Editing Turbojet (section)

== Early designs ==
Early German turbojets had severe limitations on the amount of running they could do due to the lack of suitable high temperature materials for the turbines. British engines such as the [[Rolls-Royce Welland]] used better materials giving improved durability. The Welland was [[type certificate|type-certified]] for 80 hours initially, later extended to 150 hours between overhauls, as a result of an extended 500-hour run being achieved in tests.<ref>{{cite journal|url=http://www.flightglobal.com/pdfarchive/view/1945/1945%20-%202113.html |title=Rolls-Royce Derwent &#124; 1945 |journal=Flight |publisher=Flightglobal.com |date=25 October 1945 |access-date=14 December 2013|page=448}}</ref>

[[File:J85 ge 17a turbojet engine.jpg|thumb|[[General Electric J85|J85-GE-17A]] turbojet engine from General Electric (1970)]]

[[General Electric]] in the United States was in a good position to enter the jet engine business due to its experience with the high-temperature materials used in their turbosuperchargers during World War II.<ref>{{cite book |first=Robert V. |last=Garvin |title=Starting Something Big |isbn=978-1-56347-289-3 |page=5}}</ref>

Water injection was a common method used to increase thrust, usually during takeoff, in early turbojets that were thrust-limited by their allowable turbine entry temperature. The water increased thrust at the temperature limit, but prevented complete combustion, often leaving a very visible smoke trail.

Allowable turbine entry temperatures have increased steadily over time both with the introduction of superior alloys and coatings, and with the introduction and progressive effectiveness of blade cooling designs. On early engines, the turbine temperature limit had to be monitored, and avoided, by the pilot, typically during starting and at maximum thrust settings. Automatic temperature limiting was introduced to reduce pilot workload and reduce the likelihood of turbine damage due to over-temperature.