Southern California's foray into ducted fan aircraft: the story of the Doak Aircraft Company

One day at the Western Museum of Flight, I was touring a hangar of the museum housing a Harriet jump jet, gazing at memorabilia and desktop models of aircraft pertaining to the history of aerospace industry in southern California, when I happened to come upon a collection of artifacts and documents regarding vertical take-off aircraft built by the Doak Aircraft Company. One of those artifacts was a wind tunnel model marked with the label "Doak XF-1", and on the wall of the glass casing for Doak-related artifacts and memorabilia was an engineering drawing. I had remembered hearing the name Doak because it built one of several US Army VTOL research aircraft, but even though the Bell X-22 is the best-known tilt-duct fan aircraft, I didn't know until recently that the Doak company was based in Torrance, the very home of the Western Museum of Flight. Given that the location of the Western Museum of Flight was also the headquarters of Doak Aircraft, I thought it might now be a perfect idea to dedicate this post to telling the history of the Doak company and the VTOL aircraft designed and built by that manufacturer.

The Doak DRD-1 prototype primary trainer (civil registration NX25698)

The Doak Aircraft Company was founded in Hermosa Beach, on October 7, 1940, by Edmund R. Doak, Jr., a self-taught engineer and vice president of Douglas Aircraft Company. The company later moved to a new factory based in Torrance in 1941, undertaking the construction of wooden parts and products for aircraft under a contract signed with the furniture firm Kroehler in 1942. Every major US aircraft manufacturer was subcontracted to build subassemblies for military aircraft, including molded plywood fuselages for the North American AT-6 Texan and Vultee BT-13 Valiant trainers, and doors, hatches, and gun turrets for a wide variety of combat aircraft. Throughout the course of US involvement in World War II Doak reached a peak employment of 4,000 workers (Lobb 2006). Doak also built a primary trainer aircraft of its own, the DRD-1, of which a single example (civil registration NX25698) was built and flown in 1941. The DRD-1 was powered by one Continental R-760 radial piston engine and featured laminated plywood construction, with a length of 26 feet (7.92 meters) and a wingspan of 35 feet (10.67 meters); despite being submitted for evaluation by the US Army Air Corps, the DRD-1 was not ordered into production or given a military designation.   

 

Doak ducted-fan flight/wind tunnel test vehicles on display at the Western Museum of Flight in Torrance, California, from left to right: XF-1 wind tunnel model, Model 11 wind tunnel model, full-scale XF-1 tethered test vehicle. On the upper left is a three-view engineering drawing for what would later become the VZ-4. 

Doak's real aviation interest, however, lay in vertical take-off and landing aircraft. Since 1935, he had been experimenting with ducted fans and similar air-moving concepts, and Cold War tensions in the late 1940s and early 1950s raised worries among the US military top brass that the Soviet Union and its Warsaw Pact allies in Central and Eastern Europe might carry out air attacks on unprepared airbases in West Germany, Italy, or France with combat aircraft or theater ballistic missiles. These concerns, along with recognition of the built-in speed limitations of helicopters thanks to the aerodynamics properties of the rotor blades, prompted Edmund Doak to submit a VTOL aircraft design to the US Army's Army Transportation and Research and Engineering Command at Fort Eustis in Newport News, Virginia, in 1950. He believed that his design could take off and land in a small area, hover and loiter over an enemy target, and fly backwards like a helicopter without the usual noise and vibrations of a helicopter, while incorporating the forward flight mode, high speed, weapons load, and combat mission flexibility of a fighter aircraft. Since conventional combat planes were susceptible to Soviet attacks on unprepared airbases with concrete runways, the US Army found Doak's idea attractive, and on April 10, 1956, Doak received an Army contract to build a VTOL research aircraft to test the ducted fan method of vertical take-off and landing. As a first step in testing his ideas for ducted-fan aircraft, Doak built a plethora of ducted-fan vehicles for flight and wind tunnel testing from 1950 to 1957, including the Model 11, Model 13, and XF-1. 

The Doak VZ-4 (VZ=VTOL Research), known by the company designation Model 16, was an aircraft of all-metal construction, powered by a single Lycoming T53 turboprop engine buried in the fuselage that drove two wingtip-mounted tilting ducted fan propellers through a T-shaped box on the engine that transmitted power to the propellers via a 4 in (10.2 cm) aluminum tubular shaft and two smaller shafts. Each propeller had a diameter of 4 feet (1.22 meters) while the fan ducts each had a diameter of 5 feet (1.52 meters), swiveling upwards during vertical take-off and landing with a liftoff rotation speed of 4,800 revolutions per minute, and tilting forwards during forward flight. The wings and tail unit were of all-metal construction, and the cockpit had the pilot and observer seated in tandem, with the pilot using a standard stick and rudder to control the VZ-4 in flight. The VZ-4 was originally made of uncovered welded steel tubing, but later molded fiberglass was installed over the nose section and thin aluminum sheeting placed over the aft fuselage. To save time and money, some component were used during the construction of the VZ-4, including seats from a P-51 Mustang, landing gear from a Cessna 182, and the duct actuators from a Lockheed T-33 (trainer version of the P-80/F-80 Shooting Star jet fighter).

