The second operational aircraft designed around a stealth aircraft shape and materials, after the Lockheed A-12, the SR-71 had several features designed to reduce its radar signature. The SR-71 had a radar cross-section (RCS) around . Drawing on early studies in radar stealth technology, which indicated that a shape with flattened, tapering sides would reflect most energy away from a radar beam's place of origin, engineers added chines and canted the vertical control surfaces inward. Special radar-absorbing materials were incorporated into sawtooth-shaped sections of the aircraft's skin. Cesium-based fuel additives were used to somewhat reduce exhaust plumes visibility to radar, although exhaust streams remained quite apparent. Kelly Johnson later conceded that Soviet radar technology advanced faster than the stealth technology employed against it.