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Now you've completed your contract, and haven't wasted any useful stages. After activation, manually shut down the it tested engine (you'll need to manually re-enable/activate it later). This can be handy for contracts that require an engine to be tested through staging, but you don't want to discard the usable stage before it: Kill thrust with X, move the engine to be tested to the next stage, then activate the engine through staging. You can disable engines via control groups, or by right clicking on the engine to manually shut it down or activate it. This will prevent accidental engine activation from a mistyped key. It is advisable to shutdown your engines if you don't plan on activating them, such as before resource transfers, or if maneuvers have been completed. For example, Macs and some Linux devices such as ThinkPad, require Fn+ F5. Some keyboards also require to press a Fn key to access some special keys like F5.
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