Lets come back to Dyna Soar...
After 15 years of successfull flight with USAF, NASA and CIA, the DynaSoar I was free for export markets. After the outbreak of WWIII in 1983, many countries needed high performances recon system to overfly USSR.
The US governement wanted to sell the DynaSoar spaceplane to its allies but a big problem quickly appeared. The DynaSoar launcher, Titan V was unafordable for exports markets, because of its costs, launch pad, and assembly line. As a consequence, Boeing proposed a much simpler way of launching the space plane.
The idea was using a 747 and an expendable upper stage to push the small shuttle in orbit. Boeing first thought was using the Saturn V S-IVB but it was impossible to put it piggy back on the 747 because of its big diameter. More, its J-2 engines were using hydrogen, a dangerous fuel to handle.
As a consequence, Boeing changed its plans and created a cheap expendable stage. They used the H-1 engine of the Saturn I rocket; it had the same thrust as the J-2 but used Kerosen, like its 747 carrier!
More, Boeing ingeneers had a brilliant idea : they adapted the diameter of the stage to the 747 cargo bay. This idea allowed the foreign users of the system to use the 747 not only as launcher, but also as ferrying aircraft!
The 747 launcher was basically similar to the 747-200F, but part of the fuel (and range) was cut to boost the payload at take off. Payload was now 160 tons at take off, range was 4500km only (but sufficient)
The system was quite cheap because the 747, H-1 and DynaSoar were well-known technologies... so DynaSoar started to wear foreign markings (stay tunned!)