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 Wright T35
Once Whittle’s work was made known (Lockheed’s and Northrop’s work was kept very secret at that time) Wright began its own gas turbine research in 1942 without United States Army Air Forces (USAAF) support, since the USAAF forbade the big engine manufacturers from doing anything except direct support of wartime needs. Once WWII ended and the USAAF could focus on future engines, it embraced the Wright design, which would become the T35, as well as other turboprops.
Wright was highly qualified in centrifugal compressor design, having been building its own superchargers for several years. Wright was also familiar with turbine design from having a close look (probably reverse-engineering) General Electric’s turbochargers for some time too, and Wright was undertaking the design of the compounding turbines for the R-3350 Turbo-Compound at the same time as the T35 program.
By the middle of 1944, Wright had the preliminary design of a 5,000-shp engine to show the USAAF. This size was chosen to be twice the power of Wright’s biggest engine of that time, the R-3350, and comfortably above that of the Pratt & Whitney R-4360. It had two centrifugal compressors on a single shaft driven by two axial turbines also on that same shaft. The USAAF gave Wright a development contract November 22, 1944 (W33-038ac-6247), when the Northrop B-35 and Convair B-36 were the only really-big bombers under contract with range requirements that could only be met by propeller-driving engines. The Wright T35 and Northrop T37 were considered follow-ons for these bombers, which originally used the R-4360.
The T35 was a big engine, with a diameter of 59 inches, versus about 42 inches for the T37, and 55 inches for the R-4360. The two centrifugal compressors conferred massive weight to the T35, 4,450 lb for the complete engine, versus less than 4,000 lb for most R-4360s, and 2,550 lb for the T34 (which had the same power goal as the initial T35. The original weight goal for the T35 was 3,800 lb.
The XT35-1 was first run June 24, 1946, putting out about 4,000 shp, and by this time had been selected in Boeing studies for the next-generation big bomber. These coalesced into the Boeing XB-52 program in March 1948, and the T35 was the number-one choice for power (the Northrop Turbodyne, designated XT37, was the USAF’s second choice). Amazingly today, the T35 had been the highest priority engine in development for the USAF for several years. Before the first run, Wright believed the engine could deliver 5,500 shp, and by the time 1,500 operating hours had been run (before a Preliminary Flight Rating Test, or PFRT), Wright was able to rate the engine at 6,600 shp.
The AAF was not satisfied with the slow (410 mph or less) speed the XB-52 could obtain with six T35s, and was demanding 10,000 eshp to bring a four-engine version up to 450 mph. Wright responded by adding a third centrifugal compressor stage and a third axial turbine stage to create the T35-3, initially rated at 7,300 shp, and weighing 5,950 lb. The contract for this version (W33-038ac-14145) was signed December 15, 1946 to continue development of the engine, and interest in the T35-1 declined rapidly. By January 1948, the USAF was declaring the T35 to be its top-priority engine.
Sometime before April 1948, the T35-3 had been uprated to 8,900 shp. Parts were fabricated for the -3, and first run was projected for September or October 1948. Wright was studying hollow turbine blades and recuperation in early 1948, and incorporated hollow blades in the Turbo-Compound’s turbines. Recuperation was not used.
Wright’s B-17 test aircraft under XT-35-1 power with all R-1820s feathered.
Shortly after -3 contract signing Boeing was realizing that propeller technology was inadequate to properly absorb power at 35,000 feet, and asked for contra-rotating propellers. This required a new gearbox design, and combined with changing the engine installation from tractor to pusher for a backup B-36 installation, resulted in the need for a new remote propeller-reduction gearbox, and the T35-5 model was eventually created to feature that gearbox change. Contract W33-038ac-21718 was signed on May 19, 1947 for the pusher/contra-rotating-propeller version.
