RocketSat V: AirCore

Transcription

1 RocketSat V: AirCore Jessica Lauren Brown Emily Logan Steven Ramm Mackenzie Miller University of Colorado at Boulder Colorado Space Grant Consortium Brian Sanders March 30 th, 2009 Abstract Global warming has become a major concern in today s society. By collecting more information about the atmosphere, scientists can examine the effects and possible ways to prevent the process from advancing any further. RocketSat V is working in collaboration with the National Oceanic and Atmospheric Administration (NOAA) to collect and validate an atmospheric sample with emphasis above 22 kilometers. Using this AirCore sample, a profile of the concentrations of carbon dioxide and methane in the upper atmosphere can be created. The AirCore payload is comprised of approximately 90 m of stainless steel tubing and will be launched on a sounding rocket in order to collect a stratified sample past the range of a standard balloon flight. Very little is known about this region of the atmosphere, but it is expected that as altitude increases, the concentrations of the two greenhouse gasses, CO 2 and CH 4, will decrease. The data will be used to check the validity of theoretical models of the composition of carbon dioxide and methane in the upper atmosphere. Furthermore, this will establish one of the highest collected samples of the atmosphere while contributing towards the studies on global warming. Table of Contents 1. Background AirCore Science Background AirCore Design Background Science and Theory Layers of the Atmosphere Diffusion Viscosity Design Overall system view Structural components Electronic components Expected Results References Appendix Appendix A Appendix B COSGC Space Research Symposium Page 1

2 1. Background 1.1 AirCore Science Background NOAA developed the AirCore idea for balloon flights in order to collect a continuous, stratified sample of the atmosphere. NOAA s AirCore payloads are opened just before launch and during assent the pressure inside the tubing has enough time to equalize with the outside pressure due to the slow assent. At apogee, the pressure in the tubing is very near a vacuum. Then, as the tubing descends, the atmospheric pressure begins to increase again and the pressure differential forces air to flow into the tubing. The air quickly diffuses over the small diameter of the tubing but due to the extensive length of the tubing the air remains relatively stacked in the order of collection. On the ground the tubing is closed by hand and immediately returned to NOAA to be analyzed. The AirCore can then be run through a laser analyzer before the air has time to diffuse through the entire length of the tubing. Although some diffusion has taken place during this short time, it is minimal and does not cause an appreciable error in the analysis. By knowing the approximate speed at which the AirCore fell the flow rate into the tube can be characterized. From the flow rate, the amount of mass in the tubing at any given altitude can be backed out. Each data point taken by the laser analyzer corresponds to 6ccs of a gas and therefore each sample can be correlated to an average altitude or range of altitudes. From this, the profile of carbon dioxide and methane throughout the atmosphere can be created. In previous balloon flights, the average altitude reached by AirCore has been ~100,000 ft (~30 km). The highest collected sample of atmosphere with a focus on carbon dioxide was another balloon flight from Japan which reached an altitude of 118,000 ft (36 km). RocketSat IV reached 220,000 ft (67 km) while RocketSat V will reach apogee at 449,500 ft (137 km). The most important sampling will take place between 131,200 ft (40km) and 72,178 ft (22 km). 1.2 AirCore Design Background Each NOAA payload consists of approximately 150 meters of loosely coiled tubing and very few electronics as shown in figures 1. The NOAA payloads fly with a hand turn ball valve on both ends of the tubing. One end is opened prior to flight and closed again by hand upon landing. Figure 1: NOAA AirCore balloon payload [1] Since RocketSat V: AirCore is being flown on a sounding rocket which lands in water, the tubing must be tightly coiled to a much smaller diameter to fit the rocket and must be sealed completely before landing to keep the water out. In order to close the tubing, a solenoid valve is used and is triggered by a pressure sensor. Also being recorded is temperature and acceleration for post flight analysis of the air sample. This requires more electronics than typically flown in NOAA payloads and these electronics are placed inside the center of the coil of tubing as seen in figure 2. The design modifications needed for the rocket flight may also be applied to balloon flights with return gliders and more confined aircraft flights. Also, the closing mechanism allows the AirCore to be closed before the clouds, keeping out a lot of moisture that may adversely affect the high altitude samples COSGC Space Research Symposium Page 2

