Flying far is more important than flying fast, Japanese fighter technologists have found in studies aimed at defining their country’s next combat aircraft. Looking for ways for their air force to fight outnumbered, researchers are also emphasizing that Japan’s next fighter should share targeting data, carry a big internal load of large, high-performance missiles and be able to guide them while retreating.
The results of this work may be committed to full-scale development within four years. Japan is holding open the possibility of a joint international program, which the finance ministry would surely prefer, but the defense ministry looks wary of being trapped in a late-running cooperative effort over which it has little control. Specifically national requirements such as the preference for range over speed may also nudge Japan toward going it alone.
Engineers from the defense ministry’s Technical Research & Development Institute (TRDI) and IHI Corp. are well into preliminary development of a surprisingly powerful turbofan for the twin-engine fighter, which would enter service around 2030 as the F-3. TRDI is also handling the studies into the airframe, probably with strong engineering support from Mitsubishi Heavy Industries, which would build the airframe, and Mitsubishi Electric, the country’s dominant military electronic systems supplier.
The work is intended to give Japan the option of developing a fighter to replace the Mitsubishi Heavy Industries F-2, says the ministry. The country is not committed to doing so, but by the fiscal year beginning April 2018, “the final decision for development will be made and necessary measures will be taken,” the ministry says in answer to Aviation Week’s questions.
The most likely, perhaps only, candidates for joint development are the still undefined U.S. Air Force and Navy ambitions for fighter programs to succeed the Lockheed Martin F-35 Lightning. But the ministry says that, in contemplating a joint effort, “it needs to be considered whether the development would be concluded by the time F-2 retires.” It is obviously thinking of the huge delays in the F-35 Joint Strike Fighter program.
About ¥120 billion ($1 billion) has been spent since fiscal 2010 on preliminary work for the F-3, with ¥41.2 billion requested for fiscal 2015. In this effort, which has been called i3, TRDI and industry are preparing key technologies for a future fighter, extending the progress made in developing the ATD-X stealth demonstrator, which is due to fly this fiscal year (AW&ST July 21, p. 32).
A further ¥14.2 billion yen is requested for fiscal 2015 to fund development of the F-3’s engine, which is moving ahead well in advance of the airframe. In 2012, its thrust was known to be a lavish 33,000 lb., a figure that has probably not changed, at least for the preliminary development stage (AW&ST Feb. 14, 2011, p. 33).
25DMU is the latest annual iteration of TRDI’s design studies for the F-3. Credit: TRDI
Prototypes of the engine’s combustor, high-pressure compressor and high-pressure turbine are in testing. Evaluation of the turbine, at least, is supposed to be completed next financial year. Prototypes of the low-pressure compressor and low-pressure turbine will be tested until fiscal 2017. A full prototype engine should be demonstrated in fiscal 2018.
Key aims of the engine project are to achieve the extremely high temperature of 1,800C (3,272F) and to keep the powerplant slim in order to reduce airframe frontal area. The latter point is one of several features that suggest an intention to build a supercruising fighter, which now looks doubtful amid the emphasis on range over speed.
Whether Japan will build the aircraft at all is another question. On the one hand, the country feels its security is increasingly imperiled by rising and bellicose China. On the other hand, developing a heavy stealth fighter would have to cost tens of billions of dollars.
“The expense necessary for development of the fighter aircraft hasn’t been determined at all at this moment,” says the ministry, adding that although the air force has 90 F-2s, the number of successor aircraft is also not settled. And no specification for the next fighter has been set.
Still, TRDI’s work, most recently presented at an official seminar this month, gives a pretty good indication of the direction in which Japan wants to go.
TRDI produced annual concepts in 2011, 2012 and 2013, successively designated 23DMU, 24DMU and 25DMU. (The number in each designation is the corresponding regnal year of Emperor Akihito; “DMU” stands for “digital mock-up.”) Judging from the modest 40-deg. leading-edge sweep of their mainplanes, none of these designs is intended to supercruise—to fly supersonically without afterburning.
