Cubesat Midterm

CubeSat Midterm

Kevin Scott

Robert Kelly

Austin Rogers

Joseph Kingett

Matthew Degroff

Old Dominion University

MAE 435

Dr.Bawab

19 October 2016

Table of Contents

Abstract2

Introduction3

Completed Methods5

Proposed Methods 7

Results9

Discussion13

References15

Appendix 116

Appendix 217

Appendix 3 18

Appendix 4 19

Appendix 520

List of Figures

Figure 1: Computer rendering of CubeSat structure6

Figure 2: Temperature ranges for March9

Figure 3: 2-D Orbit View11

Figure 4: 3-D Orbit View12

Abstract:

With the low cost of CubeSat manufacturing and launching, opportunities are now available to smaller budget organizations to be able to reach space. For over a decade now, picosatellites have been used in place of large, costly satellites. They are ideal for short-lived missions to collect data without creating much space debris due to their size and small orbital lifetime. For this mission Old Dominion University, the University of Virginia and Virginia Tech will each develop a 1U CubeSat in the constellation, while Hampton University will compile and interpret the atmospheric data collected by the constellation. Each 1U CubeSat will be deployed directionally, with respect to the primary orbit of the host vehicle. One CubeSat will be designed to be the primary communication satellite and will first establish a communication link with the ground. Subsequently, the other two CubeSats will establish communication links with the primary CubeSat. After the three CubeSats have established their primary communication links, the CubeSat serving as the communications hub will signal the ground station, initiating mission operations and performing systems checks. Since an actual satellite will be fabricated (by Feb. 2018), the project must span multiple semesters of ODU mechanical and aerospace engineering and electrical and computer engineering senior capstone design classes.

Introduction:

It is difficult to predict a satellite’s deorbit in the thermosphere because density in this stratum does not conform to standard atmospheric models [1]. There are many models for estimating atmospheric density in this region [2]; however, the most accurate models of these are only accurate to within ± 15 percent [3]. The Old Dominion University (ODU) CubeSat, which will be launched in a constellation with other CubeSat satellites designed and built by students at the University of Virginia and Virginia Tech, is designed primarily to improve our understanding of density variation in the thermosphere using data obtained from a micro-G accelerometer or a global positioning sensor. The ODU satellite will use these instruments to attempt to measure its negative acceleration. The CubeSat position data, along with the drag coefficient, will be used to determine local instantaneous atmospheric density. The constellation will likely be launched from the International Space Station roughly 400 km above Earth’s surface. ODU’s plan is to separate from the other CubeSats and use an onboard drag brake in order to change its ballistic coefficient.

Communication is necessary in order to collect and use the data from the CubeSats. To accomplish this, the ODU CubeSat must be able to communicate with the ground station at ODU as well as with the other satellites in the constellation. The three CubeSats will all be equipped with a Astrodev Lithium Li-1 radio. Using a standard radio for the three universities will allow easier communication to the ground station and the other CubeSats.

There is a size limitation in the design, especially for ODU. The CubeSat will be equipped with an Electronic Power Supply (EPS) that will require 30 percent of the overall 1000 cm3 volume. In addition to this requirement, the ODU CubeSat’s drag brake will place another space restriction on the size of the communications equipment, because the drag brake will require approximately 25 percent of the available volume.

The CubeSat can only function if it has enough power to operate its equipment. For this to happen, the onboard solar arrays must be oriented properly; otherwise, the battery will charge too slowly and would have to wait until the next orbit to recharge. To control the CubeSat, three-axis attitude control, which allows the satellite to be stabilized and oriented as desired, is required for flight. This research will focus on using a simple proportional derivative controller for the attitude control system because of its ease of use [7]. In the past, CubeSats have used passive systems, which is generally a bar magnet mounted inside the cube. This passive system would allow the CubeSat to orientate to the earth's natural magnetic field; unfortunately, the ODU CubeSat will require more specific attitude determination due to the added drag brake.

Waste heat from the communications and attitude control systems, in combination with external temperature changes of up to 200 C, requires that the satellite include thermal controls to prevent components from overheating. Most of the heat the CubeSat will be exposed to will come from the sun [8]. For now, the design team and the electrical engineering teams are keeping track of where components go and how much heat they produce. Since there is only conduction and radiation in the thermosphere, the danger of overheating comes from the sun and the individual components heat. From design to delivery, a Cubesat requires multiple engineering disciplines. The structure, communications, attitude determination and control, programming, and thermal control systems are all vital to the development and proper functioning of the ODU CubeSat. The purpose of this design project is to develop a CubeSat prototype with a working attitude control system necessary to allow the satellite to complete its scientific objectives.

