AQT-D (AQua Thruster-Demonstrator)
AQT-D is a 3U CubeSat for a demonstration of a water resistojet propulsion system developed by The University of Tokyo. The small propulsion system, named AQUARIUS-1U (AQUA ResIstojet propUlsion System 1U) is installed into a 1U volume using water as a propellant . The project completed the design and assembly of the AQT-D flight model. AQUARIUS-1U was fired on a pendulum-type thrust balance, and its performance was directly characterized in both a stand-alone test and an integrated test using an entire spacecraft system. — The AQT-D 3U CubeSat was delivered to JAXA to be launched to the ISS by the HTV-8 (HII Transfer Vehicle-8) flight of JAXA in September 2019. 1)
When the Aerospace Corporation launched the OCSD (Optical Communications and Sensor Demonstration) mission of two 1.5U CubeSats in November 2017, one mission objective was to test water-fueled thrusters. At the time, the idea was fairly novel. Two years later, water-based propulsion is moving rapidly into the mainstream. — Both propulsion systems experienced anomaly operation such as liquid water exhausting from the nozzle or ice plug forming at a nozzle. In addition, they have only a small amount of water less than 26 g. Further research and on-orbit demonstration are necessary to establish a water micro-propulsion technology including not only resistojet but also advanced electric propulsion such as ion thruster for sustainable CubeSats utilization.
The AQUARIUS (AQUA ResIstojet propUlsion System) was proposed as the water propulsion system using a room temperature evaporation system. AQUARIUS consists of a tank, a vaporizer, and two types of thrusters, designed within a 2.5U volume and storing 1.2 kg of water. It is to be installed on the 6U deep space exploration CubeSat: EQUULEUS (EQUilibriUm Lunar-Earth point 6U Spacecraft) and will be launched by SLS (Space Launch System) EM-1 in 2020. 2) 3)
The AQT-D 3U CubeSat mission will be the world's first ISS-deployed CubeSat installing the propulsion system. The safety of water enables for ISS-deployed CubeSat to install a propulsion system. The deployment of CubeSats from the ISS is quite attractive because of the constant launch opportunity, low-cost, and user-friendly launch environment. However, ISS-deployed CubeSats have short lifetime due to the low altitude and air drag force. Installing a water micro-propulsion system on CubeSats can overcome this crucial issue. In addition, AQT-D has a role as a precursor of the EQUULEUS mission in terms of propulsion technology demonstration.
AQT-D is a 3U CubeSat, which has the 3-axis attitude control and the propulsion system. The design and development of its bus-system are based on TRICOM-1R Tasuki (a 3U CubeSat). TRICOM-1R was already launched by the smallest orbital rocket SS-520 and operated in 2017 – 2018. 4) The bus-system newly installed reaction wheels, 3-axis control software, and S-band communication transceivers. The specifications of AQT-D are listed in Table 1.
Structure: The 3U CubeSat is based on requirements for ISS-deployment CubeSat. AQT-D has a deployment system of store and forward antenna. Therefore, the size becomes 260 x 260 x 340.5 mm after antenna deployment. The propulsion system is mounted on the PZ side and the store and forward antenna is mounted on the MZ side. Figure 1 shows a CAD model of AQT-D spacecraft and Figure 2 shows a photograph of AQT-D spacecraft.
Size of standard 3U CubeSat
100 x 100 x 340.5 mm
< 3.78 kg
AOCS (Attitude Orbit Control Subsystem)
3-axis attitude control using acceleration sensors, gyro sensors, geomagnetic sensors,
EPS (Electric Power Subsystem)
power of < 8.6 W, solar cells: 24,
Battery capacity: < 7600 mAhr, 8.4 V (charged), battery cells: 4
Water micro-resistojet thruster
AOCS (Attitude Orbit Control Subsystem): The spacecraft features 3-axis control. The geomagnetic sensor and the sun sensors are used for an attitude determination. AQT-D does not install a start tracker due to volumetric restriction. Especially, the gyro sensor was calibrated before the spacecraft integration to evaluate a dependency on temperature and output drift. It was cleared from the calibration results that the attitude determination accuracy can be estimated to <30º. This value is enough to characterize the performance of the ΔV thruster.
EPS (Electric Power Subsystem): Four solar panels are mounted on the MX. PX, MY, and PY sides. The maximum energy power obtained from the solar panels is estimated to be 8.6 W in the sunlit phase of the orbit. The battery capacity is designed within a range from 4940 m - 7600 mAhr. The maximum battery capacity is limited depending on the battery temperature and the voltage control circuit.
