My dad worked for Thiokol in the 60's - he was a chemical engineer. I saw 1,000,000 pound thrust engines static tested in the Salt Lake valley in the early 60's. Everyone there knew that liquid fuel 1st stages were needlessly expensive - the U.S. could have gotten to the moon much sooner had they been adopted. Proof of the intrinsic effectiveness and safety of solid fuel rockets is attested by the success of the Minuteman , Polaris and Posidon. The Challenger tragedy was due to the misguided requirement that the solid fuel boosters be reusable, not to any intrinsic lack of safety in solid fueled rockets.

Challenger resulted largely from managements decision to launch the shuttle in weather too cold for rubber "O" rings to work effectively. I recall NASA reluctantly choosing Solid Rocket Boosters (SRB) due to man rating concerns. The original phase "A" shuttle study called for reusable liquid flyback boosters and a shuttle capable of lifting approx. 20,000 lbs to LEO. The AF wanted 60,000 lb capability and 2,000 mile cross range. Of course, solid or liquid boosters have proven extremely reliable. The Nixon Admin. capped shuttle development budgets unrealistically low which caused NASA to scrap all liquid booster shuttle designs. SRBs were cheaper as far as up front development costs, but higher in operational costs for obvious reasons. NASA had always required rockets to be man rated and solid rockets were a little less forgiving than liquid rockets in that they cannot be shut down once fired. Solid rocket propellant is also a little less powerful pound for pound than certain liquid propellants. This can be seen in a measurement called "Specific impulse". A 1 million lb thrust liquid fueled rocket is more powerful than it's 1 million lb solid equivalent.

Marvelous image !!!