An adjective used to describe one way to get a missile out of a missile silo. The opposite of cold launched, this describes any missile whose flight begins with its own engines lighting. If this sounds frightfully dull, or if you can't imagine a missile starting its flight any other way, check out what is involved in getting a cold launched ICBM out of the silo.
Hot launched systems tend to be used as strategic deterrents--nukes--or for spacelift, since rapid reload isn't a priority, and system reliability is. A hot launched system is ignited while it sits in the silo, and when it comes out, its engine is already going full tilt. This gives a few key advantages:
- Fewer moving parts
- Easier maintenance
- Lower per-unit cost, especially in countries with cheap labor--most hot launched costs are associated with silo repair
- Marginally higher reliability
- Much smaller penalty for failure (read about cold launched failures to understand this distinction).
The trade-offs for this slightly more robust system are not trivial. The missile is heavier and slower out of the blocks than a cold launched system, since it has to carry the first 5 seconds of fuel internally, instead of leaving it behind in the sabot, and because it needs to withstand the intense heat from its own engines. Because it starts its engines while it's sitting still, the first five seconds of flight (while it overcomes inertia) are very fuel-inefficient--this inefficiency is slightly balanced by the fact that the tightly-sealed silo helps to milk all of the energy out of the hot, high-pressure gases (which, unfortunately, scorch the missile quite badly). Finally, the silo almost always requires weeks of maintenance and repair after launch. For an ICBM, one assumes that the silo isn't going to be around that long after a "real live" launch. Similarly, for a space launch, one usually doesn't require two launches in a short time, so this is considered part of the cost of doing business.
The website http://www.FAS.org credits the Soviet SS-8 as being the first hot launched silo-based missile. Technically, anything launched off a gantry is also hot launched, but this is a trivial set of systems to include. Under that definition, the Space Shuttle is hot launched--somewhat absurd, but technically true. Hot launched systems are easier and cheaper to build, but are generally considered an intermediate step towards building a cold launched system, especially from a military perspective.