Completed methods:

The orbital lifetime simulations were done in Satellite Toolkit (STK) (Analytical Graphics Inc., Exton, PA). The International Space Station (ISS) is a likely point of satellite deployment, and has a desireable orbit with well-defined characteristics that were used for simulations: an altitude of 400 kilometers, an orbital inclination of 51.6 degrees, and an orbital period of approximately 92 minutes (Appendix 1). The orbital lifetimes were roughly calculated using the ISS initial orbital parameters, along with an assumed CubeSat mass of 1.3 kilogram. The drag brake is a disc with a nominal diameter of 1 meter, and resulting wetted (front-facing) surface area of 0.785 m2 post-deployment, while that of the other CubeSats was defined as the area of one cube face, or 0.01 m2. The drag coefficient Cd was assumed to be 2.2 for all satellites, giving a ballistic coefficient[1] of 59.1 kg/m2 for all satellites prior to drag brake deployment, and 0.753 kg/m2 for ODU’s satellite after brake deployment.

Thermal control was an area that the group did not have experience with so research was necessary. A custom MATLAB (The Mathworks, Natick, MA) program to simulate the effect of solar radiation on a CubeSat was produced to help with thermal modeling (Appendix 2). Thermal tapes such as teflon tape can provide the protection that the CubeSat will need, so these tapes were acquired for further testing. In addition to studying the effects of the solar radiation on the CubeSat, work was started on creating a thermal model to verify that the electronics in the CubeSat can all operate without elevating the temperature beyond the safe operating conditions. A geometry of the CubeSat was created in Patran (MSC Software, Newport Beach, CA) for the thermal analysis. To help model the boundary conditions, the team created a power consumption budget for the electrical components.

Three design iterations of the satellite structural geometry were completed in AutoDesk Inventor (Autodesk, San Rafael, CA), two of which were printed from a 3D printer using standard ABS plastic. The final prototype design, Figure 1, incorporates four nanorack rails, a separate compartment for the drag brake, and removable panels for the front and side faces of the CubeSat.

In addition to designing the structural geometry, the team started on a structural analysis of the CubeSat. The team researched the mechanical properties of aluminum and windform XT 2.0 to determine the advantages of each material. The aluminum was found to have a greater strength than windform, but was over two times the weight. A basic model of the final prototype design was created out of steel to test the load and boundary conditions on the CubeSat, while a more detailed model is still being built.

Figure 1

When developing the attitude control system, the first step was to determine the flight dynamics. The basic equation for attitude dynamics was determined through research to be the following [7].

Iω'=-ω×Iω+Tcontrol+Tdisturbance(1)

A code was implemented from SparkFun library that can be used to find roll, pitch, and yaw. It has been adapted for future testing code.

A prototype magnetorquer was created for proof of concept testing. The magnetorquer has a conductive core and approximately 800 turns of 26 AWG wire in 2 layers.

Proposed methods:

The date required for a completed CubeSat is 28 April 2017. This date is past the scope of this 435 class. The 435 group’s next major milestone is the critical design review due 24 February 2017. This critical design is where the design and function of the ODU CubeSat must be completed. With the integration of the 434W and the 435 groups, the progress for the Fall 2016 semester has been expedited. Currently the structure is undergoing minor changes that allow for the substructure stanchions that hold the boards to be attached. The structure and prototyping group will be moving next to material selection and part manufacturing.

The structure is still undergoing modifications as the satellite component orientations are being determined. The material selection for the structure has been narrowed down to either Windform (CRP Technology, Modena, Italy) or aluminum.

The electrical team has completed part selection and is moving towards ordering the board components. These roughly take eight weeks to deliver. The structure will be completed in the next eight weeks, and the complete CubeSat will begin to be assembled. This puts ODU ahead of schedule by three months; as the assembly date is not scheduled until 28 February 2016.

The next phase of ADCS testing will be a rudimentary single axis test of the kill roll program using the prototype conductive core magnetorquer. This will be the proof of concept trial for future ADCS controller development. After the single axis kill roll test, a more thorough three axis control program will be written. In addition, the prototype conductive core magnetorquer will be replaced with a flat-wound air core magnetorquer. This will more accurately simulate the magnetorquers that are integrated in the solar panels chosen for the project. Performing a test of the three axis control program will be much more involved than the previous test, requiring an air bearing test bed not available through the university.

The STK simulations will be expanded to more accurately include the satellites from the other teams as their characteristics become available. A major focus of the STK simulations is also to upload a 3D model of the satellite so that flight coordinates can be simulated, for reasons of attitude control and sun vector acquisition. In addition to the STK simulations, there are more precise simulations of atmospheric density that will allow accurate recreation of the conditions that the CubeSat will experience during its lifetime. The local mean free path, or the mean distance traveled by a gas molecule before colliding with another molecule, is closely related to the aerodynamic drag the satellite will experience in the low-density environment of the thermosphere. The mean free path was simulated by the previous senior design team using software from NASA, and the simulations will be expanded upon as well. Power simulations are being done using both STK’s Solar Panel Tool, and using open-source MATLAB code.

