D Tibbets wrote:Uranium is very heavy. I don't know if any fission could be utilized and what it's contribution would be to the temperature of the plasma.
I don't think there's much doubt some of the fission energy could be realized, the question is how much. In Winterberg's usual monomaniacal fashion this would be equivalent to the secondary and tertiary stages of the Teller-Ulam design. IIRC, most energy derived from that is the fission.
D Tibbets wrote:But from an ISP, efficiency viewpoint the ISP is inversely proportional to the square root of the mass of the ejecta. ie: trituim/ helium 3 fusion products have a mass of 3, and lithium a mass of 6 or 7. Uranium or uranium fission products have a mass of ~ 100 to 238. The square roots of those numbers is ~ 2 to 2.5 versus 10- 15. This means that if uranium is used as a major component of the ejection mass, then the ISP would be ~ 6 times less. Instead of an ISP of perhaps 2400, it would be ~ 400 (at the same temperature)- not good . There might be more power- ie: more thrust and thus faster acceleration, but the total acceleration would be ~ 6 times less.
cites an Isp of 5140 seconds on the "standard" Slough FDR, but yes, the increased foil mass alone drops the Isp down to the solid core nuclear rocket range or so, 800-1000 seconds. Tho the additional fission energy would boost the Isp again before the increased liner "propellant" mass cut it.
D Tibbets wrote:Near the bottom of the link, some of the numbers are interesting. Confinement times of ~ 100 micro seconds- or perhaps up to several milliseconds at densities of ~ 10^22 particles/ M^3 (?) are promising. But the temperature of ~ 2.3 KeV still needs to be increased perhaps 10 fold or more. The D-D fusion cross section is ~ 500 to 1000 times less than the cross section at the higher temperature. Just to exceed the Bremmstruhlung losses with D-D fusion fuel a minimum temperature of perhaps 8-9 KeV is needed. The rocket engine is not a power source . Apparently solar panels provides the power.
Yes, tho fission reactors can as well.
D Tibbets wrote:The engine also does not appear to match the VASMIR ISP capabilities, but perhaps it provides greater thrust levels (pounds of thrust), and acceleration rates are very significant when talking about low thrust engines and travel time. As an externally powered rocket engine this may be closer to the best compromise. Note that Miley has proposed a Fusor as a rocket, better ISP than an ion (Hall effect?)rocket, perhaps a little more thrust and not much, if any weight penalty. Miley's proposal is for maneuvering thrusters on a satallite. I don't know how much more thrust this FRC or VASMIR can provide (limited by the input power from solar sails or thermal nuclear batteries, fission reactor, of ideally a fusion reactor like Bussard's concept, or a successful DPF or FRC system with net positive energy production.
The pure-IEC non-Polywell engines I've seen papers on all seem to be low-thrust and turn on future technical refinements that would make or break polywell as well - and polywell rockets in ARC or CSR modes well surpass the performance of the notional pure-IEC rockets.
D Tibbets wrote:Note that by my understanding, a Polywell operates at up to ~ 10 times greater density and perhaps 10 times or more confinement times as those mentioned in the link. It may also have significantly greater capacity to heat the plasma to higher temperatures, benefiting the triple product considerations, and the capacity to exceed the losses due to Bremmstruhlung. Not only is there upper limits where Bremmstruhlung losses overtake fusion rates (especially important in the P-B11 reaction), but there are also lower limits where the lower fusion cross sections trail behind the Bremmstruhlung rates. Bussard gave values of ~ 5 KeV for D-T fuel, ~ 12 KeV for D-He3 fuel, and I extrapolate ~ 8-9 KeV for D-D fuel. I suspect the minimum for P-B11 fuel is closer to 20 KeV or more. Consideration of thermalized vs non thermalized conditions complicates the situation further.
I was merely speculating on a possible way to quickly and cheaply bootstrap higher performance from a promising rocket engine design. Replacing lithium foils with uranium or thorium foils just isn't a difficult step given this design, and fission-fusion hybrid cycles are "well proven
" shall we say.