Minimize SloshSat

SloshSat-FLEVO (Facility for Liquid Experimentation and Verification in Orbit)

SloshSat-FLEVO is a technology demonstration minisatellite for the experimental study of liquid dynamics and liquid management problems in space. The objective is to investigate fluid dynamics in microgravity conditions by monitoring the behavior of water in an instrumented tank. The dynamics of fluid slosh (e.g. in fuel tanks) is a recurring problem for the control and stabilization of spacecraft Hence, knowledge and insight in the prediction and control of the dynamic behavior of “liquid filled” satellites is of growing importance. 1) 2) 3) 4)

SloshSat-FLEVO is a joint program between ESA, the (Dutch) Ministry of Economic Affairs, NIVR (Netherlands Agency for Aerospace Programs), and NLR (National Aerospace Laboratory). NRL is the prime contractor of the project with Dutch Space, Leiden, The Netherlands, providing the spacecraft structure and power systems.

The SloshSat Investigators' Working Group (IWG) is composed of academic researchers from Groningen State University (NL), Delft Technical University (NL), Technion (Israel), and investigators from ESA, NASA and NLR.

Background: The SloshSat proposal was initially submitted to ESA by NLR in 1989, in response to an Announcement of Opportunity for the Technology Demonstration Program. Initially there were plans to launch the satellite as an Ariane-4 ASAP (Ariane Structure for Auxiliary Payloads) secondary payload. In 1992, an opportunity for a Shuttle launch became the baseline (accommodation of SloshSat as a Hitchhiker payload). Manufacturing took place in 1997 and 1998 and integration started in early 1999. In the end, no flight opportunity materialized (the tragic accident of the Shuttle Columbia (STS- 107) mission on Feb. 1, 2003 forced an end to the scientific flights). Eventually, an alternate solution was found and Sloshsat-FLEVO was accepted for launch on Ariane-5 as part of the MaqSat-B2 test (SloshSat bolted to the top of the MaqSat-B2). This turn in events required considerable adaptations in the existing SloshSat-FLEVO hardware and software.


Figure 1: Artist's view of the SloshSat FLEVO in orbit (image credit: NLR, ESA)


The spacecraft is a cube of 90 cm side length with a launch mass of 129 kg. The outside of the satellite is covered with solar cells. After separation from Ariane 5, SloshSat's attitude control is being provided by a cold gas nitrogen system (12 small thrusters). The thruster subsystem is activated at 30 Hz. S/C power is provided by solar panels and a battery of 120 Whr.

An OBC is being used for data collection. The OBC receives data from the MSS (Motion Sensing Subsystem) that samples three fiber optic gyroscopes (Litef ìFORS 36/6) and six accelerometers (Allied Signal QA-3000-010) at 30 Hz. The accelerometers are in three pairs at corners in the SloshSat structure, i.e. they record accelerations from angular rates in addition to translational motion. Their output is passed through a high-frequency filter with cut-off at about 3 Hz. 5) 6) 7)


Figure 2: Illustration of the SloshSat spacecraft (image credit: ESA and NLR)


Launch: A launch of SloshSat-FLEVO as a secondary payload took place on Feb. 12, 2005 from Kourou on the Ariane-5 ECA (Étage Cryogen A) qualification flight (heavy lift version of Ariane-5).
The primary payloads on the flight were: XTAR-EUR, a military communications satellite of USA and Spain (mass of 3,631 kg, 12 X-band transponders, 1300 series platform of SS/L), and MaqSat-B2, built by Kayser-Threde, Munich, Germany. MaqSat-B2 (3,600 kg) represents a commercial passenger payload, but also serves as technology platform for several European experiments: SloshSat-FLEVO, Boucle-Fluide (a French heat pipe experiment), DVCAM (a French autonomous camera system monitoring key events of the launch) and a Telemetry Assembly (TMA). The MaqSat-B2 payload remains mated to the Ariane-5 vehicle throughout the mission.


Figure 3: Mounting Sloshsat on MAQSAT-B2 (image credit: NLR)


Figure 4: Sloshsat mounting on Ariane (image credit: NLR)

Orbit: GTO (Geostationary Transfer Orbit), a standard Ariane GTO with a perigee of 250 km and an apogee of 39,918 km at injection, inclination of 7º, period of 10.5 hours.

RF communications were provided in S-band using an omni-directional antenna on the spacecraft. The commands are part of the telemetry, as are the low pressure data that determine the actually delivered thrust. Housekeeping data include temperature measurements at many locations and allow correcting instrument data for thermal effects.

