Lockheed XP-58 Chain Lightning
In early 1940, the US Army reserved for itself the right to refuse permission for its American military aircraft suppliers to export these aircraft to overseas customers. This was particularly true if the Army felt that the foreign order might possibly result in undue delays in deliveries of these aircraft to its own squadrons. Consequently, when Britain and France wanted to purchase the Model 322 Lightning from Lockheed, the Army was reluctant to give its approval since it had already ordered YP-38 Lightnings for its own use. However, the USAAC finally did grant authorization to export the un-turbosupercharged Lightning to Britain and France, but only under the condition that Lockheed agree to develop and produce at no cost to the U.S. government a prototype of an advanced version of the Lightning. The formal agreement was signed on April 12, 1940.
This advanced Lightning was given the company designation of L-121, and James Gerschler was named as project engineer. The L-121 was to be powered by a pair of turbo-supercharged Continental IV-1430 liquid-cooled engines. It was to be offered in two versions, a single- seater and a two-seater. The single-seat version was to retain the standard P-38 armament of one 20-mm cannon and four 0.50-inch machine guns. The two-seat version was to have an additional armament of a single 0.50-inch machine gun mounted in a remotely-controlled barbette situated at the end of each tail boom. Gross weight was estimated at 16,500 pounds, and the aircraft was expected to attain 450 mph at 25,000 feet.
During a meeting at Wright Field in May 1940, it was decided to drop the single-seater and proceed with the two-seat version, which was assigned the designation XP-58. In July 1940, it was concluded that the XP-58 would be underpowered with the Continental engines, and the decision was made to switch to a pair of 1800 hp Pratt & Whitney XH-2600-9/11 liquid-cooled engines. The re-engined XP-58 was given the company designation of Model 20-14, and revised specifications were issued by Lockheed on September 10, 1940. A second 20-mm cannon was added to the forward-firing armament. The tail boom guns were deemed to be highly impractical, and were replaced by a single remotely-controlled dorsal turret containing a pair of 0.50-inch machine guns. The serial number 41-2670 was assigned to the prototype. Estimated gross weight had crawled upward to 24,000 pounds, and estimated top speed had fallen to 402 mph. Range on internal fuel was anticipated to be 1600 miles.
Scarcely a month after these revised specifications had been issued, Lockheed's new project engineer, Neil Harrison, was told that Pratt & Whitney was suspending development of the XH-2600 engine. The XP-58 was without an engine and attention focused on the XH-2470, the Continental XH-2860, and on the Pratt & Whitney R-2800, as possible choices for the XP-58 power plants.
Lockheed preferred the Pratt & Whitney R-2800 radial engine, and estimated that with these power plants the XP-58 would have a a loaded weight of 26,000 pounds and a maximum speed of 418 mph at 25,000 feet. However, the USAAC considered this performance to be inadequate, and suggested that Lockheed turn to the experimental Wright XR-2160 Tornado forty-two cylinder, six-row engine offering a power output of 2350 hp. This engine had an extremely small frontal area but the Tornado engine development was fraught with problems from the start. Still, in March of 1941 the USAAC announced that it was going to go with the Tornado for the XP-58.
Two months later, the USAAC issued an change order for the installation of cabin pressurization for the pilot and the aft-facing gunner, and for the addition of a remotely-controlled ventral turret to supplement the dorsal turret. These changes caused the estimated gross weight upward to 34,242 pounds. Estimated range had dropped to 1300 miles. Nevertheless, the top speed of the Tornado-powered XP-58 was estimated to be no less than 450 mph.
The XP-58 was scheduled for delivery to the USAAF in August of 1942, and to meet this deadline the project team grew to a peak of 187 people by October of 1941. However, following Pearl Harbor, the XP-58 was assigned a lower priority and most of the engineering staff were moved off to other more pressing projects. By early 1942, the XP-58 staff was down to twelve people.
In March of 1942, Lockheed suggested that the USAAF order a second XP-58 prototype using Government funds. Since the Tornado engines were already experiencing serious delays and were now not expected to be delivered until the spring of 1943, Lockheed felt that there was sufficient time to redesign the second XP-58 machine in order to provide it with enough fuel capacity to increase the range to 3000 miles. The USAAF agreed to this request and placed the order in May of 1942.
