Another NASA/US Army programme involved the S-72 helicopter, used to test various integrated rotor and propulsion systems. After the first flight on 12 October 1976, the S-72 completed the first experimental phase in February 1977.

The fuselage was entirely new compared with the other models by Sikorsky. It had traditional, airplane-type swept tail surfaces with a five-blade main rotor and five-blade tail rotor. Various types of blades (rigid, articulated) and transmission systems were tested.

A small, swept wing was fitted to one of the S-72 prototypes, plus two General Electric TF-34 turbofans, in two outboard nacelles. An interesting detail was the fact that the entire crew of the helicopter had ejector seats. With the supplementary turbojets, the S-72 increased its speed to about 450km/h. In late 1983 Sikorsky received a contract to modify one S-72 for X-wing "stopped rotor" research, a concept whereby a rigid rotor is stopped in cruise flight with the blades then acting as wings to provide lift, whilst engine power is diverted from the rotor system to give pure jet thrust.

G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984

Under contract to NASA and the US Army Sikorsky also built two S-72 Rotor Systems Research Aircraft (RSRA) designed to serve as test vehicles for a variety of rotors/rotor systems. The configuration included wings of 13.74m span, a tail unit with conventional control surfaces, two 1044kW General Electric T58-GE-5 turboshaft engines to power the rotors and auxiliary powerplant of two 4207kg thrust General Electric TF34-GE-400A turbofan engines. The wings and auxiliary turbofans, which were attachable/detachable on each side of the fuselage, made it possible to test rotors that would be too small to lift and support the S-72 in flight. This then progressed into trials involving an X-wing in place of the main rotor. Using a compressed air system this airfoil could be blown to affect cyclic and collective pitch. The wing could be stopped in flight and Sikorsky believed the design had potential for future use as a high-speed 'convertiplane'. First flight took place on 2 December 1987, but after three further flights all funding was suspended and the S-72X1 was placed in storage at Edwards AFB.

D.Donald "The Complete Encyclopedia of World Aircraft", 1997

Certainly one of the more exotic-looking rotorcraft yet developed, the Sikorsky S-72 was designed in response to a joint Army/NASA requirement for a high-speed helicopter propulsion and rotor systems testbed. The S-72 was selected in October 1973 over a competing Bell Helicopter design, and in January 1974 Sikorsky was awarded an Army/NASA contract for the construction of two machines. The Rotor Systems Research Aircraft (RSRA), as the S-72 was officially named, was intended from the start to be capable of operating with podded auxiliary turbofan engines and airplane-style wings, and the initial production contract therefore also covered the fabrication of one set of wings and the adaption of two General Electric turbofans for later installation on the completed aircraft. The first RSRA made its maiden flight as a helicopter in October 1976 and the second prototype, built from the start as a compound with wings and turbofans already attached, made its initial ascent in April 1978.

In designing the S-72, Sikorsky's engineers mated a new, highly streamlined glass fibre and alloy fuselage to the twin 1400shp GE turboshaft engines and five-bladed main rotor system of the proven Model S-61/H-3 Sea King. The new craft's sharply-swept T-tail supported a conventional rudder, a five-bladed anti-torque rotor, a low-set tailplane equipped with elevators, and a ventral fin incorporating a non-retractable tail wheel. In the compound configuration the S-72 was fitted with low-set, variable-incidence wings, each of which incorporated conventional ailerons and flaps. When fitted, the RSRA's turbofan engines were carried one on either side of the forward fuselage just aft of the cockpit. The fitting of the conventional wings, tail surfaces, and auxiliary engines allowed the S-72 to flight test experimental rotor systems that would not, by themselves, support the aircraft's weight. In an emergency the crew was able to use explosive charges to jettison any main rotor system being tested and either eject from the aircraft or, if conditions permitted, fly it back to base as a conventional airplane.

Testing of both RSRA helicopters was conducted jointly by the Army and NASA until 1980, when the latter agency assumed overall control of both machines. In early 1984 Sikorsky was awarded a NASA/Defense Advanced Research Projects Agency (DARPA) contract to convert one of the S-72s into a demonstration testbed for the company's experimental X-wing system.

S.Harding "U.S.Army Aircraft since 1947", 1990

Technical data for Sikorsky S-72 RSRA Crew: 2-3, engine: 2 x General Electric T-58-GE-5 turboshaft, rated at 1045kW and 2 x General Electric TF-34-GE-400A turbofans, 4180kg of thrust each, main rotor diameter: 18.90m, wingspan: 13.74m, fuselage length: 21.50m, height: 4.42m, take-off weight without auxiliary jets: 8300kg, empty weight without auxiliary jets: 6535kg, take-off weight with auxiliary jets: 11815kg, empty weight with auxiliary jets: 9480kg, max speed without auxiliary jets: 296km/h, cruising speed with auxiliary jets: 258km/h, max speed without auxiliary jets: 581km/h, cruising speed with auxiliary jets: 370km/h, ceiling: 3050m