A major source of aerodynamic drag with a hypersonic vehicle is due to skin friction. If you increased the length of the combuster from 3 meters to 30 meters then the skin friction goes up by a factor of ten. It's not clear that the thrust produced by a scramjet would exceed the skin friction for even a 3 meter combuster. Also there is the issue of mass and thermal protection. Obviously you want the vehicle to be as light weight as possible. However the vehicle's skin must also be cooled. This requires some sort of hot metal thermal protection system probably based upon regenerative cooling, i.e. using the fuel as a coolant. This means the metal skin must be extensively plumbed with thin metal tubing that has propellant pumped through it.

It is my opinion that the best short term technology for a "low cost" fully recoverable launch vehicle is a two stage to orbit (TSTO) system based upon the "biamese" architecture. The biamese architecture is where both the booster and orbit vehicles use the same air frame design thus requiring only one vehicle to be developed. The booster and orbit vehicles would be mounted keel-to-keel. It is also my opinion that turbo pumped kerosene fuelled engines are best. Kerosene engines have a lower Isp than hydrogen rockets but the fuel has a higher storage density. A higher density means smaller fuel tanks and less skin friction (it's a design fallacy to focus exclusively on Isp and ignore the issue of fuel tank volume). For thermal protection, I'd use densified ceramic tiles with reinforced carbon-carbon on the leading edges. This sort of TSTO would probably get the cost to LEO down to $1000/kg. Such a system would enable Mars colonization but would not open outer space to private economic development. People say the magic number for that is $100/kg. The only way I see doing that is through something really funky, e.g. rockets riding on a multi-gigawatt laser beam. I doubt that we will see that in our life times.

Gary