Launching material off the moon for use in space is a problem in itself. Many methods are feasible technically. Far less are economically sound. Sending the propellant from Earth for instance is something that should only be done for the very early missions for extreme value payloads like an astronaut or a pure helium 3. The more that can be done with an in situ technique the better. Especially if it can be done on the real cheap, relatively of course.
Launch cannons are discussed for Earth launch on occasion and dismissed just about as often. Lunar gun launch is a different story. With an extruded tube twenty kilometers long it is possible to reach Lunar orbital velocity with accelerations averaging under ten gee. One bar of pressure could accelerate three tons at ten gee with a tube two meters in diameter. More or less pressure would be used for different masses with a thirty ton payload only needing ten bars pressure to hit the ten gee average acceleration.
Two hundred meters of launch tube would be sufficient for hardy payloads capable of tolerating thousand gee accelerations. Early in colony development chunks of frozen oxygen might fit that bill with the metal containers as part of the product delivered to a Lunar orbiting factory. Firing the gun against vacuum instead of an atmosphere really allows guns to show their best side.
All that is hardly new. One thing I haven’t seen as much of is methods of conserving the drive gas from the launch. I suggest that a reusable sabot be part of the standard equipment to support the payload and seal against gas leakage. That also has been thought of. I don’t know of any proposals to conserve all the drive gas for use again and again.
What may be original is the concept of using electromagnetic braking to stop the sabot inside the tube after it has imparted sufficient velocity to the payload. Electromagnetic braking can be done without the massive electrical generation equipment of the various mass drivers often suggested. It can in fact generate electricity while stopping the sabot. The massive power generated during that braking second or two should be usable somewhere in a colony that is still under construction.
The drive gas will start to be evacuated from the gun barrel into storage tanks as soon as the payload reaches design velocity. The gas scavenging should continue as rapidly as possible to relieve pressure on the sabot that hopefully extended the mechanical brakes as soon as possible to reach a complete stop well before the end of the gun barrel. The scavenging tanks should have large volume at low pressure to evacuate the barrel quickly. The drive tanks should be high pressure and low volume to facilitate good acceleration control. The sabot is returned to the gun breach area after the barrel has been evacuated to set up for the next shot. The drive tanks can be repressurized from the scavenge tanks at leisure as long as it is quick enough for the next shot.
The extrusion process should be able to turn out a gun barrel of continuously increasing length early in a colony development. As soon as the first two hundred meter section can handle ten atmospheres it could be used to send 300 kg payloads to lunar orbital velocity. The payload would either need to be caught into a circular orbit or provide a circularizing burn in order to avoid lithobraking at the launch site. The payloads would gradually increase in mass as the barrel extended until reaching the 20 kilometer length that would allow the sabot four kilometers of braking room after loosing the payload.
Muzzle loading air rifle for lunar launch with 100% volatile recovery. Low tech and cheap enough enough for you?
johnhare
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I’m sticking with this idea:
http://forum.nasaspaceflight.com/index.php?topic=5420.0
“Muzzle loading air rifle for lunar launch with 100% volatile recovery. Low tech and cheap enough enough for you?”
Yeah, could be. But it seems one needs enough market, first.
So if main market is rocket fuel, doesn’t seem it pay for itself if less than 100 tons per year.
One might think of it as alternative to having lunar rocket fuel- lunar sample return. So by sample return I mean volume 10 to 100 tons per year.
But probably best one expect in near term is buying lunar fabricated steel at $500 per kg.
But I think lunar rocket fuel is best in terms of being the first market
and I think problem is not having enough market volume for lunar rocket fuel at the Ls or lunar orbit. Though if you get 50 tons per year for first few years that could be enough market.
And in terms of price of lunar rocket fuel at lunar orbit, you simply have to beat the cost of shipping the rocket fuel from Earth.
If use lunar rocket fuel to lift lunar rocket fuel to orbit and can sell it cheaper than it can shipped from Earth, that means lunar rocket fuel at lunar surface is a lot cheaper than you ship from Earth and if can sell as much 50 tons at lunar orbit per year, then total production 50 at orbit, 50 to bring to orbit and 50 for use at lunar surface. So using fuel to ship increases amount market for total lunar rocket fuel sold per year.
But if need 1000 tons in orbit, then by point in time, it could significantly lower the costs.
Andrew,
The sling launch is certainly a viable candidate system for use on the moon. If I hadn’t known you were a serious commenter though, I wouldn’t have followed the link. May I suggest you expand on good ideas enough that we at least have an idea of what is being discussed?
