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Date: 26 Aug 1993 21:09:22 GMT
From: Jordin Kare <>
Subject: Idle Inquiry: Explosive Bolts

In article <> (Henry
Spencer) writes:

>In article <> ("F.Baube[tm]")
>>	How are they detonated ?
>Electrically, by sending a strong pulse of current through an initiator,
>which responds by exploding, firing the main charge.  I'm not sure whether
>the initiator uses an exploding-wire setup -- where you just run so much
>current through a thin wire that it explodes thermally -- or a sensitive
>compound that's set off by heat or spark.

Generally the ignitor is a heat-sensitive compound with a filament.  
Exploding wires take excessively big current pulses.  Increasingly popular
are laser-initiated detonators, which use a pulsed laser diode to fire
an ignitor compound thru an optical fiber.  Insensitive to electric fields, 
lightweight, and much easier to seal than electrical connections.

>>	What are the alternatives for stage separation ?
>Alternatives to explosive bolts, you mean?  Mechanical latches of some
>kind.  Unfortunately, the explosive bolts typically are lighter and
>more reliable.  For all their hassles, explosives are a good way of
>building lightweight hardware that functions firmly and positively,
>because they can put a lot of energy into it for very little weight.
>"Every time I inspect the mechanism     | Henry Spencer @ U of Toronto Zoology
>closely, more pieces fall off."         |  utzoo!henry

There are other kinds of explosive separators, including a nice linear
separator (flat strip with a thin explosive "string" embedded in it, in a
rubber liner -- the liner bulges and breaks thru the metal, but retains
all the explosive products inside.  Double "strings" for redundancy, and
can be "fired" from either end for more redundancy).  An up-and-coming
alternative is something called a frangibolt:  it's a standard bolt
(say, 1/4x20) with a notch so it will fail at a specific place, plus
a block of Nitinol alloy (shape memory) that tries very hard to lengthen
when heated.  An electric heater warms the Nitinol, which stretches the
bolt until it breaks.  Much less shock than a pyro, and no explosive hazard, 
but it takes a few seconds to work, and requires more energy (few x 100 J
total) to run the heater than a pyro takes to fire.

	Jordin Kare

From: (Henry Spencer)
Subject: Re: What exactly is an explosive bolt?
Date: Thu, 8 Jun 2000 17:11:11 GMT

In article <01bfd124$15fea120$0bd41fac@HKTUT0005231758>,
R. Klompe <> wrote:
>> Where did this idea come from?
>Don't know where the idea originated. However, it was not invented for the
>space program. They were in use before that. Anyone got an idea on these
>bolts's origins???

I'd expect it came out of mounts for things like drop tanks on aircraft,
but that's just a guess.

>...The bolt is hollow
>and contains of course an explosive charge. Not a very large one, just big
>enough to blast trough the bolt's thin walls.

In fact, just enough to *break* the bolt's thin walls -- usually they are
made especially thin at just the point where it is supposed to break.
Explosive bolts are not intended to explode, just to break firmly and
reliably (although that invariably involves some amount of overkill).

>> DO explosive bolts fail?
>You bet they do. Like any explosive, one-in-so-many screws up. The SRB's of
>the Shuttle for instance are bolted to the launch platform by explosive
>bolts. If one of these fails, no problem, the SRB just rips it out of its
>attachment point.

Two corrections here...  First, the SRB mounting bolts are not explosive;
they are secured by explosive nuts.  Second, none of the nuts has ever
failed to blow on command; everything involved, including the detonators,
is multiply redundant.  The SRB *would* just rip free if it happened, but
it never has.

>On the other hand, if these things fail on, say for
>instance, a Saturn V S-IC/S-II interstage ring, than that thing remains
>attached to the S-II. This happened on launching the Skylab orbital

The ring did stay attached, but that was not a bolt failure.  If memory
serves (references aren't handy), when Skylab's micrometeorite shield tore
loose on the way up, it's thought to have damaged external wiring as it

Also, I don't think the ring was held on by explosive bolts.  The usual
scheme for the Saturn hardware was to use a linear shaped charge to cut
through a mounting flange or something similar.  Using bolts implies
beefed-up structure around them to concentrate the loads on a few bolts,
and weight-conscious designers don't like doing that.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

From: "Kim Keller" <>
Subject: Re: What exactly is an explosive bolt?
Date: Thu, 8 Jun 2000 12:44:41 -0400

"R. Klompe" <> wrote in message
> Wire, transmitter/receiver  etc... it's all possible. The bolt is hollow
> and contains of course an explosive charge. Not a very large one, just big
> enough to blast trough the bolt's thin walls.

Since I just completed a training course on STS pyrotechnic devices at work
recently, let me correct your explanation a bit.

The bolts aren't hollow: they're frangible. That means they have a shear
plane where an externally applied force can break it in two, allowing
separation. Usually, the force is actually applied through the bolt in the
form of a shockwave that breaks the pre-defined shear plane. Because the
bolts are torqued to a preset tension, the two ends spring away from each
other when they are sheared. You can't really use a hollow bolt since that
would undermine the strength of the bolt.

Nuts can also be used, and are in the case of the SRB hold-down posts. The
nut has two holes drilled longitudinally in it, 180* appart. Booster
cartridges are inserted into these holes and detonate at the appropriate
time, spliting the nut in two halves, releasing the bolt and allowing its
tension to hurl it away from the SRB.

Another type of pyrotechnic device is the Confined Detonation Fuse. It's a
tube filled with an explosive charge. When the charge is blown, the metal
tube expands enough to shear the fasteners it is routed next to. An example
of where this is used  the orbiter's side hatch.

All these devices are inited by a highly reliable device called a NASA
Standard Initiator (NSI). It's a sort of blasting cap that has two redundant
firing circuits running through it. When the Pyrotechnic Initiator
Controller (PIC) send the firing signal (which is actually just a high
amperage DC voltage) the NSI fires, triggering whatever device is attached
to it- the CDF, the booster cartridge or some others I didn't go into.

These devices are industry standards and are used on all launch vehicles,
not just Shuttle.

Kim Keller
Inspector, Fraternal Order of the Pad Rat

"Neither rain nor sleet nor snow...well, okay, maybe sleet and snow"

From: (Henry Spencer)
Subject: Re: What exactly is an explosive bolt?
Date: Fri, 9 Jun 2000 02:03:46 GMT

In article <>,
Kirk Voelcker  <> wrote:
>What efforts, if any, are undertaken to mitigate orbital debris from explosive

The Confined Detonation Fuses that Kim mentioned -- and similar devices --
are now strongly favored over simple explosive bolts for use in orbit.
Debris is actually often a secondary issue; contamination of the payload
is frequently a bigger concern.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka


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