Author Topic: Fusion with space related aspects thread  (Read 477100 times)

Offline Cinder

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Re: Fusion with space related aspects thread
« Reply #2505 on: 03/18/2010 12:29 AM »
Well you can build a 100 MW fission package in less space, but its certainly going to be far more massy than polywell, but I couldn't say exactly how much.

Does that include the heat engine necessary to get electricity out of a fission pile?  (Okay, maybe I'm jumping the gun on the whole pB11/direct conversion thing...)
Yes I do mean the closest fission analog to what this Polywell package includes.  As close to an apples to apples comparison as possible.
« Last Edit: 03/18/2010 12:30 AM by Cinder »

Offline mlorrey

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Re: Fusion with space related aspects thread
« Reply #2506 on: 03/18/2010 12:34 AM »
Well you can build a 100 MW fission package in less space, but its certainly going to be far more massy than polywell, but I couldn't say exactly how much.

Does that include the heat engine necessary to get electricity out of a fission pile?  (Okay, maybe I'm jumping the gun on the whole pB11/direct conversion thing...)
Yes I do mean the closest fission analog to what this Polywell package includes.  As close to an apples to apples comparison as possible.

That polywell reactor should have the direct conversion grid inside it. So in that case, no if were going to include turbines and generators, plumbing, valves, pumps, and cooling tower, well, polywell wins hands down.

When I was at Worcester Polytech we had a 150 kW fission reactor for the nuke engineers to learn their trade with, and it was easily larger than this polywell design, including everything.

Offline mlorrey

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Re: Fusion with space related aspects thread
« Reply #2507 on: 03/18/2010 12:38 AM »
So that is what, 7.8 meters on a side? Assuming the man is 6 ft. tall.

Guess so. Whats nice tho is that once you've reached net power, a little more scaling upward produces a LOT more power output. Note also that its not a big solid mass like you'd see with a fission device, most of the interior is empty space. If simon is around maybe he can let us know how heavy this puppy is supposed to be.
I did a spread sheet using simon's estimate of 5 tons/cubic meter of superconductor in the magnets and 1 ton /cubic meter for coolant in the magnet channels. I came up with about 6.3 metric tons for the magnets, again using simons estimate of a 2 meter radius. That fits nicely in my estimated 7.8 meter chamber. The way a truncube Magrid is configured, 2 * (2 meters+20%) = 4.8 meters on a side, leaving 1.5 meters space around the periphery of the chamber.
The WB8 image on the EMC2 website shows the configuration quite well.

Are they actually designing it to use superconductors and not just copper coils? I would think they'd want to do a proof of concept scale model with superconductors first.

Offline Cinder

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Re: Fusion with space related aspects thread
« Reply #2508 on: 03/18/2010 12:44 AM »
Well you can build a 100 MW fission package in less space, but its certainly going to be far more massy than polywell, but I couldn't say exactly how much.

Does that include the heat engine necessary to get electricity out of a fission pile?  (Okay, maybe I'm jumping the gun on the whole pB11/direct conversion thing...)
Yes I do mean the closest fission analog to what this Polywell package includes.  As close to an apples to apples comparison as possible.

That polywell reactor should have the direct conversion grid inside it. So in that case, no if were going to include turbines and generators, plumbing, valves, pumps, and cooling tower, well, polywell wins hands down.

When I was at Worcester Polytech we had a 150 kW fission reactor for the nuke engineers to learn their trade with, and it was easily larger than this polywell design, including everything.
Thank you.

Offline aero

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Re: Fusion with space related aspects thread
« Reply #2509 on: 03/18/2010 02:04 AM »
Quote
Are they actually designing it to use superconductors and not just copper coils? I would think they'd want to do a proof of concept scale model with superconductors first.
No one really knows what EMC2 is doing. There hasn't been any hard data released beyond the navy contracts. That said, there is a common thought that because of the way it scales (7th power of radius) a full scale device is expected to be only slightly larger physically than a sub-scale device, so why bother with a sub-scale machine? They could of course run a full scale machine at sub-optimal drive power to avoid the generation of fusion energy, if that were desirable.