Top: The Doak VZ-4 performing one of its first hover flights, 1958. Note that the first two digits of the VZ-4's serial number are incorrectly reversed (the serial number on the vertical stabilizer should have read 56-6942, not 56-9642).
Bottom: Desktop models of the Doak Model 20 and Model 22 ducted-fan transport projects.

The VZ-4 (serial number 56-6942) was completed by the end of 1957. It carried out its first hover flight on February 25, 1958, and the transition from vertical to forward flight was conducted on May 5 of that year. Although initial flight tests were successful, the overall short take-off and landing performance was less satisfactory than was hoped for, as the VZ-4 itself tended to make a nose-up while transitioned to forward flight mode. After 18 hours of tethered hovering and 32 flight testing hours, in October 1958 the VZ-4 was transferred to Edwards Air Force Base for a new round of flight tests, and it underwent another 50 hours of test flights, attaining a maximum speed of 230 mph (370 km/h), a cruise speed of 175 mph (282 km/h), a range of 250 miles (403 km), a flight endurance of one hour, and a service ceiling of 12,000 feet (3,658 meters) with the turbines (Stevenson 2014, pp. 14-15). The US Army formally accepted the VZ-4 in September 1959, leading to it being called VZ-4DA (the code suffix DA denoting the Doak Aircraft Company), and the aircraft was subsequently passed on to the NASA Langley Research Center in Hampton, Virginia, on May 5, 1960 for additional testing. Even before the Army acquired the VZ-4, Doak proposed two ducted-fan transport designs in 1959, the Model 20 with two ducted-fan engines and a butterfly-shaped tail, and the larger Model 22 with four ducted fans, of which two were at the tips of the low-mounted wing near the crew cabin and the other two were situated at the tips of a high-mounted wing just ahead of the vertical stabilizer. By this time, however, pressure from Congress meant that the Army could only field helicopters and that only the Air Force could operate airplanes, and even though the VZ-4 was classified as a plane, the USAF showed little interest in developing an operation ducted-fan VTOL aircraft, so in late 1960, Doak Aircraft laid off 90 percent of its employees and went out of business in 1961. The VZ-4, meanwhile continued to serve with NASA until 1973, when it was retired and eventually donated to the US Army Transportation Museum in Fort Eustis, Virginia, where it remains on display today.

Top: Designs by Douglas for transport derivatives of the VZ-4 (1-ton payload design on left, 4-ton payload design on right)
Bottom left: Douglas D-828 four-engine tilt-duct transport project 
Bottom right: Douglas D-850 four-engine tilt-duct research aircraft (which lost out to the Bell X-22) 

After Doak Aircraft vanished into history's dustbin, Douglas acquired the patent rights and engineering files for Doak's VTOL designs, while hiring four former Doak engineers to work at its naval aircraft division based in El Segundo. In 1960 Douglas had proposed two transport derivatives of the Doak VZ-4 for submission to the US Army, one with a 2,000 lb (907 kg) payload and another with a payload of 8,000 lb (3,628 kg), but these designs were rejected by the Army. On January 27, 1961, the Department of Defense announced the Tri-Service Assault Transport Program for a large VTOL transport to be jointly operated by the Army, Air Force, and Navy, and Douglas responded in late 1961 with the D-828 ducted-fan transport, which was similar to the Doak Model 22 but had shoulder-mounted wings, with tilt-duct fans at the housing 8 feet (2.44 meter) diameter six-blade propellers, each powered by four General Electric T64 turboshafts. The wings spanned 48 feet 5 in (14.77 meters) when unfolded or 30 feet (9.15 meters) when folded, and the fuselage was 50 feet (15.25 meters) long. However, the D-828 was not proceeded with because US armed services disagreed on the best method of vertical for a large VTOL transport, with the Air Force and Army favoring the tiltwing and tiltrotor arrangement and the Navy preferring the tilt-duct system. (The D-829 tiltrotor proposal envisaged in parallel with the D-828 initially seemed acceptable to the Air Force and Navy, but was rejected by the Air Force along with a North American tiltwing proposal in favor of the Ling-Temco-Vought VHR-477 tiltwing design [which became the XC-142] and thus is not considered here as it was not a tilt-duct project.) After withdrawing from the Tri-Service VTOL transport program, in the spring of 1962 the Navy announced a competition for a tilt-duct technology demonstrator. In response, Douglas proposed the D-850, which featured the same tilt-duct engine layout and placement as the D-828 but had a cockpit similar to that of the Bell X-15 experimental tiltrotor and a shorter wingspan, while eliminating the mechanism for folding wings and tails. However, Bell's rival design to the D-850, known as the D-2127, won the Navy competition for the tilt-duct research aircraft context in 1964, eventually receiving the designation X-22 and making its first flight on March 17, 1966.

The Doak Aircraft Company remains an unsung aircraft manufacturer from southern California when it comes to the development of vertical take-off machines, being largely overshadowed by Hiller Aircraft of central California in terms of name recognition. However, Doak helped to put to the test the concept of a VTOL aircraft with tilting ducted fans by building the VZ-4 tilt-duct technology demonstrator, and even though the VZ-4 never led to an operational production machine of its design, not to mention that VTOL designs envisaged by the El Segundo division of Douglas derived from Doak's unbuilt tilt-duct transport projects either did not make the cut due to wrangling among the US armed forces over what form of VTOL was suitable for large VTOL transport or lost out to design proposals put out by other manufacturers, the volumes of aerodynamic data gathered by the VZ-4 helped influence development and testing of the Bell X-22 (as a matter of fact Bell has historically led the pack of American aircraft manufacturers in building and testing various types of VTOL aircraft besides helicopters, from vectored thrust jet aircraft to convertiplanes). 

