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Hi! I came across a very well thought-out design for re-usable launch vehicles, describes in this article: VISION. The concept involves solving the problems SSTO spaceplanes have by separating the vehicle into two sections: a hypersonic carrier plane and a nose-mounted rocket-plane. The innovative part is to mount them end-to-end. The booster plane carries the rocketplane to a high altitude and gives it about 3km/s initial velocity. The rocketplane stages and completes the climb into orbit, unburdened with air-breathing jet engines. The propellants selected were RP-1 and H2O2 for their density and non-cryogenic storage, important factors for an airplane that needs to fit its propellant tanks into wings. For overall simplicity and technology readiness, no scramjets are used. Instead, simple turboramjets bring the booster plane up to supersonic speeds. Rocket engines take over at high altitude and add about 2km/s until the booster stage runs out of propellants. The second stage is a pure rocketplane that can deliver 6km/s of deltaV. Payload is about 450kg. Re-entry is made easier with the massive airbrakes the rocketplanes has on the back. https://exospace.files.wordpress.com/2017/03/a14.jpg My suggestion is that we slightly modify the propellant choices and the ascent profile for a massive boost in performance. Due to the propellant choices, air cannot be cooled and liquefied like in a SABRE engine. There needs to be a reserve of liquid hydrogen to provide the heatsink for this to happen. However, there is a way to liquefy air without using liquid hydrogen. Heat pumps can be used to cool a metal heat sink down to cryogenic temperatures, using evaporation and compression cycles. When air is run through the heat sink, which acts as a heat exchanger, oxygen condenses on it and can be collected. This costs power to run... power which can be derived from the turbo-ramjet's turbines. Of course, equipping heat pumps large enough to fill the entire booster and rocketplane with liquid oxygen as it flies up through the atmosphere instead of simply using liquid hydrogen as a heat sink is a massive mass and power penalty. This penalty can become minor if we only equip pumps capable enough to fill up only the rocketplane with liquid oxygen. About 5 times less liquid oxygen would be needed. By collected liquid oxygen this way, you can launch the upper stage with RP-1 tanks full and liquid oxygen tanks empty. The liquid oxygen tanks would be larger than peroxide tanks by about 22% in volume and be slightly heavier due to insulation. The benefit is a 30 to 50 second jump in Isp and about a 50% drop in rocketplane mass on liftoff. This means smaller booster plane engines and wings, larger payload and so on.