Deployment of a Shape Memory Composite in Near Space: An Interdisciplinary Platform for Undergraduate Student Research

Transcription

1 Deployment of a Shape Memory Composite in Near Space: An Interdisciplinary Platform for Undergraduate Student Research Abstract The establishment of the Wright State University (WSU) high altitude balloon program creates an opportunity to conduct experiments in near space environments. Recently developed materials such as shape memory polymer composites are ideal for space applications due to their high specific stiffness (elastic modulus divided by density) and ability to be stowed into a small size for launch in conventional space vehicles, then deployed to full operational size and form once in space. Such characteristics allow for space based structures to become larger and lighter than structures currently used in space. There are various issues associated with deploying shape memory polymer composites in space, such as deployment characteristics in zero gravity, activation of the shape memory properties, and achieving the power required for activation and deployment of the shape memory polymer composite. However, testing materials in space based environments is often impractical due to the complexity and cost involved. This creates an opportunity to conduct testing of a shape memory polymer composite, provided by ILC Dover, in near space environments using a high altitude balloon platform. By the end of this project, the design team plans to have accomplished the deployment of a shape memory polymer composite during the flight of a high altitude balloon platform. Introduction The goal of this project is the deployment of a shape memory polymer composite in a near space environment using a high altitude balloon and associated modules as the deployment platform. Such a project is necessary to conduct research into techniques for deploying smart materials such as shape memory polymers in space and other extreme environments. Very little information concerning the performance of shape memory polymer composites in space is known. The purpose of this project will be to learn more about shape memory polymer composites and their viability for space based environments by deploying a shape memory polymer (SMP) composite boom at approximately 100,000 feet in altitude on a high altitude balloon platform. The ever changing engineering curriculum mandates an emphasis on interdisciplinary project, where students are exposed to a curriculum that allows them to work in teams of interdisciplinary members with focus geared towards integrated technologies. This effort requires collaboration from multiple disciplines, and provides students an opportunity to learn from several other disciplines. The design of a High Altitude Balloon (HAB) is one such project that accommodates for scientific research in near space at about 100,000 feet above ground. The senior students from electrical, and mechanical departments at Wright State University have collaborated together to work on the HAB system. The first goal of the HAB team is to launch and test the wireless communication and tracking equipment, and define the launch procedures. Here, the HAB payload contains both experiment and tracking equipment such as a Global Positioning System (GPS) receiver and an amateur radio. Through the experience and confidence gained in the first launch, subsequent launches incorporate several high-altitude experiments.

2 There are several areas of interest in HAB experiments. These include radiation effects on solar cells, wireless communication, and guidance systems. This wide span of information could be used in many areas such as military aircraft, and for natural disaster rescue teams. Project Description Mechanical Engineering Design The rapidly evolving communication technology will necessitate satellites with large aperture systems into the m diameter range, which are humongous and heavy to launch into nearspace. A Shape Memory Polymer (SMP) composite satellite could be heated and formed to fit into a small storage device capable of fitting into a space launch vehicle, launched into space to the desired orbit, unpackaged, and then heated to deploy to its memorized operational shape. SMPs are in rigid plastic and elastomer states below and above their glass transition temperature (T g ) respectively. However, research needs to be performed in order to assess SMP composite performance in space. True space based testing often requires large amounts of time and can also be expensive. This creates a need for an inexpensive and relatively easy to work with platform for simulated space based testing, such as a HAB system that can commute to altitudes of 100, ,000 ft. This environment has 99% vacuum, 98% to 99% gravity, and temperatures in the range of -60 o C to -20 o C. Figure 1 below shows the various elevations above ground with their respective common names. HABs have a fairly standard configuration involving a latex balloon, parachute, reducing ring, and command module. Figure 2 below is a schematic of a typical high altitude balloon stack. The command module is a capsule that contains the tracking and communications equipment necessary for tracking and recovering the balloon system. It can also contain cameras for recording images during flight and a microprocessor for operating the various systems onboard. In order to achieve optimum deployment of the SMP composite, every point should be heated to at least 158 o F (70 o C), which requires an extensive amount of energy. The composite has to be maintained at 6psi in order to assist with the deployment and rigidization. Simultaneously, the payload box holding the composite has to comply with other requirements. The payload box has to withstand a temperature close to 250 o F (121 o C), which is the maximum operating temperature of the composite, and a minimum of -76 o F (-60 o C), which is the lowest temperature measured outside. The final box design must also have minimal heat losses to the environment. It should also be large enough to hold the composite material, power sources required to heat the SMP composite, and cameras to characterize the shape of the composite during flight.

