SteveO said:
It looks pretty feasible and practical to me, what are your views on it?
At first glance it seems like a no-brainer that an air-breathing vehicle makes sense. A launch vehicle sitting on the pad is 90+ percent propellant, half or more of which is oxygen. If you could get all of that oxygen "for free" from the atmosphere, then the lift-off weight would drop dramatically. Not only would you not need to carry all of that oxygen and tankage for it, but for each kilo of oxygen you didn't carry, you'd need less fuel and hence smaller fuel tanks. Now that you've got a much smaller vehicle (less than half the size), it should be cheaper to operate.
But when you start to think about the details, things don't look so rosy. IIRC, scramjet SSTO designs usually switch from scramjet mode to rocket mode at a few kilometers per second, well below orbital velocity. Things are better than they seem for the scramjet, because a conventional rocket will have burned more than half of its propellant just to get to, say, 1.5 km/s, but we're still a long way from orbit at the stage that a rocket has to take over anyway.
Then there's the fact that launch vehicles try to get out of the atmosphere quickly, to avoid drag losses and heating. A scramjet-based vehicle, on the other hand, must remain in the atmosphere longer. It will suffer higher drag losses and its structure will need to cope with substantial heating.
There is a fundamental limitation on how much the scramjet can help. Getting to orbit is a matter of lifting yourself up to, say, 200 km and accelerating to about 7 km/s. The first takes an energy of about (9.8 m/s2) * (200,000 m) or about 2 MJ/kg. The second takes about 0.5 * (7,000 m/s)^2 or about 25 MJ/kg. Now just think about all of that oxygen that your scramjet scoops up. It is stationary with respect to the earth's surface. Hence, although you don't have to
lift the oxygen, you do have to
accelerate it to your own speed, just to avoid slowing down, and then you have to accelerate it some more in order to get some thrust out of it. Taking oxygen from the air helps with the 2 MJ/kg of lifting that has to be done to reach orbit, but it doesn't help with the much larger 25 MJ/kg's worth of accelerating that has to be done.
Even though scramjet SSTO designs switch from air-breathing mode to rocket mode at relatively low speeds, the stationarity of the air reduces the efficiency of the engine. Suppose we're burning hydrogen, which gives us an exhaust velocity of about 4,000 m/s in a rocket engine, where the oxygen that is delivered to the combustion chamber is more or less stationary with respect to the chamber. At 4,000 m/s, the oxygen atoms in the exhaust contain a kinetic energy of about 0.5 * (4,000 m/s)^2 = 8 megajoules per kilogram. That's about how much useful energy we're extracting from combustion. In the case of the scramjet, our oxygen come screaming in at, say, 1,500 km/s, so it has a kinetic energy of 1.125 MJ/kg. Now we add 8 MJ/kg through combustion (assuming we can burn as efficiently as in a rocket engine, which is unlikely) to get a specific energy of 9.125 MJ/kg, corresponding to an exhaust velocity with respect to the atmosphere of 4,270 m/s. So, with respect to the vehicle, the exhaust velocity is not 4,000 m/s, but just 4,272 m/s - 1,500 m/s = 2,770 m/s. The simple fact that the oxygen we scoop up is stationary makes our engine less efficient. [NB: This quick-and-dirty analysis ignores the hydrogen in the mass-energy balance, but it's a modest fraction of the total mass.] The faster we go, the worse this problem gets. The problem will be worse, too, if we use a hydrocarbon fuel, which has a lower exhaust velocity. And this ignores the fact that the air we scoop up is mostly nitrogen, not oxygen. I know there are ways of dealing with the nitrogen, but they add complexity and inefficiency.
IMHO scramjets are interesting and deserve further research. But for SSTO applications, scramjet technology will have to be very much more mature than it is now to be worth the trouble.
