Rocket thrust

If polywell fusion is developed, in what ways will the world change for better or worse? Discuss.

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taniwha
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Postby taniwha » Sun Nov 15, 2009 6:11 am

What about the X-rays and Gamma rays? They're the real problem. I don't know about X-rays, but Gamma rays are expected to be about 0.1% of output power, so you need to factor in 6MW of gamma radiation.

DeltaV
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Postby DeltaV » Sun Nov 15, 2009 6:20 am

This is depressing. My VTVL Polywell-powered space flitter is starting to look like an ozone/neutron/gamma-ray spewing humanoid annihilator during low altitude (non-QED/ARC) operation, if I can believe what you're telling me.

OK, time to dig deep into the well of engineering creativity, to come up with...
Magnetically diffused REB-heated, hermetically-sealed, rotary Stirling engines directly driving the lift fans and turbines!

[That takes care of the ozone at least. I think the other nasties can be ameliorated somewhat by throttling down the fusion rate.]

93143
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Postby 93143 » Sun Nov 15, 2009 6:52 am

taniwha wrote:What about the X-rays and Gamma rays? They're the real problem. I don't know about X-rays, but Gamma rays are expected to be about 0.1% of output power, so you need to factor in 6MW of gamma radiation.


Aw dammit, I forgot about the X-rays. 5% of net power. 300 MW. 60 rem per second at 50 metres. Fortunately X-rays aren't that hard to stop.

16 MeV gamma rays (at 600 kW, or about 0.12 rem per second, if the Wikipedia article is correct about the 0.01% branching probability) are much harder to stop. I can't see 10% of a tank of hydrogen making much of a dent, but I guess it won't be that low for long enough to matter... isn't the occupational limit 50 rem per year?

DeltaV wrote:This is depressing. My VTVL Polywell-powered space flitter is starting to look like an ozone/neutron/gamma-ray spewing humanoid annihilator during low altitude (non-QED/ARC) operation, if I can believe what you're telling me.


Bah. Google "Project Pluto"...

Magnetically diffused REB-heated, hermetically-sealed, rotary Stirling engines directly driving the lift fans and turbines!


I don't see what's wrong with a Brayton cycle, using the airstream as a working fluid... it neatly solves the problem of what to do with the waste heat (it leaves with the airstream), and you avoid having separate power cycles for electrical production and thrust. Plus you don't have to deal with multiple GW of electricity, which is difficult even at low voltage.

I am sort of a fan of the idea of just shielding the whole reactor and taking the weight penalty, and making up for it with high-performance airbreathing engine designs. Bull-headed maybe, and in need of some BoE calculations to validate the possibility, but hey - back in the '60s both the Americans and the Russians designed nuclear-fission-powered jet airplanes (yes, they did what I suggested and piped the reactor heat straight into the jets in lieu of combustion). Surely with the relatively high performance of a BFR, not to mention the low levels of high-energy radiation, we can do better than that if we just throw away the idea of making everything as light as possible and focus on making it work - throw gigawatts at a problem rather than shaving kilograms...

Yeah, this attitude got me into trouble with my old SSTA design. But this one uses regenerative cooling (with no mass thrown overboard, even!), so the power balance is unlikely to go negative on me...

kunkmiester
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Postby kunkmiester » Sun Nov 15, 2009 7:10 am

Someone on the other board we were talking about with this suggested the sealift(dragon?) method, which assembles the rocket horizontally on a ship--take it out, sink one end so you're vertical, and let fly. A fusion powered VTO would be easier, since it'd be smaller. Once you're out in the ocean you can get far enough away that you don't have to worry about full coverage shielding.
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DeltaV
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Postby DeltaV » Sun Nov 15, 2009 7:12 am

93143 wrote:I don't see what's wrong with a Brayton cycle, using the airstream as a working fluid...

Doesn't that lead you right back to the problem of how to heat the airstream?

