Airless Aerobraking
Nov 7th, 2008 by johnhare
guest blogger john hare
Aerobraking is possibly the most mass efficient means of matching velocity with a planet. The problem with aerobraking is that so many places of interest have no atmosphere to use for the purpose. The moon is first on the list of places that make you carry considerable quantities of rocket fuel if you want to match velocities with the ground, without resorting to lithobraking. Asteroids and comets are also inconsiderate about carrying a useful atmosphere to make your visit more economical, or even possible in some cases.
It would be really convenient if those places could manifest an atmosphere for at least the few minutes of deceleration needed for a rendezvous. For the moon, aerobraking could about double the payload to the lunar surface of a given vehicle coming from earth. However many dollars a pound you figure to land on the moon, aerobraking could cut it nearly in half. For an asteroid rendezvous that is close in space and far in V, the factor could be ten or more. We need a way to convince these places to help us out if they are going to be visited on a regular basis.
If a lunar cannon fired a ball of frozen oxygen (if gaseous oxygen is GOX, and liquid oxygen is LOX, is solid oxygen SOX?) straight up at 500 meters per second, it would rise to a height of 75 kilometers. If that LOXball had a tiny bursting charge in it that fired just in front of an incoming spaceship, the cloud of vaporised oxygen would impact the heat shield at 2,300 meters per second. The oxygen would heat and rebound from the impact at around half the impact velocity on the average for 1,150 meter per second rebound velocity. The impact and rebound are equivalent to firing a rocket with an exhaust velocity of 3,450 meters per second. A pound of lunar oxygen could replace a pound of rocket propellant from earth without even reaching lunar orbit.
A stream of these LOXballs launched vertically could lower a ship from lunar escape velocity nearly to the lunar surface on its’ heat shield without it using significant on board propellant. The last few hundred meters per second would need to be done on rocket power both for landing accuracy and because aerobraking is inefficient at low speeds. It would take roughly 2 pounds of lunar oxygen to stop each pound of incoming spaceship. The rough four gee ride should be an acceptable trade for nearly doubling mass to surface compared to pure rocket landing. An inflatable heat shield or shock absorbers would be needed to make it merely a rough ride.
If this method works, the question is, is it cheaper to mine and refine lunar oxygen, and fire it up in a precise trajectory, or to carry half as much propellant all the way from the earths’ surface? For the first several missions, on board propellant is obvious, later it may not be. Just throwing it in front of the incoming ship at lunar suborbital velocities will be much cheaper than carrying the oxygen to lunar orbit to refuel landers. Among other things, the oxygen slugs don’t even need a ship to get them somewhere useful.
An odd thing about using in situ resources this way is that the higher the closing speed of the oncoming vessel, the higher the performance of the aerobraking method. At 1,000 m/s closing rate,’Isp’ is around 150, at 2,000, it is 300. This is about opposite what anyone familiar with the rocket equation would expect. For asteroids with high closing rates, this is important. For an asteroid with equipment in place, it takes about one sixth as much material to slow an incoming vehicle from 10 to 9 km/sec as it does to slow from 2to 1 km/sec. The effective ‘Isp’ goes up as a linear function of closure rate. At 10 km/sec it is 1,500 ‘Isp’ for material that doesn’t even enter a rocket engine.
Another odd thing is that the aerobraking material can be any volatile material that the body of interest can supply. In this case, argon has the same ‘Isp’ as hydrogen or carbon dioxide. If the NEO has water or hydrocarbons, you are in business. It is not even necessary to purify them beyond removing solid contaminants.
The other side of the idea is aerobraking propulsion. If a mars mission is planned, and a suitable earth grazing asteroid has the volatile processing equipment in place, then the ship could get a boost on the heat shield it needs at mars anyway. Eccentric orbit bodies could become quite useful.
Now that’s thinking out of the box!
– Tom
> aerobraking propulsion
This is a fantastic way to accelerate people and equipment from low earth orbit into the highly eccentric earth orbits that are the low energy way moving into and out of cis-lunar space! Once launched to HEEO a spacecraft would never have to return to LEO.
If you use something like Zupperno’s scheme (www.neofuel.com) to bring huge volumes of water from a NEA back to a HEEO water depot, then you can trade 2 pounds of depot water for every pound of tools/people you want to move from LEO to HEEO! Returning from HEEO to LEO is a matter of aerobraking against the atmosphere.
Five minutes of thought brings to mind some inherent safety features to this scheme. If something goes wrong, any payload which is incompletely accelerated will be in an orbit with the same perigee as the depot but a shorter period. Orbital phasing makes it extremely unlikely that the depot and payload will be in position to try a second time in the short run, but some minor Maneuvering at perigee will drop the payload’s apogee into the atmosphere and allow for more aerobraking into reentry or back to LEO. It would suck to sit around in a capsule for two weeks waiting for a chance to get home, but the physics would be on your side.
This is just the propellant depot idea mentioned in an earlier post, of course, with the addition of banking potential energy as well as propellant.
Someone point out the flaw in this idea before my head explodes…
Jsuros,
Someone point out the flaw in this idea before my head explodes…
It won’t work because it has never been done before. There, feel better now?
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? 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. 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)?
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.
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.
Thanks, John. I needed that.
Eric, extra terrestrial volatiles are only precious if we consider the moon as the only possible near term source. Each of the Near Earth Asteroids that are large enough to see mass millions of tons. When the Pan-STARRS telescope completes its asteroid survey in about ten years we will know where all of them are.
Zuppero pointed out in 1998 (http://www.neofuel.com/moonice1000/) that that an expedition using very low performance nuclear steam rockets could make the trip from LEO to an asteroid. Once there, the same reactor can bake 100,000 tons of water from the asteroid material in about a year. If most of that water is allowed to freeze into a big ice cube, then the remainder can be used to accelerate the whole mass back to earth. He calculated that an expedition could bring 100 times the mass that left LEO back as water. I’m sure you could improve on that ratio by bringing it back only as far as HEEO.
Throw in John’s idea for shuttling back and forth from LEO to HEEO using airless aerobraking and we’re getting somewhere.
Of course, it’s never been done before…
Eric,
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.
Jsuros,
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.
Here’s an idea for the “bursting charge”. Let the vehicle fire a piece of ice from a gas powered cannon toward the LOXball to vaporize it. The vehicle arrives shortly afterwards and deflects the expanding cloud of gas. The Orion designs fired propulsive charges through a hole in the center of the blast plate. This is the equivalent using kinetic energy rather than fission.
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.
Hi John
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
Jsuros,
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.
Jörg,
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.
This is much more far-fetched, but I’m curious;
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”.
Roger,
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.
john,
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.
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.
Roger,
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.