References: 

Cox, G., and Kaston, C., 2020. American Secret Projects 3: U.S. Airlifters Since 1962. Manchester, UK: Crécy Publishing.

Harding, S., 1997. U.S. Army Aircraft Since 1947. Atglen, PA: Schiffer Publishing.

Lobb, C., 2006. Torrance Airport. Charleston, SC: Arcadia Publishing. ISBN 978-0-7385-4662-9.

Stevenson, R., 2014. "Doak's One-Off." Aviation History (July 2014): 14–15.

Douglas A2D Skyshark: Heinemann's turboprop-powered flop

Much has been written in the US naval aviation literature about the Douglas AD/A-1 Skyraider piston-powered attack aircraft (nicknamed "Spad" by many airmen) and the prolific A4D/A-4 Skyhawk (better known as "Heinemann's Hot Rod"), both of which were designed by Ed Heinemann (1908-1991), chief designer at the El Segundo Division of Douglas, However, lost in talk regarding the design of naval warplanes by Heinemann in the early years of the Cold War is the fact, Heinemann himself undertook the first Douglas design for the US Navy with gas turbine power, the A2D Skyshark. Recently, I have seen desktop models of the Skyshark at a number of aviation museums in southern California, the Western Museum of Flight in Torrance and the Planes of Fame Museum in Chino. Unfortunately, for all of its good intentions in being a first step in bringing US naval attack aviation into the gas turbine age, the Skyshark turned out to be a disaster for Ed Heinemann due to technical problems with its engine. Therefore, I will discuss in detail Heinemann's failed effort at a turboprop-powered successor to the Skyshark.

Top: Douglas AD/A-1 Skyraider (BuNo 126997, photographed by me at the Planes of Fame Museum in Chino in April 2019), the aircraft that the A2D Skyshark would have replaced
Bottom: Three-view drawing of the Douglas D-557C design with one Westinghouse X25D2 turboprop

On June 25, 1945, the US Navy's Bureau of Aeronautics asked Douglas to design a turboprop-powered attack aircraft as a long-term successor to the new BT2D (later AD/A-1) Skyraider. A few months earlier, on January 25, Douglas had proposed the D-557 turboprop-powered attack aircraft, which was powered by two General Electric TG-100/T31 turboprops mounted side-by-side in the bottom of the fuselage below the pilot, both driving counter-rotating propellers. Armament would consisted of two 20 mm cannons and a 2,000 lb (907 kg) bomb or torpedo under the fuselage rack, plus 1,200 lb (544 kg) rockets or twelve 5 in (12.70 cm) rocket projectiles in wing racks. This proposal was unofficially called BT3D-1 by Douglas in anticipation of gas turbine propulsion rendering the Skyraider obsolete, and BT3D thus never became an official designation. By the second half of 1945, three subsequent designs for the D-557 were worked out. The D-557A was a tricycle design powered by two T31s in wing nacelles, with two 20 mm cannons and wing racks for carrying bombs, unguided rockets, or a torpedo, while the D-557B had two side-by-side T31s below the fuselage driving counter-rotating propellers and provisions for two 20 mm cannons, two 2,000 lb (907 kg) bombs mounted below the wing roots, and two 1,000 lb (454 kg) bombs mounted on mid-wing racks. The D-557C was similar to the D-557B in appearance but had one Westinghouse X25D2 turboprop driving counter-rotating propellers. By April 1946, mockups of all three proposals had been inspected by the Navy and were deemed satisfactory, but none of these projects progressed beyond the drawing board due to developmental problems with the T31 and X25D2.

Model of the Douglas A2D Skyshark (photographed by me at the Planes of Fame Museum in July 2021)

Although the initial D-557 proposals remained paper projects only, Douglas received a Letter of Intent from the US Navy on June 11, 1947, for a turboprop-powered attack plane capable of operaring from the Casablanca-class escort carriers with a combat radius of 690 miles (1,110 km) and a reduced bomb load. Heinemann himself went back to the drawing board to propose a derivative of his earlier D-557 designs powered by one Allison T40 turboprop driving counter-rotating propellers. The new design, measuring 50 feet (15.24 meters) long with a wingspan of 41 feet 2 in (12.55 meters), a height of 17 feet 1 in (5.21 meters), and a wing area of 400 square feet (37 square meters), had a slightly lower wing root thickness compared to the Skyraider and taller and larger vertical stabilizer with greater area, and armament comprised four 20 mm cannons and 5,500 lb (2,500 kg) of bombs, unguided rockets, or a torpedo mounted under 13 hardpoints. A mock-up was inspected in early September, and on September 25 a contract was signed for two prototypes designated XA2D-1 (BuNos 122988/122989). Because the T40 faced technical problems during development and test runs, the first A2D prototype did not make its first flight until March 26, 1950, with test pilot George Jansen at the controls. Although flight tests were marred by vibration problems with the T40, the outbreak of the Korean War prompted the Navy to place an order for 10 production A2D-1s (BuNos 125479/125488) on June 30, followed by another production order on August 18 for 81 more Skysharks (BuNos 127962/128042). Additional orders for 258 more production A2D-1s (BuNos 132793/133042, 134438/134445) were placed on February 10, 1951.