3 Figure 1: Various altitudes and names and designations 1 The approach followed to establish reliable heating of the composite is by determining the appropriate mode of heating, calculating the amount of power required, selecting the appropriate types of heaters, model the heating of the composite, validating the model, using the model to optimize the heating system design, and conducting full scale heating system experiments to assess the system s performance. Calculating the amount of power required to heat the composite involved studying the worst possible case scenario for heating. The total power required for 100,000ft exposed to the environment was calculated to be 147.5W, and the power required for lab based experiments was calculated to be 217.3W. As the composite will not be stationary during flight, flexible conductive heaters are used. Based on the operating conditions of 121 o C near SMP and the operational environment near space with a 99% vacuum, the polyimide/fep heaters have been selected as the ideal choice. The payload box holds the power source, electronics, characterization, and pressurization equipment along with the composite. Dow Thermax insulation was found to be ideal choice for the payload box that meets the design criteria. The only forseen design changes to the payload box will be to investigate and implement methods of reducing weight. Pictures of the current box design can be seen below in Figure 3.

4 Figure 2: Typical HAB stack Since the SMP composite needs to be pressurize to 6psi to assist in rigidization of the composite during deployment, an optimal way to seal the composite tube needed was required. Lightweight end caps were designed and fabricated to seal the ends of the composite tube. The end caps aid towards holding the pressurization system, and allow access to the interior of the tube. The best option for a light weight pressurization system at this time is to use small off-the-shelf pressurized canisters connected to a flow regulator. As the main goal of the project is to fully deploy the SMP composite in space, characterization can be used to determine its shape and size before and after deployment. The most common method for characterization is Photogrammetry. Photogrammmetry is used to create and measure two or three dimensional models using photo-grammes, which are photos of the object from multiple views using high resolution digital cameras. The object in the photo-grammes is often marked with several circles contrasting in color to the background color of the composite material. To mitigate the noise and uncertainty effects, characterization will be required at high altitudes. This system has to be integrated into the payload box, in order to allow for better deployment.

5 Figure 3: Payload box design Figure 4: Multiple Point Triangulation2 In order to achieve the most accurate model possible, photo-grammes should be taken from multiple views of the composite in a given shape. If the same camera is needed to take the pictures then a mechanism that allows for translation a single camera to multiple locations needs to be de develop. If the same type of camera, but not the same exact camera can be used, then two or three digital cameras need to be mounted in the payload box at strategic locations so sufficient photos can be taken for photogrammetry. Efficient and precise placement of the photo-grammes is required to obtain the most accurate model of the composite. Also, the composite should be characterized in the payload box before initial packing, using the same method that the composite will be characterized at altitude. Electrical Engineering Design The microcontroller that was chosen for the project was the BS2P24 Basic Stamp. This microcontroller in the HAB system comprises of four GPS chips running to it and a program that cycles through each chip in one minute intervals. The data arriving to the stamp from the GPS

6 chip is formatted to a custom format containing longitude and latitude, altitude, temperature, time, and speed which is defined here as a custom packet. The custom packet is streamed to a HAM radio which transmits the information to the base station, allowing the design team to track the balloon during flight. The custom packets received are formatted and fed into a mapping program that plots the balloons course in real time. The second method used to track the balloon is, using a singe GPS chip connected to a HAM radio that transmits information over the national Automatic Position Reporting System (APRS) frequency. This information is relayed on the internet through the APRS towers, allowing anyone with internet access to monitor the position of the HAB. Other electronics that were added to the HAB system besides the tracking equipment include a digital camera with a timer circuit to take pictures during flight, a screamer circuit, a temperature sensor to collect temperature inside of the box, and a mores code beacon to help locate the box once it had landed. A key component to the recovery, detection and avoidance of any unmanned airborne package is the ability to visually locate it. Variables of the viewer and the package determine the distance at which the observer is able to notice the object. These variables include visual acuity, position of the object relative to the viewer, ambient lighting conditions, position of the sun relative to the object, and coloration and gloss level of the airborne object. Careful selection of colors and materials of the system increased the detection ranges of the HAB system. Based on the discussions with personnel in civil air patrol, the coloration for the HAB system is decided to be neon orange. In addition to coloration of an airborne object, detection may be aided by the reflection of sunlight by surfaces on the article, commonly referred to as glint. To better serve the recovery, detection and avoidance, the HAB team worked towards maximizing glint. With a small rescue signal mirror (3.875x5.25 inches) having a visibility of approx. 20 miles in bright sunlight, the future launches will have a hard surface that can be painted and polished. This will improve glint production over the current non-reflective nylon shell, thereby increasing the potential detection distance. In meeting with the guidelines provided by Federal Aviation Administration (FAA), a radar reflector will be included in all future HAB launches. This lightweight radar reflector is constructed from a metallized foil over a foam base and provides ft 2 of radar cross sectional area (RCS). This RCS along with the flight path predictions provided the air traffic control information to vector affected aircraft in the vicinity accordingly. Additionally, the metallic outer surface of the reflector provides a glint point that aids in the visual of the HAB system, both from ground and air. There have been many goals set by the electrical engineering team for this project. Some of these goals include: learning and improving last year s communication and tracking techniques, successfully launching, tracking, and retrieving a HAB, assisting in the SMP project in multiple areas, improving antenna design, incorporating a solar cell experiment, addition of video capability to the command module, incorporation of a gyro for in-flight motion analysis, proper

