You can't use Solid fuel with MIRV bus because you wont be able to control the MIRV bus speed to insert multiple warheads at different orbits. Solid fuels doesn't give you an ability to throttle up or down the speed. Fuel liquids does that. Some technical explanations below from web
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https://www.quora.com/When-is-a-sol...et-LVM3-by-ISRO-use-a-solid-rocket-propellant
Many of the rockets use both solid and liquid propellant stages.
Generally solid propellant is used in lower stages.The reason is rocket requires higher thrust during lift off due to high atmospheric pressure which is easily given by a solid motor compared to a liquid engine for the same mass and size.
And liquid propellant in upper stages because it offers fine control for precise injection of satellites into the orbit.
Solid-fuel advantages:
- More thrust for a similar size rocket
- solid motors are easier to manufacture, store and handle a year before.
Solid-fuel disadvantages:
- Can't be turned off- once the burn starts, it goes until fuel is used up
- Lower specific impulse.274.5 sec.
Liquid-fuel advantages:
- Variable thrust- the amount of fuel and rate of burn can be changed in flight i.e.more control
- Can be turned off and on whenever required.(THIS CONCEPT IS USED BY ISRO TO SEND MULTIPLE SATELLITES TO DIFFERENT ORBITS BY SHUTTING OFF PS4 ENGINE.)
- earth storable propellants(UH25+N2O4): around 340sec
- Semi-cryogenic (Kerosene - LOX) : 350s - 360sec
- Cryogenic (LH2 - LOX) : around 450sec
Liquid-fuel disadvantages:
- Difficult to store, maintain and service the propellant to liquid tanks.
- So many complex parts making it difficult to design and fabricate the engines (Cryogenic engine took 20 years for INDIA to design making it 6th in the world to have the technology)
- Vulnerable to leaks (GSLV-D05 LAUNCH CALLED OFF FOR THE SAME REASON)
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https://www.quora.com/When-is-a-sol...et-LVM3-by-ISRO-use-a-solid-rocket-propellant
Due to high propellant density, solid rocket motors produce high amount of thrust, but it has lower specific impulse i.e. it will burn out very fast.
Liquid propellant has higher specific impulse but they produce lower thrust, that means it can burn continuously for longer duration but will provide lower thrust amount. So, to produce more thrust more no of engines will be required.
Due to presence of high atmospheric pressure, lower stages of rockets are designed to give very high thrust, so that the rocket could climb to high altitude very fast where atmospheric pressure will be low. To do that either solid rocket engines or multiple stages of liquid or cryogenic engines are used.
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https://www.quora.com/Why-does-NASA-use-solid-rocket-boosters
Solid boosters suck in a number of ways. First, they can’t be stopped, started, or throttled up and down. This is bad for accuracy and makes them bad for most deep space missions, which often need many burns.
Second, their specific impulse (essentially, mpg for rocketry) is meh compared to most liquid propellants. Thirdly, when they fail, there’s rarely any warning, unlike liquid propellant systems, in which many failure modes give warnings and at least a slight chance of shutting down or aborting. And if you do want to escape from a failing solid it’s hard, since acceleration can’t be shut off, so you’d need an even stronger solid escape motor to pull you away from the still-accelerating rocket.
What solids are really good for is the ability to store then and fire them at a moment’s notice, which is why our missiles are all solids. But neither of these is a huge priority for NASA’s application, since NASA rarely has to deal with surprise launch needs. They also have a lower development cost, which is good for political/programmatic reasons. But in the long term, they are more expensive than liquids, especially if we make the liquid boosters reusable. So still a bad idea.
Now _hybrid_ rockets, which usually pair solid fuel with liquid oxidizer, and which are throttleable, are another story, and are worth some consideration. I suspect they still lose out, however, because they are probably harder to make reusable.
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Solid fuel Pros
- Can achieve very high thrust (each of the Shuttle’s SRBs produced more than twice as much thrust as the three Shuttle main engines combined).
- Much simpler (they’re basically a long tube with solid fuel inside and a simpler metal skirt as nozzle at the end), thus in general more reliable.
- Because of their simplicity they’re cheaper to develop/test/build.
- Very easily storable fully-fueled for very long times.
Cons
- Horrible Isp (mass/thrust efficiency) compared to liquid-fueled engines: the Shuttle’s RS-25 has an Isp of 366 seconds a sea-level, while the SRBs had an Isp of 242 seconds — that +100 s difference is huge in rocketry
- Can’t be throttled or shut down once they’re lit, so they’re one-time affairs. That means they’re no good if you need the ability to perform multiple burns, if you want launch abort capability after lighting, if you need precise real-time throttle control (like for soft-landing a rocket stage), etc. (Note that their thrust does not have to be constant, but the thrust profile is determined at design time and can’t be changed during flight.)
For very small rockets (think hobbyist or missiles) and “light-lift” orbital rockets (like the
Minotaur-C , a four-stage all-solid launch vehicle with a 1320 kg capacity to LEO) their simplicity and low cost makes them the preferred choice. In such mission profiles you don’t need throttling capacity, and the light payloads mean the low Isp isn’t too much of a problem.
For launching heavier payloads into orbit their low Isp really starts to hurt, so the extra cost and complexity of liquid-fueled engines becomes justified. Still, building very large liquid-fueled engines is difficult, so many past and current launch systems use a combination of both solid- and liquid-fueled engines in an attempt to balance each’s advantages and disadvantages.
