NASA Engineers Propose Combining a Rail Gun and a Scramjet to Fire Spacecraft Into OrbitRev Up The Rail Gun
A 240,000-horsepower linear motor converts 180 megawatts into an electromagnetic force that propels a scramjet carrying a spacecraft down a two-mile-long track. The craft accelerates from 0 to 1,100 mph (Mach 1.5) in under 60 seconds— fast, but at less than 3 Gs, safe for manned flight.Fire The Scramjet
The pilot fires a high-speed turbojet and launches from the track. Once the craft hits Mach 4, the air flowing through the jet intake is fast enough that it compresses, heats to 3,000ºF, and ignites hydrogen in the combustion chamber, producing tens of thousands of pounds of thrust.Get Into Orbit
At an altitude of 200,000 feet, there isn’t enough air for the scramjet, now traveling at Mach 10, to generate thrust. Here spaceflight begins. The two craft separate, and the scramjet pitches downward to get out of the way as the upper spacecraft fires tail rockets that shoot it into orbit.Stick The Landing
The scramjet slows and uses its turbojets to fly back to Earth for a runway landing. Once the spacecraft delivers its payload into orbit, it reenters the atmosphere and glides back to the launch site. The two craft can be ready for another mission within 24 hours of landing.
24 hours! Hahahahahaha! They said that about the Shuttle, too... Still, what's not to love about maglev scramjets? It's even fun to say!
I didn't know Yoyodyne Propulsion Systems was a NASA sub-contractor.
Going that fast low in the atmosphere is a big waste of energy - huge drag and aerodynamic heating problems.
But the nice thing about "that low" (on the ground) is energy is so cheap here you can waste it. 5MJ of energy on the ground costs almost nothing. 5kJ of energy 2km up is really expensive, you will probably have to bring it with you. So even if it's really inefficient energy-wise it may be a cost saving to put lots of kinetic energy in at ground level.
Mach 1.5 is fast and wasteful in the low atmosphere, but it will just trade speed for altitude. You don't have to go very high to be above the majority of the atmosphere. In minutes, it can be above 80% of the atmosphere, and it's not going to be much more wasteful than, say, a supersonic bomber. That's are expensive compared to jet airliners, but it's very cheap compared to the cost of launching a large rocket into orbit.
Even if it does waste energy by going fast in low atmosphere, that's a very very small amount of waste compared to the win you get by using atmospheric oxygen. A very large percentage of the fuel used by normal rockets is simply used to lift the rest of the fuel. For the space shuttle to lift an extra 1 lb of payload takes something like 4 extra lbs of fuel. Around half of that fuel is oxygen. The weight savings for using oxygen from the atmosphere are dramatic. Being able to get your second stage 37 miles up going half the necessary orbital speed using only atmospheric oxygen would be huge.
When that rocket separates from the scramjet, it is still less than half way to orbital speed. You need about 70% less fuel to complete boosting to LEO too.
But "less than half-way" means less than a quarter of the kinetic energy. If takeoff is at Mach 1.5, and the end of atmospheric propulsion at Mach 10, only 2% of the energy comes from the rail gun part.
It sounds an awful lot to me like the idea was to skip the turbojet part entirely, going straight from the rail gun to scramjet flight, but that didn't work, so they put it back in. It's probably easier to optimize the turbojets for working in the mach 1.5-4 range rather than starting them out at zero, but it's still a rocket on a scramjet on a turbojet on a railgun car, and the railgun part is the equivalent of the turducken's pigeon.