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

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Abstract

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.

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Abbreviations

A :

cross area (m2)

a :

acceleration (g)

a d :

drag acceleration (m/s2)

a x ,a y ,a z :

components of gravity acceleration in three directions (m/s2)

B 0 :

magnetic flux density at the entry edge (T)

B rem :

magnet remanence of the magnet (T)

C D :

drag coefficient

d e :

entrefer length (de = dm + dg) (m)

d g :

the depth of the air gap (m)

d m :

the depth of the magnet (m)

d sc :

half the thickness of the conductor plate (m)

F B :

the total braking force of the capsule (N)

F D :

drag force(N)

F e :

the braking force on the conductor plate from the entry-edge field (N)

F g :

gravity force of the capsule (N)

F r :

the contributions of the rth harmonic to the tangential (braking) force (N)

g:

gravity acceleration (m/s2)

g m :

measured gravity level (g)

K 0 :

offset value (10-3 g)

K 1 :

the 1st order scale factor (10-3A/g)

K 2 :

the 2nd order scale factor (10-6/g)

k :

k = π/τ (m-1)

L :

relative displacement distance (m)

l m :

magnet length (m)

l p :

the length of primary plate (m)

M pr :

magnetization amplitude of the rth harmonic (A/m)

p :

number of pole pairs

R :

resistance (kΩ)

r :

the rth harmonic

U out :

output voltage (V)

u :

velocity (m/s)

u d :

velocity of drag shield (m/s)

u 0 :

u0 = 1/μ0σdsc (m/s)

v :

the fractional velocity (v= u/u0)

θ :

rotation angle (rad)

μ 0 :

permeability of vacuum (T∙m/A)

ξ :

correction factor of the magnet field (ξ =1.3)

σ:

effective conductivity of the conductor plate (S/m)

σp :

physical conductivity of the conductor plate (S/m)

τ:

pole pitch (m)

ψ, φ :

misalignment angle (rad)

ωp :

the width of primary plate (m)

ωsc :

the width of the conductor plate (m)

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Acknowledgement

This project is primarily supported by the start-up research funding of Prof. Yu Cheng Liu at Tsinghua University. Partial support from National Science Foundation of China (NSFC)- under Grant No. 51706120 and 52076117 for experimental apparatus on the inner rig is gratefully appreciated. We appreciate Profs. Claus Lämmerzahl, Claus Braxmaler, Michael Dreyer of ZARM (through connection of Prof. Qiu-Sheng Liu of Institutes of Mechanics, Chinese Academy and Sciences), Prof. Osamu Fujita of Hokkaido University, and Profs. Mitsuhiro Tsue and Shinji Nakaya of The University of Tokyo for offering visits to their microgravity facilities. Professor Yang Gao of Sun Yat-Sen University was involved in the preliminary planning of this facility while he was at Tsinghua. Professor Mark Weislogel of Portland State University was consulted during his visit to Tsinghua concerning the use of safety cables and magnetic brakes. We also thank Prof. Weislogel for providing updated materials of the Dryden Drop Tower in Table 1. Mrs. Binglei Duanmu, Xiaosong Bai, Mengxin Yang, Qingwu Zhang and many other engineers from AVIC CAPDI Integration Equipment Co., LTD are acknowledged for their devotion to implementing the designs. We thank Mr. Xiaogang He and Xiaohong He from School of Architecture, Tsinghua University for their viability analyses from the perspective of building structure. We appreciate Haiyong Yang of Tsinghua University for his coordination during the construction of LSKSTB. Many thanks to Prof. Wenjun Kong form Beihang University, Prof. Jianfu Zhao and Prof. Shuangfeng Wang from Institute of Mechanics, Chinese Academy of Sciences, Dr. Yang Yang and Dr. Jianquan Zhang from Technology and Engineering Center for Space Utilization, Chinese Academy of Sciences, Prof. Yuqun Zhuo and Prof. Hai Zhang from Department of Energy and Power Engineering of Tsinghua University for their pertinent suggestions for the development of TUFF’s operation procedure and safety regulations. Finally, we appreciate Prof. Qiang Yao of Tsinghua University for his persistent support for the development of microgravity combustion research and infrastructure in China, including the construction of TUFF.

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Authors and Affiliations

  1. Center for Combustion Energy, Tsinghua University, Beijing, 100084, China

    Lei Luo, Hengyi Zhou, Yuhang Sun, Wenyi Zhang, Tao Chen, Shuqing Chen, Yuzhe Wen, Suyuan Yu & Yu Cheng Liu

  2. Department of Energy and Power Engineering, Tsinghua University, Beijing, 100084, China

    Lei Luo, Hengyi Zhou, Yuhang Sun, Wenyi Zhang, Tao Chen, Shuqing Chen, Yuzhe Wen, Suyuan Yu & Yu Cheng Liu

  3. Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University, Beijing, 100084, China

    Lei Luo, Hengyi Zhou, Yuhang Sun, Wenyi Zhang, Tao Chen, Shuqing Chen, Yuzhe Wen, Suyuan Yu & Yu Cheng Liu

  4. AVIC CAPDI Integration Equipment Co., Ltd, Beijing, 100011, China

    Shilong Xu

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  1. Lei Luo
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Luo, L., Zhou, H., Sun, Y. et al. Tsinghua University Freefall Facility (TUFF): A 2.2 Second Drop Tunnel for Microgravity Research. Microgravity Sci. Technol. 33, 26 (2021). https://doi.org/10.1007/s12217-021-09877-5

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