The demands on space simulation chambers from the extremes of temperature and vacuum were discussed in Part One along with the development of space simulation chambers (aka space simulators) for testing of components used on vehicles for space travel such as sensors, landers and ion thrusters (Fig. 1) under the same conditions they will encounter during deployment in space. We continue this discussion by exploring the vacuum systems used to make these chambers function properly.
Space Simulations Chambers
Commercially available space test chambers can typically attain a vacuum of 10-8 mbar (7.5 x 10-9 Torr) or higher. They employ a roughing pump such as a rotary vane style, followed by one or more turbomolecular, cryo pumps. For applications that can tolerate a slight amount of oil contamination in the chamber, an oil sealed rotary vane pump with gas ballast is used for initial pumpdown. Oil sealed pumps include a dive valve to immediately reduce pressure in the foreline when power is removed, an isolation valve for leak checking, and a heated molecular sieve trap to limit the potential for oil back streaming in the event of power failure. When oil free operation is required, the preferred roughing pump is a rotary piston pump, scroll pumps and for much larger systems, Roots boosters and screw pumps.
For systems sensitive to even the lowest level of back streaming, scroll pumps are the most common selection. They are completely oil free and complement the turbomolecular or cryogenic pumps. In addition, they require no filters or foreline traps. The different types of high vacuum pumps offer different advantages and disadvantages in space simulation chambers (Table 1).
One example of a space test chamber utilizing multiple pump technologies is the Space Simulation Facility (S2F) at the AITC, the Australian National University’s Advanced Instrumentation and Technology Centre (Fig. 4). It was installed in 2013 to perform plasma thruster testing and thermal vacuum testing and qualification of satellites and instrumentation. The chamber features internal dimensions of 3.0 m diameter and 4 m long (9.8 ft. diameter x 13.1 ft. long).
The S2F chamber offers pressures down to 10-5 mbar and has a high pumping speed to maintain a high vacuum despite the injection of argon propellant into the chamber during thruster testing. It is able to accommodate the introduction of 70 cubic centimeters of argon per minute while maintaining a 10-5 mbar (7.5 x 10-6 Torr) vacuum. In order to achieve this level of vacuum, the S2F utilizes a dry roughing pump and blower, a turbomolecular pump, and a set of cryogenic pumps. The turbomolecular pump serves to keep the chamber under vacuum during cryogenic pump regeneration and helps to pump the chamber down rapidly after the roughing pump has reached its capacity and before the cryogenic pump takes over. This multiple pump design can draw the chamber down to 10-6 mbar (7.5 x 10-7 Torr) in 8 hours (Fig. 5).
With respect to thermal performance, the S2F simulation chamber has an operating range of -170°C to 150°C (-274°F to 302°F). It achieves heating and cooling through the use of a radiative shroud and a conductive thermal platen within the chamber, each powered by a 0.2 m3/s (400 ft.3/min) gaseous nitrogen temperature control system. This design was selected as it provides good control at an economical price. The thermal system must maintain temperature (Fig. 6) while absorbing a 2,500 watt radiant load and 500 watt conductive load generated by the plasma thrust system.
When a space chamber is under vacuum, there is no atmospheric pressure inside to press outward and balance the force of the atmosphere pushing inward. This results in an inward pressure on the exterior of the chamber of up to 10,200 kg/m2 (21,000 lb./ft2). Although the most common configuration is a cylindrical design due to its inherent resistance to collapse under this inward pressure, heavily reinforced rectangular designs are also available (Fig. 7).
When a larger system is required, it is custom designed using the same principles as smaller units, but to a larger scale (c.f., Part One, which illustrates the space power facility at Plum Brook Station in Sandusky).
The temperature in space is close to absolute zero due to the fact that any material in space not exposed to direct sunlight will radiate its heat until it cools to that temperature. There are very few molecules in space, and their temperature is about -270°C (-459ºF) or approximately 3°C (5ºF) above absolute zero. In order to simulate these extremely cold conditions, space simulation chambers use thermal shrouds, also referred to as cold walls or cryoshields (Fig 8). In practice, thermal shrouds can achieve temperatures down to 20° C (36° F) above absolute zero.
