High Altitude Imaging Platform (HAIP). The electronics system must survive the launch

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1 6.0 Canister Design The avionics canister houses all of the electronic components necessary to run the High Altitude Imaging Platform (HAIP). The electronics system must survive the launch of the rocket, the ejection from the rocket, and the landing of the payload. However, the components are fragile and the connections between them are delicate. Therefore, the canister must provide protection for the internal parts from the loads of the flight and from the environment. The electronics canister needs to be structurally durable and airtight. It also must provide vibration isolation and temperature control. In this section, the past semesters work, Initech s accomplishments, and the necessary future work will be discussed. 6.1 Previous Work When the HAIP project was begun, the focus was on the payload ejection systems, as well as the Montgolfier balloon. Until the spring of 2001, the electronics were to be contained in a rectangular structure. In the spring of 2001, Brian Kilpatrick discovered that a square box would not be the optimal design for ejection from a circular rocket bay. The avionics equipment was selected and ordered for the project. Mr. Kilpatrick then designed the first canister to house all the components. The canister was constructed of a PVC pipe ~4 in diameter and 12 long. The pipe was then covered with high strength carbon fiber to increase structural integrity. The bottom of the canister was constructed from a 4.25 diameter circular piece of Plexiglas. The clear plastic was used so the camera could clearly view the ground. The opposite end was fitted with a 45

2 specially machined aluminum top and secured with eight screws. The electrical components were secured inside the canister with bubble wrap. Figure 25: Original Canister Initial Avionics Canister In the summer of 2001, the SOL group made several suggestions to improve the practicality of the canister. The first notion was to surround each individual component with foam. The bubble wrap provided little protection from vibration during the rocket launch, during the ejection, and during the landing. Foam was considered better suited to prevent the components from hitting each other and the canister walls. Other suggestions for vibration isolation were proposed, including the design currently being considered, discussed below. SOL also discussed the challenges of making an airtight canister and of having temperature control. In the fall of 2001, the LOST group designed, but never built, a new canister with vibration isolation in mind. The design consisted of a 9.5-inch board that fitted into slots in the sides of the canister. Another board perpendicular to the first would provide a 46

3 mounting area for the camera. The slots that the board fit into would be lined with rubber or foam to further soften the vibrations. The components would then be mounted to the board with nut and bolts. In between each nut and bolt would be rubber or foam washers to provide more cushion. The figures below illustrate LOST s Pro/Engineer drawings of the new design. Figure 26: Exploded View of Proposed Canister Design [Mauldin 2001] The LOST group used the original carbon fiber canister to perform their ejection tests. The group put lead shot inside the canister in place of the electronic components. During the first ejection test the canister did not withstand the stresses and was damaged. The damage forced the issue of building an updated canister. Therefore, the task of building a new canister became one of Initech s main goals. 47

4 6.2 Completed Semester Work During the project, several design problems were conquered. The past work was analyzed and a new design was prepared. The new design incorporated structural integrity, vibration control, airtight sealing, and temperature control. The canister was redesigned for practicality purposes and then built Evaluation of Past Work The Initech team needed to design and build a new canister to be utilized in the test launch taking place in the late summer. The process of designing the canister required some research into why the previous canister did not function properly. This necessitated carefully examining all the past design suggestions, as well as learning the issues that had yet to be addressed with the earlier design. Four main problems were found. The first problem was the material used to construct the original canister: carbon fiber attenuates radio waves. The GPS receiver and the transmitter are housed inside the canister and both use radio waves to communicate with the satellite and the ground, respectively. Although carbon fiber provides increased strength for the canister, it needed to be replaced with a material that is invisible to radio waves, yet still strong enough to protect the equipment. Next, the inside of the canister was examined. Neither bubble wrap nor foam provided enough vibration control for the safety of the avionics package. Even after SOL suggested foam, the avionics were not secure enough in the foam to withstand the expected g loading. The launch of the rocket will create the highest loading and will 48

5 range from 10 to 12 g s. The vibration caused during the flight and the ejection from the payload bay of the rocket at apogee could also damage the components. The canister will also land with the drogue chute traveling at 10 ft/sec. The landing could also cause breakage, even at this low speed. The foam would not sufficiently secure the components for these conditions. The LOST group s design was more conventional. Their design, with some modifications, was similar to the first proposed redesign. A typical avionics mounting in a canister is shown below. Figure 27: Typical Avionics Mounting and Canister [Mobley 2002] 49

