Linear electromagnetic accelerators

Tom Kalbfus · 2015-10-05 17:08:38

Here is a radical concept, what if there was an electromagnetic accelerator in orbit, and you could just accelerate spaceships, to the required delta-v to arrive at any point in the Solar System?

Lets say you are in orbit around the Earth at 8.5 km/sec with a linear electromagnetic accelerator and you wanted to accelerate to 18.5 km/sec? It would take 1019.3680 seconds to reach this velocity from orbit or 17 minutes of accelerating at 1g to reach this velocity, traveling at an average velocity of 5 km/sec while accelerating for 17 minutes would require the accelerator to be 5100 km long. Would this be a great way to get to Mars or other points in the Solar System? If the orbit was 300 km above the surface of the Earth, you could have a linear accelerator that was 42,097.34 km long which completely encircles the Earth, align this ring with the ecliptic and you could pretty much travel to any point in the known Solar System.

RobertDyck · 2015-10-05 19:56:07

That's big. You would need to mine a metal asteroid to get enough metal to build that. I have argued that asteroid mining is practical, but you have to mine in place. Miners don't move a mountain in one piece to the outskirts of a city, then mine. They mine in place where the ore is. Mine a metal asteroid for gold, silver, platinum, and platinum group metals. Make Inconel aeroshells out of the leftover nickel and chrome, and just drop aeroshells filled with precious metal bullion on a desert on Earth. Industrial metals would be left at the asteroid as a byproduct, to be used for some future construction project in space. Transporting industrial metals to Earth is not profitable. Well, looks like you found an application for that leftover industrial metal.

I have a more near-term idea. Nuclear propulsion. Years ago we talked about open cycle gas core nuclear thermal rockets. The issue is how do you test them? Solid core rockets don't release radiation in their exhaust. At least as long as everything works right. NERVA was tested at Jackass Flats, where the military detonated thousands of nuclear bombs. It was run in open air. Today nuclear activists would have a cow at that. And although solid core rockets do not release fission fragments in their exhaust stream, open cycle GCNTR do. So how do you test it? You need to contain the exhaust of the rocket. How do you do that? Well, looks like NASA solved that problem. This is the A3 test stand at Stennis. Built to test the J-2X rocket at air pressure of the upper atmosphere (laboratory vacuum). This is how you test GCNTR. (The image is clickable for insanely high resolution.)

Now imagine what you could do with an engine that delivers Isp=9000 seconds, and equivalent thrust of SSME/RS-25.

NERVA of the early 1970s produced Isp=825 seconds. SSME provided Isp=453 seconds in vacuum. Saturn V first stage engine was F-1, it provided Isp=263 seconds at sea level. NSTAR ion engine was used on Deep Space One, it provided 3100 seconds in vacuum, but only has much thrust as the weight of a single sheet of 8.5"x11" bond paper sitting on your hand. Glenn Research Centre developed NSTAR, but they later developed MagnetoPlasmaDynamic (MPD) with Isp=8400 seconds. But that requires a lot of electric power. One reason they did that is the Russians developed Thruster Anode Layer Hall Thrusters equal to the best ion engines that NASA had, and designed a high power version that could theoretically produce Isp=8400 seconds. So Glenn built one that has the same Isp, but MPD uses liquid hydrogen instead of xenon, a much more plentiful propellant. VASIMR is advertised to provide Isp=9000 seconds, but it requires so much electric power that the weight of a nuclear reactor to provide that would defeat the high Isp. And the best VASIMR demonstrated in the lab so far has Isp=5600 seconds. Still pretty good, but not better than Glenn's best.

But now imagine Isp=9000 seconds from an engine that provides as much thrust as SSME/RS-25, and self-contained so it doesn't need a heavy power supply.

SpaceNut · 2015-10-05 19:59:23

Equal and opposite reactions....since it must push against something in order for the rocket to accelerate. This is something simular to how a rail gun works.....

