University of Colorado Department of Aerospace Engineering Sciences ASEN 4018 Conceptual Design Document ACES Airbreathing Cold Engine Start

Transcription

1 University of Colorado Department of Aerospace Engineering Sciences ASEN 4018 Conceptual Design Document ACES Airbreathing Cold Engine Start Monday 2 nd October, 2017 Project Customers Name: Captain Ryan Petrie ryan.petrie.1@us.af.mil Phone: Name: Alex Bertman albe5773@colorado.edu Phone: Positions: Electronics Lead Name: Tristan Isaacs tris9069@colorado.edu Phone: Positions: Heat Transfer Lead Name: Matthew McKernan matthew.mckernan@colorado.edu Phone: Positions: Testing and Safety Lead Name: Nicholas Moore nimo8741@colorado.edu Phone: Positions: Software Lead Name: Matthew Robak maro1934@colorado.edu Phone: Positions: Fluids Lead Name: Nicholas Taylor nita8293@colorado.edu Phone: Positions: Systems Engineering Lead Team Members Name: Jake Harrell jaha4210@colorado.edu Phone: Positions: Manufacturing Lead Name: Alexander Johnson aljo7168@colorado.edu Phone: Positions: Electrical Hardware Lead Name: TR Mitchell thmi9790@colorado.edu Phone: Positions: Financial Lead Name: James Nguyen jang5868@colorado.edu Phone: Positions: Structures Lead Name: Lucas Sorensen lucas.sorensen@colorado.edu Phone: Positions: Project Manager

2 Contents 1 Project Description Project Purpose Objectives Functional Block Diagram Concept of Operations Functional Requirements Design Requirements 6 3 Key Design Options idered ECU and Sensor System Stock ECU and ESB Sensor Bypass Unit Custom ECU and ESB Initial Energy Supply Chemical Reaction Mechanical Start Cold Soak Active Heater Low Temperature Battery Electronics Heating Conductive Heating Element Radiative Ceramic Heater Fluid Heating Element Fuel Delivery System Resistive Heating Fuel Additive Circulating Fluid Pressurize the Fuel Trade Study Process and Results Trade Study 1: Engine Control Unit/Engine Sensor Board Trade Study 2: Initial Energy Trade Study 3: Heating of the Fuel Delivery System (FDS) Trade Study 4: Electronics Heating Selection of Baseline Design Engine Control Unit/Engine Sensor Board Initial Energy Heating of the Fuel Delivery System (FDS) Electronics Heating Monday 2 nd October, of 27

3 1.1. Project Purpose 1. Project Description The objective of this team is to take a JetCat P90-RXi miniature turbojet engine and create an engineering solution that would enable it to successfully complete its start-up procedure, defined below, in under 4 hours, after being coldsoaked to -50 F. Such a design would enable a UAS (unmanned aerial system) propelled by the aforementioned engine to be restarted in sub-zero conditions where that would normally be impossible. Upon completion of this project, the potential usability of the JetCat P90-RXi in cold-soak conditions would greatly increase Objectives The subsystem goals for this project are broken down into three levels of success described in Table 1 below. The third and final level of the table is representative of a complete project success as described by the primary objective. With little data provided from JetCat Americas, the operational temperature range will have to be determined through testing the limits of the sensor package throughout the semester. Similarly, the fuel flow rate will have to be measured in order to determine a specific standard rate. Level 1 Level 2 Level 3 Table 1. Levels of Success for the JetCat AFRL Project Fuel Delivery System (FDS) Electronics Structures Time - The electronic system will - FDS will regulate Jet-A fuel regulate fuel flow - Compressor and turbines at the flow rate of standard and monitor the exhaust will rotate freely when cold operation when cold soaked to 30 temperature sensor and soaked to 50 F within a F. hall effect sensor, at T-VAC chamber. room temperature. - FDS will regulate and ignite Jet-A fuel at the flow rate of standard operation when cold soaked to 40 F. - FDS will regulate and ignite Jet-A fuel at the flow rate of standard operation when cold soaked to 50 F and integrated into the Jetcat system. - The electronic system will regulate fuel flow and monitor the exhaust temperature sensor and hall effect sensor, cold soaked to 50 F. - The electronic system will regulate fuel flow and monitor the exhaust temperature sensor and hall effect sensor, cold soaked to 50 F. and will be integrated into the larger Jetcat system - Structures will properly interact with, and not impede the functions of, either the FDS or electronics sub-systems when cold soaked to 50 F and integrated together. The entire engine design system shall complete its start-up procedure in less than 4 hours, the allotted time as stipulated by AFRL. The entire engine design system shall complete its start-up procedure in 8 minutes 42 seconds, the glide time of an average drone dropped from 30,000 ft. For the fuel delivery system, different temperatures were chosen based on low temperature behavior of elements in the FDS system. The stock Life-Po batteries produce minimal power at -30 F. At this temperature with the current engine design, the fuel pump will stop working and throttle commands will no longer be sent by the engine control unit (ECU). For this reason, this temperature was chosen for the level one success of the FDS. Similarly, the fuel will freeze at -40 F and will no longer flow nor ignite in the engine. Finally, the third level of success is the temperature specified by our client, -50 F. This third level of success includes integration into the system as a whole. For the electronics, the first level of success is simply that the designed electronics system performs the current nominal operations of the stock system. The second level is an electronic system that functions at -50 F, but isn t necessarily integrated into the larger system. The third level is full electronic function integrated into the system at large. For the engine structures levels of success, the only first level goal is only that the turbine and compressor rotate freely at -50 F. The second level of success is basically that the engine structural components work properly within the full system at -50 F. Since the engine could be dropped while attached to a conventional drone from 30,000 ft, the time taken to initiate its start-up procedure will also be a design consideration. Assuming a lift-to-drag ratio of 5:1, the glide time taken by the drone to fall to 100 feet above sea-level was found to be 8 minutes and 42 seconds. An altitude floor of 100 feet was chosen in order to avoid most ground objects. The first level of success for the time constraint will be to complete the full engine design start-up procedure in under four hours, the time limit stated by AFRL for the testing procedure at Wright Patterson Air Force Base. Level two will be to complete the start-up procedure in under 8 minutes and 42 seconds, the theoretical glide time stated above. In addition to the objectives listed above, the customer has also provided scoring criteria with which competing design groups will be evaluated against one another. These include minimizing added volume and weight to the system, as well as minimizing the modification costs. While keeping these additions to a minimum helps with the final competitive score, they aren t necessarily considered clear objectives. The customer also indicated that groups should Monday 2 nd October, of 27

