Since most of the theoretical and conceptual designs of Single Stage to Orbit (SSTO) aircraft are depicted through Scramjet-based vehicles (such as the X-30 concept from the National Aerospace Plane Program - https://en.wikipedia.org/wiki/Rockwell_X-30), would it actually be possible (and also economically feasible) to reach an orbit (at least a LEO-grade orbit) using an air-breathing engine as a Scramjet or even an hybrid approach like the Turboramjet in the SR-71 ?
Best Regards.
Hi Jorcy. In principle, it should be possible. See these two links:
http://www.universetoday.com/85322/low-earth-orbit/
http://www.answers.com/Q/What_factors_determine_the_maximum_altitude_of_an_airplane
LEO in theory could be as low as sea level. In practice, you need to be above the atmosphere, to avoid drag. Which means, at least above an altitude of roughly 85 km, or 280,000 feet. So the air breathing engine would have to boost the vehicle to a high enough speed that it could then coast into orbit, in essence.
I agree with the previous posts – in theory, it’s possible at any orbit; however, practically it’s currently very unlikely to completely sustain required orbital speed with an air-breathing engine… Just as a rough estimate by the numbers:
If we assume a practical limit to air breathing propulsion due to air density, let’s say, at 200,000ft (the current altitude record for non-rocket engine propelled aircraft is at ~120,000ft in a MIG-25) and an Earth radius of roughly 20,900,000ft, then we would need an orbital velocity of roughly:
F=mv^2/r and F=m*g (the centrifugal force has to offset the gravitational force)
v=SQRT(g*r) with g=g0*(re/r)^2 (gravitation at altitude being somewhat less than at surface)
v=SQRT{32.2*(20,900,000)^2/(20,900,000+200,000)}=25,818.6ft/s
which is roughly Mach 26 (just for comparison – shuttle re-entry speeds at that altitude are roughly at 16,000ft/s)
However, I think it’s worth at least some consideration whether lift requirements can be partially offset by centrifugal force for high altitude, high-speed jet flight…
Dear Jorcy,
In principle it is possible for a SSTO scramjet-powered design to reach orbit.
However, in addition to numerous problems such as materials, propulsion, size and cost, as exemplified in the US National Aero-Space Plane Program, there is the problem of aerodynamic heating.
Using an air-breathing scramjet engine the vehicle would have to reach orbital velocity while flying within the atmosphere.
This flight profile leads to enormous amounts of aerodynamic heating which cannot be dealt with the present level of materials technology.
Best wishes,
N. Kehayas
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