I was expecting that a kilogram of *anything* would be too much to hit at orbital velocity. I chose aerogel because of the it’s very small mass – hopefully giving the lightest mass that could be individually aimed and thrown.

The masses have to stay the same as it is the quantity of mass that does the work. Aerogel actually solves a several of the problems with the concept if it can be made to work. No longer wasting volatiles, safer, storable, and precision expansion is no longer required.

I was also chose a rail gun (*hoping* that it’s possible) so that no propellent/explosive would be needed. Not just so that propellent need not be shipped from earth, but to be “green”:

I wasn’t thinking green, but if I was, a rotary aerogel launcher would be relatively low tech and low risk. 500 m/s is just not that bad a problem for launching material.

It is possible that your suggestion could go one step further. Build a 100 km long braking field of aerogel blocks on the surface in such a way that an incoming spacecraft simply plowed into them. There ought to be at least one flattish area that could work an intercept trajectory. Or do a 3 km strip for 100 gee cargo catching.

“The lunar missions increased the mass of the lunar atmosphere by 30%, which was enough to impact the sensitivity of some of the experiments. After several weeks the atmophere returned to normal having been swept clean by the solar wind that keeps it in relative stasis. ”
http://www.xefer.com/2005/06/moon

With astronomy being one of the major ways to justify lunar activity, it’s best not to annoy the astronomers with extra atmosphere.

While I am not sure that this is a valid objection, I am equally not sure that it is not. One would definitely want to study that issue in detail before spending any development money.

I was also chose a rail gun (*hoping* that it’s possible) so that no propellent/explosive would be needed. Not just so that propellent need not be shipped from earth, but to be “green”:

“The lunar missions increased the mass of the lunar atmosphere by 30%, which was enough to impact the sensitivity of some of the experiments. After several weeks the atmophere returned to normal having been swept clean by the solar wind that keeps it in relative stasis. ”
http://www.xefer.com/2005/06/moon

With astronomy being one of the major ways to justify lunar activity, it’s best not to annoy the astronomers with extra atmosphere.

Wouldn’t it be vacuugel if you made it on the moon? This would be a way around the waste of volatiles if the manufacturing wasn’t too tough. I don’t see why you couldn’t fire them from a cannon. Firing into vacuum, bore size can be a meter for a kilogram chunk. There was something on the net a few years back about pellet stream propulsion.

Could a shield on a spacecraft be made to withstand impacts at the same speed from beads of silica aerogel?

I’m thinking of a continuous stream of them aimed at the spacecraft from the lunar surface, instead of expanding balls of gas. We’ve gotten pretty good at “hitting a bullet with a bullet” – so we should be able to make most hit.

It may be more mass-efficient – compared to an expanding gas ball, a higher percentage of the mass fired from the ground would hit and slow the spacecraft.

If I understand correctly, the main components of silica aerogel are silica and sodium, and both are found in lunar regolith.

You’d need a good gyro to keep the spacecraft from tumbling on off-center hits. Perhaps with multiple streams they’ll average out – instead of hitting like a machine gun, they hit like a sandblaster.

Firing them at much higher velocity you could use a stream to accelerate a departing spacecraft. (But then stray beads wouldn’t fall back to the lunar surface. Other spacecraft operators would get annoyed.)

I’m not sure how to launch the things. The Wikipedia entry for Aerogel mentions metal-aerogel nanocomposites – perhaps these are affected by a magnetic field, and can be launched from a rail gun.

And of course the whole thing falls apart when you get sued on trademark violations for using the term “Gellobraking”.

Your idea need some heavy equipment on the moon.

An underlying assumption in the idea is that LLOX mining is already going to take place for both life support and rocket fuel. If that assumption is valid, then the heavy equipment on the moon consists of a cannon with high rate of fire. The muzzle velocity I suggest is about half that of hunting rifles, and a third of a tank gun. Firing into vacuum lets you use a very low density projectile, which lets the cannon operate at very low pressures relative to fire arms on earth. Electromagnetic launch may be more desirable.

So i would rather build a skyhook from the surface of the moon through L1 to minimize fuel consume.
Its doable with present material.

