The Bloodhound SSC uses an 18inch hybrid rocket as one of its engines to propel it to 1000MPH. Two of these rockets are needed, as the vehicle has to do two runs in opposite directions in under an hour.
Acting as a ‘quiver’ for the rockets, the Supacat vehicle would carry them to the turn around point of the vehicle, where they could be changed quickly and safely.
You can find out more about the Bloodhound SSC Project here.