A huge percentage of a rocket’s fuel is used just in overcoming its inertia and getting off the ground, and then more just to reach sub-sonic speed. So instead of useful payload the bulk of the rocket weight is the fuel needed to get the thing moving. Launching from a jet that’s already going that fast changes the economics quite a lot. This behemoth of a carrier plane just increases the scale of the kinds of payloads that are already being launched in this manner.
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