4 strive to restart the engine in conditions that simulate pressure at 30,000 ft, but the primary goal of this project is to focus on the temperature at this altitude Functional Block Diagram The functional block diagram (FBD), shown below in Fig. 1, shows the normal operation of the unmodified engine inside the black boxes. The green borders around these boxes represent unknown modifications to allow the engine to start at 50 F. The blue arrows denote physical contact between different systems, such as for fuel and lubricant lines. The red arrows denote electrical commands from the engine control unit (ECU) to the engine starter and fuel/lubricant pump. The purple arrows represent data being sent from the engine to the sensor package then to the ECU. Figure 1. Functional Block Diagram 1.4. Concept of Operations The Mission concept of operations (ConOps) diagram, shown below in Fig. 2, depicts the series of events which put the efforts of this project in larger context. While this is not the purpose of this project, a small turbojet engine capable of a cold-soaked start would be used to deploy from a larger transport aircraft. This UAS system would be deployed under the assumption that it would be able to start its engine(s) quickly enough to provide the thrust necessary to avoid impacting the ground. For more specifics into how the engine operates and starts in these conditions, please refer to the later project ConOps diagram. Monday 2 nd October, of 27

5 Figure 2. Mission Concept of Operations The project ConOps diagram, shown below in Fig. 3, shows how the engine will be tested at Wright Patterson Air Force Base. Starting at point 1, the engine package containing the turbojet, electronics, and fuel will be cooled to a temperature of 50 F. This will be followed by the initialization of any active cold temperature mitigation at point 2. Once the engine is operable, it will ignite at point 3. At point 4, the engine will continue with its start-up procedure, spinning up the turbine until it reaches idle. After the engine is brought to an idle, it will power down and prepare either for an immediate restart or another cold soaked restart. Figure 3. Project Concept of Operations Monday 2 nd October, of 27

