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McDonnell Model 78 assault transport helicopter project (U.S. Marines XHRH-1)

Mark Nankivil

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Spook

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Design of the Model 78 was started in 1950 in answer to a request for proposals for an assault transport helicopter which had been issued by the Navy Department on behalf of the US Marine Corps. Intended to operate from CVE-105 class (USS Commencement Bay) carriers, the new helicopter was to fit on 13.41m by 12.80m deck elevators. Its primary mission was to carry 30 troops to objectives up to 100 naut miles away and return to the carrier without the need to refuel ashore.

To fulfill what were then most demanding requirements, McDonnell designed a compound helicopter with a three-bladed rotor and short wings on which were mounted two 3507shp Allison XT56-A-4 turbines. For vertical flight, air from engine-driven auxiliary compressors was to be ducted to 726kg thrust McDonnell 12JP20 pressure jets, one at the extremity of each rotor. For forward flight, each engine was to drive a three-bladed propeller, with transition from helicopter flight mode to conventional flight being made by transferring power from the auxiliary compressors to the propellers as soon as the forward speed exceeded the stalled speed of the wing. Normal accommodation was to have been provided for a crew of two and 30 fully-equipped troops. Alternatively, accommodation could have been provided for 36 troops or 24 litter patients, or two Jeep-sized vehicles could have been carried internally. Larger loads (up to a maximum weight of 5224kg) could be carried externally. Another noteworthy design feature was the inclusion of a hydraulically-operated loading ramp in the forward fuselage, beneath the cockpit. Folding wings, rotor blades, and tail unit were to have enabled the Model 78 to fit on the deck elevators of CVE-105 carriers.

The Navy selected the McDonnell design on 15 March, 1951, and two and a half months later issued a Letter of Intent for three XHRH-1 prototypes (Bu Nos 133736/133738) to be built under Contract N0a(s)-51-1201. A full-scale mock-up of the XHRH-1 was inspected on 22 and 23 October, 1952, and construction of the prototypes was begun. However, after the Korean War had ended, budget cuts and concern over anticipated developmental problems led to the cancellation of the contract before completion of a single XHRH-1. All work on the project ended in April 1954.

Technical data for XHRH-1

Rotor diameter: 19.81m,
wingspan: 13.72m,
length with rotor blades and tail folded: 16.33m,
height: 5.09m,
wing area: 30.84m2,
empty weight: 8695kg,
loaded weight: 13795kg,
maximum weight: 16329kg,
maximum speed at sea level: 444km/h,
climb rate: 12m/s,
hover ceiling out of ground effect: 3050m,
combat radius: 185km
 

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Stargazer2006

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Topic merged with earlier contribution on the subject by Mark Nankivil. Here are slightly better resolution versions of the pics you provided (from this old topic), as well a two photos of a desktop model of that era (from this other topic - apparently still offered on eBay).
 

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fightingirish

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McDonnell XHRH-1 (Model 78)
McDonnell Model 78 Marine Assault Transport - Twin Engine Jet-Rotor helicopter, designated XHRH-1. Report No. 2738, dated 1 October 1952. Credit:Greater St. Louis Air and Space Museum.
Click here to download report (16 Megs)
Click here to download Mock Up photos in one Zip file (3.3 Megs)
Source: http://aviationarchives.blogspot.com/2017/08/mcdonnell-xhrh-1-model-78.html

Edit 2017-08-14:
McDonnell XHRH-1 (Model 78) Mock Up Photos
More mock up photos of the McDonnell XHRH-1 Marine Assault Transport - Twin Engine Jet-Rotor helicopter. Part of report No. 2775, dated 20 October 1952. Credit: National Archives via Bill (Phantomologist) Many thanks, Bill!
Click here to download photos in one Zip file (55.3 Megs)
Source: http://aviationarchives.blogspot.com/2017/08/mcdonnell-xhrh-1-model-78-mock-up-photos.html

See also the picture attachments in the following posts. :)
 

yasotay

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Having the ramp in front might have been a smart idea. Pilots would have been more circumspect where they dropped the "crunchies" (aviator term there).

Brilliant set of pictures! Thank you Sir.
 

Pioneer

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Thanks Bill S, those picture give me a great appreciation of a design which was miles ahead of its time.

Regards
Pioneer
 

FighterJock

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Was the Model 78 ever designed to be a competitor to the Chinook? Pity it was never built.
 

Mark Nankivil

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The Model 78 is 8+ years ahead of the Chinook and more in the same generation as the Piasecki H-21 which went into service about 2 years earlier. McDonnell proposed a number of evolutionary developments using the tip powered rotor such as the Model 99 and 113 - see: http://www.secretprojects.co.uk/forum/index.php/topic,16674.0 but the tip powered rotor was a dead end design (noise and fuel burn) and the tandem rotor proved to be the layout of choice to go forward with.

Enjoy the Day! Mark
 

Tailspin Turtle

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FighterJock said:
Was the Model 78 ever designed to be a competitor to the Chinook? Pity it was never built.
The Navy issued contracts to both McDonnell and Sikorsky for this marine assault mission, designated HRH and HR2S respectively. McDonnell's contract was subsequently terminated. The resulting Sikorsky helicopter is better known as the H-37, its Army designation. As Mark noted, this program was almost a decade earlier than the Chinook.
 

Silencer1

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Hi Tailspin Turtle!

Thanks for info about this competition between McDonnell and Sikorsky, with the later as a winner. The resulting product - heavy H-37 was my long time favorite. It has a distinctive shape and some interesting developmennts, like Sikorsky S-60 and (new to me) British
Westland Westminster.

Seems, that idea of heavy helicopters became more clear with turbine engines' intriduction.
 
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