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ARA's Ablatives Laboratory Tests Ablative Heat-Shield Aeroshell Systems for NASA

By Bill Congdon, ARA Ablatives Laboratory Manager and Director of NASA Ablator Testing at Solar Tower

The ARA Ablatives Laboratory in Centennial, CO performs heat-shield system development and qualification testing for the National Aeronautics and Space Administration (NASA). The major portion of this effort is funded by NASA’s In-Space Propulsion Technology Project.

Testing is done in different phases using different national facilities. Developing and characterizing an ablator material and verifying how the material will perform against severe entry heating environments is typically done in an arc-jet facility such as the Interaction Heating Facility at the NASA Ames Research Center.

Once the ablator has been fully characterized and verified by extensive arc-jet testing, then a second phase of aeroshell system-level testing is required that includes both the ablator and its supporting structure. System-level testing involves larger test articles that cannot be tested in arc-jet facilities because of their bulk and because such facilities do not have sufficient power to deliver flight-magnitude heating over their large surface areas.

For system-level testing of large heat-shield panels and shaped aeroshells, the Ablatives Laboratory has developed test hardware and methods using Sandia National Laboratories’ National Solar Thermal Test Facility in Albuquerque, NM. This “Solar Tower” facility has 212 heliostats that collect solar radiation and focus this radiation onto ablator test articles mounted atop the facility’s 200-foot tall experiment tower.

Each heliostat contains 25 mirrors that are 4 foot-square, and so a single heliostat has 400 square feet of mirrors. The full array of 212 heliostats therefore has a total area of mirrored surfaces amounting to 84,800 square feet, or 2.12 acres. Assuming 80 percent efficiency, Solar Tower can deliver up to 7,900 kW of power to an ablative heat-shield test article. It is this high power level that makes Solar Tower unique and very useful for aeroshell system testing. That is, enough power is available to subject large heat-shield aeroshells to sufficiently high heating for system-level validation.

On Oct. 3, 2007, the Ablatives Laboratory completed a Silicone-20 ablative aeroshell test at the Solar Tower. The underlying structure was a lightweight, sandwich composite supplied by ATK Space Systems. The ARA ablator was applied to this structure via a honeycomb-packing approach. Large-cell, composite honeycomb was bonded to the composite structure and then the cells were packed with pre-mixed Silicone-20 ablator compound. The ablator was cured at elevated temperature and pressure and then final-machined to its smooth aerodynamic shape.

The primary purpose of testing such aeroshells at Solar Tower is verification of their structural integrity during and after an intense thermal exposure. The bondline between the honeycombed ablator and the structure has a defined maximum allowable limit for flight. For the aeroshell shown at right, that limit was 250°C (481°F).  (For other ARA aeroshell systems this limit could be 325°C or 400°C.) The test goal is to produce flight-like thermal gradients through the aeroshell system and raise the bondline to a temperature equal to or greater than the defined limit (i.e., apply high heating over relatively short durations of several minutes).

The test shown in the photos at right was highly successful in that: 1) the goal of reaching or exceeding the 250°C bondline temperature allowable was achieved; and 2) the Silicone-20 aeroshell showed excellent performance and robustness against this type of a thermal environment with the internal thermal gradients and thermal stresses that it produced.

From 2006 to 2007, the ARA Ablatives Laboratory has tested more than 20 of the 2 foot-square ablator-system panels at Solar Tower with highly successful results. In addition, a second shaped aeroshell made from Phenolic-20 ablator was also successfully tested on Oct. 23, 2007. Today, the Laboratory has several aeroshell-system development programs in progress for NASA and these programs will continue at least through 2009-2010.

For more information, contact Bill Congdon at 303-699-7737 or wcongdon@ara.com.

 

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heat
An ARA 4-square-foot ablator panel in a mid-day, system-level test at Sandia Solar Tower Facility. National Geographic photo by Mark Thiessen, October 2007 issue.

Ablative
Arc-jet testing of an ARA Silicone-20 ablator sample in the Interaction Heating Facility convective heating environment. The test condition was 153 W/cm2 for 125 seconds. This is a 4.0-inch diameter sample instrumented with in-depth thermocouples to monitor the ablator's performance.

Silicone Aeroshell
Pretest appearance of Silicone-20 1.0-meter (39.4-inch) diameter aeroshell. Ablator development and all ablator production and milling are done in-house by the Ablatives Laboratory.

Aeroshell Test
Rear and side views of ARA Silicone-20 aeroshell in test at Solar Tower at a heating rate of 153 W/cm2 for 210 seconds. (This is a heat flux of roughly 1,500 times the maximum ground-level strength of Albuquerque sunlight.)