Skyhooks will certainly be better when they come on line. The problem is that a skyhook through L1 will be tens of thousands of kilometers long and will require a lot of material shipped up from earth. That material will cost hundreds of billions in shipping costs until launch costs from the earths’ surface drop considerably. For early tether operations, a lunar rotovator would be more economical. With a tip speed of 1,600 m/s, a very small unit could pick up and deliver cargo fairly early in the game.

That would certainly be more reliable than a bursting charge, and cheaper in the bargain. At those closure rates, a BB gun could probably vaporize a kilogram chunk of volatiles. The dispersion would probably be an elongated torus.

I wonder if the hypervelocity impact you suggest would vaporize rock. If it would, volatile mining may not even be required. As Eric pointed out, wasting volatiles may not be a good idea.

Your idea need some heavy equipment on the moon. So i would rather build a skyhook from the surface of the moon through L1 to minimize fuel consume.
Its doable with present material.

jörg

This gives the vehicle some control of its fate, especially if some sort of close defense system monitors everything approaching the heat shield and can fire something like a claymore mine to vaporize incoming chunks.

Throw in John’s idea for shuttling back and forth from LEO to HEEO using airless aerobraking and we’re getting somewhere.

Your idea, so take credit where credit is due. When I have an idea, and it doesn’t die of loneliness, I’ll certainly take credit for it.

Of course, it’s never been done before…

Exactly, that’s how we know it won’t work. Ugh, me go cave now, keep fire stupid wheel.

Why would you want to waste such a precious commodity as extra-terrestrially mined volitiles to simply gain a mass factor of two or three in delivered payload?

It depends on whether it is precious in the place that it is used compared to the alternative. If LLOX costs $100.00 lb to mine, process, and launch, then it makes sense only as long as it costs well over $200.00 lb for the propellant that is inbound from earth. If LLOX costs $2,000.00 lb, and the inbound stuff costs $3,000.00 lb, then it does not make sense to do this. If the LLOX costs $50.00 lb, and the incoming costs $20,000.00lb, then it makes excellent sense.

I’d much rather see that same material be launched to an orbiting propellant depot where it could be burned much more efficiently in a rocket engine.

By the time you ship it to an L1 depot and burn it in a rocket engine to make the landing, you will not only use more LLOX, you will also have to supply a fuel to go with it. If that fuel has to come from earth, costs go way up.

It also sounds a bit dangerous. Do you really want to run the risk of pelting your heat shield with chunks of frozen material (assuming that the incoming snowball may not all vaporize precisely as you have envisioned)?

I agree here. There are at least a dozen things that can go wrong. It may not vaporize at all, which leaves a large compact mass coming at you at near orbital velocity. The LOXball launcher could malfunction with the ship on impact trajectory and no maneuvering capability. One could hit off center spinning the ship into a sideways series of gas impacts. The relative masses could be miscalculated with extreme gee forces or insufficient braking.

Could something similar be accomplished by doing some kind of in-flight refueling. In other words, pre-launch a tanker on a slower trajectory, but one that would be more energetically favorable to rendezvous with the cargo ship’s transfer orbit.

I don’t see this as competition for tankers. There will always be a need for tankers and propellant depots. This is a scheme to get serious benefits out of cheap volatiles that happen to be in convenient locations.

My personal choice would be to use spinning tethers to capture the excess momentum of inbound spacecraft, and then transfer it to outgoing spacecraft in the same manner. For perhaps the same amount of investment in infrastructure, you have a much more sustainable network of momentum banks which can be reused without the need for additional propellant resupply. Of course they will not be able to totally replace fuel depots, but perhaps they could be put to good use on regularly traveled transfer orbits.

Properly placed tethers are an excellent transportation system. It is incomprehensible that more attention hasn’t been paid to them. 3,000 m/s tips speed is my understanding off current capabilities. A tether in LEO could do a suborbital pick up and throw the craft to eccentric near escape. A tether in HEO could pick up the craft and send it almost anywhere including the moon. A rotovator around the moon could pick up and drop off at the surface without any propellant at all.

Under some conditions though, aerobraking will require much less earth launched infrastructure.