14 November 2015

Refining Anti-Submarine Warfare: The Grumman AF Guardian

Grumman XTB3F Guardian prototype
(San Diego Air and Space Museum)
By 1944, the United States was already laying down plans for the invasion of Japan and if the stiff resistance in the island hopping campaign across the Pacific was any indication, the Japanese were far from defeated and planners expected the worst may yet to come. Navy torpedoes delivered by Grumman TBF Avengers were notoriously unreliable and required a relatively slow approach of 120 mph during their drops at low level. Navy torpedo research languished in the years before the Second World War and torpedo bomber crewman paid the price in their lives. Wartime urgencies caused a reinvestment of torpedo development by the Navy. New air-launched torpedoes in the works could be dropped at higher speeds and further stand-off distances from enemy warships, but by this point in the war, in order to take advantage of the new designs, something with twice the engine power of the venerable Avenger was needed. The first design Grumman submitted for a new carrier-borne torpedo bomber was for the large XTB2F. With two Pratt & Whitney R-2800 Double Wasp radials, the XTB2F was a big aircraft with a maximum takeoff weight of nearly 43,000 lbs which was 8,000 lbs heavier than the maximum takeoff weight of a late model B-25 Mitchell medium bomber! The XTB2F reached the mock up phase before the program was canceled in June 1944 as it was simply too large of an aircraft for the Essex-class fleet carriers. 

The XTB2F mockup before the program cancelation
(Wikipedia/Grumman Archives)
Under the G-70 in-house designation, Grumman offered several variations of a new single-engined design, some of which were mixed-propulsion designs with a big radial engine (either the Wright R-3350 or the Pratt & Whitney R-4360) and a jet engine in the rear fuselage for extra speed on the target runs. When it became clear the R-3350 and R-4360 radials wouldn't be ready for the planned production schedules for what was designated the XTB3F, the Navy asked Grumman for a redesign with the more widely available Pratt & Whitney R-2800 engine which was the same engine used on the B-26 Marauder and P-61 Black Widow. A Westinghouse J30 would be installed in the rear fuselage- the J30 was the first all-American jet engine to run and was only the second production axial-flow engine in the world after the German Junkers Jumo 004. Using the R-2800 engine only cost the reconfigured XTB3F 30 mph of speed which was acceptable to the Navy. Despite the ending of the war by this point, the Navy still wanted the XTB3F, now named Guardian, as a replacement for the Avenger and on 23 December 1946, the prototype aircraft made its first flight. 

As designed, the J30 turbojet in the rear fuselage of the aircraft had 1,330 lbs of thrust and was fed by air intakes in the leading edge wing roots that fed ducts that ran to the engine. During ground test runs, the intake ducting wasn't big enough and strong enough to handle the mass flow to the J30 and would collapse. The engine was only run on ground tests and never used during the flight test program. Ultimately the intake duct problems resulted in the engine being removed and would be absent from the production Guardians. 

With a spacious weapons bay for the Navy's latest torpedoes, the Guardian also had two 20mm cannons in each wing as well as provisions for underwing bomb racks and rocket launchers. Most remarkable about the Guardian, though, was its size. It was the largest single engined piston aircraft to be operated from an aircraft carrier and was 2/3 the size of a Douglas DC-3. In fact, its production maximum take off weight was nearly that of a DC-3! The big R-2800 radial up front was canted down slightly to improve pilot visibility in the carrier approach pattern and it was also canted 3 degrees to the right to help offset the torque of the big engine. It's large tail was to help its stability in low altitude regimes but it did make the Guardian difficult to handle in a crosswind. 

