DRAG AND ATMOSPHERIC NEUTRAL DENSITY EXPLORER (DANDE)

Spherical Spacecraft Design Challenges

Submitted to the Colorado Space Grant Consortium Undergraduate Research Symposium

March 31, 2009

____________________

Lee Jasper COSGC/ASEN Student

Lee.Jasper@colorado.edu 303-204-1156

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Kyle Kemble COSGC/ASEN Student

Kyle.Kemble@Colorado.EDU 303-895-5971

The Drag and Atmospheric Neutral Density Explorer (DANDE) is a unique project performing a beneficial science mission for the international community. DANDE is designed to study the drag and wind induced on satellites as they orbit the Earth. On-orbit the primary variable that changes in the drag equation is the atmospheric density, which can vary by as much as 800%. Density variations of Earth’s atmosphere are caused in large part to the solar radiation from the Sun. By studying the effects of drag and wind on-orbit requires that DANDE maintain a constant cross-section spherical shape. Designing a spherical spacecraft presents unique challenges, including mounting solar panels, attaching to the launch vehicle, and wireless communications. Challenges like these have been overcome to bring DANDE to national acclaim by winning the fifth iteration of the University NanoSat Competition on January 20, 2009

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1 The DANDE Project The Drag and Atmospheric Neutral Density Explorer (DANDE) is a 50 kg, spherical spacecraft being developed by students at the University of Colorado at Boulder by the Colorado Space Grant Consortium (COSGC) in partnership with the Aerospace Engineering Science Department (ASEN). The goal of the DANDE (http://spacegrant.colorado.edu/dande) mission is to provide an improved understanding of the satellite drag environment in the lower-thermosphere at low-cost.

In addition to being a unique educational forum for teaching design and systems engineering, the mission is a response to government and industry needs for near-real time space-weather and drag prediction. These products are important to operators of low earth orbiting satellites with precision navigation needs and include both government and industry. DANDE government collaborators interact with CU engineering students and include the Air Force Office of Scientific Research (AFOSR), Air Force Research Lab (AFRL), Air Force Space Command Space Analysis Center (AFSPC A9A), Naval Research Lab (NRL), National Oceanographic and Atmospheric Administration (NOAA) Space Weather Prediction Center, and NASA Goddard Space Flight Center (GSFC). The DANDE project has helped

foster and strengthen these relations. DANDE is a part of AFRL’s University Nanosat Program

(UNP)1 which has as its goal the development of satellite-design and research capability at universities as well as the education of the future space-engineering workforce. DANDE was selected as the winning satellite from a national competition with 10 other universities to build and test a working satellite. DANDE has been awarded further Integration and Testing (I&T) funds and support as well as a launch to place the satellite into low earth orbit. DANDE has won for the University of Colorado at Boulder and plans to deliver the satellite later this year for a launch in 2010 or 2011. To achieve its goals, DANDE will make measurements at

altitudes between 200 km and 350 km using spacecraft radar tracking and two on-board instruments. Tracking will be done through a collaborative agreement with Air Force Space Command Studies and

Analysis Division (AFSPC) A9A providing high-priority precision tracking for drag. The dual instrument approach made here involves a novel accelerometer instrument capable of micro-g resolution and a neutral mass spectrometer to sense the wind speed and density on orbit. In order for DANDE to be operationally useful for the scientific mission, it must be near spherical imposing challenges to the design of the structure, power, and communication subsystems. These technical challenges provide unique learning opportunities for student engineers where engineering-excellence development takes place by contextualizing and implementing knowledge learned in class and research.

1 http://www.vs.afrl.af.mil/UNP/

Figure 1.1: The DANDE spacecraft showing solar cells (blue).

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2 Power Generation and Fabrication Challenges Since the science mission that DANDE is based around requires the constant circular cross-section many of the spacecraft subsystems have to bend to maintain this characteristic. A dramatic example of this creative design challenge surfaces with the power system. The internal components share the same need to conserve space, but the external power generation takes a much different impact. While DANDE has a low mission lifetime it is still important for to generate its own power, versus just having batteries that survive for the specified mission lifetime. The only viable method to do this would be to use solar power, and this is by no means a pioneering territory, where the engineering challenge arises is the need to maintain DANDE as a modified sphere, similar to Starshine.

