# Magnetic sail using radioactive decay

by udtsith
Tags: decay, magnetic, radioactive, sail
 P: 19 What are the thoughts about using a magnetic sail propelled by a powerful alpha emitter (e.g. polonium)? Imagine a 1kg sphere of polonium radiating alpha particles in all directions. Attached to the sphere, via boon, is a powerful magnetic field emitter that would repulse or even deflect forward thrusted alpha particles. Would the net momentum propel the sail in the direction of the magnetic field? Also, perhaps this could be made more effecient by having magnetic fields emitters attached around the sphere and deflecting the alpha particles at an angle away from the forward momentum. Even a rear magnetic field which is attracted to alpha particles could assist in effeciency. The benefit of this is that the density of alpha particles will only fall off over time versus distance (e.g. the sun). Also, the high velocity alpha particles wouldn't damage the magnetic field (but yes, perhaps they would damage the magnetic field emitter).
 P: 642 Hmm. The possible problem I see here is designing a magnetic field to deflect all (or almost all) particles in a certain direction. Is such possible?
PF Gold
P: 2,599
In magnetic fields charged particles tend to move in circles. This is because the force is always at right angles to the direction the particle is moving at each instant as it curves.

so
 Even a rear magnetic field which is attracted to alpha particles could assist in effeciency
isnt going to work

Dave

HW Helper
P: 2,280
Magnetic sail using radioactive decay

 Quote by Whovian Hmm. The possible problem I see here is designing a magnetic field to deflect all (or almost all) particles in a certain direction. Is such possible?
Yes. At least in principle.

However, the problem is how much can high speed particles be deflected? Plus there's the problem of how big and heavy the magnets are or where the electrical power is going to come from to power a really powerful electromagnet (perhaps by converting the heat from the nuclear reaction?) Or is that the part you're wondering is possible - making the magnetic field powerful enough to do what he wants?

You could definitely get some net propulsion from the system. The momentum of the rearward moving particles definitely increase the forward momentum of the spacecraft, however the forward moving particles decrease the forward momentum by the same amount. Catching or deflecting the forward moving particles recovers at least a portion of that momentum, leaving you with a net increase in forward momentum. The efficiency (how much of the momentum of the forward moving particles is recovered) is a little questionable, but the principle is similar to using a parabolic dish antenna to increase the effective power of a transmitter.

But if the efficiency gets too low, you'd be better off sending ionized inert particles through an electromagnetic accelerator. At least all of the accelerated particles would be moving the direction you wanted.
 P: 76 Isn't this like putting a fan facing a sail in a boat? I must be confusing this arrangement with a solar sail with the "solar" part attached to the craft.
HW Helper
P: 2,280
 Quote by surajt88 Isn't this like putting a fan facing a sail in a boat? I must be confusing this arrangement with a solar sail with the "solar" part attached to the craft.
Yes. And No.

The fan is emitting (or blowing) particles in a forward direction pushing the boat backwards. The sail is catching the particles, pushing the boat forward, with the net being zero.

This is emitting particles in a spherical pattern, which should get you no net change in momentum. However, if you "catch" the forward moving particles in your "sail", the momentum of the forward moving particles is cancelled out, just as in the fan blowing the sail. That means nothing cancels the momentum of the rearward moving particles.
 P: 1 In your example, less is more. If your alpha emitter is placed into an absorbent cylinder closed at one end, the only particles that escape will pass through the open end, thus imparting an impulse on the cylinder in the direction of its closed end. This arrangement would be an atomic rocket, and develop a thrust based on Newton's 3rd law. In a similar fashion, although the sailboat with a fan blowing against its sail won't generate a net force ("net force" = the algebraic sum of forces in the sail and fan system), taking down the sail will allow the fan to produce a thrust resulting from the air it is accelerating in the opposite direction . This precise method is commonly used in swamp running fan boats.
Mentor
P: 15,153
 Quote by udtsith What are the thoughts about using a magnetic sail propelled by a powerful alpha emitter (e.g. polonium)? Imagine a 1kg sphere of polonium radiating alpha particles in all directions. Attached to the sphere, via boon, is a powerful magnetic field emitter that would repulse or even deflect forward thrusted alpha particles. Would the net momentum propel the sail in the direction of the magnetic field?
It won't work, for a huge number of reasons.

First, you need to review your concepts of how magnetic fields works.

Next problem: Where are you going to get 1 kilogram of polonium (presumably 210Po)? The total worldwide production is 100 grams per year.

Next problem: You're not going to be able to launch a 1 kg sphere of polonium into space, at least not without a lot of protections. A whole, whole lot of protections. That's one billion lethal doses you are talking about!

Next problem: Almost all of the alpha particles emitted by your 1kg sphere of polonium will be captured by your 1 kg sphere of polonium. You can't have a 1 kg sphere of 210Po because of this. The heat generated by the decay would rather quickly cause the sphere to melt and then boil.

