The latest set of requirements for the Joint Strike Fighter (JSF) has been released to industry in draft form. As expected, the US Air Force has added requirements for an advanced integrated gun and internal weapons bays able to accommodate 900kg (2,000lb) air-to-surface weapons.

The changes could reduce commonality between the USAF's conventional take-off and landing (CTOL) JSF and the short take-off and vertical landing (STOVL) version for the US Marine Corps and Royal Navy.

Of the three JSF variants planned - the third is the US Navy's carrier-based (CV) aircraft - the CTOL and STOVL now have the most commonality.

Lockheed Martin JSF deputy programme manager Harry Blot says the company will now offer two weapon bays and two gun installations. The Marines, concerned about the STOVL variant's weight, wants 450kg-capacity weapon bays and a "clip-on" gun, he says.

Based on the latest iteration of the Joint Interim Requirements Document, both teams will now refine their JSF designs. Before the latest changes, the structural commonality between variants for Lockheed Martin's design was over 80% for the CTOL and 70% for the STOVL, but less than 35% for the CV. Use of similar, but not identical parts reduced the unique structure to less than 6% for the CTOL, 18% for the STOVL and 31% for the CV.

Lockheed Martin's design confines major structural differences between the CTOL and STOVL variants to the forward fuselage module which houses the shaft-driven lift fan in the latter and a fuel tank in the former.

Boeing says an advantage of its direct-lift system is that "-there is very little additional work done to the airframe to accommodate the STOVL configuration", according to JSF programme manager Frank Statkus.

First flight of the CTOL demonstrators is planned for early in 2000.

Source: Flight International