Figure 2.1 Starshine I (Left) and DANDE (Right) shown next to each other, both are considered to be modified spherical spacecraft. Both are designed for testing the variability of the thermosphere. DANDE picture courtesy of University of Colorado at Boulder. The challenge at hand for the DANDE team was to mate the flat surface of a solar panel with the curved surface of the structure. Since providing the maximal amount of energy would require a large surface area for the solar panels, area which DANDE can not afford to give up due to the constraints of the system. Great strides have been made in solar cell technology that has influenced the power generation capability of DANDE. Spectrolab, a subsidiary of the Boeing Company, has been aiding CubeSats in their efforts to produce more power by marketing the residuals of the efficient triple junction cells they have from major contracts to the public. These cells are rated to be 27% efficient and take up a small surface area. This research allowed for DANDE to have a good deal of the area of the hemisphere covered by solar cells, without drastically changing the shape of the sphere.

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Figure 2.2 Master assembly of the DANDE Satellite including the initial design of the solar panels each consisting of only two cells that would put out 2.5V compared to the final design where a panel has an output that meets the 15V powerbus. An unforeseen benefit to this confined space with the use of the small Spectrolab cells allowed the team to eliminate the need for a boost converter which increased the power efficiency. The large cells that were also triple junction would only provide 2.5V, but their larger surface area provided more current. The final design of the hemisphere actually strung 8 of the smaller cells together in series and then string all these in parallel to generate the most current available. With this current configuration factoring in the loss based on the angle of incidence to the sun along with the high efficiency of the cells the each hemisphere is capable of generating 23 watts of power, though on orbit only one hemisphere at a time will face the sun at any angle. When the concept of the solar arrays was to use the large solar cells, the traditional method of fabricating a solar array was going to be used. The traditional method is to individually solder up each cell together and then immerse the cell and honeycomb substrate into an epoxy and allowing the entire assembly to harden before testing the panel. Keeping this in mind to take this technique and use it with the new configuration of solar cells would have been very tricky. Since there are roughly 39 panels on each hemisphere that would mean that the same number of epoxied panels would need to be fabricated, the number becomes even more daunting when the shear number of cells is considered, of which there are almost 600 on each hemisphere. It was quickly determined that the labor was not available to perform this sort of fabrication let along the quality assurance. A new hybrid method needed to be developed to maintain the structural integrity of the solar panels along with conforming to the near spherical shape. Generating this hybrid method required what became a semester long trade study to develop an engineering unit hemisphere that fully implemented a new method. To do so DANDE leveraged an extensive amount of knowledge from the Hermes CubeSat developed at COSGC and SwissCube2. These two pico-satellite class systems have looked heavily into soldering solar panels onto PCBs and have done extensive testing into the development of cracks and air pockets. DANDE’s final design concept used these methodologies and incorporated the traditional epoxy as a structural member. With this hybrid design the solar panels on board

2 Roethlisberger, Guillaume. "Advanced Methods for Structural Machining and Solar Cell Bonding Allowing High System Integration and their Demonstration on a Pico-Satellite." 22nd Annual AIAA/USU Conference on Small Satellites (2008):