One way to quickly dump drive gas pressure is vents in the side of the barrel at the max speed point into a large dump storage tank, opened by the sabot passing. Maybe even diverters dropping in behind the sabbot to direct the onrushing gas into a dump tank. A quick close ball valve come to mind, but the propellant gas behind the sabot has considerable kinetic energy that needs to be handled.
One issue with a gas gun is the need for the gas to expand fast enough to keep up with the round being fired. Lower molecular weight and heating the gas help.
I’d prefer the launch-sling, since you’re not wasting your export-product on leakage in the system. (1 bar against an open vacuum, you are going to lose a lot of hydrogen.)
For the curious-but-cautious, a launch sling is a horizontally rotating tether. Hub is a tower. Slowly spin it up while you feed out the tether, once the tip speed reaches the desired velocity, release the payload.
(You need to counter-balance the tether+payload, and you need to release this counterweight when you release the payload to stop the tower from being jerked over. But the counterweight can be an inert dumb mass, like rock. And if it is much heavier than the payload, it doesn’t have to be let out as far, so it won’t have as much velocity when released. Ten times the mass, 1/10th the release velocity.)
The tower only needs to be tall enough to allow for bit of droop in the tether-tip at full stretch, to keep it off the ground. And the only propellant is for circularisation of orbit.
[And you can reduce that by having another rotating tether in orbit. Where the tip speed matches the incoming payload. Since the outer-tip speed flings the payload out of lunar orbit, a series of tethers (lunar orbit, L1/2, HEO, LEO) could fling payloads back and forth. Probably no better than having a decent SEP ion-drive through, I just like the image.]
Actually the rotovators in various orbits could be well superior to SEP in translunar space. Incoming loads from the moon could supply the orbital energy to lift loads from suborbital to superorbital Earth velocity. On the other end incoming loads from Earth could supply the orbital energy to lift payloads off the moon. A well choreographed system could reduce propellant requirements to the first 5,000 m/s off the Earths’ surface.
For the outward journeys the rotovators could be almost as good. A cycling rotovator in dropping from lunar orbit to Earth grazing could launch a 3,000 m/s payload at perigee while traveling at 11,200 m/s Earth relative to give 14,200 m/s Earth relative at perigee. The payload would be on the order of 9 km/s after gravity losses in solar transfer orbit to Mars, Venus or wherever. That payload could be a Lunar refueled high mass ratio probe to anywhere this side of Saturn without gravity assist.
The sling’s electric motor could be in the counter weight. It would run round a circular track on a train. The connecting arm would need to be able to move horizontally to take any recoil.
I see no need for a counter weight, it’s very easy to built an anchored structure that can resist lateral loads that are many, many times its own weight.
I’m a Rotovator / launch sling fan myself; sorry to move away from the main topic, but some of you might enjoy a spreadsheet detailing the rotovator idea a bit:
http://bit.ly/131qd37
And here is a little illustration of the concept.
http://bit.ly/YfCMpC
One question that I haven’t found the answer too: is there necessarily a capture system in space to pick up the payload, since to go into orbit would require some form of orbital injection, and therefore propulsion? And if it doesn’t go into orbit, the payload will crash down again, and if it is going faster than escape velocity, well, it escapes, doesn’t it?
So this is an air gun system. Compressors and vacuum pumps are awfully inefficient machines, you may be wasting quite a bit of energy re-compressing your gases, like, about 90% or more?. Not to mention all that waste heat. On the other hand, the gas expansion cooling may freeze up nozzles if you use compressors and suchlike.
If we use the sabot idea: As a muzzle loader, I guess this is a kind of single piston compressor with a very long stroke? Do you really need to evacuate the gas? Doesn’t the barrel act as the air tank? Have you considered the sabot friction with the canon walls? What will hold the seal at maximum compression?
Is the sabot really necessary? How about a valve that closes the barrel as soon as the payload leaves the canon? Won’t the gas have transferred most of its energy to the payload, so the valve may be able to withstand the shock of closure? After all, won’t the pressure be down to about 1 bar, i.e 15 psi or for a 20 inch gun, or about 300 x 15 = 4500 pounds, 2000 kg?
What is the compression ratio of the gas? In a sense aren’t you replacing the energy stored in capacitors or magnetic coils by energy stored in a compressed gas? I think the barrel may end up being rather heavy…
The barrel is extremely heavy and is basically a long metal pipe laynig on the ground. Sabot friction is an obvious factor that would have to be addressed in the initial design phases as well as the sabot itself being the check valve. Inefficient would be wasting all those volatiles. Any of these ideas would need a capture or burn at apolune to stay in orbit.