Offline steve kelsey

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Re: Fusion with space related aspects thread
« Reply #2510 on: 03/19/2010 07:43 PM »
I know there is a way go go yet before this technology is proven,however as the EMC2 team appear to be making real progress towards a demonstration design, and as this is the Advanced Concept  thread, is it time to start thinking about how this technology might extend capabilities.
If the energy production does scale by the 7th power then would Orion class vehicles be feasible?

Offline mlorrey

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Re: Fusion with space related aspects thread
« Reply #2511 on: 03/19/2010 08:38 PM »
I know there is a way go go yet before this technology is proven,however as the EMC2 team appear to be making real progress towards a demonstration design, and as this is the Advanced Concept  thread, is it time to start thinking about how this technology might extend capabilities.
If the energy production does scale by the 7th power then would Orion class vehicles be feasible?

Orion class vehicles use explosive nuclear pulses from small fission or fusion shaped charge warheads so the explosive energy pushes against a giant pusher plate with giant shock absorbers. So Polywell would not be useful in this class of vehicle because Polywell produces constant or near constant fusion reactions (there has been reported a pulsing effect in polywell fusion activity, but this is extremely minor compared to nuclear explosives).

However, a polywell reactor in space would, if scaled up to produce 2 GW of power, be able to power a variety of propulsion schemes. 2 GW is enough to power the full scale VASIMR engines that would allow for 39 day trip times to Mars from Earth.

Then you have the reactor itself as part of the engine...

Firstly, hydrogen, or water, could be used as a reactor coolant and then ejected as a propellant much like fission engines like NERVA, PLUTO, etc. however as most of the energy produced by pB11 fusion, the preferred fuel choice for low radiation fusion, is in the form of electrons, this allows for direct conversion of fusion reaction energy into electric power, with no need for turbine generators, brayton cycle converters, or humongous radiators. Whereas fission reactors have higher thermal power output than electric, due to the inefficiency of converting thermal energy to electricity, polywell reactors would produce significantly more power as electricity, with a small fraction as heat.

You could therefore use a concentrator system to use your coolant to concentrate the heat in less mass via a heat pump system, then subject that mass of propellant to electron beams, which would convert it into a very energetic plasma at such high temperatures that you would need magnetic nozzles like VASIMR has, and would result in very very high Isp, in the hundreds of thousands.

Theoretically polywell fusion permits efficiencies as high as several million Isp if you simply took the fusion plasma products, dumped the electricity back into them, and expelled them.

Carrying your own on-board fuel, you could potentially reach up to 20% of light speed with this technology, enabling trips to Alpha Centauri in 22 years trip time.

If you were to use this technology as the core of a bussard ramjet spacecraft, you would carry an onboard store of Boron 11 fuel, and use the hydrogen collected from interstellar space as your source of protons. Since brehmsrahlung thermal issues require your reactor run hydrogen rich, you could collect a mass of hydrogen equal in mass to the on board boron fuel, which would double your effective Isp. This combined fuel ramjet stage would potentially enable you to light up the ramjets fusion from a dead stop and reach velocities high enough where the amount of hydrogen collected, and speed its collected at, would permit pure H-H fusion to be achieved, and the bussard ramjet could potentially reach near to light speed depending on trip length and acceleration rate.

At this point you run into issues of radiation from the relative velocity of your ship vs the gasses in interstellar space hitting your ship and/or the magnetic field around it thats used to scoop up that gas. There are some calculations showing that at some high fraction of light speed, no amount of mass or magnetic shielding would protect a human crew from this radiation that would be feasible for enabling a ship capable of any significant acceleration.

Now, beyond fusion propulsion, there is also the potential to use polywell reactors to power more exotic field drives or mach effect drives. Mach-Effect thrusters at low power levels are believed to generate simple thrust from an input of electricity. Laboratory evidence supports this part of the theory so far, though this is not yet rock solid and experiments are ongoing. Furthermore, this sort of technology is also supposed to allow for generating warp fields like those described by Alcubierre and shown in Star Trek. Finally, they potentially (and this is far future speculation given a lot of advancement of the technology and no show stoppers that have not yet been thought of) are the technology needed to generate stargate type wormhole travel. These three levels of propulsion are at different levels of power and efficiency with this sort of device, and a lot of this is at this point just theory, as I previously said. However, if current low power experiments confirm that part of the theory, then it is likely that the rest of the theory is also valid and only depend on future advancements to achieve (much akin to the distinction between ancient roman steam wheel toys and modern turbine jet engines).
« Last Edit: 03/19/2010 08:45 PM by mlorrey »

Offline steve kelsey

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Re: Fusion with space related aspects thread
« Reply #2512 on: 03/19/2010 10:51 PM »
And there I was just thinking about getting a significant tonnage to LEO :)

I have read Dr Woodwards work with great interest and have my fingers crossed for that one as well. I still don't fully understand the efficiencies though,my math isn't up to it yet. What I had in mind was, assuming known technology, how much more capable would a 'fusion HLV'  be if we could scale up the Pollywell to several Gigs.Would it approach the lift capability that the Orion team were proposing. As a minor aside I remember reading the Dyson history of that project and from memory it was only when Freeman Dyson was talking about vehicles of several million tons during design studies that he was told to 'calm down'.

Microwaves are an efficient way of transforming electrical energy to heat perhaps as an air breather for the initial phase then converting to heating a propellant mass to reach orbit.At the ISP's suggested the mass fraction for the payload could be significantly higher.

But I agree with you,a Woodward-Mach impulse engine would be far more effective if the thrust scales.
« Last Edit: 03/19/2010 10:52 PM by steve kelsey »

Offline mlorrey

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Re: Fusion with space related aspects thread
« Reply #2513 on: 03/20/2010 01:41 AM »
And there I was just thinking about getting a significant tonnage to LEO :)

I have read Dr Woodwards work with great interest and have my fingers crossed for that one as well. I still don't fully understand the efficiencies though,my math isn't up to it yet. What I had in mind was, assuming known technology, how much more capable would a 'fusion HLV'  be if we could scale up the Pollywell to several Gigs.Would it approach the lift capability that the Orion team were proposing. As a minor aside I remember reading the Dyson history of that project and from memory it was only when Freeman Dyson was talking about vehicles of several million tons during design studies that he was told to 'calm down'.

Microwaves are an efficient way of transforming electrical energy to heat perhaps as an air breather for the initial phase then converting to heating a propellant mass to reach orbit.At the ISP's suggested the mass fraction for the payload could be significantly higher.

But I agree with you,a Woodward-Mach impulse engine would be far more effective if the thrust scales.

Ah ok, well first we need to look at the power of existing rocket engines in terms of Wattage to get some idea of how much power we need. This has been discussed in the past. An SSME is said to produce the equivalent of about 9-10 GW of power which translates to 400klb of thrust. Tom Ligon has posted here about the amount of shielding tonnage needed for a polywell reactor in this range. Polywell pB11 fusion is only aneutronic in relation to other forms of fusion, it still produces a good amount of radiation, including brehmstrahlung radiation that needs to be shielded against.

Given the scaling rules with polywell, it seems best to design big so that the power scaling overwhelms the less accelerated shielding and size scaling.

Lets say we have a 50 GW power budget for 791 tons of thrust. This power level requires a 7 meter diameter polywell which should come out to about 14-15 meter reactor. Reactor mass should be 225 metric tons, or 450klb.

Now, using a air augmenter around the nozzle, you'll combust the post-expansion hydrogen with atmospheric oxygen in a ramjet environment which will boost your Isp by another 3500 seconds over the 1250 seconds of the pure hydrogen exhaust phase, and the chemical combustion will give you about one more SSME worth of thrust while in atmosphere, which is a nice kicker for getting over those gravity and atmospheric losses.

So, reactor diameter about 50 feet, imagine a torus hydrogen tank around the reactor with permanent shielding only on top of the reactor to shadow the crew behind. The hydrogen tank will provide surround shielding at takeoff. Build the vessel to look like the SERV, or, ghu forfend, a giant flying saucer... (danger Wil Robinson).

This puppy will reenter like a feather, so metallic TPS is all thats needed, lets use the base of engine configured as a plug nozzle (aerospike) for the reentry TPS as the SERVE also used.

Any questions?