RobertDyck · 2015-10-05 20:14:36

Tom's electromagnetic accelerator would be pushed back with as much momentum as the ship is pushed forward. But a big heavy accelerator would not gain much velocity. Still, to keep it stable, you would have to expend propellant to push it back up. But that propellant would be in LEO, not with your ship. Is it worth it?

An alternate idea that was proposed many years ago is an electromagnetic accelerator on the Moon. That uses the Moon itself as mass to push against. You won't change the Moon's orbit much, and the Moon is slowly but constantly spiralling up out of Earth's orbit, so pushing it back would be a good thing. And you could mine the Moon itself for metals to build the accelerator. Would the Moon have the right metals?

Last edited by RobertDyck (2015-10-06 10:39:14)

Tom Kalbfus · 2015-10-06 04:28:23

RobertDyck wrote:

Tom's electromagnetic accelerator accelerator would be pushed back with as much momentum as the ship is pushed forward. But a big heavy accelerator would not gain much velocity. Still, to keep it stable, you would have to expend propellant to push it back up. But that propellant would be in LEO, not with your ship. Is it worth it?
An alternate idea that was proposed many years ago is an electromagnetic accelerator on the Moon. That uses the Moon itself as mass to push against. You won't change the Moon's orbit much, and the Moon is slowly but constantly spiralling up out of Earth's orbit, so pushing it back would be a good thing. And you could mine the Moon itself for metals to build the accelerator. Would the Moon have the right metals?

One advantage of the electromagnetic accelerator is that it could use something like an ion drive to stay in orbit, and it could have big solar panels on either side to power that ion drive, but it can accelerate its payload rather quickly from orbit, thus it would be transferring the momentum from the ion drive through itself to the payload it is accelerating at 1g for 17 minutes. Usually an ion drive can't accelerate anything at 1g, but in this case you can. Another thing if its in low Earth orbit, you could launch something up to it, and use the accelerator to accelerate up to Orbital velocity and beyond. Say you launching something up from the ground to an altitude of 300 km, the accelerator slows something down to dock with the payload as it attains its 300 km altitude and then it could accelerate it from that point to 18.5 km/sec from stand still relative to the ground. Then you would use its ion drives to recover the lost momentum.

The final kicker is that it would be easier to build than a space elevator!

Last edited by Tom Kalbfus (2015-10-06 04:33:48)

Terraformer · 2015-10-06 10:34:29

Eh, I suspect it will make more sense to accelerate easily ionisable pellets, vaporise them into plasma when they near the ship, and use a magsail to catch the plasma. Especially since you could accelerate them at a much higher acceleration.

Tom Kalbfus · 2015-10-07 00:16:50

Terraformer wrote:

Eh, I suspect it will make more sense to accelerate easily ionisable pellets, vaporise them into plasma when they near the ship, and use a magsail to catch the plasma. Especially since you could accelerate them at a much higher acceleration.

Problem is its hard to track the pellets, and if you fail to ionize them, they may hit your ship like a bullet!

Terraformer · 2015-10-07 17:25:56

First, include a transmitter then, it's not all that difficult or expensive. Or, given that you know their speed and direction, use radar. Tracking should not be an issue.

Secondly, the pellets are low mass, and would be moving fairly slowly relative to the ship as far as things in space go. There's no need to have them all going the same velocity, and in fact you wouldn't want them to do so. If they hit your whipple shield, they'd impact with the same energy as 0.1 kg of TNT, assuming the projectile is 0.1 kg in mass and moving at 3 km/s wrt the ship. Though you probably wouldn't target them directly at the ship anyway, instead just a bit to the side - it shouldn't matter much, as long as the magnetic field catches the plasma.

Tom Kalbfus · 2015-10-07 19:18:22

The idea has merit, maybe a little harder than a straightforward linear accelerator though, past a certain final velocity, the pellet gun makes more sense than a linear accelerator, such as in a starship for instance, for getting anywhere in the Solar System however, a linear accelerator would work fine for getting to most places, and there is still the unsolved problem of stopping once you get there. The thing about pellet guns is that you can accelerate the pellets much harder than you can accelerate people, thus the accelerator to launch the pellets can be shorter than anything you'd use to launch crewed spaceships with. So you can accelerate pellets at 1000s of gs and achieve the same acceleration of pellets in the space of only a few kilometers instead of thousands of kilometers you'd need to launch people with. You do need a lot of pellets though to accelerate something for 17 minutes at 1g.

