Orion testing heating up

  • Published
  • By Philip Lorenz III
  • AEDC/PA
Arnold Air Force Base, Tenn. --  Workers at Arnold Engineering Development Center (AEDC) recently concluded aerothermal testing in support of NASA's Orion Crew Exploration Vehicle missions to the International Space Station by 2014 and the return to the moon by 2020.

NASA engineers specializing in arc heater testing teamed with their counterparts here to conduct facility validation runs at the center's High Enthalpy Aerothermal Test H2 test facility.

The June tests helped pave the way for successful screening and performance evaluation testing of Orion's heat shield candidate materials, according to Dr. Don Ellerby, Orion TPS Advanced Development Project Block 2 (Lunar Direct Return) Heat Shield Materials lead. 

As a follow-on to the successful testing in June, additional material evaluations are planned at AEDC in the future, including a test entry in December, said AEDC's Mark Smith, the test project manager.

"Orion's environment during reentry from a lunar mission is going to be pretty severe," Ellerby said. "The spacecraft is similar to Apollo in shape, but Orion is bigger and its thermal protection system or heat shield will be subjected to heating and shear beyond what was experience by the Apollo command module. These environments are outside the test envelope of what our Ames arc heater facility can operate and resulted in our collaboration with AEDC where it was anticipated the H2 facility could provide some coverage, but until we completed the validation test series we didn't know for sure.

"We operated the heater at AEDC in a different mode than has been done in the past," he explained. "These tests served to validate the test environment and calibrate the facility for the heat shield material samples - necessary for us to conduct the testing of candidate materials at AEDC this June."

The earlier test runs successfully demonstrated a suitable test environment for future high-shear materials testing of TPS candidates for Orion. The team was also able to obtain flow-field calibration data for the conditions necessary for the upcoming pre-production tests.

"That earlier work was a watershed test," said Smith. "The test was initially discussed as a facility validation project and it developed into something more than that. AEDC modified the H2 arc heater configuration to provide a new test condition tailored specifically for testing these TPS candidate materials. This allowed us to subject the materials to the higher enthalpy conditions that are more representative of what would be encountered during Orion's return and re-entry from a lunar mission."

A spacecraft's heat shield materials are designed to absorb and dissipate the severe heating and shear forces caused by hypersonic flight or re-entry by slowly ablating or vaporizing in a controlled manner, allowing the spacecraft to survive the journey.

Center engineers reproduce reentry conditions in a two-stage process. The arc heaters use a high voltage, electric arc discharge to heat air to temperatures up to 13,000 degrees Rankine (thermodynamic temperature scale). High-pressure test flows are achieved by confining the electrical arc discharge in a water-cooled channel capable of withstanding high chamber pressures. The heated air then flows through a nozzle and over the arc-heated test sample in an evacuated chamber.

Another NASA program, the Mars Science Laboratory Planetary Probe Program, has also executed initial testing in H2 and has plans for further testing at AEDC.

AEDC's previous support of the Orion CEV has included aerothermal testing at their Hypervelocity Wind Tunnel 9 complex in Silver Spring, Md. Engineers there made use of conventional and advanced measurement techniques in the testing process.

The primary objective of that work was to obtain heating data over the model's surface, covering the full operational range of the facility at Mach 8 and 10 freestream conditions.

Unlike the development of the Apollo capsule where the database was populated entirely using experimental data, the Orion database was developed using advanced computational fluid dynamic (CFD) modeling techniques. The experimental data was used to validate the CFD models for NASA's Orion database development.

Tunnel 9 also supported the CEV testing by pushing the use of Temperature Sensitive Paint (TSP) to its limits during the project's final phase.

TSP, which is similar to Pressure Sensitive Paint (PSP), is a system that includes a special paint, an ultra-violet illumination source and a sensitive charge coupled device (CCD) camera to obtain surface temperature data. The paint is applied to the model in two layers - a white undercoat and the TSP layer. The white undercoat provides a uniform reflective surface for the TSP. The illumination source excites the TSP layer, which glows a bright red color with its intensity inversely proportional to the surface temperature on the model.

The space agency also contracted with AEDC to provide PSP support for CEV testing test at NASA Ames' supersonic and transonic wind tunnels in March 2006.

"We sent our portable PSP system, which was developed for 4T and Tunnel A, to NASA Ames and installed the system in their wind tunnels and provided NASA with the PSP results," said Marvin Sellers, senior engineer at PWT, and the center's primary PSP development specialist. "The system was connected to the NASA facility so that automated data acquisition could be performed under their control. The PSP data provided a source for CFD validation that couldn't be done with the limited conventional pressure instrumentation available."