Fluid Actuator Conduits for CubeSat Applications

Design of pFDA conduits starts out from the conduit presented by Noack in [108]. Due to its nature, the conduit of Noack’s laboratory demonstrator is not fit to be directly used on CubeSats.

5.4.1 Conduit Considerations

A circular conduit was designed by Noack against the requirements of having a maximum diameter of 80 mm, a maximum fluid mass of 20 g, and being able to store at least 150 µN m s of angular momentum [108]. Application of eq. (5.7) leads to a required mass flow rate of at least 15 g/s.

pFDA conduit geometry, that better fits the cubic shapes of CubeSat subsys-tems and structures, has square outer dimensions and square cross section.

Prospected mounting of three pFDAs on CubeSat solar panels calls for square outer dimensions with a maximum length of 80 mm to leave enough space for CubeSat rails and satellite structure. With fluid mass being limited to 20 g and taking the conduit wall thickness into account, conduit cross section width is calculated. This width is found to be 13 % smaller than the diameter of the laboratory demonstrator conduit. Applying eq. (5.7) again under the requirement for 150 µN m s of angular momentum yields a required mass flow rate of at least 13.4 g/s. This is a reduction of about 10.6 % in comparison to the conduit presented by Noack in [108].

A reduction in required mass flow rate should lead to a reduction in power demand of the pump. This is contrasted by the increased friction losses due to the increased length of the channel and the sharp bends introduced by the square conduit. To mitigate the influence of the sharp bends, a square with rounded corners is used as outer shape of the conduit. The resulting conduit geometry was first presented by Grau et. al in [63].

5.4.2 Pump Housing

Inside the pump, conduit cross-section needs to change from square to flat rectangular (cf. [107]). The large magnetic flux needs to be shielded and is therefore guided using two iron pole pieces. Both, housing and pole pieces, require openings to feed the electrodes into the pump. Electrodes and pole pieces need to be electrically isolated from each other.

Design of pump inlet and outlet geometry is intricate: the circular or square conduit cross-section needs to be transformed to the flat rectangular shape while maintaining a constant cross-sectional area. To produce the pump, inlet, and outlet geometry using traditional manufacturing processes like milling, the pump housing needs to be assembled from two pieces. The two pieces need to have additional features for mechanically aligning and fastening them. Additionally, this will result in redundant housing material and therefore increased actuator mass. If the conduit is made from tube, additional contact surfaces are required at the inlet and outlet to the pump in order to glue the tube to the pump housing, which further increases actuator mass.

As a solution to this, the author proposed the monolithic integration of the pump housing into the conduit geometry in [63]. In this design, the pump sits in one of the corners of the square/square conduit, and free-form inlet and outlet directly connect the pump with the channel. Features to align the magnets, feed the electrodes to the pump, align the pole pieces, and electrically isolate the pole pieces from the electrodes are also directly integrated with the conduit structure. The monolithic, integrated design allows to reduce the amount of redundant housing material, as no additional features for mechanically aligning and fastening the pump housing or connecting the channel to the pump are required.

5.4.3 Actuator Electronics

Actuator electronics of the laboratory demonstrator consist of a DC/DC converter patented by Noack in [106], a microcontroller, a MEMS gyroscope, and peripheral circuitry like voltage regulators (cf. [108], figure 5.1 on page 58).

Design and development of a miniaturized version of the actuator electronics is described in this work in section 5.5 from page page 74 onwards. At least the components of the DC/DC converter need to sit very close to the pump housing, in order to keep the high-current feed lines as short as possible. This requires support features integrated with the monolithic conduit structure, where the pump driver PCB could be inserted and attached, which is addressed by the author in [109].

5.4.4 Manufacturing of Monolithic, Integrated Conduits

Manufacturing of monolithic, integrated conduits is difficult, due to their complex shape. Having only the electrode openings left for access to the inside geometry, injection molding with lost core would be a traditional manufacturing technology allowing to produce this geometry. Injection molding allows to use plastics as conduit material, which fulfills the requirement of electrical isolation between the fluid, conduit walls, electrodes, and pole pieces (cf.

section 5.4.2). In addition, injection molding results in good surface finish, which is important for the inside walls of the conduit in order to not increase the flow resistance due to excessive wall roughness. However, due to the multiple molding stages required, manufacturing costs are too high to be used in the scope of this project.

