Most horizontal-launch systems envisioned in the past, that used electromagnetics as a launch-assist, were subsonic. In order to conserve fuel during launch and to keep engine-weight to a minimal a combination of electromagnetic-propulsion, i.e. maglev or rail-guns and scramjet engines may offer a viable alternative for manned systems to LEO. A lot of electrical-energy storage systems would be required to create the required velocity in the launch infrastructure such that the orbital vehicle scramjet engines could ignite. An example of a lower speed system is here. If the launch velocity could be increased to between Mach-2 and Mach-4 a manned system, using only scramjet engines, could be constructed. Final orbit insertion would need some additional rocket engines. The launch-track would need to be at least 10 miles in length to keep acceleration under 3 g's, and offer launch-abort opportunities.
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$\begingroup$ The problem lies not in the theoretical doability, but in the complexity of such a system, compared to just building a larger conventional rocket to reach the same speed. $\endgroup$– Rikki-Tikki-TaviCommented Sep 15, 2016 at 15:45
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$\begingroup$ Building larger rockets runs into two major problems: Specific-Impulse en.wikipedia.org/wiki/Specific_impulse and Return-on-Investment en.wikipedia.org/wiki/Return_on_investment . Larger rockets aren't necessary a better investment. $\endgroup$– D. WadeCommented Sep 15, 2016 at 16:08
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1$\begingroup$ While not a duplicate, this answer to the question Why aren't all satellite-carrying rockets launched from airplanes? looks like it might be relevant. $\endgroup$– Dan PichelmanCommented Sep 15, 2016 at 17:36
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$\begingroup$ I'm not sure what this site could add to an article that lays out the idea, and was proposed by NASA engineers. Someone must think it's viable. $\endgroup$– SteveCommented Sep 15, 2016 at 20:09
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The potential of a horizontal launch within the earth's atmosphere is very much limited by the atmosphere itself. At a low height the maximum speed is limited by the high atmospheric pressure and at medium height there is not enough oxygen left for the scramjet. But to get into a low orbit, much more speed and height is necessary, about 8 km/s speed instead of some 1.3 km/s for Mach-4 and 200 km height instead of only about 25 km. The difference in kinetic energy is impressive, about 38 times more for 8 km/s instead of 1.3.
