Skipjack wrote:Uri assumes D+He3 as the idea fuel for a SFS- Z- Pinch rocket engine. The Q and T/W are higher than with an engine using PB11 (which they also examined).
You are totally right, indeed the pB11 has theoretical worst characteristics as a fuel, but is also the fuel that has been less investigated so far; I do believe it is still the field where "surprises" can come out once some real experimental test start to be done.
I didn't even take into consideration 3He route because right now on earth the only 3He source is from the natural decay of Tritium (which has about 12 years half life) thus is not a fuel that we have ready available and we will not have it in the coming future, so whatever engine we will use in the first stage of solar system exploration will have to be based on available commodities.
Regarding the paper you are referring to, I believe is this one, correct me if I am wrong:https://www.nasa.gov/sites/default/files/atoms/files/niac_2016_phasei_thomas_fusiontopluto_tagged.pdf
For the radiators mass I am not convinced by the assumptions in their paper.
They assume to reject to space only about 25% of the fusion power generated with a D-3He fusion.
They assume radiator weight based on a new carbon-carbon radiator with 5 times less mass than actual technology (but still is just theoretical).
They assume (very optimistically) that both sides of the radiators will be in a optima radiate situation.
These assumptions are offsetting the needed mass of radiators by a factor of 10 and the needed area by a factor of 3 compared to existing technology that would require about 1400 Kg of mass and 140 M2 for the radiators; and this just to get 5 N of thrust.
Now let's imagine the mass and area needed for the radiators for a 50 KN engine for a human rated vessel.......
I greatly hope to be proven wrong in the immediate future, but radiators technology for space applications is in my opinion one of the most underestimated and underfunded field of space research but whose importance I place on the same level as the reactor. That is, unless we have a reaction that can get rid of most of these thermal issues.
And if you look it under this light p-B11 fuel starts to look very attractive, even if it is greatly more difficult to ignite.
Skipjack wrote:One could also trade some Isp by injecting extra cold fuel into the flow or in between pulses to cool the engine and generate extra thrust (or more continuous thrust if pulse frequency is indeed a problem). That would make it work a bit like a "conventional" nuclear thermal engine.
One could double or triple up the number of reactor cores to increase the pulse frequency that way.
I like both ideas, even if the thermal dissipation requirements will still be quite big there could be a sweet spot somewhere that could make the design more affordable.
Worst case, let's fit the ship with a set of NT reactors, find some water rich asteroid to mine and just steampunk our way around the solar system. Maybe not greatly efficient, but what an amazing view would such a ship be!
A society of dogmas is a dead society.