I’m not sure. What about all the fuel needed to get the combination of rocket AND plane to 50k? Is that really less fuel than getting a rocket alone to 50k feet? Are aircraft really THAT much more efficient at lifting payloads than rockets?
Yes.
Instead of brut force to go up 9.5 miles (to 50,000ft.), you use the lift of air over a wing. You’re expense in fuel is for forward motion.