Jumping Jets! - The Joint Strike Fighter

Jet engines, and the planes that they power, are under constant development.  There are commercial pressures to make them quieter, more powerful and more fuel efficient, as well as pressures from the military for speed, manoeuvrability and stealth.

One major change has been the development of Vertical Take off and Landing (VTOL) or Short Take Off and Vertical Landing (STOVL).  This ability, probably best known from the Harrier Jump Jet, makes it easier for military aircraft to launch from, and land on, naval aircraft carriers.

The newest generation of STOVL aircraft, the Joint Strike Fighter or JSF programme, has at its heart a Lift Fan designed by Rolls Royce.  I spoke to Neil Mehta, programme director for the JSF Lift system...

Neil -   We've taken the Harrier concept that we developed here at Rolls Royce, taken their best ideas and built on top of those really and to give an aircraft now which is supersonic.  It is fully stealthy which means it can't or it's very, very difficult to be seeing through a radar and has a much bigger load bearing capacity and range than the original Harrier.

Ben -   So how do you make an aircraft take-off as if it were a helicopter straight up?

Neil -   Well, the hover part, the vertical thrust part comes from the lift system and the lift system is made up of 4 different modules.  The first one is the lift fan and the lift fan is kind of like the front part of a traditional turbo jet but it pops through 90 degrees. So it will suck air from the top of the aircraft and blow it out at high pressure at the bottom of the aircraft through a vain box which allows that flow to be directed. And that gives about 20,000 pounds of thrusts and creates one of the 4 legs that keeps the aircraft in the air during a hover. At the back of the aircraft, we direct thrust from the main engine through 90 degrees when we're in hover, through a module called a 3-bearing swivel duct.  And this is a fantastic piece of equipment.  In normal flight, it will point backwards, in hover, it will point down and it can transition between one and the other in under 2 seconds and turn 20,000 pounds of thrust through that angle.  So that's two of the legs and between those two legs, they take the most of the weight to the aircraft.  But then roll control would stop the wings dipping up and down.  Embedded in each of the wings is a roll post duct which bleeds air off the main engine and creates two, much smaller jets of air, really to give the roll control on the aircraft.  So effectively in hover, the aircraft's balanced on four pillars of air - one from the lift van at the front of the aircraft, one from the 3-BSM at the back of the aircraft, and then one on each of the wings.

Ben -   So, taking the fan itself, how does that compare to a jet engine?  You said it's like the front part of it, but we know that jet engines work.  Could you not just put a jet engine rotated through 90 degrees and use that for your thrust?

Neil -   Certainly, some of the concept studies around the world did just that, but ours is a much simpler and refined design.  The power from a jet engine comes from burning fuel and running it through a turbine which then drives the fans and the compressors at the front end of the engine.  Instead of having the fuel and turbines in the lift fan, we actually take through a shaft and a clutch in a gearbox arrangement, we take that power off the main engine.  So the very same engine that would power aircraft when it transitions into normal flight.  So that means we don't need a duplicate turbine, we don't need a duplicate fuel system, we don't need a duplicate combustor which all adds weight and cost, and would take up a lot of room in the aircraft.  So, our concept is really the only feasible concept for this type of aircraft.

Ben -   I assume that because you don't have the complications of say, the extreme heat inside the jet engine, that means that actually, the materials you put in the lift fan can be optimised to be even lighter and even better doing that lifting, without having to worry about functioning in extreme conditions.

Neil -   It's true on the lift fan that we don't have extreme heat conditions although that can't be said of the 3-BSM which is at the back of the main engine.  However, we put the lift fan through some pretty extreme conditions. We are always mindful of the weight thrust balance in there, and aerodynamically as you might imagine, having an engine at 90 degrees to the airflow rather than directly on, poses some incredible challenges to our aerodynamicists.  

And to cope with flying it around about 250 knots with the engine at 90 degrees to it has required us to push our computational fluid dynamics and understanding of aerodynamics to a next level.  We operate in areas that no other aircraft or no other blade would operate in.

Ben -   Neil Mehta, Programme Designer for the JSF lift system.  And that brings us right up to the present day, but engines in aeroplanes are still in constant development.  The new alloys and composite materials being designed in universities across the world will pave the way for new types of jet and push that current limits even further, keeping us flying into the future.