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From: Henry Spencer <>
Subject: impact shielding (was Re: Alternative space station)
Date: Sun, 13 Apr 1997 06:06:40 GMT

In article <>, Bill Baka <> wrote:
>That is probably a necessity of construction, metal first breaks the sand grain,
>then the Kevlar spreads out the impact. I think at 150,000 MPH a grain of sand
>would go right through the Kevlar.

Not quite.  At 150,000MPH, when the grain of sand hits *anything*, it
vaporizes, as does some of what it hits.  The result resembles an
explosion just below the surface much more than it resembles an ordinary
low-velocity impact; intuition is misleading here.  It's a lot easier
to soak up the force of the explosion if you let it expand across a gap
first, hence the two-layer shield.

The Giotto designers did experiment with using a composite outer shield.
It does work, but not quite as well as aluminum -- more of it vaporizes
and the impact on the inner shield is greater -- so they used aluminum for
the outer shield.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

Subject: Re: The perfect? shielding material?
From: Henry Spencer <>
Date: Sat, 28 Jun 1997 16:28:42 GMT

In article <>,
Gregg Eshelman <> wrote:
>Just imagine a 1/16-1/8" layer of this stuff on the outside of
>a shuttle ET. Space debris would either bounce off or get stuck
>in it...

I'm afraid you've got the wrong model here.  Space-debris collisions are
not like a thrown rock hitting a wall.  The energy in all but the mildest
debris hits is high enough to completely vaporize the debris.  A debris
hit is basically an *explosion*, often accompanied by shrapnel.  You can't
get useful answers for this problem based on rock-throwing analogies.

Much the most effective way to shield against debris (and micrometeorites)
is to put a thin outer shield a short distance out from the object being
protected.  Hitting the shield vaporizes smaller impactors and explodes
bigger ones, and the gap between shield and inner hull lets the cloud of
vapor and fragments expand so that it doesn't all hit in one place.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

From: Henry Spencer <>
Subject: Re: Alternative space station
Date: Thu, 10 Apr 1997 15:27:57 GMT

In article <>, Bill Baka <> wrote:
>> Making a sheet of Kevlar as difficult to puncture
>> as an inch of glass is easy, and despite the various cracks,
>> nothing's ever punctured the Shuttles' windows.
>A grain of sand at a differential velocity of maybe 40,000 MPH would
>puncture just about anything man made.

Not if it's a well-designed multi-layer structure.  Giotto's forward shield
held up well against several grains of sand at 150,000 MPH during its Halley
encounter.  The trick is to use a light outer layer that essentially causes
the incoming grain of sand to explode, followed by a gap that lets fireball
and fragments spread out, followed by a thicker inner shield which soaks
up the spread-out impact.  Extra shield layers can also be useful, but a
wider gap is often more mass-effective.

In any case, the point is that metal has no particular advantage over Kevlar
here.  In fact, Giotto's inner shield was mostly Kevlar.

>Most impacts are likely to be from some man made junk...

Depends on the size range, actually.  When you get down into the very small
stuff, the natural micrometeorite population dominates the space debris.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

From: (Dani Eder)
Subject: Re: Space Station Freedom
Date: May 10 1996

"Muhammad A. Ali" <> writes:

>The Space Station Freedom will be soon be constructed but I just want to
>know what material they use on the outside walls. I know there is a lot of
>debris out there traveling very fast, is there any material that can stand
>up to this?
>Thanks for taking your time to read this message. 

The meteoroid/debris shield is made of aluminum plate, as is the pressure
wall of the modules.  A typical impact speed is 8 km/s for orbital
debris.  This comes to 32,000 joules per gram of material, which is
sufficient to raise the temperature of the particle plus 10 times it's
mass in shield wall to 3200 K.  In other words, a typical particle can
melt or vaporize 10 times it's mass in whatever it hits, no matter what
you make it out of.

The principle of the meteoroid/debris shield is to use a first layer
a distance away from the pressure shell.  The particle hits this layer
and melts or vaporizes itself plus whatever bit of the first layer
it hits.  The melted/vaporized stuff sprays out onto the pressure
shell in the form of small droplets.  These have enough energy to make
small pits in the pressure wall, which is much better than a hole.

The thickness of the debris shield (around a 1/16 inch) and the
pressure shell (around 1/2 inch) were sized to minimize the chances
of penetration based on the known concentration of debris in orbit.

There is junk in orbit that is big enough to punch through both the
debris shield and the pressure shell.  In this case the kinetic
energy gets deposited in whatever equipment happened to be behind
the wall.  The effect will be like setting off a M-80 firecracker,
hand grenade, or artillery round, depending on the size of the junk.
At 8 km/s, whatever hits you has 8 times it's weight in TNT in
energy, so a 1 lb piece of junk acts like 8 lbs of TNT.  This type
of explosion is really bad news for the station, and anybody who
happened to be in that module.

They will try and avoid large junk by tracking it with radar, and
moving the station out of the way if they have to.  The tracking 
will be done on the ground, and if a piece of junk looks like it
will intersect the station's path, the station can fire it's reboost
thrusters and move out of the way.  The warning has to come well
in advance, since the station can only pump out about 1/1000 gee
in thrust.  So it can't dodge very fast.  

Dani Eder

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