Sloshsat-FLEVO has been controlled by operators at the ESA/ESOC Diane ground station in Kourou (15 m antenna on the ground), with the support of ESOC in Darmstadt, Germany. The SloshSat experiment was operated by the NLR spacecraft operations team whenever visible from the Diane ground station (roughly 10 hrs per day). 8)


Figure 5: Schematic view of the FACT (Flight-demonstrator Advanced Crew Terminal) archticture (image credit: NLR, Ref. 11)



The satellite includes one experiment, a cylindrical tank of 86.9 liter volume with 33.5 liter of deionized water. The composite tank has 270 sensors mounted inside on the tank walls to measure the sloshing behavior by observing the distribution of the water. At the same time the temperature, pressure and fluid velocity are being measured at 17 locations while three accelerometers and fiber-optic gyroscopes monitor the resulting spacecraft motions. Thrusters, powered by a cold gas nitrogen system, provide linear and rotational movement to excite fluid motion. - What's important is to understand how the motion of the liquid influences the motion and orientation of the spacecraft

Mission sequence: ESA's spring-loaded ESAJECT mechanism, developed and built by Verhaert of Kruibeke, Belgium, for satellites in the 50-150 kg class, ejects Sloshsat-FLEVO from MaqSat-B2 once the launcher reached geostationary transfer orbit. The satellite then transmits data on the behavior of the water in its tank under different motions controlled from the ground, for a test period of about a week. The total experiment time lasts until the gas supply of the cold-gas reaction control system is exhausted. Between experiment runs the water is allowed to settle, and the battery is recharged using the solar panels.


Figure 6: Schematic of the SloshSat tank (image credit: NLR)

The sloshing experiments are being supported by a theoretical/computational model that has been developed over the last few years at SRON (Space Research Organization Netherlands). This CFD (Computational Fluid Dynamics) model, called ComFlo, is based on the Navier-Stokes equations for incompressible flow and includes a description of the capillary surface physics. The model is capable of simulating three-dimensional free-surface flow in solid containers, including the interaction with the container dynamics. 9) 10)

Initial post-flight results: The spacecraft was operated until all propulsion gas was exhausted on February 21, 2005, for a total 57.5 hours of experimental data. During this period, SloshSat was being rotated periodically with its thruster system. Six accelerometers and three gyroscopes were tracking the spacecraft's motion with 270 sensors monitoring the water's temperature, pressure, and velocity inside the tank. The accelerometers and gyroscopes - measuring the motions of the satellite -performed very well, allowing good scientific results. Also the rather complicated slosh control whereby the satellite translated and rotated around the center of mass of the liquid in the experiment tank worked well. - A setback was the lack of any data on the position and velocity of the water in the tank, which complicated the control of the experiments. As a consequence some experiments will provide less definite results. 11) 12)

Based on MSS (Motion Sensing Subsystem) data that have been corrected only for bias on the accelerometer signals, and for relatively large rotation rates the following has been concluded:

1) On the Sloshsat mission:

• precise calibration promises to lead to high resolution of significant forces from various
physical phenomena

• no sign of tank leakage as a cause of the major anomaly of the mission is seen

• the real-time data distribution system FACT (Flight-demonstrator Advanced Crew Terminal) worked fine

• detailed analysis of a large manoeuvre indicates that a flight mechanics model can become a valid and useful tool for slosh control on spacecraft.

2) On the performance of SMS (SloshSat Motion Simulator):

• the predicted time histories of variables have signatures that are typically shown also by the measured data. Then, predictions of variables that cannot be measured should be valid

• introduction of a potential force field in the tank allows achieving a fit between predicted and actual behaviour. The Weber number dependence of the potential is to be analysed, as is the effect of other variables

• mechanisms for damping are to be reviewed and modelled anew, following evaluation of low Weber number data.


The spacecraft was switched off after the propellant of SloshSat was depleted. The satellite is expected to re-enter into the atmosphere in less than 25 years.

This was the first time that a satellite has been dedicated to studying fluid behavior in weightlessness.


2) G. Schilling, “Sloshing in Space - Analyzing how Liquids Affect the Motion of Ships,” Scientific American, August 2004, pp. 12-13

3) J. J. M. Prins, “Sloshsat FLEVO project, flight, and lessons learned,” Proceedings of the 56th IAC 2005, Fukuoda, Japan, Oct. 17-21, 2005, IAC-05-B5.3./B5.5.05

4) J. J. M. Prins. “Sloshsat FLEVO Facility for Liquid Experimentation and Verification in Orbit,” Proceedings of the 51st IAC (International Astronautical Congress), Oct. 2-6, 2000, Rio de Janeiro, Brazil, IAF-00-J.2.05, URL:

5) “SloshSat FLEVO,” ESA, Feb. 7, 2005, URL:

6) “SloshSat FLEVO,” Dutch Space, 2007, URL:


8) Information provided by J. J. M Prins of NLR, The Netherlands

9) J. A. Helder, “Sloshing SloshSat FLEVO: Numerical Simulation of Coupled Solid-Liquid Dynamics in Micro-Gravity,” Master's Thesis, University of Groningen, The Netherlands, Sept. 2005, URL:


11) J. P. B. Vreeburg, “Measured states of Sloshsat FLEVO,” NLR-TP-2005-518, This report is based on a paper presented at the IAF Congress, 56th IAC 2005, Fukuoda, Japan, Oct. 17-21, 2005, IAC-05-C1.2.09, URL:

12) Jan P. B. Vreeburg, “Liquid Dynamics from Spacelab to Sloshsat ,” Microgravity - Science and Technology, Volume 21, Numbers 1-2, Januar 2009, pp. 11-20

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.