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Low Cost Mission Enabling Cold Start for Small Engines

Description:

TECHNOLOGY AREA(S): Air Platform 

OBJECTIVE: Develop and demonstrate a low cost high altitude cold start ignition system for expendable small gas turbine engines. 

DESCRIPTION: The ignition environment of a gas turbine during high altitude (10,000 ft. to 35,000 ft.) air drop missions is very challenging. At the point of the drop, the pressure, temperature, and air mass flow are all low, and the entire gas turbine and fuel system are cold soaked to atmospheric temperatures (cold start). The ignition system has to light the combustor and spool up the engine to a self-sustaining condition over a finite period of time. For a given gas turbine and ignition system the resulting startup envelope is limited by Mach number, altitude, and environmental conditions as well as time from start of ignition to self-sustaining operation. Small engines are more challenging to ignite due to the constraints on the combustor design, primarily the high surface to volume ratio and low fuel flow injectors. The most common system utilized today is expensive pyrotechnic start cartridges. They result in fast spool up times and reliable high altitude starting with a wide startup envelop, but are expensive relative to the cost of the entire gas turbine and introduce handling, fragility, and aging issues. As the gas turbine decreases in size the percentage cost of the pyrotechnic start cartridge grows, representing almost 1/3rd of the entire cost of the system at the smallest scales. Commonly used lower power ignition systems, such as spark ignitors or glow plugs, have restrictive startup flight envelopes and long spool up times. This is very mission limiting due to the need for dive-to-start/climb to cruise mission profiles resulting in significant inefficiencies and longer times to target. It also decreases the survivability of the ordinance. Novel ignition approaches for the high altitude wing drop application are sought that result in a wide startup envelop, with a reasonable spool up time, and with reduced costs compared to existing approaches. Due to the nature of the mission, low cost is the most important attribute. The new approach must also be compatible with existing carriage resources and still meet the all operational requirements of the mission, such as carriage/launch loads of the system, thermal cycling, and long term storage. 

PHASE I: Create a preliminary design and show the feasibility of the novel ignition approach to achieve a wide startup envelop for a small engine in the air drop application. An initial estimate of the system’s weight, volume, viability to meet operational requirements, and the scalability of the approach should be included. Additionally, special attention should be made to estimate the resulting systems cost compared to current ignition approaches. 

PHASE II: Design, build, and test a prototype of the ignition approach developed in Phase I for a specific small gas turbine system in both a laboratory setting and in the gas turbine. It is recommended that collaboration with either a turbine engine manufacturer or a Government lab to perform the ignition test in the gas turbine. A vision system design should also be developed during Phase II. A refined estimate of the system cost and performance parameters should be made based on the vision system design. 

PHASE III: Develop a vision system level design for a specific existing expendable small gas turbine engine. Demonstrate/assess the startup envelop, spool up time, and projected final system cost. 

REFERENCES: 

1. “Comparative Analysis of Gas Turbine Engine Starting”, A. Beyene and T. Fredlund, American Society of Mechanical Engineers, 98-GT-419, (1998).; 2. "Gas Turbine Performance", P.P. Walsh and P. Fletcher, Chapter 9, Second Edition, Blackwell Science Ltd, Oxford UK (2004).; 3. "Experimental Investigations into High-Altitude Relight of a Gas Turbine", R.W. Read, Doctoral Thesis (2008).; 4. "Research on Windmill Starting Characteristics of MTE-D Micro Turbine Engine", Chen Xia, Xin Fu, Zhaoyun Wan, Guoping Huang, and Jie Chen, Chinese Journal of Aeronautics, Volume 26, Issue 4, Pages 858-867 (2013).

KEYWORDS: Low Cost, High Altitude, Cold, Start, Ignition, Expendable, Small Gas Turbine Engine 

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