3 Figure 2: RocketSat V AirCore payload 2. Science and Theory 2.1 Layers of the Atmosphere Particles of air take decades to diffusion upwards in the atmosphere. Therefore, the air in the upper atmosphere is the older air that was at the surface decades ago. By using an AirCore profile, a history of atmospheric concentrations can be discerned. The profile collected shows a time history of carbon dioxide and methane in the atmosphere over the past few decades. There are multiple aspects of the atmosphere that affect the concentrations of air particles at specific altitudes. To start off are the main layers of the atmosphere: the planetary boundary layer, the homosphere and the heterosphere. The planetary boundary level extends from the surface of the earth to 6,500 ft (2 km). It contains turbulent wind currents based on the drag from the earth s surface. In this layer, the wind currents keep the concentrations of gasses essentially consistent throughout. The homosphere extends from 6,500 ft (2 km) above the earth s surface to approximately 263, ,000 ft ( km) above earth s surface. The wind currents contained in this layer are horizontally along isobars. These wind currents mix the air molecules and keep the concentrations homogenous. As a result changes in concentration of air molecules from this layer can be assumed to be due to diffusion and not vertical wind mixing or separation by mass. The heterosphere extends from 263, ,000 ft ( km) and beyond. Throughout the atmosphere, the gas molecules have a tendency to sort by mass, the heaviest molecules tend to sink while lighter gases tend to rise. At lower altitudes, this effect is not readily observed because wind currents keep the atmosphere nearly homogeneous. In the heterosphere the wind currents in this layer are negligible and gases are free to separate by molecular mass. At this altitude the concentration of gasses is solely a function of molecular mass. There is particular interest in the gasses in the homosphere. Above that, there is very little atmosphere, less than 1%, and it is difficult to collect gasses. Also the gasses in the heterosphere have very little effect on earth climate. Below the homoshpere, in earth s boundary layer, it is easy to sample the atmosphere and much is known about this zone. Daily measurements of the concentrations of gasses are taken and therefore it is not the focus of RocketSat V. The homosphere however, extends past the capabilities of daily measurements and above approximately 72,178 ft (22 km), very little is known about the atmosphere. Also throughout the homosphere, gasses are separated by age more than weight and so as altitude increases, it is like looking back in time at Earth s atmosphere. 2.2 Diffusion One of the most important concepts in the AirCore mission is diffusion. Diffusion is the displacement of gas due to molecular motion. It is responsible for keeping the gas in the order that it flowed into the tubing and for the change in the concentration of gasses as altitude changes. Equation 1 shows the calculation used to determine the distance gasses are expected to travel in a given period of time. X rms = 2DT [1] X rms is the root mean squared of the displacement of the molecules in a gas. This serves as a measure of the mean displacement of the molecules of a gas from an initial point which is defined when time is zero. D is a displacement constant that depends on the gas. T represents the time interval in seconds. The distance that a gas molecule moves in a period of time is proportional to the square of the time. Inside the tubing, the sample will diffuse quickly across the diameter but take much longer for the gas to diffuse along the length of the tubing. The analyzer has a chamber volume of 6ccs, therefore a sample in the tubing is defined as a 6cc volume of air. A 6cc volume in the tubing will correspond to a certain length, this length defines an acceptable amount of diffusion and then the time it takes for it to diffuse over that length sets the requirement for how long the AirCore can sit before being analyzed. Within this time frame the concentrations measured for each sample will 2009 COSGC Space Research Symposium Page 3

