Ferrofluid Reaction Wheel

To work correctly, most satellites require the possibility to stabilize and control their orientation. It is usually achieved by ADCS system composed of magnetotorquers or reaction wheels. The idea for the mission experiment is to develop a reaction wheel in which ferrofluid would rotate. The first fluid-based reaction wheel using the induction pump mechanism was successfully constructed and tested by students of Technische Universität Berlin in their satellite TechnoSat. We are proposing a different approach to fluid acceleration. Ferrofluid Reaction Wheel uses 8 coils placed in a circular pattern around a pipe filled with ferrofluid. Letting electricity through the coils attracts the ferrofluid to them. Provided correct control sequence on the coils, the ferrofluid is put into motion. Acceleration of ferrofluid provides momentum which can be used to control satellite orientation. More information about construction and functioning of the Ferrofluid Reactio Wheel can be found in: Dominik Markowski, "Construction of the stabilization system for nanosatellites using ferrofluid", BSc Thesis, AGH University of Science and Technology, Cracow 2017 supervisor: Alberto Gallina and it the thesis: Jan Życzkowski, "Teoretical simulation of stabilization system for nanostelites based on the ferrofluid.", BSc Thesis, AGH University of Science and Technology, Cracow 2018 supervisor: Alberto Gallina

Ferrofluid

Ferrofluid is a colloidal liquid, and in terms of the size of dispersed particles (less than 10 nm) it is a suspended matter. A mixture of this type is stabilized by surfactants, where apart from the dispersive liquid, the main component is a spinel containing ferrosoferric oxide – FeO·Fe2O3, commonly known as magnetit. In those types of oxides, the occuring iron is characterized by two different electron configurations, representing the 2nd and 3rd level of oxidation of iron cations for which we can observe unpaired electrons. The presence of unpaired electrons endowed with a constant magnetic moment and near atomic spacings in the spinel lead to the occurence of magnetic domains. As a result, the interaction of an external magnetic field with those types of materials results in the change of length and orientation of magnetic domains. The effect of the process on ferromagnetic liquid located in a magnetic field is the organization of magnetic moments in the direction of the field strength H. This leads to the magnetization of the ferrofluid and examination of the resultant movement of its particles along with the dispersive liquid. Another crucial component of the ferrofluid is the stabiliser of magnetically active particles in the dispersing phase; this includes non-polar dispersing agents. The action of this type of surfactant can be described as surface coating of magnetic molecules, i.e. accumulation at the interface: solid – liquid, leading to their mutual repulsion in the volumic quantity of the fluid. These types of surface surfactant interactions lead to system stabilization and inhibition of the aggregation and sedimentation process. The magnetic particles together with the surface adsorbed stabilizing agents are dispersed in the dispersion liquid, which is a light distillate of crude oil treated with ICP hydrogen. This type of system, obtained based on the components presented, is characterized by high stability at operating temperatures below 150 °C (boiling point around 223 °C) and low vapor pressure of 0.1 kPa at 20 °C and also low viscosity at 6 cP. The presented mixture is characterized by a high magnetic susceptibility of the active substance and thermal stability as well as a relatively low vapor pressure and viscosity at the working temperature, making it a stable, safe and suitable environment for further tests under appropriate measuring conditions. The physicochemical data of the material are presented in tables bellow.

The dependence of pressure on temperature was determined in the apparatus shown on the image bellow. The 12 mL measuring chamber was entirely filled with the test liquid. Then, the Furnace was heated and temperature was recorded at certain pressure values.

The results from the pressure measurement experiment at given temperatures presented in the table.

The results from table are presented in the graph below. Based on the presented measurements, it is stated that the dispersed system will not achieve a pressure higher than than 3 Bar at the operating temperature.

F.R.W. mechanical construction

KRAKsat payload consists of following parts:

8 C-shaped electromagnets grouped in 4 separate phases,
steel ferrofluid container,
aluminium ferrofluid wheel case,
aluminium ferrofluid wheel case cap,
two core separators,
temperature sensor.

Wires are routed through central hole in FRW case and connectors on PCB.

F.R.W. electromagnets

F.R.W. electromagnets were made of Armco steel. Huge effort was taken to choose the right materials. After conducting a large number of tests, Armco steel was selected as a material with appropriate magnetic properties and capacity to survive rocket launch and work in an orbit environment. The elements were fabricated using CNC milling. In order to optimize power consumption and to avoid electric cable burnout, coil wire was chosen with the following parameters: ‌

number of turns: 320
wire diameter 0.3 mm
coil resistance 5 Ohm

Mechanical details

Material properties

Armso steel is used largely as the basic material for (re-)melting of low-carbon, stainless and acid-resistant steels, materials with a high nickel content, magnetic alloys as well as stainless and heat-resistant steel castings in induction and vacuum furnaces. Has many applications in aviation construction, nuclear technology, production of magnets (pole cores, yokes and armatures), automotive construction, as magnetic shielding, as welding rods and fuse wire, as gasket in the chemical and petrochemical industry, power station construction, as anti-corrosion anode and as galvanizing tank including equipment.

Assembling details

The subchapter will be presented in the next release of the documentation.

F.R.W. ferrofluid container

Ferrofluid container was made of stainless steel 316L. It was machined as two separate parts, which were later connected together using laser welding. In order to manufacture the tank, individual welding technology was developed and several companies were involved. More information about manufacturing process of the container can be found in: Rafał Janiczak, "Realization and examination of technology of welding a ferrofluid reservoir used in cosmonautics", Master Thesis, AGH University of Science and Technology, Cracow 2019 supervisor: Krzysztof Pańcikiewicz.

Assembling documentation

The container was welded using the 521-TruLaser Cell 7040 TRUMPF device. First, the beam parameters were selected as in the samples shown below.

The tank was welded using the appropriate parameters (WPS) of the laser beam.

WPS - Welding Procedure Specification

Closing the container

Before closing, the tank was filled with ferrofluid.

The container was closed with an aluminum screw. The thread was sealed with space-ready Loctite 638. More information about the conteiner can be found in Rafał Janiczak, "Realization and examination of technology of welding a ferrofluid reservoir used in cosmonautics" , Master Thesis, AGH University of Science and Technology, Cracow 2019 (supervisor: Krzysztof Pańcikiewicz).

Tests

Weld tests

Tests of welded joints were carried out. These were radiographic, penetration, macroscopic tests, as well as mechanical stretching and bending tests. Protocols for these tests are provided in the attachment below.

After filling the tank with ferrofluid, pressure, vibration and temperature-vacuum tests were carried out.

Pressure Tests

To check the tank for leaks, an underwater leak test was performed. A hose was connected to the tank under an air pressure of 0.8 MPa and the tank was placed in a container filled with water. The analysis was conducted for the occurrence of one of two indications: escaping air bubbles and a decrease in air pressure on the manometer.

Temperature-Vacuum Test

The tank filled with ferrofluid placed in low vacuum chamber was placed in a temperature chamber. To check the tightness of the tank, the temperature was changed twice from 85 °C to - 40 °C. Varying temperature showed no effect on the tank in vacuum conditions (10e-3 Pa) .