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It's been a while since I read about the sabre engine so someone please correct me. One of the most promising developments in aviation and (one day) rocketry a

It's been a while since I read about the sabre engine so someone please correct me. One of the most promising developments in aviation and (one day) rocketry are ram jets and scram jets which breathe in atmosphere for fuel oxydizer instead of keeping it on board along with the fuel. Scram jets operate above the speed of sound so the air rushing into any intake will essentially be superheated which is a problem since the fastest you go the more fuel you need to burn and thus you need more and more oxidizer the faster you go (meanwhile, your intake air is explosively expanding). A passive supercooler aplows you take in more air, compress it, and feed the oxygen to your engine. Considering that the space shuttle booster rockets used a 6:1 mass ratio of oxydizer to fuel, air breathing engines powered by these supercoolers can change the economics of rockets.

jessriedel 11y ago
> Considering that the space shuttle booster rockets used a 6:1 mass ratio of oxydizer to fuel, air breathing engines powered by these supercoolers can change the economics of rockets. It doesn't change it too much because it's not feasible to go above Mach 10 or so in the atmosphere. (In fact, another comment on this page says air-breathing typically cuts out before Mach 6.) Low Earth orbit is something like Mach 25, so most of the accelerating is done in a vacuum where there's no air anyways. My vague understanding is that the performance enhancement is fairly modest on an absolute scale, but it happens to be important because it is just enough to make single-stage-to-orbit feasible.
- curtis 11y ago
  The benefits may be small or large depending on how you look at them. Suppose you need 95% of your launch mass to be propellant for a pure-rocket SSTO design, and suppose you only need 85% of your launch mass to be propellant for a hybrid SSTO like Skylon. This seems like a small savings, but if you look at the "dry-weight" instead -- the mass that you put all the way into orbit -- the air-breathing design puts three times as much mass into orbit. This simplifies the design greatly, allowing you to add things like wings, a more robust fuselage, thermal protection tiles, and covers the extra mass-cost of the heavier engines that you need. So you may end up with two designs with about the same launch mass and the same payload to orbit, but one of them is re-usable and one isn't. I'm personally more than a little skeptical that the economics will ever work out, but it's an interesting idea, and it's not completely crazy.

jessriedel 11y ago

> Considering that the space shuttle booster rockets used a 6:1 mass ratio of oxydizer to fuel, air breathing engines powered by these supercoolers can change the economics of rockets. It doesn't change it too much because it's not feasible to go above Mach 10 or so in the atmosphere. (In fact, another comment on this page says air-breathing typically cuts out before Mach 6.) Low Earth orbit is something like Mach 25, so most of the accelerating is done in a vacuum where there's no air anyways. My vague understanding is that the performance enhancement is fairly modest on an absolute scale, but it happens to be important because it is just enough to make single-stage-to-orbit feasible.

curtis 11y ago
The benefits may be small or large depending on how you look at them. Suppose you need 95% of your launch mass to be propellant for a pure-rocket SSTO design, and suppose you only need 85% of your launch mass to be propellant for a hybrid SSTO like Skylon. This seems like a small savings, but if you look at the "dry-weight" instead -- the mass that you put all the way into orbit -- the air-breathing design puts three times as much mass into orbit. This simplifies the design greatly, allowing you to add things like wings, a more robust fuselage, thermal protection tiles, and covers the extra mass-cost of the heavier engines that you need. So you may end up with two designs with about the same launch mass and the same payload to orbit, but one of them is re-usable and one isn't. I'm personally more than a little skeptical that the economics will ever work out, but it's an interesting idea, and it's not completely crazy.