The Ultimate Superfortress: The B/RB-54A

Concept art of the B/RB-54A in flight
(Boeing Historical Archives)

During the Second World War Boeing worked extensively on further improvements to the B-29 Superfortress. The most important of these improved variants was the B-29D that involved swapping out the Wright R-3350 radial engines with the more powerful Pratt & Whitney R-4360 Wasp Major radial engine. In July 1945 the USAAF signed a contract for 200 B-29Ds, but with the end of the war and the rapid postwar demobilization, the B-29D contract was canceled. With the creation of an independent United States Air Force in 1947, there was a need for interim bombers pending the arrival of more advanced jet bombers. The USAF was already getting the Convair B-36 which took on the mantle of the heavy bomber, but the USAF also wanted the B-29D which would be redesignated as a medium bomber. The USAF had the B-29D redesignated as the B-50 to avoid the appearance of ordering a "wartime" bomber. Making its maiden flight on 25 June 1947, the B-50 Superfortress would eventually result in 320 examples of all variants produced.

Boeing, however, was working on an even more powerful and longer-ranged development of the B-50. Designated the B-50C, this evolution into the ultimate Superfortress was designed to extract as much speed and performance as was possible using a new version of the Pratt & Whitney R-4360 Wasp Major engine that added what was called a "variable discharge turbine" (VDT) to the engine. The standard Wasp Major used on the B-50 developed approximately 3,500 horsepower and a Wasp Major with a VDT could easily produce 4,000 horsepower, making it one of the most powerful production piston engines in the world.

The Wasp Major VDT
(from the Engine History website)

The VDT consisted of two General Electric CHM-2 turbosuperchargers that collected the hot exhaust gases from the 28 cylinders of the Wasp Major. A portion of the hot gases were diverted through an intercooler to provide turbosupercharging at high altitudes. The bulk of the hot gases went through the CHM-2 turbines and were exhausted out a variable area nozzle that resembled a set of eyelids. By adjusting the size of the nozzle, jet thrust could be achieved that had the potential to add as much as 15% to the speed of the B-50C over the production standard B-50. The Wasp Major VDT was already flying at this point on the Republic XF-12/XR-12 Rainbow long range reconnaissance aircraft. 

The scope of the changes needed to for the B/RB-54 resulted in a redesignation to B-54 with the planned reconnaissance variant being the RB-54. The jump in power output from the use of the Wasp Major VDT resulted in a redesign of the wings that resulted in a wingspan that was over 20 feet longer than that of the B-50 with a chord increase as well- an additional six feet of chord at the wing root and an additional four feet of chord at the wing tip. This provided additional fuel capacity along with external fuel tanks which were three times the capacity of the external tanks used on the B-50A on the outboard wings. The wingspan increase was so much that outrigger gears were needed under the outermost engine nacelles. Wind tunnel testing had shown that the new wing and powerful engine output also required a longer fuselage and the B/RB-54's fuselage was stretched 10 feet. Instead of the plexiglass domes used by the gunners on the B-29/B-50, low drag hemispheric sights were used. These used a fish eye hemispheric optical element that the gunner sighted through. Glenn's Computer Museum has some great pictures of the R/RB-54 hemispheric gunsight. The tail gunner also had a hemispheric gunsight but also had a radar to direct the four-gun turret as well which was mounted in fairing above the gun turret but below the hemispheric gunsight. Fairings were also present on the nose and under the forward fuselage for bombing and navigation radars. 

As a comparison, the B-29 weighed 120,000 lbs fully loaded and the B/RB-54 would weighed in at 207,000 lbs at takeoff. The Wright R-3350 engines of the B-29 developed 2,200 horsepower and the bomber had a range of approximately 3,250 miles. The B/RB-54 would have been able to push 8,000 miles of range. The mockups were completed in 1948 and the contract was signed for 43 bombers as an initial production lot. While the Secretary of the Air Force Stuart Symington and the USAF Chief of Staff General Hoyt Vandenberg were supportive of the B/RB-54 project, General Curtis LeMay, the head of the Strategic Air Command, felt that the B/RB-54 was inferior to the Convair B-36 Peacemaker particularly the B-36D that added four J47 jet engines under the outer wings. Pending the arrival of the B-52 Stratofortress, LeMay felt deterrence was better served by the B-36 which could fly faster, farther, higher, and carry a significantly larger bomb load. In the postwar atmosphere of austerity, more B-36s couldn't be accommodated in the Air Force budget and Secretary Symington offered LeMay more B-50s instead of increased numbers of B-36s. This was even more unsatisfactory to the outspoken SAC commander who then argued that if he couldn't get more B-36s, then the funding set aside for the B/RB-54 should be shifted over to get more of the Boeing B-47 Stratojet which made its first flight in December 1947. This was agreeable to all involved, even for Boeing as it meant more funding for the Stratojet program. The B/RB-54 project was cancelled with the prototype approximately 75% complete (it was converted from a B-50A) at Boeing's Seattle facilities. In addition, the addition of the outrigger gears wasn't popular with SAC as many of its bases would need widened taxiways and runways to accommodate the B/RB-54. 

The B-29 lineage would live on, though, in the C/KC-97 Stratofreighter (the last examples being retired in 1978) and in the commercial Boeing 377 Stratocruiser. But neither would have matched the leap in performance of the B/RB-54, the "ultimate" Superfortress.

The Retromechanix page has a series of superb photos via the National Archives that show the B/RB-54A mockup in detail as well as some schematic drawings. It's well worth the time to browse them!

Source: Boeing B-29 Superfortress (Crowood Aviation Series) by Steve Pace. The Crowood Press Ltd, 2003, p166-168. Boeing B-50 (Air Force Legends Number 215 by Geoffrey Hays. Ginter Books, 2012, pp 118-121.

The XB-15: Getting Boeing Back On Its Feet

Clairmont Egtvedt led Boeing during its most precarious times
(Boeing Historical Archives)

The Boeing Company of the mid-1930s was existing by only a razor-thin margin and at any moment in those times, could have shut down for good with the Boeing name a mere footnote in aviation history. Why one of the dominant aviation companies of today nearly ceased to exist eighty years ago we have to move the clock back to 1929 when President Herbert Hoover appointed Walter Fogler Brown to be the Postmaster General. Within a year, Brown had lobbied Congress for more authority to improve the air mail system which he felt was inefficient and piecemeal in the organization of the existing airline networks that carried the mail. The Air Mail Act of 1930 (also known as the McNary-Watres Act after its Senate sponsors) gave Brown the authority to rationalize the US air mail system, which in effect, led to the rationalization of the US airline system of the day. The details of that controversial move will of course will be the topic of a future blog article here at Tails Through Time- but the key event here was the so-called "Spoils Conference" of 1930 where Brown consolidated the existing air mail routes which by extent led to the consolidation of the US airline system to just three airlines- United, TWA, and American- essentially forcing out of business the other airlines. One of the effects of this move was that Bill Boeing teamed up with Frederick Rentschler of Pratt & Whitney to form a large business conglomerate with diverse interests in aviation called United Aircraft and Transport Corporation. United Aircraft was the holding company for both Boeing Aircraft and Pratt & Whitney and also held ownership of Hamilton Standard, the propeller manufacturer, as well as the aircraft companies of Chance Vought, Sikorsky, and Stearman along with United Air Lines. In late 1933, Senate investigations opened on the 1930 Spoils Conference which led to the Air Mail Scandal of 1934 which led to the Roosevelt Administration canceling all the air mail contracts and handing the air mail delivery to the US Army Air Corps with disastrous results. With no choice but to return air mail service to the airlines, the government did so with punitive conditions that large consolidated aviation firms with airline interests had to be broken up in order to win back air mail contracts. United Aircraft was broken up back into its constituent companies and Bill Boeing resigned from aviation altogether. 

