Question about Polywell assembly in space

[energyfan]

Here's a question someone asked me(and I couldnt answer) :

How much cheaper would a break-even polywell be if it were built in space? It's pretty easy to make a vacuum there. Would it also be colder, making it easier to use superconducting magnets? I'm pretty sure it wouldn't be, because the reactor would heat faster than it could radiate (I'm assuming, of course, that the polywell would be in a shadow). But still, it could be possible to cool nitrogen (or another suitable gas) over an area, and run it in a cycle through the magnets. The cooling side of it seems just as complex, and not significantly cheaper, but there has to be an advantage in having an open reactor.

[93143]

It wouldn't be cheaper.

Let's say the reactor, plus supporting equipment, power supplies, thermal handling, etc. could be done in 20-25 tons, which I'm not at all sure is true. (Remember that WB-4 weighed the better part of a ton all by itself.)

Launching a 20-ton ground-assembled automated test rig on a DoD rocket would eat most, if not all, of the $200 million budget all by itself, without even paying for said automated test rig (space-rated electronics anyone?). Doing it any other way (except using a cheaper 25-ton-class launch vehicle, like a Falcon 9 Heavy or something) would be more expensive. ESPECIALLY using the Shuttle, which has a full manifest out to retirement anyway. Don't even think about trying to get astronauts up there to operate it, never mind build it in situ... and something like this would have to stay far, far away from the ISS, so that's no help...

The Falcon 9 Heavy hasn't even flown yet (I believe the Falcon 1 has yet to get a payload into orbit), and it's supposed to be an incredibly cheap $94.5 million per flight...

Also, cooling is way hard in space. A substantial fraction of the mass would have to be radiators, even if most of the neutron flux was unobstructed. I don't think the full 100 MW worth of radiators could be done in 25 tons at all, even if the rest of it could. (Just a gut feeling - I haven't done any math...)

There are heavier-lift rockets on the horizon (Ares V (VI?), Jupiter-120/232), but the Jupiter-120 would be the cheapest by a factor of two or so at $140 million dollars a flight for maybe 45 tons to orbit. The other two are true heavy lift vehicles (>100 tons) but would cost far more. If Energia were still operating you might be able to use it for cheaper, but it isn't...

Besides, designing a reactor to work in space is way more difficult anyway, and correspondingly more expensive... What would you use for power? A fission reactor? (Any idea how much a 10 MW fission reactor would weigh?) Solar won't be enough for more than intermittent pulses...

Also, if a piece of space debris were to hit the coils and produce a sharp-edged divot, it could instantly render the reactor useless...

[blaisepascal]

Here's a question someone asked me(and I couldnt answer) :

How much cheaper would a break-even polywell be if it were built in space? It's pretty easy to make a vacuum there. Would it also be colder, making it easier to use superconducting magnets? I'm pretty sure it wouldn't be, because the reactor would heat faster than it could radiate (I'm assuming, of course, that the polywell would be in a shadow). But still, it could be possible to cool nitrogen (or another suitable gas) over an area, and run it in a cycle through the magnets. The cooling side of it seems just as complex, and not significantly cheaper, but there has to be an advantage in having an open reactor.

Ignoring the "access too space" issues, the reactor wouldn't be simpler. The main thing which easy access to a vacuum would seem to get you is the ability to remove the vacuum chamber. But you are going to need something to stop the alphas, or you aren't going to recover any energy. That means you need the chamber, although it (theoretically) could vent to the outside without a problem.

There is also a problem of vacuum quality to worry about. Wikipedia says that atmospheric pressure at the Karman Line (boundary of "outer space") is around 1e-3 Torr, and other sources suggest that in the thermosphere (where any low-earth-orbit satellites would go) the pressures are in the range of 1e-6 atmospheres, which is around 75e-6 Torr. I seem to recall reading somewhere that Dr. Bussard said that 1e-6 Torr was about the maximum pressure outside the polywell, which is less than you'd get in low earth orbit. So you'd still need vacuum pumps and a vacuum chamber.