6 1.5. Functional Requirements Main: 0 The JetCat P90-RXi mini turbojet engine shall complete the start-up sequence when all components of the engine system, with the exception of the hand held wireless transmitter, have been cold soaked to -50 F. ECU 1: The engine control unit (ECU) shall control the functions of the JetCat engine during the start up and operational procedures. FDS 2: The Fuel Delivery System shall provide fuel for a successful start-up sequence and continued operation of the engine. ENERGY 3: An initial energy source shall begin the start-up sequence of the engine. 2. Design Requirements Main 0: The JetCat P90-RXi mini turbo jet engine shall be modified such that it is able to complete the start-up sequence within 4 hours when all components of the engine system, with the exception of the hand held wireless transmitter, have been cold soaked to -50 F. ECU 1: The engine control unit (ECU) shall control the functions of the Jetcat P90-RXi engine during the start up and all operational procedures. ECU 1.1: The ECU shall communicate with the engine sensor board (ESB) via wired connection. Motivation: This is based on the current hardware architecture. Potential solutions will require wired connections and transmission of data between the ECU and ESB. The ESB measures the RPM and temperature of the engine and sends this information to the ECU. This allows the ECU to determine necessary changes to engine control. This requirement is important as at low temperatures such as -50 F, micro-controllers and batteries will cease to function. To mitigate this problem, potential solutions focus on bringing the temperature of these systems back into operable conditions. Verification & Validation (V & V): Inspection: If stock unit is used the team will verify that the signals between the ECU and ESB are not compromised. Test: If a custom ECU or ESB is used, show that a command from the ECU is executed by the ESB and that an output from the ESB is processed by the ECU. ECU 1.1.1: The ESB shall monitor engine temperature with electronic sensors and report temperatures to the ECU. Motivation: The engine temperature is a key measurement for monitoring the condition of the engine. Current hardware architecture monitors exhaust gas temperature to determine if an engine shutdown is required or if the start-up sequence has been completed. It will be essential to design the ECU so it accurately measures temperature. V & V: Test: Confirm that temperature measurements throughout the engine are correct and being received by the ECU. This shall be done through software and physical tests. ECU 1.1.2: The ESB shall monitor RPM through the use of a Hall Effect sensor and report the measurements to the ECU. Motivation: This is based on current hardware architecture. Monitoring the RPM is crucial in determining if an engine shutdown is required or if the start-up sequence has been completed. V & V: Test: Verify that when the engine has reached its idle state, the RPM measurement received ECU is 38,000 RPM (the value from the manual). ECU 1.1.3: The ECU shall interpret the signals from the ESB and issue an engine shut-down command under under potentially harmful operating conditions. Motivation: Derived. Safety during this project is a large concern so safeguards are put in place to ensure damage to person or equipment is avoided. It should be noted that for this project, it is assumed that the engine will need to function at temperatures below the stock ECU s safe range. V & V: Test: Feed the ECU signals to simulate potential hazards and verify the ECU responds as expected. ECU : The ECU shall not issue an engine shut-down command for violating its default minimum operating temperature threshold. Motivation: While the team is unaware of the exact value of the default low temperature threshold, it is assumed that all testing temperatures are below the operating thresholds of the engine. The team will perform tests to determine this exact threshold. If the stock ECU is used, the final design will require a Monday 2 nd October, of 27

7 method of mitigating the low temperature shut-down command. If a custom ECU is made, a minimum operating temperature will be neglected when creating shut-down command parameters. V & V: Inspection: A visual inspection of the software would be sufficient to determine if this is feature of the ECU or not. ECU 1.2: The ECU shall operate and monitor the fuel delivery system (FDS) via a wired connection to the fuel pump. Motivation: The ECU is able to shut off the power to the fuel pump in the event that any abnormalities are found within the temperature or RPM measurements. This ensures the engine will operate safely. Due to the temperature requirements for this project, however it is known that the engine will initially be below the temperatures that the stock ECU considers safe. Therefore, once again the potential design solutions focus on modifying these safe operating temperatures, bringing the subsystems of the engine into their operable range, or some combination of both. V & V: Test: Verify the ECU can control the flow and turn off the fuel pump when it is required. ECU 1.2.1: The ECU shall provide the necessary amount of electrical power to the fuel pump. Motivation: Having the ability to set the power used by the fuel pump will be necessary to control the mass flow rate to counteract changes in viscosity caused by the cold soaked temperatures. Potential solutions to address the increase in viscosity include increasing the pressure within the fuel lines via the fuel pump or increasing the temperature via various heating methods. Both of these types of solutions will require control of the fuel pump and regulation of the amount of power it receives. V & V: Test: Verify the specified amount of power from the ECU is going to the fuel pump using oscilloscopes and other electronic measuring devices. ECU 1.2.2: The ECU shall control the fuel flow rate via the fuel pump. Motivation: The ECU is able to control the performance of the engine by regulating the fuel flow. When more thrust is required, the ECU provides more power to the pump to increase the fuel flow. Alternatively, in the event of a required engine shut-down, the ECU must be able to command the fuel flow to 0 in order to stop the engine. V & V: Test: The team will input commands to ECU and verify the FDS responds as expected. ECU 1.3: The ECU shall send and receive electronic signals from the GSU via a wired connection. Motivation: The GSU displays critical engine parameters, such as temperature and RPM, as well as error and warning messages. It is also responsible for beginning the start up sequence and test spooling. V & V: Inspection: Verify the GSU displays expected temperature and RPM information and can spool the engine when commanded. ECU 1.4: The ECU shall receive wireless signals regarding throttle position via user input from an R/C controller. Motivation: The start up procedure requires the throttle controller be at its maximum position. Then once the idle RPM of approximately 38,000 is reached the throttle is decreased to the idle position. This signifies that the start up sequence is complete. The ECU needs to be capable of distinguishing this difference in throttle placement. V & V: Test: Show the throttle controller can increase or decrease the engine RPM as desired. FDS 2: The Fuel Delivery System shall provide the fuel necessary for a successful start-up sequence and continued operation of the engine. FDS 2.1: The fuel pump shall regulate fuel flow rate into the engine to reflect the commands from the ECU. Motivation: The mass flow rate into the engine is the determining factor is how hot the engine can get as well as how fast the engine will rotate. This is crucial for a proper start-up sequence as well as a safe engine shutdown. V & V: Test: Verify the fuel could be pumped into the engine utilizing physical tests or analogs. FDS 2.2: A temperature sensor shall monitor the temperature of the fuel within the FDS. Motivation: The viscosity of the fuel depends on its temperature. Managing the fuel s viscosity is crucial for a successful start-up procedure. V & V: Test: Verify the fuel line temperatures can be read by the temperature sensor and that signals are sent to the ECU and given to the user for monitoring during the start up process. FDS 2.3: The fuel delivery system shall deliver fuel to the combustion chamber in a manner that allows for proper and continued ignition. Motivation: The ignition of fuel is required for the operation of jet engines. Without fuel, the combustion cannot take place and the engine would not operate. To be capable of ignition, the fuel must be atomized into a very fine mist. This mist provides the necessary fuel-to-air mixture for efficient combustion to take place. Monday 2 nd October, of 27