The designers have moved back and forth in balancing stealth and other characteristics, but appear to have consistently rejected the challenging measure of eliminating vertical tail surfaces, a move that would help defeat radars operating at lower frequencies. Size seems to have varied, lately moving up, and is probably not at all modest, considering the thrust of the engine. Two engines of 33,000 lb. each imply an aircraft approaching the class of the Lockheed Martin F-22 Raptor. Conceivably, the engine might be scaled down, however.
The 2014 airframe concept has not been revealed, but by last year TRDI’s work had evolved a design, 25DMU, that emphasized large internal missile stowage and especially range, with an unusually big wing of high aspect ratio (span relative to average chord). The results of studies presented at the seminar endorsed the 25DMU’s emphasis on range, so this year’s undisclosed 26DMU concept may not be much different. Design 25DMU is at least still relevant, since it will be used as a benchmark next year for assessing 26DMU. Altogether, it sounds as if the Japanese are zeroing in on a final configuration.
The 2011 design, 23DMU, looked somewhat like a scaled up ATD-X. As is common in stealth aircraft, snaking inlet ducts shielded the engine faces from radar energy, which they would otherwise reflect strongly. The tail of 23DMU had the usual four surfaces, with the fins angled outward.
Internal, side-by-side weapons stowage would have accommodated four “medium-range missiles”—which TRDI’s drawings show to be very large, implying more than medium range. Douglas Barrie of the International Institute for Strategic Studies in London notes that the missiles in all TRDI’s drawings have inlets for ramjet propulsion, suggesting a greater kill probability than offered by weapons with only rocket engines. All TRDI’s published designs also include two short-range missiles in the sides of the fuselage, large passive radio receiver arrays on the sides of the fuselage, supplementing the nose radar, and infrared sensors below and forward of the cockpit.
The result of the 23DMU design effort was quite a deep fuselage and a lot of radar-reflecting side area, which the designers sought to reduce in 24DMU by flattening the aircraft. They moved the engines outboard and fed them with straighter ducts, relying on blockers—radial baffles mounted ahead of the engines—to help obstruct radar energy. The four medium-range missiles were carried in tandem pairs. Just two stabilizers were mounted as a V-tail much like that of the Northrop YF-23, the aircraft that the U.S. Air Force rejected when it chose the F-22.
Having produced 24DMU, TRDI assessed the impact of these changes in a simulated engagement. It found that a pilot flying a 24DMU instead of a 23DMU would be able to fire about 13% more missiles and the enemy about a third fewer. (These figures are judged from a bar chart, without numerical values, which TRDI presented at the seminar.) The time available for taking a shot was shorter for both, but the enemy’s firing interval suffered more. A modified 23DMU with a different sweep angle produced intermediate combat results. TRDI comments: “Different sweep angles have little effect on peak radar cross section.”
Credit: Colin Throm/AW&ST
In the next step, devising 25DMU last year, the developers restored the fully snaking ducts but kept the side area lower than in 23DMU. They moved the engines inboard and left a broad space for side-by-side stowage of six medium-range missiles under the ducts, which twisted upward and inward. The additional missiles, even at the expense of greater size and cost, make good sense for a country that must contemplate fighting against far more numerous enemy forces, Barrie says.
In another change, the four tail surfaces reappeared in 25DMU, but the fins remained highly canted and were kept shorter than those of 23DMU, while the tailplanes were angled down, perhaps to provide a sufficient vertical component for the tail.
Wingspan and aspect ratio increased markedly—the latter to 3.8-3.9 from 3.2-3.3 in 24DMU, judging from the imprecise drawings that are available. The aspect ratio of the F-35A is 2.4; the Boeing F-15’s is 3.0. If TRDI’s drawings are to scale, as they appear to be, span increased almost 20% in 25DMU. Clearly, the point of these wing changes was to increase range with an improved ratio of lift to drag and a greater volume for fuel. The fuselage looks larger, too, offering more space for fuel. Consistent with that, TRDI confirms that range has increased, although it gives no figures. Speed and acceleration must have suffered, especially if 25DMU is at least 10% larger than its predecessors, as it appears to be. These changes reflect the results of studies that show extreme flight performance will have less effect on winning battles than range and, implicitly, endurance on station, at least under Japan’s strategic conditions.
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