6 The third concern with previous designs involves the change in pressure at higher altitudes. When the components are loaded at sea level humidity is present. With the reduced pressure at higher elevations, the water vapor in the canister will condense and form water droplets. The water could damage the sensitive avionics workings. The condensation will also fog up the Plexiglas, or other clear viewing cap, inhibiting the camera s ability to record useful images. The last concern is the low temperature restrictions on the components, which may be exceeded during the flight. Increase in altitude causes a decrease in the temperature. Several of the delicate instruments on board the payload cannot tolerate the dramatic change in temperature. These matters have been considered in the current design. Initech has incorporated all of the past concerns into our design. The most important objective this semester was to construct a structurally sound, airtight canister that limits the vibration of the components. The four issues discussed in the above section will be tackled below Initial Canister Redesign The Initech canister was initially designed without considering the availability and the cost of materials. Although the final canister was altered, the initial canister is discussed below to ensure future groups learn from our oversights. Below, the four main design considerations are discussed with respect to the first Initech canister design. Structural Integrity Although the canister can no longer be made of carbon fiber, the structure must withstand the loads of the launch and ejection. Five material options were explored for 50

7 the external structure of the canister. Metal was rejected immediately due to the possibility of shrapnel in the event of an explosion. In addition, a metal canister would be extremely heavy. Next, wood was considered and then rejected due to its porous nature. The project requires an airtight seal for successful execution. Plexiglas was considered but cracks easily on impact. Acrylics, such as Lexan, were then investigated. The choice for the initial redesigned canister s body was Lexan polycarbonate. Lexan transmits radio waves well. It is a clear polymer that insulates well and is lightweight. Lexan is resistant to abrasions and breakage. Most notably, Lexan has one of the highest impact strengths when compared to similar materials, as shown below. Lexan was thought to be readily available. However, it is only available in sheets from local distributors. Other acrylics were considered, but they proved to be beyond our set budget. Less costly options were later explored. Figure 28: Lexan s Impact Resistance Compared to other Materials [General Electrics 1997] 51

8 Vibration Control The next important step in the design of the new canister was preventing vibration. The components are delicate and cannot withstand the g-loads of the rocket flight. In addition, the connections between the components are not secure enough to stay connected with extreme shuddering. The following components needed to be mounted in the canister. The weight of each component was taken into consideration while placing them on the board. Table 3: Weights of individual components Component Weight in ounces 2 Battery Packs 26.1 oz. GPS Antenna 4.8 oz. Transmitter 4.1 oz. GPS Overlay 2.4 oz. Wires 2.4 oz. Accelerometer 2.25 oz. Video Camera 2.2 OZ. To prevent the vibrations and the g loading from damaging the avionics, the components were originally designed to be mounted on two plywood boards. The boards were in the configuration proposed by the LOST group. Rubber washers were to be placed between the nuts and bolts holding each of the pieces. While that would have prevented transverse vibration, most of the loading will occur axially along the board. Therefore, springs were to be positioned between the caps and the board to provide more flexibility. Two long rods through the board were to secure it inside the canister. The proposed design could have greatly decreased the chance of avionic damage occurring during the mission. A rough schematic of the initial design is pictured below. 52

9 Figure 29: Simple drawing of the redesigned canister Airtight Sealing The sudden change in air pressure will cause condensation to form on the window of the camera and on the electronic components. To prevent this, the canister needed to be airtight. Initially, the ends of the canister were to be sealed with O-rings. The extruding rods would have then been sealed with Teflon tape. The break line that will trigger the power of the system upon ejection will also need to extend outside the canister. The wire will need to travel through a hole that is capable of closing again when the wire is cut. Other lines may also need to exit the canister. The method of sealing these connections was still being evaluated and will be discussed further, below. Two valves will be placed at the end of the canister. One valve will pump in nitrogen and the other release valve will allow the air to escape. The nitrogen, an inert gas, will provide a dry and stable atmosphere for the avionics. 53

10 Temperature Control The other design constraint is the sensitivity of the components to temperature. The HAIP project will eventually be deployed to 60,000 feet above sea level. Several of the current components are ineffectual at low temperatures. The average temperature at 60,000 feet is -57 C, well below the operating range of three of the components, as shown in the table below. The balloon is neutrally buoyant so the canister will be at a high altitude for a long time. The inside of the canister would reach temperatures similar to the air around it. In the future, insulation or a safe heating device inside the canister could ensure the proper operation of the equipment. Another possibility is to purchase more robust equipment. Table 4: Operating temperature ranges for sensitive components Component Operating Range in C Reference Camera -1.1 to [wireless 2002] GPS Overlay Board -10 to 70 [icircuits 2002] GPS Receiver -30 to 85 [Garmin 2002] For the mission during the summer, there is no need for temperature control because the rocket is expected to fly to only 8,000 feet. At this altitude, the temperature is about -1 C. The camera is the most limiting of the components. Without protection the camera could only operate up to approximately 8,500 feet, where the temperature drops below 1.1 C. The Lexan canister would have offered some protection, as it is a good thermal insulator. However, these design goals where met using the alternative design that is described below. 54

