Rail Gun Help Please

Factual Questions
1

Yo,

I have a bet with my dad (who is a nuclear physicist but was very wrong about some ‘simple’ relativity things, thank god he only only makes cell phone towers instead of nuclear warheads now…) Any way heres the deal: I say it is ‘cheaper’ to use a Rail Gun to launch a satelite into space than it is to use a chemical rocket to launch it.

The rail gun: The projectile consists of a cylindrical aluminum chassis, with a non metalic and non conductive nose and ‘butt’ cones. It also has a smallish chemical booster rocket.

According to my current understanding, the repellant force of the aluminum ‘circuit’ against a rotating magnetic field (Ie an electro mag ran off of AC) is more powerful/efficient than the standered ‘positive attraction’ electro mag force used by such roller coaster rides as ‘Superman’, ‘Superman: The Great Escape’, and ‘The Outer Limits’(all by 6flags)…

2 assumptions, a ‘cheap’ way to produce electric energy, either Hydro-electric, or Nuclear…and a ‘Cheap’/efficiant way to store the electric energy, what im thinking about is a technology developed for the (may it rest in piece) Super Conducting Super Collider. The way I understand it a vast array of superconducting coils is used to ‘trap’ the electric energy. IE you pump electricity into this unit and it gets ‘lost’ till you want to pull it out…Kinda like a VERY VERY VERY huge (next to) loss-less capaciter…I think one of the last McGyver(sp?) episodes (or movies) dealt with this. (and I had a friend that actually worked on this project…SHHHH the feds may be looking…)

Anyway, I need scientific formula that it is cheaper to launch a sat into earth orbit from earth using a rail gun than by using a chemical rocket…I have a bet with my Nuclear Physicist dad.

Im guessing the formulas needed are the escape velocity of the earths grav well + the air resistance encountered and alot of others…

If any brave souls feel like they can figure this out, based on physics and other mathematical formula (JS Princeton PLEASE HELP!!!), help me please…
Id be glad to share the winnings of the bet with ya…unfortunatly, the prize of the bet is a night on the town on the others expense…

Help Please,

-BLah

2

I can’t even begin to answer your question but doesn’t the “smallish chemical booster rocket” sort of negate the whole debate? Sounds to me like you are making his argument for him.

3

What’s with the free electricity and super wonder capacitor? Does dad, THE NUCLEAR PHYSICIST, get to assume free propellant and booster rockets? What, exactly, do you mean by ‘cheaper?’

For the formula try putting, oh, I don’t know, Escape + Velocity + Earth into google. I bet you will find your answer.

4

It’s impossible to put something in orbit from a planet using any type of gun, if the planet has an atmosphere*.

So there has to be a rocket or scramjet or something that can change the object’s trajectory while in flight. So no, I don’t think that immediately disqualifies using a gun to launch something into orbit, it’s an issue of big rocket vs. gun and small rocket, since the bet is about cheapness, not one or the other.

*On a planet with no atmosphere, you could stick the gun up on a tower, point it horizontally, fire it, then move the tower before the projectile ran into the back of it.

Blah, have you seen ARLA? It might reinforce your argument, since it’s meant to be built by hobbyists, rather than a government. http://www.alt-accel.com/arla/

5

Yes, if you use just a railgun the resulting orbit will intersect with the earth (or at least its atmosphere). But you launched the projectile fast enough so it gets out of the atmosphere at orbital speed, you’d only need a small rocket motor to circularize the orbit. On the Space Shuttle the main engines put the orbiter on an elliptical orbit, and the small OMS engines (the little ones next to the vertical stabilizer) powered by internal tanks (the big external tank has been disposed of by this time) are used to circularize the orbit.

As for energy efficiency, my WAG is that chemical rockets are more efficient. As I said, a railgun projectile must be launched at speeds much faster than orbital speed so it can overcome air resistance using momentum alone and still have 7km/s of speed left. That means flying at at least Mach 30 at low altitudes - not a very energy-efficient thing to do.

In any case, the cost of energy is insignificant compared to the cost of designing, building and testing the launch system. The cost also depends on how many launches you do. Currently the market for launches is limited, so you can’t build a $5-billion railgun system (I think it’d be at least that much) and hope to recover the investment. It’s more economical to build a $20-million expendable rocket for every launch.

6

Short answer: Need more information on exactly how one should go about figuring out how to answer this fairly subjective question. The rest of this post is just attempts to address salient points in application.

Formulae for calculating trajectories and energies required to launch a satellite into orbit are meaningless for this comparison since it will require exactly the same amount of energy to do one or the other. You have to have exactly the saming launching velocity for any process you want in order to achieve the same orbit (except the chemical rockets can actually go faster because they’re allowed to turn around and burn the other direction to slow down as well as speed up, you can’t do that in the rail gun). So the question really is, “Which is cheaper energy: a rail gun or a chemical rocket?”

