Magnetic Ring Could Launch Satellites, Weapons

MattSparkes writes, "A new study funded by the US Air Force has suggested a cheaper method of sending satellites (possibly missile weapons) into orbit. A 2-km-wide ring of superconducting magnets would contain and propel a payload, accelerating it over a period of hours, before suddenly flinging the satellite into space at 23 times the speed of sound. The satellites would be engineered to withstand the g-forces encountered (2,000 g), and be cased in an aerodynamic shell. A two-year study has been commisioned and will begin within a few weeks at LaunchPoint Technologies in Goleta, California." New Scientist points out that if such a launch ring were built, it would instantly become "one of the most important targets on the planet."

Comments

Here:

[jbeaupre]

http://en.wikipedia.org/wiki/M712_Copperhead

Now you're aware...

Not a rail gun.

[MoralHazard]

Your lapse is forgivable, but only because the proliferation of terms like "Gauss gun", "rail gun", and "mass driver" in SF has overwhelmed their usage as technical terminology. But the point is, THIS IS NOT A RAIL GUN.

A rail gun is a parallel, non-touching pair of conductive rails, joined at the back-end by a partial circuit capable of generating an extremely high current flow (amps) of electicity in a very, very short time. A conductive projectile is injected into the gap between the rails (so that it touches both rails at once), which completes the circuit. As current flows from one rail to the other, through the projectile, it generates a powerful magnetic field. The Lorentz force causes the projectile to be pushed toward the far end of the rails--the magnitude of the force depends on the current flow.

Rail guns can achieve extremely high velocities, far higher than conventional explosive-charge guns. The velocity of a firearm projectile is limited by the velocity of the expanding explosive gasses that propel it out of the barrel; the gas velocity is in turn limited by the speed of sound in the gas medium, which has a physical upper limit for any type of explosive. Rail guns don't suffer from this limitation.

I have seen references to a 'Gauss gun' which consists of a series of solenoids stationed along a tube barrel, timed to trigger so that a ferrous metal projectile will be pulled faster and faster down the barrel by each of the solenoids in turn. I don't know how valid this terminology is, though.

Re:Lost in space

[megaditto]

You are confusing pressure with acceleration. These are not the same.

a_c = - \omega^2 r

[Kadin2048]

Except that the proposed design accelerates the payload around in a circle -- using magnets arranged inside a torus -- not a long straight runway. I doubt a linear runway would be practical; it would just be too long. The advantage of a torus is you can keep using the same magnets to accelerate the payload, over and over, until you've reached sufficient speed to let it fly.

Unless the circle was ridiculously large (probably the size of a continent or better), you're not going to be able to get up to escape velocity before you'd (as a human being) would be crushed by the effects of the centripetal acceleration.

I'm not going to do the math right now, but I'm pretty confident that of the 6,000 Gs they're quoting, most of them are in the radial direction and not in the tangential, so that even if you brought the payload up to speed slowly, you'd still be crushed. It would be just like being in a centrifuge.

Re:a_c = - \omega^2 r

[radtea]

I'm not going to do the math right now,

The speed of sound at sea level is 330 m/s, and a = v*2/r, so at 23*330 = 7590 m/s you would need r ~ 600 km to get a under 10 g.

Of course, there's going to be a bit of bump when the capsule hits the atmosphere, and there's also the bit of a trick about getting the thing oriented so the capsule if flung upward...

As a satelite launcher this sounds like a great technology, although I'm not sure who would be "targeting" it or for what purpose...advertisers, maybe? Painting thier logos on it or something? Or some guy hiding in a cave someplace that we're supposed to be all afear'd of?

Re:a_c = - \omega^2 r

[radtea]

The advantages of being high up are... ...negligable. Realistically, you can only get a few kilometers up, unless you're proposing to build it in the Himalayas. It is well known from other mass-driver studies that the aerodynamic advantage of hitting 80 bar at Mach 23 are no big improvement over hitting 100 bar at Mach 23.

