26 February 2015

The Boeing PBB Sea Ranger: The Best Flying Boat at the Worst Possible Time

The Boeing XPBB-1 Sea Ranger prototype
Despite being a dated pre-war design, the Consolidated PBY Catalina was already successful before America's entry into the Second World War. Martin Aircraft decided to go one step further than the Catalina realizing the potential of newer, more powerful engines with its 1937 offer to the US Navy for what became the PBM Mariner. With a bigger hull but the same speed and payload of the PBY Catalina, Martin's proposal was eagerly received by the US Navy and the Martin XPBM-1 prototype made its first flight in February 1939. Seeing what Martin had accomplished with the design, in the following month the Navy issued a new specification for an even more powerful twin engine flying boat using the new powerful Wright R-3350 Duplex Cyclone radial which had made its first bench run in May 1937. The Consolidated PBY Catalina used the Pratt and Whitney R-1830 Twin Wasp radials developing 900 hp each. The Martin PBM Mariner used Wright R-2600 engines developing 1600 hp each. The Wright R-3350 in development developed 2200 hp, so the potential for an even better twin engine flying boat was obvious to the Navy. With Martin and Consolidated busy with their respective flying boat designs, the Navy invited Boeing and Vought-Sikorsky to submit designs that used two R-3350 engines. Vought-Sikorsky had extensive flying boat expertise with pre-war designs used by both the Navy and civilian airlines. Boeing had just flown what many considered the pinnacle of commercial flying boats, the Boeing 314. 

Note the long tapered wings that were based on the B-29's 
Boeing's initial submission featured a tapered straight wing using the Davis airfoil with retractable outboard floats and a bomb bay within the hull. The flying boat was also pressurized to allow high altitude transit to patrol areas. The Navy preferred the Vought-Sikorsky design but the company had little resources to spare was it was busy with other priority projects. The Navy then asked Boeing if they would be willing to build the Vought design on 24 February 1940, but Boeing expeditiously redesigned their submission and the following month responded the US Navy's request with a presentation of an improved design that had a narrow, low drag hull, fixed outboard floats, bomb bays moved into the inner wings and deletion of the cabin pressurization. The new design was larger and more capable and dispensed with the Davis airfoil with a new Boeing in-house design that was also used on the B-29 Superfortress project. The Navy was suitably impressed and ordered one prototype for evaluation as the XPBB-1 on 29 June 1940. The mockup review went quickly in January 1941 with the XPBB prototype starting construction in June 1941. With the potential of the new design to be superior to both the PBY and PBM, the Navy went ahead and ordered 57 PBB-1s on 8 October 1941 despite the XPBB prototype not having made its first flight yet. To accommodate production for the PBB Sea Ranger as it was called, a new Boeing plant was built on the southern shores of Lake Washington at Renton. With the clouds of war on the horizon that fall, the Navy indicated to Boeing that as many as 500 Sea Rangers would be needed before 1943. 

Note the bomb bay doors on the underside of the inner wings
The PBB Sea Ranger was a remarkable clean aerodynamic design for a flying boat. The outer wing and horizontal stabilizers were near-identical to that used on the B-29 Superfortress (which would make its first flight in September 1942). The Wright R-3350 engines developed 2300 hp and drove a 16.5-foot diameter three bladed prop. The original proposal was for counter rotating props, but designing the gearing for a contraprop proved to difficult at the time. Instead of fuel bladders, the wing was wet with integral tankage which not only saved weight, but gave the Sea Ranger an enormous fuel capacity that would make 72-hour patrols possible. As the tanks were emptied, carbon dioxide gas under pressurization would purge and inert each tank. Each wing had five bomb bays that were between the wing ribs with a roller door covering each bay. The payload of the ten bays would have been 20,000 lbs of bombs (identical to the B-29 bomb load). Pylons could be fitted between the bays to carry torpedoes or other weapons too larger for the wing bomb bays. The defensive guns were eight 50-caliber guns with a twin dorsal turret and twin tail and nose ERCO turrets that were similar to the nose turret of the Consolidated PB4Y Privateer. A single 50-caliber gun was used on two waist mounts which were teardrop shaped also like the waist mounts on the Privateer. Below the nose turret sat the bombardier whose front window could be protected by doors when the Sea Ranger was landing or taking off. In addition to the bombardier, two pilots, the flight engineer, navigator and radio operator sat in the flight deck with the remainder of the crew rounded out by five gunners. 

