The roll out of the Stratolaunch aircraft started a considerable amount of criticism on the sources that I read. Basically it has the same problem as the White Knight, superb aircraft with no viable rocket to mate up with. I wonder though if that is the reality.
The Stratolaunch aircraft is supposed to have a lift capacity of 500,000 pounds under that center wing. It would seem to be ideal for the three barrel launch vehicle that Gary Hudson was suggesting a few years back for air launch. Visualize a Falcon 1 heavy slung under that wing with all three Merlins the vacuum variant. That’s just to create a visual. Now realizing that Musk isn’t involved, I go to the vehicle that I believe could exist somewhere Real Soon Now.
Paul Allen apparently went through three different big name booster companies before settling on the Pegasus. Except the Pegasus doesn’t make sense for such an aircraft. It would seem possible that somewhere there is a very quiet development effort going on. I can think of a few companies capable of developing the vehicle I am going to suggest without feeling the need to Branson about it.
When launching from over 34,000 feet, more than 3/4 of the back pressure losses from sea level are gone. This means you can have a higher expansion ratio nozzle, or lower chamber pressures, or some optimum combination of both. With lower pressure engines that still have good performance possible, pumps become simpler to develop, or even unnecessary with pressure fed by modern materials. Simpler is cheaper. Three barrels with one engine each with all large expansion ratio nozzles. Probably methane and LOX for the self pressurization aspects even at the cost of higher residual pressurant mass than with helium. Very much an operational cost conscious design.
I start with a GLOW of 500,000 pound as maximum for the aircraft. Suggesting an exhaust velocity 3,300 m/s throughout the flight. 8,000 m/s from drop to orbit. Stage mass of 8% at cut off. Total mass ratio of 11.3. Mass ratio to outer stages drop 2.72. Mass ratio of core stage to orbit 4.15. Cross feed from outer stages to core until they burn out.
These are the numbers I came up with starting at the drop from the carrier aircraft in pounds.
weight outer stages 343,543
propellant outer stages 316,060
weight core stage at sep 156,456
propellant core stage 118,798
mass in orbit 37,557
stage mass 12,516
payload mass 25,140
It should be obvious that these peanut gallery numbers are speculation that I put together with a TI 30 at lunchtime. Real vehicles won’t hit these exact numbers as they are just what I got out of a calculator. You would need to round up or down or change the assumptions as you feel necessary to get something realistic. Look at he last number though, over 11 metric tons of LEO payload from three low pressure engines, two of which can be recovered after separation just as the Falcon9 first stage is recovered now. Actually simpler as the Stratolaunch will be from up range so that the outer boosters RTB (Return To Base) without needing a boost back burn. The Falcon9 is rated for more payload than this, but before shouting too loud, I suggest going back and looking at the actual loads orbited and find that every one of them to date is well under what I have speculated here.
Cost could beat the Falcon9 depending on assumptions. An aircraft to maintain instead of a launch pad. Two simple engines and small stages to refurbish before next flight against nine engines and a larger stage. An expended core stage comparable to the Falcon9 upper stage though simpler by design.