Results:

Preliminary research was conducted for the chassis design, thermal control, attitude control, and trajectory simulations. Sensors and other parts were purchased by the group for prototyping and testing purposes, and to become acquainted with the process of assembling some of the components of a small satellite’s hardware suite.

Early designs for the CubeSat chassis were completed using Autodesk Inventor (Figure 1) and then 3D printed out of standard acrylonitrile butadiene styrene (ABS) to assist with visualization of any changes that needed to be made, and visualization of the amount of space that will be available after installation of the drag brake compartment. This chassis allows experimenting with potential interior layouts to find the optimal configuration.

Hardware was chosen to be used in the thermosphere density measurement experiment. A very powerful accelerometer and GPS system can be used to track the CubeSat as it slows in orbit. This will be used to measure the drag, which will allow calculation of the atmospheric density at those data points. The GPS system will be a challenge because it can be a drain on the limited power budget, as well as the logistics of working through ITAR to get approval. Other hardware used to determine the orientation of the CubeSat, a sun sensor and a magnetometer, were chosen for the final CubeSat. These will need to be tested with magnetorquers to make sure these pieces of hardware work together with the attitude control system code once it is finished.

Following the research into thermal control methods, a program was created and used to simulate the thermal environment the satellite must survive. The unprotected CubeSat was simulated to have outer surface temperatures ranging from 330°K and 250°K (Figure 3). Several different kinds of thermal tape that met the mission’s requirements were chosen to be tested for efficiency in the future: a golden mylar tape, an aluminum tape, and a teflon tape.

Figure 2

This figure shows what temperature each face experiences over time in the month of March. It is worth noting that face 6 is on the side of the CubeSat that does not experience direct sunlight. That is seen when the maximum temperature that side experiences after it has stabilized is only 260°K and it can get as low as 255°K. Each side must be able to withstand very low temperatures like this, and survive warmer temperatures as high as 315°K experienced by the faces in more direct sunlight. These calculations are now being used for further finite thermal analysis for the internal components.

Simulations of approximate orbit were created in (STK) using the mean orbital characteristics of the International Space Station. Rough simulations were also made for the accompanying CubeSats, which will have no drag brakes. The lifetimes of all three CubeSats were estimated to be between 1.2 and 1.5 years before deployment of the drag brake. The orbit lifetime of the ODU CubeSat after deployment of the drag brake was estimated to be between 6 and 7 days, depending on full drag brake deployment, the changing mass of the satellite which is still being designed, and the atmospheric density model used. Figures 3 and 4 are snapshots from the STK simulation, showing the orbits of ISS in green, ODU’s CubeSat in cyan (subsequent to its drag brake deployment), and the remainder of the constellation in red. Figure 3 is the 2-D view, and Figure 4 is the 3-D view. Appendix 3 contains the tabulated results of the STK orbit lifetime estimations for each atmospheric model.

Figure 3

Figure 4

Coding has begun for attitude control testing on the Arduino. First the breadboard was assembled with a teensy 1.3.2 board to act as the Arduino. This board was connected to a 9 degrees of freedom chip (9DOF) which has a magnetometer, gyroscope, and accelerometer, all of which will be necessary to determine the orientation of the satellite. The library for the chip was obtained and connected to the arduino app to be used for the code for the 9DOF. The code was implemented and tested, and it outputted the magnetic field in 3 directions, the gyroscopic roll rate in 3 directions, the acceleration in three directions, as well as the roll, pitch, and heading. Then the library was used to develop a kill rotation code that is still in development, and should have its first tests at a later date.

Discussion:

For this mission Old Dominion University, the University of Virginia and Virginia Tech will develop a 1U CubeSat in the constellation, while Hampton University will compile and interpret the atmospheric data collected by the constellation. One CubeSat will be designed to be the primary communication satellite and will first establish a communication link with the ground. Subsequently, the other two CubeSats will establish communication links with the primary CubeSat. The goal is to gather data on the atmospheric density. will be accomplished by gathering accelerometer and GPS data, which can be used to calculate the coefficient of drag as well as density.

There are several challenges that will affect the design of the CubeSat. These include size constraints placed on the CubeSat which restrict the overall size of the pre-deployed satellite package to a 1000 cm3 volume, a total mass of as little as one kilogram depending on the ultimate deployment system, and hazardous space conditions. Magnetic field orientation changes, solar activity, and extreme temperature variations are all potentially hazardous to spacecraft and on-board electronics. Atomic oxygen is also a thermospheric hazard that must be accounted for in the design process. The communications gear, attitude determination and control systems, and thermal controls will be designed to overcome these challenges. Reduction of the size of the drag brake could increase orbit lifetime and therefore the amount of data collected by the satellite, and also leave more space available for the rest of the components.