polywell reactor for booster rocket applications?

Discuss how polywell fusion works; share theoretical questions and answers.

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93143
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Post by 93143 »

I thought NERVA was cancelled because Apollo was cancelled (it was supposed to provide Saturn V with a high-performance upper stage) and because the budget for such things went through the floor under Nixon. There are tons of non-nuclear NASA-related projects that have suffered a similar fate.

Skipjack
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Post by Skipjack »

I thought NERVA was cancelled because there was no political will to do it. You know with the environmentalists and all that.
Also people thought that it was not really needed and that the disadvantages (difficult handling, radioactivity, environmentalists going nuts over it, etc, etc) were outweighing the benefits. That was IMHO not really true, but anyway...
If a polywell reactor could replace a gas core reactor then we would have a winner. Otherwise... guess we will have to hope for some other miracle... Been doing that all my life... Its not been going to well so far ;)

icarus
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Post by icarus »

http://www.fas.org/nuke/space/c04rover.htm

16 - Phoebus 2A

The most powerful nuclear reactor of any type ever constructed, with a design power level of 5,000 MWt. Operations in June 1968 were limited to 4,000 MWt dur to premature overheating of of aluminum segments of pressure vessel clamps. At total of 12.5 minutes of operations at temperatures of up to 2310 K included intermediate power level operations and reactor restart.

Skipjack
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Post by Skipjack »

So well, anyway. I am pretty sure that Dr. Bussard talked about Polywell enabling SSTO and I also remember seeing concepts.
So would it, or wouldnt it?

Mike Holmes
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Post by Mike Holmes »

I think we've been over this before, no? That is, the answer is "it all depends on what Polywell ends up being, if it ever does."

That is, based on the most optimistic extrapolations of what the technology might do, yes, it could do everything that Bussard and we imagine. Based on other extrapolations, it will do precisely nothing. With a lot of room in between for "it might be useful for just this and that."

Mike

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Post by ohiovr »

I've been reading about Bussard's ARC relativistic electron beamed plasma thruster. An issue came to mind. According to what I've read the nozzle would be protected from the high temperature exhaust by a magnetic field similar to how the vasimr engine operates. What I don't understand is the pressures. All the plasma confinement schemes I am familiar with deal with pressures far less than an atmosphere. But for boosting though the atmosphere to get any thrust at all the engine must have positive pressure. The space shuttle's lox lh2 main engines have a chamber pressure of over 190 atmospheres.

I've read many times that plasma confinement is difficult. How much more difficult does it get with several atmospheres of pressure? Or is it not so bad since it is just a cylindrical solenoid? I've calculated my exhaust temperatures to be from 15,000-60,000 kelvin. Propellant would be mono atomic hydrogen plasma.

How strong would the solenoid have to be in magnetic field strength?

Say this scheme is impossible, could it be possible to cool the rocket's internals with sweat cooling? Of course this definitely would come as a performance hit.

KitemanSA
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Post by KitemanSA »

Presuming that Polywell does in fact work as DrB professed, and
Presuming that Polywell is approximately as dense as has been described, then
Yes it should be able to provide for SSTO. But even if it is not quite up to DrB's dreams, but still is within the same order of magnitude it could still provide for an effective booster system via a hypersonic rotovator (or skyhook) with MXER boost. The main need for any tether boost system is momentum which with an electric drive powered by a Polywell, becomes a massless (or Earth mass) proposition.

D Tibbets
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Post by D Tibbets »

I think the relativistic electron beam and other acellerater based schemes that Bussard talked about were low thrust high efficiency designs that would not come close to the thrust needed for a booster. For a booster a working fluid like hydrogen is heated by the reactor and ejected much like the fission rocket designs. Heavier molecules would provide greater thrust, but at lower efficiency (also would probably be more dense so smaller tanks would be needed-there are trade offs). I believe that one of Bussard's proposals was an air breathing design like a scram jet, except the heat came from the reactor instead of chemically burning a fuel (or perhaps a combination of both). Onboard reaction mass was carried for the final exoatmospheric acelleration.
The main advantages of a Polywell fusion approach (especially P-B11) is the lack of radiation concerns, especially from an accident stand point (I presume alot of weight dedicated to containing the dangerous plutonium in the case of a crash or reactor breach). Because the Polywell fusion reactor (or perhaps DPF or reverse field pinch approach) presumably would have a much higher energy density that an equivalent Tokamak (very large and heavy) it would be much more pratical. Would it be lighter than an equivalent fission reactor and associated equipment (ignoring safty concerns)?