Left: First Douglas XA2D-1 Skyshark prototype (BuNo 122988)
Right: One of the ten completed production Douglas A2D-1s (BuNo 125480) in flight

Despite the Navy anticipating the success of the Skyshark, the A2D flight test program suffered a major setback on December 19, 1950, when the first XA2D-1 prototype crashed on its 15th flight during a landing attempt because of a failure of the gearbox of the Allison T40 turboprop, killing Navy test pilot Hugh Woods. Consequently, the second XA2D-1 prototype was fitted additional instrumentation and an automatic decoupler for the T40, making its first flight on April 3, 1952. Even so, the T40 continued to be bedeviled by gearbox problems, and the Navy was having doubts about the Skyshark overcoming all problems with the T40 turboprop, not to mention that carrier-based jet aircraft were starting to become commonplace. By mid-1952, the Navy project office in charge of the Skyshark recommended that the A2D program be cancelled, and later that year, all production orders for the A2D except for the first ten production aircraft under construction were cancelled. The second production A2D-1 (BuNo 125480) became the first production aircraft to fly, making its first flight on June 10, 1953 with George Jansen at the controls. Five more A2D-1s (BuNos 125479, 125481/125484) were completed and delivered to the Navy later in 1953, and the remaining production A2D-1s (BuNos 125485/125488) were built but not flown. During one A2D test flight in 1953, test C.G. "Doc" Livingston was pulling out of a dive when the T40 gearbox failed, caused oil from the engine to cover the airplane's windscreen and the propellers to fall off the A2D, but Livingston safely landed the plane. The second A2D-1 was lost in an accident near Lake Los Angeles on August 5, 1954 after suffering a gearbox failure, and the pilot bailed out of the plane. By this time, the new Douglas A4D Skyhawk had just begun flight tests, and the escort carriers that the Skyshark had been designed to operate from were phased out of service.

The only extant Skyshark (BuNo 125485) on display at the Gillespie Field Annex of the San Diego Air & Space Museum in San Diego

Following cancellation of the A2D program, the second Skyshark prototype was loaned to Allison for help in solving problems with the T40 turboprop in April 1953, and two production aircraft (BuNos 125481 and 125484) were intended to be flight ready and delivered to Allison; BuNo 125484 had originally been slated to be delivered to a naval storage facility in Litchfield Park, Arizona, but was instead flown to Edwards Air Force Base for use by Allison. After the completion of engine tests, the second XA2D-1 as well as the third and sixth production A2Ds were transferred to Navy facilities in Maryland and Rhode Island for various tests, including armament and barrier tests. The fate of these aircraft is unknown, but they may have been broken up for scrap. Meanwhile, the remaining A2D-1s at the Douglas factory in El Segundo were scrapped except for BuNo 125485, which was eventually used for ground radar calibration tests (minus its engine) at Los Angeles International Airport before being donated to the Ontario Air Museum in Ontario, California in the 1960s. It eventually languished in storage at Chino Airport near the Planes of Fame Museum in Chino, until 1993, when it was trucked to Idaho Falls Airport in Idaho Falls, Idaho, and restored to display status by Pacific Fighters. In the early 2010s, the aircraft was moved to the Gillespie Field Annex of the San Diego Air and Space Museum for static display, where it resides today.   

Despite offering better performance over the Skyraider, the A2D Skyraider turned out be an engineering disaster because the Allison T40 turboprop failed to live up to expectations in performance, much to the frustration of not only the US Navy but also Douglas and other US companies that had designed several naval aircraft around the T40. In any case, Ed Heinemann learned from the failures of the A2D program and designed a lightweight jet-powered attack plane as an alternative, which became the A4D Skyhawk and eventually supplanted the Skyraider along with the later Grumman A2F/A-6 Intruder all-weather attack aircraft. Although no longer in service with its country of origin, the Skyhawk still serves with the militaries of Argentina and Brazil.

References:

Allen, F., 1994. "Shark With No Teeth: The Story of the Douglas A2D Skyshark". Air Enthusiast 53 (Spring 1994): 69–75. 

Buttler, T., and Griffith, A., 2015. American Secret Projects 1: Fighters, Bombers, and Attack Aircraft, 1937 to 1945. Manchester, UK: Crecy Publishing.

Francillon, R., 1988. McDonnell Douglas Aircraft Since 1920: Volume I. Annapolis, MD: Naval Institute Press.

Markgraf, G., 1997. Douglas Skyshark, A2D Turbo-Prop Attack (Naval Fighters No. 43). Simi Valley, CA: Ginter Books.