4 still be representative of the atmosphere at that mean altitude. Diffusion also affects the atmosphere on a large scale. Above the boundary layer, where the winds are horizontal, the primary force causing a change in the altitude of molecules is diffusion. Because diffusion is time related, the majority of the air that diffused out of the boundary layer in 1978, for example, will be at a higher altitude than air that diffused out is It is for this reason that a profile of carbon dioxide and methane concentration is so interesting. It serves not only as a measure of how these gasses change with altitude, but also how these gases have changed with time. 2.3 Viscosity One of primary obstacles that AirCore flights on rockets face is viscosity. Ideally, the tube starts at a vacuum at apogee. As the tubing descends the pressure outside increases and begins to push into the tube. In an ideal situation the pressure inside would equalize to the pressure outside instantaneously. However, this does not happen. Instead, the air enters the tube as a function of the flow rate. The flow of a fluid is described by equation 2. 4 ρπd Q = 128η l ( P) [2] In equation 2, Q is the flow rate, d is the diameter of the tubing, and l is the length. The density of the fluid is and the viscosity is. The driving force behind the flow is the pressure differential, P. At high altitudes, the density decreases and the change in pressure with altitude is smaller. This means that the flow is slower at higher altitudes and therefore it takes a longer time for the tubing to reach equilibrium. During a balloon flight, the descent is slow enough for the tubing to get close to equilibrium at the high altitudes. On a rocket flight however, the payload section is dropped from a much higher altitude and is not slowed by the atmosphere until it reaches low altitudes. Also on a rocket flight, the parachute is deployed at 20,000 ft (6 km) while a balloon s parachute opens as soon as there is enough atmosphere to cause significant drag. These two factors contribute to the higher descent velocities seen in rockets. This means that the rocket AirCore spends less time at any particular altitude than the balloon AirCore. For the same flow rate, the rocket AirCore will have less time for the gas to flow in and therefore there will be fewer samples. In order to combat this problem, the tubing needs to have as large a diameter as possible. However, due to volume and mass constraints, an increase in tubing diameter would correspond to a decrease in tubing length. With a smaller length of tubing, the percentage of the tubing a molecule can diffuse over for a certain period of time increases and the accuracy of the analysis decreases. This dictates that a length of smaller diameter tubing be added at the end of the large diameter tubing. This tubing would hold the most important, high altitude samples in a volume where the length of a 6cc sample is spread over a greater length of tubing. Figure 3 illustrates a rough layout of the two different diameters and the flow of air. Figure 3: Layout of tubing with different diameter tubing 3. Design 3.1 Overall system view The RocketSat V system focuses on preserving the scientific mission of the project. In following this objective, the AirCore tubing is entirely composed of stainless steel. The unique chemical properties of stainless steel reduce the likelihood of carbon dioxide and methane sticking to the sides of the tubing or being absorbed by the material and thereby skewing the analyzed sample. RocketSat V is using ft (43.4 m) of 3/8" stainless steel tubing and ft (41.3 m) of 1/8" stainless steel tubing, both with a wall thickness of 10 mils. Throughout the tubing, there are three OMEGA PX209 pressure sensors. One pressure sensor is located before the larger diameter tubing, one between the two different diameter tubing, and one before the hand turn valve. The purpose of the hand turn valve is to allow for easy access to the atmospheric sample after collection while allowing the system to be sealed during flight. The hand turn valve will remain closed until the gas in the tubing has been retrieved. A Parker Hannifin solenoid valve is located at the beginning of the tubing. One additional solenoid, controlled by Wallops is placed on the skin of the rocket as redundancy to prevent the rocket from flooding. The Wallops solenoid opens and closes at 6,000 ft (1.8 km) while the AirCore solenoid opens at 131,200 ft (40km) and closes at 20,000 ft (6 km). Figure 4 shows a basic layout however in Appendix A, there is a more detailed and labeled layout of the overall system COSGC Space Research Symposium Page 4

