Space Cover #122: The X-2 Rocketplane
Well, some clown named micropooz started this on SCOTW #111 with a flown cover from the Douglas D-558-II Skyrocket in 1951. Then stevedd841 upped the ante on SCOTW #118 with an X-15 flown cover from 1967! Well, it's hard to surpass Steve, so...
...instead, let's fill in the gap between the Skyrocket and the X-15.
The Bell X-2 rocketplane incorporated swept wings (as did the Skyrocket) but used an engine with 2 to 3 times the thrust of the Skyrocket. Once the teething problems of the bigger engine got solved, the X-2 rapidly started setting speed records in the mid-1950's.
On July 23, 1956 the X-2, in the hands of pilot Frank K. "Pete" Everest, set a record speed of 1900 mph — 2.87 times the speed of sound (Mach 2.87 — and see above)! Everest carried an unknown number of covers on this flight (as he had on previous flights), in the pocket of his pressure suit. He mailed all of the covers carried on this July 23 flight on July 24, hence the day-after postmark from Edwards, CA.
This is an example of Ellington-Zwisler #81, each of which were postmarked at Edwards on July 24, and each example has a different cachet dependent upon the addressee who mailed their blank cover to be carried by Everest. I've held three or four of these in my hands, and have seen several more offered for sale, and each is a little different.
After this flight, pilot Ivan Kincheloe took the X-2 to a record 126,000 ft altitude (24 miles). Then on September 27, 1956, pilot Mil Apt exceeded Mach 3 in the X-2. Unfortunately the X-2 went into an irrecoverable spin at that point, killing both Apt and the X-2 program.
After that, it was up to the X-15 to push the way into higher/faster winged flight, and to pave the way to a future Space Shuttle.