Optimal fuel for air augmentation

[sevenperforce]

If you're building an SSTO spacecraft intended to make maximum use of air augmentation, using a single engine cluster from liftoff to orbit, what are some of the pros and cons of the best fuel combinations?

The way I see it, fuel density is going to be fairly important because you want your SSTO to be fairly compact, and deep cryogens probably should be avoided to save tank weight because you can't drop the tanks. You'll also want something that can react at least a little with atmospheric air so that you can run fuel-rich from Mach 0.7 up to Mach 4 or so and get a little airbreathing assist. And you can't sacrifice too much ISP because air-augmentation is going to run out when your velocity equals your actual exhaust velocity, and at that point you're still depending on the same engine for orbital insertion.

But if your primary working maas from launch to Mach 10+ is going to be diatomic nitrogen and diatomic oxygen, should you use a low-molecular weight fuel or something with really high heat capacity? Air augmentation is about delivering as much thermal energy to an airstream as possible, which is different from the usual vacuum rocket optimization parameters. What fuel will have optimal mixing in a duct? Would it be feasible/advantageous to use a triprop design, with a denser oxidizer for takeoff and max air augmentation and cryolox for the upper portion of the flight?

Any thoughts/analysis would be appreciated.

Edited February 12, 2016 by sevenperforce

[lobe]

For Earth the combination is nearly unethical in all regards. If a spaceplane is made it will fly via booster like Atlas-V or Skylon. Compared to a Kerbal SSTO it is so much mor difficult.

[sevenperforce]

This is an Earth SSTO, not a Kerbal SSTO. I know it's not easy.