5 Figure 4: System Layout 3.2 Structural components The structural design of RocketSat V is constrained by the dimensions of the canister provided by the RockOn Workshop, along with a weight constraint of pounds. The canister has a height of 9.5, a diameter of 9.75 and all components of the payload must remain with in these dimensions. To maximize the space available, the C&DH components are mounted upon four 5.25 diameter Makrolon plastic plates that are stacked upon each other. The design also includes one larger Makrolon plate that has the same inner diameter of the canister, 9.5, and attaches to the bottom lid of the canister. Figure 5 illustrates this layout. Figure 6: Bottom plate with Batteries The next plate, shown in figure 7, houses the AVR board, which is held in place by four nuts and bolts. To keep the components from shorting, nylon spacers are included to hold the board off of the plate. The nylon spacers also serve to protect the AVR board by dampening vibrations felt during flight. Figure 7: Second plate with AVR board Figure 5: CDH Stack Each of the four Makrolon plates has at least one half-circle hole cut into the outer portion of the plate to allow wires to pass easily from one level to another. Each plate is separated by 1½ stainless steel standoffs. The power source for the electrical components is situated on the bottom small plate and consists of four 9V batteries that are paired together. The two pairs of batteries are secured to the plate by brass brackets as shown in figure 6. The batteries are also hot glued to the brass brackets prior to flight to ensure they remain in place. The third plate, shown in figure 8, is separated from the second plate by 1 standoffs and houses three separate CDH components: the AirCore board, the zaccelerometer, and the g-switch. The PCB board is situated in the center of the board, to one side of the PCB board is the g-switch, and on the other side, the zaccelerometer. Since both the g-switch and the zaccelerometer must be mounted vertically, L-brackets were used. One side of the L-bracket is attached to the plate using nuts and bolts, while the g-switch and zaccelerometer are attached to the other portion of their respective brackets. Nylon spacers were, again, placed between the electronic boards and their respective mounting surfaces. Figure 8: Top plate with AVR board 2009 COSGC Space Research Symposium Page 5

6 The top plate in the system does not house any electrical components, but instead serves as an extra protection for the electrical components on the plate below, as well as a place for the hand turn ball valve and pressure sensor to be secured. Since the ball valve and pressure sensor must be easily removable, they are only secured using Dacron string threaded through small holes drilled in the plate. Finally, because there are no wire connections on this top plate, there is no need for a large hole in the board for wires to pass through; making this plate is a complete circle. The flight canister also houses the ft (43.4 m) of 3/8" stainless steel tubing and ft (41.3 m) of 1/8" stainless steel tubing. In order to fit the entire lengths of tubing within the canister, the tubing was bent into two separate coils, with the 1/8" tubing encompassing the outer coil and the larger, 3/8 diameter tubing as the inner coil. These two separate coils of tubing are connected using a Swagelok connector. Figure 9 illustrates the final tubing for RocketSat V fully coiled. Figure 9: Coiled Tubing Figure 10 is an exploded view of the solid works model of the entire system. The coil of tubing fits around the CDH stack which attaches to the bottom Macralon plate. That bottom support plate is bolted to the bottom of the canister, shown in blue. The skin of the canister and the lids are designed to easily interface with other identical canisters and Wallops rocket. Figure 10: Exploded view of final payload configuration 3.3 Electronic components In order to open and close the tubing at the desired altitudes during the flight, a solenoid ball valve is being used. In addition there are several sensors used to gather data necessary to back out the altitudes of each sample and identify the altitude during flight. These sensors include several pressure sensors, temperature sensors, and accelerometers. The pressure sensors will be used to identify what altitude the rocket is at and will trigger the opening and closing of the solenoid valve. The data recorded and stored by the pressure sensors and the temperature sensors will help identify what altitudes each sample was from as well as characterize the flight environment COSGC Space Research Symposium Page 6

