A unique high temperature modal test and model correlation/update
program has been performed on the composite nozzle of the FASTRAC
engine for the NASA X-34 Reusable Launch Vehicle. The program was
required to provide an accurate high temperature model of the
nozzle for incorporation into the engine system structural dynamics
model for loads calculation; this model is significantly different
from the ambient case due to the large decrease in composite
stiffness properties due to heating. The high-temperature modal
test was performed during a hot-fire test of the nozzle.
Previously, a series of high fidelity modal tests and finite
element model correlation of the nozzle in a free-free
configuration had been performed. This model was then attached to a
modal-test verified model of the engine hot-fire test stand and the
ambient system mode shapes were identified. A reduced set of
accelerometers was then attached to the nozzle, the engine fired
full-duration, and the frequency peaks corresponding to the ambient
nozzle modes individually isolated and tracked as they decreased
during the test. To update the finite-element model of the nozzle
to these frequency curves, the percentage differences of the
anisotropic composite moduli due to temperature variation from
ambient, which had been used in the initial modeling and which were
obtained by small sample coupon testing, were multiplied by an
iteratively determined constant factor. These new properties were
used to create high-temperature nozzle models corresponding to 10
second engine operation increments and tied into the engine system
model for loads determination.
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