A rocket engine test facility is a location where rocket engines may be tested on the ground, under controlled conditions. A ground test program is generally required before the engine is certified for flight. Ground testing is very inexpensive in comparison to the cost of risking an entire mission or the lives of a flight crew.The test conditions available are usually described as sea level ambient or altitude. Sea level testing is useful for evaluations of start characteristics for rockets launched from the ground. However, sea level testing does not provide a true simulation of the majority of the operating environment of the rocket. Better simulations are provided by altitude test facilities.