8 V & V: Test: Verify through modeling and analog testing that the fuel will have the capability of igniting when the glow plug is present. ENERGY 3: An initial energy source shall provide enough power to initiate the engine start-up sequence. ENERGY: 3.1 The initial energy source shall provide a TBD amount of energy at -50 F. Motivation: At -50 F most batteries and electronic systems will not operate. Therefore a small amount of initial energy is required in order initiate the start up procedure. The amount of energy is not known due to being dependent on the type of solution chosen. This value will be determined before PDR. V & V: Test: Verify that initial energy is sent to the system and that the system will start at temperatures below their operable range. 3. Key Design Options idered Once key design areas have been identified, several design methods are considered to define a viable system solution. ACES can be broken into four main design categories: ECU and Sensor System, Initial Energy Supply, Fuel Delivery System Heating, and Electronic Heating. ECU and Sensor System includes the Engine Control Unit and Engine Sensor Board (ESB), how they will communicate with each other and how they will interface with the engine itself. The ECU will control the engine through regulation of fuel flow, power consumption, data storage, and error analysis. The ESB will provide raw data for the engine s performance during operation to the ECU. The Initial Energy Supply will encompass a system that will provide an initial energy surge to the system at extreme cold temperatures, where JetCat s stock batteries break down and hinder power consumption. The Fuel Delivery System will regulate the heating of the fuel lines for delivery and ignition of fuel once cold soak conditions have been reached. Finally, electronic heating will necessitate a system that will regulate the temperature of the engine s electronics system such that the components stay in a safe operational range. These four areas will be designed with any combination of electrical, mechanical, chemical, or software solutions. The design tree below in Fig. 4 shows the breakdown of the design areas stated above. Figure 4. Design Tree 3.1. ECU and Sensor System The Engine Control Unit and Engine Sensor Board are vital to starting, controlling, and monitoring the engine. The ECU controls the functions of the engine, while the ESB executes the commands of the ECU as well as relays the outputs from the sensors. These boards exist to ensure proper and safe use of the engine under any conditions. The design team is already in possession of the stock ECU and ESB, therefore they already are a viable solution for engine control. However, the designers at JetCat implemented programming safeguards to prohibit the use of the engine outside of nominal conditions. As our -50 F design constraint surely falls outside of nominal conditions, Monday 2 nd October, of 27

9 design alternatives must be considered. The three alternatives which are being considered are warming the ECU/ESB system up to operating temperature, sending the ECU false data to replicate nominal conditions, or designing a custom ECU/ESB system which would be capable of operating in the adverse conditions required of the project Stock ECU and ESB Fig. 5 and Fig. 6 below depict the current hardware in place for the ECU and ESB, respectively. The ECU is a self-contained micro controller and I/O port system which is roughly 2.5 long, 1.5 wide, and 3/4 thick. This small system has numerous ports on both sides of its structure. These ports are connections for the battery, receiver, fuel pump, I/O board (for GSU display), output connections to potential ailerons/rudders/elevators etc, and an output to the engine. On the other hand, the ESB only has the one external connection which connects it too the ECU. However, from within the engine, the ESB connects to the exhaust gas thermocouple, Hall effect sensor, and the internal fuel pump. The physical shape of the sensor board is a roughly 1.5 wide circular arc that encompasses 1/3 of the circumference of the inlet cowling of the engine. Figure 6. Stock Engine Sensor Board Figure 5. Stock Engine Control Unit As alluded to previously, the exhaust gas thermocouple, ESB board, and ECU board, fuel pump/fuel lines/injector would have to be increased to the standard starting temperature of +20 F 10. Table 2. Stock ECU/ESB / Engine is known to work in nominal conditions. Simplifies the electronic work for this project. Monday 2nd October, 2017 Detailed knowledge of ECU/ESB system required. Heaters may be difficult to place within the tight spaces of the inside of the engine. Engine components are no longer manufactured. High power consumption by the heaters due to large change in temperature. Long start-up time due to the huge change in temperature. 9 of 27