Space complicates cooling, since you can't dump excess heat into the environment via conduction/convection. You have to radiate it off, which isn't trivial.

[StevePoling]

Is the quality of lunar vacuum sufficient? Luna has plenty of He3 to burn. Heat sinks would be simpler, too.

Maybe in a hundred years lunar colonist kids will build BFRs in their back yards for high school science projects.

[93143]

Is the quality of lunar vacuum sufficient?

Dust + high voltage = no. Sorry.

But a good vacuum chamber might be easier to build...

[kcdodd]

Of course, Bussard envisioned using the polywell for propulsion purposes similar to the bussard ramjet. Using magnetic fields to direct the exhaust out the back and not even try to convert it to electricity.

[93143]

Of course, Bussard envisioned using the polywell for propulsion purposes similar to the bussard ramjet. Using magnetic fields to direct the exhaust out the back and not even try to convert it to electricity.

I just re-read the original post, and it doesn't actually say anything about building WB-100 in space. It just says a "break-even" Polywell, which I assumed meant the test unit...

Naturally we all hope that net-power Polywells will eventually be used for propulsion. However - "cheaper" it's not...

[Roger]

Here's a question .

I'm guessing the first polywell in space will be a QED style rocket ship. At some point ya drop one on the moon.

[Yitzhak]

Hi, I was the person who asked the original question. I knew that the cost to take the materials for the reactor into space would be a lot more than anything saved by having it built there, but I thought it might be possible to build a very basic test reactor using resources already in orbit. It seemed to me that the major limit for Dr Bussard was the size of his reactor, and with the chamber volume unlimited, a large reactor could be built with less hassle. What I didn't know was the composition of the atmosphere - I had assumed it was very close to a vacuum, but can now see that a pump would still be needed to operate the reactor. Also, since reading through this forum, I have realised the enormous length of wire necessary to make useful electromagnets.

Really, my ultimate intent in asking the question was to judge the plausability of building a fusion reactor to produce thrust in space. If it were simpler to make the reactor in space, this technology could have more incentive for funding as a means of thrust for the space industry than as a competitor for the Tokamak.

The design is so simple I'm sure it's been thought of before. Basically I imagine an open polywell (assuming for now using Boron-Proton fuel) with a hemispherical positively charged shell over one side. Any alpha particles produced would either be decelerated by the shell (causing thrust and heat, the latter of which could be used for power generation) or simply released through the other side. To decrease thrust, extra shells overlaying the major hemispherical shell would cover the exhaust side, causing some of the alpha particles which were being emitted to be decelerated, cancelling out some of the forward thrust, and simply producing heat energy.

What I need to know is if the reactor can be altered in some way for the pressure in space to be low enough for the reactor to work. Is the necessary pressure increased for a greater size?

I originally assumed Boron-Proton fuel, but I also wonder if lighter elements can be used. I've heard that Helium-3 can be produced by D-D reactions. This nucleus, when at high energies, could be used as thrust, and then be recycled into the Deuterium fuel to produce and alpha particle and a proton, which could also be used for thrust. Could it be possible to get the deuterium fuel (or even helium-3?) directly from solar wind or flares or the like? I doubt it - the content of useful particles in the atmosphere of space would probably be miniscule by comparison to the other particles floating around.

[MSimon]

Most of the fuel flow (80 to 95% or more depending) in an operating reactor is just to maintain operating pressure.

To do it right recirculation would be required.

For a rocket throwing away that amount of fuel would probably be acceptable. It would not be good with tens of thousands of ground reactors.

In any case the big cost for a pB11 unit would be the start/maintenance power supplies and power conversion eqpt. I'd say (BOE) that vacuum pumps would be under 20% of costs and possibly well under that figure.

And as some one pointed out up thread - "free" vacuum at this time is very expensive.

[MSimon]

As I recall space wind is on the order of 3 protons/cc not very dense.

http://spaceweather.com