Early vertical take-off and landing fighters from southern California, part 3: Lockheed VTOL flat riser projects

In the late 1950s the US Air Force and Navy had come to recognize the operational impracticalities of the tail-sitter concept and instead looked to the concept of lift jets and lift fans being pioneered in the United Kingdom by Rolls-Royce in the 1950s as the most suitable method of VTOL for a combat jet. With the lift jets (or lift fans) situated in the center fuselage, the pilot would easily land his plane on an airfield or any unpaved surfaces. In addition to lift jets/lift fans, the US aircraft industry looked at equipping VTOL jet fighters with jet engines housed in nacelles that could swivel upwards during vertical take-off and landing but tilt longitudinally in forward flight. The Bell Aircraft Corporation of Buffalo, New York, was the chief leader in developing flat-rising VTOL jet aircraft in the US during the late 1950s, as exemplified by the Model 65 experimental tilt-jet and the X-14 vectored thrust technology demonstrator, before turning to the design of the D-139 and D-188 VTOL combat jets, the latter whose design philosophy influenced West Germany's EWR VJ 101C prototype VTOL jet fighter. But Bell wasn't the only US aircraft manufacturer to undertake design studies for flat-rising VTOL combat jets. As will be emphasized in my third and final post in my three-part blogpost series on early American VTOL fighter designs, as interest in the tail-sitter concept gradually evaporated on part of the Air Force and Navy, Lockheed decided to envisage flat-rising VTOL jet fighter designs of its own after becoming aware of the impracticality of the tail-sitter idea; the resulting spree of proposals for flat-risers from Lockheed in the late 1950s would range from designs with lift jets/lift fans to aircraft with swiveling jet engines.

Top: Lockheed CL-346-1 (left) and CL-346-31 (right) designs
Bottom: Artist's conception of the CL-407 heavy fighter/light bomber

Around the time that design studies for the CL-295 and CL-349 tail-sitter projects concluded, Lockheed proposed a tilt-jet VTOL interceptor/tactical strike aircraft under the designation CL-346-1. This design retained the airframe of the F-104 Starfighter but differed in eliminating the fuselage engine and instead having two General Electric J79 turbojets in wingtip nacelles, and the horizontal stabilizer was situated on a ventral fin below the tail empennage. The J79s would tilt upwards for vertical take-off and when the CL-346-1 transitioned to forward flight, the engines would tilt horizontally so the aircraft could cruise to Mach 2 at 60,000 feet (18,288 meters). The CL-346-1 would have carried four Sidewinder air-to-air missiles below the wings in its role as an interceptor or a single nuclear bomb below the fuselage in tactical strike role. Another CL-346 design study, the CL-346-31, was an F-104 airframe with the entire propulsion housed in the inner wing section, with swiveling units and four additional lift jets in the fuselage, and the tail empennage had a tailwheel and long front undercarriage struts to provide good exhaust clearance during vertical take-off. A series of VTOL heavy fighter/light bomber designs were worked out by Lockheed under the designation CL-407, including the canard delta wing CL-407-37/40 with two jet engines in the rear fuselage that could swivel for VTOL and three lift jets situated behind the cockpit, and the canard delta wing CL-407-47-2 with two jet engines that could swivel backwards for VTOL and wingtips that could fold downwards. The CL-407 would have had a top speed of Mach 3 and an altitude of 70,000 feet (21,336 meters).

Three-view drawing of the CL-704

Beginning in 1958, Lockheed envisaged a VTOL derivative of the F-104G, the CL-521-1. which was similar to the F-104G but differed in having two large wingtip pods, each containing four Rolls-Royce RB.108 turbojets. The fuel in the wingtip pods was optionally transferred to the fuselage for use in the General Electric J79 and the engine pods could easily be removed, returning the aircraft to non-VTOL form and allowing the carriage of wingtip fuel tanks or missiles. The CL-521-1 would have a combat radius of 287 miles (460 km) and a top speed of Mach 1.4 when carrying the engine pods, although its range/payload could be increased if VTOL wasn't required, and armament comprised the 20 mm M61 Vulcan cannon and 4,000 lb (1,814 kg) of Sidewinder missiles, air-to-ground rocket, free-fall bombs, and tactical nuclear weapons. The CL-704 was a reconnaissance version of the CL-521-1 designed to conduct reconnaissance missions from NATO bases in Europe, with a secondary strike capability, and it was similar to the RF-104G in housing three KS-67A spy cameras. A total of 34 design variations of the CL-704 were probably worked out, even though documentation is lacking. 

CL-802-12 VTOL jet fighter design

From January 1962 to May 1963, Lockheed undertook design studies for a single-seat carrier-based multi-role VTOL combat jet, the CL-706, of which one proposal, the CL-706-13, was powered by two General Electric X-84 turbofans and ten 4,000 lb (17.79 kN) thrust Continental Model 365 lift jets, of which six were housed in the wingtip pods and four were situated in the forward fuselage section. Armament for the CL-706-13 consisted of a rotary 30 mm cannon, free-fall bombs, rockets, and air-to-surface missiles. A series of additional flat-rising VTOL supersonic jet fighter designs was devised under the company designation CL-802. The CL-802-12 was an F-104 Starfighter with the General Electric J79 turbojet moved further back and twelve 4,000 lb (17.79 kN) thrust Continental Model 365 lift jets, of which eight were housed in the wingtip pods and four were situated behind the cockpit, and it could be used for close air support, cargo transport, and light observation. The performance of the CL-802-12 would have been only Mach 1 or less, slower than other VTOL Starfighter design studies. The CL-802-7 was a two-seat Mach 2 fighter design with a single tail fin, two afterburning turbofans, and eight lift fans in the center fuselage section, and it would have carried four underwing munitions plus a variety of weapons in an internal weapons bay. On the other hand, the CL-802-14-3 proposal featured a two-seat Mach 2 VTOL jet fighter with two General Electric J79 turbojets in nacelles on the rear fuselage and ten Continental Model 365 lift jets (four behind the cockpit, four behind the wing's trailing edge, and two towards the tail). Armament for the CL-802-14-3 would have comprised a rotary cannon, underwing air-to-air or air-to-surface missiles, and a tactical nuclear weapon in the internal weapons bay.