7 The system uses two main boards: the RockOn Workshop AVR board and the AirCore board. The AVR is a general board that has been used on previous RocketSat flights and is used in the RockOn Workshop. It has flight heritage and works well for our purposes, however there is a need for a second, smaller board, which is specific to the AirCore mission. The AirCore board is designed to interface with the multiple pressure sensors and temperature sensors needed for the flight as well as power the solenoid. The AVR main board, shown in figure 11, contains an AVR microprocessor, 16 MB flash memory, RBF interface, G-switch, and accelerometers. All of the components on this board will be powered by 5V and 3.3V, regulated from a 9V battery source. The Atmega- 32 is the main brain of the system that processes C base codes. The AVR will control data flow of the entire CD&H system, for example the AVR samples all the pressure sensors and then writes the data to the flash memory. The AVR will also be used to send signals to open and close valves and do minor analog to digital conversions, but an external Analog to digital converter is used on the Aircore board. On top of the processor the AVR board also consists of the accelerometers and a temperature sensor. Also two regulators are included on the AVR to regulate 9V power. The communication to the AVR is done by writing software onto the ATmega- 32 micro-controller. Another part that needs to mention and also a vital to them mission is the internal clock, which will be our main source of timing. Aircore board. The solenoid valve will be connected between an 18V source and the source pin of an nmos transistor. Once the open conditions are met, the AVR will send a voltage to the gate of the nmos, allowing current to flow through, and thus open, the solenoid. This board is specific to the AirCore mission. The schematics for the AVR and AirCore boards can be found in appendix B. 4. Expected Results On previous balloon flights from NOAA, the concentrations of carbon dioxide and methane decrease with altitude. Figure 12 shows, in red, the concentrations of carbon dioxide as a function of altitude. The black line indicates the temperature profile of the atmosphere. RocketSat V expects to see similar results from the main rocket flight and on a test balloon flight in mid April. The Balloon flight will verify the functionality of the electronics system and help to validate the air sample collected on the later rocket flight. The RocketSat AirCore will be compared to a similar AirCore payload flown by NOAA. Figure 11: AVR Board As mentioned, the AirCore board is a peripheral interface between the AVR, solenoid, pressure sensors and, temperature sensors. The model for this external analog to digital converter is AD 974 from analog devices. This board will have 2 4-channel ADCs and a control circuit that controls the solenoid. The first ADC will govern the data from the temperature sensors, and the second ADC will govern the data from the pressure sensors, both outputting to the AVR for processing. The solenoid valve is large part of the Figure 12: Carbon Dioxide Profile with Altitude For both flights, it is expected that the data collected will be slightly different than the actual 2009 COSGC Space Research Symposium Page 7

8 concentrations originally collected. This is due to the inner walls of the tubing capturing the air particles. The walls will absorb or emit molecules based on the conditions the sample is submitted to. Offsets occur while the gas flows through the tubing, while it is stored, and when there are changes in temperature and pressure. The tubing has been characterized with many tests for all of the conditions that cause changes in the concentrations of carbon dioxide and methane, as shown in the figures below. The data collected during the flights will be analyzed while taking into account the measured changes in concentration that occur during testing. 5. References [1] Rick von Glahn, Russ Chadwick. Announcement of EOSS-102, Edge of Space Sciences. 24Mar [2] Aaron Russert, Evan Coeffy, Justin Simmons, Frankie Ning, Kristen Brenner, Carolyn Maurus, Jessica Brown, Mackenzie Miller, Emily Logan, Steven Ramm. RocketSat V CDR. Dec2008. [3] Pieter Tans, Colm Sweeney, Anna Karion. Interviewed by Aaron Russert and Jessica Brown. October 2007 through present. Tank Coils Tank Figure 13: Flow Test showing offsets in ppm of CO2 Tank Coils Figure 14: Storage Test showing offset in ppm of CO2 Figures 13 and 14 are from two tests performed in order to characterize the tubing. In figure 113 the test indicates a small offset of approximately 0.2 ppm of carbon dioxide, which can easily be taken into consideration. The test performed for figure 14 is a storage test showing that over time the offset increases, however, in a 24-hour period of time the offset is still very close to 0.2 ppm. For every sample collected approximately 0.2 ppm will be subtracted from the final concentration measured for that altitude. Additional testing is being performed on the tubing and all of the results will go towards validating the final air collected from the rocket flight. The final rocket flight will be this summer on June 26 th COSGC Space Research Symposium Page 8

9 6. Appendix 6.1 Appendix A Pressure Sensor 3/8 to 1/8 connection 1/8 solenoid valve Pressure Sensor 24 inches of 3/8 drop down tubing Skin of Rocket Safety solenoid 1/8 port 1/8 Hand turn ball valve 1/8 tubing 3/8 tubing 3/8 to 1/8 connection 3/8 to 1/4 Quick Connect 24 inches of ¼ O.D. Teflon tubing Figure 15: System Layout 2009 COSGC Space Research Symposium Page 9

10 6.2 Appendix B Figure 16: AVR Schematic 2009 COSGC Space Research Symposium Page 10

11 Figure 17:AirCore Board Schematic 2009 COSGC Space Research Symposium Page 11