[Edit] An interesting link-
http://www.ibiblio.org/lunar/school/Int ... stems.HTML


Dan Tibbets
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MSimon
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Post by MSimon »

D Tibbets wrote:I think the relativistic electron beam and other acellerater based schemes that Bussard talked about were low thrust high efficiency designs that would not come close to the thrust needed for a booster. For a booster a working fluid like hydrogen is heated by the reactor and ejected much like the fission rocket designs. Heavier molecules would provide greater thrust, but at lower efficiency (also would probably be more dense so smaller tanks would be needed-there are trade offs). I believe that one of Bussard's proposals was an air breathing design like a scram jet, except the heat came from the reactor instead of chemically burning a fuel (or perhaps a combination of both). Onboard reaction mass was carried for the final exoatmospheric acelleration.
The main advantages of a Polywell fusion approach (especially P-B11) is the lack of radiation concerns, especially from an accident stand point (I presume alot of weight dedicated to containing the dangerous plutonium in the case of a crash or reactor breach). Because the Polywell fusion reactor (or perhaps DPF or reverse field pinch approach) presumably would have a much higher energy density that an equivalent Tokamak (very large and heavy) it would be much more pratical. Would it be lighter than an equivalent fission reactor and associated equipment (ignoring safty concerns)?

[Edit] An interesting link-
http://www.ibiblio.org/lunar/school/Int ... stems.HTML


Dan Tibbets
The higher ISP of H2 means a lower total mass. I was thinking of an ISP equivalent to H2 @ 2,300 K - Space Shuttle main engine temps. An ISP of around 2,000.
Engineering is the art of making what you want from what you can get at a profit.

93143
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Post by 93143 »

The relativistic electron beam was the heating mechanism proposed for the ARC booster engine and the external-heating airbreather. The reactor doesn't generate heat (mostly); it generates high voltage DC - so that's what you put into the propellant...

The magnetic shielding doesn't have to be perfect; it just has to reduce the wall heat load to a manageable level.

ohiovr
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Post by ohiovr »

MSimon wrote:
The higher ISP of H2 means a lower total mass. I was thinking of an ISP equivalent to H2 @ 2,300 K - Space Shuttle main engine temps. An ISP of around 2,000.
The lower molecular weight of the hydrogen propellant does indeed improve speciffic impulse but I don't think by quite that much at least according to this:

http://www.braeunig.us/space/propuls.htm

(the equation in particular is this:)

Image

With that I got an isp of about 1093 seconds..

I figure that for for a specific impulse of 2000 seconds an exhaust temperature of 8000K would be needed. Unfortunately I don't know what temperature hydrogen molecules dissociate into individual atoms. If that were to occur at 8000K the specific impulse would be over 2800 seconds, different by a great margin.

And if we were to use water as our propellant, assuming a molecular weight of 18, our specific impulse would be a mere 679 seconds at 8000K.

All my calculations were also assuming a zero pressure atmosphere (running in space). At 1 atm things look a lot different...


At least that is what my open office spread sheet is telling me.

MSimon
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Post by MSimon »

I just used a rocket calculator that NASA provided. I can't be sure the math was correct.
Engineering is the art of making what you want from what you can get at a profit.

D Tibbets
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Post by D Tibbets »

93143 wrote:The relativistic electron beam was the heating mechanism proposed for the ARC booster engine and the external-heating airbreather. The reactor doesn't generate heat (mostly); it generates high voltage DC - so that's what you put into the propellant...

