I am going to compose this particular post in Word, so that I can save something that I have had to say again, and again, and again!
Listen up people, large solid rocket motors are NOT as safe as large liquid propellant rocket motors are!!
You would say of course, but such liquid engines are far more complicated, and have far more moving parts (in the turbines and turbo pumps of the propellant feed systems). And I would concede that you would be correct there.
But that particular point is far over ridden by another pertinent part of the necessary manufacturing procedure for these two systems.
And that is in the testing of the final product as follows:
When you test a large SRB, that particular motor IS history, it is gone, never to return. This is an understandable result of such testing!
OK, now because of the nature of the homogeneousness of liquid propellants on the other hand, a large liquid rocket engine that is tested and runs well within its design parameters can (and indeed should) be used as the very engine that is then used on the launch vehicle itself. In fact, tweaking that can be done to make sure that the engine performs well can even sometimes be done right after the test, and then the particular engine is run again to ensure its safe operation.
This run that makes sure that the engine operates safely and well is called a “Green run”, at least at Rocketdyne, (which has built and tested some 95% of all the liquid rocket engines ever built for the US) but it may go by other names elsewhere. This has been done even on the engines destined for the one time use of launching commercial satellites and NASA space probes.
This is kind of akin to the automobile industry making a similar “Green run” on all of its automobile engines. And you certainly do not expect your new car’s engine to explode the fist time ( or even many times afterwords) that you turn the key in the ignition, now do you?
On the other hand a large solid rocket motor MUST depend totally upon its manufacturing integrity to make sure that motors subsequent to a particular successfully tested design have absolutely no flaws at all. For without this rather extreme version of Statistical Process Control (called STP in Quality Assurance circles) then it is just as likely that the motor(s) have now become very large potential bombs instead of the propulsion elements that they were originally meant to be.
Now, please don’t get me wrong here, the excellent employees of ATK in Utah have truly done a great job of such STP that the SRB’s of the shuttle have been safe (the Challenger incident would not be generally a part of this inherent unsafe condition, as that was an original design problem coupled with a NASA management mess that caused that).
But, this inherent testing problem still does make large liquid rocket engines inherently safer than large solid rocket motors. And no less a great rocket scientist that Wherner Von Braun so believed this that he would not under any circumstances even consider using large solid motors for the principle propulsion source for the venerable Saturn series of rockets (even though there were those that wished to do that even back then).
Besides which none of the other rockets, (some like the Delta II with an almost flawless record) of the US used for space launches use SRB motors. Although they are used for ICBM’s (which it is hoped will NEVER have to be used anyway). And also, no other country that launches such rockets uses anything other than liquid engines for their primary propulsion units either.
So, hopefully I have used facts and logic to bust the myth of the relative safety of solid rocket motors when compared to liquid rocket engines of comparable thrust!
So, thanks for bothering to read this relatively long post, and have a great day!
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