If only it were that simple...Unfortunately, delta v to orbit increases with higher Isp as gravity losses increase (propellant mass decreases at a slower rate), and drag losses increase with lower density propellants. LH2 NTR is the worst case for these factors.Also low density propellants require much larger tankage and bigger engines, using much more of that remaining GLOW fraction than a high density, but lower Isp, would.You really need to examime each case using reasonable aindividual ssumptions for each propellant, for things like: engine T/W, tank mass, delta v to orbit.
I've played with nuclear air-breather numbers before as well and what I found was that the crucial factor is the engine T/W at the switch to full rocket mode as that sizes the core NTR which flows through the design. With that in mind there are a couple of engine designs you've missed. Adding Lox-augmentation that tapers off through the trajectory after the transition to full rocket mode allows for a smaller core as well as atmospheric compensation in the nozzle and relatedly a SABRE precooled design that also does ACES and collects air for that thrust augmentation later in flight. Also, assuming this is technically possible, a NH3/H2 slush fuel is possibly slightly more efficient. The key design question is however, manned or unmanned. Unmanned requires very little shielding and would seem very build-able while manned requires a lot and is potentially more marginal depending on core and shielding technology.
I was surprised to find just how ridiculously well a simple water NTR performs once you move from a solid-core to an optimized pebble-bed.
NH3 should be much better than H2O as it decompose to N2 and H2
Assumed specific impulses:- Hydrolox: 366 s at SL, 452 s in vacuum- Methalox: 334 s at SL, 382 s in vacuum- Kerolox: 282 s at SL, 348 s in vacuum
Quote from: sevenperforce on 04/16/2017 03:59 PMI was surprised to find just how ridiculously well a simple water NTR performs once you move from a solid-core to an optimized pebble-bed. Provided of course you can build a flight weight PBR.
Quote from: sevenperforce on 04/15/2017 01:32 PMAssumed specific impulses:- Hydrolox: 366 s at SL, 452 s in vacuum- Methalox: 334 s at SL, 382 s in vacuum- Kerolox: 282 s at SL, 348 s in vacuumDon't look like a fair comparison. Using FRSC for hydrolox,FFSC for methalox and GG for kerolox (without mentioning it).Difference between kerolox and methalox is MUCH SMALLER when using an equivalent engine cycle and equal pressure, and still considerably smaller when using optimal cycle for each. (if FFSC is impossible for kerolox, then use ORSC for that, instead of GG)
Quote from: john smith 19 on 04/16/2017 08:36 PMQuote from: sevenperforce on 04/16/2017 03:59 PMI was surprised to find just how ridiculously well a simple water NTR performs once you move from a solid-core to an optimized pebble-bed. Provided of course you can build a flight weight PBR.And also provided it doesn't melt down. Both Earthbound pebble beds and propulsion prototypes were.... problematic at best.
To pile on this a bit: only the hydrolox numbers have been demonstrated in a single engine - RS-25. The others are probably not feasible with a single engine since they require expansion ratios of 150 or better. A double-expansion nozzle might get close. I'm not sure what aerospikes get in practice, but they might be close also.I suppose that's the least of the technical concerns when talking about nuclear SSTOs, though...
Yeah, sorry; I just ripped isps from the SSMEs, the Merlin D, and the Raptor (projected). Is there a better list?
You might be surprised. The liquid-metal TaHfC pebble-bed reactor with a water-based exhaust of 5.4 km/s has a bare-bones TWR of 260:1, including the weight of the reactor, the turbopump, the nozzle, and the gimbal system, at a 12 kN vacuum thrust rating. Of course, that's without safety margins or shielding. But it's not outside the realm of possibility.Using liquid water makes it very thrusty.
The primary problem with the NERVA prototype was, if I recall, hydrogen corrosion. Not a problem (or not as much of a problem) with water. Meltdown isn't a problem for TaHfC encapsulation; the fuel is supposed to melt. Casting the fuel pebbles might be rough, though.
Quote from: sevenperforce on 04/17/2017 04:06 PMYeah, sorry; I just ripped isps from the SSMEs, the Merlin D, and the Raptor (projected). Is there a better list?Well I suppose people might be arguing for using the RD-180 instead of the Merlin 1D. However the trade is that the TWR of M1D is more than 2x better than that of RD-180. It would be reasonable to look at both, I think.
Quote from: Katana on 04/17/2017 03:17 AMNH3 should be much better than H2O as it decompose to N2 and H2NH3 + H2O2 or O2?Isp?
The idea was to create a restriction on SSTO designs. E.g., even if you had an altitude-compensating engine with full SL and vac performance to work with, you'd STILL have to beat this particular fuel fraction to make SSTO.To that end, though, I should probably use the max isp of each engine type.