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From the ATK website (www.safesimplesoon.com)
#4 — 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 if the engine is designed to
 enable in-flight shutdown, there are failure modes that 
will be catastrophic for both liquid and solid rocket motor 
designs. The advantage of a solid rocket motor is that the 
chance of having a catastrophic failure is less likely. 
This is due to its simplicity relative to the liquid 
design. In the event of a catastrophic failure, a solid 
rocket motor actually provides more reaction time and 
better survivability for a launch escape system to protect 
the crew. Most catastrophic failures of a solid rocket 
motor actually result in a phenomenon referred to as thrust
 augmentation, which is easily detected by an In-Vehicle 
Health Monitoring System (IVHM), which can be used to 
signal the Launch Escape System.