That is fascinating. Why is that the case?
Is it simply easier to build something disposable? Or are there fundamental restrictions like the inability to stop a solid rocket motor?
Is it simply easier to build something disposable? Or are there fundamental restrictions like the inability to stop a solid rocket motor?
Secondly you have issues with fuel/oxidisers; cryogenic fuels/oxidisers need to be regularly stirred in low gravity because convection doesn't work the same as it would in the presence of gravity. This is why most RCS systems use non-cryogenic hypergolic bi-propellant fuels or mono-propellant fuels as the systems are much simpler; pressurised tanks, one or two one-way valve(s), possibly a catalyst, a combustion chamber and bell for the simplest of systems.
Using hypergolic propellants solves this problem. The trouble is that hypergolic propellants are inherently dangerous (they'll explode if they come in contact, that's the whole point) and are typically highly toxic. They're OK in small quantities, thus they show up in RCS systems, but making an entire orbital launcher using hypergolic propellants is a massive pain. It has been done, but the result tends to be more explodey than usual.
I think Russia might disagree with you. Wikipedia describes the Proton launcher as "one of the most successful heavy boosters in the history of spaceflight."
The SR-71 used TEB to ignite its engines, as JP-8 was so hard to ignite.
While the rocket is running, the acceleration caused by the motor causes the propellant to pool at the bottom of the tank where the outlet is. As soon as you stop the motor, the acceleration stops and the liquid starts floating around inside the tank. If you tried to restart it now, you would get an unstable mix of propellant and pressurizing gas which might not allow the rocket to startup properly.
To restart reliably, you need to apply a force to the tank to force the liquid back down to the outlet. During Apollo, this was done with many small solid rocket motors called Ullage Rockets. These would fire for just a few seconds just immediately before the rocket was restarted. The force was just enough to get the propellant down to the outlet in time to be sucked into the turbo pumps before ignition.