Space simulation needs multiple chambers

There is perhaps no environment more demanding than space. Not only is it nearly a complete vacuum, but the temperature swings from very hot to very cold, certainly hotter and colder than any place on earth. When auto engineers need to thoroughly test vehicles in extreme conditions, they can take their designs into the desert or to the Arctic. Aerospace engineers, responsible for satellites and space vehicles, don't have the same luxury.

To test their designs, aerospace engineers must rely on test chambers that simulate the conditions of space. These chambers must simulate several environments, including: The thermal/ vacuum environment, the radiation environment, and contamination effects.

While some companies, such as Boeing, operate their own test facilities, most companies rely on test labs. There are commercially run test labs, such as those operated by Wyle Labs and National Technical Systems, and there are labs run by the government, such as the space simulation test center that is part of the Arnold Engineering Development Center (AEDC) at Arnold Air Force Base near Chattanooga, TN.

As you might expect, these facilities need a variety of test chambers and associated equipment to simulate the space environment and to run tests. The smallest chamber at the AEDC, for example, measures 1 ft in length and 1 ft in diameter; the largest, called the "Mark I," measures 82 ft high and 42 ft in diameter (Figure 1). The AEDC uses the small chamber to test small components of satellite systems, and it uses the Mark 1 to test system-level hardware.

Figure 1 Measuring 42 ft in diameter and 83 ft high, the Mark I is one of the largest space simulation chambers in the US. Engineers at AEDC use the Mark I to test entire satellite systems and large space-vehicle components.

Because the Mark I test chamber is so tall, the AEDC can simulate zero gravity for up to 2 s. The chamber has mechanisms to catch the unit under test at the bottom of the chamber and prevent any damage. Typical space-dynamics tests require zero gravity for only a few tenths of a second and 1x10^–4 torr vacuum levels (simulating altitudes of 150,000 to 350,000 ft). Some of the tests that the AEDC has run in the Mark I include full-scale separations of the Titan 34D, Titan IV, and Delta III payload fairings.

For testing smaller systems and system components, the AEDC has several smaller chambers. The Ultra High Vacuum (UHV) chamber uses Leybold-Hereaus RPK-10000 cryopumps to produce vacuums as low as 1x10^–8 Torr. This chamber measures 7 ft in diameter and 8 ft long, and the AEDC has used it to test the Space Station's Kevlar 29 Debris Shield.

The vacuum found in space varies depending upon the orbit you are in and somewhat upon the spacecraft itself. In geosynchronous orbit, the vacuum is on the order of 10^–11 Torr, while in low earth orbit is significantly less than that. Typically, vacuum levels on the order of 10^–6 Torr do a good job of simulating the vacuum conditions of space, and for those applications that require higher vacuum, engineers can use the UHV chamber.

The UHV chamber also provides a low radiometric background (<10¹⁰ photons/s) for use in infrared source calibration and characterization. The low background provides a test bed for infrared detectors and cold electronics evaluation.

For slightly bigger systems, the AEDC has the 10V and 12V chambers. The 12V chamber measures 12 ft in diameter and 35 ft high. Like the Mark I, it has a liner that uses liquid nitrogen to cool it to 77 K. In addition, the chamber has a GHe-cooled inner liner, or shroud, which can be cooled and maintained at 10 K and evacuated to 1x10^–8 Torr.

The 12V chamber is equipped with an off-axis solar simulator system consisting of an array of seven xenon arc lamps, an integrator lens system, and a mirror to produce a well-collimated, uniform beam in a test volume 8 ft in diameter and 8 ft high. The xenon arc lamps produce an energy spectrum similar to that of the sun. The array has a multi-element integrating lens that it uses to attain a beam uniformity factor of greater than 95%. Technicians can adjust the beam irradiance from 0 to 1.5 solar constants by varying the number of lamps and the lamp voltage.

NASA engineers have used the 12V chamber to perform thermal-vacuum tests for several Space Station components. One component tested in the chamber was the hatch covering a passageway between Space Station elements. These hatches allow the astronauts to seal off any section from the rest of the Space Station. The engineers ran the hatches through several thermal cycles to determine the durability of a latching mechanism.

NASA also used the 12V chamber to test the Space Station cupola, which serves as an observation area for the astronauts. The tests included several thermal cycles using heaters to control the cupola's internal temperatures.

The most recent Space Station component to undergo testing in the 12V chamber was the Active and Passive Common Berthing Mechanism. The test for this component included long-term thermal balancing using the solar array for heating and using liquid nitrogen for cooling. During the test, the mechanism was rotated to characterize the thermal load requirements while simulating orbital conditions.

AEDC engineers also use the 12V chamber to measure the infrared (IR) signature of space vehicles or vehicle components. Using the infrared signature, engineers can remotely measure the temperature on the surface of the vehicle or component.

The chamber walls and floor are cooled with liquid nitrogen to maintain a simulated background radiation level of 77 K. A two-degrees-of-freedom vehicle-handling system controls the pitch and spin of the UUT to simulate orbital vehicle dynamic motion with respect to the sun, and it enables the tester to attain proper solar radiant heat-transfer profiles. While the UUT rotates, a circular variable filter spectrometer (CVFS) measures the UUT's infrared signature from 5 to 22 µm.

To collect data while running these tests, the AEDC uses a variety of data-acquisition systems. Depending upon the chamber, there are anywhere from 100 to 500 channels of data acquisition available, with data rates ranging from DC to GHz.

Organizations can use their own data-acquisition equipment if they prefer, or some combination of their own gear and AEDC equipment. In a recent Electric Propulsion Test, for example, an AEDC customer maintained three of its own systems, while using two systems owned by AEDC. And in a recent test run in the Mark I, the customer used AEDC data-acquisition equipment to gather chamber data, such as pressure and temperature, while collecting system data with its own gear.