The Sea Ranger's wings were its key to long endurance patrols
On 9 July 1942 the XPBB-1 Sea Ranger prototype made its first flight from Lake Washington. There were remarkably few issues that arose during the flight test program. The aircraft was formally delivered to the Navy on 12 January 1943 but the prototype remained in the Puget Sound area for the Navy's trials. The production Sea Ranger would have differed only in details from the XPBB-1 prototype. It was during the flight test program that the fortunes of the Sea Ranger began to wane. It was clear by that point that the B-29 Superfortress program was a national priority for the war effort. This was taking on more and more of Boeing's resources and worse yet for the Sea Ranger, it used the same engines as the Superfortress and production priority for the R-3350 was earmarked for the B-29. This was also the same time that the Navy began shifting patrol missions to land-based aircraft like the PB4Y-2 Privateer. The same year that the Sea Ranger made its first flight, the Navy had also asked Consolidated about a variant of the B-24 Liberator that was more dedicated to the patrol bomber mission than the first PB4Y-1s which were Liberators with modest changes for the naval mission. The PB4Y-2 was the definitive patrol bomber version and it made its first flight in the same year as the Sea Ranger. As the Privateer didn't use the same engines as the B-29, it didn't have to compete for R-3350 production like the Sea Ranger. As a result, the Sea Ranger was canceled with only a single aircraft built. 

The Sea Ranger prototype was flown anyway further by the Navy to its flight test center at Patuxent River, Maryland, on 5 October 1943 and given a formal evaluation despite the cancellation. The Navy was immensely impressed with the Sea Ranger, believing it to be the best flying boat ever developed. Even though the Renton plant where the Sea Ranger was to be built was turned over to B-29 production, the Navy reconsidered its cancellation and approached Martin Aircraft about producing the Sea Ranger for Boeing. The R-3350 engines would have been swapped out with Pratt & Whitney R-4360 Wasp Major engines for a power boost along with two booster jet engines for added performance. The Wasp Major was even more powerful than the R-3350 and production was to begin on that engine in 1944. The Sea Ranger, even though a Boeing design, would have had the Navy designation P4M in recognition of Martin's production of the type. For obvious reasons, Martin wasn't enthused about building someone else's design and at the time of the Navy's proposal, work had already started at Martin on a land-based patrol bomber with two R-4360 Wasp Major engines and two Allison J33 booster jet engines. That design was what became the P4M Mercator which made its first flight in October 1946. 

News reel footage of the Sea Ranger's maiden flight

While the Sea Ranger presented a promising opportunity for Boeing, it ended up being a dead end that arrived at the twilight of the flying boat as a maritime patrol aircraft. Stuck competing with resources with the much more important B-29 Superfortress, the Sea Ranger's historical legacy is not so much its design but rather the Renton facility that was built in anticipation of production. The Renton facility became home to the B-29 program but in the post-war era was where Boeing was launched into the jet age with production of the KC-135, 707, 727 and 737 taking place at Renton. In fact, the Navy's newest maritime patrol aircraft, the Boeing P-8A Poseidon, is built at the Renton alongside commercial 737s.

Source: American Bomber Aircraft Development in World War 2 by Bill Norton. Midland Publishing, 2012, pp 117-120. Photos: Boeing, San Diego Aerospace Museum.

22 February 2015

Flying High This Past Week

This is a new little feature for the blog I thought I'd start doing on a weekly basis called "Flying High This Past Week". I've noticed in following the stats and page views for this blog that sometimes some unexpected past posts become popular for a variety of reasons ranging from an external link on a forum or another website to Google searches that direct folks to a particular topic. Think of this little feature as a way of finding some past articles I've written that you may have missed that might feed your inner avgeek!

