Rocket Science for Earthlings #42
a continuing series for the gravitationally impaired
A rocket firing and just lifting off the pad is accelerating, but it isn't moving very fast. What is moving very fast is the rocket exhaust, the jet. So all of the energy is really going into the jet and not into the rocket. This is why rocket powered automobiles are very noisy and spectacular, but not really an efficient means of transport. You need to get the vehicle up to the same speed as the exhaust jet to get maximum efficiency from a rocket motor. ( for further reading see "Space Science and Engineering" by Ernst Stuhlinger ) What you really need to get efficiency at liftoff is an exhaust jet that is cool, slow moving, and has a lot of mass, sort of like pushing against something big and heavy. The way to do this is to suck a lot of the surrounding air into the exhaust jet. The drawing below shows a couple of the methods commonly proposed to accomplish this. The high velocity jet from the rocket nozzle acts as a jet pump pulling in additional air from the front opening and shoving it out the back. A ram effect also works to increase the mass flow through the engine.
No rocket that I know of has used air augmentation to increase it's thrust, although the English Black Prince rocket had doors that opened to allow air to flow through the engine bay and mix with the exhaust. Jet airplanes have used air augmentation to good effect. Turboprop and turbofan aircraft engines use part of the heat energy from the jet to pull more air into the exhaust stream. That makes them more efficient.