MSimon wrote:In a rocket, reaction mass is available as shielding.
No, it isn't. The shielding has to still be there at burnout.
And reserves are still useless for this, because (a) the shadow shield will probably be at or near the top of the fuel tank and will thus get drained first, and (b) you might need the reserves.
Far better to just use a dedicated shield.
And for gammas the only thing that counts is mass. (There are some details for low Z species that complicate the calculation but roughly - gammas are shielded by mass).
I think I checked it once, for that specific energy of gamma ray. The post is here:
viewtopic.php?p=10250#10250
IIRC I read the numbers off a graph somewhere, so they may be a bit shaky... Lots of low-Z nuclei in concrete, though...
The shielding doesn't have to be a full 360 degree shield. Only passengers need to be shielded.
Maybe for a big launcher that no one is going to get anywhere near during the launch. A single-core spaceplane would be far easier to use if it had a full 360° shield, though unfortunately the shield is probably too heavy. Ground crews are going to need bunkers...
If you could get the core size down to 6 m diameter instead of 10, the shielding mass would be less than 150 mT... still pretty heavy; more than half the mass of Bussard's notional SSTO...
A large fusion-powered airplane (which is actually what I was talking about) needs to be able to operate from an airport. So it would probably absolutely
need 360° shielding on the reactor core(s).
Remember, you're trying to get more than 6 orders of magnitude reduction in neutron radiation intensity. Taking a 3-metre core radius as a baseline, this means an unshielded core has to be over 3 km away, neglecting the effect of the atmosphere.
For larger clusters the shielding mass efficiency starts going up even if the reactor size doesn't increase. Multiple cores in a single linear sleeve doesn't get you anything, but I calculate that a 5x5x6 cluster (150 cores, 900 GW) would require about 35% as much shielding mass as 150 individually-shielded reactors. So a Hypernova-class launcher might not have as much trouble getting a plausible power-to-weight ratio... not that anyone should be standing close enough to worry about neutrons during the launch anyway...
...but in space the shielding would be very useful, as you could get near the thing while it was under power. Maybe to land a starfighter in one of the docking bays... Another nice thing about such a large cluster is that it gives you fine-grained throttle control even if individual cores can't be throttled at all...
Yes, I know I sound like I'm smoking something. No, you can't have any...
DeltaV wrote:If no heat exchanger material could withstand the REB
That's probably a safe bet.
...
Free electron lasers would probably make great Polywell-powered weapons. For propulsion applications, the trouble is that a laser beam doesn't get absorbed that easily by air; a fair chunk of it will keep on going and hit something solid, with tragic results.
I was thinking of REB-heating an intermediate fluid, which would then be run through the heat exchanger in the engine at extreme temperatures.
It doesn't have to be a thermodynamic cycle, because you don't have to get any power out. If you want power out, you take it from the Brayton cycle represented by the heat-exchanger-driven airbreathing engine.
You could even use a refrigeration loop to dump waste heat into the intermediate fluid before it gets REB-heated. If the engine heat exchanger is a counterflow design, it might get the final temperature of the intermediate fluid tolerably low (yes, counterflow is heavier, but the reduced refrigeration requirements should help make up for that; this is quite a refrigerator we're talking about here)...
Considering that the energy in question is used rather than dumped, perhaps that would make it a heat pump rather than a refrigerator? This is essentially a zero-mass-overboard All Regeneratively Cooled design, at least until things start getting too fast and hot for it to work properly...