  • The Marine Corps Bet On the Harrier: This is the latest article posted on 21 February, this one takes a look at how the Marines adeptly navigated the treacherous waters of military procurement to get their hands on an aircraft that would not be picked up by any other of the US military branches. "We want to buy a whole slug of them and get started and have a meaningful program! - General McCutcheon, USMC Deputy Chief of Aviation in 1969, in his testimony to the Senate Armed Services Committee.
  • The Sud-Ouest SO 6000: France's First Jet Aircraft: Posted 16 February, this article takes a look at the first French jet aircraft that had a rather unique development story and unique among pioneering jet aircraft of the day, might be well considered the very first VLJ.
  • The Messerschmitt Me 163 Komet Takes To the Air Again: Posted on 1 February 2011, this article looks at a Komet glider replica built by Josef Kurz, a glider enthusiast in postwar Germany. The replica glider was as close to the original Komet as possible (minus the Walter rocket engine, of course) and was first flown in 1996 and is part of the EADS Heritage Flight. 
  • Operation Ranch Hand and Agent Orange: I had posted this article on 7 April 2010 on the background and operation of the airborne spraying of defoliants in Vietnam. The unfortunate legacies of Ranch Hand still linger in the health problems of many who were exposed to the Agent Orange herbicide during the flights that were conducted from 1962 to 1971.
  • The Genesis of the Predator UAV: Posted on 26 February 2011, this article took a look at Abraham Karem, an Israeli immigrant and inventor in the United States, who in 1982 secured DARPA seed money to create a long endurance UAV that was quite unlike any of the prior long endurance UAVs that had flown before. Please also note the comments from some of my readers in this article that corrected some of my errors in the original article in regards to the operational deployment of the first Predators to the Balkans.
  • The Turboprop B-17 Flying Fortress: From 19 June 2010, this is what happens you take a surplus B-17 and fit it with the Rolls Royce Dart engines from a Vickers Viscount!

21 February 2015

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.

16 February 2015

The Sud-Ouest SO 6000 Triton: France's First Jet Aircraft

Sud-Ouest SO 6000 Triton prototype. Note the nose intake.
France's first jet aircraft has its roots during the German occupation during the Second World War. Sequestered away in a small Paris apartment a group of French engineers led by Lucien Servanty began work on what would become the Sud-Ouest SO 6000 Triton. Servanty was a graduate of the prestigious engineering school Ecole des Arts et Métiers and prior to the war started out working at Breguet before moving to SNCASO (Société nationale des constructions aéronautiques du sud-ouest) which was a conglomeration formed in 1936 out of several French aircraft companies including Blériot, Bloch, and Lioré et Olivier but was better known as Sud-Ouest. Sud-Ouest was one of the precursor entities to the post war Sud Aviation that would later become Aerospatiale. It was here at Sud-Ouest that Servanty worked on the last variants of the Bloch MB.150 fighter before the fall of France in 1940 to the Nazis. Some of the French aeronautical establishment fled to the Great Britain, some went south to work with the Vichy regime, some were imprisoned for refusing to collaborate (like Marcel Bloch, who later changed his name to Marcel Dassault), and others like Lucien Servanty went into hiding. This was the sort of environment that the Triton was developed, in secret places tucked away from the Nazi occupiers. Servanty's team even built small scale models and tested them in wind tunnels they constructed in secret. 

Lucien Servanty
Uniquely for pioneering jet aircraft of the time from other nations, the Triton was to be a side-by-side two seat aircraft with dual controls and a retracting tricycle undercarriage. A two seater was quite a departure from the first jets in the United States, Great Britain, Germany and the Soviet Union which were all single seaters. The cabin of the Triton wasn't really that much different from a two-seat cabin general aviation aircraft- this was Servanty's idea that the Triton might find use as more than a research aircraft but a training aircraft to introduce pilots to jet propulsion. As designed, the all-metal Triton was to use an indigenous French engine called the Rateau (named for Auguste Rateau who had done much work in the first half of the century on industrial turbines) which was also being developed clandestinely during the occupation. The intake for the Rateau engine was under the nose with the intake duct passing between the pilots to the centrally-mounted engine abreast the wings. Furthering the impression of the Triton as a general aviation aircraft was its crew access via car door-like hatches on each side of the cockpit. In fact, one writer even suggested the Triton may well be the first VLJ-class aircraft thanks to its design!

