What does "a rocket in your pants" mean?

The rocket has been synonymous with speed in everyday language for centuries: phrases like "to take off like a rocket," "to go like a rocket," or more colorful variants describe someone who starts off suddenly and doesn't stop. The expression doesn't appear in flight manuals or technical glossaries; it's a popular hyperbole. Even so, the metaphor picked the right machine, because few devices built by humans accelerate so continuously and so steadily.

An orbital launcher lifts off with modest acceleration, barely 1.2 to 1.5 times gravity, because it starts out with full tanks. As it burns propellant it gets lighter, and the same thrust accelerates it more and more, until the flight computer throttles down to avoid exceeding about 4 g. In about nine minutes it goes from standstill to the 7.8 km/s (about 28,000 km/h) that low Earth orbit requires. A firework rocket is a different thing: a gunpowder motor that burns for one or two seconds and launches the casing to a height of 100 to 300 m, where it explodes.

The rocket's speed comes at a steep price. For one ton to reach orbit, twenty to thirty tons of propellant are needed, and the entire vehicle is designed around that imbalance: tanks that take up almost all the volume, structures pushed to the limit of their strength, and stages discarded in flight. Not even the fastest aircraft comes close: a fighter jet exceeds Mach 2, about 2,400 km/h, twelve times slower than a rocket in orbit. The popular image captures the blazing takeoff, but leaves out the engineering that makes it possible and the years of preparation behind every minute of ascent.