Minimize ISS Utilization: RUBI

ISS Utilization: RUBI (Reference mUltiscale Boiling Investigation)

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Liquid vapor flows exist in a wide variety of applications in both normal gravity and reduced gravity environments. As it is usually the case, there are many benefits and drawbacks in the use of two-phase systems and, consequently, serious considerations are needed before deciding on whether or not to proceed with the design, construction and use of these systems, particularly in a reduced-gravity context. 1)

RUBI is a fluid science experiment developed and built by Airbus for ESA (European Space Agency). The overall objective is to investigate the fundamentals of boiling fluids in microgravity. RUBI will study the phenomena of phase transition and heat transfer during the evaporation of fluids in microscopic and macroscopic dimensions.

On Earth – thanks to the effect of gravity – only small bubbles form, quickly detaching from the heating surface and masking other physical effects. The scientists want to optimize their numerical models of the boiling process with a series of tests conducted under microgravity conditions and corresponding reference tests on Earth. In the future, this could contribute towards the production of more efficient and environmentally friendly household appliances (stoves, radiators) and heat exchangers for industrial manufacturing processes.

Boiling is a two-phase heat transfer process where large heat fluxes can be transferred with small driving temperature differences. The high performance of boiling makes the process very interesting for heat transfer applications and it is widely used in industry for example in power plants, refrigeration systems, and electronics cooling. Nevertheless, due to the large number of involved phenomena and their often highly dynamic nature a fundamental understanding and closed theoretical description is not yet accomplished. The design of systems incorporating the process is generally based on empirical correlations, which are commonly accompanied by large uncertainties and, thus, has to be verified by expensive test campaigns. Hence, strong efforts are currently made to develop applicable numerical tools for a reliable prediction of the boiling heat transfer performance and limits. In order to support and validate this development and, in particular as a precondition, to enhance the basic knowledge about boiling the comprehensive multi-scale experiment RUBI (Reference mUlti-scale Boiling Investigation) for the Fluid Science Laboratory on board the ISS is currently in preparation. 2) 3)

The scientific objectives and requirements of RUBI have been defined by the members of the ESA topical team "Boiling and Multiphase Flow" and addresses fundamental aspects of boiling phenomena. The main objectives are the measurement of wall temperature and heat flux distribution underneath vapor bubbles with high spatial and temporal resolution by means of IR thermography accompanied by the synchronized high-speed observation of the bubble shapes. Furthermore, the fluid temperature in the vicinity and inside of the bubbles will be measured by a micro sensor array. Additional stimuli are the generation of an electric field above the heating surface and a shear flow created by a forced convection loop. The objective of these stimuli is to impose forces on the bubbles and investigate the resulting bubble behavior such as bubble sliding on and detaching from the surface. The experiments benefits from the absence of vapor buoyancy and natural convection in the high quality and long-term microgravity of the ISS. Effects and phenomena like thermocapillary convection that are hardly observable in normal gravity conditions can be investigated. Clearly predefined conditions particularly of the thermal layer at the heating surface can be established without disturbances by natural convection. Vapor buoyancy as the main detaching force in normal gravity is missing. Hence, it is possible to study stationary, attached bubbles and alternative detaching forces. With RUBI a long history of boiling experiments is perpetuated that used microgravity as a tool for a deeper understanding of the fundamental phenomena. Several precursor experiments closely related to the RUBI project have already been conducted on parabolic flights. The subject of the paper is to provide an overview on the RUBI project, its scientific objectives and the corresponding experimental principle. The current design of the experiment container that is under development at ASTRIUM Space Transportation in Friedrichshafen will be introduced. Furthermore, results from the precursor experiments are presented. The industrial activities of the RUBI project are funded and the science team is supported by ESA.

RUBI instrument

RUBI’s core element is a cell filled with fluid, which can be heated and cooled thermoelectrically. The boiling process is then triggered on a metal-coated glass heater using a laser. High-resolution cameras record the formation and growth of vapour bubbles in both the visible and infrared spectrum. By taking up to 500 images per second, RUBI’s cameras can create a three-dimensional representation of the bubble shapes and analyze the temperature distribution on the heater, enabling the scientists to precisely determine evaporation conditions and heat flux densities. The boiling process can be systematically influenced using a high-voltage electrode (up to 15,000 V) and an adjustable convection loop.

