In terms of overall bang for your buck, solid-fuel rockets are pretty great: They’re dead simple, very reliable, and offer respectable efficiency in a very small form factor, as long as you’re prepared to handle a lot of thrust all at once and then never again. While some robots have attempted to use rocketsto jump from place to place, controllability has always been an issue, since solid-fuel rockets give you a fixed amount of thrust whether you want it or not, and that thrust isn’t always directed in exactly the way you’d like.