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This inflight photograph of the X-2 (46-674) shows the twin set of shock-diamonds, characteristic of supersonic conditions in the exhaust plume from the two-chamber rocket engine. The Curtiss-Wright XLR-25 rocket engine caused one of several problems that delayed flight of the X-2. At one point, people in the project suggested its replacement. It was the first "man-rated" (in the terminology of the day) rocket engine that was throttleable, and the technology was not yet mature. Other problems included the X-2's landing gear and the replacement of the planned electronic flight controls with a conventional hydromechanical system like that used in the F-86.