Since the subject is about internal fuel and its hoped for increase whenever possible, some basic knowledge would be helpful...
For an aircraft design like a jet fighter where fuselage internal volume is maximized for fuel, it is not as simple to increase internal fuel just because some methods increased internal volume space. The engine is fed straight from fuselage tanks and as fuel leave these tanks, outer (not external) fuel sources are transferred inward to replace departing fuselage fuel.
Fuel management specialists prefers the word 'outer' over 'external' even if we are talking about hanging wing tanks. Fuel management specialists are people that us regular folks most likely will never meet in our lives, but their effects are always present during the design stages of a fighter aircraft. Often, they can veto a concept before it is even modeled. For these people, the word 'internal' mean inside a container, such as a fuselage bladder/tank or a hanging wing tank or the wing itself. The words 'inner' and 'outer' are preferred because they are directional in the various fuel management schemes. So if a wing has several discrete tanks that are physically separated by walls, the tank closest to the fuselage is 'inner' and the rest are 'outer'. If the wing itself is one continuous fuel tank, then the wing itself is 'outer'. If there are hanging wing tanks, then they are even more 'outer'. But these people will tolerate us regular folks if we use 'internal' and 'external'.
Anyway...The scheme's intention here is to reduce the physical effects of sloshing fuel inside a container, which in this case is the fuel tank itself. For a hanging wing fuel tank, not only will we have a mass but we also have a liquid mass in motion while the aircraft is maneuvering and that could create all sorts of performance issues -- usually negative. So because we have an aircraft that is designed to carry at most two human beings, to be rapidly maneuverable, and there is the need to keep mechanical items to a minimum, this fuel management scheme was deemed to be the best. To refuel, the reverse occurs: the fuel management system will transfer fuel from inner (fuselage) to outer tanks, if there are hanging wing tanks, they will be filled first, then internal wing tanks, then fuselage tanks last.
Not only there are negative physical effects of sloshing fuel, we have a phenomenon called 'hydraulic jumps' when a liquid is in motion inside a container and the greater the ratio of empty space to liquid inside this container, the greater the potentiality of 'hydraulic jumps'...
Dynamic hydraulic jumps in oscillating containers
When the liquid in a tank undergoes sudden movement, as in the case of a fuel tank in an aircraft or in a marine vessel, it may be subjected to as many as 6 degrees of freedom.
It has been found that under such motions, typical of those obtained within the flight envelope of military, private and commercial aircraft, a dynamic hydraulic jump can occur.
So...If we have these 'hydraulic jumps' we will also have greater the potentiality of fuel-air vapor being formed, which can result in the 'you-know-what' effect...
Aircraft Fire Protection
Recent articles in the Air Force Times discussed the change from JP-4 to JP-8 fuel. As one of the major proponents for the change and the co-author of the following document citing many of the safety reasons for the change, I would like to shed additional light on this subject. In the report (AFAPL-TR-74-71) titled, "Assessment of JP-8 as a Replacement Fuel for the Air Force Standard Jet Fuel JP-4," June 1975, factors including fuel properties, aircraft vulnerability, aircraft crash fires, fuel system and aircraft performance, fuel handling, maintenance, and environmental impact were addressed related to the fuel change. By the mid 1990s most Air Force bases changed over to JP-8 with resulting reductions in aircraft mishaps and damage caused by fuel combustion as the initial cause or as a secondary effect.
6. The fuel/air vapor in the ullage of the TWA flight 800 center wing tank was flammable at the time of the accident.
7. A fuel/air explosion in the center wing tank of TWA flight 800 would have been capable of generating sufficient internal pressure to break apart the tank.
That is not saying this is what caused the TWA 800 disaster but only the implication that such an effect, if it did happened, would have contributed to the severity of the disaster. The higher the volatility of the fuel, the greater the blast strength of a fuel-air vapor explosion -- thanks to the 'hydraulic jumps' phenomenon that created that mixture in the first place. The 'hydraulic jumps' phenomenon was an obvious life-threatening concern for the Space Shuttle because of the nature of the fuel and the radical change in environment: gravity to weightlessness and back to gravity again.
Tank geometry will contribute to this 'hydraulic jumps' phenomenon. The greater the departure from ideal shapes such as squared or rectangular cubes, the greater the phenomenon during violent maneuvers, which increases the odds of creating said dangerous fuel-air vapor mixture. The proper phrase here 'ignition vulnerability' and fuel volatility, as shown of the difference from JP-4 to JP-8, increases said vulnerability.
Aircraft Fuel Tank Inerting .: Nitrogen generators .: Nitrogen Generation .: Oxygen generator .: Oxygen Generation .: Gas Generation Systems .: Onsite Gas Systems Inc.
For years, nitrogen has been used to inert the headspace in combat aircraft fuel tanks. In 1996, the crash of TWA flight 800 brought the issue of explosive fuel vapors to the forefront for commercial aviation as well. Early on in the development of the OBIGGS (On-Board Inert Gas Generation System), On Site Gas engineers worked with a number of companies, and the FAA, to determine the best method for inerting the empty tanks of commercial aircraft.
A fuel tank inerting system increases overall weight and complexity for the aircraft, as in 'just another thing we have to fret over and work on'. An increase in fuel tank capacity will require an increase in the inerting agent.
This is not to discourage anyone from believing that the JF-17's internal fuel capacity cannot be increased, only that this is important knowledge for the laymen to think about when discussing these things of interests. Certainly the JF-17's internal fuel can be increased, but if possible, it will done with care with the above life-threatening factors considered. I would rather be downed by an enemy weapon than by my own faulty designed or supposedly 'improved' aircraft.
The best way to improve fuel economy in a small aircraft is to reduce weight, specifically
NON-FUEL RELATED mass. We want to give the same quantity of fuel less mass to motivate throughout all flying conditions and maneuvers. This means reexamining the current design with new technology to see if we can trim some fat here and there with care taken to see how this would affect fuel quantity and its management.
If we increase internal volume thanks to composites and restructuring, do we fill that volume with fuel or with superior avionics? Increased fuel quantity that will motivate a lower aircraft mass sounds attractive, but so would the argument that the same fuel quantity to motivate a lower aircraft mass but with superior avionics. One option will give us greater range only. The other option will give us not as great an increase in range but also an increase in combat lethality. Keep in mind that avionics mass is constant mass while fuel is decreasing mass.
Who wants to be in the chair to make the final decision and take any possible blame if something goes wrong?
