The McDonnell HRH, company designation Model 78, was a 1950s transport helicopter proposal for the

United States Marine Corps The United States Marine Corps (USMC), also referred to as the United States Marines or simply the Marines, is the maritime land force service branch of the United States Department of Defense. It is responsible for conducting expeditionar ...

(USMC) by

McDonnell Aircraft The McDonnell Aircraft Corporation was an American aerospace manufacturer based in St. Louis, Missouri. The company was founded on July 6, 1939, by James Smith McDonnell, and was best known for its military fighters, including the F-4 Phantom II ...

, designed to operate from ''Commencement Bay''-class

escort carrier The escort carrier or escort aircraft carrier (U.S. hull classification symbol CVE), also called a "jeep carrier" or "baby flattop" in the United States Navy (USN) or "Woolworth Carrier" by the Royal Navy, was a small and slower type of aircraf ...

Development

was awarded a contract for the aircraft in March 1951 after responding to a 1950 USMC requirement for an assault transport helicopter to carry 30 fully-equipped marines and two pilots from an

. The HRH emerged as a

compound helicopter A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attribut ...

with a three-bladed rotor driven by McDonnell-designed

tip jet A tip jet is a jet nozzle at the tip of some helicopter rotor blades, used to spin the rotor, much like a Catherine wheel firework. Tip jets replace the normal shaft drive and have the advantage of placing no torque on the airframe, thus not re ...

s fed by auxiliary compressors driven by the main engines, which were also equipped with three-bladed propellers that would provide forward thrust once the stall speed of the aircraft's stub wings was exceeded. The aircraft would be equipped with a hydraulic loading ramp under the cockpit, and could alternately carry two smaller vehicles or 24 stretchers instead of 30 troops. The rotors, stub wings, and tail would fold to allow the large aircraft to use the small deck elevators on ''Commencement Bay''-class escort carriers. A full-scale

mockup In manufacturing and design, a mockup, or mock-up, is a scale or full-size model of a design or device, used for teaching, demonstration, design evaluation, promotion, and other purposes. A mockup may be a ''prototype'' if it provides at lea ...

was inspected and approved by the

U.S. Navy The United States Navy (USN) is the maritime service branch of the United States Department of Defense. It is the world's most powerful navy with the largest displacement, at 4.5 million tons in 2021. It has the world's largest aircraft ...

in October 1952 and construction of three prototypes (

bureau number In the United States, all military aircraft display a serial number to identify individual aircraft. These numbers are located on the aircraft tail, so they are sometimes referred to unofficially as "tail numbers". On the Northrop Grumman B-2 Spi ...

s ''133736'' to ''133738'') was authorized; however, officials were concerned about potential developmental problems, and the end of the

Korean War The Korean War (25 June 1950 – 27 July 1953) was an armed conflict on the Korean Peninsula fought between North Korea (Democratic People's Republic of Korea; DPRK) and South Korea (Republic of Korea; ROK) and their allies. North Korea was s ...

led to substantial US military budget cuts. Although the

Bureau of Aeronautics The Bureau of Aeronautics (BuAer) was the U.S. Navy's material-support organization for naval aviation from 1921 to 1959. The bureau had "cognizance" (''i.e.'', responsibility) for the design, procurement, and support of naval aircraft and rela ...

had expected the XHRH-1 to fly in December 1955, the design was cancelled in 1953 in favor in of the

Sikorsky HR2S The Sikorsky CH-37 Mojave (company designation S-56) is an American large heavy-lift military helicopter of the 1950s. It entered service as the HR2S-1 Deuce with USMC in 1956, and as the H-37A Mojave with the U.S. Army that same year. In the e ...

, the first prototype remaining uncompleted. All work on the aircraft was terminated in April 1954.

Specifications (Model 78 / XHRH-1 estimated)

References

{{Authority control United States military helicopters 1950s United States military transport aircraft 1950s United States helicopters HRH Tipjet-powered helicopters Compound helicopters Twin-turboprop tractor aircraft Cancelled military aircraft projects of the United States Aircraft with fixed conventional landing gear