AF-2S and AF-2W hunter/killer team
(San Diego Air and Space Museum)
On the day following the Guardian's maiden flight, the Navy imposed a stop-order on the program to give funding priority to the Douglas AD Skyraider and Martin AM Mauler programs which were also capable of dropping torpedoes and as optimized attack aircraft, were more flexible than the Guardian not to mention smaller in size. It might have been end of the Guardian had it not been for the rapid postwar expansion of the Soviet Union's submarine force. Navy carrier forces needed a specialized anti-submarine aircraft and the XTB3F Guardian best fit that need. As a result, in January 1947, the Navy redesignated the Guardian as the AF (this would be a whole topic for a complete article on how the Navy consolidated the scout, bomber and torpedo roles in the new "A for Attack" designator). Given the state of the art of anti-submarine warfare of the time, though, a single AF Guardian couldn't carry both the necessary detection equipment and weapons, so the Navy brought back the hunter-killer team concept that had worked well during the Battle of the Atlantic were Avengers hunted U-boats and were assisted in their attacks by F4F/FM-2 Wildcats. One Guardian would be equipped with the AN/APS-20 radar in a large under fuselage radome and this would be the AF-2W "hunter". The other Guardian of the pair would be the AF-2S "killer". In operational service, the AF-2W was nicknamed "Guppy" while the AF-2S was nicknamed "Scrapper". 

Interestingly the AN/APS-20 radar started out as a crash program to give the Navy its first airborne early warning aircraft to warn of incoming Kamikaze attacks. Fitted to an Avenger designated the TBF-3W under Project Cadillac, the first AEW Avengers were in Hawaii conducting carrier qualifications when the war ended in 1945. The radome and radar installation on the Guardian was for all intents and purposes, pretty much just moved over from the TBF-3W to the AF-2W. With the J30 engine and its intake ducting gone, space was available for the radar and electronics on the AF-2W and more fuel on the AF-2S. 

Production was launched in October 1947 with an order for 23 early examples which would be devoted to operational testing as well as further flight testing. The first operational examples were delivered to the fleet in September 1950 with VS-24 being the first ASW squadron to get their Guardians. The first carrier qualifications took place that November and in December 1950, VS-24 embarked on the USS Palau (CVE-122) on the Guardian's first operational cruise. Now imagine an aircraft as heavy as a DC-3 that's just 2/3 the size of a DC-3 operating off the decks of escort carriers like the Palau! While a tough and reliable aircraft, a significant number of AF Guardians were involved in deck accidents. Most deck crews were used to handing aircraft half the size of the Guardian and one ASW squadron commander got so frustrated at the incidence of handling accidents just moving the Guardian on the carrier that he required a pilot to be present and in the cockpit if needed when an AF was being moved, even if it was just in the hangar deck!

Operationally, the AF hunter/killer team would form a protective ASW screen around the carrier battle group with the idea to detect and attack any submarines as far away from the carrier as possible. During the Second World War, the protective ASW screen was handled primarily by destroyers and destroyer-escorts- with the arrival of the Guardian, aircraft could now provide the ASW screen for the task force. Guardian teams would fly up to 500 miles out from the carrier with the AF-2W "Guppy" flying a search pattern using its radar to either detect a surfaced sub or a snorkel. Once detected, the "Guppy" would summon the AF-2S "Scrapper". Once in the area, the AF-2S had it's own pod-mounted radar under the right wing (and a search light under the left wing if it was needed at night) to prosecute the attack using vectors from the "Guppy". A periscope sight aft of the wings in the belly was used to release depth charges. If needed, the "Scrapper" could come back around and use rockets and bombs to finish the job. Late model Guardian "Scrappers" designated AF-3S were fitted with magnetic anomaly detection (MAD) gear to improve the chances of note just detecting a sub but sinking it. 

A total of 386 Guardians were built and from 1945 to 1954, it was the US Navy's premier front-line carrier-based ASW aircraft. Of that nine year span, only three of those years was the Guardian truly operational as part of anti-submarine squadrons at sea. Its replacement, also from Grumman, the S2F Tracker, would combine the hunter and killer roles in the same aircraft and the design of the Tracker was heavily influenced by the canceled XTB2F, the design that was replaced by the AF Guardian. Of all the Guardians built, five survived and were used by Aero Union as 800-gallon capacity water/retardant bombers against forest fires from 1957 to 1974. That final year the US Forestry Service instructed its water bomber operators that single engined aircraft could no longer be used. Most were scrapped but a single Guardian was saved and restored to its original configuration and is now on display at the Naval Aviation Museum in Pensacola, the last of its breed in existence. 

Sources: Ironworks: The Story of Grumman and Its Aircraft by Terry Treadwell. Tempos Publishing, 2000, p150-154. "Grumman's Guardian" by Budd Davison. Flight Journal, September 2011.

No comments:

Post a Comment