Terraformer · 2015-10-08 04:51:20

Plus, because of the harder acceleration, you don't necessarily need it to be linear. A coilgun could work as well.

A large 'linear' accelerator built around an airless body might have advantages at directly accelerating craft, but it also has an incredibly high capital cost.

Antius · 2015-10-08 13:28:03

Why not instead use a much lighter mass driver as an engine? You could ferry a spacecraft to Earth escape velocity and then cycle the engine back to low earth orbit. This was pretty much Gerard O'Neill's favourite propulsion concept. It has the advantage of being able to use literally anything as propellant. You could even grind up spent upper stages and use them as reaction mass. It would work especially well if those upper stages were made from carbon steel, as you could accelerate the iron reaction mass ion coil guns as per Tom's suggestion.

Tom Kalbfus · 2015-10-08 19:10:02

I think the linear electromagnetic accelerators would use up less reaction mass. If you expel gas out of a rocket nozzle, you generally can't recover much of the gas. Gas rockets use lots of reaction mass. If you use a mass driver, and throw rocks in one direction, it generally isn't worth recovering all those rocks so you can use them again. A linear accelerator accelerates the payload along its length and the payload upon release coasts to its destination, the ion rockets the accelerator uses makes efficient use of reaction mass, the ions expelled have a higher velocity so less of them is used, the acceleration provided by the accelerator is in the range of chemical rockets launched from the ground so it quickly attains cruising velocity and then releases the payload on its trajectory.

Void · 2015-10-09 12:35:03

Gerard O'Neill also had pellet catchers in "L" locations if I recall.

If asteroid mining will start by bringing back water, I would suppose for large mining efforts eventually they might try to dump the Oxygen, and just bring back the Hydrogen.

And then there is the Moon, which is a big Oxygen tank among other things.

Just maybe you could all have what you said, if there were significant economic activity on the Moon. I don't see Mass drivers as being worth while to launch humans off of the Moon. Also, if metals are available from the asteroids for orbital construction, I don't think launching metals off of the Moon is a good plan, with or without Mass Drivers (Linear Accelerators).

But Gerard O'Neill also had pellet catchers in "L" locations if I recall.
and Terraformer said Coil Gun.
and I like Oxygen pellets, I presume that they are paramagnetic.https://www.google.com/search?q=Solid+Oxygen+paramagnetism&biw=1366&bih=673&tbm=isch&tbo=u&source=univ&sa=X&ved=0CCkQsARqFQoTCOSlmp6FtsgCFQTMgAodEM0Bfw
and Tom likes to launch spacecraft with a massive linear accelerator.
and antius likes to shoot objects out of a spaceship to propel it.

So do all of it maybe?

Add electromagnetism to Gerard O'Neill's pellet catcher.
Also turn it into a Magnetic Plasma Bubble device.

The interior of the catcher must generally be kept below the freezing point of Oxygen? Or can you hold some Oxygen in place with the plasma bubble? Even temporarily might be enough if you eventually condense the gas inside the catcher.

So Terraformer is pushing pellets at the pellet catcher, but they might evaporate in the sunlight? You might still catch the gas if it sticks together with magnetism. I don't know if it would. Anyway, perhaps the pellets need a minimal coating, to reflect sunlight, and to allow the interiors of the pellets to self pressurize. That would be another method. Another method would be to only shoot pellets when the Moon or Earth were blocking the sunlight. Anyway...

If I recall the Gerard O'Neill concept of the collector, the pellets would slow down while climbing up the Moons gravity well, and would not impact at a very high speed.

They should shatter, and perhaps even partially vaporize and that vapor expand and rapidly cool, its heat radiated away. The cup or cone shaped container, which would also be magnetic with one open end, might be able to retain much of the Oxygen.

