The SRBs (Solid Rocket Boosters) cannot be shut down, does that make it less safe than using liquid rocket engines?
This is a very commonly asked question. In reality, once the vehicle is launched, the last thing the astronaut crew would want to do is shut down the main engines. The most reliable liquid rocket engine manufactured today is the Space Shuttle Main Engine (SSME), and the most reliable solid rocket motor is the SRB. Recent Probabilistic Risk Assessment (PRA) analysis for the Space Shuttle show that the contribution to risk during launch from the SRBs is an order of magnitude less than from the SSMEs. Also, shutting down a liquid rocket engine is not trivial. An important parameter used to look at the effects of shutting down a liquid rocket engine, which is suffering a malfunction, is what is referred to as the “catastrophic failure ratio.” This is defined as the percent of time that an engine will fail catastrophically. The accepted value for current rocket engines is 20-30%. The SSME and the J-2 are the only engines with in-flight shutdown capability in response to malfunctions. Even i
