Originally Posted by tdracer
"as long as we continue to throw away a significant portion (or all) of a rocket every time we use it, space access will remain insanely expensive (currently between $2,600 and $10,000 per pound - and the shuttle was ~ $24,000/lb )
This is the standard argument; it has some validity but is misleading. A good example is the reusable shuttle cost about $10,000-$25,000 per lb to LEO. The expendable SpaceX Falcon Heavy currently costs under $1,000 per pound, and that was achieved with no reusability, no exotic airbreathing, no wings, etc.
In his presentation to the Augustine Commission, shuttle program manager John Shannon called reusability a myth. He explained when a vehicle is made in very small numbers, you cannot just shut down all manufacturing capacity after it's built. The vehicle and subsystems require ongoing R&D, fixes, testing, etc. Parts wear out, you have failures, design issues become apparent during use. You essentially have to keep the production line available, even if nothing is being manufactured, along with much of the associated industrial infrastructure. Then you must buy "one of" pieces to fix things which is expensive.
Airliners achieve lower costs through mass production, not just reuse. They also benefit from a smaller "standing army" of support personnel relative to the fleet size.
Any envisionable air breathing SSTO would be far more complex and expensive than the space shuttle. If built in small numbers, there's no mass production. It would be costly to operate, regardless of having wings and airbreathing propulsion.
Originally Posted by tdracer
"Something around 75% of the liftoff weight of an orbital capable rocket is oxidizer - get rid of that for the first stage and you can add a whole bunch of hardware to make the thing reusable while still reducing the overall vehicle size and weight."
An air breathing design would save oxidizer. However LOX costs about $0.01 (one penny) per pound. The LOX for a shuttle launch cost only about $14,000. You are right much of the liftoff mass of an orbital rocket is oxidizer. On the shuttle the tankage cost for that was maybe about 1/2 of the total $50 million ET, so eliminating the LOX would enable smaller, cheaper tankage.
OTOH the additional cost and mass of a hypersonic airbreathing design offsets the gain from losing the oxidizer. An air breather must fly a depressed ascent trajectory, like a shuttle reentry in reverse. You must have active cooling of the vehicle skin and wings, multiple propulsion systems, fuel, structure and thermal protection for those systems, etc. Since scramjets cannot reach orbit, conventional rocket propulsion must be used for the final ascent. All the mass, structure and thermal protection for that must be hauled to and from orbit.
Originally Posted by tdracer
...True, the current state of the art for Ramjet/Scramjets is no where near making this viable, but technology marches on.
This is true so there is always hope. The NASP research made significant progress in various areas. With today's more advanced CFD and material science it might be more achievable.
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."
This is not quite like it seems because of the v^2 term. Mach 7 is only 7.8% of orbital kinetic energy. Being at 120,000 ft actually helps relatively little since gravity and air drag losses to reach that altitude are not severe. If launched at that altitude and velocity the orbiter would still have to do over 80% of the work to reach orbit, yet a Mach 7 mother vehicle would be fantastically heavy and expensive. Optimum staging velocity for lowest total vehicle mass on a two-stage launcher is about Mach 10, and the total vehicle mass curve is very steep around that minimum. IOW deviate much from Mach 10 staging and the whole stack get a lot heavier, quickly.
A lot of smart people have worked on airbreathing launchers for many years. It is a compelling concept but in the real world it is much more difficult than first appears. The current trend is that optimization of conventional rocket technology through lean production and partial re-use are a more effective way forward.