2006 $11.40 X-Plane
Issue Date: March 17, 2006
City: New York, NY
Printed By: Banknote Corporation of America for Sennett Security Products
Printing Method: Lithographed with hologram
Perforations: Die cut 10 ¾ x 10 ½
This Express Mail stamp features a side view of the X-15 plane. Only three X-15 planes were built. They made 199 flights from 1959 to 1968. Thirteen of those flights exceeded 50 miles in altitude, the U.S. criterion for spaceflight. The pilots on those flights qualified as astronauts.