drachefly wrote: sevenperforce wrote: Wolfkeeper wrote:
Merely cooling and compressing atmospheric air for use as working mass would be a lot easier, though still probably too difficult to be realizable.
Nope, that's exactly what Skylon/SABRE will do, and the technical reviews have come back 👍 looks good.
Well, Skylon/SABRE isn't cooling/compressing atmospheric air to...
TWEEET tweet yellow flag.
Compressing air quickly achieves the opposite of cooling. What did you mean here?
Quite; SABRE does both. The SABRE engine uses an advanced, compact closed-cycle helium precooler (using liquid hydrogen fuel as the terminal coolant) to supercool the air between inlet compression and turbine compression. The vaporized and heated hydrogen fuel is burned with bypass air in a series of small spill duct ramjets so that it's not totally wasted. Once the inlet compression becomes so high that the amount of liquid hydrogen needed to cool the airstream becomes prohibitive, the front inlet is closed and the engine proceeds under pure rocket propulsion.
It's a promising design. It's just that any liquid hydrogen rocket requires extremely high-volume tanks, which ends up dictating all your design factors. That makes both re-entry and overall reusability much more challenging. Hence the desire to design an SSTO vehicle using a higher-density, non-cryo fuel using an air bypass to vastly exceed the specific impulse AND the dry thrust of a pure hydrogen/LOX rocket.
Wolfkeeper wrote:There's this myth that jet engines need oxygen from the air. I mean, they certainly do use it; it's free, why wouldn't you. But that's not the main thrust. 80% of the thrust of jet engines comes from lobbing nitrogen out the back at high speeds. If the air contained only nitrogen, jet engines, including SABRE would work fine, you'd just need a tank of LOX as well as your fuel. Performance would drop quite a bit, but it would still be much better than just burning the fuel and LOX in rocket mode. I mean, turbofans don't burn most of their air at all for example.
As a further emphasis of this line of reasoning, it should be noted that Billig ("Propulsion Systems from Takeoff to High-Speed Flight", High-Speed Flight Propulsion Systems
1990) found that a pure ducted rocket has a better mass fraction from takeoff to Mach 3 than both turbojets and afterburning turbojets. The slightly better specific impulse of turbojets, while making them better for cruising flight, is outweighed by the enhanced thrust capability of a ducted rocket and thus loses in terms of acceleration-stage mass fraction.
The same article found, incidentally, that a reusable ducted-rocket SSTO spaceplane would have truly fantastic cruise capabilities if the proper trajectory is chosen, making it quite suitable for serving either as a hypersonic transport or a hypersonic strike vehicle with a larger payload. Of course, he didn't consider the added benefits of a central-bypass ducted rocket.