Well, the question is, “What do you mean by cheap?” Do you have to consider, parts, labor, development, maintenance, etc? Is it based purely on economics? Are you talking about what is cheaper in terms of which uses the least amount of energy? (The answer is they both use the same amount of energy.) You might ask which is more efficient? That last one’s hard. It really depends on how efficient your capacitors and your electric generation is. I would tenatively guess there is generally pretty good comparison between electric generation and storing of chemical energy on these scales; it’s fairly complicated to really pin down which form of energy would be most efficient. Electrical energy has won the day for the private consumer because it can be generated on larger scales, but we aren’t talking about simple internal combustion engines and motors here; we’re talking about rockets and rail guns, both being fairly large scale. In general, the larger the scale the more efficient the energy and as our scales are exactly equivalent by definition… well, now we’re running in circles.

If cheaply means more easily and quickly (cost benefit analysis), at this point the answer is a chemical rocket. If you gotta get the orbitting object up there, the only way to do it right away is to use a rocket. New technologies need to be tested and refined before being easily applied. And time is money.

One more thing to consider…

Although you gotta tow a lot of weight with your rocket’s fuel reserves, you get a force applied over a much longer period of time. While your impulse and total energy will ultimately be the same for either process, the burst of power needed will necessarily be greater for your rail gun. You’ve gotta be able to pull a heck of a punch to get that rail gun to give something enough oomph. In principle that means you gotta build this fandangled capacitor. The R&D on that’ll set ya back. Say that the technologies are all available… it really depends on how much it costs to run the darn thing. Do you need a staff of a particle-collider lab on top of your regular space technitians just to keep your rail gun operational?

The flip side of that coin is this: Let’s say an efficient rail gun is developed. In terms of strictly MATERIAL resources it would definitely win hands down. Rockets toe their fuel along with them that you never get back. A rail gun stays on the ground and can be reused (no assembly line required… just an army of physicists). Depending on how complex the system is, it may be faster to run the rail gun, but I wouldn’t bet on it. Complex systems (like particle accelerators pr rail guns that are in existence today) are subject to many delays and demands of testing. I don’t know how you’re going to get around the “cutting edge” nature of the rail gun. Generally, the more mundane and straightforward the technology, the easier (and in arguably REAL ways, cheaper) it is to run it.

7

Aside from being poorly written, the previous paragraph is subject to wild misinterpretation. As has been pointed out rightly, the railgun will require more energy for the launch. However, the railgun can also launch less. I don’t know quite how this trade-off looks, other than all the railgun proposals I’ve seen use a rail gun and rockets together in a kind of minimizing the Lagrangian technique.

Also, above I said, “saming launching velocity” when I should have said “same escape velocity”. This is what I was really getting at. The rocket can actually go at a higher escape velocity and slow down if it wants to (though that’s just an error nicety, not really germane to the discussion).

Honestly those few sentences should have been left out because they only confuse the issue further. I need to stop late-night posting nonsense. Sorry!

8

For some reason I recall maintenance as being non-trivial for railguns. These are guesses about maintenance costs:

  • Expensive to build a long rail that’s flat, well-supported, and can handle the extraordinary speed of the rail car as it reaches the end of the track.

  • Expensive to keep it calibrated.

  • Expensive to devise a system to stop, recapture, and refurbish the part of the accelerated vehicle which stays on the ground.

  • Railgun itself needs lots of space, near the equator. There aren’t good sites near huge amounts of electricity.
    And there are sonic booms, of course. Really big ones, enough to disrupt 10s, if not 100s of square miles.

Finally, railguns can’t be repointed. Spacecraft are orbited in all sorts directions, occasionally even 90 degrees away from the spin of the Earth.

A railgun on the moon is a different prospect: no air, no geological movement of the ground, no sonic boom, lower gravity, so the railgun can launch at much more reasonable speeds, ability to use a very dirty nuclear plant to power it…

9

Isn’t escape velocity for a ballistic object 11 kilometers per second? That’s pretty speedy for something that isn’t providing constant thrust, Blahman… don’t forget to take atmospheric friction into account.

I recall an old thread a while back about this… http://boards.straightdope.com/sdmb/showthread.php?threadid=82098&highlight=space+launch

One person figured that to get an object into orbit using a rail, and assuming 8 G’s in doing so, you’d need about 20 miles of rail track… and this won’t even get you up to escape velocity (the plan was for a scramjet to do the rest).

Are you suggesting that it’d be cheaper to build a 20+ mile rail track than it would be to just use the good ol’ conventional rocket fuel?