7 addressment of the FAA, improving visibility of HAB for easier recovery/easier avoidance by aircraft, and proper documentation for smoother transition to next years HAB team. All of these goals have been achieved or are striving to be achieved by using a design methodology which comprises of 10 basic steps: 1. Assess what needs to be done 2. Research into how it will be done 3. Design of multiple solutions 4. Decision of which solution best fits the needs 5. Attainment of supplies 6. Build the solution 7. Test the solution 8. Modification of solution 9. Repeat 7 and 8 as necessary 10. Final product HAB Launch and Results The most important job of any operation not matter the size or complexity is leadership. Success of a team on an operation is dependent upon clear direction, strategy, communication, a common goal, and dependability. Every team member must know what they are supposed to be doing and how it fits into the big picture of the operation. This strategy for leadership was implemented during launch. Each person was assigned a task before and during launch. Pre-launch operations included weather checking, FAA notification and approvals, module assembly and testing, equipment gathering and transportation, and flight predictions. During launch each person was assigned to a certain area, and each person was also given a copy of the launch procedure so they could understand where and how their part of the launch fit in.

8 Figure 5: Schematic of the Microcontroller with GPS chips

9 Once the balloon was launched, a command center was established in Dayton, OH as the coordination center, and the lead chase car containing tracking and mapping software was assembled and given the responsibility to track the balloon to its landing area. Upon arrival to the landing area, three teams were created to triangulate the position of the balloon. The first launch for the HAB team occurred the day before Thanksgiving The balloon was launched from a small airport in New Castle, IN. Before arriving to the launch site, the FAA was notified and balloon flight predictions were made. Initial setup started with the two mechanical engineering students filling the balloon and doing all non electrical work, and one electrical student (other two students were not able to attend due to schedule conflicts) along with the advisor doing all the electrical/communication setup. With all electronics tested numerous times before arriving at the launch site, the setup goal at the launch site was to only have to make sure that all electronics had fresh lithium ion batteries. Though this was the projected goal of on-site setup, many other problems occurred. Figure 6: Davis Emergency Radar Reflector Once the balloon was filled, all electronics were turned on and the command module was closed. The mores code beacon was attached in a small cube beneath the command module, and the tracking system was tested for operation. During the test, it was noticed that the incoming transmissions were not accurate. Troubleshooting showed that two Sony GPS chips used for the custom packets were locked up and were not operational. It was found that the two other Garmin GPS chips were working. As daylight was burning by then, and the two chips were working with one chip on each form of communication, the design team decided to go ahead with the launch. Once the balloon was let go, tracking of the custom packets received by the laptop commenced. Only once during flight, was a reconnect command sent to the command module to reestablish the broken communication link with the balloon. The last coordinate transmitted by the command module before landing led us to within one square mile of its location. During search operation using directional antennas, the design team received a phone call from a local fisherman stating that he had spotted it. Given time, the design team would have triangulated the search area and found the HAB package by themselves. Table 1 below shows the sample of data received from the HAB package during flight.

10 Table 1: Sample data received during HAB flight Time Latitude Longitude Alt hh:mm DDMM.mmmm DDDMM.mmmm feet 9: N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W : N W Conclusions While working on HAB project, students got the opportunity to work with others from different disciplines. One essential aspect students learned is, effective communication of technical concepts and ideas to students from a different departments. During working on the SMP composite deployment project, an electrical engineering student was assigned to design the circuitry and triggering mechanisms for each component in the system. This provided a good learning experience to all participating students. For instance, mechanical engineering students learned how to explain the various parts of project to the electrical engineering student such as; operation of SMP, power calculation for heating, and pressure relationships for designing the pressurization system etc. The most valuable experience students gained from the interdisciplinary project is development of teaming skills required to work in the real-time projects, where individual engineer rarely works alone on a project. References 1. P. Verhage, Near Space: How Some Hobbyists are Getting Around the Difficulties Associated with Amateur Space Exploration, Nuts & Volts, Feb The Basics of Photogrammetry, Internet: