Giorgio wrote:True, SC-CO2 cycle can work at 300C, yet efficiency does not go much over 40%.
Add losses from capturing the whole x-Ray fraction and you do not end much far away from my projected claim.
That, and
only that, is what I've been disputing. You seemed to be claiming that almost half of the waste heat would be lost to the surroundings rather than being captured by the cooling system. I think this is unjustified pessimism, particularly given the effort that would go into this segment of the design if it turned out high-efficiency x-ray capture was critical to close the power balance.
Everything else you've said about efficiency and power recirculation is true, but irrelevant to this point.
Keep in mind that while x-ray
shielding requires thick walls, x-ray
heating is basically a first-wall problem; you don't have to worry about significant power being dissipated in blocks of concrete several feet away from the vacuum vessel. Almost all the x-ray power comes out in the vacuum vessel wall itself, or in the equipment within it, and after that it's a straightforward matter of heat collection.
93143 wrote:Also, this is another illustration of how the comparison to the launch vehicle reactor is pertinent - the waste heat has to come off that thing at a much higher temperature, perhaps as high as 1500°C, to get a decent Isp.
How can this be pertinent?
This thread assumes an high fraction of power as X-Ray that needs to be collected, converted and replenished (in energy terms) by the e-guns.
The other thread assumes polywell to work with little to no bremsstrahlung.
You keep mixing different threads, different applications different assumptions.
If (as improbable example) bremsstrahlung are near to zero you can even manage them at air frame level.
The reason the coolant peak temperature matters should be obvious, considering that you've been claiming it can't be very high. If you can't get 900 K on the ground, you certainly can't get 1800 K in a launch vehicle. A Polywell is a Polywell; there's no new physics that steps in to save you when you put one in a rocket.
As for the differing assumptions re: Bremsstrahlung fraction...
It doesn't matter what the bremsstrahlung fraction is for a launch vehicle reactor - there are other sources of waste heat (the direct conversion system, for example, cannot possibly be 100% efficient), and the power level is so high that even 5-10% percent waste heat, captured at 90% efficiency, is far too much. No one, not even the proponents, is claiming the waste heat level could be much lower than that, and it is likely to be higher, so the only option is to have near-perfect active cooling.
My point in bringing up the rocket example is that it is inconsistent for you to hold both (a) that launch vehicle application of Polywell will happen quickly and be relatively easy (assuming reasonably low bremsstrahlung fraction), and (b) that
if bremsstrahlung fraction is high, close to half the thermal output of the reactor will be lost as leakage, not even caught by the cooling system.
Do remember that Polywell scaling means that a 5 GWf reactor will not be vastly larger than a 100 MWf reactor. In the 100 MWf case, a direct conversion efficiency of 75%, an electron/ion loss power of 20 MW, and a bremsstrahlung fraction of 1.74 (the thermal scenario) results in 219 MW of heat that needs to be dissipated. (It also requires ~54% thermal conversion efficiency in order to break even, but that's beside the point.) In the 5 GWf case, a direct conversion efficiency of 95% and
zero bremsstrahlung or electron/ion wall losses results in 250 MW of heat, over
at most triple the surface area, likely much smaller. With 85% DC efficiency in both cases (actually, due to beam spreading, the higher-power reactor will likely be worse than the low-power one, especially since it has to be compact), neglecting electron/ion wall losses, we have 189 MW in the 100 MWf case and 750 MW in the 5 GWf case.
In other words, under reasonable assumptions, it looks like the launch vehicle reactor (with low bremsstrahlung) would have a higher thermal power flux than the ground reactor (with high bremsstrahlung). So why is it so hard to cool the ground reactor, and so easy to cool the LV core?
As above, fix the discussion and the discussion terms.
Is it about feasibility of a Polywell power plant machine with a high bremsstrahlung rate or feasibility of a polywell rocket with high bremsstrahlung rate?
The original discussion was about the achievable efficiency of collection of x-ray energy as useful heat, that could then be fed through whatever thermal conversion system you like. I said it would likely be pretty high; you disagreed.
