Tuesday, August 9, 2011

Dryden F-104 Flight-Tested Shuttle TPS Materials

During the formative years of the space shuttle program, NASA Dryden F-15 and F-104 jets were used to flight-test various advanced Thermal Protection System (TPS) materials for the shuttles.

These tests included TPS materials from different locations on shuttle orbiters, and they were tested for everything from rain impact integrity, to air-loads strength and surface bonding.

During one such effort, NASA Dryden engineers conducted flight-testing of the orbiter’s advanced, flexible Felt Reusable Surface Insulation (FRSI) and Advanced Flexible Reusable Surface Insulation (AFRSI) TPS materials. These were the soft, sewn blanket-like materials that covered most of the upper surfaces of the orbiters, while black silicon tiles covered the underside, and reinforced carbon-carbon materials protected the nose and leading edges of the wings.

Up until the space shuttle, only disposable, one-use-only ablative materials were used as TPS materials on spacecraft. Ablative materials are layered and are designed to burn off, carrying heat with them in order to keep the heat away from the spacecraft. The idea of using reusable materials was radical, especially lightweight and flexible materials, to withstand the super-hot friction heating that spacecraft encounter while returning through Earth’s atmosphere.

The objectives of the FRSI and AFRSI tests were to evaluate the performance of the materials at simulated shuttle launch aerodynamic loads, and also to provide a database for future advanced TPS flight tests.

These flights were flown mostly on Dryden’s F-104 test bed aircraft in the 1980s, with the TPS materials attached to a fin-like structure called the Flight Test Fixture (FTF) underneath the F-104.

During this series of tests, the material samples were exposed to 40 percent higher aerodynamic loads than they were designed to withstand. The test articles required tailoring of the airflow over them to accurately simulate shuttle conditions over the FTF.

To accomplish this tailoring, an elliptically shaped nose was designed for the FTF to produce a high-pressure shockwave at the location of the TPS material samples attached on the sides of the FTF.

Data-wise, it was extremely important that the required flight conditions be maintained. This was accomplished by using a flight trajectory guidance system called the Uplink Guidance System (UGS). The UGS used an analog cockpit display to alert the pilot, in real-time, of any deviations from the desired flight conditions. For example, one parameter displayed on the UGS was sideslip, which is the flight condition in which an airplane is no longer flying straight along the path of its longitudinal axis.

During the FRSI and AFRSI flights, the pilots could keep precisely on track by keeping an eye on the UGS indicator.

The FRSI and AFRSI flight-test projects were a success, both in terms of accomplishing their test objectives and that the TPS materials passed these tests with no material failures noted during post-flight inspections.

NASA Dryden’s expertise in such work continues today, as the center uses F-15 aircraft to flight-test the next generation of aerospace sensors and materials.

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