one or both cartridges drives one of a pair of pistons to shear the bolt, with the second piston acting as a hole plugger to fill the cavity left by the sheared bolt. A centering mechanism rotates the unit from the displacement position to a centered position, aligning the bearing flush with the adjacent thermal protection system mold line.

The aft structural attachment consists of two special bolts and pyrotechnically actuated frangible nuts that attach the external tank strut hemisphere to the orbiter's left- and right-side cavities. At separation the frangible nuts are split by a booster cartridge initiated by a detonator cartridge. The attach bolts are driven by the separation forces and a spring into a cavity in the tank strut. The frangible nut, cartridge fragments and hot gases are contained within a cover assembly, and a hole plugger isolates the fragments in the container.

The mechanical and electrical connections passed through two umbilical plates which each had three bolts equipped with frangible nuts.

The aft separation involves right and left umbilical assemblies. Each assembly contains three dual-detonator frangible nut and bolt combinations that hold the orbiter and external tank umbilical plates together during mated flight. Each bolt has a retraction spring that, after release of the nut, retracts the bolt to the external tank side of the interface. On the orbiter side, each frangible nut and its detonators are enclosed in a debris container that captures nut fragments and hot gases generated by the operation of the detonators, either of which will fracture the nut.

Total - 1 bolt, 8 nuts

enter image description here

Other pyrotechnic devices

I've described the explosive bolts and nuts which fired on every mission. There were a number of other non-bolt-and-nut-type pyrotechnic devices used, a few of which fired every mission (e.g. the forward landing gear thruster) but most of which were provided for contingency cases (e.g. the backup main gear deployment system, the manipulator arm jettison system, range safety, etc)


  • NSD - NASA Standard Detonator
  • RSS - Range Safety System

References (see each for primary sources)

  • 2
    $\begingroup$ Interesting that both answers have a roughly 2:1 ratio of nuts to bolts. $\endgroup$
    – DrSheldon
    Mar 23, 2021 at 3:16

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