Replacing Bill Boeing was his general manager, Clairmont "Claire" Egtvedt, who had risen up the ranks at Boeing after starting out as a draftsman in 1917 fresh out of college. Of all the pieces of the now-broken up United Aircraft, Egtvedt had to take charge of the smallest and weakest piece- Boeing Aircraft itself which included its Stearman subsidiary in Wichita, Kansas. The first year of Boeing's new independent existence found it operating at a loss of over $200,000, a crushing loss in those days. Egtvedt wanted to get Boeing back into business by building a twin-engined bomber and twin-engined transport, but there was just barely enough money in the company's accounts to make payroll. By the late summer of 1934, Boeing's workforce had fallen from 2,275 a year earlier to only 600. The remaining employees as a testament to their faith in Boeing, offered up a plan to Egtvedt where half the group would work two weeks and then the other half of the group would then work the next two weeks and keep alternating so that only half the employees needed to be paid but the core experience of Boeing could be kept intact. Egtvedt naturally approved of the plan and true to the loyalty of the Boeing employees, some of those where not assigned to work in a particular two week block came to work anyway to help out without asking for payment. 

What Boeing products were available needed to be sold overseas to earn income for the company, so Egtvedt hired a young engineer, Wellwood Beall, who had been an instructor at Boeing's own school of aeronautics. Beall was an enthusiastic and gregarious individual who Egtvedt tasked with selling the Boeing P-26 Peashooter to the Chinese, who ended up ordering eleven aircraft in 1935. But it wasn't enough, and even the in-house company newspaper shut down publication to save money. Ever looking for opportunities to turn Boeing's fortunes around, Egtvedt had always thought the Boeing 247 airliner was too small, which as it turned out, became its biggest liability to the roomier and bigger Douglas DC-3 which proved to be an immense success. Several years earlier Egtvedt actually watched flight operations with Boeing fighters aboard the first US aircraft carrier, the USS Langley. A Navy rear admiral who was hosting the Boeing delegation remarked that battleships made more sense than bombers (this was in the wake of General Billy Mitchell's demonstration where he sank captured German warships with bombers) for US defense because a battleship could defend itself. When pressed further by Egtvedt, the admiral responded that the only practical bomber would be one that would be able to defend itself like a "flying dreadnaught". The remark must have stuck with Egtvedt that bigger was the way to go, but the only large aircraft of the day in the United States was the Barling bomber of 1920 that had a 120-foot wingspan, six engines and needed a tailwind to break 90 mph and had a range of only 300 miles on account of its heavy weight. Large aircraft simply had a lousy track record in those days. But that didn't stop Egtvedt from thinking about the large aircraft problem. Bill Boeing himself once said: 

"I've tried to make the men around me feel as I do, that we are embarked as pioneers upon a new science and industry in which our problems are so new and unusual that it behooves no one to dismiss any novel idea with the statement that 'It can't be done'. Our job is to keep everlastingly at research and experiment, to adapt our laboratory results and those of other laboratories to production as soon as practicable, to let no new improvement in flying and flying equipment pass us by."

The XB-15
(Boeing Historical Archives)

Boeing wasn't just thinking about bigger aircraft. There were also some officers with the US Army Air Corps that saw Billy Mitchell's 1921 demonstrations as prescient and the key to making the United States an aviation power. At the lowest point of Boeing's fortunes in the spring of 1934, Egtvedt got a phone call from Brigadier General Conger Pratt who was head of the Air Corps Materiel Division at Wright Field in Dayton, Ohio (predecessor to today's USAF Systems Command). Pratt wanted Egtvedt's presence at a secret conference in Dayton to which he also invited the heads of several other aircraft companies. Within the Air Corps, many of Mitchell's subordinates were imbuing the USAAC with ideas of long range bombers which received high level support with the command staff in Washington. At the meeting on 14 May 1934, General Pratt informed the aircraft company leaders that the Army wanted proposals for a long range bomber that could carry at least 2,000 lbs of bombs at least 3,000 miles and weigh no more than 32 tons. Nothing like it had been done before and the invited guests were quite startled. 

Egtvedt immediately put Boeing engineers to work on a 150-foot wingspan bomber called "Project A". The speed at which Boeing got to work on Project A resulted in a design contract from the Army. A large hangar at Boeing Field was set aside with a partitioned area only cleared personnel were allowed along with Army representatives from Wright Field. Excitement built quickly at Boeing with word of a mammoth bomber in the works. Despite the Army contract, Boeing's financial state was still quite precarious, but Egtvedt pressed on- Boeing, in his mind, had to learn how to build large aircraft if it was going to survive and Project A would be the company's school house, even if the company ran in the red- which it did. 

With work proceeding on what would become the XB-15, a second proposal was issued by Air Corps Materiel Division at Wright Field. While the first project was to be a technology demonstrator of a large bomber aircraft, the second proposal that was issued on 8 August 1934 just months later called for a production bomber with a 2,000 lb bomb load, a top speed of 250 mph, a range of 2,200 miles and a crew of around six. Companies were asked for designs and bids for an initial production run of just over 200 aircraft. You can imagine Claire Egtvedt's excitement with this second proposal- he had been thinking about larger aircraft and Boeing was learning all about designing and building larger aircraft with the XB-15 demonstrator. He had been thinking about a flying dreadnaught that a Navy admiral referred to years earlier and now the Army was asking for that very aircraft. Egtvedt wanted the Boeing proposal for the production bomber to be a four engine aircraft, but in those days, four engine aircraft were rare and seen as risky from a technological standpoint. He even flew to Wright Field to get further clarification on the production bomber specifications to be sure a four-engined design was acceptable. 

