Rocket thrust

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

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kunkmiester
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Postby kunkmiester » Wed Nov 11, 2009 6:21 am

cryonitro is cheaper. Provided you can get the same thrust with the same weight, it might be preferable, since it would need a smaller tank to carry the same weight.

My concern with air and neutrons was basically the same as using water. Even at .1% or whatever, you're still getting a decent neutron flux out of a GW+ reactor. I suppose the lighter elements in air would have less trouble with making nasty stuff, but no one addressed that.

Rather than messing with a second set of engines, use the reactor to run electron guns or whatever to provide the head for the engines. This means you don't have to carry fuel at all. If you can set up a bypass to get a ramjet at higher altitudes and speeds, you can get a ways up before you have to go to internal fuel.

I've still not sat down and figured out what I was originally asking about. :roll:
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DeltaV
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Postby DeltaV » Wed Nov 11, 2009 7:15 am

D Tibbets wrote:If you need to run the superconducter equipmet for longer times (deorbit and landing after up to several weeks in orbit) the increased storability and density and heat capacity (?) of liquid helium or especially liquid nitrogen might serve better.


kunkmiester wrote:cryonitro is cheaper. Provided you can get the same thrust with the same weight, it might be preferable, since it would need a smaller tank to carry the same weight.


Assuming, of course, that your superconductors work at liquid nitrogen temperature. If it was vertical takeoff via lift fans, I'd also want it to be vertical landing via lift fans. For a shuttle-style reentry, a powered transition to hover at the end would be WAY less energy needed than it took to get to orbit. Allowing powered atmospheric flight after reentry might not require too much more energy, either. So I'm thinking the amount of cryogenics needed for descent would be a lot less than needed for ascent.

93143
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Postby 93143 » Wed Nov 11, 2009 7:50 am

kunkmiester wrote:Rather than messing with a second set of engines, use the reactor to run electron guns or whatever to provide the head for the engines. This means you don't have to carry fuel at all. If you can set up a bypass to get a ramjet at higher altitudes and speeds, you can get a ways up before you have to go to internal fuel.


Once you start the reactor, you need a certain LH2 flow rate to cool it. I suppose you could subcool the LH2 and transfer the heat into the tanks prior to main engine startup, but unfortunately the energy transferred is rather large; given the waste heat from a 6 GW reactor, 100 tonnes of LH2 subcooled by 10°C would reach the boiling point in about 10 seconds.

If you could throttle the reactor (by either reducing the voltage or easing up on the POPS), and use an external air-cooled fin system or something for dumping heat, it might work...

...but then you still have the problem of ozone generation at ground level. Or the problem of having to lug hundreds of megawatts worth of voltage stepdown equipment all the way to orbit and back. One or the other. Unless you can think of a way to use the power at full voltage that doesn't generate ozone and doesn't require onboard propellant...

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Postby KitemanSA » Wed Nov 11, 2009 12:01 pm

kunkmiester wrote:cryonitro is cheaper.
LOX is cheaper still, has a greater heat of vaporization, and has a number of other uses. If eutectically mixed with LN2, it also has a lower freezing point and boiling point. IIRC, the eutectic is about 20% N2 + 80% O2.

Mindblast
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Postby Mindblast » Wed Nov 11, 2009 3:54 pm

I was contemplating about the same things a while back for an Addon i'm working on for Orbiter. At first i wanted to use Superconducting electric ducted fans too but then it occured to me that the equipment to step down the voltage for a couple hundred megawatt would be insanely heavy. The alternative i thought of was to design a polywell reactor that essentially has a heat mode, where the alphas are not used to generate electric power but impact the collector grids at full speed, creating heat. This could then power the fans through a cooling loop and ordinary gasturbines. Not sure if it is feasible but seeing that i.e. the turbines of the A380 engines are somewhere around 50MW shaft power at a couple tons weight or so it seems doable.. at least i thought it would be a more promising way than the electric fans.

clonan
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Postby clonan » Wed Nov 11, 2009 5:59 pm

Mindblast,

Would you really want all that spattering on your grids?

kunkmiester
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Postby kunkmiester » Wed Nov 11, 2009 7:01 pm

We basically use jet fuel the say way right now. A "small" turbine runs the great whopping fan at the front of an airliner's engine, which produces most of the thrust.

Pop Sci had an article about thorium, and some quantum change in the nuclues that produced gamma rays but didn't involve fission--can't remember the specifics. They were planning to put the gammas in a ceramic heating element in an turbine engine, which would heat the air and produce power without burning anything.