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conform to the small surface area available and modify the exterior of the sphere to a point that can still be modeled in terms of its effect on the overall drag of the satellite. However this is only the one piece of the puzzle to create a panel that led to the current solar array. Physically mounting the panels to the DANDE superstructure is another challenge that had to be overcome. Since it was decided that the cells would be mounted to the solid PCB there would be very little chance to perfectly mate the aluminum hemisphere to the PCB directly. Since most solar panels have a substrate as an intermediary between the cells and the structure. DANDE solved this challenge in a similar manner by machining delrin substrates with one side to match the curvature of the hemisphere and the other side flat to mate with the solar panel. This created an interesting thermal condition since the delrin is a good insulator. Since panels are in direct sight of the sun’s rays they represent the warmest part of the satellite and conducting that heat away is a necessity. With the delrin as a good insulator the best heat path is through the four bolts acting as an attachment. This condition of the thermal model dictates the final surface coatings of many subsystems. However this has shown the great response of industry to aid student projects like DANDE since many organizations have offered their facilities to actually test how this heat path actually acts. Due to the constraints placed on the DANDE mission by the science that is being tested the power system felt a strong need to conform to the system. This has uncovered subtle benefits for power generation, and the development of a hybrid system that takes much of the labor out of the picture. Additionally by having to avoid the traditional wing like solar arrays DANDE always has panels pointed at the sun for power generation when in daylight.

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3 Attachment to the Rocket Body A major focus of the structural design is the interface between the payload and the rocket body. Being a spherical spacecraft, DANDE inherently has issues attaching to almost any rocket interface. This has caused significant development into the method for attaching DANDE to the rocket. As a secondary payload DANDE has some of the legwork done already since large companies have developed the ESPA ride share adaptor. This is a universal attachment for a secondary payload. Another, fairly universal attachment mechanism is the Lightband, which is a spring loaded ‘kick-off’ mechanism between the spacecraft and the ESPA adaptor. Still the Lightband mechanism is an annular shape and this could not directly attach to DANDE due to geometry. Further, the Lightband would interfere with DANDE’s designed spherical shape which is unacceptable. The Lightband is the desired and recommended mechanism to use. This lead the team to design around the device. It is necessary to keep DANDE spherical during main operations. Thus the Lightband needs to be ejected from the main body of DANDE. As stated before, the Lightband also does not attach to DANDE well. Due to these constraints, the Lightband Adaptor Bracket (LAB) was developed. The LAB is an annular shape to fit the Lightband. However, the LAB is not only a ring, but a complete structural piece that includes support for the mechanism that detaches the LAB and Lightband. A SpaceDev Low-Shock Release Mechanism (LSRM) has been selected to allow the release of the LAB and is shown in Figure 3.1.

Figure 3.1 graphic of the attachment mechanism to the rocket body. The staging of the assembly is to be kicked off the ESPA Ring and then after leaving the vicinity of the rocket and charging the batteries aboard DANDE the SpaceDev Mechanism will be activated and eject the LAB. After that sequencing DANDE will have regained its modified sphere shape. The release mechanism made by SpaceDev was nominally intended for use in conjunction with multiple points of restraint. However, the DANDE connection point created unique challenges

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since the DANDE sphere is essentially a moment arm. The mechanism design is a two ‘pin’ attachment, which does not support off-axis motion well. During a vibration test, the lowest fundamental frequency was found around 80 Hz, and was a rocking motion back and forth. This has led to a change in the LAB which has made it stiffer. There are also three unique parts involved with the system named the Kinematic Mounts. These mounts restrict the motion that DANDE can have, thus making the overall connection stiffer. The mounts limit the motion by restricting upward vertical, and rocking motions in all directions (Figure 3.2). The flat surface provides a solid contact point and helps to stop rocking motion in one direction while the ‘V’ shape prevents motion along in the spinning and opposite rocking motion of the flat plates. Finally, the half ball shape aids all of this by limiting translational and rotational motion. These mounts also are very lenient to tolerancing errors in the machining and are therefore very easy for integration purposes.

Figure 3.2: Kinematic Mounts

Finally, the LAB has undergone a stiffening redesign to increase its natural frequency, shown in Figure 3.3.

Figure 3.3: Lightband Adaptor Bracket. Old design (right) and new design (left)

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4 Communications and Radiation Pattern Challenges In spacecraft design, one of the most challenging parts of the system is the communications. Not only must this system be extremely reliable, it is also often the most power hungry. Creating an efficient and robust system is costly as well. Balancing all of these trade offs can be difficult, and it is only compounded when a desirable antenna cannot be placed away from the spacecraft body. On DANDE, the science requirements drive the shape of the structure. Much like the constraints placed upon the solar panels, DANDE must maintain its spherical shape as much as possible to maintain a constant and predictable cross section and drag coefficient. Thus, no antenna can leave the surface of the structure. To begin, the DANDE communications system is made up of the components shown in Figure 4.1.