The thermal shroud is located inside the space test chamber, and the test object is placed inside the shroud. The shroud therefore forms a radiant heat barrier between the test object and the interior of the test chamber. Thermal shrouds are constructed of a polished stainless steel or aluminum shell designed to have a low emissivity (highly reflective) exterior to reflect radiant heat. With an emissivity in the 0.02 to 0.05 range, this surface absorbs very little heat from the interior of the space chamber walls. The interior of the shroud is coated with a high emissivity black paint to effectively absorb as much heat as possible from the test object inside the shroud. The paint uses special pigments to absorb radiant heat energy in the 5 to 50 micron wavelength range. Different cooling media are used to cool the thermal shroud depending on the temperature range required. The shroud is fitted with tubing that carries the coolant and provides distributed cooling of the surface of the shroud.
For thermal shroud temperatures from -180°C to 150°C (-292°F to 302°F), gaseous nitrogen is used as the coolant. Since gases have lower heat capacity than liquids, a larger volume of gas must be circulated in comparison to a liquid coolant. Blowers are used to circulate the nitrogen through the cooling circuit. Gaseous nitrogen systems have the advantage of being able to add heat as well as remove it. When this is desired, the nitrogen is heated using electric heaters located in the path of the recirculated nitrogen. For temperatures below -180°C (-292°F), liquid nitrogen is used, and temperatures below -196°C (-321° F) are achieved with refrigerated helium gas. Temperatures down to -253°C (-423°F) can be obtained with these systems, just 20° C (36° F) above absolute zero.
Testing of the James Webb Telescope
The next generation of space telescope, the James Webb Space Telescope (JWST), is under construction and scheduled to launch in October 2018. The JWST is unique in that it will provide very high resolution in both the visible and infrared electromagnetic ranges. In comparison to the Hubble Space Telescope, which gave the world historic insights into deep space using its 2.4 m (7.9 ft.) mirror, the JWST features a 6.5 m diameter (21.3 ft.) mirror.
In order to prove the space-worthiness of the JWST, it was tested in Chamber A located in the Space Environment Simulation Laboratory at NASA’s Johnson Space Center in Houston (Fig. 9). Originally designed for the Apollo command and service modules, Chamber A was retrofitted with a thermal shield system to simulate the extreme cold the JWST will encounter in space 1,600,000 kilometers (1,000,000 miles) away from earth.
Chamber A accommodates specimens up to 22.9 m (75 ft.) tall and 7.6 m (25 ft.) in diameter, and weighing up to 68,100 kg (150,000 lbs). It features a revolving work table that can rotate the payload 180° at up to 1.6 RPM. The chamber utilizes a vacuum system incorporating a series of mechanical and diffusion pumps, and a 20K (-253°C or -423°F) gaseous helium crypump. The system can attain a vacuum of 1 x 10-5 mbar (7.5 X 10-6 Torr) in 19 hours, with a gas leak load of 36.8 mbar-liters per second. It has a pumping capacity of 2 x 10-7 liters per second condensables and 3 x 105 l/s non-condensables at 1 x 10-6 mbar (7.5 X 10-6 Torr). At atmospheric pressure prior to pumpdown, the weight of the air in the chamber is 7,771 kg (17,097 lbs.). After pumpdown at full vacuum, only 7.77 g (0.017 lbs.) of air remains, less than the weight of two sheets of paper.
The chamber simulates solar radiation via heating panels with a power density variable from 654 to 1,549 watts/m2 (60 to 144 watts/ft.2), emitting in the range of .25 to 3.0 microns. After addition of the retrofit thermal shielding system for the JWST, the interior of Chamber A is lined with black, liquid nitrogen cooled panels that totally surround the test specimen and cool down to -233°C (-387°F), simulating the cold temperature of space