10 Sensor Bypass Unit Due to the proprietary nature of the JetCat nature, detailed spec sheets, electronic schematics, and engine control software information are not available. Therefore, reprogramming the existing the ECU to tolerate adverse start-up conditions would most likely prove to be too challenging for this project. Instead, fake signals could be generated by a second micro-controller and sent to the ECU. This second micro-controller, which will be referred to as the SBU (Sensor Bypass Unit), would be in charge of reading and interpreting the output of the sensor board and relaying information onto the ECU, as well as relaying instructions from the ECU to the ESB. An important distinction of the interpretation task is that the SBU must be able to distinguish between sensor readings normal for the low-temperature environment, and readings requiring an immediate engine shutdown. This interpretation ability is crucial in order for safe operation of the engine. Figure 7. Flow Diagram for the ESB bypass solution Engine would not have to be fully warmed in order to function. Simplifies the electronic work for this project. Shorter engine start-up time. Table 3. Sensor Bypass Unit / Detailed knowledge of the communication standard between the ECU and ESB would have to be known. ECU power system would be burdened as there is one more device to power. Longer development time. Heater system would still be required Custom ECU and ESB Given the lack of information provided with the JetCat, another design option would be to remove Jetcat s electronics altogether, replacing them with a custom ECU and custom ESB. While this would avoid the issue of deciphering the black box of the JetCat hardware, it will pose to be quite difficult to successfully complete, as past JetCat groups have attempted to create their own ECU with no complete successes to date. One example of a past group which decided to create their own ECU is the MEDUSA team, whose custom ECU is shown in the below Fig. 8. Monday 2 nd October, of 27

11 Figure 8. MEDUSA s Custom ECU 2.0 This ECU was a custom printed integrated circuit. According to the team s Spring Final Review presentation, their ECU/ESB system was able to function, but the ESB was only able to be completed last minute. This shows how complex a custom electronic solution is. This is a large concern for this design option because the team has very limited electronics hardware experience. On the other hand, the team has extensive software experience, which could make this design choice feasible. Table 4. Custom ECU/ESB / Team in control of the software. Team in control of hardware connection types and communication. Shorter engine start-up time due to less heating being required. Community support is available 3.2. Long development time Unknown communication standard between ESB and Stock sensors. Limited team experience in electronic hardware Initial Energy Supply One of the central problems in running this engine is producing initial power to run critical functions of the engine. These functions include warming the electronics, fuel delivery system, and key structural components of the engine, as well as the operating the engine. The ECU, receiver, and fuel pump are currently powered by a LifePo-4 battery, which isn t recommended for use below -4 F. Because of this issue, the power sources must either be warmed sufficiently so they provide the necessary power to the ECU/receiver/fuel pump. In this section the team includes possible solutions that either allow the main power source to function or that function as the main power source themselves. There are several potential solutions to this problem, listed below Chemical Reaction There are numerous chemical compounds that when combined follow a known, exothermic (heat producing) reaction. More specifically the type of chemical reaction being considered is a neutralization between a strong acid and strong base, such as hydrochloric acid and sodium hydroxide. The equation for the heat producing chemical reaction is given below. + H(aq) + OH(aq) = H2 O(l) Monday 2nd October, of 27 (1)

12 Reactions of this type are known to release KJ of energy per mole of water produced. If those compounds were combined in close proximity to system batteries heat could be transferred to bring them to a working temperature. In order for a sufficient amount heat to be transferred into the batteries rather than the chemical solution. The acid and bases must be prepared in very high molarity as to limit the amount of solvent. Another consideration for the chemical solution is if the reactants will be liquids at -50 F. The freezing point of a solution of HCl and and water is shown in the below Fig. 9. This figure shows that a solution which is 25% HCl by weight would easily be satisfy the temperature restriction of the project 12. Figure 9. Freezing point of a Solution of HCl and Water The other chemical, the sodium hydroxide would need to be formed in an aqueous solution of methanol. The reason this alcohol would have to be the solvent is because it would not react with the water or other reactants and provides adequate solubility of the NaOH. Furthermore, the freezing point of methanol is F and so with the added benefit of the freezing point depression of the solute, would remain a liquid at -50 F. The pros and cons for this solution are tabulated in the below Table 5. Table 5. Chemical Heating / Reliably produces heat. Cheap and easy acquisition. Quick start-up. Potential harm to engine and personnel Limited team experience One of the advantages of this system is that its a reliable, proven way of producing heat, and it can be easily acquired. Unfortunately, this solution greatly adds to the safety considerations of the project, as the chemical reaction used could harm the batteries or electronics and pose as a great danger to personal harm. This will weight heavily in the later trade study Mechanical Start Another potential solution to the initial energy problem is the use of an auxiliary power unit (APU) that would be started using a physical mechanism. The APU would consist of a miniature internal combustion engine connected Monday 2 nd October, of 27

