Advanced Medium STOL Transport (AMST)
YC-14 and YC-15
The issue of replacing the C-130 first surfaced in 1963, as Project Forecast was recommending development of the CX Heavy Logistics Support Aircraft, which became the C-5A. The study also called for developing a vertical-short-takeoff-and-landing (VSTOL) aircraft to take the place of the C-130. The VSTOL project proved overly ambitious, however, and required technological innovations which, if theoretically possible, would require unrealistic outlays of money. In 1970, following Deputy Secretary of Defense David Packard's criticism of unnecessary features on the C-5A, the Air Force reduced the performance required of the VSTOL. Scaling down the VSTOL's performance characteristics did not necessarily signal the end of the project. Also in 1970, the Air Force endorsed the Tactical Air Command's request for the "urgent development of a short-takeoff-and-landing (STOL) aircraft with greater payload and operational capability than the existing C-130."
In accordance with Secretary Packard's policy of using prototypes to control weapons development costs, the Air Force Systems Command prepared proposals for an Advanced Medium STOL (Short Takeoff and Landing) Transport (AMST). In 1972, the Air Force issued a request for proposals (RFP) for an Advanced Medium STOL Transport (AMST) that would ultimately replace the C-130 tactical transport. The program emphasized innovative technologies and the capability of conducting STOL operations from 2,000-ft runways.
In 1973, Boeing and Douglas received contracts to build and test two prototypes each. Boeing's YC-14 competed with the McDonnell Douglas YC-15. Both aircraft were designed to a common cargo specification, and each utilized off-the-shelf engines to achieve a Coanda effect to maximize lifting capability. Both the YC-14 and the YC-15 satisfied the AMST performance requirements, but the program was canceled before either one of the innovative cargo planes could be chosen for production.
The engines designed for the YC-14 and YC-15 STOL aircraft both used a very low fan pressure ratio to keep jet-flap noise about 3 dB below total system noise. Other noise reducing features discussed are the low tip speed fans and a carefully selected number of fan blades and vanes with adequate spacing between them. Attention is also given to the development of a low emissions combustor, and reduction of fan frame weight, through the use of graphite/epoxy material. The YC-15 engine also employed variable pitch fans to provide thrust reversal, thus saving weight. Tests have proven that the engines could be configurated to meet the needs of a powered lift system without excessively compromising performance or weight.
In March, 1976, the Air Force Chief of Staff Gen. David C. Jones asked Air Force Systems Command to see if it was possible to use a single model of the AMST for both strategic and tactical airlift roles, and if it was possible to develop non-STOL derivatives of the AMST prototype to meet strategic airlift missions. It appears that this strategic study originated with a note from the Chairman of the Joint Chiefs of Staff, Gen. George S. Brown, that asked "Is it practical to have an AMST with a slightly higher box pick up much of the C-5 outsized load for Europe_with air refueling as necessary?".
Gordon Taylor and Gordon Quinn from the Aeronautical Systems Division at Wright Patterson Air Force Base, Ohio, were leaders in a conceptual design analysis to determine if DoD could use the AMST for strategic missions. The analysis included reviewing the ability to carry the M-60 Main Battle tank, weighing 110,000 to 117,000 pounds, on a routine basis with ranges from 2,000 NM, 3,000 NM, and 4,000 NM. Taylor and Quinn concluded that using a derivative aircraft in a routine strategic airlift role would increase AMST weight and cost significantly. To restructure the AMST from a tactical to a strategic program would require full-scale development (a larger wing, heavier structure, and different aerodynamics). Even in a non-STOL capacity the wing was the major airframe component that the study said must undergo considerable change.
In May 1976, Brig.Gen. Philip Larsen, Deputy Chief of Staff, Systems, Air Force Systems Command, wrote: "It would not be cost effective to incorporate a STOL capability in a strategic airlift derivative aircraft. A strategic derivative could employ a less complex conventional flap system which would permit CTOL [conventional takeoff and landing] operations from an 8,000 foot hard surface runway under sea level standard day conditions. The aircraft would be stretched eight feet to provide a 55-foot-long cargo compartment. This would permit routinely carrying the M-60 tank and single item payloads up to 112,500 pounds, or 14 463L cargo pallets, for distances up to 3,000 NM without refueling. In this particular example, it would be necessary to increase... YC-15 wing area 69 percent and gross weight 115 percent...."
On December 10, 1979, Program Management Directive (PMD) No. R-Q 6131(3) formally cancelled the AMST program. On that same day PMD No. R-C 0020(1) provided formal direction and guidance for activities leading to Full Scale Engineering Development of the C-X. PMD R-C 0020(1) directed that the C-X skip Milestone I and the Demonstration and Validation phase because the new aircraft will use existing technology since the Air Force had demonstrated and proved advanced technology concepts and operational utility in the AMST program.
When Secretary of Defense Harold Brown made the decision to pursue a new strategic transport in November 1979, President Jimmy Carter's emphasis on reducing military expenditures had just brought about the end of the AMST. Besides concerns over costs, developing the AMST engendered little support as it only offered tactical airlift capabilities. As a result, the C-X (Cargo Transport Aircraft-Experimental) - the future C-17 - evolved from discussions between Office of the Secretary of Defense (OSD) and Air Staff officials on expanding the role of the AMST to include strategic capabilities.
The demise of the AMST in late 1979 did not surprise the Air Force. The high unit cost of the proposed transport prompted the decision, but requirements of cost-effectiveness played a part. The AMST, like the C-130, operated as an intratheater aircraft, but a new transport capable of both tactical and strategic airlift would be more cost-effective.
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