A long sought after alternative is to launch and go to high altitude with atmospheric engines (thus saving mass of oxidizer) and aerodynamic lifting force (so you can use lighter engines, having overall TWR < 1), basically an aeroplane. Only after reaching high altitude and speed switch to rocket mode, or separate a rocket-based next stage. Scaled Composites' SpaceShipTwo goes far into that direction, but it is still limited to low-supersonic speeds by using jet engines. The problem is, plain jet engines are not very efficient at high Mach, due to both having to slow air down to sub-sonic speed and thermal limitations.
The next logical step is to use scramjet engines, which perform in high supersonic (`hypersonic') flight regime. This is technically challenging, because you have to work with high supersonic airflow inside engine (hypersonic gas behavior is significantly harder to model than at the usual subsonic), and also you have to maintain steady flame in its hypersonic airflow.
1. Rocket engines have high thrust to weight ratio, typically around 100. Jets can have about 10. Scramjets can be really bad.
2. You can speed up quite quickly since the atmosphere is there for the first 30 km or so only.
3. White Knight Two that launches Spaceshiptwo is subsonic, not supersonic. The air launch is beneficial roughly:
- primarily because your rocket engine's expansion ratio can be bigger and you get more thrust for same fuel flow
- secondarily because then you can launch flexibly by flying to a location, that for example has less population or air traffic or is easy to launch to the right orbit from
- thirdly because you can always glide to a landing if something goes wrong with the engine. With a vertical takeoff there's a time right after takeoff where it's hard to do an engine out abort with something like a parachute.
Note all the above advantages are not dependent on carrier aircraft speed. In fact supersonic separation is a hard problem, never mind something at Mach 3. (See D-21)
If you look at something like dry mass, rockets will practically always win over air breathers in studies because of the lightness and simplicity of simple tanks and high thrust to weight ratio of rocket engines.
This whole scramjet affair is very misguided if you're trying to save something like liquid oxygen which is extremely cheap. Even when it weighs something, it doesn't matter since tanks are light and simple and (did I mention already:) rocket engines have high thrust to weight ratio.
I'll choose that any day over a huge scramjet inlet that needs to adjust to variable speeds by having big movable ramps, a big burner and a big exhaust, all experiencing large aerodynamic heating problems, multiple load paths (a rocket has basically only vertical loads).
Horizontally launched rockets, such as Pegasus [1], are launched from a parent aircraft at 50,000' altitude. The mass fraction that you can get into orbit is limited by the weight of the oxidizer the rocket is required to carry. A scramjet on this rocket would allow the rocket to accelerate to Mach 15 or 20 while in the upper atmosphere, and then use an oxidized insertion motor to establish the desired orbit.
edit: ...which I see is mentioned in the actual article, once I got around to reading it.
It's mostly useful for weapons at this point.
(Actually, if you look at Navaho, rocket engines and navigation at least, but that's a whole another story...)