Tsinghua University Freefall Facility (TUFF): A 2.2 Second Drop Tunnel for Microgravity Research

Ground-based freefall facilities are to create microgravity environment on earth for studying the space-related or fundamental sciences, which have been widely used in the field of combustion, fluid, physics and material sciences. Aiming to serve for the broader microgravity science and preliminary tests for projects onboard the upcoming assembly of Chinese Space Station, a 2.2 second freefall facility was designed and built in the Lee Shau Kee Science and Technology Building on the campus of Tsinghua University in Beijing. This facility is composed of five systems: (I) freefall tunnel and safety cables; (II) release and retrieve system; (III) the capsule; (IV) brake system; (V) electrical control and safety interlock system. The capsule consists of an outer drag shield and an inner rig of which the total weight is 280 kg. The capsule is released by a pneumatic chuck that minimizes disturbance of the release operation. The eddy current brake modules made of several permanent magnets are applied to decelerate the capsule without power supply. This paper primarily discusses the tests designed and conducted to characterize the performance of such facility in many aspects. The results show that during freefall, the microgravity level of 10(-3) g can be achieved for 2.2 second. Concerning movement of the inner rig relative to the drag shield during freefall, the clearance of 0.43 m and relative velocity of 0.62 m/s were specified. The deceleration can be controlled within 15 g. The terminal velocity of capsule in the brake system ranges from 0.4 to 0.5 m/s depending on the actual weight of capsule. The noise level in the entrance section of brake system is about 75.6 dB. The duration of this noise is very short (<1 s), suggesting that the operation of facility is environmentally friendly to the operators and people working in the building.

Automated summary

The 2.2 second freefall facility designed and built at Tsinghua University in Beijing serves the broader microgravity science and preliminary tests for projects onboard the upcoming Chinese Space Station assembly. This facility consists of five systems including freefall tunnel and safety cables, release and retrieve system, capsule, brake system, electrical control and safety interlock system. The tests conducted show that a microgravity level of 10(-3) g can be achieved for 2.2 seconds during freefall, with controlled deceleration within 15 g and a terminal velocity of capsule ranging from 0.4 to 0.5 m/s. The noise level in the brake system entrance section is about 75.6 dB, suggesting environmentally friendly operation.