Three decades ago, the US Air Force was flight-testing two potential advanced short take-off and landing replacements for the Lockheed Martin C-130 - Boeing's YC-14 and McDonnell Douglas's YC-15.
The Advanced Medium STOL Transport programme was cancelled in 1979 when the air force shifted its focus from tactical to strategic airlift. This resulted in the Boeing C-17, and left the C-130 to soldier on.
Now, 30 years later, the USAF is again trying to develop an advanced STOL replacement for the C-130 - the Advanced Joint Air Combat System (AJACS) - and using the YC-14 versus YC-15 competition as a model for its plan to fly competitive demonstrators.
AJACS is the air force's solution to the army's requirement to move its Future Combat Systems vehicles around the battlefield. But it is also intended to replace the C-130 in its other roles, including special operations, combat rescue, gunship and tanker.
The Air Force Research Laboratory (AFRL) is leading the technology acquisition for AJACS, with the aim of flying concept demonstrators by 2015. The effort has been under way since 2002.
AJACS is intended to enable the army's vision of mounted vertical manoeuvre, says the AFRL. The aircraft must be able to operate from short improvised airfields, carrying the army's heavier FCS vehicles, but cruise efficiently at transonic speed.
Key technologies being demonstrated include integrated propulsion, lift and control transonic planform optimisation inlets for embedded high-bypass turbofans multi-role structures and immunity to electromagnetic interference.
Two key projects are the Advanced Composite Cargo Aircraft (ACCA), awarded to Lockheed in 2007 and the Speed Agile cruise-efficient STOL demonstration, award of which is imminent. Under ACCA, a Dornier 328Jet modified with composite fuselage and tail is to fly in October.
Speed Agile will involve low- and high-speed windtunnel testing of an AJACS concept combining extreme STOL capability with transonic cruise performance. "We are looking for one platform that can fly efficiently at 90kt for STOL and also cruise above Mach 0.8," says programme manager Cale Zuene. "It's not VTOL, but very STOL - less than 2,000ft [610m]. We want to get away from established airfields."
The goal is to take off and land in 460-610m on an improvised airstrip carrying a 30t (65,000lb) payload. This will require a powered lift system, soft-field gear, and integrated propulsion that can prevent or tolerate foreign object damage, says Zuene. The air force is also looking for "balanced survivablility" against an array of sensors - "infrared, radio-frequency and human".
Northrop Grumman has windtunnel-tested a tailless flying-wing airlifter using a blown-flap powered lift system to provide STOL performance. The design resembles the company's X-47B naval unmanned combat air vehicle demonstrator.
Northrop believes an all-wing design offers payload efficiencies, structural robustness, lower weight, better survivability and longer range. And, while a tailless design presents low-speed flight control challenges, avoiding the drag of a tail increases cruise speed.
"Speed is an obvious force multiplier," says Scott Collins, Northrop's director future mobility systems. "When transporting the same-size force, the faster delivery system requires fewer flight hours and potentially a smaller fleet size. This directly translates to more rapidly achieving military objectives with less risk, and this is something the air force and army prefer."