A particular challenge for the Airbus-led industrial team was to shrink RUBI down to the size of a ‘shoe box’ (40 x 28 x 27 cm) weighing just 34 kg that would then be suitable for use in space. By comparison, a terrestrial laboratory setup would be approximately the size of a wardrobe (2 x 1 x 1 m) and would have a mass of ~300 kg.


Figure 1: RUBI (in front at right) under test with Airbus Project Manager Olaf Schoele-Schulz. RUBI, a fluid science experiment developed and built by Airbus for ESA (European Space Agency), addresses the fundamentals of the boiling of fluids (image credit: Airbus)

RUBI will study the phenomena of phase transition and heat transfer during the evaporation of fluids in microscopic and macroscopic dimensions. RUBI’s core element is a cell filled with fluid, which can be heated and cooled thermoelectrically. The boiling process is then triggered on a metal-coated glass heater using a laser. High-resolution cameras record the formation and growth of vapour bubbles in both the visible and infrared spectrum.


Figure 2: Thumbs up: RUBI project manager Olaf Schoele-Scholz of Airbus (right) signals RUBI is ready to fly, built by Airbus for ESA (image credit: Airbus) 4)

Launch: RUBI was launched on 25 July 2019 as part of the SpaceX CRS-18 (Commercial Resupply Service) mission to the ISS on 25 July 2019 (22:01 UTC). SpaceX launched its CRS-18 mission from Space Launch Complex 40 (SLC-40) at Cape Canaveral Air Force Station, Florida. Dragon separated from Falcon 9’s second stage about nine minutes after liftoff and will attach to the space station on Saturday, July 27. 5)

Orbit: Near circular orbit, altitude of ~400 km, inclination = 51.6º.

ESA astronaut Luca Parmitano is set to install RUBI in the Columbus module of the ISS during his five-month ‘Beyond’ mission (from July to December 2019). The fluid experiment will then be operated and controlled by the Belgian User Support and Operation Center (B-USOC) in Brussels.

Mission status

• June 9, 2020: Did you know that in microgravity you can better study the boiling process? — Boiling is a very common process in our everyday life. For instance, we usually boil water to cook or to clean. The boiling process is common in many engineering fields such as environmental applications and industrial chemical processes. 6)

- Understanding the dynamics of boiling is essential to improve energy production and conversion in power plants, and to design future space applications like cryogenic fuel storage and propulsion.

- On Earth the process happens too fast to be accurately observed and measured. But experiments conducted in low gravity environments, like on the International Space Station, allow us to observe phenomena like phase transition and the onset of bubbles much more clearly.

- Such studies may lead to increase the energy efficiency of several application also here on Earth, from Power plants to thermal management systems used in electric vehicles, laptops, and smartphones just to cite a few examples.

Figure 3: This video interviews Peter Stephan of the Technical University of Darmstadt in Germany talking about the Reference mUltiscale Boiling Investigation experiment, known affectionately as Rubi. Paolo Di Marco of the University of Pisa in Italy talks about pulsating heat pipe experiments and Catherine Colin from the Institut de Mécanique des Fluides de Toulouse in France talks about heat transfer flow boiling and how to keep electronics cool. Lastly Giuseppe Zummo, of Italy’s National Agency for New Technologies, Energy and Sustainable Economic Development, ENEA, explains how parabolic flights are used to test new two-phase flow heat transfer in weightlessness (video credit: ESA - European Space Agency)

- Do take advantage of the opportunities and capabilities that are available for your research and development to further grow and extend your achievements. Take the next step ... the step to Space. We did it already.

• November 13, 2019: Two months ago the first space bubble marked the beginning of the Multiscale Boiling experiment, known affectionately as Rubi. Since then over 200 science runs on bubble formation and growth with shear flow and electric field experiments continued. 7)

- Rubi generates bubbles using a special heater to expand our knowledge of the boiling process in microgravity. Boiling is relevant for many industrial applications, such as energy conversion, food and chemical processes.

- Larger and slower bubbles will allow scientists to observe and measure effects that are too fast and too small on Earth. Results could be key to design cryogenic fuel storage, more compact laptops and cooling electronics in space.