With the end of the war and the liberation of France, the new French government immediately placed an order for five Triton aircraft plus one static airframe as part of broader effort to rebuild the nation's aeronautical industry. Construction began in somewhat humble facilities outside Paris that were once the factories for Nieuport biplane fighters before moving to a more suitable facility that was once used by Farman. Like many pioneering jet aircraft of the day, the airframe work proceeded much more quickly than the engine development effort. Engineers working on the 3250 lb-thrust Rateau engine were running into difficulties and the Triton prototype was slightly modified to accept a German Jumo 004 engine as used on the Messerschmitt Me 262. This was something of a fortuitous coincidence for Lucien Servanty's team as the Jumo plants in Germany happened to be located in the French occupation sector. The first two Triton airframes would be powered by the Jumo 004. The first flight took place on 11 November 1946 with test pilot Daniel Rastel at the controls. Prior to his first Triton flights, he acquainted himself with jet propulsion by flying captured Me 262s. With this first flight, France become the fifth nation to join the Jet Age after Germany, Great Britain, the United States and the Soviet Union. The flight lasted only 10 minutes and never got about 1000 feet and it was quite clear the Jumo 004 wasn't enough engine for the Triton. 

The third Triton. Note the new lateral intakes and reprofiled cockpit.
The first Paris Air Show after the war took place only three days after the Triton's first flight and the third Triton airframe minus its engine was displayed. Not happy with the reliability of the Jumo 004 engine, Servanty's team switched to Rolls Royce Derwent engine, the same engine that powered the Gloster Meteor fighter. They then switched again to a more powerful Rolls Royce Nene engine which was the engine of the De Havilland Vampire fighter. The French company Hispano Suiza had just gotten a license to build the Nene for the French Air Force's order of Vampire fighters. Because of the increased mass air flow of the Nene engine, the third and fourth Triton featured lateral intakes and improvements to cockpit visibility. In addition, ejection seats designed by Heinkel were installed. The fourth Triton flew first on 19 March 1948 with the third Triton flying for the first time on 4 April 1950. In fact, the fifth Triton flew before the third Triton on 23 May 1949. The sixth airframe was the static test article. Of the last three flying Tritons, it was the fourth one that flew the most as the third and fifth airframes only made a few flights before getting grounded for mechanical issues. The airframe summary is as follows: 
  • Triton 01: Prototype, Jumo 004 powered. Eight test flights. Retired in November 1947.
  • Triton 02: Identical to prototype, but never flown as it was set aside for the French Rateau engine which never became available for use. 
  • Triton 03: Modified for the Nene turbojet. Only two test flights before getting grounded for mechanical issues. It can be seen today at the French aerospace museum at Le Bourget. 
  • Triton 04: Most successful one of the group. 189 test flights. Final flight in November 1950.
  • Triton 05: Damaged due to a forced landing after only eight test flights. 
  • Triton 06: Static test article not intended to fly. 

While modest in performance compared to the jet aircraft of the late 1940s, the Triton gave French industry its first jet experience and many of the prominent test pilots of the time got their first jet time on the Triton given it's trainer layout. Not a bad accomplishment for an aircraft that was designed in secret not out of security but out of fear of the Nazi regime. Lucien Servanty stayed on with Sud-Ouest which became Sud Aviation in 1957. He rose to engineering prominence at Sud Aviation and headed the French design team for the Concorde before his death in Toulouse in 1973. 

Source: X-Planes of Europe: Secret Research Aircraft from the Golden Age 1946-1974 by Tony Buttler and Jean-Louis Delezenne. Hikoki Publications, 2012, pp 22-26. Photos: Wikipedia, Walter Van Tilborg Collection. 

11 February 2015

The Corvus Missile- Laying the Foundation for US Anti-Radiation Missiles

Wind tunnel model of the Corvus missile
In the Second World War, the Radioplane Company had built thousands of remote controlled drones for target practice. In the late 1940s, Radioplane was asked by the USAF for a jet-powered drone for target practice and in 1950 first flew the YQ-1 which was powered by a pulse jet. The drone only had a flight time of about 60 minutes which wasn't terribly useful for the USAF's needs. One drone was converted to use a small Continental J69 turbojet but by the time this version had flown in 1953, the USAF had settled on the Ryan Firebee series of drones for its needs. Radioplane's work transferred over to GAM-67 Crossbow missile program that started in 1953 to meet a USAF need for an anti-radiation missile for the Boeing B-50 Superfortress and B-47 Stratojet bombers. The Crossbow would have been the world's first missile designed to kill radars and was to have had a lightweight 10-kiloton nuclear warhead. A Stratojet could carry four Crossbows under its wings. The first successful guided flight took place in 1957 but the program was soon canceled due to technical difficulties. At the same time the Crossbow flight test program was gearing up, the Navy decided they wanted their own anti-radiation missile and in 1955 issued a requirement for a rocket-powered stand-off missile for its carrier attack aircraft. That missile was designated the ASM-N-8 Raven, but a parallel project going on at the time with the Navy duplicated the Raven's role, so the Raven program was canceled early on and efforts shifted (along with the ASM-N-8 designation) to what was called the Corvus missile. 