I then pointed out that you were very optimistic about the launch vehicle application working, and that this was logically inconsistent with your pessimism regarding heat recovery fraction because a launch vehicle reactor would need near-perfect heat recovery regardless of what the bremsstrahlung fraction was.
It's sort of a separate line of argument, in which I claim that you have made contradictory assertions without realizing the contradiction. Basically, I assert that you must pick one or the other; you cannot continue to claim simultaneously that LV application is easy and that high-efficiency waste heat recovery (by which, at the beginning of the argument, we both meant % of heat collected, not % of heat turned back into electricity) is impossible or prohibitively difficult.
As for myself, I believe neither.
93143 wrote:It strikes me that you don't seem to have much engineering understanding of this stuff. The role of insulation in controlling heat flux, for instance. The role of coolant flow rate in controlling peak temperature. That sort of thing.
Great news. I guess this means that I will have to drop the Pixie Dust formula on the design of our next heat exchanger and use a coolant flow control instead.
I wonder how all the ones we have already designed and installed are working thought....:roll:
I said "seem". Your comments to date do not give me the impression that you are actually an engineer. Are you?
And do you have any idea what a multi-megawatt uncontrolled heat leak would look like on a machine with only half a dozen or so square metres of uncooled surface area? It would be glowing pretty brightly...
93143 wrote:Even better, a polywell based rocket to lift goods in space might even be only thermal with no direct conversion at all.
You'd get an awful Isp without a ridiculously high peak temperature. NERVA-type nuclear thermal is a lot lighter than Polywell per GW
and has peak temperatures in the range of 2000-2800°C, but it
still isn't anywhere near good enough for ground launch.
If you think a conventional rocket I do agree, but for an hybrid spaceplane it might be feasible.
Someone made some calculations a couple of years ago, I will look back for them.
Okay. While you're at it, please remember that the stagnation temperature of high-altitude air reaches 600 K at about Mach 3, and 1800 K not much past Mach 6 (so you can't plausibly heat it directly for most of the way to orbit),
and that the amount of energy required to get liquid hydrogen up to 1800 K (do you believe the cooling loop can hit that temperature? Bussard said so, but I'm not so sure) is
about a quarter of the energy released by its combustion. In other words, the fusion reactor only adds about 25% more energy on top of what you'd get by deleting the reactor entirely and dumping the hydrogen straight into the airstream. What you're describing is sort of like an RBCC engine with a very fuel-rich rocket section, but far heavier due to the mass of the fusion reactor. You'd be better off with straight chemical.
Not to mention that to get to orbit, a significant fraction of the launch will have to be in vacuum, where an 1800 K hydrogen temperature gets you an Isp of about 725 seconds (63,000 lbf per GW), which is pretty miserable for such an appallingly heavy machine. Restrict the temperature to 600 K (the hot-side temperature we were discussing for the SC Brayton cycle) and you get worse Isp than a hydrolox chemical rocket, but with much larger/heavier tankage.
93143 wrote:Not at all. It's simply a reminder that when designing a launch vehicle, your options for passive, non-regenerative cooling are limited.
But your options to have different engines for different applications are not limited.
Apparently you didn't follow the line of thought I was trying to head off. Never mind.
Anyhow, I am ready to drop any other topic and just stay on this thread topic:
The need of recovery of a high bremsstrahlung fraction can render the reactor useless as a power generator?
I have not yet understood your point of view on this issue.
I haven't given you my point of view on this issue. The discussion started with you asserting that x-ray recovery would take a significant hit from uncollected heat loss. I countered with an assertion that heat collection would likely be efficient enough that it wouldn't have a major impact on the power recovery efficiency, and I backed up my view with explanation. That is what we've been fighting about, though you seem to have forgotten this. Bringing other factors into the discussion, while it may be technically 'on topic' wrt the thread title, muddies the waters regarding the specific sub-thread we're engaged in here.