Should there be a need for higher-than-normal velocities, a set of two booster rockets can be fitted to the superstructure to augment the main propulsion system.
The vehicle consisted of a spaceplane for orbit and re-entry, fueled by expendable liquid hydrogen and liquid oxygen tanks, with reusable strap-on solid booster rockets.
Instead, a partially (not fully) reusable design was selected, where an external propellent tank was discarded for each launch, and the booster rockets and shuttle orbiter were refurbished for reuse.
The test makes it five times that this same booster rocket has flown successfully, then landed again at the launchpad to be reused for the next flight.