"Fuel everywhere" aircraft design

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yeetmahboi

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Inspired by the Soviet's attempts on using spaced armour as holding cells for fuel ( this topic has some nice references) I thought that the design of traditional aircraft could yield higher density effciency by using the voids within to contain fuel. So, say, a 777 would have fuel within both the fuselage (double hull submarine expertise could help immensely) and the wings... and everywhere.

And apparently some drones have their avionics submerged in fuel (no sources for this, sorry!).

I wonder how would successful implementations of this philosophy mold future aircraft designs, if at all. Higher fuel fraction of course, but how would mant. issues work out. Opening a panel could lead to massive spillage, and alot of stuff prefers to stay dry.
 
Thanks for that! A wet wing is exactly what I'm thinking of, but only applied to the wing. Across the entire structure is what I'm thinking about, but the tube-wing-tail configuration is so established and used already, and many probably dont want to get way too far ahead (CYA is understandable, keep your job!)

I wonder, how about a BWB? That would have alot of surface area which could translate to a heck lot of unused space. Say, above the passenger compartment? Whenever I look at LockMart's HWB, that "holy moly how much spare space is inside that" always pop up, and if we can get morphing wings to work, even more space for fuel!

And looking at the ol Warthog, how much fuel could we add by using the titanium armour as a holding compartment for even more fuel? And since diesel actually performs quite decently as space armour filler... Gaddamit I might have to get back to @Scott Kenny's CAS design topic...
 
What is your intention??? If you would fill up the fuselage of any commercial airliner with fuel, it would be far to heavy for take of! The MIG 25 can indeed be filled with fuel until most of it's free volume is tank, but this makes use of very powerful jets with afterburners....
 
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Yes, having fuel everywhere in the fuselage would be dangerous, inconvenient and heavy.

The range of an aircraft is related to fuel fraction, the fuel weight compared to total weight. It's usually in the 0.25 - 0.5 range, with the B-2 about 0.49 being close to the practical maximum, with fighters at the lower end and bombers and long haul airliners at the top end.

(* I think world record is probably the https://en.wikipedia.org/wiki/Virgin_Atlantic_GlobalFlyer at 0.82)

There are structural limits to how high the fuel fraction can go because fuel is quite heavy, and the structure of the aircraft has to be able to cope with increased load, which means heavier structure which reduces the fuel fraction. When you get to 0.45, you can forget about having structures stressed to allow 9g turns, it would simply be too heavy.
 
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Plenty of aircraft have integral fuselage tanks as well as wing tanks. The main limits are that you need to fit other things in the fuselage as well, and small bays are very inefficient because of the pipes and pumps needed to move the fuel around towards the engine.
 
yeetmahboi said:
and if we can get morphing wings to work, even more space for fuel!
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I want a contract which holds you and all your descendants personally accountable and liable for any and all incidents for perpetuity resulting from incorrect trim changes and/or weight/mass distribution as the wings morph while carrying that fuel weight, then and only then will I consider the concept to have more legitimacy than some kind of Kerbal add-on.

(dang, man, I'm starting to sound like Martin) :oops::rolleyes:
 
@overscan (PaulMM) The MIG 25 could carry 14820 kg of fuel and had a max. take off mass of 36700 kg, which means that astonishing 40% of the take off mass is fuel! The welded steel fuselage shurly helped in creating the tank volume.

If I would have to design an aerial tanker, I would build it as flying wing, this would enable high lift and strctual efficiency for carring a large quantity of fuel.
 
A lot of the fuel would have been burned before reaching the enemy, but of course, this has never been plane for dogfighting.
 
yeetmahboi said:
Across the entire structure is what I'm thinking about, but the tube-wing-tail configuration is so established and used already, and many probably dont want to get way too far ahead (CYA is understandable, keep your job!)
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Boeing tried a different configuration with the Sonic Cruiser, the airlines weren't interested. (Which was probably predictable, getting there slightly faster didn't have a massive value for most travellers).

how about a BWB? That would have alot of surface area which could translate to a heck lot of unused space
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The BWB likely has a lower SA to volume ratio than tube and wing designs. SA and Volume aren't directly related.

