from flowing through to the exterior of the fuel
The most commonly used types of self-sealing
cell (fig. 4-4.)
fuel cells are the standard construction type and
The mechanical reaction results because
the type that uses a bladder along with the self-
rubber, both natural and synthetic, will "give"
sealing cell. Of the two, the standard construction
under the shock of impact. This will limit damage
cell is used the most. It is a semiflexible cell, made
to a small hole in the fuel cell. The fuel cell
up of numerous plies of material.
materials will allow the projectile to enter or leave
The combination bladder and self-sealing cell
the cell, and then the materials will return to their
is made up of two parts. One part is a bladder-
original position. This mechanical reaction is
type cell, and the other part is identical to the
almost instantaneous.
standard construction cell. It is designed to self-
The chemical reaction takes place as soon as
seal holes or damage in the bottom and the lower
fuel vapors penetrate through the inner liner
portions of the side areas. The bladder part of
material and reach the sealant. The sealant, upon
the cell (nonself-sealing) is usually restricted to the
contact with fuel vapors, will extend or swell to
upper portion. This type of cell is also semi-
several times its normal size. This effectively closes
flexible.
the rupture and prevents the fuel from escaping.
The sealant is made from natural gum rubber.
SELF-SEALING CELL (STANDARD CON-
The retainer material is the next material used
STRUCTION).-- There are four primary layers
in fuel cell construction. The purpose of the
of materials used in the construction of a self-
retainer is to provide strength and support. It also
sealing cell. These layers are the inner liner, nylon
increases the efficiency of the mechanical action
fuel barrier, sealant, and retainer. All self-sealing
by returning the fuel cell to its original shape when
fuel cells now in service contain these four primary
punctured. It is made of cotton or nylon cord
layers of materials. If additional plies are used in
fabric impregnated with Buna N rubber.
the construction of the cell, they will be related
to one of the primary plies.
SELF-SEALING CELL (NONSTANDARD
The inner liner material is the material used
CONSTRUCTION).-- One variation from the
inside the cell. It is constructed of Buna N
standard construction, self-sealing fuel cell
synthetic rubber. Its purpose is to contain the fuel
previously discussed is shown in figure 4-5. It has
and prevent it from coming in contact with the
four primary layers--an inner liner, a nylon fuel
sealant. This will prevent premature swelling or
barrier, two sealant plies, and three retainer plies.
deterioration of the sealant.
The cords in the first retainer ply run
Buna rubber is an artificial substitute for crude
lengthwise of the cell. The cords in the second
or natural rubber. It is produced from butadiene
retainer run at a 45-degree angle to the first. The
and sodium and is made in two types, Buna S and
cords in the third retainer run at a 90-degree
Buns N. The Buna S is the most common type
angle to the second. The outside is coated with
of synthetic rubber. It is unsuitable for use as
inner liner material in fuel cells. It causes the
petroleum fuels used in aircraft to swell and
eventually dissolve. The Buna N is not affected
by petroleum fuels, making it ideal for this
application. However, the Buna N is slightly
porous, making it necessary to use a nylon barrier
to prevent the fuel from contacting the sealant.
The nylon fuel barrier is an unbroken film of
nylon. The purpose of the nylon fuel barrier is
to prevent the fuel from diffusing farther into the
cell. The nylon is brushed, swabbed, or sprayed
in three or four hot coats to the outer surface of
the inner liner during construction.
The sealant material is the next material used
in fuel cell construction. It remains dormant in
the fuel cell until the cell is ruptured or penetrated
by a projectile. It is the function of the sealant
Figure 4-5.-Self-sealing fuel cell (standard construction).
to seal the ruptured area. This will keep the fuel
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