Lockheed CL-757 VTOL test rig

To test the flight behavior of its VTOL jet fighter designs, Lockheed built an experimental VTOL test rig, the CL-757, which had an open cockpit for the pilot and observer and measured 23 feet (7 meters) long and 32 feet (9.75 meters) wide, with a gross weight of 7,920 lb (3,592 kg). Power came from six upright General Electric turbojets on each side of the craft, and those engines were supplied and fitted by US Air Force technicians. A total of 19 test flights of the CL-757 were carried out at Edwards Air Force Base in mid-1963, of which the initial ones were tethered. Although the Lockheed flat-rising VTOL jet fighter designs envisaged in the late 1950s and early 1960s looked good on paper and offered operational advantages over the earlier Lockheed tail-sitter designs in terms of landing characteristics, not a single one of Lockheed's flat-rising VTOL jet fighter projects progressed beyond the drawing board, largely because the US Air Force had lost interest in VTOL jet fighter technology and Lockheed itself was over-committed to other projects, including the A-12/SR-71 Blackbird and L-2000 supersonic airliner. Nonetheless, the VTOL propulsion systems utilized by Lockheed for its design studies for flat-rising VTOL combat jets would find their way into the EWR VJ 101C, Hawker Siddeley (later British Aerospace) Harrier, and Hawker Siddeley P.1154 combat aircraft.

References:

Buttler, T., 2007. American Secret Projects: Fighters and Interceptors 1945 to 1978. Hinckley, UK: Midland Publishing.

Rose, B., 2013. Vertical Take-off Fighter Aircraft. Hersham, UK: Ian Allan Publishing. 

Early vertical take-off and landing fighters from southern California, part 2: the jet-powered tail-sitters

As I discussed in my previous post, the Convair XFY and Lockheed XFV were touted by their respective companies as magic-bullet solutions to overcome the speed limitations of helicopters by combining the vertical lift of a helicopter with the combat performance of a fighter plane. However, these aircraft were handicapped by the pilot having difficulty landing the plane (in other words, figuring out how close he was approaching the ground), and the Convair and Lockheed "Pogos" also would have been no match for Soviet fighter planes even if they had gone into production due to the turboprop powerplant around which were designed. However, US aircraft manufacturers in southern California did not completely give up on the tail-sitter concept, instead merely choosing to design tail-sitting VTOL fighters powered by a jet engine rather than a propeller engine. While the Ryan Aeronautical Company of San Diego is best known for taking the lead in jet-powered tail-sitting VTOL aircraft design, the very Convair and Lockheed that had created the turboprop-powered "Pogo" aircraft also undertook design studies for tail-sitters with jet propulsion. Therefore, this post will focus on jet-powered designs for tail-sitting VTOL fighters from southern California.

Artist's impression of the Ryan Model 38 VTOL jet fighter project

In January 1947, the Ryan Aeronautical Company of San Diego began design studies for a single-seat VTOL fighter, the Model 38, in response to a Pentagon requirement for a prototype jet-powered, single-seat VTOL fighter. The first Model 38 design, the Model 38-1, was to use either one Rolls-Royce Nene or Allison J33 turbojet (both of which had a thrust of 5,000 lb [22.2 kN]) and had a gross weight of 7,700 lb (3,492 kg). Because the weight of the proposed aircraft exceeded the available thrust of the turbojet, Ryan's chief designer, Benjamin Salmon, proposed to equip the Model 38-1 with four rocket-assisted take-off (RATO) boosters that would be used in take-off mode until the aircraft reached sufficient altitude in order to conduct a transition to forward flight. By February, two additional Model 38 proposals were envisaged, the Model 38-2 with an X-shaped tail and tip-mounted jets, and the Model 38-3 with a tail empennage featuring four fins. In April 1947, the US Navy awarded Ryan a contract for further development, with the aim being to construct two prototypes. A test rig was built by Ryan to use an Allison J33 for VTOL testing in late 1948, and Ryan carried out wind tunnel tests of various configurations for the Model 38 for the next few years. By September 1951, a somewhat larger Model 38 design was envisaged with delta wings, a weight of 17,500 lb (7,937 kg) and one 21,000 lb (93.4 kN) thrust General Electric J53 turbojet. The proposed production Model 38 would be armed with four 20 mm cannons and air-to-surface missiles. Unusual features for vertical take-off and landing included a vectorable exhaust outlet for providing low-speed pitch and yaw control, plus ducts feeding engine compressor air to outlets in the wings for low-speed roll control. The aircraft would have flown at subsonic speeds at an altitude of 50,000 feet (15,240 meters). By late 1951, however, the Navy chose not to fund full-scale development of this proposal, largely due to concerns about cost and performance, not to mention that the J53 had earlier begun test runs. Ryan then sought to win back Navy support in 1953 by conceiving the Model 38R with a T-tail delta wing planform and one Pratt & Whitney J67 turbojet, but this design did not progress beyond the drawing board (Miller 2001). 