Egtvedt didn't just want to submit a proposal- he wanted to something daring. He wanted to proceed with a prototype of Boeing's proposal and fly it to Wright Field to demonstrate its performance. This was an immense gamble- Boeing was already operating in the red, it was busy working on the XB-15 project and now Egtvedt wanted to build and fly a prototype of Boeing's submission within one year. Egtvedt turned to his friend for guidance- Boeing's company lawyer and future CEO, William M. Allen. Allen came from the University of Montana but had a Harvard law degree- he first worked with Boeing in setting up Boeing Air Transport in 1926 (the predecessor to United Air Lines). Allen was an unusual sort for the Boeing team- he wasn't a pilot and he wasn't an engineer. But he had a remarkable clarity of thought that many at Boeing came to rely on to help with tough decisions and he asked Egtvedt bluntly "Do you think you can build a successful four-engined airplane in a year?" Egtvedt's response evoked the spirit of Bill Boeing: "Yes I know I can." So with Bill Allen's support, on 26 September 1934, Boeing's board of directors authorized Egtvedt to borrow money to the limit to begin work on the Model 299. 

Model 299 prototype
(USAF Museum)

In less than three months the production drawings for the Model 299 were in the hands of assembly shop who began building the prototype, all while work continued on the XB-15 program. As assembly of the Model 299 prototype began, the company ran out of money and the board, putting its faith in Claire Egtvedt, arranged to borrow more money. While it was clear that the XB-15 would never be a production aircraft, what had been learned from the design effort paid dividends for Boeing in the speed at which the design and fabrication of the Model 299 proceeded. Final assembly began in June 1935 and soon workers disregarded their shifts in an all-out effort to finish the Model 299 prototype. At sunrise on a Sunday, 28 July 1935, the Model 299 prototype was rolled out at Boeing Field for its successful maiden flight. Keep in mind the go-ahead to launch the Model 299 program was on 26 September 1934! After a stunning first flight, the Model 299 was then flown to Wright Field on 20 August 1935, taking only nine hours to cover over 2,000 miles with an average speed of 252 mph. Boeing's other competitors also built flying prototypes of their submissions but they paled in comparison to the Model 299. Martin entered the YB-12 which was just a re-engined version of the B-10 twin-engined bomber and Douglas entered the B-18 Bolo which was based on the DC-2 airliner. 

Then tragedy struck. During a flight test out of Wright Field in October, the control locks were left in place and the Model 299 prototype stalled at takeoff and crashed with the loss the pilot and varying degrees of injuries to the rest of the crew. Under the rules of the competition laid down by the Army, the Model 299 had to be eliminated from competition as it was unable to complete its flight tests. It was a somber Christmas 1935 when Douglas won the contract with the B-18 Bolo. But there was a silver lining in the loss of the Model 299 prototype- the flight test program won the Model 299 many advocates within the US Army Air Corps who saw the Boeing design as the future of long range bombers. A service order was placed for thirteen aircraft and a fourteenth aircraft that would be a structural test article. The aircraft's designation would be a name that would echo in the annals in the history of aviation- B-17. When the Seattle Times reporter Richard Williams dubbed the Model 299 a "flying fortress" on account of its six gun turrets, Boeing astutely saw the value of the name and the B-17 became the Flying Fortress. 

Work did continue on the XB-15, though- it made its first flight on 15 October 1937. It was 26 feet longer than the B-17 with a 36-foot greater wingspan and at the time of its rollout, it was the largest bomber ever built. The aircraft was christened "Old Grandpappy" in recognition of the role its design played in the B-17 Flying Fortress. The service history of the XB-15 will also be the subject of a future article here at Tails Through Time, so stay tuned! 

Sources: Boeing: The First Century by Eugene E. Bauer. TABA Publishing, 2000, pp 59-69. Legend & Legacy: The Story of Boeing and Its People by Robert J. Serling. St. Martin's Press, 1992, pp 27-35.

The Last Savoia-Marchetti Airliner

Italian aeronautical engineer Alessandro Marchetti

The Italian aircraft manufacturer Savoia had a history dating back to its founding in 1915 by Umberto Savoia and after the end of World War I, it merged with SIAI (Società Idrovolanti Alta Italia), another firm known for its seaplanes. The company became Savoia-Marchetti (sometimes also referred to as SIAI-Marchetti) when the designer Alessandro Marchetti became its chief engineer in 1922 and quickly became famous for his work on the S.55 twin-hull flying boat. Many of Marchetti's designs during the interwar period would set speed and endurance records in flight. Most of what Alessandro Marchetti is best known for, though, was his line of three-engined aircraft that began with the SM.79 Sparviero that first flew in 1934 as a fast eight-passenger transport capable of air racing. With the storm clouds of war fast coming to Europe, the Sparviero became Italy's primary bomber aircraft and one of the few Italian designs produced in significant quantities during the Second World War. The trimotor layout of the Sparviero set the pattern for a whole seriesof aircraft from Savoia-Marchetti, In 1934 the Italian airline Ala Litorria asked Marchetti for a modern long range airliner to which the SM.75 Marsupiale transport resulted. The Marsupiale had its inaugural revenue flights with Ala Littoria in 1938 with the Italian air arm, the Regia Aeronautica, taking interest in the aircraft as a transport. 

With war embroiling Europe in 1941, Marchetti began work on a four-engined derivative of the SM.75 that accommodated 18 passengers on long-range flights. It was a departure from Marchetti's land plane designs which were nearly all trimotors save the obscure SM.74 of which only three were built of this pre-war shoulder-wing airliner. The new aircraft was designated the SM.95 and a prototype and two pre-production examples were built in 1942. In addition, work began on a long-range bomber version designated SM.95B. The original design called for either 14- or 18-cylinder Piaggio radial engines, but wartime availability meant that Marchetti had to settle for a 9-cylinder Alfa Romeo engine producing 780 horsepower. Typical for Marchetti's designs of the period, the SM.95 was of mixed construction with a welded steel tube fuselage with metal alloy skin for the nsoe section and underside and fabric covering for the rest of the fuselage. The wings were plywood-skinned with three wood wing spars. The mixed-material construction likely also made the aircraft much lighter given the fact that lower-powered engines were used instead of what was originally planned. 

The prototype first flew on 8 May 1943 and was immediately impressed into transport service by the Luftwaffe. The fate of the first pre-production aircraft is unknown but is believed to have also been impressed into Luftwaffe service. The second pre-production aircraft was stretched and designed SM.95GA for "Grande Autonomia", featuring increased fuel capacity and revised cockpit instrumentation. Work on the SM.95 was soon hampered by the Italian Armistice in September 1943, but work was completed on the SM.95B bomber prototype with had the wings, engines, and empennage of the transport variant married to a new fuselage that was deepened to allow a bomb bay below the wing spar carry-through structure. A glazed nose accommodated the bombardier with the flight deck moved forward with defensive armament consisting of 12.7mm Breda guns in a turret aft of the flight deck and lateral positions in the aft fuselage and a ventral position forward of the bomb bay. No known photographs of the SM.95B are known to exist though the bomber prototype did fly at least once in 1945. 

