F-35 Cancelled, then what ?

[Engines]

Perhaps I could help throw some light on the issue of conformal fuel tanks.

All fast jet combat aircraft face a common challenge - making space within the aircraft for enough fuel to do all the required missions. This challenge arises due to the iron laws of physics, and the achievable values for specific fuel consumption and engine thrust to weight ratios. The main solutions usually available are:

1. Reduce the fuel load and accept a shorter range/endurance - which nobody likes
2. Add fuel internally to the aircraft inn places you normally wouldn't.
3. Add fuel to the aircraft externally, using extra tanks.

Option 2 above has been used occasionally in the past, and is a good indicator of how hard the struggle has been. If aircraft have fuel tanks in their fins, tailplanes, flaps, or even their undercarriage doors, they've had a problem with internal fuel volume during their initial design phase.

The third option is almost always adopted, and it's unusual to find a tactical combat jet that hasn't used drop tanks. However, these generate a lot of drag, and it's common for tanks to generate so much that most of the fuel they carry is used up hauling the tank around the sky. I don't have precise figures to hand, but I think that any 'productive drop tank fuel fraction' over 50% would be very good indeed. I do know that the large tanks on the F3 (2250l?) gave only about 10% 'productive fuel' in some sortie profiles.

So, conformal tanks have often been adopted as a way to add fuel with less drag. Actually, some were around before conventional tanks, examples being the Bf110, Seafire belly tank, Lancaster spine mounted tank, and the Supermarine Attacker. They've appeared on Soviet aircraft (Mig-21, etc.) and later on US aircraft (F-15, F-16 and now F-18). Now they're appearing on the Typhoon. Essentially, what you do is add a new bit of airframe on to the outside of the aircraft to hold fuel, picking up on a number of attachment points along the fuselage. Most conformal tanks aren't jettisonable - they are a semi-permanent addition.

So why put them on? I don't know the precise reasons for each decision by type, but here are my (hopefully) informed guesses:

1. In service weight increases - most aircraft get heavier as time goes on. Extra this, extra that. all lead to more weight, so more lift required, so more drag. (Darn those pesky physics). Eventually, you just need more fuel and you can't lose another pylon.
2. Extra drag - you build a nice clean aircraft and then start adding lumps and bumps all over it (EO turrets, extra antennae, RWR systems, etc.). Or bigger external stores. More fuel required for the same range/endurance.
3. Changes to requirements. Here's an example. Might be Typhoon, or not. You build an aircraft that's basically an out and out high altitude BVR death machine, pulling tons of G and firing off telegraph poles of death on JTIDS data at the dastardly foe coming at you over the North German plain. A few years later, you then get told that the primary role is to haul tons of draggy bombs around at medium level for hours on end, as your available bases are hundreds of miles from the action. You end up needing more fuel, and lots of it. Losing a few 'G' off the envelope is now not so important.

I have seen claims that adding conformal tanks reduces drag. With all due respect, I find those slightly unlikely. However, it might be true at the new sortie profiles now being demanded. More likely, what they are actually saying is that a set of conformal tanks generate less drag than the equivalent set of drop tanks. That would be very likely. Conformal tanks usually appear toward the end of an aircraft's service life. Perhaps driven by the 'in-service' factors I outlined above. Sometimes, they appear during development - the V-22's external fuselage fairings grew massively during development to house more fuel as they struggled with weight increases.

LO aircraft designs make the design challenge I outlined at the start of this post even tougher - you don't have the 'add drop tanks' option available to you for many of the expected missions, and the internal volume of the aircraft is also being taken up by many of the avionics bits and pieces often carried in pods outside the skin. (I'm not making excuses for the F-35 here, just stating the facts). It's even harder if you also stress the internal volume with big lumps of powered lift gear. In the case of the F-35 (all variants, note) the aft fuselage 'hump' got noticeably larger during the SDD phase, driven by the need for more fuel. They just avoided carrying fuel in the fins - the fins had been designed to carry fuel, but during the weight reduction phase it was found that this imposed too much of a weight penalty in extra fin structure, and the fuel volume was found elsewhere.

To wrap up - designing tactical combat aircraft is really hard. keeping them relevant throughout their service lives is just as hard. Conformal tanks are a useful way of solving some of those problems. But for my money, the better option has to be to get as much fuel into the aircraft as you can first time around. Trading off 'high end' performance might just be a price worth paying. But try telling single seat pilots that.

Best regards as ever to all those juggling with the iron laws of physics,

Engines