the first unit to receive the Su-17. The Su-17 was produced until 1990, at a total of 2867 units produced. The Su-17 largely resembles its predecessor, the Su-7, with weight-saving measures added at the cost of combat survivability, an example of which is the removal of armored protection for the pilot. The prototype S-22I differed little from the Su-7 except for the wing, being essentially a technology demonstrator for the variable-geometry wing. It was later lost in an accident. Following the S-22I, two pre-production prototypes were constructed, designated the S32-1 and the S32-2, respectively. The two aircraft mounted updated avionics, and replaced the older AP-28I-2 autopilot with the newer SAU-22 automatic control system. The next series of prototypes were the Su-7-85, with 85 indicating the batch number, continued from that of the Su-7. The batch of ten aircraft incorporated a completely redesigned fuselage, a streamlined cockpit (similar to that of the Su-7U), extra and more accessible maintenance hatches, and an upwards-opening canopy. The front of the cockpit was protected with a windshield and two electrically heated side windows. The first three aircraft of the 86th batch that followed further incorporated clear windshields with warm air blown at it, taken from the 9th stage of the engine compressor. However, this new windshield was dropped in favor of the more traditional glazed windshield following tests by the 4th Combat Use and Retraining of Air Force Personnel Center in Lipetsk. The Su-7-85 was equipped with a modified KS4-S32 ejection seat, capable of safely ejecting the pilot at speeds above 140–170 km/h in the event of an accident. The fuel system of the Su-17 was modified from that of the Su-7 as well - fuel was now stored in three lightweight tanks, with provisions for up to four disposable auxiliary tanks each with 600 litres of capacity (itself used on the Su-7B), or two PTB-1150 tanks with 1150 litres each, mounted on "wet" pylons underneath the fuselage. The wing was largely unmodified from that mounted on the S-22I prototype. The stationary part of the wing being half as long as the rotating part. With wings at maximum sweep, the Su-17 would look virtually identical to the Su-7. A slide-out flap was installed on the stationary part of the wing, while a slat, a rotating flap and aileron are mounted on the rotating part. The sweep angle of the wing could be configured between 30°