The North American X-15 rocketplane flew at up to Mach 7 and at very high altitudes, it was also capable of spaceflight but these were quite steep parabolic patterns. At Mach 7 it flew leveled and my question is whether the X-15 ever landed on another airport than the one it launched from (attached to its B-52 carrier plane so actually the airport where the B-52 launched from). Wikipedia seems to be silent on this.
my question is whether the X-15 ever landed on another airport than the one it launched from
That depends on what you mean by "landed", "airport", and "another".
On its free flights, it never landed on another airport than the one it started from. However, it also never landed at the airport it started from. In fact, it didn't land at an airport at all.
It always landed on a dry lakebed near the airport. Whether you consider them part of the airport or not, is a matter of definition. As is the question whether you consider a dry lakebed an airport. I believe that the lakebeds were prepared with temporary runway markings, and they also may have cleared the "runway" from rocks and debris.
However, on aborted flights or captive carry flights, when it was never released from the carrier aircraft, it obviously landed attached to the carrier aircraft back at Edwards AFB.