Figure 4: The Multiscale Boiling experiment recently conducted on ESA's 71st Parabolic flight campaign studies boiling process in altered states of gravity. While the boiling process on Earth takes place quickly, in weightlessness the liquid to vapour phase change happens more slowly, allowing researchers to study this seemingly simple process in greater detail. Understanding heat transfer will lead to precise models to create more heat efficient products (video credit: Novespace)

• September 10, 2019: Things got heated on the International Space Station this week after the Multiscale Boiling experiment, known as Rubi, was successfully switched on. 8)

- ESA astronaut Luca Parmitano installed the shoe box-sized container studying the boiling process in the Fluid Science Laboratory of the Columbus module after its arrival on a Dragon cargo resupply mission in August.

- The experiment is now in full swing and generated its first bubble under controlled conditions.

- Scientists will also observe and quantify the effect of external forces on the boiling process.

- Rubi is equipped with an electrode to assess the effect of the electric field on the bubbles, as well as a small pump that, when activated, will get the liquid moving to evaluate flow on the boiling process.

- Why space bubbles? - Scientists are investigating the boiling process in space mainly for two reasons.

- Running this experiment in weightlessness has the advantage that the boiling process takes place in slow motion and the bubbles generated are much larger than on Earth allowing researchers to investigate details of the process in high resolution.

- Boiling is a common process in many applications such as refrigeration or cooling of high-performance electronic devices. This research will provide valuable information for designing thermal management systems in a more efficient way, both in space as well as on Earth.

- Rubi will run for five months on the International Space Station, during which time more than 600 test runs are planned.

- Follow the Rubi experiment on social media for regular updates and more bubbly images and videos.


Figure 5: While the bubbles form, a number of measurements are taken. The temperature sensor in the left of this image measures bubble temperature while a high-speed camera records how the bubbles behave and an infrared camera tracks the temperature of the heated region (image credit: ESA)

• On 27 July 2019, two days after its launch from Florida, the SpaceX Dragon cargo spacecraft was installed on the Earth-facing side of the International Space Station’s Harmony module at 12:01 p.m. EDT. 9)


Figure 6: July 27, 2019: International Space Station Configuration. Five spaceships are parked at the space station including the SpaceX Dragon cargo craft, Northrop Grumman's Cygnus space freighter, the Progress 72 resupply ship and the Soyuz MS-12 and MS-13 crew ships (image credit: NASA)

- The 18th contracted commercial resupply mission from SpaceX (CRS-18) delivers more than 5,000 pounds of research, crew supplies and hardware to the orbiting laboratory.

- A key item in Dragon’s unpressurized cargo section is International Docking Adapter-3 (IDA-3). Flight controllers at mission control in Houston will use the robotic arm to extract IDA-3 from Dragon and position it over Pressurized Mating Adapter-3, on the space-facing side of the Harmony module. NASA astronauts Nick Hague and Andrew Morgan, who arrived at the station Saturday, July 20, will conduct a spacewalk in mid-August to install the docking port, connect power and data cables, and set up a high-definition camera on a boom arm.

- Robotics flight control teams from NASA and the Canadian Space Agency will move the docking port into position remotely before the astronauts perform the final installation steps. IDA-3 and IDA-2, which was installed in the summer of 2016, provide a new standardized and automated docking system for future spacecraft, including upcoming commercial spacecraft that will transport astronauts through contracts with NASA.

- After Dragon spends approximately one month attached to the space station, the spacecraft will return to Earth with cargo and research.

• On July 27 2019, Dragon was captured at the ISS. While the ISS was traveling over southern Chile, astronauts Nick Hague and Christina Koch of NASA grappled Dragon at 9:11 a.m. EDT using the space station’s robotic arm Canadarm2. 10)


Figure 7: The SpaceX Dragon is in the grips of the Canadarm2 robotic arm shortly after it was captured over southern Chile (image credit: NASA)

- Ground controllers will now send commands to begin the robotic installation of the spacecraft on bottom of the station’s Harmony module.

RUBI Science

• August 8, 2019: Bubbles are soon to be made in space as part of an experiment that combines scientific insight with an objectively cool process on International Space Station. 11)

- The Reference mUltiscale Boiling Investigation experiment, known affectionately as RUBI, aims to expand our knowledge of the boiling process.

Figure 8: Bubbles in altered states of gravity. Understanding how boiling behaves in weightlessness is imperative because gravity plays an important role in this process. Without gravity, boiling takes place in slow motion and produces larger bubbles. This will allow scientists to observe and measure effects that are too fast and too small on Earth. With this insight and more accurate calculations of the boiling process, products such as laptops can be improved and made more compact (image credit: Technical University Darmstadt)

Plug and boil

- A lot of science will take place in a container the size of a large shoebox. Built by Airbus for ESA and housed in the Fluid Science Laboratory in the Columbus module, RUBI will generate bubbles under controlled conditions using a special heater.

