Newsgroups: sci.space.policy,sci.space.shuttle From: Henry Spencer <henry@zoo.toronto.edu> Subject: impact shielding (was Re: Alternative space station) Date: Sun, 13 Apr 1997 06:06:40 GMT In article <N.041097.212205.13@syix.com>, Bill Baka <bbaka@syix.com> 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 | henry@zoo.toronto.edu Newsgroups: sci.space.shuttle Subject: Re: The perfect? shielding material? From: Henry Spencer <henry@zoo.toronto.edu> Date: Sat, 28 Jun 1997 16:28:42 GMT In article <MPG.e1d18d0a20c4e7d9896a3@news.cyberhighway.net>, Gregg Eshelman <eshelman@cyberhighway.net> 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 | henry@zoo.toronto.edu Newsgroups: sci.space.policy,sci.space.shuttle From: Henry Spencer <henry@zoo.toronto.edu> Subject: Re: Alternative space station Date: Thu, 10 Apr 1997 15:27:57 GMT In article <N.040897.002501.93@syix.com>, Bill Baka <bbaka@syix.com> 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 | henry@zoo.toronto.edu From: ederd@bcstec.ca.boeing.com (Dani Eder) Subject: Re: Space Station Freedom Date: May 10 1996 Newsgroups: sci.space.tech "Muhammad A. Ali" <mali01@dakota.isd77.k12.mn.us> writes: >Hi, >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 |