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[b]Space Cover #550: XF-91 Rocketplane[/b] Just over 70 years ago, Air Force test pilot Richard Johnson carried the cover above on one of his test flights of the Republic XF-91 Thunderceptor. This cover is listed in the Ellington-Zwisler Rocketmail Catalog as US #72C1. It was flown and then postmarked on February 1, 1950 at Muroc, Ca. (the post office at Edwards AFB had not yet changed from its former "Muroc" name at this time). So, what's a XF-91? Let's take a look at a rendition of the XF-91 used as the cachet art by Cachet Craft for their first day cover of the 50th Anniversary of Powered Flight stamp (Sc #C47), below: The XF-91 was designed by Republic Aviation to be a jet and rocket powered interceptor fighter plane for the Air Force. Its body had the cigar shape used in Republic's earlier F-84 fighter planes, with new wings (wider at the edge than at the root), a beefier jet engine, and provisions for a new Curtis-Wright rocket engine. Looking at the rendition above, we can see squared off "blisters" just above and below the tailpipe, and these were where the two barrels of the rocket motor were to be. The XF-91 first flew on May 9, 1949 using jet power only because the rocket engine was not yet ready. Republic test pilot Carl Bellinger carried a cover on that flight. The XF-91 made a number of other jet powered flights (including the February 1, 1950 flight above) while waiting for the new rocket engine. Finally, the Air Force got fed up with the delays on the new rocket engine and outfitted the XF-91 with the venerable Reaction Motors XLR-11 rocket engine that had been powering the X-1 rocketplane for 5 years. The XLR-11 was installed in the lower "blister" and the XF-91 made its first jet and rocket powered flight on December 9, 1952. The XF-91 went on to perform a number of combined jet and rocket powered flights, but by that time the Century Series of fighter planes were just around the corner and could outperform the XF-91 as a fighter plane. And the X-1A and upcoming X-2 could far outfly the XF-91's research capability. So it got cancelled soon afterward. When we study the history of the rocketplanes, we will see mention of the XF-91. But bear in-mind that its flights were mostly jet powered only, and it did not push the boundaries of speed and altitude the way the X-1, X-2, and D-558-II did. In fact, we could say that Ellington and Zwisler actually erred by listing the cover above as a rocketmail cover since there was no rocket engine aboard the XF-91 at that time...
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