The reason why I mentioned pointing it up is that there is a big advantage to passing through the atmosphere as quickly as possible. Firing a capsule out normal to the local vertical will result in minutes being spent in getting to the top of the atmosphere, by which time you will have lost most of the initial velocity, to say nothing of broken all the windows for kilometers around. If you do the math, it takes about 13 seconds to travel 100 km at Mach 23 (just under 8 km/s). So a 30 degree incline nearly doubles that (you get some benefit from the curvature of the Earth) and things get rapidly worse from there on.

As the whole point of my calculation was to show how big the thing would have to be to keep the acceleration below 10 g there is no way a 30 degree incline is going to happen--you've have to have a curve so long that the top of it really would be above a significant fraction of the atmosphere.

Bad math?

[Bender0x7D1]

Am I crazy, or did they get the math wrong in the article?

The acceleration equation for circular motion is: a = v^2 / r

We are given:

Velocity: 10 kilometers/s

Width of ring = 2 kilometers, so radius = 1 kilometer

So:
v = 10,000 m/s
r = 1,000 m

a = (10,000 m/s * 10,000 m/s) / (1,000 meters) = 100,000 m/s^2

The acceleration due to gravity is about 10 m/s^2

This gives: (100,000 m/s^2) / (10 m/s^2) = 10,000 g

So it seems that their 2,000 g is way off. Even if we use 2 km for the radius it is still 5,000 g.

Re:Bad math?

[doctor_nation]

Your math is correct. I have an abstract from a presentation these guys gave last week and it lists the radial force at 20 MN (that's mega-Newtons) for a 200 kg projectile = 10,000 G. They don't list the acceleration in G anywhere so it's probably a New Scientist math error.

Re:Sounds Good, except

[NoData]

I know there's a relationship between bird migration and magnetic fields, too, as a lot of them blindly smack into the brick walls at a local MRI center.

Cute, but you gotta be kidding. I work with a 3T research MRI magnetic. Both the machine and the facility are heavily shielded, and the field drop-off is very steep. While the isocenter of the bore is at 3 Tesla (30,000 Gauss), the 5 Gauss line is only a few meters (about 5 in the axial direction, 3 in the radial direction) from the isocenter. By comparison, a kitchen magnet is maybe 100-250 Gauss.

Re:"Moon is a Harsh Mistress" anybody??

[dgatwood]

Peak for shuttle launch is 3Gs, and for Apollo reentry, exceeded 7Gs (source paper with cited sources). For a launch abort on the Apollo design, stress would have exceeded 16Gs, and this was deemed uncomfortable, but survivable (albeit with an assumed inability to operate controls during the process). (source LBJ Space Center.)

So limiting it to 2Gs of total stress is very arbitrary and unnecessary.

Fast dead mass is still REALLY useful if its cheap

[Big_Breaker]

This ring could fling mass up to a skyhook to recharge its orbit. Imagine a LEO skyhook that catches dozens of dead weight shots from this gun and uses that momentum to promote its orbit to a highly eccentric one. Then the satellite can exchange this orbit potential with a target at its low altitude point through a tether or skyhook style method. The target could be a large satellite in LEO or even a suborbital payload. Once the potential is transfered the target can have its orbit promoted to GEO or other significant altitude.

This method saves a lot of reaction mass in a heavy lifter because you can aim for a high alitutde but a suborbital trajectory. IE it's easier to shoot straight up than curve towards an orbital path at sufficient speed. For instance the X prize is all about sub-orbital. LEO is much harder and GEO is even harder still.

Re:"Moon is a Harsh Mistress" anybody??

[Rei]

I was reminded of Gerald Bull, one of the great "mad scientists" of our day, and Project HARP. :) Check out the plume leaving the barrel of their research gun. That had to be quite something to see in person.

Of modern ballistic launch mechanisms, there are lots of neat options ranging from light gas guns to ram accelerators. I also find the concept of ballistically-launched scramjets to be pretty nifty. :)