On the other hand, deep space probes like the Mars rovers seem to be generally on time, and at least reasonably within budget (and fabulously successful scientifically). I wonder what is making the difference.
NASA Science Directorate, which operates Mars rovers and sends planetary probes, is mission driven. NASA human spaceflight is a jobs program (it invents make work in order to excuse spending money) that is fatally afraid of taking on a project they might fail at, because that might endanger the spigot of endless government money for all the pet technologies (for the people inside NASA) and jobs in districts (for the funders in congress).
"is fatally afraid of taking on a project they might fail at, because that might endanger the spigot of endless government money for all the pet technologies "
A lot of projects fail without taking human lives with it. Ares was dropped after one single test flight of a simpler Ares I.
Arguably it shouldn't even have been attempted - the sheer stupidity of the whole idea of having a single large solid rocket propelling you up without the possibility of being throttled down and with the chance of having burning solid fuel raining on your parachute in case you needed to bail out was probably swept under the rug too many times for comfort.
And that clusterfuck was stopped before it'd be able to kill anyone. In the meantime, it produced some knowledge on large solid fuel 1st stages that may someday compete with SpaceX's tech.
It won't, solid rockets are inherently limited in their safety (for the reasons you pointed out), fuel efficiency and ability to be rapidly reused.
The fuel efficiency will never be that of kerolox, methalox or hydrolox engines [1].
Landing a solid the way Falcon 9 lands is impossible (there is no engine on the way back, and if you can shut it down safely and then relight it then you can't throttle it), and when you recover the parts of the rocket that are not fuel, "refueling" solid rocket is basically re-manufacturing (the STS boosters were re-manufactured, without any economy).
In general, the use of solid boosters for crewed NASA missions is a folly that was mandated by congress to funnel money into ATK (manufacturer of many military solid rockets and explosives from hand held to ICBM/SLBM, now Northrop Grumman). Ares I, Liberty and OmegA, and the SLS boosters, are just more of the same.
Indeed. The SRBs are a stupid idea. Someone must build a decent rocket factory in the same congressional district so that NASA can make safe rockets.
It makes sense if refueling is just filling them with fuel in an automated way (think a giant extrusion device printing layers of fuel inside a rotating empty rocket). That way, the cost of flying a rocket becomes downtime+fuel.
Wasting billions of dollars trying to prevent a few astronauts from dying, while that money spent on saving lives more efficiently could have saved thousands of lives, is called "statistical murder".
If you miss your launch window for going to Mars, the delay is two years; since you can't be 1 month late and have things still be okay, the initial schedule is more realistic and delays are much less tolerated.
