The increasing demand for a low-cost, reliable way to launch commercial
payloads to low-Earth orbit has led to the need for inexpensive,
expendable propulsion systems for new launch vehicles. This, in
turn, has renewed interest in less complex, uncooled rocket engines
that have combustion chambers and exhaust nozzles fabricated from
ablative materials. A number of aerospace propulsion system manufacturers
have utilized NASA Lewis Research Center's test facilities with
a high degree of success to evaluate candidate materials for application
to new propulsion devices.
Because of their knowledge of Lewis' extensive past experience
in experimentally evaluating ablative materials (such as nozzle
sections for rocket engines), TRW, Thiokol Corporation, and other
companies submitted requests to evaluate new material compositions
for Lewis. NASA Space Act Agreements were established to carry
out these evaluations, utilizing samples supplied by the interested
organizations, and the versatile rocket engine test facilities
(Test Cell 22) at Lewis. A number of different composite materials
were fabricated into nozzle inserts, with and without typical
thrust chamber throats, and into as-throated disk samples. These
samples were exposed to combustion gas temperatures of gaseous
hydrogen and gaseous oxygen propellants from approximately 4400
°F to approximately 6000 °F at thrust chamber pressures from 500
to 900 psia.
Silica composite material samples provided by Thiokol were initially
analyzed according to space shuttle experience, to predict the
erosion of the nozzle liners to be tested. The primary goal was
to minimize the ablative liner thickness and corresponding weight.
The results of liner response from the rocket engine test stand
show a wide range of erosion rates over test periods up to 341
sec, from negligible erosion for some ceramic composite materials
to approximately 3.4x10-3 in./sec for some phenolic
composites. These data provided key performance information for
the application of low-cost, low-density materials. The test data
from Cell 22 were also crucial to obtaining better "melt
layer" models for silica composite materials.
The samples provided by TRW and others included phenolic and ceramic
matrix composites, some with protective throat coatings. As expected,
the evaluation resulted in a wide range of durabilites for the
composites under these severe operating conditions: the results
verified some analytical predictions, and experimentally showed
the limitations of other compositions.
These experimental evaluations were conducted in an expeditious
and very productive manner, with excellent cooperation between
personnel from NASA Lewis and the other companies involved. Thiokol
Corporation is currently under contract to the Rocketdyne Division
of Rockwell, which is the rocket engine contractor to McDonnell
Douglas. The rocket engine test stand, Cell 22, will provide continuing
investigation of test coupons, liners, and nozzles with a very
cost effective, fast-paced schedule. The other companies involved
have expressed interest in providing additional samples for evaluation
in the near future.
Previous articleLast updated April 30, 1997
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