Offline steve kelsey

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Re: Fusion with space related aspects thread
« Reply #2514 on: 03/20/2010 06:58 AM »
Only 'how do we go bigger?' ;)
 I am assuming that there is some engineering limit on building the reactor -material limitations must apply at some point. If so we may need to cluster engines of the style you define?  I like the approach you suggested very much, it’s very elegant but does it scale with the tank as a torus?
If not the tanks may be better placed above the reactor.
Clustering the reactors may give problems, for example I am not sure if the Pollywell is subject to neutron coupling like fission reactors. If so then it would be difficult and expensive to combat this using shielding so it may be necessary to adopt a spaced configuration. The inverse square law may help us out here?
The potential well is pretty compact and occupies a small volume of space compered to the reactor structure so this may not be a problem.If that is true we can cluster away and build it as big as the nearest shipyard can handle. Of course I am making the assumption that we can’t apply ship building technique to the Polywell itself. You could build a really big rector if the power density did not demand exotic materials. The reactor isn’t massy and I am guessing the major engineering challenge would be building a vacuum chamber of sufficient strength to avoid bulking up the mass. Perhaps the submarines in space thread needs a revival?

I do like the idea of keeping the FHLV a fluffy design to ease re-entry and I have always like the annular aerospike concept

Will Robinson may not recognise the Jupiter 3 but I am sure he would get used to it quickly when he realised he could have an Olympic size swimming pool on board.
.
« Last Edit: 03/20/2010 08:38 AM by steve kelsey »

Offline PMN1

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Re: Fusion with space related aspects thread
« Reply #2515 on: 03/20/2010 06:27 PM »
The work that EMC2 is currently doing, could it have been started any sooner?

Offline Cinder

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Re: Fusion with space related aspects thread
« Reply #2516 on: 03/20/2010 09:35 PM »
That's such an open ended question...

Offline PMN1

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Re: Fusion with space related aspects thread
« Reply #2517 on: 03/20/2010 11:21 PM »
That's such an open ended question...

OK, so what i'm trying to say, is in terms of materials etc, when is the earliest this research could have been started?

Offline 93143

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Re: Fusion with space related aspects thread
« Reply #2518 on: 03/21/2010 05:57 AM »
I am assuming that there is some engineering limit on building the reactor -material limitations must apply at some point.

Well, the superconducting coils can only stand so much magnetic field.  If you could get the coils up to their maximum field, a smaller reactor would actually be more powerful, since power goes as B^4 but only R^3, and the effective B is inverse with size for a constant near-coil field.

There's also the structural issue of supporting very large, very powerful magnets with insulated standoffs.  A more compact system might have an advantage, but then again the magnets would be that much closer together...

There's also the thermal issue.  The magrid will be absorbing electron losses, and the direct conversion system will be heated by the slight nonzero energy of the decelerated alpha particles hitting the plates in the Venetian blind assembly.  You might need exotic cooling techniques to get much past 1 MW/m² of cooling power, and 10 is probably about the limit.  Not knowing the geometry of the direct conversion system, or the exact figures for plasma leakage, I can't do better than order-of-magnitude estimates on these parameters.

Quote
I am not sure if the Pollywell is subject to neutron coupling like fission reactors.

...no.  Fusion reactors are not subject to neutron coupling.  You really should be able to figure this out, since neutrons don't cause fusion.  Aneutronic fusion reactors, like the p-ııB version of the Polywell, are especially not subject to neutron coupling - even a nearby fission reactor would feel nothing.

You do have to be careful shielding a Polywell.  According to one source, which I fervently hope is wrong, there's a 1 in 10,000 chance of any given proton-boron fusion producing 16 MeV worth of gamma radiation (in the 4, 12, and 16 MeV bands) instead of alpha radiation.  This takes something on the order of a foot of lead to shield effectively.

(Note:  my earlier calculations assumed 1 in 10,000 power output fraction for gammas.  This is optimistic; it's more like 1 in 5400 for the branching probability described.  Not that it changes the shielding requirements much...)

Offline mlorrey

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Re: Fusion with space related aspects thread
« Reply #2519 on: 03/21/2010 11:00 PM »
The work that EMC2 is currently doing, could it have been started any sooner?

Yes, it could have been funded from the close of Bussards WB-6 experiments, which was as I recall 4 years ago.

Bussard was working on polywell for some 15 years or more under Navy contract, if he'd been funded more it is possible they could have gotten more done with more manpower available sooner.