【X15試驗機】人類飛行速度最快飛機，採用火箭動力，飛行速度接近7馬赫，飛行員必須要穿壓力服

北美X-15（North American X-15）是一架由北美航空所承製開發的火箭動力實驗機。 X-15可能是在貝爾X-1之後，美國空軍／NASA／美國海軍X系列試驗機中最重要的一架。在1960年代，X-15打破了許多速度與高度的記錄，由於其飛行高度到達大氣層的邊緣，為之後的研究提供了重要的資料。它同時也是美國建造的第一個有人亞軌道飛行器。

在整個X-15計劃的飛行項目中，有13架次到達了80千米以上的高度，這是美國空軍所製訂宇宙飛行的標準高度，駕駛這13架次的八名飛行員因此獲得宇航員的身份。另外還有兩名飛行員獲頒NASA的宇航員之翼勳章。

一些權威的航空學研究者設定的太空的起始高度比美國空軍和NASA的標準高度都低，所以還有許多X-15飛行員也可被視作宇航員。在距地球19英里（31千米）的上空是適航大氣層，這裡的環境與太空基本相同，而許多X-15飛行員駕駛飛機都到達此高度以上。

另外還有兩架次（由同一飛行員駕駛）到達國際航空聯合會的宇宙飛行標準高度，即到達62.1英里高度（100千米）。
North American X-15 (North American X-15) is a rocket-powered experimental machine developed by North American Aviation. The X-15 may be the most important of the X-series test aircraft of the US Air Force/NASA/US Navy after the Bell X-1. In the 1960s, X-15 broke many speed and altitude records. Because its altitude reached the edge of the atmosphere, it provided important information for subsequent research. It is also the first manned suborbital vehicle built by the United States.

In the entire X-15 flight program, 13 sorties reached altitudes above 80 kilometers, which is the standard altitude for space flight established by the US Air Force. The eight pilots who flew these 13 sorties were thus granted the status of astronauts. In addition, two pilots were awarded NASA's Astronaut Wings Medal.

Some authoritative aeronautics researchers set the starting altitude of space lower than the standard altitudes of the U.S. Air Force and NASA, so there are many X-15 pilots who can also be regarded as astronauts. The airworthy atmosphere is 19 miles (31 kilometers) above the earth. The environment here is basically the same as that in space, and many X-15 pilots fly above this altitude.

Another two sorties (flighted by the same pilot) reached the International Aviation Federation’s standard altitude for space flight, which reached an altitude of 62.1 miles (100 kilometers).