Rocket thrust

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

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Postby 93143 » Mon Nov 16, 2009 7:40 am


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Postby IntLibber » Mon Nov 16, 2009 9:53 am


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Postby kunkmiester » Mon Nov 16, 2009 5:44 pm

Evil is evil, no matter how small

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Postby 93143 » Mon Nov 16, 2009 6:12 pm

Uh...

One SSME at 104.5% develops about 5 GW of jet power in vacuum. The combustion power (and thus the energy per second needed to generate the H2 and O2) is more.

That gives you 488,800 lbs of thrust, or roughly enough to lift a 10 m diameter lead shadow shield capable of protecting the astronauts from gamma rays during the burn... and nothing else. Forget the reactor itself. Forget the electrolysis system. Forget the structure of the vehicle, and the payload, never mind the huge tank of water...

Unshielded, the gamma rays deliver a potentially fatal dose in under a minute. From 50 m away. You'd need about three metres of water (at a guess) to offer the same protection as the 10" lead shield; maybe more (lead is slightly better at gamma shielding than its mass indicates). Reserve propellant might do it, but this time it'd really better be an emergency if you dip into the reserve...

Need I remind you that the 10% propellant reserve in this example weighs more by itself than the thrust available from the rocket?

The upshot is that it's not even close to worth it, for the tankage mass you'd save... 450 seconds isn't really all that good; there's a reason we're having so much trouble getting into orbit with hydrolox rockets...


I suppose I should redo the analysis for a large reactor cluster, to see where the breakeven point is (if there is one). Either way, it doesn't look good for Polywell-powered launch vehicles that take off vertically on internal propellant - to get more thrust you'd have to decrease the Isp...

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Postby 93143 » Mon Nov 16, 2009 6:23 pm

It would be really awesome if rnebel showed up right about now and told us that the gamma-emitting branch probability is an artifact of the thermal plasma assumption, and that a Polywell is expected to produce gammas at a rate five orders of magnitude lower...

But I doubt he will...


And really, even if he did, a vehicle powered by a single reactor would still not be remotely capable of vertical liftoff on internal propellant at a useful Isp...

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Postby 93143 » Mon Nov 16, 2009 6:39 pm


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Postby DeltaV » Mon Nov 16, 2009 7:47 pm


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Postby 93143 » Mon Nov 16, 2009 9:04 pm

Last edited by 93143 on Mon Nov 30, 2009 7:53 pm, edited 1 time in total.

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Postby DeltaV » Mon Nov 16, 2009 9:29 pm


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Postby 93143 » Mon Nov 16, 2009 9:54 pm


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Postby 93143 » Sat Nov 28, 2009 4:45 pm

Well, after doing a dose calculation, it seems to me that 1½" of lead should suffice for the neutron absorption gammas. However, this will probably not suffice for the bremsstrahlung, which may require ~2-5" depending on energy spectrum (based on attenuation of 300 MW of ~200+ keV X-rays to 2 rem/year at 5 m from the core, or 11 orders of magnitude). If the gamma rays are as frequent as the Wikipedia article on aneutronic fusion says, they still dominate, requiring about a foot of lead. I can imagine the rate being distribution-dependent, though...

I'm going to try to conceptualize an SSTA with unlimited supersonic cruise, using optimistic assumptions. I'll assume POPS and/or strong magnets can get the 6 GW core down to 3 metres in radius, and that it only needs 3" of lead shielding on top of 6" of water and a thin layer of boron-10. This yields a shield mass of about 115 tonnes.

The vehicle might be designed to take off vertically using rotated jet engines, Osprey-style - but with four of them rather than two. I wonder if it's possible to have the blades of a very large turbofan retract, pivoting into slots in the annular wall (or the shaft, but that might be an even worse idea structurally), the idea being to clear the bypass for QED ramscram operation... almost certainly the fan would have to stop before attempting this, which requires controllable blade angle at the very least...

...naw, that's a dumb idea. To land, you'd have to re-extend them and lock them in place while flying at a substantial forward speed... darn it, that's a perfectly good bypass going to waste there...

The wings could double as high-temperature radiators. I will assume that only about 700 MW of waste heat (~10%) needs to be dealt with, but since the thing needs to operate in space it needs to be capable of dumping that in the absence of atmosphere, while in high-Isp mode.

There's a major tradeoff between radiator size and reactor efficiency, because pumping the heat up to a higher temperature uses a lot of power that is then unavailable for anything useful. Given a 350°C primary reactor cooling loop, a radiator system at 1200 K would require (I think) over 1 GW of refrigeration power per core, in the form of shaft power to high-temperature compressors. The use of turbines on the cool side could improve this somewhat, but they'd need to be capable of dealing with a saturated two-phase flow of liquid metal...

I don't know how feasible it is to significantly increase the temperature of the primary cooling loop, but it would sure help... I'd really like to use lithium as the refrigerant (it's lighter and can carry more energy), but its boiling point at 1 bar is over 1600 K - great for radiator size, less great for refrigeration power...

Question: Given that we're talking hundreds of tonnes here anyway, how much exactly would it weigh to have a gigawatt of voltage step-down capacity from whatever final voltage the direct conversion and conditioning system puts out?

Or would it be better to try to produce an electric motor that can run on 1.5 MV?

This is what happens when I watch soft sci-fi. Maybe I should keep SSTO in mind as a fallback in case I can't make this work...

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Postby DeltaV » Sun Nov 29, 2009 12:16 am

Last edited by DeltaV on Sun Nov 29, 2009 3:22 am, edited 1 time in total.

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Postby D Tibbets » Sun Nov 29, 2009 1:03 am

To error is human... and I'm very human.

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Postby ANTIcarrot » Thu Dec 03, 2009 2:12 am

Some light reading material: Half Way To Anywhere, The Rocket Company, Space Technology, The High Fronter, Of Wolves And Men, Light On Shattered Water, The Ultimate Weapon, any Janes Guide, GURPS Bio-Tech, ALIENS Technical Manual, The God Delusion.

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Postby 93143 » Thu Dec 03, 2009 5:17 pm

You can calculate the minimum power required for a given thrust/Isp combo by simply computing the jet power:

(F [kgf])/(Isp [kgf/kg·s]) = mdot [kg/s]

0.5*mdot*(g*Isp)^2 = P [kg·m²/s³]

For F = 50,000 kg and Isp = 1400 s, this gives a jet power of 3.37 GW.

At the quoted efficiency of 28% (from your first linked paper), this requires almost exactly the total output of two 6 GWe BFRs.

This is why I want to go airbreathing for most of the ascent. Maybe carry LOX for orbital insertion, if the subsequent REB heating doesn't just dissociate the H2O...

It would be nice to at least be able to land on the Moon. This requires a minimum vacuum T/W of maybe 0.2, which is pretty stringent for a fusion vehicle. I don't want to have to go to shadow-shielding, but I may have no choice...


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