# Composite fuel tank question

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
I checked one online supplier today who wanted about $400 for one 4' x 8' x 1/8" extruded vinylester/fiberglass sheet cut into four 4' x 2' sections plus another$75 for shipping, so I contacted another for a quote and that one wanted $450 +$100. That's just for the one sheet, I would still need resin and FG cloth for the curved decking/tank top and FG tape to put it all together. :-( So it sounds I'd end up making the sheet myself on a plate of glass.

If going down that road, I wonder if I'd be better off making a foam plug (less the 3/8" spacing above the deck per the plans) on which to form the bottom and ends of the tank including the flanges for gluing to the tank top/decking. I could see tacking the "funnel" on the main plug to form the sump at the same time. Then I'd add the internal baffles and glue it all together, at least, that's the theory.
I built five of the six sides on a male mold of white foam, coated in dry micro to keep the vinylester out of of the foam, then release taped it. Did an open wet layup against the release tape to make a well sealed tub. Vacuum bagged on the foam cores, the aluminum inserts for pipe threaded penetrations, and the outer layer of glass and resin. Baffle stock was vacuum bagged on a table. Lid will be made separately and the flange transfer technique to give it sturdy bonding surfaces. If I were building in with the firewall and forward deck already in place thus needing to mold four surfaces, I would do it that way too.

Billski

#### lr27

##### Well-Known Member
I've read that gas stations sometimes use polyester, vinylester, or epoxy glass tanks. Presumably, any materials they used would stand up to almost anything likely to be found in gasoline or diesel fuel. So maybe there are special grades of each that would hold up?

#### Vigilant1

##### Well-Known Member
Another consideration might be future fuels and additives. The eventual replacement for tetraethyl lead, etc. Staying with a material that is common in commercial tank construction is probably the safest way to avoid this problem.

An approach to the tank issue that I've considered is use of a commercial rotomolded PEX tank (already very tough) with a composite laid-up shell (inner layer, foam or corrugated/web core, outer layer). The shell protects the inner (more flexible) tank against puncture and deformation and provides a handy spot for hold-downs/restraints. The PEX tank is definitely fuelproof, very resistant to fracture, and all the standard fittings are already there. The PEX tank would serve as the male mold for the layups.

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#### lr27

##### Well-Known Member
Sounds like a good idea. Probably safest to use static dissipative PEX, which I assume is used in gas tanks. I wonder how hard it would be to fabricate something like that in a shape to fit in a particular spot in a particular aircraft.

#### Vigilant1

##### Well-Known Member
I wonder how hard it would be to fabricate something like that in a shape to fit in a particular spot in a particular aircraft.
Most of the stock racing fuel cells are blocky rectangular things, though some are slim.
It would cost a lot to have a custom mold made.

#### dog

##### Well-Known Member
the major cost in rotomolding is the mold,which for me and others here is just the doing.
the most obvious question is how the outer shell bonds or doesnt to the rotomolded inner layer.
Or is a high temp cured composite tank suitable for use as its own mold?That would be convienient.
Still have the details of how to attach filler ,and fittings that were compatible with the process.

#### Vigilant1

##### Well-Known Member
.
the most obvious question is how the outer shell bonds or doesnt to the rotomolded inner layer.
.
If going with a composite case (foam core or corrugated) around a PE rotomolded tank, I don't think there would be any advantage to bonding them together. The outer case is there to help the inner tank keep its shape (to avoid bursting) and to reduce punctures from other items. Keeping the inner tank de-bonded would reduce any crack propagation and would allow the inner tank to flex if the outer tank does get penetrated or is dented/locally crushed, etc.

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#### lr27

##### Well-Known Member
Does anyone know if there are static dissipative additives for epoxy, vinylester, tank sealer, etc?

BJC