Posted: 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.
a discussion forum for Polywell fusion
https://www.talk-polywell.org/bb/
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.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.
Bah. Google "Project Pluto"...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.
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.Magnetically diffused REB-heated, hermetically-sealed, rotary Stirling engines directly driving the lift fans and turbines!
Doesn't that lead you right back to the problem of how to heat the airstream?93143 wrote:I don't see what's wrong with a Brayton cycle, using the airstream as a working fluid...
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).93143 wrote:Plus you don't have to deal with multiple GW of electricity, which is difficult even at low voltage.
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.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 think that is the lifetime dose. IIRC 2 rem is the yearly occupational limit.isn't the occupational limit 50 rem per year?
Sounds right.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.
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.DeltaV wrote:Doesn't that lead you right back to the problem of how to heat the airstream?93143 wrote:I don't see what's wrong with a Brayton cycle, using the airstream as a working fluid...
500,000 lb? A fully shielded 6 GW BFR weighs twice that on its own.DeltaV wrote: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).93143 wrote:Plus you don't have to deal with multiple GW of electricity, which is difficult even at low voltage.
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: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.
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?93143 wrote:500,000 lb? A fully shielded 6 GW BFR weighs twice that on its own.