13 to a generator or a generator and internal combustion engine contained in the same unit that would provide electrical power to the electronics heating system, warming the batteries that power the ECU and engine. In order to avoid using batteries to start the APU, a human-initiated mechanical system (such as a switch) that starts the engine by creating a spark would be used. Additionally, this engine would use a fuel with a lower freezing point, so it would be more usable at low temperatures, thereby avoiding another engine cold start problem. The pros and cons for this solution are tabulated below in Table 6. Easy acquisition Doesn t rely on batteries Safe Table 6. Mechanical Start / High system complexity Expensive acquisition Difficult to convert APU power to electrical power The major downside of this solution is the introduction of a very complex component to the system. The energy from the APU would have to be converted into useful power with the right voltage and current so that it could be used to safely power the electronics heating system. The mechanical system to start the APU would also need to be developed. The APU would also have to reliably start and run at -50 F. On the upside, acquiring the APU would be relatively easy, if expensive, and the system would likely be safer than a solution that utilized volatile chemicals. A simple functional block diagram showing the basic layout and connection of this system is given in figure 10. Figure 10. APU functional block diagram Cold Soak Active Heater Since the primary issue with providing initial energy to the system is that the provided batteries do not function at very low temperatures, this could be mitigated by creating a system that would keep the batteries at a functional temperature at all times. This system would be designed to include a heating control unit (HCU) that would regulate the temperature of the electronic components and batteries, keeping them well within their operational temperature range during the cold soak process. The pros and cons for this solution are tabulated below in Table 7. Relatively simple No need to reheat battery Low Cost Table 7. Cold Soak Mitigation System must be constantly operating Risk of damage to battery due to high temperatures High energy use This solution may be problematic due to the fact that the heating system must be constantly running, including during the cold-soak process, meaning that it will utilize a large amount of electrical power and some components Monday 2 nd October, of 27

14 of the system will never truly be cold-soaked as they will never reach -50 F. This means that for this solution to be feasible within the context of the concept of operations shown in figure 2, the UAS must always have an internal or external power source and its batteries can never be allowed to drop below their operational temperature range. This means that this solution will not be successful if there is a power failure at any point during the start-up process. However, this solution ha a high likelihood of success as long as sufficient power is supplied since it involves keeping the batteries at nominal conditions under which it is known that they operate Low Temperature Battery Although most batteries do not produce usable electric power past about -30 F, there are some options for batteries that are operational at -50 F. These batteries would need to be acquired and tested, and likely would require voltage regulation. The power generated by these batteries would not be enough to power the engine system as a whole, yet could provide initial electrical power to the electronics heating system, thereby bringing the stock batteries back into their operational temperature range. A potential option for this battery is the 3.6v mah Primary Lithium Battery shown below 11. Figure 11. Cold Temperature Battery This battery has a minimum temperature of -76 F, making it a viable option for starting this engine at -50 F. 13 The pros and cons for this solution are tabulated below in Table 8. Very simple Little modification required Cheap Table 8. Low Temperature Batter / Unreliable to acquire Battery may not function as intended Battery may not be compatible with system electronics This is the simplest solution available due to the fact that the batteries could power the electronics heating system directly, allowing the stock batteries to power the electronics and engine once they reach their operational temperature, with very little modification with the exception of a voltage regulator circuit. However, these batteries do not provide much voltage and thus several of them will need to be used in order to accommodate the power requirements for the electronics heating system Electronics Heating As stated above, electronic components such as circuits and batteries begin to have significant performance deterioration once their temperature (ambient and internal) reach below -30 F. At this point they are not considered usable but do not completely lose functionality until their temperature reaches below -40 F. This is mainly due to the physical properties of the electronic components being altered as a result of either diminished or complete loss in electronic potential. Thus, it is imperative that a heating method be used to preheat the battery and the electronic components (such as the ECU and ESB) to a temperature above -30 F in order for them to reliably operate. The battery is also a source of risk for this phase, as it is volatile. Also of particular note is that the ESB is housed within the cowling of the engine. This puts it right in the path of the airflow, and in close proximity to the compressor blades. Because of its location, it will be in constant contact with freezing atmospheric conditions, and so space will be an additional constraint for designing a heating solution for the aforementioned circuit. This does not apply to the ECU and the battery as they are housed outside of the engine. It is assumed that there will be a source of initial energy (see 3.2), but this still requires heat addition. For all of the above mentioned components, it is important to consider design solutions that will correspond with their individual functional requirements. For this reason, the electronic heating design solutions below may be used in concert with each other. Monday 2 nd October, of 27

