Cubesat Final Paper

CubeSat Final Paper

Kevin Scott

Robert Kelly

Austin Rogers

Joseph Kingett

Matthew Degroff

Old Dominion University

MAE 435

Dr. Bawab

05 December 2016

Table of Contents

Abstract2

Introduction3

Completed Methods5

Proposed Methods 8

Results10

Discussion15

References17

Appendix 118

Appendix 219

Appendix 3 20

Appendix 4 21

Appendix 522

Appendix 623

List of Tables and Figures

Figure 1: Computer rendering of CubeSat structure8

Figure 2: Temperature ranges for March11

Table 1: Comparison of aluminum and Windform12

Figure 3: 2-D orbit view13

Figure 4: 3-D orbit view14

Abstract:

Old Dominion University, the University of Virginia and Virginia Tech, working in cooperation with support from NASA and the Virginia Space Grant Consortium, are each designing 1U CubeSats that will be placed in low Earth orbits, flying initially in a constellation. These design projects exploit the relatively low-cost access to space provided by these secondary payload systems. Hampton University is leading the scientific aspects of the three satellite systems, focusing on the influence of solar activity on space weather. Each of the 1U CubeSats will be deployed from a host or primary orbit employing either a Poly-Picosatellite Orbital Deployer (PPOD) or Nanoracks delivery device. The CubeSat being designed and fabricated by Virginia Tech will serve initially as the primary communication satellite and will establish a communication link with one of several ground stations. 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 April. 2017), and must be delivered (by March 2018), the project must span multiple semesters of ODU mechanical and aerospace engineering and electrical and computer engineering senior capstone design classes. This is a status report of the overall design effort for the ODU 1-U CubeSat.

Introduction:

It is difficult to predict the terminal descent trajectory of satellites entering the lower atmosphere. A primary reason is the sensitivity of the thermosphere (atmosphere at altitudes between 90 and 1,000 km) to solar activity. It is not possible to employ a “standard atmosphere” model [1] in this altitude interval because the density at a given altitude and time of day can vary by more than one order of magnitude because of variations in solar activity. Those density variations translate directly to a similar order of magnitude change in drag forces acting on a satellite thereby altering their rate of descent into the lower atmosphere. 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 contribute to our understanding of the relationship between density variation and solar activity in the thermosphere using data obtained from a micro-G accelerometer and a global positioning sensor. The ODU satellite will employ these instruments to measure the micro-gravity negative accelerations that cause the CubeSat to descend. 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 (ISS), roughly 400 km above Earth’s surface. The plan is for the ODU CubeSat to separate from the other two CubeSats in the constellation, deploying an onboard drag brake to change its ballistic coefficient.

Communication with accessible ground stations will be critical to collect and use the data. 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 an Astrodev Lithium Li-1 radio. Standardizing the constellation radios will allow easier communication to the ground station and between the other CubeSats.

Each CubeSat must be equipped with an Electronic Power Supply (EPS); these modules require approximately 30 percent of the overall 1000 cm3 volume. In addition, the ODU CubeSat incorporates a drag brake design that will restrict further the volume available for the communications hardware. Currently, the drag brake design is expected to require approximately 15 percent of the available volume.

The CubeSat can only function if it has sufficient electrical power. Power will be produced using integrated solar arrays, but they must be oriented properly to produce maximum power. Since the probable orbit will expose the ODU CubeSat directly to the sun for about two thirds of each orbit, battery power will be required. If the solar arrays are not oriented properly, charging will be too slow and, in the best case, it may be possible to wait until the next orbit to recharge. To control the CubeSat orientation, three-axis attitude control, which allows the satellite to be stabilized and oriented as desired, is required for flight. Attitude control system research will focus on using a simple proportional-integral-derivative (PID) controller, exploiting its ease of use [7]. In the past, CubeSats have used passive systems, usually a bar magnet mounted inside the cube. This passive system can allow the CubeSat orientation to be maintained with respect to the Earth's natural magnetic field. The ODU CubeSat will require a more active attitude determination and control system due to solar array and drag brake orientation requirements.