I expressed myself poorly; when I said compressors are inefficient, I meant that the work done to compress the gas in mostly lost as heat. The energy that is available from the compressed gas is not all that high. The gas is acting as a kind of battery. The real energy source is either a nuclear reactor or solar panels. So the question would be: what technology has the lightest infrastructure capable of storing the energy and delivering it to the payload?
Chemical
Chemical would store the energy in split water, hydrogen and oxygen. To make sense, the water would need to come from the moon. So the base would be either the poles or (perhaps) a lava tube. The efficiency of the splitting is, what, 50%?. It requires oxygen and hydrogen compressors, so some cooling system as well. The fuel is stored in the cryogenic tanks of the ascent/descent vehicle. This would need to be entirely reusable. The payload to mass ratio on the moon is very good, so this is probably a very economical system. If the water is there and easy to mine (not too many salts!)
Magnetic
The magnetic idea suffers a lot from the weight of the magnets and their support structure.
Electric
The electric storage would use some form of ultracapacitor. If these could be built on the moon, great, but from the earth?
Compression
The air gun stores the energy in a tank in the form of compressed air. if the tanks (or the barrel, if it’s the only element used) were built from lunar materials, very little would need to come from Earth. The energy might be put into the air by some kind of electric motor that would winch the projectile down the barrel (with a cable?), and compress the gas. How efficient is very slow compression? Not sure. I’ll check the equations and send in a spreadsheet. The only weight would come from heat dissipation equipment, and the extra solar cells required if the system in inefficient. Put this way, the idea suddenly makes a lot of sense….
Mechanical
The rotovator stores the energy in the payload and the cable in the form of kinetic energy. This seems at first glance the most efficient since there are very few transformation losses. However, there is a lot of energy in the cable that has to be dissipated to wind it back in, requiring some form of cooling, or, less likely, energy storage. So, overall, perhaps not so good.
You nailed the concept with your compression section except that I see the compression by a more conventional type of compressor while the sabot acts to prevent gas loss. Use of Lunar materials would need to be towards 100% to make it feasible.
Rotovator makes sense on several levels with a few modifications to the conventional thinking. Somewhere in the last few years I posted about a compound tether on this blog. Modified for Lunar surface use would be a main tether rotating at half orbital velocity tip speed. A second hub at the tip would carry the secondary tether rotating in the same direction and hub relative velocity as the main tether. Once per secondary revolution, the tip of the secondary would be momentarily motionless relative to the Lunar surface as the velocities canceled. Half a revolution later the velocities would be additive with the tip of the secondary at lunar orbital velocity. You could pitch and catch to a stationary base with that system. One advantage being that incoming payloads would spin the system up adding the energy to throw equal payload out. No need at all to stop and start the system.
Hi John!
Here is a messy spreadsheet of the air gun: http://bit.ly/10vVTs0
The calculations lead to the following arrangement: A 1 km long, 30cm bore, 1cm wall, 78 tons cannon. The canon is closed by a quick opening valve at the muzzle, and is initially filled with air at 1 atmosphere, a shirtsleeve environment. The projectile is fed into the gun by a breach very near the muzzle, just behind the valve, and attached to a 1 km cable, 1cm wire high tensile Zylon. The breach is then closed, and the end valve opened. A compressor raises the pressure in the canon to 10 atmospheres. A winch then starts reeling in the cable, and the projectile acts like a piston, compressing the gas to higher and higher pressures. When there is only 33m left of travel in the gun, the pressure has been raised to 300 atmospheres or about 4500 psi. The other side of the projectile is in vacuum.
Then you release the cable attachment to the projectile.
You hope the winch doesn’t self destruct. This is a very iffy part.
The projectile is propulsed out of the barrel at escape velocity.
The pressure in the barrel is down to 10 atmospheres. The instant the projectile is out, you slam the valve closed. 10 atmospheres is only 150 psi, it can take it!
So one valve, one compressor, one air tank, one barrel, one cable, one very tough winch and a very high torque motor, probably a very heavy speed reducer.
No sabot, no braking.
The spreadsheet doesn’t take into account the fact that the rapid expansion of the gas will cool it, and that some of the energy will be lost. I’ve added a fudge factor which I hope is enough. If you think it’s worth it, I can look up the proper adiabatic equation. On the other hand, you could simply suppose that there is a heating element in the gun that heats the compressed gas before firing, to compensate.
Details.
A carbon fiber gun would be much lighter.
The initial acceleration must be rather scary.
Worth exploring, do you think?