FAA had objections to the A321 XLR design because they say the extra long range fuel tank is too close to the skin of the aircraft.

airinsight.com

FAA’s Special Conditions for A321XLR fuel tank |

The Airbus A321XLR uses a unique fuel tank to obtain its long-range performance, with an integral rear center tank, not covered by existing regulations.
airinsight.com airinsight.com
 
DWG said:
The BWB likely has a lower SA to volume ratio than tube and wing designs. SA and Volume aren't directly related
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BWB or Flying wings are structurally more efficient by largly avoiding the wing root bending moment and involved heavy structure also they enable a mass distribution of the fuel which equals the lift distribution (keep in mind, no pressurisation will be needed exapt for the cockpit). Also, flying wings are usually built as a flying "thick wing" to enable sufficient internal room and height, so they are well suited for producing a lot of lift.
 
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Nicknick said:
What is your intention??? If you would fill up the fuselage of any commercial airliner with fuel, it would be far to heavy for take of! The MIG 25 can indeed be filled with fuel until most of it's free volume is tank, but this makes use of very powerful jets with afterburners....
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Nothing too serious, just a thought experiment. I'm trying to get my "blended body, edged aligned A-10" to work and intergral fuselage and wing tanks would work. Commercial aviation hits different..
 
southwestforests said:
I want a contract which holds you and all your descendants personally accountable and liable for any and all incidents for perpetuity resulting from incorrect trim changes and/or weight/mass distribution as the wings morph while carrying that fuel weight, then and only then will I consider the concept to have more legitimacy than some kind of Kerbal add-on.

(dang, man, I'm starting to sound like Martin) :oops::rolleyes:
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If you do that three times in a row, I will haunt your dreams.
 
overscan (PaulMM) said:
Yes, having fuel everywhere in the fuselage would be dangerous, inconvenient and heavy.

The range of an aircraft is related to fuel fraction, the fuel weight compared to total weight. It's usually in the 0.25 - 0.5 range, with the B-2 about 0.49 being close to the practical maximum, with fighters at the lower end and bombers and long haul airliners at the top end.

(* I think world record is probably the https://en.wikipedia.org/wiki/Virgin_Atlantic_GlobalFlyer at 0.82)

There are structural limits to how high the fuel fraction can go because fuel is quite heavy, and the structure of the aircraft has to be able to cope with increased load, which means heavier structure which reduces the fuel fraction. When you get to 0.45, you can forget about having structures stressed to allow 9g turns, it would simply be too heavy.
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Dah! I forgot MTOW. Though can structure OML get more unconventional to reduce stress? Or is the tube body the most efficient design today?
 
yeetmahboi said:
Inspired by the Soviet's attempts on using spaced armour as holding cells for fuel ( this topic has some nice references) I thought that the design of traditional aircraft could yield higher density effciency by using the voids within to contain fuel. So, say, a 777 would have fuel within both the fuselage (double hull submarine expertise could help immensely) and the wings... and everywhere.
Click to expand...
I'm not sure it's worth dealing with for the extra fuel. Gotta add extra piping and pumps to move fuel from the fuselage tanks to the wings for the engines. Then, if those tanks are "empty", there's still some quantity of unusable fuel in there, more dead weight.


yeetmahboi said:
And apparently some drones have their avionics submerged in fuel (no sources for this, sorry!).
Click to expand...
I believe that's for cooling. The best way to deal with airframe or avionics heat is to dump it into the fuel and then out the engines.
 
DWG said:
Boeing tried a different configuration with the Sonic Cruiser, the airlines weren't interested. (Which was probably predictable, getting there slightly faster didn't have a massive value for most travellers).
Click to expand...

What really killed the Sonic Cruiser proposal was that Boeing gave airlines an option; the SC or the slower, conventional, slightly more efficient Yellowstone. Naturally the airlines flocked to the latter because 1% fuel saving is worth more to them on the short-term stock cycle than advancing design. Boeing undercut their own innovation.

If the market choice had been Sonic Cruiser vs A330/340 there wouldn't have been the production capacity for the airlines to exhibit their usual conservatism and the SC could have flourished.
 
Kiltonge said:
What really killed the Sonic Cruiser proposal was that Boeing gave airlines an option; the SC or the slower, conventional, slightly more efficient Yellowstone. Naturally the airlines flocked to the latter because 1% fuel saving is worth more to them on the short-term stock cycle than advancing design. Boeing undercut their own innovation.

