Phil said:

>I'm no aero engineer but it seems to me that the technical challenges here are
>myriad and complex. Hi-temp body heat shields. Hi-temp scramjet engine parts.
>All to save the weight from not carrying O2.

I am an aeronautical engineer and years ago I worked on scramjets for a living. What you're saying is largely correct.

>Why not simply build a bigger vehicle that carries its own O2 that''s a rocket?
>Fly a ballistic lob with maybe some added glide range. Forget the complex holy
>grail of hypersonic scramjet engines. How expensive is several added tons of
>O2? This solution would be orders of magnitude cheaper to develop with higher
>operating costs reflected in the fuel costs but maybe engine maintenance of a
>simple rocket engine vs non-existant scramjet engine) would save operating
>costs.

Building an inexpensive recoverable launch system is really hard to do. If the thing flies too fast in the lower atmosphere then it needs to carry more mass as thermal protection against aerodynamic heating. For an Expendable Launch Vehicle (ELV), most of the energy required for getting into orbit is simply to get out of the Earth's gravitational well (NOT to overcome skin friction). However if you make your vehicle too big then skin friction can become an issue. A good way to make a vehicle too big to be practical is to use liquid hydrogen as a propellant (very poor volume efficiency). So the designer might choose kerosene as the propellant and use F-1 engines. F-1s have great thrust but relatively poor specific impulse. Exhaust velocity scales with specific impulse. Rocket momentum directly scales on exhaust velocity. Von Braun developed a clever hybrid design by using kerosene F-1s for the first stage of the Saturn V and then liquid hydrogen fueled J-2 engines for the second and third stages. The F-1s got the Saturn-V stack off the ground and past maximum dynamic pressure. The vehicle then shifted over to the lower thrust / higher specific impulse J-2 engines. The Space Shuttle sort of does the same thing with its Solid Rocket Boosters (SRBs) and liquid hydrogen fueled Space Shuttle Main Engines (SSME).

A better design might forfeit specific impulse for the lower cost and propellant volume (lower skin friction) of kerosene. However if hydrocarbon fuelled launcher were being frequently used to access space then environmental issues might come in (soot in the upper atmosphere might(?) cause ozone depletion). Hydrogen and liquid oxygen burn to water which is environmentally benign.

If it was up to me I'd use a biamese TSTO (Two Stage To Orbit) Recovarable Launch Vehicle (RLV). The booster and orbit vehicle would use identical air frames with identical thermal protection systems. The air frame would be worn like a suit of clothing (or a knight's armor) over a propulsion system or payload. The booster's propulsion system would be a couple of F-1s with kerosene and oxygen fuel tanks. The orbit vehicle's propulsion system would be a J-2 or SSME and an F-1. The F-1 would be plumbed to suck kerosene from the booster's fuel tank (like the Shuttle's SSMEs suck fuel from the external tank). The single J-2 or SSME would suck fuel from the orbit vehicle's own fuel tank. My design would normally fly unpiloted. If people were on-board, they'd fly as cargo in an escape capsule. I'd design the vehicle such that the air frames could be installed over their respective propulsion systems a couple days before launch. If there was a fault with a particular propulsion system, then a new one could be swapped in within a few hours. The booster and orbit vehicles would taxi on their own landing gear to the launch pad. Two vertical erectors would mate the booster and orbit vehicle keel-to-keel for vertical launch. The air frames used for boosters would be out-of-test orbit vehicles. Orbit vehicles would be younger, fresh-off-the-assembly line hardware. Really old booster propulsion systems could be strapped on without reusable airframes to payloads requiring heavy lift. These older propulsion systems would end up in the ocean.

Gary