DeltaV
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Postby DeltaV » Sun Nov 15, 2009 7:46 am

93143 wrote:Plus you don't have to deal with multiple GW of electricity, which is difficult even at low voltage.

Looking at large helicopters (CH-53K), the max takeoff weight power-to-weight ratio is somewhere in the neighborhood of 200 W/lb. A heavy 500,000 lb vehicle (widebody airliner class) would need (very roughly) 100MW of lift fan power to leave the ground (ignoring among other things thrust efficiency differences between a single large rotor and multiple smaller fans). I'm assuming the Polywell fusion rate can be modulated (maybe a bad assumption). So I'm not thinking of multiple GW for low altitude operation. If you MUST run a Polywell at full blast, always, then I would use part of the diffused REB to run the Stirlings and dump the rest of the REB into the air (upwards, to avoid lawsuits).

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Postby MSimon » Sun Nov 15, 2009 8:03 am

93143 wrote:...huh. According to my calculations, at 50 metres from an unshielded 6 GW BFR at full power, it should take about half an hour to absorb one rad.

Maybe this isn't so bad after all...

That's assuming 60e12 neutrons per second, with a neutron energy of 2.9 MeV, range 50 m yielding 31416 m² over which to distribute the neutrons. Result is 190986 n/cm²·s, and for a sitting person estimated at 4500 cm² and 70 kg, that's 12277667 n/kg·s, or 5.7e-6 J/kg·s, or 0.00057 rad/s. Assuming all the neutrons are absorbed, because I don't know how to calculate the fraction.

Is that about right?

Of course, if the reactor is closer the dose gets larger... but structure should help mitigate...

Just using water and boron-10, without the extra (heavy) gamma absorber, increases the time to a given dose by a factor of about 6 due to the lower energy of the gammas as compared with the expected neutron energy. Also, neutrons in the expected energy range are apparently 10-20 times as damaging as gammas for a given dose...


I haven't run the numbers to check but that sounds about right. For neutrons assuming 100% absorption is not too far off and is conservative.

There is also the possibility of shield suits or a shield room during burns. Also consider that arms and legs do not need to be as well shielded.

Ship electronics will need to be shielded as well.
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DeltaV
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Postby DeltaV » Sun Nov 15, 2009 8:40 am

On the other hand, a REB that is magnetically or electrostatically diffused/defocused might reduce the destructive impingement effects so that a REB-to-air/propellant heat exchanger is possible (no direct REB-air contact to avoid ozone generation), in which case the Stirlings, fans and turbines could be eliminated and the heated exhaust flow would provide low altitude lift/thrust until QED-ARC altitude/speed is reached. Simpler.
Last edited by DeltaV on Sun Nov 15, 2009 8:42 am, edited 1 time in total.

MSimon
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Postby MSimon » Sun Nov 15, 2009 8:42 am

isn't the occupational limit 50 rem per year?


I think that is the lifetime dose. IIRC 2 rem is the yearly occupational limit.
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taniwha
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Postby taniwha » Sun Nov 15, 2009 12:28 pm

According to atomic rocket, the dose limits are:

general public: 0.04Rem, 5Rem (0.5 for minors)
occupational: 0.4Rem, 5Rem, ~200Rem
astronaut: 150Rem, 300Rem, 400Rem

That's 30day limit, Yearly limit, Career Limit.

Also, I did the conversions from Sieverts to Rems in my head, so there may be mistakes.

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Postby MSimon » Sun Nov 15, 2009 12:30 pm

taniwha wrote:According to atomic rocket, the dose limits are:

general public: 0.04Rem, 5Rem (0.5 for minors)
occupational: 0.4Rem, 5Rem, ~200Rem
astronaut: 150Rem, 300Rem, 400Rem

That's 30day limit, Yearly limit, Career Limit.

Also, I did the conversions from Sieverts to Rems in my head, so there may be mistakes.