Left: Ryan X-13 Vertijet prepared to hook itself to a flatbed launch/transport trailer after a test flight, 1957
Center: Second X-13 Vertijet (serial number 54-1620) on display at the National USAF Museum in Dayton, Ohio.
Right: First X-13 Vertijet (serial number 54-1619) on display at the Gillespie Field Annex of the San Diego Air and Space Museum in San Diego, California.

Although the Navy lost interest in Ryan's VTOL jet fighter design, the US Air Force was very interested in Ryan's proposal, so Ryan itself proposed the Model 69 VTOL technology demonstrator with one non-afterburning 10,000 lb (44.4 kN) thrust Rolls-Royce Avon turbojet, a high-mounted delta wing spanning 21 feet (6.4 meters), small wingtip stabilizers, and a large vertical stabilizer. In 1953, the US Air Force awarded Ryan a contract to build two experimental aircraft (serial numbers 54-1619/1620) designated X-13, and the aircraft was officially christened Vertijet. Assembly of the first X-13 began in early 1954 and was completed by the fall of 1955. The Vertijet made its first flight on December 10, 1955 with Ryan test pilot Pete Girard at the controls; this was a brief horizontal flight to prove airworthiness, with an improvised, fixed tricycle landing gear. As flight tests progressed, the tricycle undercarriage was replaced by the tail-mounted Roller Skate apparatus with dampened wheels, and on May 28, 1956, the first X-13 vertical take-off and landing was conducted with the Roller Skate. The second X-13 (54-1620) was completed in early 1956, differing from the first aircraft in featuring a modified cockpit with improved visibility for the pilot when the Vertijet was in an upright position. It was shipped to Edwards Air Force Base on a trailer in the spring of 1956 and fitted with fixed tricycle landing gear, making its first flight on May 28 in horizontal takeoff and landing mode. After initial flawless flights in horizontal mode, the aircraft was fitted with necessary attachment gear that would allow it to operate from a special 'pogo rig', prompting the addition of an attachment hook below the X-13's nose that would engage with a small trapeze when the aircraft was at rest. A trailer system featuring a vertical flatbed was built to serve as a launch and recovery platform for the second X-13, and on November 28 the aircraft made its first transition from level flight to hover at 6,000 feet (1,828 meters), before returning to level flight. On April 11, 1957, a vertical take-off from the trailer was carried out followed by a transition to horizontal flight and a vertical return to the pogo rig. By July, the second X-13 was ferried to Pentagon via the Panama Canal aboard the USS Young America to conduct demonstration flights for the military top brass, with Pete Girard as well as Ryan test pilots William "Lou" Everett and William Immenschuh taking part in those flights. After several short demonstration flights at Andrews Air Force Base in Maryland, Girard himself carried out a demonstration flight on July 30 before 3,000 Pentagon officials and media reporters lasting 7 minutes, aiming to prove to the Air Force the potential combat applications of the X-13. The flight unexpectedly ended in mishap just as the X-13 was to land in front of the Pentagon, as Girard noted that the fuel meter was low, so he decided to land the plane in a rose garden. The X-13 program was terminated in early 1958 and the two aircraft subsequently were given to museums, with the first aircraft now on display at the Gillespie Field Annex of the San Diego Air and Space Museum in San Diego, and the second aircraft displayed at the National Museum of the US Air Force at Wright-Patterson Air Force Base in Dayton, Ohio.

3-view drawing of the Ryan Model 115C tail-sitting supersonic VTOL jet fighter

Although the Model 38 and X-13 dominated much of Ryan's focus on VTOL jet fighter design, in early 1954 the company was asked by the US Air Force to study the notion of a tail-sitting supersonic VTOL jet fighter. Drawing upon experience in designing the Model 38 and X-13, Ryan proposed the Model 84 hook-suspended jet fighter, of which 14 different single- and twin-engine configurations were worked out. I was initially proposed to power the Model 84 with one Pratt & Whitney J75 turbojet, but Ryan later felt that two side-by-side General Electric J79 turbojets were essential to give the Model 84 a better combat radius. A modified version of the Model 84 was also proposed, the Model 84F-7, which had two 20,000 lb (89 kN) thrust General Electric X-301 afterburning turbofans, a top speed of Mach 2.5, and armament comprising 20 mm cannons and air-to-air missiles or one free-fall nuclear weapon. The Model 84 design resembled the Convair F-102 Delta Dagger and Dassault Mirage IIIA in having engine inlets on the sides of the forward fuselage, a delta wing, and a single vertical stabilizer. A hook would be fitted below the forward fuselage, and the pressurized cockpit was fitted with a swiveling ejector. Ryan submitted the Model 84 design to the Wright Air Development Center, and after this proposal was well-received, it began work in 1955 on an improved supersonic VTOL tail-sitting fighter design, the Model 112, which had two General Electric J79s. The Model 112 design study continued into 1956, and Ryan offered two versions of the Model 112 to the US Navy, the Model 113 and Model 114. Meanwhile, the Air Force decided to continue funding Ryan's supersonic VTOL tail-sitter studies, and thus Ryan proposed the Model 115, which was similar to the Model 112 but had a longer fuselage to accommodate a larger bay housing a single tactical nuclear weapon or four air-to-air missiles. The final version, the Model 115C, envisaged in 1957, had a slightly stretched fuselage, greater operating range, circular inlets for the J79s, and greater range, and the airframe would be made from stainless steel. Alternative methods of boosting take-off performance were considered, including water injection and use of exotic fuels, and the Model 115C also was to used a tricycle landing gear. Despite promising lower operating costs, none of the Ryan designs for supersonic VTOL tail-sitter designs progressed to the hardware phase.