- A high-speed camera will record how the bubbles behave, while an infrared camera measures the temperature of the heated region.

- It sounds simple enough, but what makes RUBI complex is that scientists are eager to observe and quantify the effect of external forces.

- With no gravity to disperse the bubbles, the science teams installed an electrode to observe the effect of an electric field on the bubbles.

- The experiment container also contains a small pump that, when activated, will get the liquid moving to evaluate the effect on the boiling process.


Figure 9: Illustration of the RUBI experiment (image credit: Airbus)

Why space bubbles

- "Making sure equipment and computer chips stay at the right temperature is of vital importance, otherwise their lifetime, as well as their performance, could decrease abruptly," says ESA project scientist Daniele Mangini.

- “Boiling is an extremely efficient way of getting rid of excess heat. It could therefore be used to keep components of future spacecraft at their optimal temperature,” continues Daniele.

- Back on Earth, better heat transfer technology means a lower impact on nature, as products such as laptops can cool down more efficiently.

- ESA astronaut Luca Parmitano will install RUBI on 9 August and the experiment will run for five months on the International Space Station, during which time more than 600 test runs are planned.


Figure 10: Multiscale Boiling experiment science team. Members of the RUBI (Reference mUltiscale Boiling Investigation) science team with ESA project scientist, BUSOC (Belgium User Support Operations Center) operators, and Airbus during a meeting at ESA's technical center in the Netherlands (image credit: ESA)

Institutions of the Multiscale Boiling Science Team:

• Technische Universität Darmstadt, Institute of Technical Thermodynamics

• Aix Marseille Université

• University of Pisa

• Institut de Mécanique des Fluides de Toulouse

• Institute of Thermal-Fluid Dynamics, ENEA

• Transfers Interfaces and Processes, Université Libre de Bruxelles

• LAboratoire PLAsma et Conversion d’Energie Université Paul Sabatier

• Università degli Studi di Padova

• University of Thessaloniki

• University of Ljubljana

• Institute of Thermophysics, Novosibirsk, Russia

• Kobe University

• Hyogo University

• University of Maryland

1) Catherine Colin,Olivier Kannengieser,Wladimir Bergez,Michel Lebon,Julien Sebilleau,Michaël Sagan,Sébastien Tanguy, ”Nucleate pool boiling in microgravity:Recent progress and future prospects,” Science Direct, Volume 345, Issue 1, January 2017, Pages 21-34,, URL:

2) Nils Schweizer, Marco Stelzer, Olaf Schoele-Schulz, Gerold Picker, Hans Ranebo, Jan Dettmann, Oliier Minster, Balazs Toth, Josef Winter, Lounes Tadrist, Peter Stephan, Walter Grassi, Paolo di Marco, Catherine Colin, Gian Celata Piero, John Thome, Oleg Kabov, ”RUBI -a Reference mUltiscale Boiling Investigation for the Fluid Science Laboratory,” 38th COSPAR Scientific Assembly, 18-15 July 2010, Bremen, Germany

3) CatherineColin, OlivierKannengieser,WladimirBergez, MichelLebon, JulienSebilleau, MichaëlSagan, SébastienTanguy, ”Nucleate pool boiling in microgravity: Recent progress and future prospects,” Science Direct, C. R. Mecanique, Volume 345, Issue 1, January 2017, Pages 21-34,

4) ”RUBI – Full steam ahead for the ISS,” Airbus DS Press Release, 2 July 2019, URL:

5) ”SpaceX Soars Sending a Dragon on a Falcon to Resupply the ISS ... CRS-18 Mission Success So Far,” Satnews Daily, 25 July 2019, URL:

6) ”Boiling. We research. You benefit,” ESA Science & Exploration, 9 June 2020, URL:

7) ”Behind the scenes – space-age physics — Space bubbles,” ESA, 13 November 2019, URL:

8) ”Space Bubble,” ESA, Space in images, 10 September 2019, URL:

9) Mark Garcia, ”Dragon Installed to Station’s Harmony Module for Cargo Operations,” NASA, 27 July 2019, URL:

10) Mark Garcia, ”Dragon Captured With New Science Experiments,” NASA, 27 July 2017, URL:

11) ”Bubbles in space,” ESA human and robotic exploration, 8 August 2019, URL:

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 (

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