Alitalia's SM.95 I-DALL "Marco Polo"

The third SM.95, the SM.95GA, finally made its first flight on 28 July 1945. It and the next aircraft built were put into military service with the Aeronautica Militare. The stretched fuselage of the SM.95GA became the production standard, the nine-foot fuselage stretch allowing for the carriage of 30 passengers in three-abreast seating. The military transports entered operational service starting in May 1946. The new Italian flag airline Alitalia had just been established in September 1946 and orders for six SM.95s were placed. The first two were I-DALJ "Cristoforo Colombo" and I-DALK "Amerigo Vespucci", delivered at the end of 1947 to Alitalia and promptly put into airline service. The production SM.95s had upgraded 9-cylinder Alfa Romeo radials that had increased power output from 780 horsepower on the wartime prototypes to 930 horsepower to accommodate the increased weights of the increased fuel and stretched fuselage. The balance of Alitalia's order, though, was completed with British Bristol Pegasus engines that delivered 1,000 horsepower- these aircraft were I-DALL "Marco Polo", I-DALM, I-DALN "Sebastiano Caboto" and I-DALO "Ugolino Vivaldi". The first two Alitalia SM.95s were subsequently re-engined with the more powerful Bristol Pegasus. It was I-DALN "Sebastiano Caboto" that inaugurated Alitalia's first postwar services to Great Britain on 3 April 1948. 

One of LATI's three SM.95s, I-LATI "San Francesco"

Another Italian airline also ordered the SM.95- Linee Aeree Transcontinentali Italiane (LATI), which had operated air services between Italy and South America prior to the Second World War. LATI had ordered three SM.95s which were all delivered by 1949- I-LAIT "San Antonio", I-LATI "San Francesco" and I-LITA "San Cristoforo". When LATI ceased operations in 1950, their three SM.95s were assumed by Alitalia. Interestingly the only other airline operator of the SM.95 was SAIDE of Egypt, which operated three aircraft to connect Cairo with European capitals. While Alitalia configured its aircraft for 20 passengers, LATI flew shorter routes than Alitalia and configured its aircraft for 26 passengers but SAIDE operated even shorter routes and packed in 38 passengers on their aircraft. Both LATI and SAIDE's aircraft were powered not by the Pegasus radial engine but Pratt & Whitney R-1830 Twin Wasp 14-cylinder engines producing 1,200 horsepower. 

Only a total of 12 production SM.95s operated commercial services out of a total of 20 airframes built including the prototypes. The mixed material construction wasn't terribly robust with the rigors of scheduled passenger services and lacking pressurization also limited their usefulness. Compared to the Douglas DC-4 and the Lockheed L-749 Constellation of the time, the SM.95 was an outdated design. The last passenger flights took place in 1950 less than a year after the last production aircraft was completed. 

Interestingly, there was a plan by the Regia Aeronautica called "Operation S" prior to the 1943 armistice that would have used a modified SM.95GA to fly at very long ranges to bomb New York City. Benito Mussolini, however, would only allow the mission to drop propaganda leaflets as he didn't want to alienate the large population of Italian-Americans in the city. The mission was under preparation when the 1943 armistice occurred. 

Source: Air International "Plane Facts" Volume 10 Number 2, February 1976. Photos: Wikipedia, Alitalia, Air International

The Marine Corps Bet on the Harrier

Hawker P.1172 Kestrel in KES markings

For a large part of the postwar history of American military aviation, the procurement of non-American aircraft was an unusual exception. I had written back in 2011 about the efforts that led to the selection of the Dassault Falcon 20 jet as the basis of the US Coast Guard's HU-25 Guardian medium range search-and-rescue aircraft.  The two most significant prior examples of non-American aircraft procurement were the Martin B-57 Canberra selected by the USAF as its new interdiction bomber in 1951 and the selection of the Hawker AV-8A Harrier which entered service with the Marine Corps in 1971. The story of the Marine Corps' evaluation and procurement of the Harrier is one that readily demonstrates the Marines' political savvy in navigating the treacherous waters of Congressional funding as well as a single-minded commitment to efficient close air support to the Marines on the ground. The predecessor of the Harrier was the Hawker-funded P.1127 Kestrel demonstrator. Two prototypes and four development aircraft were built and then followed by nine more developed P.1127 airframes which in 1963 were to be part of what was called the Kestrel Evaluation Squadron (KES). The aircraft were designated Kestrel FGA Mk1 and the KES was staffed with test pilots from the Royal Air Force, the Royal Navy, the US Navy, US Army, US Air Force, and the German Luftwaffe. Because there were three nations in the Kestrel Evaluation Squadron, it was also referred to as the Tripartite Evaluation Squadron (TES). The unit was formed in 1965 for the purpose of exploring the possibilities of a V/STOL combat aircraft. 

At the time of the KES flight program, Lt. Col. Thomas Miller was assigned to the US Marine Corps' Air Weapons Systems Requirements Branch at the headquarters. It was the job of the staff of this department to review all the latest research and development to see what sort of equipment would be useful to the Marines. Miller and a fellow officer, Lt. Col. John Metzko, had gotten hold of film footage from the British Embassy in Washington DC of the Kestrels in action. By this point, the RAF had committed to getting the Kestrel operational with a more developed aircraft called the Harrier. They had monitored the P.1127 Kestrel program despite not having any Marine pilots in the evaluation squadron and when it became clear the RAF was going to go forward with the Harrier, they immediately briefed the USMC Deputy Chief of Aviation who was none other than General Keith McCutcheon. I had written about him recently as the "Father of Modern Close Air Support" and needless to say, given General McCutcheon's background as a passionate advocate for the Marines' own close air support, he was readily on board to find out more about the Harrier. The next step was the brief the Commandant of the Corps, General Leonard F. Chapman. With the enthusiastic support of the Commandant, the Marines then set about on getting flight time on the new Harrier. The British were adamant that anyone who would be evaluating the Harrier be a qualified test pilot and working through the defense liasons at the British Embassy and Hawker Siddeley, Miller and Marine test pilot Lt. Col. Bud Baker were chosen to head to the UK. Miller's test flying experience was getting the A-4 Skyhawk and F-4 Phantom into Marine service, so he was well versed in what an aircraft had to be able to do to support the Marines on the ground. At the request of the British, the two Marines would clandestinely make 10 flights each in the Harrier and would wear civilian clothing during their stay in Britain during their evaluation. Test pilot John Farley of the Royal Aircraft Establishment worked with Miller and Baker to prepare them for their Harrier flights. It was Farley who made the first flight of the P.1127 Kestrel prototypes in 1964 and he would come to amass 19 years of experience as a Harrier test pilot. 

