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Rocket engine ignition systems are especially critical. If prompt ignition does not occur, the combustion chamber can fill with excess fuel and oxidiser and significant overpressure can occur (a "hard start") or even an explosion. Rockets often employ pyrotechnic devices that place flames across the face of the injector plate, or, alternatively, hypergolic propellants that ignite spontaneously on contact with each other. The latter types of engines do away with ignition systems entirely and cannot experience hard starts, but the propellants are highly toxic and corrosive.