XV-15 litho - NASA's History Office

to build “safety” into the design. In the 1960s, the Army and civil rotorcraft
operators were experiencing loss of life and property due to post-crash fires.
Studies that examined the statistics from these crashes showed that injuries
and fatalities were significantly reduced when rupture and tear resistant fuel
cells were installed. The fuel cells, basically flexible rubberized fabric bladders
that held the fuel, were less likely to burst and release fuel upon impact
with the ground than rigid metal tanks or fuel-containing wing structures that
did not include the bladders. By the early 1970s, the use of fuel cells, in particular
in Army helicopters, had dramatically reduced the incidence of postcrash,
fuel-fed fires.
The original Bell Model 300 design (predecessor to the XV-15) incorporated a
“wet wing,” which used the volume within the wing to hold the fuel. While
crashworthy fuel bladders would significantly increase the cost and weight of the
fuel system and would reduce the available fuel volume by about five percent,
the potential safety benefits were believed to be high enough to accept penalties,
and the fuel cells were made part of the XV-15 design.
Bell then contracted with Uniroyal Inc., of Mishawaka, Indiana, the manufacturers
of fuel cells for Army helicopters and Air Force fixed-wing fighters, to
fabricate the cells for the XV-15 TRRA. With no background in the design of
fuel cells for a research aircraft, a method for the selection of the thickness of
the rubberized fabric (i.e. the number of the rubberized fabric laminates used
in the bladder material) had to be defined. Thinner fabric would be lighter and
easier to install in the wing (through small openings in the aft wing spar) but
it would be more susceptible to impact damage than the thicker-wall material.
To resolve the issue, a standard test was conducted at the Uniroyal facility on
December 3, 1974. Two test bladders, in the shape of cubes measuring three
feet on each side were fabricated, one with a light gage material and one with
a thick wall material. The bladders were filled with water and dropped from a
height of 65 feet onto a concrete surface. The lighter-gage material bladder
ruptured on impact, while the thicker-walled bladder material did not. This
not-so-scientific method, along with the previously qualified seam and fitting
designs and validation of acceptable tear and puncture material characteristics,
provided the basis for the qualification of the thicker-wall fuel cells for use in
the XV-15.
In addition to the fuel bladders intended to provide fuel containment in the event
of damage to the wing structure, the interconnecting fuel lines between adjoining
cells (there are two cells in each wing) were provided with breakaway fittings
which sealed in fuel when the lines were broken on impact.
The fuel system, like all other critical XV-15 TRRA systems, was designed with
adequate redundancies (such as dual fuel pumps with the capability to feed both
engines) so that a single failure would not result in the requirement to terminate
the flight.
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