15 Figure 12. Batteries Conductive Heating Element The simplest option that fulfills the design goal of heating the electronics to operational temperatures would be to use a resistive heating system. This is simply composed of wire that conducts electricity, but with a high internal resistance. Typically these kinds of heating elements are very reliable in nature, and due to the first law of thermodynamics, it is very efficient as well. This option makes use of the concept of conduction, which requires a direct interface between the two surfaces. This method makes use of Ohm s law and it s relationship to the Joule-Lenz law, which states that P= I V (2) Because this circuit would only be composed of resistors, virtually all of the current passing through it will be converted into power. When rearranged, Ohm s law becomes P = I2 R (3) showing that it is possible to calculate heat addition from resistance. The equation that will govern the heat transfer into our system from a conductive heating element is shown below: Q k A (T 2 T 1 ) = t d (4) Table 9. and for Conductive Heating Element Highly reliable Low cost Fast start-up time Complex to manufacture High power consumption Possible risk of electronics damage due to high temperatures Radiative Ceramic Heater Much like the conductive heating element, the radiative ceramic heater acts on Ohm s law to turn electrical current into heat. In addition, all of the current passing through the circuit is turned into heat. This makes it extremely efficient. This option differs from the previous design solution, however, because its method of heat transfer is through radiation instead of conduction. The system will take the heat directly emitted by the resistor and pass it into a ceramic brick, which absorbs the heat and emits it through radiation. Radiative heat transfer is governed by the Stefan-Boltzmann equation: P = e σ A (T 24 T 14 ) (5) Monday 2nd October, of 27

16 While this method has similar benefits of the conductive method described above, it is susceptible to heat loss because of directional constraints, and the potential for heat loss due to dimensional constraints. For example, this method will require the design of dedicated housing to ensure its operational success. Table 10. and for Radiative Ceramic Resistor Highly Reliable Complicated Manufacturing Moderately Safe High Power umption Low Cost Moderately slow Start-up Time Fluid Heating Element The third option uses the method of convection to transfer heat into the system. This method involves putting a fluid in motion in order to transfer its heat into the surrounding environment. Unlike the two methods described above, it does not rely on electrical power to generate and convey heat. Instead, the fluid itself stores heat directly from an external source before transferring it through the use of Newton s Law of Cooling: q = h c A (T 2 T 1 ) (6). The fluid will be conveyed through a system of tubes that will pass by the critical electronics systems. This option has the potential to utilize a contained chemical reaction to provide heat instead of electrical power, which could be an extremely powerful means to fulfill operational requirements. On the other hand, it will require the manufacture of an external system, as well as the mass and volume additions that accompany it. Table 11. and for Chemical Liquid Convection Reliable High Power consumption Reasonably Safe (Completely enclosed) Complex and manufacturing intensive Only requires thermal heat source Very long start-up time 3.4. Fuel Delivery System The fuel delivery system consists of all components involved in the storage, delivery, and injection of the Jet-A fuel being used to start and continuously run the JetCat P90-SXi jet engine. These components include the fuel tank, the fuel hopper, the fuel pump, the fuel injector, and all of the fuel lines. As Jet-A fuel has a freezing point of -40 F, fuel lines ceasing up and becoming solid is a primary concern after a -50 F cold soak. Additionally, Jet-A has a pour point that is identical to its freezing point of -40 F, therefore fuel crystallization is not a concern until -40 F. This creates a need for a system to either heat the fuel to above its freezing point post cold soak or maintain the fuel s temperature at a nominal value above the freezing point throughout the entirety of the cold soak and start up process. Three different methods of fuel heating are being considered and each is summarized in the following subsections. A functional block diagram of the FDS is shown below in figure 13. Monday 2 nd October, of 27

17 Figure 13. FDS functional block diagram Resistive Heating A resistance heating solution would consist of a series of resistive wires, powered by a source separate from the ECU and receiver power sources, that would enclose each of the fuel system components. When powered, the resistive wiring would heat up to a predetermined temperature in order to transfer heat to the various fuel delivery system components via means of conduction or convection. This heat transfer process would lower the temperature of the fuel within the various system components to a point where the fuel temperature can be maintained above its freezing point throughout the course of the start up procedure. Due to the variation in fuel distribution and volume in each of the FDS components, it is possible that separate sets of resistive wire enclosures with variations in power may be required for different FDS subsystems. Table 12 summarizes the pros and cons for of this solution method. Table 12. and for Resistive Heating Simple Reliable Controllable Low cost Requires constant power source Potential safety hazard The resistive heating solution for the FDS would work in a manner almost identical to that described in section above, with equation 4 governing the heat transfer from the resistive wiring to the various fuel delivery system components. A functional block diagram of the resistive heating solution for the FDS is shown below in figure 14. Monday 2 nd October, of 27

