Space Shuttle Orbiter Model / Medium Experimental and Prototypes and the Air Force grounded

and the Air Force grounded 57 of the planes on 10 occasions due to problems with the engines

aerobatics

the NS-1 proved its worth as a primary trainer

the plane has been flight tested at more than one million pounds at the Air Force Test Center at Edwards

Officially

Space Shuttle Orbiter Model / Medium Experimental and Prototypes and the Air Force groundedThe Space Shuttle takes off from Earth as a rocket and maneuvers in orbit as a spacecraft. After reentry into Earth's atmosphere it is controlled and brought in to an unpowered landing in the mode of a conventional fixed wing airplane. The Shuttle takes off vertically under the power of three Rocketdyne main propulsion engines, each developing 375,000 lb. thrust, plus two Thiokol solid propellant booster rockets, each producing 2,900,000 lb. thrust,