The flash boiling process of superheated liquid nitrogen (LN2) under high-altitude conditions was investigated with the help of a cryogenic temperature adjustment system and injection system at the test bench M3.3 at DLR Lampoldshausen. The test fluid LN2 reaches its superheated condition by varying the injection temperature or by a low-pressure environment in the vacuum chamber of the test bench. This cylindrical chamber with an inner diameter of 300 mm provides four optical accesses to visualize the sprays and their morphology by high-speed shadowgraphy and by laser-based Phase-Doppler diagnostics (PDA). For the present testcase, highly superheated LN2 sprays with constant injection conditions were generated. By means of PDA the local distributions of the mean droplet velocities and diameters within the flash boiling sprays were measured.