Gen. McCutcheon, USMC Deputy Chief of Aviation

Miller and Baker realized very quickly during their flights that the Harrier was a new breed of combat aircraft that Marines had to have. To them, it could do everything the A-4 Skyhawk could do but not need a 6,000 foot runway to do it. All it needed was a 1,000 foot strip for rolling STOL takeoffs with an increased weapon load or the deck of an amphibious assault carrier. It was clearly close air support that could not only go where the Marines were, but be readily based close to where the Marines were in action. To get their hands on the Harrier, the Marines needed funding. Not only did the Marines have to deal with the US Navy since the Marine Corps is a department of the Navy, but they weren't sure how the US aircraft industry would react to the Corps wanting a British aircraft. After briefing Commandant Chapman, they met with the Presidential Scientific Advisory Board to get their support.

Fortunately the Navy was receptive and sent over their own team to fly the Harrier as well which allowed them to compare with their earlier Kestrel flights as part of the KES. Also fortuitous for the Marines was that a Marine was in charge of the Navy's A-4 Skyhawk program, Col. Edwin Harper, and he got to fly the Harrier as well, giving him the unique perspective of comparing the Skyhawk with the Harrier. With the ready support of the Navy in 1969, the Marines now had to lobby Congress for the funding. The FY1970 budget didn't have any money for Harrier procurement, but Harper, Miller, Baker and General McCutcheon briefed the Senate Armed Services Committee anyway. McCutcheon made his pitch to the chairman, Senator Barry Goldwater, that he didn't want just a handful of Harriers and do an evaluation. That'd been done already. "We want to buy a whole slug of them and get started and have a meaningful program!" 

Rep. Mendel Rivers (D-South Carolina) crucial to the Harrier program

Though the FY1970 Department of Defense budget was set at their time of their briefing to the Senate Armed Services Committee, the support of the Presidential Scientific Advisory Board insured that supplemental funding was secured as an amendment to the FY1970 budget bill. The supplemental funding was enough to procure 12 Harrier jets at a cost of $57.6 million. But there was a catch- the money was secured via the Marines canceling procurement of 17 McDonnell Douglas F-4 Phantom IIs. It would be necessary to win over McDonnell Douglas. As part of getting Congressional support, the House Armed Services Committee was also briefed on the Harrier plans and the chairman of the House committee, Representative Mendel Rivers of South Carolina, would support the procurement of the 12 Harriers as long as future Harrier buys were aircraft built in the United States. Realizing that the Marines was a significant ground breaking sale into the US defense market, Hawker Siddeley immediately sent representatives to the United States to canvas the aircraft industry and find an American partner for the AV-8A Harrier program. Hawker's team met with eight aircraft manufacturers and narrowed the list down to three- Ling-Temco-Vought, Grumman, and McDonnell Douglas. Hawker felt McDonnell Douglas was the best fit given their naval aircraft experience and at the time, the A-4 Skyhawk program was starting to wind down and the AV-8A Harrier would be good transition for McDD. On 29 September, Hawker Siddeley and McDonnell Douglas signed a 15-year agreement to cooperate on the AV-8A Harrier program. In tandem with this agreement came one from Rolls Royce to team up with Pratt and Whitney on the Pegasus engine. The teams developed a plan to transition production of the Marine's AV-8A Harrier flight as well as the Pegasus engine from UK production to US production over a span of five years. With a planned buy of 114 AV-8A Harriers, it was found by Representative Mendel Rivers' staff that it was cheaper to stick with UK manufacture instead of phasing in production in the United States. During the FY1971 budget debate, discussions centered on the pros and cons of moving production to the United States and eventually Congress agreed with Mendel Rivers' analysis that there was no need to phase in production of the AV-8A in the United States. Though the agreement never resulted in US production, it did lay down the foundations for the later AV-8B Harrier II program. 

AV-8A Harriers of VMA-513, the first USMC Harrier unit

With Mendel Rivers' support now behind them, McDonnell Douglas agreed to become the engineering group responsible for product support of the AV-8A Harrier which was more than adequate compensation for the 17 canceled Phantoms. The AV-8A Harrier first entered service in 1971 at the Navy's Flight Test Center at NAS Patuxent River, Maryland while the first Harrier squadrons prepared for the transition to the AV-8A. The first operational Marine Corps squadron was VMA-513 "Flying Nightmares" which had been flying the F-4 Phantom since 1963. VMA-513 become operational with the AV-8A Harrier in May 1971 at MCAS Beaufort in Representative Mendel Rivers' home state of South Carolina. 

Source: Harrier II: Validating V/STOL by Lon O. Nordeen. Naval Institute Press, 2006, pp 23-30. Photos: Wikipedia, USMC.

The PT1: Pratt & Whitney's First Turbine Engine

Animation showing the operation of a free piston turbine engine

Most airborne turbine development during the Second World War wasn't focused at first on jet engines but on superchargers that were driven mechanically by the engine to compress the thinner air of higher operating altitudes so that aircraft engines operated as if they were in the richer air of lower altitudes. Turbochargers operated on the same principle and tended to be more complex do the exhaust ducting used to drive the turbines that compressed the air for the engines. During the war, General Electric was one of the top firms in supercharger and turbocharger development. However, as early as the late 1930s, Pratt & Whitney had been sponsoring small research programs at the Massachusetts Institute of Technology (MIT) to forward its own efforts in supercharger development. In early 1941, one of the MIT engineers, Andrew Kalitinsky, along with his Pratt & Whitney liason, John Marchant, began discussing a new sort of propulsion system based on a free piston engine- basically two opposing pistons would compress air in a combustion chamber between them with the exhaust being ducted to drive a turbine which in turn drove a propeller. Turbochargers already were in use that collected exhaust gases and drove the turbine for the supercharger- aircraft like the P-47 Thunderbolt used this method. The difference in a free piston engine is that the pistons aren't connected to a crankshaft, hence being "free" but are used to compress air for combustion to drive a turbine which is what provides the rotational power to a propeller. 

Free piston engines were already in use as air compressors. On German U-boats, for example, a four pairs of free pistons in series were used to generate compressed air for launching torpedoes. Kalitinsky and Marchant's idea for an aircraft free piston turbine engine went up their respective chains of command at both MIT and Pratt & Whitney with all involved interested in the concept. On 6 September 1941 a formal report based on MIT's studies was submitted to Pratt & Whitney titled "Free Piston Gas Turbine Power Plant for Aircraft". The proposed engine had eight pairs of free pistons (eight stages) as a gas generator that drove a turbine that in turn drove a propeller through reduction gear. The exhaust gases after spinning the turbine were discharged through a variable area nozzle for additional propulsive thrust. In addition, the duct work incorporated a burner for extra power like an internal afterburner. The report suggested that such an engine could drive a fighter aircraft to Mach 0.75 at 40,000 feet or power a four-engine bomber at Mach 0.6 at 40,000 feet. 