In January 1957, development of the Corvus missile was awarded to Dallas-based Temco Aircraft. Temco (Texas Engineering and Manufacturing) was founded in 1952 by Robert McCullough who was the plant manager of North American Aviation's plant in Grand Prairie next to NAS Dallas. McCullough and his group took over the plant from North American under the Temco name and kept it open doing subcontracting work for other aircraft manufacturers. Soon Temco got into electronics and missile guidance systems and by the time of the Corvus contract, nearly half of Temco's revenue came from their missile guidance work. The Corvus award was a boost to the company that they were experienced enough to take on a complete weapons system like the Corvus on their own.

XLR-48 engine on display at the Smithsonian
The Corvus was a big missile. It was 16 feet in length and weighed about 1750 lbs. It had delta wings to extend its range and cruciform tail surfaces set at 45 degrees from the delta wing. Instead of a jet engine as was used on the Crossbow, the Reaction Motors subsidiary of the Thiokol Corporation developed the XLR-48 Patriot pre-packaged liquid fuel rocket motor. Reaction Motors was already well established making rocket engines for the X-plane programs and to overcome the Navy's hesitation about having to store liquid rocket fuels aboard the carrier, the XLR-48 was pre-packaged with storable propellants so the missiles would never have to be fueled. This was a Reaction Motors innovation and for the Corvus, the XLR-48 offered greater thrust over a longer period of time than a solid rocket motor sized for the Corvus airframe. The propellants were hypergolic, meaning they ignited on contact which resulted in a surprisingly simple rocket motor. 

Drawing of an A4D Skyhawk with two Corvus missiles
The Corvus could be armed with either a conventional warhead or a 10-kiloton nuclear warhead very similar to what was planned for the Crossbow missile. Being rocket-powered, the Corvus was a lot faster than the jet-powered Crossbow, flying out at Mach 3 on a ballistic trajectory to 85,000 feet. The seeker head was designed by another Dallas-based electronics firm that would itself soon become famous in electronics, Texas Instruments. The seeker would lock onto an enemy radar emitter. In this mode, the Corvus had a range of 170 miles. It also could lock onto a target that was illuminated by the launch aircraft in a semi-active homing mode against non-emitting targets, but in this attack mode the range was shorter at around 100 miles. A data link would have been used to guide the Corvus in semi-active homing mode until it picked up the radar returns from the target. 

Corvus missile being launched from an A3D Skywarrior
The first airborne launches of the Corvus took place from a Douglas A4D Skyhawk in July 1959 from NAS Point Mugu in California. About twenty missiles were built and the program had progressed to fully guided flights when it was abruptly canceled in August 1960. The Navy had decided that nuclear-tipped air-to-surface missiles didn't quite fit with its operational doctrines and the program was transferred to the USAF. Finding the Corvus redundant to its own AGM-28 Hound Dog missile program that had been awarded already to North American Aviation in 1957, the program was canceled in late 1960 despite the Corvus being significantly lighter than the Hound Dog.

The work wasn't all for naught, though. Texas Instruments, who designed and was tapped to produce the Corvus radar seeker guidance head, gained significant experience from the program and would go on to work with the US Navy on the AGM-45 Shrike became the first American anti-radiation missile to be fielded and was used in combat for the first time in the skies over Vietnam against SAM sites. Texas Instruments would then go on in 1974 to develop the successor to the Shrike that is still in use to day, the AGM-88 HARM (High Speed Anti-Radiation Missile). 

As for Temco, their work on the Corvus made them an attractive merger target. Just prior to the Corvus cancellation in 1960, Dallas businessman Richard Ling, head of Ling-Altec Electronics acquired Temco to form Ling-Temco and shortly thereafter gained controlling interest in Chance Vought Aircraft, the new company becoming Ling-Temco-Vought, or LTV. 