Waste heat will be produced by the data acquisition sensors and hardware, radio communication, and attitude control systems. Furthermore, the external surface temperatures will vary by up to 200 K because of radiant exchange with the sun and with deep space during portions of each orbit. It will be necessary to control the internal CubeSat temperatures as much as possible to maintain the electronic components in their prescribed operating temperature ranges. The primary CubeSat heat source is the sun [8]. Presently, the mechanical design team and the electrical engineering design team are allowing for variable component location specifications to allow for distributed heating control. The absence of meaningful convection heat transfer in the thermosphere makes the danger of overheating from the sun more severe.

From design to delivery, a Cubesat requires multiple engineering disciplines. The structure, communications, attitude determination and control, programming, and thermal control systems are all integral and 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 and solar power simulations were done in Satellite Toolkit (STK) (Analytical Graphics Inc., Exton, PA). The ISS orbit is the most probable CubeSat satellite deployment orbit and has desirable, 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 1.33 kg, which would be the lowest total mass of the CubeSat possible and therefore would give the most conservative lifetime estimates. The drag brake is a four panel “flower petal” design, and resulting wetted (front-facing) surface area of 0.0302 m2 post-deployment. As a point of comparison, the other CubeSats in the Virginia CubeSat constellation have estimated cross sectional areas (one cube face) of 0.01 m2. The drag coefficient Cd was assumed to be 2.39 for a standard 1U CubeSat and 4.04 for a CubeSat with drag brake deployed, giving a ballistic coefficient[1] of 54.5 kg/m2 for all satellites prior to drag brake deployment and 10.9 kg/m2 for ODU’s satellite after drag brake deployment.

Spacecraft 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). In addition to studying the effects of the solar radiation on the CubeSat, work was started on creating a thermal model to estimate the thermal influences on the electronics within the CubeSat and ensure that they could 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 acrylonitrile butadiene styrene (ABS) plastic. The final prototype design, Figure 1, incorporates four NanoRack rails [9], 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 (CRP Technology, Modena, Italy) to determine the relative advantages of each material [6, 7]. The aluminum obviously had a greater strength than Windform, but is twice as dense and therefore double the mass. The structure of the CubeSat prototype was modeled in PATRAN to run a finite element analysis to estimate the stresses and deflections in the structure. The CubeSat must be able to withstand a maximum of +8 g’s axially and +3 g’s laterally during rocket launch [8]. Two models of the frame were completed under these load conditions. The two structures were simulated using the mechanical properties of aluminum or Windform XT 2.0 for the frame and polyimide for the the sides of the CubeSat. The results of the finite element analyses obtained from PATRAN showed that the aluminum would provide more structural rigidity than the lighter Windform (Table 1).

Figure 1 Three-dimensional model of the ODU 1-U CubeSat

When developing the attitude control system, the first step was to characterize the anticipated flight dynamics. The basic equation for attitude dynamics was determined through research to be the following [10]:

Iω'=-ω×Iω+Tcontrol+Tdisturbance(1)

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

Proposed methods:

The required delivery date for our flight-ready CubeSat is 28 April 2017. This date is outside the schedule for this MAE 435 class. The next MAE 435 class’ next major milestone is the NASA Critical Design Review (CDR) scheduled for 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. The structure is currently undergoing minor changes that allow for the substructure stanchions that hold the boards to be attached. The structure and prototyping group will be moving to material selection and part manufacturing next.

The structure is still undergoing modifications because the satellite flight coordinate orientation requirements are still being defined, based on the assumed ISS orbit. The material selection for the structure has been narrowed to either Windform or aluminum.

The electrical team has completed part selection and is moving toward ordering the board components. These components will likely have an eight-week delivery schedule. The structure will be completed in the next eight weeks, and the complete CubeSat assembly will be initiated. This puts ODU ahead of schedule by three months; as the assembly date is not scheduled until 28 February 2018.