If the market choice had been Sonic Cruiser vs A330/340 there wouldn't have been the production capacity for the airlines to exhibit their usual conservatism and the SC could have flourished.
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Don't forget that the Convair 880s and 990s tried higher speeds in the 60s, and only saving 15 minutes travel time was not enough to convince people to buy and fly on them.

It takes something more akin to a Mach 2 speed increase to almost make it worth building and using. And even then, Concorde was a transport for the very rich, not for a regular working stiff.
 
Kiltonge said:
What really killed the Sonic Cruiser proposal was that Boeing gave airlines an option; the SC or the slower, conventional, slightly more efficient Yellowstone. Naturally the airlines flocked to the latter because 1% fuel saving is worth more to them on the short-term stock cycle than advancing design.
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Worth far more to them on any length of time you care to specify, for that matter.
 
martinbayer said:
If you do that three times in a row, I will haunt your dreams.
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One rumour has it that Concorde used fuel tanks in the nose and tail to trim the airplane for supersonic flight. Trim and balance for sub-sonic are vastly different than supe-sonic. Adjusting fuel weight for balance shaved off a few knots fo trim drag created by slighlty deflected control surfaces.
But again, you need rigid control of fuel flow and two or three back-ups that help you re-balance for landing configuration.
 
Another rumour has it that one of the Boeing twin-jets has a fuel tank in the vertical stabilizer.
 
Consider the structural weight of various shapes of fuel tanks. Orbs are the lightest followed by cylinders. Thin fuel tanks are the least structurally-efficient shapes. IOW double-walled fuselages would be the heaviest way to build fuel tanks.

If you can use some other structural component as a fuel tank wall, you might be able to save a few pounds. That is why fuel is often carried in center wing boxes. Fuel is rarely carried in leading edges because of the risk of spills and fires in the event of a collision. Trailing edges are too thin to carry significant amounts of fuel, besides trailing edges are usually festooned with control surfaces. That is the same reason that fuel is not carried in horizontal stabilizers. Many airliners tilt their entire horizontal stabilizers for pitch trim. Moving surfaces present far too many opportunities for leaks. OTOH one or two Boeings have auxilliary fuel tanks in their fixed vertical stabilizers. Since vertical stabs are much smaller volume - than wings - they are mainly used to care "trim" fuel.

As for surrounding avionics with fuel .... I suspect that is more for cooling. Avionics generate plenty of excess heat that must be off-loaded. For comparison, consider the triple-sonic SR-71 which's airframe suffers from air friction heating at faster air speeds. They circulate fuel through skins before routing it into the engines. Adding a bit of heat to incoming fuel helps a bit with propulsion.

Finally, consider the structural advantages of carrying fuel on/in wings. When they developed the B-47, Boeing learned that they could build very long slender wings that were great for high-altitude, high sub-sonic cruise by distributing engine and fuel weight along the entire span. Spreading out engine and fuel weight reduces point-loading on wing roots. Installing fuel tanks inside wings makes for zero drag compared with external fuel tanks (see dH Comet airliner). Many airliners have wet-wings where structural components (spars and ribs) form the walls of fuel tanks. Military airplanes are more likely to add rubber fuel bladders, but that is more about damage-tolerance. Rubber fuel bladders are easier to build self-sealing.
Boeing also learned that if they hung engines in front of the leading edge, they could tailor wing flutter.
 
riggerrob said:
One rumour has it that Concorde used fuel tanks in the nose and tail to trim the airplane for supersonic flight. Trim and balance for sub-sonic are vastly different than supe-sonic. Adjusting fuel weight for balance shaved off a few knots fo trim drag created by slighlty deflected control surfaces.
But again, you need rigid control of fuel flow and two or three back-ups that help you re-balance for landing configuration.
Click to expand...
Details can be found here: https://www.heritageconcorde.com/fuelgeneral
 
You only put fuel everywhere in the airframe when the fuel is your payload. (Or when the flight crew is the payload and the fuel is to get them there).

Sometimes you put a couple of fuel tanks into a design as trim tanks, to adjust your CG while flying. But that's usually things like the vertical fin tank and a fuselage tank well forward.
 
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