The magnetic shielding doesn't have to be perfect; it just has to reduce the wall heat load to a manageable level.
I concede that seems to be what Bussard is saying, though it seems (to me at least) that the realitivistic electron beam would be more appropiate for acellerating small masses to high speed- high ISP, low thrust. Perhaps an extreamly powerful (GW's of beamed power) electron beam has a high enough efficiency in transfering energy to a working fliud so that it would offset all the presumed additional weight needed for conversion and generation of the beam. Even if no direct heat came from the alphas the reacter would be producing alot of heat from x-rays that would require large radiaters- or fuel flow to cool. Also, the alpha kinetic energy can directly heat a large flow of working fluid (fuel) through mixing to produce the large thrust needed for boosters with little more needed structure beyond the reactor, except for a magnetic nozzel and fuel pumps.

Dan Tibbets
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93143
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Post by 93143 »

The trouble is that you need a wall between the high-pressure propellant flow and the interior of the vacuum chamber. The alphas can either hit that wall and heat it, and pollute the interior via sputtering, or they can be stopped by a direct conversion system. You can't just pipe them out at full speed and mix them with propellant unless your vehicle is already in a hard vacuum (deep space DFP drive, Isp ~1e6).

Yes, reactor waste heat goes into the propellant in an ARC drive (hence the name) - I suspect some form of exotic refrigeration (liquid metal?) may prove necessary to get the Isp high enough, because there's a nontrivial amount of waste heat to get rid of, and we don't want to run into Skylon's problem (admittedly solved rather elegantly in that specific case, but the solution they used isn't available to us) of the optimum coolant flow rate being significantly larger than the optimum propellant flow rate... If the required refrigeration power is high enough, this raises the question of whether it would be lighter to use an MHD turbine on the exhaust rather than trying to step down that much power directly from the reactor... you can't use the propellant preflow in a normal turbine because that would constitute a perpetual motion machine of the second kind... unless you brought along some LOX for a preburner... or just preheated it with a mini-REB, that might work too... It's late and I'm tired; shut up...

D Tibbets
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Post by D Tibbets »

93143 wrote:The trouble is that you need a wall between the high-pressure propellant flow and the interior of the vacuum chamber. The alphas can either hit that wall and heat it, and pollute the interior via sputtering, or they can be stopped by a direct conversion system. You can't just pipe them out at full speed and mix them with propellant unless your vehicle is already in a hard vacuum (deep space DFP drive, Isp ~1e6)...
All you have to do is provide more exaust pressure for the charged alpha particles than the chamber pressure that is pushing back, which presumably would be much greater than atmospheric pressure in any case. The grids to guide and concentrate the alphas into a small high flow rate (pressure) port would not be much different in principle from energy harvesting decelleration grids. Some energy would need to be harvested to run the system, but most of the alpha's kinetic energy should be aviable for heating (and ionizing a portion) of the working fluid. The working fluid molecules and ions would not be able to backstream into the reactor vacuum vessel do to the flow of the pos. charged alphas and the associated electrostatic (and magnetic?) fields that that make up the realitively small exit port. I don't know if an ion pump of this type could maintain the large pressure gradiants involved, but the alphas have to be extracted from the reactor vessel with very high pumping capacity pumps weather direct or indirect thrust is generated. You also will need to keep track of the electrons so that the ship remains neutral.

It should be obvious by this point that I am shooting in the dark, but hopefully the various competing factors can be combined in a useful fasion. An energy harvesting/ electrical conversion system will be needed irregardless. Electron guns will be needed (perhaps not relativistic electron beam guns- are they more massive than low volatage electron guns?). And the alphas will have to be removed from the reactor vessel very rapidly. Ion pumps may add the extra capacity that is needed over the iffy capacity of very large neutral molecule pumps(terbomolecular and / diffusion pumps). It boils down to what will work (if anything) and the associated complexity, reliability, mass and volume of the systems.


SN tIBBETS
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