Figure 4.1: DANDE Communications Block Diagram

There are several major components to the DANDE communication system. There are three antennas, one for receiving data (RX), and two for transmitting (TX). A receiver radio takes the signal and hands it to the Terminal Node Controller (TNC)/Modem. The TNC takes the signal and converts it into useable digital format for the spacecraft computer. The process also works in reverse for transmission. The TNC takes digital signals and converts them into analog transmission signals. This is then passed through the transmit radio out to the antennas. The spacecraft antennas are the components that are the most affected by the spherical shape.

Ground Station

CDH SYMEK TNC31

RS-232 MODEM 9,600 TX /

RX

AUDIO SPLITTER

70cm PATCH

ANTENNA

70cm PATCH

ANTENNA

HIGH POWER

RF

2m PATCH

ANTENNA

YAESU FT847

SYMEK TNC31

MODEM 9,600 TX /

RX RS-232

PC

70cm YAGI

2m YAGI

Spacecraft

AUDIO

HIGH POWER

RF

LOW POWER

RF

SPACE- QUEST

RX

SPACEQUEST TX

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Because the antennas must follow the contour of DANDE, several arced antenna patterns were developed. The latest revision, a patch antenna shown in Figure 4.2, is constructed of a long radiating surface with a shorter bend for the ground plane. The antennas are fed through the structure through a shielded cable which connects directly to the radiator. The ground plane is connected to the structure. In between the ground and the radiator, a supporting piece of Ultem plastic is used. Ultem is desired because of its non-interfering properties with

RF signals. The Ultem, antenna assembly is then bolted into the structural equatorial plate. This plate has a relief lathed into it which allows for the approximately inch deep assembly to mount flush with the exterior hemispheres. Apart from the mechanical difficulties of creating and mounting the curved antenna, the radiation pattern is of major concern. DANDE is a spin stabilized spacecraft. Then antennas are placed at the far radius of DANDE’s rotation axis. For communications, this rotational motion causes challenges form maintaining a constant link. If, for example, whip antennas could be used, they would be placed along the rotational axis of DANDE, effectively mitigating any spin motion in the gain pattern. Because of the science requirements, the patch antennas in Figure 4.23 must be

used. Since the patch antennas are used, they are more directional, which introduces problems with a spinning spacecraft. Figure 4.2 shows a radiation pattern of an average patch antenna. The figure shows that the gain pattern is fairly directional, allowing only about a 60 degree cone within the 3dB gain area. This makes it challenging to maintain a link with a rotating spacecraft. Several factors help to mitigate the antenna’s directionality issue. First, the wavelength of the receive antenna is 2 meters which effectively means that the structure does not interfere with the signals as badly as higher frequencies might. Thus, only one antenna is necessary. Second, on the transmit side, two antennas are used instead of one to achieve a more desired radiation pattern. The transmission

frequency is in the 70cm band and the size of the structure does interfere with that wavelength, thus two transmission antennas are necessary. Figure 4.3 shows the empirically determined gain pattern at 430 MHz. All the patterns have two or more lobes and each lobe has greater than a -20dBi null which will cause a loss in the communications link. However, it is expected that the link will be maintained with 1x10-5 bit error rate and the current link budget shows a positive 6dB margin.

3 Dobkin , Daniel M.. "Patch Antenna." wikipedia.org. 2009. 26 Mar 2009 <http://en.wikipedia.org/wiki/File:Patch_antenna_pattern.gif>.

Figure 4.2: DANDE Patch Antenna Design. Top view (left), isometric view (right)

Figure 4.2: Patch Antenna Gain Pattern

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Figure 4.3: DANDE Antenna Gain Pattern

The spherical shape of DANDE has added challenges to the RF component of the communications system. However, solutions to those challenges have been found, including a novel patch antenna, proper structural integration, and antenna creation to limit the effects of nulls.