It looks like this is a no-go as is. That's because the X-33 is too heavy. Since the idea is to use the microwave booster as an airbreather, the need is to keep the velocity down for an airbreather since excessive velocity will result in cooling problems. That's to say, theoretically the microwave booster can do it, but the airbreather can't handle it. You want to keep velocity down somewhere between Mach 3 and Mach 5.5. Mach 3 is where the Blackbird flew, and Mach 5.5 is where the Skylon will revert to conventional rocket mode.
Calculations for the needed boost velocity from the microwave booster were well above Mach 5.5 for the X-33. So much for the X-33.
The idea now is to use an S-IVB type rocket on top of the 747, or the catapult used on aircraft carriers. A 747 airlaunch may not be feasible if the mass gets too high. It is right there on the edge of feasibility on weight alone. Besides, this thing already weighs in much more than a Shuttle.
It looks like an S4B ( using this as it's easier to type!) with an uprated J2X engine can actually reach orbit like this without a boost from the microwave. But it wouldn't be reusable, and this is a goal. If you didn't mind throwing away the S4B after every launch, you could still use the 747.
The most favorable mass allowance would be for an S4B powered by the microwave boost. This gives nearly double the mass of the basic empty weight of the stack in order to make it resuable. But this extra weight is butting up against the weigh limitations of the 747.
Something just occurred to me. You could use a Stratolaunch concept in order to airlaunch the whole setup, as opposed to a 747. The Stratolaunch is designed to launch something that weighs almost twice as much as the S4B. Add this as a third option giving the 747, catapult, or the Stratolauncher.
It's fair to say that if Musk's rocket can get to space on a Stratolauncher, the S4B can do so as well as it is a more powerful rocket and weighs less. Musk's rocket uses RP1 jet fuel instead of hydrogen. Its ISP is much lower and therefore must carry much more fuel. This adds to its mass.
Now back to the microwave booster. Let's say that you had 500k lbs of mass allowance for the system- per Stratolaunch. That would leave nearly double the weight of the S4B which could be used to make it reusable. You would do this through using the microwave booster module attached to it in order to get it to first stage type velocity and altitude. That should leave enough mass allowance left over from these maneuvers in order to make the S4B recoverable and reusable.
The microwave booster would be an airbreather. It would carry no fuel. It would get its thrust from heating the air to superheated temperatures and then expelling it as thrust. The goal would be to reach a velocity of between Mach 3 and Mach 5.5 and an altitude of 100k feet. The Stratolaunch would release it along with the S4B still attached at lower altitudes where the microwave booster would take over to get the rest of the altitude and velocity. The airbreather module would be reusable itself and return to base like a glider.
Question: Now that we have more mass, is the X-33 airframe now a feasible option? Possible, but you need it to be bigger and heavier than the original design. Otherwise, it would still need too much of a boost from the microwave. Stick with the S4B.
You could use an X-33 type airframe for the microwave booster that could carry the S4B inside of it. You could place the S4B inside of it and leave the inside part of the X-33 empty. The S4B would take the space of the fuel tanks inside of the X-33. The rest of the X-33 could be similar- except that it must be an airbreather.
Operationally, the S4B would come out and then you would have to plug that big hole. Maybe with an inflatable device or a cover of some kind.
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| .aerospaceweb.org |
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| Wikipedia size comparisons of large aircraft v. Stratolaunch |
Update:
Part 3 of the series.
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