In reply to Robert Lynn's message of Wed, 6 Jun 2012 22:49:45 +0100: Hi, [snip] >Sadly I do understand, I am just not blind to the implied engineering >requirements. *40MW/kg !!!!!* The highest power to weight machines >(outside of bombs) that humans have ever build were the space shuttle main >engines, they did about 3MW/kg utilising a supply of LH2, the best possible >coolant, to keep the engine from melting. That is a luxury you do not have >with fusion in an ultra-high Isp engine.
Actually, a bomb is not a bad analogy. The difference would be that this would a "slow burning" bomb rather than a high percussive device. > >At 40MW/kg (and would need to be much higher than that if you wanted any >payload) even if your engine miraculously manages to eject 99% of that >energy in the exhaust you still have somehow come up with a way of >radiating 400kW for every kg of spacecraft from your engine cooling system. Much greater than 99% actually. > >And any hot fusion engine requires a driver to initiate the fusion, >typically recirculating 0.1-10% of the fusion power output. This one may be self sustaining, requiring no driver, and is not based on hot fusion. Power output regulated by rate of fuel supply. > So being >insanely optimistic with a Q of 1000 you are recirculating 40kW/kg of space >craft power (4GW for a 100 tonne craft), and somehow cooling that power >collection and driver systems as well, all for some small fraction of your >total mass budget. > >Then there are the shielding requirements for the vehicle and occupants who >are sitting next to this multiple TW output engine and its incredible gamma >ray (at minimum) output. If you use D depleted Hydrogen (i.e. near pure Protium), then there will be very little gamma output. Furthermore a small shield just forward of the reactor will cast a large shadow over the rest of the craft. In fact the current multi month missions to Mars that are under consideration may run a larger risk of radiation exposure (e.g. from Solar flares) than a two day fusion powered mission would. > >Which are just a couple of simple examples as to why anyone with even >cursory knowledge of actual engineering knows that what you suggest is so >far beyond the feasible that it is quite simply ridiculous. You may turn out to be correct, but it wouldn't cost much to construct a tiny model to see how well/poorly it performs. You seem to be under the impression that I want to construct such a craft immediately. That would be ridiculous. It's an *ultimate* goal, not an immediate one. But perhaps not quite as impossible as you seem to think. > >100 years from now it might be possible for a spacecraft to achieve 1% of >the power and acceleration levels you suggest at an Isp of 7.5e6m/s, but I >wouldn't bet on it. If we don't try new things, it will indeed take a hundred years, or longer. [snip] Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