With the rise of additive manufacturing technologies over the last decades, low-cost alternatives to injection molding exist. From the diverse 3D-printing technologies and materials, selective laser sintering (SLS) from polyamide powder was selected. The advantages of this combination are plenty:

– Full flexibility in conduit geometry, with wall thicknesses down to 0.7 mm.

– Low-cost procurement with short term delivery from dedicated suppliers over the Internet.

– Similar coefficient of thermal expansion as the used metallic fluid.

– High mechanical and temperature stability of the printed parts.

While polyamide, better known as Nylon, is widely used in space applications, its hygroscopic behavior has caused catastrophic failures on space missions due to out-gassed and condensed water vapor, as stated by Gross in [110].

The use of Nylon for space applications is therefore not recommended. With PEEK, a more expensive, alternative material is readily available for SLS printing. For functional verification, however, polyamide represents the better choice.

Additional disadvantages of using parts produced by SLS from polyamide powder are rough surfaces and removal of residual powder from the interior of the channel. Solutions to those problems are addressed by the author in [109].

While removal of the powder from the conduit is just a time-consuming task that requires manual labor, surface treatment inside the conduit is difficile.

Watzinger in his Bachelor’s Thesis [111], conducted under the supervision of the author, developed a process to improve the interior surfaces of 3D-printed polyamide parts using acids under increased temperatures. While the results regarding surface improvements are more then promising, the long term durability of the treated polyamide parts is severely impaired.

5.4.5 First Rapid Prototyping Experiences

A first specimen of a monolithic, integrated conduit is shown in figure 5.6.

The outer dimensions of the conduit are 80×80 mm, and three flanges are added for mounting onto a piece of veroboard. Conduit mass is less than 4 g, and it can hold about 18 g of liquid metal. The presented conduit is not yet equipped with mounting features for the pump electronics, as it was meant for gaining experience with 3D-printed parts and powder removal. The main usage is for functional demonstration on a gas bearing with the electronic components mounted on veroboard (cf. sections 5.5 and 5.6).

Figure 5.6:First monolithic, integrated fluid-dynamic actuator conduit [112]

5.4.6 Conduit Geometries for CubeSat Applications

Following functional verification of the first integrated conduit and the develop-ment of a miniaturized version of the pump driver electronics (cf. section 5.5), the conduit is adapted to be integrated with the pump driver and the satellite.

Gridded support structure is added to the bottom of the actuator, which can be seen in figure 5.7. This increases the contact area between the conduit structure and the solar panel, to which the actuator will be glued (cf. chap-ter 6). The gridded structure at the bottom follows the design of BEESAT’s primary structure, and therefore allows to use the same solar panel on sides with or without pFDAs.

Additional structure was added to support the electronics board and the transformer used with the pump driver. Thus preventing damage to those components due to mechanical loads during rocket launch. To increase satellite stiffness, the battery compartment on the BEESAT satellites is connected with the solar panels and the outer structure using two screws per side. To use the same screws with the pFDAs, the conduit is further equipped with two sleeves, that later incorporate metallic spacers to have a solid mechanical and electrical connection between the solar panels and the battery compartment.

Figure 5.7:Monolithic, integrated fluid-dynamic actuator conduit [112]

Figure 5.8: Circular integrated conduit used for student projects

5.4.7 Advantages of Conduit Rapid Prototyping

One of the advantages of conduit rapid prototyping is the ability to quickly design and produce new conduit variants. This ability was demonstrated with the development of a circular conduit with square cross section (cf. figure 5.8).

This actuator is used for education of aerospace engineering students in the scope of the spacecraft dynamics and control lecture at TU Berlin.

Design of the adapted version was done in less than one day, printed parts were shipped within one week. This shows the great flexibility of the selected manufacturing process and the potential for adapting conduit geometry.