The X-15 flew so fast that it already had special paint which ablated in flight. It was already hot, and it would have been sub-orbital of course.
... Or a rocket like that with your choice of fuel (or nitrogen peroxide) sling shot skyward from a magnetic rail system high in the equatorial Andes Moutains in South America. (Some SF authors like Larry Niven wrote about this concept of launcher.) ...
There are three major shortcomings with using a rail/sled launcher as opposed to a 747. Location, location, location. Just kidding. It is quite true that a mag-rail system will be highly energy efficient, and will give a substantial boost to the feasability of any assisted SSTO. However, the infrastructure costs of such a facility would be rather high. There is a substantial downside to placing a non-moveable spaceport in such a hard-to-reach location. A 747 launcher could use just about any large airport in the world as a spaceport.
The launch point from a mountain-based mag-rail would be at approximately one half the altitude of a 747 launch. The thicker atmosphere and higher gravity losses would take a bite out of your projected payload.
Two-stage fully reusable solutions launched from an Andes mountain would be particularly difficult because the first stage would probably come down out in the Atlantic ocean. A 747 could fly to a launch point where the first stage would come down exactly where you want.
Operating a cargo 747 at a small multiple of fuel costs is an established and perfected business model. Countless investors all over the world already own shares and make profit from these companies. It might be easier to find financing for a 747 based space cargo company than it would to finance a South American mag-rail system.
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There exists a properly sized kero/LOX rocket motor with the required levels of efficiency, reliability, and reusability, the RD-0124M1
http://www.astronautix.com/engines/rd0124m1.htm . It could improve its efficiency even further by using subcooled propane/propylene mix instead of kerosene. Five of these motors would be ideal for the rocket I've described. Combined with a standard blunt-cone semi-ballistic re-entry profile using a TUFI-based heat shield, the rocket I've described could launch a payload every day, and could have an operating cost under $500,000 per launch.
Potentially, an orbital tether (
http://www.tethers.com ) could work amazingly well with a system like this, doubling the payload fraction while reducing re-entry loads. It's hard to imagine cheaper access to space without a beanstalk.
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I grew up on much of the same sci-fi that you mention here, but my experience working as a NASA sub-contractor took some of the stars out of my eyes.
RE: Using a 747 for a 1st stage of space cargo system.
I suspect one of the X-Prize companies might try this after they establish themselves in the sub-orbital market. This might be a second generation design. Doing a google search revealed the cost of a brand-new 747 freighter is around 200 million dollars. Ugh! Maybe they could get a used one and refurbish it, perhaps it would only cost around 50 million then. So as soon as the profit threshhold of a space cargo company rises to the 50-100 million level, maybe we'll see something like this in the air. I wonder if Rutan is planning on creating bigger versions of his carrier craft? This will be interesting to watch unfold in the next few years.