Propulsion Subsystem: The propulsion system, named AQUARIUS-1U (AQUA Resistojet propulsion System-1U), uses water as a propellant. Water has no toxicity, flammability, or carcinogenic issues, and no explosion hazards. Therefore, the propulsion system does not require a pressure vessel for propellant storage. The safety of this system permits a low-cost development, and also a propellant filling at launch site or an orbital habitation module such as ISS or the Lunar Orbital Platform-Gateway (LOP-G) in the future. The ultimate green propellant “water” enables ISS-deployed CubeSats to install a propulsion system.
Figure 3 shows a system diagram of the propulsion system. Figure 4 shows a photograph of the propulsion system. The propulsion system consists of a tank, a vaporizer, Delta-V thruster, and RCS thrusters. Total wet mass and dry mass are approximately 1.20 kg and 0.80 kg. Inside the tank, a bladder is inserted, which is a kind of rubber balloon, storing less than 0.40 kg water. Pressurized gas and water are separated by the bladder. The vaporizer was manufactured using additive manufacturing, which allows for a flexible design of the inner feed lines. Water droplets are injected into the vaporizer by opening the regulating valves downstream of the tank and evaporates at room-temperature (290 –310 K). The vaporizer is filled with water vapor with a saturated pressure (< 5 kPa). The saturated vapor flows under its own pressure to the thrusters, which are preheated to approximately 343 K. Finally, the steam is expelled from the thruster. Table 2 lists the specifications of the propulsion system.
92 x 92 x 105 mm
Wet mass, dry mass
<1.2 kg, <0.80 kg
< 250 Ns
Number of thruster
1 x ΔV thruster, 4 x reaction control thruster
Tank: < 100 kPa (303 K); Vaporizer: <5 kPa (290-310 k); Nozzle (plenum): <5 kPa (343 K)
Thrust: 4 mN (depends on power); Specific impulse: 70 s; Thrust to power ratio: 0.22 mN/W
Reaction control thruster
Thrust: < 1 mN (depends on power); Specific impulse: 70 s; Thrust to power ratio: 0.22 mN/W; Minimum impulse bit: > 0.5 mNs
Launch: The AQT-D 3U CubeSat payload to the ISS was launched on the HTV-8 (HII Transfer Vehicle-8, known as Kounotori-8) flight of JAXA on 24 September 2019 (16:05 UTC). The HTV-8 vehicle delivers supplies to the International Space Station (ISS). Kounotori-8 provides very basic support for ISS operations by delivering up to six tons of cargo and has the world's largest transportation capacity. It also has the unique function of carrying multiple numbers of large-size experimental instruments on one flight. The HTV-8 vehicle was launched from the Tanegashima Space Center aboard an H-IIB launch vehicle of MHI (Mitsubishi Heavy Industries). 5)
Orbit: Near circular orbit, altitude of ~400 km, inclination = 51.6º.
A total of 3.5 metric tons are stored in the PLC (Pressurized Logistics Carrier) while 1.9 metric tons are stored in the ULC (Unpressurized Logistics Carrier) of the HTV-8.
Cargo in the PLC (Pressurized Logistic Carrier)
• CBEF-L (Cell Biology Experiment Facility-Left).
• SOLISS (Small Optical Link for International Space Station). SOLISS is a small-sized satellite optical communication system collaboratively developed by Sony Computer Science Laboratories, Inc. and JAXA Space Exploration Innovation Hub Center in order to conduct in-orbit technology demonstrations. For the in-orbit demonstrations, the SOLISS is installed on the IVA-replaceable Small Exposed Experiment Platform (i-SEEP), which is installed on the out-board platform for experiments of the KIBO on the ISS. The communication demonstrations with the ground are conducted by using the 1550 nm laser.
• Hourglass (Gravitational Dependence Research of Flexible Surface on a Planet). Hourglass is an experiment that utilizes the CBEF (artificial gravity generator) only possessed by the Japanese Experiment Module, KIBO. The objective is to research the effects that micro and low gravity has on the characteristics of powder and granular materials.
• JSSOD (JEM Small Satellite Orbital Deployer) and Ultra-small Satellite (CubeSat). An ultra-small satellite collaboratively developed by Kyushu Institute of Technology and National Authority for Remote Sensing and Space Science (NARSS) as well as other ultra-small satellites developed by Space BD Inc. and The University of Tokyo will be transported and deployed from the KIBO.
• NARSSCube-1 (Kyushu Institute of Technology/National Authority for Remote Sensing and Space Science (NARSS)). Development, operation of a satellite, and a demonstration experiment with a 200 m resolution camera installed on the satellite by our research partner, Egypt.
• AQT-D (Space BD Inc./The University of Tokyo). Technology demonstration of a resistojet thruster module that uses 1U size of water as propellant and communications with the mountainous terrain using a UHF antenna.
