by George Raiche

June 23, 2000

The Arc Jet Facility at NASA Ames Research Center is used for studying thermal protection materials. An example of a thermal protection material is the ceramic tile on the Shuttle Orbiter. It uses heat and supersonic speeds to simulate the energy experienced by a vehicle reentering the Earth's atmosphere. Various materials are tested to see if they provide effective thermal protection.

The arc jet sustains a standing bolt of lightning that heats the air up to 15,000 degrees Celsius. You can think of it as similar to a welding torch. There is a huge arc of electrical energy that jumps across a gap between two conductors. Air is blown through the lightning bolt and it gets heated up. The air is actually stored at high pressure to achieve supersonic speeds. The air is released once the arc is started. The air, heated by the arc, expands through a nozzle which connects to a vacuum chamber. This chamber is pumped by a building-sized vacuum pump, one of the largest in the world. As the hot air moves through the nozzle, it expands, cools, and accelerates, just like water through a garden hose. Now the air is cooler (about 1000 degrees) but moving at supersonic speed.

When this supersonic air strikes a stationary object (the test model), a shock wave forms over the model. The shock wave forms because the collision speed is greater than the speed that the air molecules can flow (the speed of sound), so the air molecules pile up and recompress. This same compression effect is what happens during reentry, except the spacecraft is moving while the atmosphere is stationary. This compression causes the temperature to rise again, just a few centimeters away from the model surface. The temperature in the shock wave is about 5000 degrees, about the same as for reentry. We measure the temperatures of the shock wave, the model surface, and the interior of the model, and compare them with computer calculations. There are several different sets of computer sensors that collect data during a test. One type of measurement taken records the caloric heat or the energy of the gas and the other type of data collection uses laser diagnostics. We also inspect the size and strength of the model before and after the test, to study how well it withstands this simulated reentry.

Once the testing is complete, the nozzle cools and accelerates the air at the same time. Instead of using fans to move the air, it flows when you release a valve. All of the exhaust gases are collected in big tanks and are chemically scrubbed or cleaned before it is released into the atmosphere. In addition, the arc jet facility has a big cooling plant that helps to keep all of the various mechanical components from overheating.

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