The proposed fighter engine at low altitudes had nearly all the propulsive power coming from the propeller, but the proportion from the turbine exhaust increased as altitude increased. At the operating altitude of 40,000 feet. 2/3 of the propulsion would come from the prop and 1/3 of the propulsion could come from the turbine exhaust. In the bomber engine at operating altitudes 3/4 of the propulsion came from the prop and 1/4 of the propulsion came from the turbine exhaust. For comparison, consider the widely-used PT6 turboprop engine- about 85% of its propulsion comes from the prop and 15% comes from the exhaust gases. The specific fuel consumption of the proposed free piston turbine was 0.36 lbs/hr/HP for the bomber engine and 0.41 lbs/hr/HP for the fighter engine. This represented about a 30% reduction in fuel consumption over the piston engines of the day. Again, for comparison to a modern turboprop, the PT6 engine has an SFC that ranges between 0.64 to 0.59 depending upon the variant. 

The report summarized the potential advantages of the free piston turbine over the piston engines of the day: 

• 1. Improved fuel economy.
• 2. Reduced weight.
• 3. Reduced cooling requirements. 
• 4. Flexibility in installation due to the smaller size. 
• 5. Since the turbine exhaust contributed to propulsion, the propeller could be smaller.
• 6. Less fatigue stress since the engine torque would be minimal.
• 7. Use of alternate fuels than avgas. 

The PT1 test article at the Pratt & Whitney Museum

A new engine designation system would be needed since the free piston turbine was a departure from Pratt & Whitney's established business. "P" would stand for propeller and "T" would be for turbine. The free piston turbine was launched as a company-funded program on 27 October 1941 as the PT1. The PT1's pistons were the same as that used on the R-1830 radial engine. The initial test engine was built out of cast iron since it wasn't going to be a flight-worthy engine. The main challenge for the small PT1 team was getting the two pistons to oscillate symmetrically at high frequency. First run was in August 1942, but again, the technical challenge was getting just two opposing pistons to synchronize. Imagine getting eight for the proposed engine! By March 1943 the PT1 test article was running as intended. In parallel to the free piston work were two other efforts- work on the turbine and work on the burner. Two types of burners were tested, one before the turbine that could boost power to the prop and a second one that was in the turbine exhaust as a rudimentary afterburner. 

Cross section model of the PT1, showing the opposing cylinders

It's important to realize at this time, the PT1 was a low priority program despite the potential advantages. The US military didn't want resources to be diverted from established radial engine programs that were crucial to the war effort. In 1943 there were only 74 personnel assigned to the PT1 program but the engine had run for 400 hours and 280 hours were run with the turbine. Considering the modest budget, small staff and that the PT1 pushed Pratt & Whitney's metallurgical techniques and limited turbine experience, that run time is quite an accomplishment for the technology of the day. Based on the testing so far, the PT1 team concluded the bomber application was most promising and should be the focus of the team's efforts. A comparison was done with a B-29 compared with a B-29 powered by the PT1. For the same bomb load, the range was better with PT1 engines going from 5200 miles to 8900 miles. On a long distance 2900-mile mission, the bomb load was also heavier, going from 7600 lbs to 25,600 lbs! In March 1945 Pratt & Whitney proposed developing the PT1 further to a 4500 hp demonstrator engine with the military designation T32. By this point, though, jet engines were a better-known quantity and offered even more power for relative simplicity compared to a production PT1 engine. 

At war's end the PT1 program was canceled but it wasn't for nothing. Many of the engineers who worked on the turbine section of the PT1 would go on to have great influence in the design of Pratt & Whitney's jet engines like the J57/JT3 turbojet and much of what was learned in the PT1 went to the the company's first true turboprop, the PT2 program which was started at the end of the war. The PT2 became the T34 turboprop engine that powered the Douglas C-133 Cargomaster and was flown prior to the Cargomaster on a Boeing C-97 Stratofreighter, a Lockheed Constellation, and a Douglas C-124 Globemaster. The PT2/T34 was Pratt & Whitney's first axial flow gas turbine engine that set the stage for later engine designs. 

Source: The Engines of Pratt & Whitney: A Technical History by Jack Connors. American Institute of Aeronautics and Astronautics, Lockheed Martin Library of Flight, 2010, pp161-172. Photos: Wikipedia, www.enginehistory.org (Kimble D. McCutcheon)

The First Steps to a Turboprop Transport, Part Two

A week and a half ago I had blogged about how the USAF was getting turboprop transport experience by setting up a test squadron at Kelly AFB to operated transport aircraft that had been converted to turbine power: 

52-2693 and 52-2672 in flight together.

On 15 June 1954, the headquarters of the Military Air Transport Service (MATS) activated the 1700th Test Squadron (Turboprop) at Kelly AFB, Texas, with the task of developing maintenance procedures and techniques for the employment of turboprop transport aircraft pending the arrival of the C-130 and C-133 into the USAF service. The squadron had three flights with each flight dedicated to a single type for the testing of standard transport aircraft that had been converted to turboprop power. The first of the three flights to be activated would operate the Convair YC-131C. Two aircraft were converted from standard C-131 Samaritan transports (the USAF version of the CV-340 airliner) to use early test versions of the venerable Allison T56 turboprop.

Back in January 2010 I had written a short posting about the second of the demonstrator aircraft that were operated by the 1700th Test Squadron and operated in the second flight of the unit- the Boeing YC-97J, a Pratt & Whitney T34-powered version of the C-97 Stratofreighter. I had recently picked up Cal Taylor's voluminous tome on the Douglas C-133 Cargomaster and he devotes considerable space to the YC-97J and its operational use by the 1700th TS. The YC-97J made its first flight at Edwards AFB on 19 April 1955 and given that it used the same T34 engines as the upcoming C-133, the USAF was keenly interested in flight testing the engine in an operational environment with the YC-97J. From my previous posting about the YC-97J: 

Boeing converted two aircraft (52-2693 and 52-2672, both KC-97Gs) to turboprop power. Pratt & Whitney YT34 turoprop engines (which would later be used on the Douglas C-133 Cargomaster) delivering 5,700 horsepower were substituted for the four R-4360 radial engines. For a brief time the USAF considered redesignating these two Stratofreighters as C-137, but ended up assigning them the designation YC-97J (ironically the C-137 got used for the Boeing 707s used by the military, itself a development of the Model 367-80 prototype).

The conversion to turboprop power shaved nearly 5,0000 lbs off the aircraft's weight as the YT34s were much lighter but more powerful. The first flight was made on 19 April 1955 and the YC-97J demonstrated significant improvements in overall performance. The top speed was 417 mph compared to 375 mph for a regular Stratofreighter and the YC-97J took only 14 minutes to reach 20,000 feet whereas the regular Stratofreighter took 50 minutes!

Inflight study of the YC-97J during its Edwards flight test program.

