Rocket
A rocket is a vertically launched craft. Usually it uses rocket engines to propel it forward and doesn't utilise the lift of wings or winglets. It only uses control surfaces to help steering in an atmosphere. The main purpose of a rocket is to reach space and an orbit, or at least a sub-orbital flight.
With various stages, rockets can dump dry mass and become more efficient, while a spaceplane usually doesn't have stages. As they don't use lift to maintain altitude, the rocket engines need to counteract gravity. To determine if the rocket can lift off, the thrust-to-weight ratio can be determined. If the ratio is above one, the thrust generated by the engines is higher than the gravitational force and thus the rocket will lift off. Because the engines drain fuel, the craft gets lighter so the ratio also increases with the flight.
Because rockets don't need lift surface, they work in every environment and don't need an atmosphere. On the other hand, rocket engines with sufficient thrust are far more inefficient than jet engines with similar thrust.
Rockets are built in the Vehicle Assembly Building and then launched on the launch pad.
Start
The usual flight profile starts on the launchpad on Kerbin. Prior to liftoff, any jet engines need to be started to run at full throttle on liftoff. When the jet engines are running at full power, the liftoff can be initiated by activating the rocket engines and releasing any TT18-A Launch Stability Enhancers.
The rocket then starts to rise and continues so until reaching about 10 km. At this point a gravity turn is initiated by tilting in the desired direction. Depending on the targeted inclination, the direction of tilting might be different. For example, retrograde orbits, where the crafts rotates in the other direction than the body beneath it, the tilt would be to the West. With the gravity turn the craft starts to pick up horizontal velocity, which is crucial for a stable orbit. The engines continuing to accelerate the vessel to raise the apoapsis and periapsis out of the atmosphere.
As soon as the apoapsis and periapsis are outside the atmosphere (about 69 km), the crafts orbit will not decrease, and has reached a parking orbit. From this orbit it is possible to initiate orbital maneuvers like the Hohmann transfer to change the orbital shape.