In May 1948, the USAF was still saying “the T35 was imperative for the B-52”. The central problem of getting the T35 engine up to even 5,500-shp output with durability and reliability was being slowly resolved, and Wright had only been able to obtain 700 hours of engine development running by June 1948, as indicated in a report from Wright to the USAF dated June 14. Worse, it was not yet able to complete the 10-hour endurance test. However, the USAF reported that the PFRT on the -1 engine was completed in late July; so the intervening five weeks must have seen a breakthrough. This writer believes that test to have been the older 10-hour PFRT and not the 50-hour PFRT that was just coming into use.
The June report projected Wright’s test schedule would reach 1,700 hours around the time the program was cancelled, and not reach its goal of 10,000 hours of development engine operation until 1953. In that era, 10,000 development running hours were typically needed to prove an engine ready for trouble-fee production (engines would be certified/qualified sooner, with the expectation that service use would find and iron out some problems). Some writers have defined the T35’s progress as “slowness” in developing the engine (the T37 too), and that it caused the USAF to lose enthusiasm for turboprops. However, the successful T34 program proceeded at the same rate overall, and even took much longer for its first run. Although unstated in the histories of the big turboprop development programs, the gearbox was at least as big a challenge as the turbine stage. Wright’s June report also projected the cost of the T35 program to reach $56 million by the time all of the major problems had been found and fixed (10,000 hrs test work).
To obtain the greater power and improved fuel consumption, Wright increased engine shaft speed from 7,080 to 7,200 rpm, and increased turbine inlet temperature from around 1,550°F to 1,700°F. The addition of the third centrifugal compressor stage, a stackup that has only been done in water pumps, increased engine pressure ratio from about 4.5 to 9.4 (combined with the speed increase). Four engines were built for development running in the -1 program, two more were built for the -3 program, supplemented by four complete sets of spares (four equivalent engines), and only two new gearboxes ordered for the -5 program; to be run on an engine by April 1949. Wright admitted it was able to average only 8 hours of run time per engine per month, so that total run time likely did not exceed the projected 1,700 hours. Wright was projecting a steady increase in the run-time rate up to about 25 hours. Wright had also asked to be able to use the common parts fabricated for the -3 to get more -1 engines running. The expected first run of the -3 engine was delayed from late in 1948 to April or May 1949. The parts for the contra-rotating gearbox were built but not tested until after cancellation proceedings started, with 30 hours running done. It is not clear if this running was on a -1 or -3 engine.
Cancellation of the T35 had begun to loom in January 1948 when the USAF Headquarters felt the XB-52 was the wrong plane at the wrong time, but was persuaded by Boeing and Gen. Curtis LeMay to keep going. Then, in May 1948, Boeing was directed to supplant the T35 with the Westinghouse J40, and cancellation of the T35 program became more likely, as no other need was showing up. The J40 program later proved to be unsuccessful. On October 21, 1948, Boeing was directed to replace the J40 with the Pratt & Whitney J57; and the USAF began cancellation proceedings on the T35, with a 60-day stop order being issued January 4, 1949. Termination proceedings were not completed until March 1949. It is interesting to note that as late as December 7, 1948, the USAF was adding tasks to the T35 contract; in that case, parts for the contra-rotating propeller gearbox. The -3 contract was complicated by having added in a Wright J59 engine program, and some control equipment development items, but it looks like about $23.5 million was spent on the T35 for the three contracts. In mid-November 1948, the USAF said it expected program cost by the first Military Qualification Test to reach $40M.
Wright T35-1
Studies: 1942-1944
Contract Start: 22 Nov 1946
First Run: 24 Jun 1949
Contract End: Mar 1949
Number built: 4
Test Hours: 223 + ~1400
Power: 5,000 - 6,600 shp
Overall Pressure Ratio: ~4.5
Specific Fuel Consumption: 0.676 lb/shp/hr
Airflow: ~80
Weight: 4,450 lb
Turbine Inlet Temperature: ~1,550°F
Propeller Reduction Gear: Nose
Rotating Speed: 7,080 rpm
Dollars Spent: 3.0 million
Diameter: 59.0 inches
Length: 140.0 inches

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