Sounds right.
Engineering is the art of making what you want from what you can get at a profit.

93143
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Postby 93143 » Sun Nov 15, 2009 9:09 pm

DeltaV wrote:
93143 wrote:I don't see what's wrong with a Brayton cycle, using the airstream as a working fluid...

Doesn't that lead you right back to the problem of how to heat the airstream?


That's what I've been trying to get across to you. Instead of using a REB to heat the air directly, you use it to heat a fluid in a closed loop. This fluid is pumped through a counterflow heat exchanger in the engine, thus heating the air.

The closed fluid loop doesn't need to be a power cycle, so it doesn't have any inherent inefficiency. And as a bonus, on the cold side (after the engine heat exchanger and before the REB) you can put another heat exchanger, to add heat to this loop from another loop - a heat pump, with its own cold side being the cooling loop from the reactor. (Or you could just have it be the cooling loop from the reactor, if you can find a suitable fluid...) This allows waste heat to be pumped to the engines along with energy from the REB, solving the problem of what to do with it and increasing the total efficiency of the system.

The heat exchanger would need to be made of something that could take the high temperatures involved without structural compromise. It would most likely be very heavy, but the reactor is incredibly heavy anyway so this might not matter much. And the turbine in a modern jet engine has to take similar temperatures (for obvious reasons), so it's not like we've never done anything like this before...

This scheme won't work at hypersonic velocities, because the heat exchanger would need to get too hot. I suppose you could get fancy with nuclear-lightbulb-style radiative exchange, but the fact is that at the altitude you'd be hypersonic at, ozone isn't a big problem, so you can use the REB directly.

Of course, if you do that, you need another way to get rid of waste heat... all I can come up with right now is start dumping hot hydrogen into the combustion chamber upstream of the REB, like a standard airbreathing ARC-QED drive...

This scheme should give you a vehicle that can fly for an unlimited time at low Mach numbers, but needs to pack LH2 for hypersonic flight. Oh well...

DeltaV wrote:
93143 wrote:Plus you don't have to deal with multiple GW of electricity, which is difficult even at low voltage.

Looking at large helicopters (CH-53K), the max takeoff weight power-to-weight ratio is somewhere in the neighborhood of 200 W/lb. A heavy 500,000 lb vehicle (widebody airliner class) would need (very roughly) 100MW of lift fan power to leave the ground (ignoring among other things thrust efficiency differences between a single large rotor and multiple smaller fans). I'm assuming the Polywell fusion rate can be modulated (maybe a bad assumption). So I'm not thinking of multiple GW for low altitude operation. If you MUST run a Polywell at full blast, always, then I would use part of the diffused REB to run the Stirlings and dump the rest of the REB into the air (upwards, to avoid lawsuits).


500,000 lb? A fully shielded 6 GW BFR weighs twice that on its own.

A modern ultra-high-bypass turbofan can get over 50,000 lbs of thrust from less than 90 MW. 6 GW gives you 3.4 million lbs of thrust, or about one STS RSRB. That's enough to lift an aircraft weighing about three times what the reactor does. But of course a horizontal-takeoff aircraft doesn't need a 1:1 thrust-to-weight ratio to take off, so perhaps it could be larger. And/or the engines could be smaller, so as to enable supersonic flight.

Or you could just let the reactor be more than a third of the all-up vehicle weight, and get something that flies like a giant Su-37... probably easier said than done...

For military applications, shadow shielding might be worth the increased performance. For space launch, it probably is.

The reason I've been using such high power is that the shielding mass goes up so slowly with output power. Remember, Polywell gross power goes as the seventh power of the linear dimension. Shielding thickness is fairly insensitive to power output (it has to reduce radiation by so many orders of magnitude anyway that a factor of 60 is nothing), so to a first approximation shield mass scales as the second power of the linear dimension. Thus larger reactors have much better power-to-weight ratios, not just for basic structure, but also for shielding.

