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Prompted by a lengthy discussion over on @KerikBalm's "near-future scifi" thread, I created a rather beefy Excel table to model the performance and fuel fraction of a nuclear-thermal turbocharged ramrocket engine, to get an idea of what might be possible for SSTO applications. The spreadsheet worked so well that I expanded it out to provide fuel fractions for a wide range of engine types using various propellant combinations If you want to build an SSTO, you've gotta be able to fit into these fuel fractions. No getting around it. These are, within error, the absolute minimum fuel fractions you'll need to make orbit. If the table says a given fuel fraction is 85.5%, then you've gotta fit your engines, tanks, airframe, and payload (plus margins and recovery hardware) into the remaining 14.5% of GLOW. Note that I didn't even include the values for a "true" airbreather (e.g., turbojet/ramjet/scramjet), because the thrust losses of combusting the airstream while still trying to accelerate make net performance far, far worse than even a pure rocket design. With no further ado, here's the table! Fuel fractions for SSTO Precooled Turbocharged Air-augmented Rocket only NTR (H2O) N/A 72.3% 73.2% 83.1% NTR (LH2) 47.9% 48.9% 49.7% 63.7% Hydrolox 79.8% 80.7% 81.5% 89.0% Methalox 85.5% 85.9% 86.6% 92.3% Kerolox N/A 89.1% 89.7% 94.3% Visually: Each SSTO ascent profile is constructed around a base assumption of 7.8 km/s to orbit, plus 750 m/s of gravity drag and 750 m/s of air drag. The NTRs are Tantalum Halfnium Carbide pebble-bed reactors operating at slightly over 4400 K; basically the best thing we could actually build with modern tech. Assumed specific impulses: NTR (H2O): 469 s at SL, 555 s in vacuum NTR (LH2): 820 s at SL, 971 s in vacuum 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 Additional notes... Rocket only. This is provided mostly for comparison. It is assumed that a rapid climb is used to leave the atmosphere as soon as possible, with peak specific impulse and zero drag being reached around 2 km/s. This design invariably has the worst fuel fraction but allows the highest TWR engines. As mentioned above, altitude compensation is assumed; specific impulse climbs linearly to 2 km/s before plateauing at the vacuum value. Air-augmented. This is an optimally designed intake shroud/duct with area behind the duct for expansion. A 22% static thrust boost is assumed due to pressure entrainment. Thrust augmentation reaches 100% around Mach 1.75, then begins to drop around Mach 6, decreasing to zero at the exhaust velocity of the actual engine. Aerodynamic drag is higher, at 800 m/s, and is spread out over a larger range of airspeeds, with vacuum specific impulse being reached much later. This represents a large fuel fraction gain for a fairly modest decrease in engine TWR. Specific impulse starts at slightly higher than the vacuum isp, then rises rapidly before dropping gradually. Turbocharged. This adds a single-stage compressor fan to the shroud inlet. The increase in static thrust is substantial, allowing a physically smaller engine, but there is only a fractional improvement in net fuel fraction, as the fan is only useful to about Mach 2. Because the fan intake is more demanding than a ram intake, the design and vehicle integration may prove to cost significantly more in dry mass than it saves. However, the turbocharger can (in principle) be used alone as a ducted fan for a hover-light landing, which is a nice and very efficient advantage if you're looking for that (e.g., with an integrated-cabin manned launch vehicle). Aerodynamic drag increases to 850 m/s. Precooled. This is the design employed by SABRE, albeit without combustion of the airstream. Precooling the intake air allows the compressor fan to operate up to Mach 5 with hydrogen and Mach 3 with densified methane, with a corresponding improvement in fuel fraction. Water and kerosene are excluded, for obvious reasons. Dry mass penalty will be hefty, though. For those who REALLY want to nerd out, here are the specific impulse curves, using hydrolox as an example: And here are samples of the drag curves with respect to velocity: