F-35 External fuel tanks?

Hi folks

First post in the F-35 section;

Anybody know or have seen the F-35 with external fuel tanks? I know she can carry them, just curious as to wether they are similar to the F-16 600 gallon tanks, or completelly new design?

They are a new design that is optimized for transonic flight and not due till at least Blk4.

The tank is 426 gallons and the Israelis have been talking of developing a 600+ gallon version.

From probably an SWP pdf post: http://www.f-16.net/f-16_forum_download-id-16605.html

I had heard they were having separation problems.
At any rate, is there any unclassified estimate on how badly they hurt RCS, and how much extra range they give the F-35?
If the F-35 carries around 3000 gallons internally, and the external tanks add another 800 gallons, that would only be about a 25% increase in range -- but that doesn't count the extra drag.

general rule of thumb is ~60% of the fuel in a tank goes to range, the rest goes to drag.

2x426 = 852 Gal = 5500 lbs of fuel which is roughly the difference between the internal fuel carriage of a F-35A and F-35B. I'd guess 200-300nm extra range.

So, on the topic of LO, does anyone have a information on whether the tanks can be made radar transparent?
From a physics standpoint, if the jet fuel is non-conductive, radar waves should pass right through it -- but if it has a significant index of refraction in that frequency band then you could still get a reflection off of it. The same goes for whatever the tank is made out of.
Radar transparency would be nice, just because it wouldn't impact the the rest of the aircraft's LO.

What is 'radar transparent' mean?

handyman wrote:What is 'radar transparent' mean?

Radios waves pass through non-conductive material.

Why not absorb or deflect the radar energy away from its source like the rest of the LO aircraft?

IIRC Lockheed made experiments with transparent wings when looking for a replacement to the U-2. But since there is a small difference in conductivity between Air and Fuel the fuel reflected some energy back to the radar. Apparently enough to make the idea useless. Can't remember where I saw that, can anyone else confirm?

The area ruling of the tank is quite extreme, it puts a lot of fuel quite far forward, must be challenging both structural and for from a cg point.

My 5 cent

Wing flutter resistance is improved with a forward CG of external stores. That is why store CGs are always forward of the wing twist axis (also engines on transport aircraft). Tank CG location can be controlled to some extent with compartments that feed in a specific order. For example the F-16 370 tank has three compartments. The original burn sequence was forward, aft, center to minimize CG movement, but that sequence led to flutter concerns due to aft CG movement. The revised sequence, center, aft, forward, was adopted and solved the flutter problem. But as you say, structural loads (pitch and yaw moment) were increased.

OK, I thought the fuel sequence was mainly because you wanted the tank to have good ballistics at all time so that it would clear the aircraft when dropped.

Link below is probably not because of tank aft cg, but you get the point...
http://www.youtube.com/watch?v=TKlV9VvHaEY


Have never heard that flutter due to stores is a problem on the 39, but then again I were not involved in the wing that much, and/or maybe the wing is much stiffer on the 39, it didn't even break when the control system went bezerk on prototype 39-1.


my 5 cent

Safe separation is certainly a requirement equal to flutter resistance. The F-111 clip (and others) shows why. That airplane landed safely, as they were very lucky where the tank struck the airplane. The engines are separated by about two feet, with a fuel tank between them. The tank struck the airplane precisely on the centerline, puncturing the tank, but doing no damage to the engines.

Low aspect wings (like the delta) are more flutter resistant than high aspect wings, as they are stiffer.

count_to_10 wrote:So, on the topic of LO, does anyone have a information on whether the tanks can be made radar transparent?
From a physics standpoint, if the jet fuel is non-conductive, radar waves should pass right through it -- but if it has a significant index of refraction in that frequency band then you could still get a reflection off of it. The same goes for whatever the tank is made out of.
Radar transparency would be nice, just because it wouldn't impact the the rest of the aircraft's LO.

This post was lost for a while -- thanks to Spaz for getting it headed in the right direction.

So just mentally expanding on a previous posters comment about using non-ferrous materials to build the fuel tanks (which in theory, allows you to use the most aerodynamically efficient form) it brings up the follow-up

Question 1: What is the radar reflectivity of kerosene? It seems to me that it would probably be transparent also but, could be wrong about that.

Question 2: Don't the drop tanks also include internal baffles to reduce fuel sloshing about, multiple internal fuel pickup tubes, fill ports with caps, connecting parts and such? It could prove challenging to get all those pieces radar transparent. How about fuel booster pumps? Are those tucked inside the tanks to push it out, or is the fuel sucked out?
If they do have pumps those would also have to be surrounded by radar absorbent or properly angled reflective materials.

Question 3: If you used 'chines' on the nose/side of the fuel tanks to reflect RF away from the source transceiver, don't you create the RF 'corner pocket' effect that ultimately returns the radar wave anyway by reflecting the energy between the tank -> hanger -> wing?



I assume this must have been measured before because all our current and past fleets of stealth aircraft can and do on occasion dump their fuel for some reason. If dumping fuel was a signature problem then I suppose we wouldn't have heard about it but it may be public information.

Please no "Use Unleaded" jokes - thank