Source: Scooter: The Douglas A-4 Skyhawk Story by Tommy Thomason. Crecy Publishing, 2011, p79. Pushing the Envelope: The American Aircraft Industry by Donald M. Pattillo. University of Michigan Press, 2001, pp 241-242. Detecting and Classifying Low-Probability of Intercept Radar by Phillip E. Pace. Artech House, 2008, pp 554-555. DesignationSystems.Net. Photos: Wikipedia, National Air & Space Museum, NASA.

06 February 2015

How the Lockheed Hercules Helped Open Alaska to Oil Exploration

The Lockheed L-100 prototype loading in Fairbanks. Note the Alaska titles.
In the spring of 1965, there were seven different oil exploration groups prospecting on Alaska's North Slope and they had run into significant logistical difficulties in bringing in not just the equipment needed but also basic supplies for the personnel. The only way at the time was to bring it in on ocean-going barges that only had access to Alaska's northern coast a few weeks at a time during the summer thanks to the extensive ice pack of the Arctic Ocean. There were no roads or railways that connected the North Slope to the cities of Fairbanks and Anchorage further south. The Yukon River was the first geographic barrier between Fairbanks and where the oil was, but even more significant was the Brooks Range that stretched east to west across Alaska for over 700 miles with peaks as high as 9,000 feet. A straight line distance from Fairbanks to the North Slope is easily 500 miles across the most remote and untouched wilderness in the United States. Engineers for the oil companies involved with North Slope exploration had devised a massive caterpillar land train, but it would cost over $1 million to build just one and it would take a year to transit from Fairbanks to the North Slope, threading its way through the mountain passes in the Brooks Range. During the summer months, no one was quite sure if the tundra could support the weight of the proposed massive land train. That left airlift as the only solution but in 1965 there were no civilian aircraft that had the right combination of load carrying capacity, ability to take in outsize loads, the performance to get into and out of what would be very short gravel airstrips, not to mention an ability to keep working in the extreme Alaskan winters. 

The answer came from Charlie Willis, who had been the head of Alaska Airlines since 1957. The year prior, Lockheed had flown the prototype L-100, a civilian cargo version of the C-130 Hercules. The L-100 was basically a demilitarized version of the C-130E and made its first flight on 20 April 1964. Lockheed was eager for commercial sales of the L-100 and Willis struck a deal with Lockheed to wet-lease the prototype in what became known as the "Thirty Day Miracle"- with only Alaska Airlines titles on the landing gear sponsons, the wet-lease included not just the aircraft but the Lockheed pilots and crew as well. This way there would be no delay in getting an Alaska crew up to speed on the new aircraft. 

During the four-week airlift to the North Slope, the lone L-100 prototype moved over 2 million pounds of out-sized cargo to improvised air strips all along the North Slope. Charlie Willis was thrilled with the lease, exclaiming to the Alaska press covering the airlift "This aircraft can airlift heavy machinery, large earth moving equipment, and prefabricated structures on the same day at less cost by conventional surface means." The first company to benefit from the airlift was Richfield Oil who struck oil at Prudhoe Bay, sparking the Alaska Oil Rush. The Prudhoe Bay oil field is the largest not just in the United States but in North America as well. With over 25 billion barrels of oil, this field is more than double the size of the next biggest oil field in the United States which is in East Texas and extends to the Gulf of Mexico. By the end of 1965, five oil fields and eleven gas fields had been opened up on the North Slope. The challenge then became how to get the oil and gas from the North Slope and the only economically feasible option would be the construction of the Trans-Alaska Pipeline that would run from Prudhoe Bay to the port of Valdez in the south. 

One of Alaska International's Herks that supported pipeline construction.
Alaska International Airlines had ordered the Lockheed L-100 after seeing it in action during the "Thirty Day Miracle". When construction of the pipeline began in March 1974, AIA devoted six L-100s to airlift missions to support the pipeline contstruction. The six aircraft were moving 1 million pounds of cargo *each* day into improvised gravel air strips along the pipeline construction route. From heavy equipment to fuel to prefabricated housing for the workers, AIA's L-100s averaged 12 hours of flying each day, some aircraft flying as much as 21 hours in a day. The aircraft were indispensable to the construction effort and along with the "Thirty Day Miracle" part of the lore that has surrounded the Lockheed C-130 Hercules. 

Source: Herk: Hero of the Skies by Joseph Earl Dabney. Copple House Books, 1979, pp 231-249. Photos: Lockheed Martin Archives

01 February 2015

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)