Then if you have a spaceship to launch, you load it up with Oxygen as mass to propel it, (Per Antius), and then you might even give the ship a boost on it's way out, using Toms method, (Using the electro magnetism of the collector in a different way for the launch.

And the Oxygen liberated from lunar soil, leaving behind building materials on the Moon to build things on the Moons surface.

A variant of this plan would have a collector in low lunar orbit, Coil guns shooting Oxygen into the collector "Cup" from many locations on the Moon. The collector cup maintaining orbit by the impacts, and also it can have it's own propulsion, where perhaps it also shoots out Oxygen.

Low Lunar Orbit:
http://science.nasa.gov/science-news/sc … _loworbit/
This would help in the case where you don't want your Oxygen pellets to vaporize, or the vapor to spread too far before impact, but your impacts from sub-orbital pellets might be rather high.

Last edited by Void (2015-10-09 13:03:05)

Tom Kalbfus · 2015-10-11 09:00:56

Just maybe you could all have what you said, if there were significant economic activity on the Moon. I don't see Mass drivers as being worth while to launch humans off of the Moon. Also, if metals are available from the asteroids for orbital construction, I don't think launching metals off of the Moon is a good plan, with or without Mass Drivers (Linear Accelerators).

Do you see this image of the Moon? There are a lot of craters here. What do you think happened to all the asteroids that dug them? If they weren't volatiles, all the metallic asteroids are all still there. What do you think is more valuable to Earth, metals or volatiles? If you want to commercially mine the asteroids, a good place to start would be the Moon's surface both near side and far side, the Moon is easy to get to, it remains more or less at a constant distance from the Earth, and it is always possible to send missions to the Moon and payloads from the moon on whatever schedule we like. With Mars or any other object orbiting the Sun, we have to wait for certain optimal alignments, but with the Moon the window is always open. We also have near real time communication with the Moon, and all those asteroids are still their all in one spot, the Moon! It is much easier to find them there than to search out each independently orbiting asteroid around the Sun.

Void · 2015-10-11 14:12:45

I spoke in an economic sense Tom, on my hunch of where metals could best be obtained. Platinum family metals will be in those locations under a great deal of overburden on the Moon, but finding and extracting them may be pricy. I don't know. I am not the pope of what you may or may not launch off of the Moon with a mass driver. I just don't think that metals will be most economically taken off of the Moon, especially if you want to build thing out of them on the Moon anyway. But fine you can shoot metals off of the Moon.

Hey! How 'bout that Oxygen mass driver! I am wondering why I did not get a beating over that? Lots of if's there.

1) Anyway if you did get your Oxygen off of the Moon, then as I previously stated: Oxygen as propulsive mass could help to access objects beyond the Earth/Moon, possibly for mining.

2) Very likely water may be extracted from small (Tiny) asteroids before #1. That water is to be the first product that is obtained off Earth to command a commercial price.

3) Later asteroid miners hope to extract metals. Platinum family to return to Earth, and other metals to build things in space.

4) I would say that as space mining becomes more sophisticated, and infrastructure for it expands enough, instead of bringing back water, they would bring back liquid Hydrogen. They could use the Oxygen from the water as propulsion mass, by some means, for instance shooting it out of a mass driver to move the Hydrogen and Metals to Earth orbit.

So, you would have Hydrogen and Metals coming to Earth/Moon orbit from asteroids, and Oxygen coming to Earth/Moon orbit from the Moon. And I will make a concession here. Yes, if the metals are cheaper to mine and remove from the Moon, then metals could be moved from the Moon to the Earth/Moon orbits.

Linear electromagnetic accelerators are cool. (Very on topic)

Tom Kalbfus · 2015-10-11 16:05:27

Void wrote:

I spoke in an economic sense Tom, on my hunch of where metals could best be obtained. Platinum family metals will be in those locations under a great deal of overburden on the Moon, but finding and extracting them may be pricy. I don't know. I am not the pope of what you may or may not launch off of the Moon with a mass driver. I just don't think that metals will be most economically taken off of the Moon, especially if you want to build thing out of them on the Moon anyway. But fine you can shoot metals off of the Moon.