I think it's the ARC engine that you'd be using in a similar manner, using the compressed air from the front of the turbine instead of onboard propellant. Skylon's Sabre engines are designed to run from start of runway to orbit, a similar engine running off an ARC cycle instead of burning fuel should be fine.

It should be easy to throttle a polywell--you just control your ion feed. You start with just enough to run the reactor, and go from there. Might be less efficient, but then everything is less efficient when it's not running in it's comfort zone.

We have at least one superconductor that runs at LN2 temps, I'd imagine that there are more, and since this is actually a useful temperature range due to the economies of LN, I'd imagine that any commercial/industrial research is focusing on this area. So, I'd imagine that by the time you're looking to build a fusion powered shuttle, you'll have the magnets you need.

The Sabre engine is supposed to supercool the air coming in, and enrich the oxygen content to some extent, right? How hard would it be to have a part of another engine that scavenges liquid air to use for coolant?
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MirariNefas
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Postby MirariNefas » Wed Nov 11, 2009 7:57 pm

kunkmiester wrote:Pop Sci had an article about thorium, and some quantum change in the nuclues that produced gamma rays but didn't involve fission--can't remember the specifics. They were planning to put the gammas in a ceramic heating element in an turbine engine, which would heat the air and produce power without burning anything.


I think I remember that. They called it quantum nucleonics, and needed specific nuclear isomers of some sort. They thought the moon would have a large quantity of fuel for this process for some reason.

93143
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Postby 93143 » Wed Nov 11, 2009 9:43 pm

kunkmiester wrote:It should be easy to throttle a polywell--you just control your ion feed.


With a given magnetic field, and a given electron temperature, you have a fixed plasma density if you want to run in wiffleball mode.

You could add more hydrogen and less boron, I suppose... Changing the voltage slightly (moving off the cross-section peak) and adjusting both ion feeds accordingly could also work...

If you're willing to waste a lot of overhead on cooling due to a high bremsstrahlung power fraction, you could probably run the reactor quite low in terms of net power as a fraction of rated output, simply by detuning it like that...

Mindblast
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Postby Mindblast » Wed Nov 11, 2009 10:04 pm

clonan wrote:Mindblast,

Would you really want all that spattering on your grids?


Well i'm not sure.. i mean the usual figures for a polywell are like 20% thermal power anyway. I guess a lot if not most of that would be on the grids, so obviously they would have to be able to handle like 1.2GW thermal load in a 6GW reactor. For running some fans the first 5-10 minutes of the flight you don't even need 1.2GW power.

If that fails.. maybe it would be possible to put a REB/ARC heater into the coolant loop to superheat the coolant using the electricity generated by the reactor.

93143
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Postby 93143 » Wed Nov 11, 2009 10:55 pm

Mindblast wrote:
clonan wrote:Mindblast,

Would you really want all that spattering on your grids?


Well i'm not sure.. i mean the usual figures for a polywell are like 20% thermal power anyway. I guess a lot if not most of that would be on the grids, so obviously they would have to be able to handle like 1.2GW thermal load in a 6GW reactor. For running some fans the first 5-10 minutes of the flight you don't even need 1.2GW power.


I think he means "sputtering". The high-energy alphas would liberate large quantities of atoms from the collector plates, raising the population of heavy neutrals in the vacuum chamber at a rate substantially higher than the alpha flow rate. This could poison the reaction and contaminate sensitive parts.

If that fails.. maybe it would be possible to put a REB/ARC heater into the coolant loop to superheat the coolant using the electricity generated by the reactor.


...why didn't I think of that?

Basically a boiler/combustion chamber with electric heating instead of combustion. You'd have to be careful to design it so it didn't destroy itself, but that's true of a reusable QED rocket too. You could use it either as a heat source for a high-efficiency thermal power cycle, or as a way to heat incoming air without generating ozone, or both.

Any heat used to raise the temperature of incoming air doesn't have to be bound by the inefficiencies of a power cycle; the power cycle is the airbreathing engine. It seems to me that everything up-process of that could be all regeneratively cooled without expending any internally-carried propellant. Even refrigeration power could be dumped into the heating chamber of the engine, so it isn't a loss. Now all we need is a heat exchanger that can handle the required temperature without being too heavy...

