The X-15 Program Ended In 1968. 56-6671 Was In An Accident And Was Later Rebuilt As An X-15az. Created To Further Explore The Performance Envelope Of The Basic Airframe And To Expand Its Velocity To Mach 8. Also Could Carry A New Scramjet. The External Tanks Enabled The X-15 To Increase Its Burn Time By Approximately 70%. Left Tank 1,150 Pounds--Liquid Oxygen; Right Tank 648 Pounds Contained 1,080 Gallons Of Anhydoros Amonia. 56-6671 Is At The Air Force Museum, 96-6670 Is At The Smithsonian.