Can magnets be used to launch spacecraft?

Question

I assume there is some reason we don't use magnets to launch off Earth. Are they not strong enough? Isn't magnetic force technically stronger than gravity??

Would either of these work:

1. A tube tunnel deep in Earth, lined with electromagnets, leading up into a tower of more, to launch upward or across.

2. An electromagnetic ring 'track' around the Earth, using multiple 'orbits' to accelerate, then launch, craft. Like a combination of Tesla's idea & the Hadron collider, perhaps even using 'particle-sized' ships.

Also, is there any way to use Earth's magnetic field or gravity to help? For example, could the Earth's poles help at all, if launching from Antarctica?

Answer 1

Earth's magnetic field is way too weak to repel against with the force required to launch anything into orbit. Actually, it's really easy to demonstrate that. Take one fridge magnet, place it on the kitchen scale, and write down its weight. Then turn the magnet around and weigh it also with its polarity reversed. You shouldn't see any difference and the scale will show identical weight in both cases, something you'd expect to be much different if Earth's magnetic field was so strong that it would substantially negate gravitational acceleration in one case, and increase attractive force when the magnet's polarity would be flipped the other way around.

Regarding tube tunnel with electromagnets, Earth's escape velocity on its surface is 11.2 km/s (that's 40,320 km/h or 25,053.7 mph!). Some of this velocity is already provided by the Earth's own rotation, depending on how far from equator the launch site would be (1670 km/hr at equator, 0 at true poles), but the majority would still have to be provided by the launch system. That's a lot to ask of a maglev / railgun system and would require either tremendous power on a relatively short track (a few kilometers perhaps) for accelerations that no human could survive, or roughly 1,472 km (914 mi) long track to keep acceleration down to 4 g, if the launch system would be at equator.

To achieve Low Earth Orbit (LEO), required target velocity should reach approximately 8 km/s (28,800 km/h or 17,895 mph) once in LEO, but since there would of course still be all the atmosphere in the way once it would exit the launching system, the exit velocity would have to be much larger than that to negate atmospheric drag. The initial supersonic shock would also require incredibly tough materials to withstand the shock forces and aerodynamic heating, and the projectile would likely still require some means of propulsion of its own, if for nothing else, then for orbital maneuvers, trajectory corrections, and deorbit burn. Oh, and in case of a human rated system, it would have to have launch escape and safe descent systems too, all of it adding to its size and weight.

So as you see, there are some major drawbacks to such horizontal launching systems using electromagnetic propulsion, and you'd still have to carry most of the systems that are used with rocket launches onboard, even if you exited the launch ramp at high velocities. A bit more about the hurdles to using railguns as space launch systems is also discussed in two similar questions:

• What technological hurdles prevent the development of a space gun?
• Is it possible to literally shoot something into space?

Earth's magnetic field can however be useful as an in-orbit attitude control, mainly to stabilize satellite's alignment along the long axis and/or keep a particular part pointing towards nadir, using what we commonly refer to as magnetorquers. They work in a similar way to how a magnetized needle self-orients towards magnetic poles, only using powered electromagnetic coils. But, as you probably expect by now, they wouldn't really be reliable over the Earth's magnetic poles due to oscillation in the magnetic field, so they would leave their coils unpowered over those areas if the satellite is in a polar orbit. For more information on how they work, I suggest searching our site using the magnetorquer tag.

Answer 2

Given the question, as written (rather than the misleading title) - yes, a magnetic accelerator could be used to launch a spacecraft.

The issues with so doing are several.

1. the contents of the craft need to survive the magnetic fields needed for reasonable length accelerators
2. the track
   1. the linear track is extremely long
   2. the circular track causes fairly high G-forces upon the occupants
   3. the high accelerations needed to reduce track lengths to reasonable levels.
3. the deceleration of atmospheric friction
4. the lack of abort options
   1. past the halfway point, insufficient distance to bring to a stop
   2. ability to rotate payload in the tube isn't a good use of mass except for failure modes, where the inability to do so moves the safe abort point below halfway.

NASA has looked into the use of magnetic accelerators for launching - and rejected as too unreliable for what NASA is launching. Not that it won't consistently put things into orbit, but that those things may not survive the trip in working order. The IEEE has as well; feasible lengths result in too high an acceleration "for humans and fragile cargos." (McNab, 2003)

So, this leaves a number of items where it would be feasible. Food, water, fuel, life support supplies (filters, scrubbers, gasses), sample return containers, structural components, hardened electronics.

The overall costs are not yet truly finalizable, but the 2003 cost estimate, without overruns, is in excess of $1.3 billion for a railgun, and the rails have a limited lifespan. Cost per kilogram should be comparable to some current high-efficiency launcher methods proposals.

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References

• McNab, Ian (2003), Launch to Space With an Electromagnetic Railgun, http://research.lifeboat.com/ieee.em.pdf

Answer 3

To answer this portion of the question:

Isn't magnetic force technically stronger than gravity??

The magnetic force is stronger than gravity, but only at close range.
Magnetic field strength is proportional to $1/r^3$, where r is the distance to the magnet.
Earth's gravity is proportional to $1/r^2$, where r is the distance to the center of the earth.
As a result, when you get further away from the magnet, gravity quickly becomes the dominant force. You can see this in action in Maglev trains: they use powerful magnets to hover a few mm above their track. If you want the train to hover further away, the energy required quickly becomes impractical.
Similarly, if you use the magnets for propulsion, the propulsive force drops quickly when you move away from the magnet, and you're left with an unpowered projectile with the drawbacks explained by @aramis and @tildalwave.

Answer 4

The existing answers pretty much show why it's not done but I'll add a couple more points also:

1) Remember that a rocket going up these days pulls several Gs. Note how far it goes before the engines shut down--hundreds of miles. If you want the same G load you need as much distance--for humans to survive the trip your launcher needs to be hundreds of miles long. Obviously it must shoot on a very flat trajectory.

2) A rocket obtains most of it's velocity while pretty much outside the atmosphere. A linear motor launcher obtains it's velocity on the ground and thus has a major problem with drag--even more so because it's going nearly horizontally. You'll have to have an awfully big vehicle to avoid it losing ALL it's speed in the atmosphere. (You can't overcome this by adding to the launch velocity as the drag goes up as fast as the velocity does.)

Answer 5

I like your idea, but I was thinking more along the lines of using an electromagnet amplified by a centralized magnet amplifier. Explanation: A small electromagnet inside a slightly larger one, and that slightly larger one inside of an even larger one, and so on until you get the proper strength. Each magnet amplifies the magnetic field of the smaller one inside of it. You could build a silo with this theory applied, insert your "payload" into the "barrel", so to speak, turn the magnet(s) on and the payload will launch. If your trying to launch to, say, the moon, you should build your payload to be durable, because it will make a rather firm impact, obviously.