Development of a Cold Gas Propulsion System for the ... - SSL - MIT

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8 Conclusion The TALARIS CGSE is an application of mature technology to a very specific purpose. While cold gas propulsion has been used for decades, and several small robotic lunar lander testbeds have recently been designed and flown with cold gas propulsion systems, the requirements and constraints of the TALARIS program made the CGSE unique. Hopping as a mode of transportation for lunar or planetary surface exploration is a relatively new and untried idea, and the hover hop in particular is a maneuver which few if any space exploration vehicles have been purposely designed and built to perform. In addition, the dual role of TALARIS as testbed and demonstrator drove a unique design philosophy for the CGSE. Finally, the limited budget, aggressive schedule, core of student participation, and exceptional focus on safety all contributed to setting TALARIS and the CGSE apart from similar projects. Development of the CGSE to fit the unique conditions of TALARIS was a process that required the application of many tools and techniques. Familiarization with existing technologies was necessary to understand the baselines from which to create a design. Modeling was an essential tool for applying science to designing the CGSE, but it was important to understand the model’s limitations and to analyze its results in the context of real-world considerations and imperfections. Testing was critical for characterizing the performance of the CGSE but also for pointing out ways to iterate on the original design and improve the functioning of the system. And throughout the entire development process, it was necessary to keep track of requirements, to ensure not only that the CGSE could perform to meet set standards, but also that it was able to fulfill the overarching objectives for which it was originally designed. In the process of completing the work documented in this thesis, several lessons were learned which seemed to have broader significance beyond the development of the CGSE. They are presented briefly here, in hopes of positively influencing work on other parts of TALARIS or perhaps any project involving the design of a subsystem to be integrated into a larger system. Documentation is important. Many parts of the process of developing the CGSE, such as selecting the basic system architecture and writing the MATLAB model to name just two, were built on work started by previous students, and it was invaluable to have a record of not only the results of their work but the thought processes and assumptions that went into it. Documentation can help prevent the duplication of previous efforts, troubleshoot problems, and suggest (even through omission) possible new directions of work. Properly 118
documenting progress takes time which could be spent doing other kinds of work, but it is an investment which can pay off well in future. Safety is essential. Safety was a central consideration in all phases of CGSE development. Sometimes actions taken for safety reasons came at the cost of performance, such as the overall decision to develop a cold gas system rather than a monopropellant hydrogen peroxide system. Often, extra time had to be taken to ensure safety, as with the effort put in to developing, following, and updating checklists for CGSE operation. However, safety is another area in which effort put in up front pays off later. For example, at one point during CGSE flight system testing, the low side became overpressurized enough to vent the two rupture disks in the system. The root cause of the overpressurization was eventually traced to a leak in the main orifice of the flight regulator due to wear and tear on the seat in the valve, as mentioned in section 7.1.4. This specific problem had not been anticipated, but the possibility of overpressurizing the low side had been considered, and measures had been put in place to handle that situation as safely as possible. The rupture disks functioned as expected and released the pressure in a relatively controlled and predictable way; they were simply replaced after the incident, and it was possible to return to testing very quickly. Had the rupture disks not been in the system, the overpressurization might have been severe enough to cause a component to fail in an unpredictable way and to cause damage that might have been more difficult to repair. More importantly, no one was injured when the rupture disks vented, partly because everyone present at the test was following established safety procedures and staying in shielded zones. Because time and effort had been invested in safety beforehand, the rupture disk incident was a small hiccup rather than a major roadblock, and progress in the development of the CGSE was able to continue on smoothly. There is a balance to be struck between modeling/simulation and hardware testing. Modeling and testing were both important elements of the CGSE development process. However, limited resources meant that neither activity could be pursued in as much depth as might be desired, and compromises had to be made. For instance, the MATLAB model had many inherent simplifications and sources of error, as listed in section 4.1.5. For some of these, such as the lack of a heat transfer model, a method of addressing them was known but would take time to implement. Instead, it was decided to use the model as it was, make component purchasing decisions with the help of its results, and spend the time on testing instead. This turned out to be the right decision; the components selected proved to be acceptable in the single-stream tests, and CGSE development proceeded. However, testing 119
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Development of a Cold Gas Propulsio
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Abstract The TALARIS (Terrestrial A
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Table of Contents List of Figures .
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7 Ongoing and Future Work .........
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Figure 6-6. Identification of thrus
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Notation Acronyms and Abbreviations
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N newton Pa pascal psi pounds per s
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1 Introduction The TALARIS (Terrest
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the idea of hopping was born, and i
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However, as stated before, this doe
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Figure 2-3. Diagram of ACAT lander
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An alternate approach to resolving
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accurate conditions for testing GNC
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Ballistic hops tend to use less pro
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2.3.2 Comparison of Cold Gas and Mo
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Handling propellant There are sever
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and if the cold gas system was foun
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3 TALARIS CGSE Design Framework Aft
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Figure 3-1. Scaling of TALARIS terr
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(3) Providing attitude control Ther
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Figure 3-2 also shows the body coor
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4 Modeling and Flow Control Compone
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4.1.2 Rocket Propulsion Equations L
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variables in equation (4-8) deal wi
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equations. Equation (4-10) was then
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that of helium (0.227 MPa = 32.9 ps
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thruster solenoid valve, and chambe
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where
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discussed later in section 6.3.4, t
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The flight profile begins with maxi
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hop, any given valve or regulator o
Page 69 and 70: esponse time was an important perfo
Page 71 and 72: directly opens and closes the main
Page 73 and 74: If 1D isentropic flow is assumed, t
Page 75 and 76: 5 Single-Stream Component Testing A
Page 77 and 78: the solenoid valve, and a pressure
Page 79 and 80: As indicated in Figure 5-3, initial
Page 81 and 82: Figure 5-5. CGSE high side as const
Page 83 and 84: Figure 5-7 illustrates several aspe
Page 85 and 86: 6 Full Eight-Thruster Flight System
Page 87 and 88: Figure 6-2. TALARIS CGSE assembled
Page 89 and 90: stream tests revealed that changes
Page 91 and 92: Figure 6-5. Original CGSE control c
Page 93 and 94: other constraints. This was difficu
Page 95 and 96: variables (such as number of thrust
Page 97 and 98: One solution to this problem would
Page 99 and 100: One of the characterization tests w
Page 101 and 102: or more thrusters were firing toget
Page 103 and 104: Table 6-3. Valve timing metrics dur
Page 105 and 106: Figure 6-11. Redesigned CGSE contro
Page 107 and 108: The imaginary simplified thruster c
Page 109 and 110: traverse phase of a hop and involve
Page 111 and 112: Figure 7-3. GNC data from 3-DOF tes
Page 113 and 114: Tests on this vertical stand demons
Page 115 and 116: control the belay line can be put u
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Page 119: development under the supervision o
Page 123 and 124: were encountered, it was harder to
Page 125 and 126: [10] Bryant, K. M., Knight, C. J.,
Page 127 and 128: [31] Canadian Centre for Occupation
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[50] ISA - The Instrumentation, Sys
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Development of a Cold Gas Propulsion System for the ... - SSL - MIT

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