The Lycoming XR-7755
Introduction and Contents
by Kimble D. McCutcheon
Published 1 Aug 2024; Revised 7 Oct 2024

Lycoming XR-7755-3 Larry Rinek

The Lycoming XR-7755 was the largest complete U.S. aircraft engine ever to run. Capable of producing 5,000 takeoff horsepower and cruising at extremely low fuel consumption, it was intended to power huge intercontinental bombers. Its development started in 1943; by 1948, it had struggled through a 50-hr full-power test, which exposed many issues that still needed work. The Air Materiel Command abandoned the project; it was clear that the future was going to be large gas-turbine-powered bombers whose range was extended by aerial refueling. This article is taken almost exclusively from primary sources in Record Group 342 (Records of U.S. Air Force Commands, Activities, and Organizations) at the U.S. National Archives at College Park, Maryland.

Preface

The Lycoming XR-7755 is an engine that I have researched since 1995. It is time to write about it. This will be a long article, and I have decided to provide installments as I sift through the source material. Those who have not done research in the U.S. National Archives' Record Group 342 Engineering Division files cannot easily conceive how daunting a task this is.

Imagine that a truck full of loose-leaf pages in no particular order dumps its load onto you lawn. You've no idea what the pile contains or even whether its contents are related. So you start picking through the big pile and making little ordered piles whose contents seem to belong together. This is the strategy I have taken with my previous books. For this book-length article, I intend to repeat what has worked before and to reveal that process as it unfolds, which means the reader is going to experience a work in progress rather than the finished result. I plan to add new content as I process it, so things may not appear in a rational order. I plan that the coverage of this engine will be largely chronological, but suspect that story arcs will reveal themselves as work progresses; these will be treated separately. Indeed, the Contracts/Administrative and Specifications topics are already active. The good news is that much of this material is the hand-written notes of the MatCmd XR-7755 project manager, J. Glen Blackwood. This presents a rare opportunity to analyze the project manager's role and the many hats (technical, adminstrative, referee, etc.) that he wore.

Please sit back and enjoy the process. I shall be continually updating a change log that shows what has recently been added. Please feel free to email me if you have suggestions, comments, or spot errors. I hope we all learn some new things about the XR-7755. — Kimble D. McCutcheon

Background

On 1 Sep 1939, Nazi Germany invaded Poland, marking the beginning of WWII. On 3 Sep 1939, France and Great Britain declared war on Nazi Germany. Meanwhile, U.S. Army Air Corps (USAAC) planners who embraced the strategic bombing doctrine were building a bomber fleet around aircraft intended to fly from European airfields. Germany's rapid defeat of Poland tossed that idea into question.

Facing the possibility that the USAAC might have to prosecute a war against Germany from the continental United States, it became clear that a new class of bombers would be required, ones that could carry a 10,000 bomb load 10,000 miles. The studies associated with this new strategy were lumped together under classified project number MX-423. Aircraft manufacturers were invited to submit proposals; Boeing and Consolidated-Vultee complied. The USAAC was unimpressed, claiming that the proposals were too conservative (although it did eventually buy the Convair B-36). The Materiel Command's Aircraft Laboratory ultimately drew up its own design series under the heading of MCD (Materiel Command Design) 392. These huge aircraft concepts featured up to 12 engines, a 655,000+ lb gross weight, a 321 ft wingspan, 120,000 lb maximum bomb load, and a 14-man (or more) crew. Allison V-3420 engines were to power these monsters.

Realizing that 12 engines would be problematical and that fewer, more powerful engines would be optimal, the USAAC invited engine proposals. Lycoming and Studebaker designs were most attractive. The Studebaker XH-9350 did not get much beyond single-cylinder testing. However, the Lycoming XR-7755 came very close to fruition. [Cully, George. The Range Extension Problem. 2012 AEHS Convention Presentation]

Materiel Command Design Series 392

MCD 392 Model Comaprison		MCD-392-A (Courtesy Scott Lowther)	MCD-392-B
MCD-392-B
Color Images from NARA RG342 Engineering Division Memos 4261-3-4, dated 12 May 4194, and 4261-3-5, dated 29 Jun 1944 Courtesy of Secret Projects Forum User Clioman

Working Topics

1943
1944
1945
1946
1947
1948
Contracts/Administrative
Specifications
Appendices (Acronyms, Glossary, Personnel)

Additions, Change History

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