Armstrong, John C., Wilsted, H. D.
Methods of improving altitude-starting limits of turbojet engines were investigated. At a flight Mach number of 0.6, the altitude-starting limit of a production type turbojet engine was increased from 15,000 to 43,000 feet; at a flight Mach number of 0.8, from sea level to 47,000 feet. The improvements in altitude starting limit required improvement in all three phases of the engine start: ignition of combustors containing spark plugs, propagation of flame through the cross-fire tube to ignite the remaining combustors, and acceleration from windmilling speed to a useful rotor speed. Of the many variables investigated, increased spark energy together with reduced cable losses provided the greatest improvement in ignition limits. Enlarging the cross-fire-tube diameter to 2 inches raised the engine propogation limits to any altitude at which ignition could be established. Use of a variable-area exhaust nozzle to reduce turbine-outlet pressures during acceleration reduced the time required for engine acceleration about 40 percent.
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