• RWASAT-1 (The University of Tokyo/Ministry of Commerce, Industry, & Tourism Rwanda Utilities Regulatory Authority Smart Africa secretariat (Republic of Rwanda)). For human resources development of researchers in Republic of Rwanda and technological improvement. A radio wave (weak) receiver is installed and collects sensor information on the ground.
• Cargo for the ISS crew: HTV8 will also deliver fresh food.
Cargo in the ULC (Unpressurized Logistics Carrier)
• ISS battery ORUs (Orbital Replacement Units). Following HTV6 and HTV7, HTV8 will deliver new lithium ion batteries for ISS on the Exposed Pallet (EP) in the ULC (Unpressurized Logistic Carrier). New six battery Orbital Replacement Units (ORUs) consisting of new lithium-ion battery cells manufactured by a Japanese company are delivered.
- The nickel-hydrogen batteries currently used on the ISS are becoming old. An extension of ISS operations becomes possible with the supply of Japanese lithium-ion battery cells. Only the HTV is capable of delivering six battery ORUs at one time, and thus plays an important role in continuous ISS operations.
Direct thrust measurement: standalone and integrated firing test of AQT-D
Thrust performance of the propulsion system was directly measured by using the pendulum-type thrust stand developed by the authors. In addition, both standalone firing using the propulsion module and integrated firing using the entire spacecraft system were conducted.
Figure 7 shows a photo of the experimental setup of the standalone firing test. The propulsion module was mounted on a mass balance to measure mass profile during firing. Both components were mounted on the pendulum-type thrust stand designed base on the proposed one.6) Ten minutes firing was conducted by using in-house developed software. Figure 8 shows an example of firing profile of the ΔV thruster. Water droplets were injected intermittently by opening the regulating valve for 2.7s at a cycle time of 60 s. The vaporizer pressure and thrust had a peak value just after injection. Thereafter, the vaporizer temperature decreased because of the heat of the evaporating water droplets. Figure 9 shows the firing profile of the RCS thrusters.
Integrated firing test: Figure 10 shows a photo of the experimental setup of the integrated firing test. The spacecraft installed the propulsion system was mounted on the thrust stand which was similar to the stand-alone test. The mass balance was not used in the integration test because the maximum load of the mass balance was lower than the spacecraft mass. Therefore, mass consumption was calculated based on the results of the stand-alone test. Power was supplied from the battery to keep the same firing condition between ground and on-orbit. The firing tests of both the ΔV thruster and RCS thrusters were conducted by using the actual flight command and data handling system.
1) Jun Asakawa, Kazuya Yaginuma, Yoshihiro Tsuruda, Hiroyuki Koizumi, Yuichi Nakagawa, Kota Kakihara, Kanta Yanagida, Yoshihide Aoyanagi, Takeshi Matsumoto, Shuhei Matsushita, Yusuke Murata, Mikihiro Ikura, ”AQT-D: Demonstration of the Water Resistojet Propulsion System by the ISS-Deployed CubeSat,” Proceedings of the 33rd Annual AIAA/USU Conference on Small Satellites, August 3-8, 2019, Logan, UT, USA, paper: SSC19-WKV-07, URL: https://digitalcommons.usu.edu
2) Jun Asakawa, Hiroyuki Koizumi, Keita Nishii, Naoki Takeda, Masaya Murohara, Ryu Funase, Kimiya Komurasaki, ”Fundamental Ground Experiment of a Water Resistojet Propulsion System: AQUARIUS Installed on a 6U CubeSat: EQUULEUS,” Transactions of the Japan Society for Astronautical and Space Sciences, Aerospace Technology Japan, Volume 16, Issue 5, pp: 427-431, 2018, https://doi.org/10.2322/tastj.16.427, URL: https://www.jstage.jst.go.jp/article/tastj/16/5/16_427/_pdf/-char/en
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4) Kazuhiro Yagi, Seiji Matsuda, Jun Yokote, Takayoshi Fuji, Kenji Sasaki, Mitsuteru Kaneoka, Shinichiro Tokudome, Yohsuke Nambu, Masaaki Sugimoto, ”SS-520 Nano Satellite Launcher and its Flight Result ,” Proceedings of the 32nd Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 4-9, 2018, paper: SSC09-IX-8, URL: https://digitalcommons.usu.edu
6) Yuichi Nakagawa, Hiroyuki Koizumi, Hiroki Kawahara, Kimiya Komurasaki, ”Performance characterization of a miniature microwave discharge ion thruster operated with water,” Acta Astronautica, Volume 157, April 2019, Pages 294-299, 2019, https://doi.org/10.1016/j.actaastro.2018.12.031
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).