This works much less well at high speeds, because the heat exchanger needs to operate at high temperatures and airspeeds (possibly supersonic), so drag probably kills it even if it doesn't outright melt. (Actually, now that I think about it, it would probably melt first.) But at high speeds you should be at high altitude, and thus be able to explicitly REB-heat the air without worrying about ozone, and then just start dumping high-temperature GH2 coolant into it.

93143
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Postby 93143 » Thu Nov 12, 2009 3:00 am

I estimate a 6 GW reactor would require at least 350 tonnes of shielding, if it is to be operated near civilians. This represents a 3½" shell of lead on a ~10 metre sphere, to mitigate gamma radiation (about 1 cm of lead per factor of 10 reduction) from a thin boron-10 neutron absorber with maybe 6" of water (50 tonnes) ahead of it as a moderator (60x10^12 n/s coming off the reactor). If my calculations are correct, concrete would be more than twice as heavy as lead, because of the decreased stopping power against half-MeV gammas.

Is MSimon around? Does that sound halfway plausible? Or would you want more margin in a passenger vehicle?


The thing is, reducing the output power doesn't substantially decrease the amount of shielding required. It's logarithmic, so the higher the reactor power, the more mass-efficient the shielding is.

This is not for 70-seat regional jets. I'd love to see a truly monstrous fusion-powered aircraft, hypersonic or otherwise, that can afford to pack one (or two! or three!!) of these, and actually uses full power at takeoff...

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Postby kunkmiester » Thu Nov 12, 2009 5:03 am

One of the first uses will probably be to put the polywell in a 747 or airbus, and run electric fans on the pylons, instead of turbofans.

If polywell works out and scales well, this might actually be a reason to go for a nitrogen cycle, as mentioned in the sci fi question thread. Would a 5 GW boron reactor with shielding outweigh a nitro reactor that doesn't need shielding?
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93143
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Postby 93143 » Thu Nov 12, 2009 8:30 am

No shielding on a net-power fusion reactor is probably an impossible dream. Remember that p-11B, properly run, produces neutrons at a rate eight orders of magnitude below D-T. That's pretty low. And at that tiny neutron rate, it still requires fully half as much shielding as a D-T reactor would. Logarithmic.

Unless there genuinely is no way for p-15N to produce neutrons, either by impurities or by side reactions or by structural impingement or whatever, you will need shielding.

DeltaV
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Postby DeltaV » Fri Nov 13, 2009 11:25 pm

DeltaV wrote:If Polywell is ready before Mach-effect, then:

Superconducting electric lift fans (VTVL) from ground to low atmosphere,
transitioning to superconducting electric turbines from low to medium atmosphere,
transitioning to ram-air arc-jet from medium to high atmosphere,
transitioning to onboard propellant arc-jet from upper atmosphere to orbit.

Modes blended where appropriate.

All powered by direct-converted alphas.


Instead of "arc-jet", which is low-voltage, high-current, ohmic resistive, I should have written "Quiet Electric Discharge (QED) All Regeneratively Cooled (ARC) engine", as with a Relativistic Electron Beam aimed into a high-density working fluid, with magnets surrounding, per Bussard.

So, if light weight, high power, high-to-low voltage convertors are just wishware (MSimon?), let's (ignoring neutron/radiation shielding for now) keep the above high altitude modes and rethink the low altitude modes to somehow use either the alphas directly or the high-voltage REB (without venting any ozone).

93143 wrote:You'd have to be careful to design it so it didn't destroy itself, but that's true of a reusable QED rocket too. You could use it either as a heat source for a high-efficiency thermal power cycle, or as a way to heat incoming air without generating ozone

93143 wrote:Now all we need is a heat exchanger that can handle the required temperature without being too heavy...

It would be nice to keep the Polywell/direct-conversion system sealed (as far as possible) and simply use it in all flight modes as a high-voltage "battery" driving the REB. To avoid ozone at low altitude we need a heat exchanger that keeps the REB and air/propellant separated. Maybe a sealed, REB-scanned, "channellized" heat exchanger made out of thin-walled metal and ceramic, that heats the air/propellant without ozone and is moved out of the "combustion chamber" when QED-ARC kicks in at higher altitude. A conical or cylindrical heat exchanger might work, with the REB entering through a single, sealed tube and being either scanned via a rotating magnetic field or circularly fanned-out to the channels via a static field. A rectangular geometry would require some fancy ductwork.

The simplicity is appealing (practical is another matter)... REB heating of air/propellant for all four modes (I'm not giving up on VTVL!), with no intermediate closed-cycle thermal loops as it seems would be needed for low-voltage superconducting motors.


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