The famous research aircraft series is shown above: in the center, the Douglas X–3; lower left, the Bell X–1A; continuing left to right, the Douglas D-558-I, the Convair XF–92A, the Bell X–5, the Douglas D–558–II, and the Northrop X–4. In the photo below, the X–15 is shown as it drops away from its mother B–52 and starts its own 57,000-lb.-thrust engine to begin another of its highly successful research flights.
