Questions about solid fuels. The rockets that use them, the fuels that are burned, and the vendors that make them.
Solid-fuel (or solid-propellant) rockets were the first type of rockets to be developed, being used in (for instance) early fireworks. As the name suggests, they use a solid mass of propellant; this is usually an intimate mixture of finely-divided particles of fuel and oxidizer held together by a rubbery binder (the most common recipe is ammonium perchlorate composite propellant (APCP), which uses ammonium perchlorate [duh!] as the oxidizer, and - usually - aluminium powder as the fuel), although other varieties exist.
Solid-fuel rockets have a few things going for them:
- They are relatively cheap.
- They can be built to reliably provide vast amounts of thrust (far more than even the most powerful liquid-fuel rockets) without much risk of a rocket-explosion.
- They can be stored essentially forever, even out in the open, without their propellant degrading to any significant degree.
However, they also have considerably lower performance than most liquid-fuel rockets. The specific-impulse of even the best solid-fuel rockets can barely break 300 seconds even in a vacuum; this compares to sea-level specific impulses of 340 seconds or more for even relatively-low-performance liquid-fuel rockets, and of over 450 seconds for high-performance liquid-fuel rockets. Also, unlike liquid-fuel rockets, most solid-fuel rockets cannot be throttled or shut down once ignited (although it is possible to design a solid-fuel rocket that can be throttled, extinguished, and - potentially - restarted; the first extinguishable solid-fuel rocket was the U.S. Minuteman missile, dating to the mid-1950s), which can pose problems should an abort become necessary. Finally, although solid-fuel rockets don't fail often, when they do fail (often due to flaws in the chunk of solid fuel and/or the motor casing), they have a tendency to do so rather... spectacularly.
As a result of all this, solid-fuel rockets are widely used as strap-on booster engines to help get heavy launch vehicles off the pad (the titan-rocket versions III/IV, space-shuttle, ariane 3 and later, and the higher-capacity versions of the atlas-v and the delta4 Medium all use or used solid-fuel boosters, as will the under-development vulcan), as ICBMs (the military doesn't really care that using solid fuels rather than liquids means they need a bigger rocket to throw the same amount of warhead; what matters is that the missile can be stored fully-fuelled in its silo for decades and then launched on a moment's notice, all without exploding), and as small, cheap kickmotors for payloads that only need a little more delta-v; they are not generally used as the main engines for launching artificial-satellites (this task is usually left to a higher-performance liquid-fuel core and/or upper stage), although there are exceptions.