Attitude Determination and Control System (ADCS) testing will continue in a virtual environment using custom MATLAB code. The first trials will test the “bang bang” control algorithm, designed to detumble the satellite after deployment from the launch apparatus. After this test, a more precise three axis control program will be utilized to attain the proper attitude prior to drag brake deployment. Finally, a two-axis control program (used post-drag brake deployment) will be tested.

Results:

Preliminary chassis designs have been examined with respect to thermal control, attitude control, and orbital trajectory characteristics. Sensors and other prototype components were purchased for testing purposes, as well as to become acquainted with the process of assembling some components in a small satellite hardware suite.

Early designs for the CubeSat chassis were completed using Autodesk Inventor (Figure 1) and then 3-D printed using standard ABS plastic to assist with overall visualization as well as exploration of possible changes that were being considered. These low-strength prototypes permitted visualization of the assembly and interference issues that will occur, particularly after integration of the drag brake compartment. This chassis allows experimenting with potential interior layouts to find the optimal configuration.

Hardware was selected for use in determination of local thermosphere density. A very sensitive, three-axis microgravity accelerometer and a GPS module can be used to track the CubeSat as it slows in orbit. The spatially-varying near-instantaneous acceleration will be used to determine the drag force, which, when combined with estimates of the frontal area and drag coefficient will allow calculation of the atmospheric density associated with those locations and times of day. The GPS orbital position system will be a challenge because typical commercially-available units can require excessive electrical power with respect to the overall spacecraft power budget. Additionally, the logistics needed to comply with the International Traffic in Arms Regulations (ITAR) can be very tedious. Other hardware that will be employed to determine the attitude of the CubeSat are coarse sun sensors and magnetometers, with final CubeSat component specifications awaiting additional power consumption and design data from the ECE design team. These components will be tested, along with the attitude-control magnetorquers to assure overall integrated performance. While there are significant differences between the magnetic fields, the transient behavior of the magnetorquers and possibly other electronic components need to be established. Furthermore, the attitude determination and control system codes need to be validated.

A program was created and employed to simulate the external thermal environment that will be experienced by the orbiting satellite. The external CubeSat surfaces were simulated as massless, maintaining thermal balance with the external environment. While these estimates are obviously extreme, surface temperatures ranging from 330 K and 250 K (Figure 3) were predicted. Three different thermal tapes that met the mission requirements were identified as possible candidates that could be tested for efficiency in the future (golden mylar tape, an aluminum tape, and a teflon tape).

Figure 2. Predicted CubeSat face temperature histories for five orbits

Figure 2 represents the transient equilibrium surface temperature behavior of each face during 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 influence can be observed where the maximum temperature on that side is only 260 K and it can cool to temperatures 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 will be incorporated in a finite element-based thermal analysis that will incorporate thermal mass and other transient effects (such as electronic component power dissipation) in the internal components.

The structural analysis of the CubeSat was run for aluminum and Windform XT 2.0. The results obtained from PATRAN for these two models showed that the aluminum frame would provide more structural rigidity than the lighter Windform (Table 1). The maximum deflection obtained for the aluminum frame was acceptable at 0.004 cm; however, the Windform frame showed an unacceptable deformation of 0.01 cm. From these estimates, aluminum was chosen as the material of choice for the ODU CubeSat.

Material / Estimated Max. Deflection / Estimated Max. von Mises Stress
Aluminum 6061 / 0.004 cm / 6.3E+7 Pa
Windform XT 2.0 / 0.01 cm / 4.05E+7 Pa

Table 1. Comparison of Aluminum and Windform

Simulations of the nominal mission orbit were performed starting from March 1, 2018, the presumed deployment date. The lifetimes of all three CubeSats were estimated to be 330 days before deployment of the drag brake using the drag coefficient of 2.39. The orbit lifetime of the ODU CubeSat after deployment of the drag brake was estimated to be 57 days using the drag coefficient of 4.04, depending on full drag brake deployment, changes in the final mass of the satellite, and the fluctuations in atmospheric density. Figures 3 and 4 are snapshots from the STK simulations, showing the orbits of ISS in green, the ODU CubeSat in cyan (after its drag brake deployment), and the remainder of the constellation in red. Figure 3 is the two-dimensional 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.