18 Figure 14. FDS resistive heating functional block diagram Fuel Additive A fuel additive solution involves the use of a chemical compound or compounds that could either be mixed with the Jet-A fuel or used as a temporary fuel for the start up procedure only. These additives could potentially serve to lower the freezing point of Jet-A to a temperature below the -50 F cold soak temperature. Additionally, though the engine is required to run on Jet-A only, the start up procedure can use a different type of fuel. Jet-A1 for example has a lower freezing point that is extremely close to the -50 F cold soak temperature. Because of this, it may be a compound of interest for a potential start up fuel source. Once the start up procedure is complete, a valve activated manually or automatically would switch the fuel flow from the alternate source to the source of the standard Jet-A fuel. Table 13. and for Fuel Additives Does not require constant power No mechanical complexity Simple operation Quick start up time Potential harm to engine Fuel stability Engine reliability Three primary compounds have been identified as potential fuel additives or start up replacements for the standard Jet-A fuel. The first option considered is Jet-A1 fuel for the start up procedure as opposed to standard Jet-A. The benefit of Jet-A1 is that it has the same flash point and auto-ignition temperature as Jet-A fuel, but with a lower freezing point, -53 F 14 as opposed to -40 F for Jet-A to be exact. As the cold soak process is conducted at only -50 F, the benefit of using Jet-A1 is obvious. Jet-A1 is also just as attainable as Jet-A due to how much it is used in the aviation industry. A second compound that could potentially be used to replace Jet-A for the start up procedure is JP-4 or JP-8 jet fuel. Both of these fuels serve as the military equivalent to Jet-A1 but with even lower freezing point. JP-4 has a freezing point of -72 F 14 and JP-8 has a freezing point of -58 F 14. Therefore, both fuels would survive the cold soak process without freezing. However, both are fuels developed for military use while JP-4 was phased out of the military in This makes the availability of these compounds an area of concern. The third potential option would be using a fuel system icing inhibitor (FSII) such as Prist. This would be added to the Jet-A instead of replacing it during the start up procedure. FSII serves to lower the freezing point of any dissolved water in the jet fuel to -47 F, thus delaying the onset of any potential crystallization of water within the fuel. However, because the fuel will be cold Monday 2 nd October, of 27

19 soaked to -50 F, this would only be a partial solution. A functional block diagram of the fuel additive solution for the FDS is shown below in figure 15. Figure 15. FDS fuel additive solution functional block diagram Circulating Fluid This approach consists of a separate reservoir containing a heated fluid that would be fed continuously through tubing and would encase the temperature critical components of the FDS. This heated fluid would act to preheat the fuel prior to being injected into the engine and ignited. This fluid would circulate back into the thermal reservoir in order to prevent ice blockages and also help keep constantly heated fluid flowing around the fuel lines. Ideally this fluid would have a freezing point below -50 F and very high thermal conductivity. The fluid circulation method would work in a manner described previously in section 3.3.3, with Newton s law of cooling (equation 6) governing the transfer of energy from the working fluid to the various FDS components. Table 14. and for Circulating Fluid FDS Heating Safe Easy to maintain consistent temperature Low efficiency Complexity Requires additional heat source Slow start up time A functional block diagram of the circulating fluid heating solution for the FDS is shown below in figure 15. Monday 2 nd October, of 27

20 Figure 16. FDS circulating fluid heating functional block diagram Pressurize the Fuel This solution would involve increasing the pressure in the fuel to promote fuel flow. This could be accomplished by replacing the existing fuel pump with a larger, more powerful fuel pump. It could also be accomplished with another fuel pump, placed either before or after the existing pump. Although this wouldn t change the viscosity of the fuel or the freezing point, it would increase the velocity of the flow, potentially overcoming those other factors. This solution could also be combined with other FDS solutions. The biggest advantage of this solution is its simplicity and how easily it can be used in concert with another solution. The fuel could be simultaneously pressurized and heated, and replacing the fuel pump would not be a substantial problem. One other concern is that the increased pressure could damage the system in some way. The pros and cons for this solution are tabulated below in table 15. Table 15. Pressurization / Simple Unlikely to fully solve problem alone Could be combined with other solutions Could damage fuel delivery system Easy to acquire 4. Trade Study Process and Results For this project, four trade studies were deemed necessary to conduct: the engine control unit/engine sensor board, the initial power supply, the electronic component heating elements, and the fuel delivery system heating elements. Scores are given to each design considered on a scale of 1 to 5 with respect to each weighted metric. A 1 on this scale means that design solution performs poorly in regards to that specific metric while a 5 means that the design performs well in that specific metric. This is a very subjective scale and so in the process of conducting the trade study, the entire team stated numerous arguments for which each design should receive its respective score and debated until a unanimous decision could be reached. To perform these trade studies, five metrics were chosen to be standard across study categories: feasibility, reliability, team experience, cost, and safety. For each trade study other metrics were also included. This includes start-up time for every trade study except for the ECU/ESB solutions, manufacturability for the fuel delivery system and electronic heating solutions, development time for the ECU/ESB solutions, and complexity for the initial energy Monday 2 nd October, of 27