To test its experimental X-43A — an unmanned, single-use, scramjet-powered, hypersonic aircraft of which three were built — NASA piggybacked it on two other aircraft.

First was the Boeing B-52 Stratofortress, which carried under its wing the other two vehicles to an altitude at which they could be ‘drop-launched’.

Then there was the booster rocket, a modified version of a Pegasus rocket, which would accelerate the X-43A after the drop launch to a speed at which its scramjet engine could operate.