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From: (Andrew Higgins)
Subject: Re: shape charges
Date: Mon, 01 Jun 1998 23:35:43 +0200

In article <6kspgn$8e5$>, wrote:

> Someone earlier said that explosive's detonation products expand at 5 km/s.  I
> guess shape charges can "multiply" this velocity by having it push on the
> material (the "liner") at an angle thereby levering up the impulse imparted.
> Sort of like squeezing a watermelon seed from your fingertips.  Is this right?
> (It's really more complicated than this, but I think this is how the
> basic mechanics of a shape charge work.)

The shaped charge effect typically involves an explosive with a "V"-shaped
slot or conical hole at the far end (opposite the detonator).  As the
detonation front propagates into this hole, it reaches the apex or peak of
the cavity first, sending the metal liner jetting outward.  The detonation
continues to propagate up the sides of the cavity, feeding the jet with
more liner.  The details of this are complex and are still a topic of
investigation today.

> I skimmed through the book "Fund. of Shape Charges" by Walters which said some
> "jets" (the melted expelled hot metal liner) reach 10 km/s.  Is this right?

Yes, shaped charges can generate jets of 10 km/s and faster.  But these
jets are of liquefied metal and are very thin (1 mm or less).  Such jets
are also unstable and usually break up after 10's of cm of travel.

For space launch, there are more judicious ways to use explosives.  One
example is an explosive-driven gas gun.  For example, if you line the
barrel of a gun with high explosive and detonate it at one end, it will
implode the barrel and drive a "pinch" down the barrel, which can be used
to push a projectile by a slug of gas (hydrogen, helium) trapped between
the pinch and a projectile.  The nice thing about this concept is that the
projectile never "sees" the explosive; as far as the projectile is
concerned, it is just being pushed down a barrel by high pressure gas like
a conventional gun.  This concept will work up to the detonation speed of
the explosive (8 km/s or so).

For higher velocities, you can line the barrel with an explosive lens,
which can implode the barrel at much faster speeds.  With an explosive
lens, the speed at which the detonation sweeps down the barrel (the phase
velocity) can be made arbitrarily high (faster than the speed of light,
even).  Thus, the imploding barrel can push a projectile to very high
speeds (15-20 km/s).

A explosive-lens driven gun as I've described above was built by Physics
International (PI) in the late 1960's.  With a very modest development
program, PI demonstrated muzzle velocities of 13 km/s with a 2 g
projectile.  And unlike the shaped charge effect, the projectile emerges
intact (not shattered or liquefied).  Obviously, such a gun is a
one-shot-only device.

Next time you in an engineering library, check out the NASA Contractor
Reports which PI wrote on this project:  CR-982, CR-1533, and CR-2143.
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University    Montreal, Quebec

From: (Andrew Higgins)
Subject: Re: shape charges
Date: Wed, 03 Jun 1998 23:39:33 +0200

In article <6l4lph$rd3$>, wrote:
> Anybody have any more information on space-related uses of shape charges, or
> any information on shape charges themselves?  Thanks.

The most realistic application is micrometeorite and orbital debris impact
simulation.  For example, there are about 100,000 pieces of debris in LEO
which are too small to track (10 cm or less), but are bigger than the ISS
is designed to withstand (10 g - 1 kg).  It might be nice to have some
idea of just how catastrophic an impact of one of these 100,000 objects
would be on a space station module.

Unfortunately, there does not exist a technology capable of accelerating
100 g or more to orbital class velocities (10 km/s) in a laboratory
environment.  What's more, the current "state of the art" which NASA uses
for hypervelocity impact tests is a light gas gun that is older than I am
and can barely accelerate 1 g to the velocities of interest.

Shape charge and other explosive-driven accelerators are probably the most
promising techniques which have a chance of simulating such an "event"
before the real thing happens.
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University    Montreal, Quebec

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