In addition to using the same T34 engines as the C-133, the YC-97Js also used an early version of the same Curtiss turboelectric three-bladed propellers planned for the C-133. The first YC-97J completed its flight testing at Edwards and was delivered to Kelly AFB on 14 September 1955, nine months after the YC-131Cs had arrived. The second YC-97J arrived at the end of the month. After a short series of flights operating within the continental United States, the USAF authorized the aircraft to begin overwater missions with the first overwater flight being to Kindley Field in Bermuda- the aircraft covered the 1,700 mile route from San Antonio to Bermuda in 4 hours 42 minutes, the fastest time at that point by a prop-driven aircraft. On 26 January 1956, the YC-97J departed for Rhein-Main AB in West Germany staging through Dover AFB in Delaware, then Newfoundland and Scotland. Despite record breaking cold weather on the trip, the YC-97J performed flawlessly without any of the usual maintenance headaches that were commonplace for the piston-driven C-97s. On the leg between Newfoundland and Scotland, four hours were shaved off the usual flight time when using C-124s or C-118s, the run being made in only 6 hours 30 minutes. It was clear that the time savings was tremendous on long distance missions. The international aviation press covered the flight with interest. On an outbound stop in London, the YC-97J was climbing out of Heathrow at 2,500 feet per minute and London ATC asked the pilots to slow the rate of climb as the radar dish was too slow to keep up! The return flight from Frankfurt stopped in Paris, London, the Scotland (Prestwick), Newfoundland (Goose Bay) then Selfridge AFB in Michigan before returning to Kelly AFB. It was the first round-trip trans-Atlantic crossing by an American turboprop aircraft. During the mission to West Germany and back, no engine or prop maintenance was needed and the aircraft's four engines used a mere four quarts of oil for the entire trip. Needless to say, the USAF was very enthusiastic about the aircraft!

In March 1956 the two YC-97Js were put on a scheduled cargo run between Kelly AFB to Ramey AFB in Puerto Rico via Charleston AFB in South Carolina and the return routing stopped over at Brookley AFB in Alabama (now Mobile Downtown Airport). Average flying time between San Antonio and Puerto Rico was 16 hours and despite the stopovers, it was still nine hours faster than what piston-driven USAF transports took to cover the distance. But it didn't stop there- that same month the first YC-97J made the first trans-Pacific crossing by a turboprop aircraft, averaging 360 mph over the 18,000 mile round trip. The longest leg of the route to Tokyo was between Midway Island and Yokota AB outside of Tokyo- on this leg the YC-97J flew at 30,000 feet and averaged 400 mph. 

In preparation for the arrival of the Douglas C-133 Cargomaster, the first group of air crew and mechanics arrived at Kelly AFB from Dover AFB for familiarization training with the T34 engine and its Curtiss propellers. The three-week course had pilots flying an average of 38 hours on the YC-97Js to build turbine experience while the Dover mechanics worked side by side with the Kelly AFB maintenance team to keep the YC-97Js flying. The reliability of the turboprop over the piston engine was now unquestionable and in the summer of 1956, both YC-97Js would fly a total of 46 hours 35 minutes together in a single calendar day as proof of the reliability of the turboprop. The engine overhaul time (TBO) over the course of the test program with the 1700th started out at 150 hours and ended up at 1,000 hours. 

The YC-97J departs San Diego Lindbergh Field.

In addition to its scheduled cargo flights, the YC-97Js were also flown on demonstration flights for interested groups ranging form the US Navy to other defense contractors like Pratt & Whitney and North American Aviation. On a three day demonstration in Connecticut for Pratt & Whitney, the YC-97J made 78 engine starts, 19 takeoffs and landings, 7 air starts and 15 flights without any malfunctions of the engine or propellers. By October, one of the T34 engines became the first American turboprop engine to reach 1,000 flight hours since its last overhaul. It was removed from the YC-97J with 1,001 hours and 20 minutes flight time and in that time, it only needed 44 hours of unscheduled maintenance and used a miserly 392 quarts of oil in that time frame, a fraction of what the regular C-97's piston engines would have used in 1,000 flight hours. The propellers also proved to be extremely reliable and when the first C-133 Cargomasters were delivered to Dover AFB, the engines and propellers were already rated at 1,000 hours TBO, a significant feat in that day. 

The 1700th TS's flight test program with the YC-97Js concluded on 15 November 1956, six weeks ahead of schedule. However, the aircraft were kept operational until 17 January 1957 as they were used in Operation Safe Haven to fly refugees from the 1956 Hungarian Revolution from Europe to new homes in the United States. The first YC-97J, would go on to create more aviation history, though- it was modified to become a Super Guppy transport. Aero Spacelines president Jack Conroy had already flown a piston driven Super Guppy, and aware of the pending retirement of the YC-97Js, acquired one as the turboprop engines made his conversion not only faster, but more efficient. The new turbine Super Guppy used a swing nose instead of a tail break as was the case with the original design and it was put into service with NASA in 1966, its first job transporting the second stage of the Saturn IB rocket from Huntsville, Alabama, where it was built to the Kennedy Space Center in Florida. It was subsequently retired to the Pima Air and Space Museum in Tucson, Arizona. 

Stay tuned for the final installment in this series which will look at the turboprop-powered YC-121F Super Constellation!

Source: Remembering an Unsung Giant: The Douglas C-133 Cargomaster and Its People by Cal Taylor. Firstfleet Publishers, 2005, p29-43. Photos: Smithsonian Institution, SDASM.

The Centaur Upper Stage

During the early days of the American space program, there were two schools of thought in liquid-fueled rocket design. One group, represented by Werner von Braun and his team of NASA engineers at the Marshall Spaceflight Center, favored rockets with strong internal structures similar to that of monocoque aircraft. The other group, represented by the Atlas missile engineers at Convair, favored rockets with a thin aluminum shell that used pressurization of the tank and its propellants for structural integrity- what was called balloon skin construction.

The Atlas team had already proven their structural technique would work as the Atlas had already entered service as the first American ICBM and it was already being used for space launches from satellites to the manned Mercury program. The main engineers behind the Atlas, Charlie Bossart and Kraft Ehrlicke, were planning a high-energy liquid fueled upper stage that could be used on the Atlas to boost its orbital payload. This was the Centaur upper stage and it would use the first rocket engine to use liquid oxygen and liquid hydrogen- while liquid oxygen was commonly used as an oxidizer, the Centaur and its Pratt & Whitney RL10 rocket engines pioneered the use of liquid hydrogen- it was much colder and tricker to handle, but combined with liquid oxygen it offered a quantum leap in performance.

Early in the Centaur program the USAF was the main customer, but bureaucratic changes and shifts in mission funding requirements led the Centaur program to be transferred from the Air Force to NASA. As a result, the two main schools of thought in liquid-fueled rockets came to clash in 1962.

