Solids are not ideal for space exploration as they are difficult to control and reusing them is expensive (& thus usually isn’t worth it). I am fully on board t

Solids are not ideal for space exploration as they are difficult to control and reusing them is expensive (& thus usually isn’t worth it). I am fully on board to reusable liquid rockets, which can use extremely cheap propellants liquid liquid methane or liquid hydrogen fuel and (70-90% of the mass of propellant) liquid oxygen. These propellants are extremely cheap, about 25¢/kg of propellant on average, enabling very cheap launch costs, compared to around $5/kg in solid rocket motor propellant costs (and greater difficulty in reusing them and much higher dry mass and lower Isp). And the exhaust is really bad for the environment. The high acceleration and sometimes high vibration and inability to turn off in an emergency make them non-ideal for human spaceflight. And they’re basically use-once (with possibly of remanufacture) so you can’t acceptance-test the same motor like you can for liquids, and refueling them is a complicated process that can’t easily be done off-planet (unlike liquid rockets which can be and are refueled such as the ISS’s propulsion systems). So while there are some niche uses of them for space exploration (interplanetary kick stages for probes), from an energy, performance, safety, reusable cost, and ride comfort standpoint, I’m rooting for liquids.