I'll make a series out of this one. The first was the X-37C post, made on September 11th. The second post was a brief followup last month.
An idea for this post came from how the XCOR's Lynx will get to the Karman line. It will fire its engines and get only to Mach 3, and then drift upwards to the Karman line before returning to the ground. The idea here is to get to the Karman line with some momentum and let the NTR do the rest of the lifting to orbit.
It has occurred to me that the mass of the X-37C is close to the mass envisioned in James Dewar's book "The Nuclear Rocket". Let's say we want to replace his idea of using solid rocket boosters to be launched from a cargo plane--- with a launch from a Stratolauncher and an airbreather engine like the SABRE engine being developed for the Skylon SSTO.
This warrants another speculation alert, of course.
Dewar wanted his package to mass out at 91k lbs. Presumably, this would be a 17k pound payload into orbit. That's pretty close to the X-37C's mass. I didn't see any provision in Dewar's concept that would save the hydrogen tank. We should want to be able to do that.
The SABRE engines wouldn't be required to go to orbit, so we could economize on mass there by making that booster smaller. It won't be needing oxygen tanks since it is only going to be used in its airbreathing mode. The Stratolauncher helps save mass here too, because it imparts the first 30k feet of altitude to the orbiter.
Instead of going to orbit, the SABRE engines would separate at Mach 5.5 and 85k altitude, thus imparting momentum to the 91k mass of Dewar's NTR module. The module would drift upwards to the Karman line or possibly well below it and then light up the nuke booster. The SABRE powered booster would do a RTLS (return to launch site ). The 91k NTR's mass could probably be increased, since the airbreathing booster would not have to carry oxygen. The downside is that the NTR would be losing momentum as it waits for the nukes to warm up.
You could play around with the masses in order to optimize the design so that it will make it all fully reusable with a fast turnaround. For example, instead of a "cocoon" that enables the nuclear core to be fished out of the ocean, some landing gear could be supplied so that it can land like a plane. Also, the tank could be made into a lifting body shape and designed so as to survive a reentry. If more mass is needed, you may have some margin in the concept for that. Up to 500k pounds could be lifted by the Stratolauncher.
I'm thinking that it should be optimized so that that you don't have to use onboard oxygen for the first stage ascent. The SABRE provides that capability. The second stage on up to orbit would use the higher ISP capability of a nuclear thermal rocket (NTR)--- up to 1000 ISP.
Hopefully, there is a design that would enable all these advantages to be incorporated in a practical design that could be fully reusable and with a fast turnaround. The launch costs could be brought down so as to make space more accessible. That's what James Dewar believes the NTR can do. Dewar compared the chemical rocket to the "Pony Express". A nuclear rocket could take us to the next level.
Running the numbers seems to confirm that this could work. As for the rest, it would have to fit under the wing of the Stratolauncher and it would have to pass through all the red tape.
The entire mass of the thing except the X37-C and booster, could fit on a X33 airframe. You'd have to mate the NTR booster on to that, but if you could, the thing would mass at less than the X33 launcher originally planned.
The X33 failed because of the large hydrogen tanks, but those would be much reduced by an airbreathing engine for most of the trip. The X33 was a demonstrator only, not designed for orbital velocity, but suborbital velocities and altitude mentioned here.
The X33 would have to be redesigned for this, though. The imagination runs a bit too freely, perhaps.
After crunching the numbers even further, I am beginning to wonder if you even need the Stratolauncher. You may be able to use a 747 in the same way that it was used to transport the Shuttle. In addition, all of the fuel should fit inside the X-33 fuselage so that it can be made into a cargo holder for the NTR. This configuration would allow the same X-33 airframe to house the NTR/X-37C and be lifted by a 747. A minimum of new stuff would have to be invented.