Top: Convair Configuration IVa
Center: North American tail-sitting supersonic VTOL design study
Bottom (clockwise from top): Lockheed CL-295-1, CL-295-3, CL-295-4, and CL-295-2. The CL-295-2 was also offered to the Navy as the CL-349-17.

Even before testing of the Convair XFY and Lockheed XFV began, Convair and Lockheed investigated design studies for supersonic VTOL tail-sitting jet fighters, which like the turboprop-powered "Pogos" hewed to the design philosophies for the fighter jets built by Convair and Lockheed. Under a six-month USAF contract to study a lightweight supersonic tail-sitter issued in December 1953, Convair devised several proposals for supersonic VTOL tail-sitters, all of which shared a delta wing. One of the first supersonic VTOL tail-sitter design studies to emerge had a fuselage containing a jet engine fed by air flowing through a nose intake and unguided rockets carried in retracted forward-located packs. The aircraft stood on four dampened struts in wing nacelles and two vertical stabilizers, and the cockpit was housed in a large pod on the vertical stabilizer, with the pilot lying in a prone position. A revised design had the cockpit at the front of the aircraft, surrounded by the engine inlet. The Convair Configuration IVa had a fairly typical cockpit canopy design, an air inlet for the turbofan below the cockpit, and a ventral pod housing one M61 Vulcan 20 mm cannon and the landing strut. Wind tunnel tests of this design at NACA Langley in 1954 showed this proposal to be aerodynamically sound, but concerns about directional stability prompted Convair to devise a new Configuration IV design with delta-shaped canards on the nose and the ventral fin eliminated. In the meantime, Lockheed in 1954 investigated several designs for tail-sitting VTOL supersonic jet fighter under the company designation CL-295. The first two proposals, the CL-295-1 and CL-295-3, were based on the F-104 Starfighter and featured a retractable hook below the nose similar to that of the Ryan supersonic VTOL tail-sitters, as well as an exhaust flow control system, reaction jets, and a secondary stabilizing horizontal stabilizer for VTOL. The CL-295-1 was powered by one Wright TJC32C4 turbojet whereas the CL-295-3 used one General Electric X-84 turbofan and had a slightly shorter fuselage and wingspan in addition to being lighter, and both proposals would be armed with one 20 mm M61 Vulcan cannon and reach a speed of Mach 2. The next CL-295 design, the CL-295-4, was powered by two General Electric X-84 turbofans and stood on two vertical stabilizers and two wingtip nacelles when standing upright for VTOL, while featuring a canards on the nose. Successful test runs of the General Electric J79 turbojet prompted Lockheed to propose a J79-powered derivative of the CL-295-4 with twin dorsal vertical stabilizers, the CL-295-2. A version of the CL-295-2 was also offered to the US Navy as the CL-349-17 to meet the parameters of the TS-140 specification for a VTOL jet fighter. The CL-295-68 was similar to the F-104 in having fuel tanks at the wingtips but had rear-mounted backswept wings, a cruciform tail empennage similar of that of the earlier XFV, and one Wright J67 turbojet with low-speed vane control in the exhaust flow, fed by air flowing through a ventral engine intake. The final design for the CL-295, the CL-295-77, had nose canards as in the CL-295-2/4 and CL-349-17 but had rear-mounted backswept wings with two General Electric X-84 turbofans at the wingtips. Armament for the CL-295-77 comprised Sidewinder air-to-air missiles, and the CL-295-77 used vanes in the exhaust flow and compressed air nozzles in the wingtips for low-speed vertical control, with normal control surfaces for horizontal flight. North American is also known to have worked on a design for tail-sitting VTOL supersonic fighter, which stood upright on three vertical stabilizers and had three jet engines in the rear fuselage for vertical takeoff and four more in pairs in wingtip-mounted nacelles for forward flight, but this proposal is known only from drawings and no technical data is available at the moment. 

By the late 1950s, the US Air Force and US Navy had come to the conclusion that the tail-sitter idea was conceptually a dead end when it came to operational practicality, and after hearing the new of flight tests of the Rolls-Royce Thrust-Measuring Rig (nicknamed the Flying Bedstead) realized that the best way for jet fighter to achieve vertical take-off and landing was to use swiveling jet engines, lift jets, and lift fans. In other word, by allowing a jet fighter to rise vertically above the ground through means of downward air, lift fans, swiveling jet engines, and separate jets could give the pilot visibility during the process of landing his/her plane vertically. Thanks to the British, the US armed forces and the aircraft industry in southern California could now look in this new approach to vertical take-off and landing for jet fighters.

References:

Bradley, R., 2013. Convair Advanced Designs II: Secret Fighters, Attack Aircraft, and Unique Concepts 1929-1973. Manchester, UK: Crécy Publishing.  

Buttler, T., 2007. American Secret Projects: Fighters and Interceptors 1945 to 1978. Hinckley, UK: Midland Publishing.

Rose, B., 2013. Vertical Take-off Fighter Aircraft. Hersham, UK: Ian Allan Publishing.