Polywell for Space Propulsion

[mweiss]

I am new to this board. I got here because I was on another board, newmars.com/forums, which was discussing the possibility of polywell for space propulsion.
The two methods of using nuclear for space propulsion are nuclear thermal (in which a fluid is heated by passage through a nuclear reactor and then ejected) and nuclear electric (in which an electric propulsion device is powered by a nuclear reactor with a low mass per electric output). In the former case, the issue tends to center on a few MW thermal output (as in NERVA) while in the latter case the issue tends to be about 100kWe output (as in SAFE-100).
The question I would like to pose is this: if/when polywell finally passes the last milestone, will it be useful for either of the above purposes? How much improvement do you think it would have above fission? Just to nail things down (in the hope that someone out there is great at math) let us say that NERVA is the baseline for the NTR option and the SAFE-100 (400kWt, 100kWe, ~700kg including Brayton turbines) is the baseline for the NEP option. Thanks!

[MSimon]

Could you spell out the meanings of NEP and NTR?

NEP = National Elevator Powered?

NTR = New Toy Puller?

[Skipjack]

NTR= Nuclear Thermal Propulsion
NEP= Nuclear Electric Propulsion
If I am not mistaken here

[Skipjack]

Bussard never really went into much detail about the actual propulsion system in his research, but since he is talking about the ability to launch payloads into orbit, I would assume that he meant nuclear thermal.
I do think however that he did not mean a NERVA like design in which a "fuel"- mass like LH2 is directed over the reactor to become heated, expand and then exhaust through the nozzle (which would be perfectly possible with this design as well). Instead I think he meant to use the nuclear products as an exhaust mass. I might be wrong here though.
I think I also read a concept somewhere where the plasma confinement is "broken" in the direction of the exhaust to release the plasma and therefore generate thrust.
Anyway all methods seem to be feasable with a BFR (some of the BFR- experts here, please correct me if I am wrong). I think it depends on the application which one would work best.

[Art Carlson]

The question I would like to pose is this: if/when polywell finally passes the last milestone, will it be useful for either of the above purposes?

Meaningless question. There are good reasons to suspect that the polywell will never yield net energy in any way, shape, or form. If you assume it will anyway, then you can choose assumptions that will make it better than fission or assumptions that will make it worse than fission.

One comment anyway. A fission thermal system might be able to work without cooling, which is usually the biggest problem in space power systems. In a magnetic confinement fusion system, other cooling requirements are aggravated by the need to cool large superconducting magnets to cryogenic temperatures.

[mweiss]

NTR= Nuclear Thermal Propulsion
NEP= Nuclear Electric Propulsion
If I am not mistaken here

That would be it.

Follow up question: does B-p fusion produce a lot of heat?

[Skipjack]

Art, if you are so convinced that a Polywell reactor will never yield net energy, why are you still here?
If I was as certain about this, as you obviously are, then I would go and look elsewhere for my daily "science thrill"

[MSimon]

Art, if you are so convinced that a Polywell reactor will never yield net energy, why are you still here?
If I was as certain about this, as you obviously are, then I would go and look elsewhere for my daily "science thrill" ;)

Art is our resident sceptic. He keeps us honest. I hope he stays.

In fact he has a much higher value to the discussion than the average poster.

[Jeff Peachman]

Agreed. The average poster is not a trained, professional plasma physicist.

I'm acually glad Arts been around so long. I read more than I post on here but its surprising so many people keep coming back with so little news.

[Skipjack]

Dont get me wrong, I dont want him to go either, as I love reading most of his posts. They are eligthening and interesting to read.
He also provoques a reaction from R. Nebel every now and then and that is always interesting too...
So yeah please Art stay! Dont get that wrong!
My question was more rhetorical. As in "why are you (obviously) still interested in this approach to fusion, if you think that it does not work?".
The other way round:
If you still give it a chance to work no matter how small, why should we then not be allowed to speculate about the applications?

[Jeff Peachman]

I certainly don't mean to put words in Art's mouth, but obviously since he has voiced his professional opinion here about polywell's chances, it is worthwhile to follow it.

If it works, thats great.

If not, he gets the bragging rights. (In a good way).

[classicpenny]

There are good reasons to suspect that the polywell will never yield net energy in any way, shape, or form.

Art,

Ever since you came over to Talk-Polywell from the Cosmic Log discussion, I've been trying to follow the dialogue between you, Dr. Nebel and the others who seem to understand the in-depth problems with the polywell. I very much value the "gadfly" role you have been playing, and I hope you will continue to do it. Even though you still believe that the polywell will never yield net energy, there must have been a few changes in your thinking as a result of your extended discussion on Talk-Polywell. With this in mind, and for us "lurkers" with a physics background limited to the undergraduate level and a lot of time on Talk-Polywell, would you be willing to sum up those reasons (that the polywell will never yield net energy) for us - again?

Thanks!

Bill Flint

[mweiss]

...you can choose assumptions that will make it better than fission or assumptions that will make it worse than fission.

And here they (not sure who) had me believing that fusion is always better than fission at everything because it produces more energy. Fundamentally, leaving polywell per se out of the question, does it?

Bussard never really went into much detail about the actual propulsion system in his research...I think he meant to use the nuclear products as an exhaust mass. ...plasma confinement is "broken" in the direction of the exhaust. ...all methods seem to be feasable with a BFR

I wasn't aware Bussard discussed fusion propulsion, although I think I recall reading he had something to do with NERVA now you mention it.

Robert Zubrin ("Entering Space") mentions a fusion magnetic bottle that leaks the products out the back as a fusion rocket. He says that for interstellar flight the exhaust velocity would be 0.05 the speed of light, while for voyages to the outer planets the exhaust would be watered down with more than 95% ordinary hydrogen (for thrust) for a velocity of more that 250km/s. I have no idea how he derived these numbers though.

I had also assumed that this concept would not apply to polywell since polywell is electrostatic confinement of the plasma (with help from magnetic confinement of an electron "sheild") rather than a real magnetic bottle. Also, from the pictures of the polywell reactors, it is a set of rings rather than a tube shape. That being the case could you still use polywell in the leaky magnetic bottle sense? Also, am I getting my facts straight?

What is BFR? Just guessing: Bussard Fusion Rocket? If so, please refer me to source material. Thanks.

[mweiss]

Meaningless question. There are good reasons to suspect that the polywell will never yield net energy in any way, shape, or form.

Then your answer is "neither/nor" at this time.

[MSimon]

BFR = Bussard Fusion Reactor