Next problem: Even with the most optimistic and idealistic assumptions, there is still essentially no thrust. Suppose you get around the problem of internal absorption and instead make a thin (very, very thin) sheet of polonium so that almost all of the alpha particles go off into space rather than being captured internally. Suppose you somehow collimated those alpha particles so they all go in one direction. The exhaust stream of alpha particles will have a specific impulse of at most 16,000 km/s (that's over 5% of the speed of light). At 140 kilowatts per kilogram of polonium, that comes to 8.75 millinewtons from your 1 kg source.
 P: 19 well...it could be uranium, and good point about the boiling off part...the sheet arangement would be better...you are right about the launching part being tough especially since society is too cautious and cheap to even send humans into low earth orbit. I do think you left off time in the 140 watts calculation so... it wouldn't be just 8.75 millinewtons but 8.75 millinewtons/second, which over a years time would lead to a significant velocity in a near frictionless environment.
 Mentor P: 15,153 Demanding extreme protections and lots and lots of safeguards in transporting a billion lethal doses of some substance on a vehicle that has a 2% to 10% chance of catastrophic failure is not being overly cautious. It's the only sane thing to do. I gave the benefit of the doubt, many benefits of the doubt in fact, in arriving at that 8.75 millinewtons of thrust. That's ridiculously low, even by low thrust standards. In comparison, SMART 1 had a thrust of 68 mN, and that meant SMART 1 had to be very small, only 367 kg at launch. Even then, it took over a year to get from it's initial 7000x42000 km orbit about the Earth to a high lunar orbit. SMART 1 had a rather low thrust, even by low thrust standards. One of the key points I granted in arriving at that figure was that the entire alpha output could somehow be collimated. That's a huge, huge stretch, and it's an even huger stretch if the alpha emitter is stretched out in the form of a thin sheet. Fail to do that and you'll get a fraction of that 8.75 mN. Polonium is a non-starter for a number of reasons. So how about uranium? Which isotope? Even if you do manage to collimate the particles produced by the decay, a long lived isotope is not going to provide any measurable thrust. A short lived one is going to suffer the same consequences as polonium. The idea of using the light particles generated by radioactive decay as the exhaust that provides thrust just doesn't make sense.
Mentor
P: 16,319
 Quote by udtsith I do think you left off time in the 140 watts calculation so... it wouldn't be just 8.75 millinewtons but 8.75 millinewtons/second
No, it's mN, not mN/s. The force does not increase with time.

An alpha is ejected at 10% of the speed of light. If you had 1 kg of Po-210, after it has completely decayed, the alphas have a summed momentum of 600,000 kgm/s. So, by conservation of momentum, that's the largest impulse you can transmit to your spaceship, even if you used magic to get them all in the same direction.

Voyagers weighed 700 kg. So this will, at best, get them to a speed of about 800 m/s - less than 5% of the speed that they actually achieved.

This idea is, I'm afraid, ineffective, impractical and dangerous.
Mentor
P: 15,153
 Quote by Vanadium 50 An alpha is ejected at 10% of the speed of light.
I get a bit more than half that for the 5.3 MeV alphas emitted by 210Po decay. This makes your low delta V even that much worse.

This is also assuming, for the sake of argument, that (a) all of the alpha particles are emitted into space, and (b) one can somehow collimate those emitted alpha particles into a very narrow beam. Fail to do achieve both and the momentum transfer is drastically reduced.

 This idea is, I'm afraid, ineffective, impractical and dangerous.
I agree completely.
 P: 50 It's a sound idea in theory and one that should certainly be further investigated. Well done, a very interesting concept!
 Mentor P: 16,319 No, it is an unsound idea, for the reasons given. PS You might want to PM Greg and request a new username, rather than the name of a notorious crackpot.
P: 50
 Quote by Vanadium 50 No, it is an unsound idea, for the reasons given. PS You might want to PM Greg and request a new username, rather than the name of a notorious crackpot.
Very rude, Velikovsky happens to be a family name.
 P: 50 JPL and NASA had undertook extensive studies on the subject of "fission sails" and have classified them as a viable propulsion method. http://www.niac.usra.edu/files/libra...02/740Howe.pdf
P: 89
 Quote by Vanadium 50 No, it's mN, not mN/s. The force does not increase with time. An alpha is ejected at 10% of the speed of light. If you had 1 kg of Po-210, after it has completely decayed, the alphas have a summed momentum of 600,000 kgm/s. So, by conservation of momentum, that's the largest impulse you can transmit to your spaceship, even if you used magic to get them all in the same direction. Voyagers weighed 700 kg. So this will, at best, get them to a speed of about 800 m/s - less than 5% of the speed that they actually achieved. This idea is, I'm afraid, ineffective, impractical and dangerous.
It works if the alpha emitter and its inert backing film is a substantial fraction of the spacecraft mass. Say that for every kg of Po-210 there is 1 kg of inert film and 1 kg of payload. Also assume that the alpha emissions are not perfectly collimated, so that only 50% of the maximum possible momentum transfer is realized. This hypothetical propulsion system imparts 300,000 kg*m/s to every 3kg of spacecraft mass, for a final speed of 100,000 m/s relative to Earth, on the order of Voyager's speed. Use of lighter inert films, smaller payload mass fraction, and staged radioactive films with different half-lives can improve on this result.

Launching alpha emitters from Earth is a serious logistical and safety problem, of course. The radioactive films for such a system would have to be manufactured in space, or at least activated in space.

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