Von Braun's team offered a more conservative approach that was being incorporated in their work on the Saturn launch vehicle. Von Braun, now being placed in the position of overseeing the Centaur program, sent his structural engineering chief, Willie Mrazek, to San Diego to review the progress on the Centaur program. Convair's engineers by that point had already developed a reputation as being the "unruly wards" of the Marshall Spaceflight Center and while the USAF was willing to push the technological envelope, the safety-minded culture of NASA desired a less risky approach. Mrazek came to Convair in 1962 not terribly pleased with the balloon skin design of the Centaur.

The heads of Convair tried their best to keep Charlie Bossart and Kraft Ehrlicke clear of Mrazek during his visit. During a technical presentation on the Centaur's construction, unbeknown to Mrazek who was seated in the audience, Charlie Bossart managed to sneak into the presentation and took the seat right to Mrazek. As the presentation proceeded, Mrazek was constantly shaking his head and muttering to himself what he thought was wrong with the Centaur's design. By this point, Bossart had had enough despite his explanations to Mrazek (imagine these two men arguing in a loud whisper in the back of an auditorium) and took him outside. Deane Davis, Convair's deputy director for technical control and Ehrlicke's immediate superior, was convinced that Bossart was taking Mrazek outside to settle the score with fisticuffs.

Bossart took Mrazek to the outdoor factory yard where a brand-new Centaur upper stage sat, its polished thin aluminum skin gleaming in the sunlight. "What's inside it?" asked Mrazek. "Just nitrogen" replied Bossart, at which point he handed Mrazek a hammer and invited him to pound a hole into the side of the Centaur. Without the nitrogen pressurization, the Centaur would collapse under its own weight. Mrazek couldn't believe that gaseous nitrogen was keeping the Centaur's structure, so he took a swing at the rocket stage. The first hit was only a glancing blow and hardly scratched it. I can only picture Bossart at this point egging Mrazek on to hit it harder- which he did. The hammer rebounded off the skin and hit Mrazek in the face, knocking his glasses off. Again, the skin remained unscathed.

Bossart's point was made at the expense of Mrazek's ego and his glasses. The Centaur program proceeded onward from that day in 1962, and the Centaur is still in use today as a second stage on launch vehicles like the Atlas V. It was successfully used on the Titan series of launch vehicles and is currently under consideration as a high-energy upper stage on the new Delta IV rocket.

Source: To Reach the High Frontier- A History of US Launch Vehicles, edited by Roger D. Launius and Dennis Jenkins. The University Press of Kentucky, 2002, p344-345.

The last Avro Canada CF-100 Canuck flight took place on 28 June 1982, but it wasn't with an operational role. Since 1967, Pratt & Whitney Canada had operated a CF-100 on loan from the Canadian Armed Forces to serve as an engine testbed for the JT15D small turbofan. PWC needed a two seat aircraft (the rear seat of the CF-100 being occupied by a flight test engineer) and one with sufficient ground clearance for an underslung nacelle for the JT15D engine.

The CF-100's ability to cruise up to Mach 0.8 as high as 48,000 feet made it ideal for testing an engine destined for business jets. The aircraft arrived at PWC's St-Hubert test center in November 1967 for conversion. The first flight took place on 22 July 1968 with the first flight with a JT15D engine on 14 August 1968. The first air start of the JT15D took place less than two weeks later on 22 August 1968.

The loan of the CF-100 got extended several times as PWC tested different versions of the JT15D engine, ultimately making over 400 flights totalling 1,017.6 flying hours with the test engines.

The JT15D engine is unique in that it has a centrifugal high pressure compressor which was common on early generation jet engines. However, in the JT15D it made the engine more compact and simpler in terms of complexity and parts as opposed to what it would have been had it used a traditional axial-flow compressor. The JT15D first went into use on the Cessna Citation 500 (later rebranded Citation I) and has been used on the Hawker Beechjet/T-1A Jayhawk, Aerospatiale Corvette, and the Citation II, Ultra, and Citation V as well as the military T-47 and UC-35 versions of the Citation.

Source: Wings of Fame, Volume 18. AIRtime Publishing, 2000, "Avro Canada CF-100 Variant and Operator Briefing" by Jeff Rankin-Lowe, p114, 133.

Following the Second World War, engine maker Pratt and Whitney found itself lagging behind in jet engine development as the wartime and immediate postwar demand for its piston engines was an all-time high. Nearly 50% of all engines built for US aircraft during World War II were Pratt and Whitney engines. The company's head, the legendary Frederick Rentschler (who quite literally built Pratt and Whitney into the dominant engine company it had become during World War II after leaving Wright Aeronautical in 1924), directed his head of engineering, Leonard Hobbs, to begin efforts to catch up in jet engine development.

Rentschler's goal was to catch up with the competition like General Electric, Westinghouse, and Allision by 1950 and become the dominant jet engine maker. Hobbs and his boss, however, had trouble finding an aircraft design tailored to their proposed engine designs. Gaining valuable experience by license-building the Rolls-Royce Nene turbojet for the Grumman F9F Panther as the J42, they found the USAF was in the midst of deciding between a turboprop and a turbojet for its new long range bomber that would become the Boeing B-52 Stratofortress.

Hobbs' initial design was for a larger turboprop than the Wright T35 being considered. Designated the T45, it combined the gearbox of a turboprop with the compressor/turbine core of one of their own jet engine designs. To achieve the necessary specific fuel consumption, Hobbs increased the pressure ratio to an unheard of 8:1. However, this would make the engine difficult to start and sluggish in acceleration. Hobbs' technological leap was to make the engine dual-spooled, with the front of the engine running at a slower speed than the back of the engine which ran at a faster speed. By optimizing the speeds of the different sections of the engine core, the engine would run not only more efficiently, but could generate more thrust than single spool engines of the day could ever create.

When Boeing's bomber design grew in size, the USAF decided only a turbojet could meet the needs of the design and Pratt and Whitney abandoned the T45 and the jet engine core of the turboprop became the J57 engine. With its compressor ratio increased to 12:1 (which was over double the industry standard of the day), the engine ran for the first time in January 1950 and would become the first jet engine to produce more the 10,000 lbs of thrust.

The J57 would be used in many fighter aircraft including the North American F-100 Super Sabre, Vought F8U Crusader, Douglas F4D Skyray, Convair F-102 Delta Dagger and McDonnell F-101 Voodoo. It also powered the Boeing B-52 Stratofortress and the KC-135 Stratotanker. The civilian JT3 version would be used on the Boeing 707 and Douglas DC-8 jetliners. The J57 and the JT3, would become the dominant large aircraft engine as well as fighter engine for a good part of the 1950s and 1960s, realizing Frederick Rentschler's goal set in 1945.

Source: US Naval Air Superiority: Development of Shipborne Jet Fighters 1943-1962 by Tommy H. Thomason. Specialty Press, 2007, 151-152.