Shortly after, the USAAF began to rethink the ultimate mission for the XP-58. First, the USAAF suggested that the nose-mounted forward-firing armament should be changed to a 75-mm cannon with a 20-round automatic feeder plus a pair of 0.50-inch machine guns as a ground attack aircraft. This led to considerations of several different alternative configurations, including a two-seat attack aircraft with six forward-firing 20-mm cannon and a three-seat bomber with a bombardier in the nose, an enlarged central nacelle containing an internal bomb bay, and with or without the 75-mm nose cannon. In both the attack and bomber versions, the dorsal and ventral turrets were to be deleted, and un-supercharged engines were to be used.
With the Douglas A-26 Invader in production as an attack bomber, and the experimental Beech XA-38 Grizzly then under development as a low-altitude tank buster and ground attack aircraft. Consequently, the XP-58 program was then re-oriented back to its original role as a high-altitude aircraft, as a bomber destroyer rather than an escort fighter. The turbo-superchargers and the dorsal and ventral turrets were put back on.
The first prototype was to have four forward-firing 37-mm cannon, whereas the second was to have a 75-mm cannon and two 0.50-inch machine guns. Gross weight was now up to 38,275 pounds, and top speed was down to 414 mph at 25,000 feet. Range was only 1150 miles.
By early 1943, the XP-58 program was in chaos because of the constantly changing Army requirements. In desperation, Lockheed recommended in January 1943 that only one prototype actually be built, and that it have interchangeable noses that would permit the fitting of either type of forward-firing armament.
The trouble-ridden Tornado engine program finally collapsed in February 1943, leaving the XP-58 without engines once again. Lockheed and the USAAF both agreed to switch to a pair of turbo-supercharged Allison V-3420-11/13 twenty-four cylinder liquid-cooled engines, rated at 2600 hp for takeoff and 3000 hp at 28,000 feet.
The US Army contract, of 27 April 1943, tasked Lockheed to build 'one (1) airplane, pursuit, fighter, two-engine, two-place, known as the XP-58, complete and conforming to the contractor's specifications'. In the final configuration, the aircraft had two crewmen; a pilot and a rear-gunner operating two power turrets although the turrets and the forward firing armament was never actually installed.
With these Allison engines, the XP-58 (serial number 41-2670) was finally completed in June of 1944, more than four years after its design had begun. Its company designation was now Model 20-86. When the XP-58 rolled out of the factory it was really only half-finished. No cabin pressurisation equipment was provided, no forward-firing armament was installed, and dummy dorsal and ventral turrets were fitted in place of the real things. It made its initial flight from Lockheed Air Terminal at Burban on June 6, 1944, piloted by test pilot Joe Towle. Its first flight was a 50-minute shakedown which took the XP-58 from the manufacturer's Burbank plant to Muroc AAF. Twenty-five flights totalling 19.3 hours were made at Muroc prior to the delivery of the XP-58 to Wright Field in Ohio on October 22, 1944. These flights were marred by turbo-supercharger problems.
The second airframe was never fully completed but was trucked in parts to Muroc Dry Lake.
Because of the cost of continuing a marginal programme with a 'one-off' machine, even with a second for spare parts, the programme was terminated in 1945 and it was transferred for use as a non-flying instructional airframe. Derelict at Wright Field as late as January 1946, the Chain Lightning was scrapped in 1947.
The XP-58 project, which started off its career as having absolutely no cost to the US government, ended up costing the taxpayer over two million dollars. $400,000 of this money covered the USAAF requested changes to the first prototype, and the rest of the money covered the government-ordered, but then cancelled, second prototype.
Engines: Two turbo-supercharged Allison V-3420-11/13, 2,600 hp / 1950kW
Span: 70 ft 0 in / 21.33 m
Length: 49 ft 3 in
Height: 16 ft 0 in / 4.88 m
Wing area: 600 sq. ft / 55.74 sq.m
Empty weight: 31,306- lb
Gross weight: 38,874 - 42990 lb
Maximum speed: 436 mph / 700 km/h at 25,000 ft.
Cruising speed: 274 mph at 25,000 ft
Service ceiling: 38,400 ft
Initial climb rate: 2660 fpm
Normal range: 1250 miles / 2000 km
Maximum range: 2650 miles
Lockheed XP-58 Chain Lightning