11 6.2.3 Final Canister Redesign The final Initech canister was redesigned and built with cost and material availability in mind. The structure was also built with our four main concerns taken into account. Each individual requirement is discussed separately below along with the validation of the final design. Structural Integrity The expensive acrylics and the carbon fiber were abandoned for the readily available and inexpensive PVC (Polyvinyl Chloride). The manufacturer of the GPS Antenna and the Transmitter guaranteed that PVC does not attenuate radio waves. PVC is weather and corrosion resistant. PVC also has a high strength to weight ratio. The self-extinguishing property of PVC was beneficial during the ejection tests. A more flammable material could have caught on fire. In addition, PVC is a good thermal insulator. The outer structure is constructed of an 8 section of green PVC pipe. The pipe is capped at one end with a threaded female adapter. A two-inch section of the adapter was sliced off to reduce weight. The adapter was then affixed to the pipe with PVC glue. A screw-on-cap then fitted into the top of the adapter. The threaded top provides easy access to the inside components of the package. The final canister is shown in the figure below. 55

12 Figure 30: Final Redesigned Canister Originally, the camera window was made of Plexiglas as in the original carbon fiber canister. A preliminary structural integrity test was conducted on the canister during the ejection test on April 26. The PVC structure endured as expected, but the Plexiglas cracked on impact. A piece of 0.09 thick Lexan then replaced the cracked Plexiglas. The superior impact resistance of Lexan is shown in a graph below. Lexan was also used to provide vibration regulation for the internal components. Vibration Control The second constraint for this system design is the isolation of the vibration caused by the range of g loading experienced on launch, ejection, and landing. The components must be stabilized to prevent this movement. To accomplish this, two perpendicular 0.09 thick Lexan boards were cut and fitted into each other securely. The 56

13 ends of the structure were fitted with two pieces of hobby plywood. The soft plywood will offer some flexibility to soften the vibrations. Epoxy was used to attach the two ends to the Lexan boards. The GPS overlay board, the accelerometer board, and the GPS antenna were connected to the Lexan with nuts and bolts with extra washers. A hole was cut in the bottom plywood board to fit the camera lens, so it can see through to the Lexan. The wood offers a buffer to protect the camera from external forces. The two battery packs were attached to the inner structure with worm or hose clamps. Finally, the transmitter was attached to the top wooden board using NASA s invention, Velcro. The wires are connected through slots in the Lexan and one quarter of the internal structure houses the majority of them securely. The entire internal structure fits tightly into the canister to prevent superfluous movement. The internal structure is shown in the picture below. Figure 31: Internal Mounting Structure 57

14 Airtight Seal The canister then needed to be prepared for the change in pressure during the flight to prevent fogging on the Lexan and to prevent water damage to the parts. The PVC adapter was glued to the pipe itself, providing a solid seal. The threads of the top and inside of the adapter were wrapped with Teflon tape to prevent air leakage. A standard refrigeration valve was used to allow Nitrogen to be pumped into the canister. This inert gas will replace the water vapor. It was discovered that only one gas fitting, instead of two, is needed to purge the inside of water vapor. The event timer will most likely need a wire to pass to the outside of the canister wall. The hole of this wire could also be closed with Teflon tape. A superior possibility would utilize one-minute soft epoxy around the wire. The soft epoxy allows the wire to be removed. For the flight, a simple automotive silicon gasket sealer should be used in addition to ensure the epoxy has no leaks. The canister should be tested prior to use for air tightness. A simple test of the seals would entail filling a tub with water and placing the sealed canister in the water to watch for bubbles. If bubbles appear, the canister has a leak where the bubbles appear. The leak would then need to be closed and then the canister should be retested. Temperature Control As mentioned above, PVC is a good thermal insulator. Also, the altitude of the scheduled test flight being conducted this summer by the next HAIP group will only be 8,000 feet. The operating range of the components will not be compromised during this test. The canister will be ejected at 8,000 feet, but then will fall at least several hundred feet before the balloon reaches neutral buoyancy. The short time at the apogee altitude further limits the change of an equipment malfunction due to temperature constraints. 58

15 6.3 Future Work The future group(s) will need to mostly focus on preparing the canister for the actual rocket test flight. Any extruding wires will need to be sealed from the two event timer controlled actions. The timer may control the powering on of the devices and the severing of the line connecting the balloon to the canister. The canister would need to be tested for leaks, possibly by placing it in water to look for bubbles. Before takeoff, it must be purged with Nitrogen before the test flight to expel the water vapor inside the canister. The next team could also conduct some vibration and loading tests if time allows beforehand. If the HAIP requires operation at higher altitudes, the limiting temperature ranges of some of the components should be addressed in future studies. Either insulation or stronger components could be exploited. 59