Re: Navigation Budget
"SRBs are incredibly well-behaved compared to LOX plus anything with carbon and/or hydrogen in it and a lot easier to handle and store, one reason most military missiles use solid fuels these days."
Solids make excellent weapon systems. Except the filling of the Shuttle SRB's, which slumps over time.
There virtues (storage vessel is combustion chamber, mechanical simplicity, once started never go out) are either irrelevant to commercial use or active liabilities. LOX is dangerous due to it's concentration and temperature. In time it warms up and diffuses into the air. In contrast both solids and hypergolics are intrinsically dangerous. Solids don't evaporate and a cloud of UDMH is an effective chemical weapon.
When solids are built, moved or stacked they are classed as an explosive and the expenses are those of an explosive, not a set of propellants which can be moved separately.
"The material they're made of (an auminium powder and perchlorate mix plus a binder in the case of the SRBs) doesn't explode"
Under normal circumstances, but solids have exploded and the combustion of 1.1 million lbs of explosive is pretty serious. The normal physics of operation are deflagration but it can move into the detonation regime (and has) and the transition process is not that well understood. It used to be thought hybrids were immune to this, but even they are not.
"If the flame had been directed towards the outside of the stack away from the ET then the Challenger would probably have survived and made it to orbit."
Highly doubtful. At that point you have a case weak spot and increased burning area of the grain. You now have a thrust vector acting on the stack it was never designed for. If this SRB fails you've now got massive force imbalance on the stack. If it does not you have a (growing) side force until the SRB burns out (prematurely) leaving the other (because you cannot shut them down) SRB to rip the stack apart.
As for the contention that only Thiokol could build large solids and my recollection there were at least 3 companies that could perhaps you should look at this little item on Shuttle design choices and mfg selection.
http://www.tsgc.utexas.edu/archive/general/ethics/boosters.html
"I don't know what fuel the Dragon capsule's LES is going to use. I presume it will be storable hypergolics such as UDMH and N2O4, not something I'd be comfortable sharing such a compact capsule with in quantity."
Agreed but they are the common state of practice. Again what does that have to do with Grasshopper, which is mostly LOX/RP1?