Assuming the superconducting magnets can't be modulated, how deeply you can throttle a BFR depends on how the losses behave when you detune from nominal operating conditions. Increasing the hydrogen-to-boron mixture ratio would reduce fusion power and bremsstrahlung, the former probably more than the latter. Electron loss rates shouldn't change much. Altering the magrid potential changes the temperature and thus the nominal density; it also changes the fusion cross section and should affect bremsstrahlung slightly. I think you should be able to get it fairly far down, maybe as low as 20% output including waste heat, before you start having trouble running ancillary systems like the cooling refrigeration loop, but of course this is essentially a guess.

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Postby kunkmiester » Mon Nov 16, 2009 1:28 am

ya know, ozone, and the various nitrogen compounds I'm sure you have to deal with too, isn't stable. It'll break down eventually, the bit problem is making sure you don't have too much buildup in one place. You wouldn't launch it from LA, which is pretty much designed to store smog, but I'd imagine a place like Cape Canaveral does better at dispersal, and so ozone wouldn't be as much of a worry.
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93143
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Postby 93143 » Mon Nov 16, 2009 5:33 am

I wonder if you could stick a turbine in that loop.

There's an energy supply and a pump. That would make it a power cycle.

But it's a weird power cycle, because it takes on energy at a low temperature and sheds it at a high temperature, like a refrigerator. The difference is the REB heater. You've got a five-part power cycle that...

I need to analyze this. It's too late at night now.


Either way, you need something to drive the compressor and fan, which says either electric motor or turbine. If turbine, you don't need one in the REB/heat exchanger loop, because said loop isn't useful at hypersonic speeds anyway; it tops out at roughly the same speed as the turbine in the engine. If electric motor, how much weight would you have saved by just putting the turbine there instead of in the exchanger loop?

So... never mind?

There's got to be a way to avoid dumping hydrogen at high speeds. But what else can you dump heat to? Use the wings as radiators? I've been down that road before, besides which it's very dangerous to be deliberately heating your structure when flying hypersonic. A design like that could get very heavy and/or brittle very fast...

That reminds me; are huge gigawatt-range turbofans going to have a problem with bird strikes? They happen now and then even with normal-sized engines...

DeltaV
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Postby DeltaV » Mon Nov 16, 2009 6:53 am

93143 wrote:That's what I've been trying to get across to you. Instead of using a REB to heat the air directly, you use it to heat a fluid in a closed loop. This fluid is pumped through a counterflow heat exchanger in the engine, thus heating the air.

OK, speaking only of lower altitude, low-to-moderate speed (non QED-ARC) operation, let's assume that we have a heat exchanger that hermetically separates a REB, in vacuum, from the flowing air (+ any propellant), but still transfers enough thermal energy into the flow to provide sufficient thrust for lift, hover and forward acceleration until QED-ARC territory is reached. Also assume that the REB is magnetically or electrostatically diffused or defocused and that the intercepted power per unit area is low enough to avoid disintegrating the heat exchanger. While I've not done any calculations (it's been many, many years since thermodynamics class, and I've never used it since), intuitively it seems like such a heat exchanger would have to be, at a minimum, white hot, more likely blue (UV?) hot, to get sufficient energy into the flow to lift/propel a massive vehicle. So I'm wondering, what kind of fluid are you considering for the closed loop, a liquid metal such as sodium, or something else? Maybe my intuition is failing to give me a clear picture of just how much energy needs to be transferred to the flow to be practical for flight.

93143 wrote:500,000 lb? A fully shielded 6 GW BFR weighs twice that on its own.

A million pound shielded BFR? (I think less than 6 GW might do, but let's go with it.) Is this a generally accepted value? That's about the max takeoff weight of a 747-8. Polywells are basically just big spherical vacuum tubes. I know, it's mostly shielding and cooling system weight. MSimon and other nukes -- is this a reasonable weight number for full shielding?


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