The always valuable PPrune points us to a Flight story that may mean a lot more than it says.
One of the UK's most important contributions to the Joint Strike Fighter (JSF) is the technology for flight control in the short take-off, vertical landing (STOVL) mode. The Harrier is a difficult beast to land. The pilot has control of thrust, attitude and nozzle angle, all via different channels which are not connected to each other.
Long before there was JSF, what was then the UK Royal Aircraft Establishment was using a two-seat Harrier to test a simpler and more automated system in which the pilot could actually command the airplane to change its flight path. Now operated by QinetiQ, the RAE's successor, the hardworking VAAC Harrier (it stands for vectored thrust aircraft, advanced flight control) has been flying for 20 years. In its latest tests, the airplane is going to demonstrate a technique called "shipboard rolling vertical landing" (SRVL) to see whether it can be applied to the STOVL F-35B.
SRVL means landing with forward speed, over and above the wind-over-deck speed – the combined total of ship speed and headwind – and the idea, says the UK Ministry of Defence, is to augment jet lift with wing lift, increasing the load of fuel and weapons that the F-35B can bring back to the ship. Nothing wrong with that, you might think.
Some observers, however, have pointed out that SRVL has inherent risks. How fast "rolling" might be is not clear, but on take-off the F-35B is going more than 80 knots before it gets enough wing lift to make a difference. The Royal Navy's new carriers will not be fast ships, so the 15-ton JSF would probably be touching down with 40 knots or more of relative velocity, nose-up to generate lift, with the engine winding down, and with only the wheel brakes to stop. Any problems, and there is no chance of a traditional carrier-type "bolter" – that it, shooting off the front of the deck at full power and trying again – because the thrust is pointing the wrong way.
The concept wasn't part of early plans for the JSF, studies having started in 2004. The UK MoD casts it as a performance improvement in the Flight story, but the UK's National Audit Office in its November 2006 report on major projects (pdf) says that the SRVL project is part of "intensive program action" to meet the F-35B's bring-back weight goal, which is "at risk" because of "weight challenges and propulsion system integration issues".
This is not too surprising. The F-35B's bring-back load – the ability to recover with two 1000 pound bombs, two AMRAAMs and fuel reserves – was the focus of the fighter's weight problems in 2004. In the course of 2006, Pratt & Whitney disclosed that it is looking at a two-phase uprating program for the F135 engine, to take effect soon after the jet is due to enter service. General Electric and Rolls-Royce, meanwhile, have been told not to talk too loudly about the fact that the design of their alternate F136 engine – which the Pentagon keeps trying to scrap – was frozen after the first F-35 weight problems and consequently provides for a bit more oomph than its rival.
All the easy (and even moderately difficult) ways to cut weight out of the F-35B have been done in the course of fixing the 2004 weight gain. Fuel reserves on landing have been pared to a minimum. The engine is giving its best – at service entry, the stress of vertical landings will already reduce the engine's life. Moreover, running the jet hotter is not an easy option. The JSF is a stealthy aircraft and consequently dumps a minimum of heat overboard, in order to reduce its infra-red signature. Instead, it uses its fuel as a heat-sink (as the F-22 does) but reducing the fuel reserve means that there's less cooling capacity. Hence the interest in SRVL – because it allows the jet to land at a higher weight. Tests of the veteran VAAC – possibly including landings on France's carrier Charles de Gaulle – will show how well the technique can be made to work