The HRH was a McDonnell Assault helicopter project for the USMC. The design was a 30 seat turboprop-driven compound helicopter with the rotor driven by tipjets. In late in 1950, in response to BuAer's request, the helicopter manufacturers competing for the assault transport helicopter presented their proposals. Thereafter, in March 1951, the bureau selected two aircraft companies to build the helicopter, McDonnell and Sikorsky.
McDonnell Aircraft received a contract for two experimental models of an extremely advanced design. It incorporated the conventional single main rotor configuration; however, the power was to be provided by jet burners located in each rotor blade tip. This method of propulsion is also referred to as the "pressure jet principle." To produce thrust, compressed air is routed to each rotor blade tip where it is then mixed with fuel and ignited. Also unique in the McDonnell design, and a feature which made it a compound helicopter, was the installation of twin gas turbine propeller engines mounted externally in wings.
The compound helicopter was designated as the XHRH-1 (H-helicopter, R-transport, H-heavy) and was estimated to carry a useful load of 13,000 pounds at a cruising speed of 150 to 200 knots. The empty weight was estimated at a little over 26,000 pounds. The flight technique for the HRH envisioned the machine taking off as a conventional helicopter, then as its airspeed increased it would convert to fixed-wing flight, with the reverse procedure for executing the landing phase.
The awarding of dual contracts for the same operational requirement (AO-17501) appeared justified in view of the complexities involved in both McDonnell's and Sikorsky's proposals. The two-phase program was established in order to provide the Marine Corps with maximum protection in the event one of the designs failed to materialize. In this case, progress in the development of the assault transport helicopter was planned to provide two helicopters in logical sequence with the XHRH going beyond existing requirements. The procurement provided for the development of equipment to satisfy future requirements by taking advantage of technological progress beyond that incorporated in the HRS-A.
The original contract did not specify such a date nor did the CNO assign a priority number. Later, on 9 April 1952, the CNO assigned a priority of 1C to the XHRH, and subsequently BuAer published a proposed first flight date of December 1955.
Although the experimental one-man helicopter project survived for many years, others were not so successful. The McDonnell XHRH-1 was one in particular. The project was cancelled after mock-up stage. The Model 78 heavy assault helicopter was a 30,412 lb design with jet-tip rotor, stub wings, and tractor Allison T56 turboshaft/props on each wing. Three aircraft were ordered in 1951, but the project had only reached the mock-up stage when cancelled two years later, in favor of the Sikorsky HR2S/H-37.
Its short life was attributed to an economy move which required the diversion of its funds to a similar and equally unsucecssful helicopter, the Navy's XHCH-1 (H-helicopter, C-cargo, H-heavy). The demise of the XHRH-1, eventually came as a result of insufficient funds to carry out development of the power plants, rotor head, and rotor blades .
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