10

You also have to consider over how many launches you’re comparing costs. Launching payloads by conventional non-reusable rockets will have a moderate cost of developing the rocket, then a fixed cost per payload launched as each rocket has to be built from scratch. Launching payloads by railgun will have a very large initial cost (to buy the land and build the facility), then a lower cost per launch (for the electricity needed to run the plant, and the booster rocket needed for orbit circularization). So for a small number of launches, the rocket wins out, but for thousands of launches all into the same orbit the railgun looks better.

Also, the railgun will subject your payload to a much higher acceleration force than the rocket. This means the payloads have to be built to survive higher gees, which will make whatever you’re putting into orbit more expensive and heavier.

11

Even if you only have to go 7km/s that’s still about mach 21. At sea level the atmospheric friction would turn you into a cinder with a smoke trail. SR-71 Blackbirds can fly about mach 3, the pilots can warm food from the heat radiating from the cockpit windshield.

Conventional rockets do most of their high speed work up where the air is thin, and even they have to be careful not to burn up during re-entry.

12

Also, just as a side issue, how practical is it to subject any payload except for, say, a solid-metal BB/pellet (which is what little railguns shoot now) to the ridiculous magnetic field you’re generating? The fields can literally blow current railguns apart … that can’t be good for the electronics on a satellite.

13

Railguns usually shoot metal arrow-like projectiles in sabots. You’re thinking of a coil gun, a different beast.

The only reason to limit yourself to 8 G’s is if you’re launching humans. A railgun system would be better for smaller, non-living projectiles, so you’d accelerate a hell of a lot faster (and in a shorter distance).

With the nature of a rail gun, I don’t know if it’s even possible to accelerate at only 8 Gs.

15

It doesn’t matter how you accelerate the projectile. As long as you are doing the acceleration on or along a track fixed to the ground, the limitations pointed out so far all apply. As for storing energy in a superconducting coil, yes it’s possible but hardly “low maintenance.” You have to use liquid helium to cool the coil, and cryogenic equipment is expensive to build and maintain. Closeness to the equator is vital for any launch system (except for polar orbit launches) because the extra kick you get from the earth’s rotation is just too valuable to give up. The sonic boom itself may not be a problem if you choose a very remote location, but the projectile still loses a lot of energy to the sonic booms.

I think the most energy-efficient method of launch (besides a space elevator) is to use wings and air-breathing engines (i.e. take oxygen from the air instead of a tank) to get as high as possible. Then use rocket engines to hop into orbit. Currently the Orbital Sciences Pegasus rocket comes closest to this.

16

Choppy Logic, sorry your cuz ran afoul of things. Thank him for his response. I was paraphrasing from memory some studies I’d read long ago. It seems to me the railgun was assumed to used magnetic elevation/propulsion. Obviously, I can’t have much of a handle on the maintenance costs of “top secret” tech, but I still suspect maintenance would be a problem.

Also, which African or South American country would the superpowers like to give control of a major launch site? Grin. My bet: none of them.

The problem with not being able to aim launches is more serious than it may appear. A considerable number of satellites are not launched in an equatorial orbit, and moving anything in space from one orbital inclination to another is very expensive.

Here’s an interesting page with 3D realtime updates of satellites and their orbits.

http://liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html

17

True, but a lot of satellites have sensitive equipment that can’t be jostled around too much. Plus, the 20 mile-long launch track at 8 G’s wouldn’t get one up to the necessary 11 kilometers per second velocity one needs to get into orbit.

What you’re describing is a “coil gun”, which is what most people think a rail gun is. A rail gun just has two rails and a projectile nestled between them. A coil gun has all this stuff about “rotating magnetic fields” and the like.

End result is pretty much the same, however.

Maintenance: You got twenty miles of highly-sensitive track. If you just get some loony to go out there with a bag full of gunpowder, you can wind up with a dead launching system really quick.

18

Maintenance: You’ve got a giant white beast covered with tiles that it has to have, attached to two canisters of very dangerous solid rocket fuel and a huge red tank full of liquid hydrogen. One loony with a rifle could make a dead launching system from a mile away.

19

No, I’m not (though the OP is).

http://www.scienceweb.org/movies/eraser.html

http://www.scienceweb.org/movies/Eraser2.gif

20

Yes, I don’t disagree about the forces involved and the dificulty in creating a good design, I was disagreeing with your choice of projectile.

21

I prefer giant nuclear rectors which heat up a stream of water from a million-gallon tank and spit the steram out the botttom.
Easy way of reducing size of engine and fuel. I’d have to do a study on whether the radiation (and mining, purifying, etc.) would be worse for the environment than the mining, purifying, and burning of the same energy’s worth of chemical fuel.

If you could find a way to make alarge quantities of antimatter, you could do the same idea, but this time the fuel and engine system are really small.

You don’t have to worry about the radiation affecting the passengers - they are all behind a million-gallon propellant tank from the “hot” core.

If you took off from a relateivly lifeless place, you’d be fine. How’s the biodensity of a deep, wide part of the Pacific Ocean?