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Japanese Test Weapon Bay For Fast Release, Fast Flight Bradley Perrett Japanese engineers have overcome the challenge of designing a weapon bay for supersonic release of stores, a feature of a fighter that the defense ministry has proposed to develop for entry into service in the 2030s. A critical design objective has been achieving high but undisclosed speed in the four-step cycle of door opening, weapon ejection, launcher retraction and door closing. Tests have verified aerodynamic and mechanical performance of the bay, says an official of the Defense Ministry’s Acquisition, Technology and Logistics Agency (ATLA). As with a parallel program to design inlets, the objective is not to confirm a production design is ready but rather to show the engineers can execute such a development effort. ·         A full-scale system for an indigenous fighter has been tested ·         In pairs, it would carry six Meteor-size missiles Wind-tunnel tests first confirmed the test equipment for the bay was fit for purpose and assessed the aerodynamics associated with the shape of the cavity. Later work used a model with doors (in the open position) and generically shaped ducted-rocket missiles, which were lifted out to confirm they would separate properly. The bay is capable of launching at supersonic flight speeds, says the official, speaking at a technology seminar organized by ATLA. The ministry has reported release tests were conducted at Mach 1.4—a likely speed for the launch of an air-to-air missile. The faster the launch, the farther the missile will fly. Also, a pilot maneuvering supersonically does not want to slow down—losing energy—to release weapons. In the final step to confirm they know what they are doing, Japanese engineers this year have verified the operation of the hydraulic mechanism of their design. This involved building and testing a full-scale bay, the left one of two that would go into the belly of the aircraft. The external dimensions of the bay are: length, 6.2 m (20.3 ft.); width, 2 m; and depth, 1.5 m. The cavity is smaller.

Japan’s full-scale weapon bay test article. Credit: Acquisition, Technology and Logistics Agency.

Fast operation is needed to minimize the period in which the unavoidably reflective bay is exposed to enemy radar energy. The official declined to disclose the cycle period, but the mechanical performance is likely to be extreme. During the selection phase of the Joint Strike Fighter program in 2001, Boeing said the door and ejection sequence for its design would take 1–3 sec. Achieving a 1-sec. cycle seems scarcely believable, but Raytheon has said in regard to the Lockheed Martin F-22 cycle time: “If you blinked, you missed it.” As for the ejection process between door movements, Cobham says its missile eject launcher takes no more than 0.3 sec. to throw out its store and retract. The Japanese had to confirm the demonstrator bay’s doors would open and shut fast during high-speed flight. Having no surrounding airframe, the test item could not be evaluated in a wind tunnel. Instead, the developers applied actuators to resist door movement, simulating aerodynamic loads estimated with computational fluid dynamics. Resistance varies as bay doors move, but the evaluation used constant loads in each test; different loads were applied in different tests. The bay is designed to carry six missiles the size of the MBDA Meteor, a version of which, with an advanced Japanese seeker, is the likely weapon for the Japanese fighter, if it is developed. Two small side bays would each carry one short-range missile. Japanese engineers have devised a bay for subsonic release in recent years in the Kawasaki Heavy Industries P-1 maritime patroller. But supersonic release from inside an aircraft is a much more difficult technology—so much so that in the past, supersonic aircraft have generally slowed down before dropping anything from a bay. Then-U.S. Air Force Capt. Scott Bjorge summarized the daunting problems of supersonic release from a bay in a 2004 paper. “One of the most common phenomena is the formation of self-sustaining pressure oscillations, creating cavity resonance, which can lead to structural fatigue of the aircraft and the store and extensive damage to sensitive store electronics,” he wrote. On its way out, the store passes through an unsteady interface between the air inside the bay and the fast flow outside. It also meets a shock wave from the leading edge of the bay. The motion of the store is therefore unpredictable: It can fly back up and hit the aircraft. Part of the solution appears to be shoving stores out faster, achieving the necessary speed within the limited depth available for a piston stroke in a combat aircraft. Details of this aspect of the Japanese design are unavailable, but the pneumatic Harris LAU-142/A ejector in the Lockheed Martin F-22 accelerates Raytheon AIM-120 Amraams at up to 40g to achieve a separation velocity of about 9 mps (30 fps). Cobham says its missile eject launcher, powered by cartridges, imparts similar velocities.