S-54 single-engine two-seat advanced supersonic trainer and light combat aircraft which alongside with Myasichev entry lost to subsonic twin-engined Mikoyan and Yakovlev trainers. The S-54 did not meet official requirement as a replacement of Aero L-29 and L-39 because RusAF was mostly interested in twin-engined aircraft. Nonetheless, the ANTK Sukhoi saw an opportunity for a supersonic advanced trainer in the world's shrinking trainer marked and continued the refinement of the basic S-54 design. The S-54 is targeted at the T-38 and MiG-21U operators and complimented by more sophisticated and capable armed single-seater version can take a bite out of F-16 share.
Since the launch of the program in 1990 configuration has been refined at least twice, in 1992 and 1996. The original layout somewhat reminiscent of scaled-down Su-27 Flanker with all-swept mid-wing configuration and twin outward-canted fins lacked Flanker's prominent sting. An early configuration featured a single ventral engine air intake positioned under the cockpit area.
During the second iteration of S-54 development, the aircraft received larger LREX, its bifurcated reshaped engine intakes were moved forward under the wingroots while more outward-canted twin fins were moved to the wing trailing-edges akin F/A-18. The wing was modified to have wingtip mounts for two dogfight IR homing AAM (R-73) and two hardpoints under each wing for air-to~air and air-to~surface guided weapons. The avionics for new trainer were tested on modified Su-25 and Su-27.
The experience gained with T-10-24 and naval Flanker determined the direction for further refinement of the S-54. This involved an addition of the canards, referred PGO in Russian (Perednee Gorizontal'noe Operenie - Forward Horizontal Stabilizers). The PGO solved control problems encountered at high angle-of-attack (AOA) flight regimes when the tailerons lost their efficiency in the wake of the wing. The cockpit was moved to the aft to provide additional volume for radar and avionics, the LREX area was reduced and the rear seat was raised to give a superb view for both pilots, changing the shape of two-piece canopy. This changes indicated shift in S-54 marketing from combat capable advanced trainer to a capable multirole two-seat light fighter aircraft.
A final and natural step of S-54 evolution involved elimination of the second seat and fill the space with more fuel and avionics to increase the range and widen the capabilities of the aircraft. Sukhoi have vast experience in similar modifications trying it out with Su-17M3 and Su-25T/39 attack aircraft. The final configuration redesignated S-55 was presented at Le Bourget 1997. A scale model (which was stolen during the first days of the show) showed the aircraft of T-10-24 aerodynamics utilizing the tandem triplane layout (canard foreplanes, wings and tailplanes). The second seat was removed, the canard foreplanes were shifted further forward from the wing roots and the tailerons placed further aft. This baby Su-33 had a pair of rearward facing stings, most likely housing additional fuel, avionics and brake shute(s).