Originally Posted by tdracer
"An air breathing launch vehicle that could carry an orbiter to ~120,000 ft./Mach 7 (or better) would mean the orbiter wouldn't need to be huge to carry the fuel to get into orbit."
Originally Posted by joema
...because KE=1/2*m*v^2, it takes tremendous kinetic energy to reach orbit. A Mach 7 reusable booster is easier to design but it makes the orbiter much heavier. This is because the remaining delta-V is a squared term, hence energy required falls disproportionately on the orbiter.
Originally Posted by tdracer
...Wrong, quite wrong. Rockets engines are effectively constant thrust devices, not constant power....No, you're missing basic physics. Acceleration is directly proportional to force, and in this case force and thrust are the same thing...I won't bother with the rest of your analysis because it's based on a flawed assumption.
Ironically a Saturn V stages at Mach 7. You can see the event in this photograph. The remaining stack is still gigantic and massive -- 1.48 million lbs. The Mach 7 assist did not translate into a "non huge" orbital vehicle:
https://joema.smugmug.com/Aerospace/...xt/i-dCpvGLS/O
Another example is the Space Shuttle, which at SRB sep is at Mach 4.5 and 150,000 ft. About 77% of ET propellent is remaining, so the remaining stack is about 2.2 million pounds. Mach 4.5 is a good number because it's achievable with ramjet propulsion at much lower cost and technical risk than scramjets.
A hypothetical air-breathing first stage which injected the shuttle orbiter plus ET to Mach 4.5 at 150,000 ft. would reduce the required ET propellant by 23%. The remaining orbiter stack would still be a gigantic vehicle weighing 2.2 million lbs. Why? Because of the remaining tremendous kinetic energy required to reach orbit.
So these examples illustrate the difficulty of an air-breathing first stage. One key reason is the highly non-linear affect of the kinetic energy V^2 term. Boosting to Mach 4.5 or Mach 7 doesn't help that much, yet would require a titanically expensive and complex winged air-breathing booster.
The non-linear effect of how kinetic energy affects this can be easily seen in the near-vertical curve on the left side of the staging velocity chart:
https://joema.smugmug.com/Aerospace/...6t/i-nKLmZSR/O
This also explains why subsonic air launch has almost no benefit to orbiter mass. This was discussed in the AIAA paper "A Study of Air Launch Methods for RLVs
(Sarigul-Klijn, et al, 2001):
http://tinyurl.com/z7zstrm
"
"Surprisingly, a typical straight and level subsonic horizontal air launch such as used by the X-15 research rocketplane does not result in any significant changes in the delta V requirement as compared to a baseline vertical surface launch."
There are groups still working on air-breathing SSTO and TSTO launchers such as Skylon and Bristol. I wish them well but I doubt they will be successful. Even if they were successful, they would still have to compete with SpaceX price/performance, while amortizing the huge development cost of airbreathing hypersonic vehicles:
Skylon:
Reaction Engines Ltd - Space Access: SKYLON Bristol SpaceBus:
Spacebus | Bristol Spaceplanes
Air-breathing SSTO is not dumb -- the brilliant designer Tony duPont conceived the original NASP design and it first appeared possible. However DARPA recently re-evaluated this in light of technical growth and believes it is still not possible:
http://www.nytimes.com/2014/10/21/sc...ress.html?_r=0
NASP and follow-on research indicates scramjets and the associated penalties of multiple propulsion systems, active cooling, etc, cannot remotely reach orbit and are not good candidates for a 1st stage reusable launcher. It now appears the main application for hypersonic airbreathing propulsion is military -- long-range hypervelocity missiles.
However a reusable non-airbreathing 1st stage is possible for smaller payloads - the XS-1 is a good example:
US Military Awards New Contracts for XS-1 Space Plane
The problem is even with the projected reuse savings of that booster, their payload cost per lb is already higher than SpaceX on the Falcon Heavy ($1,000 vs $770). It may have other advantages such as quick reaction and flexibility.