Those asteroids buried under the Moon's surface are ancient, probably most of them are billions of years old, most of those craters were formed soon after the Moon's surface solidified, from a time where asteroids were more common in or part of the Solar system. There is not much geology on the Moon's surface, basically an asteroid hits the Moon, makes a crater and some of the material blasted aside by the impact buries the remnant of the asteroid. Part of the asteroid is part of the ejecta, as it was probably shattered too. You can not only shoot metals off of the Moon, you can shoot them all the way to Earth, you can target specific locations on the Earth's surface for pick up.platnum metal groups, especially platinum itself with its high melting point would survive entry through the atmosphere quite nicely, if it hits a body of water, it won't make a crater if delivered in small chunks. The platinum has a number of industrial uses, including in fuel cells for example.

Hey! How 'bout that Oxygen mass driver! I am wondering why I did not get a beating over that? Lots of if's there.

We could deliver hydrogen to the Moon, every kilogram of hydrogen makes 5 liters of water with the oxygen that is already there

1) Anyway if you did get your Oxygen off of the Moon, then as I previously stated: Oxygen as propulsive mass could help to access objects beyond the Earth/Moon, possibly for mining.
2) Very likely water may be extracted from small (Tiny) asteroids before #1. That water is to be the first product that is obtained off Earth to command a commercial price.
3) Later asteroid miners hope to extract metals. Platinum family to return to Earth, and other metals to build things in space.
4) I would say that as space mining becomes more sophisticated, and infrastructure for it expands enough, instead of bringing back water, they would bring back liquid Hydrogen. They could use the Oxygen from the water as propulsion mass, by some means, for instance shooting it out of a mass driver to move the Hydrogen and Metals to Earth orbit.
So, you would have Hydrogen and Metals coming to Earth/Moon orbit from asteroids, and Oxygen coming to Earth/Moon orbit from the Moon. And I will make a concession here. Yes, if the metals are cheaper to mine and remove from the Moon, then metals could be moved from the Moon to the Earth/Moon orbits.
Linear electromagnetic accelerators are cool. (Very on topic)

Void · 2015-10-11 20:16:11

OK Tom, your right, I will loosen up on it. There are too many variables to say just what pattern will turn out to be the best fit.

However, I was trying to emphasize that from the Moon perhaps could come a very large quantity of Oxygen, and that perhaps, that would require a minimum human population to maintain the production and launch system. In other words the Moon might be the best local area to get mass quantities of Oxygen. The Linear electromagnetic accelerator and the catcher, whatever their structures might be, will have to actually work though in a practical way. Lots of needs there.

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#1 2015-10-05 17:08:38

Linear electromagnetic accelerators

#2 2015-10-05 19:56:07

Re: Linear electromagnetic accelerators

#3 2015-10-05 19:59:23

Re: Linear electromagnetic accelerators

#4 2015-10-05 20:14:36

Re: Linear electromagnetic accelerators

#5 2015-10-06 04:28:23

Re: Linear electromagnetic accelerators

#6 2015-10-06 10:34:29

Re: Linear electromagnetic accelerators

#7 2015-10-07 00:16:50

Re: Linear electromagnetic accelerators

#8 2015-10-07 17:25:56

Re: Linear electromagnetic accelerators

#9 2015-10-07 19:18:22

Re: Linear electromagnetic accelerators

#10 2015-10-08 04:51:20

Re: Linear electromagnetic accelerators

#11 2015-10-08 13:28:03

Re: Linear electromagnetic accelerators

#12 2015-10-08 19:10:02

Re: Linear electromagnetic accelerators

#13 2015-10-09 12:35:03

Re: Linear electromagnetic accelerators

#14 2015-10-11 09:00:56

Re: Linear electromagnetic accelerators

#15 2015-10-11 14:12:45

Re: Linear electromagnetic accelerators

#16 2015-10-11 16:05:27

Re: Linear electromagnetic accelerators

#17 2015-10-11 20:16:11

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