It’s a common misconception that a plane can’t go faster than the exhaust from its engine. That’s wrong.’
It’s related to a common misconception that rockets need something external to push against—also wrong.
In both cases, thrust is the equal and opposite counterreaction to the acceleration of a reaction mass. Jet engines accelerate the surrounding air, while rockets accelerate their own propellant. That’s why rockets need proportionately much larger propellant tanks—and why they work in space.
In both cases also, the exit speed of the exhaust has nothing directly to do with the speed attained by the craft.
For a rocket operating in space, any thrust at all will accelerate the craft by some amount. The greater the mass (and therefore inertia) of the craft, the less the acceleration from a given amount of thrust, but since there is no air resistance in space, even a tiny acceleration can eventually build up to tremendous speed.
Similarly, whether a jet engine accelerates air by 200 mph or 2,000, the force needed to accelerate it creates an equal and opposite force thrusting the aircraft forward. As long as that counterforce is greater than the force needed to push the atmosphere out of the way, the craft will accelerate. Once air resistance upon the nose of the craft comes into balance with the force accelerating air out the tailpipe, the craft will stop accelerating.
Afterburners are not, and never were, used because you need supersonic airflow to reach supersonic speeds. You don’t. Afterburners were used, (and still are), because an engine powerful enough to meet the requirements could not be fit into the space and weight allowances of the high-performance aircraft in which it was needed. Afterburners could add a lot more thrust generation for a given amount of engine mass for a whole lot less complexity and weight—only at the cost of much higher fuel consumption.
And afterburners were not the only solution. In the B-52s my dad flew and other large jets of the time, water and alcohol could be injected into engines just ahead of the combustors to increase the mass being accelerated through the engine. It was an easy way to significantly increase thrust during takeoff without increasing the weight of the engine carried throughout the flight.
But today, high-bypass turbofan engines can be made that generate a lot more thrust per pound of engine. Such engines can do the job without the help of water injection and most of the time, without afterburners, and being far more efficient, they save the mass that would have been wasted on a temporary boost in performance and use it to increase payload capacity and/or range.
Again, as long as engine thrust exceeds drag, the aircraft accelerates—even into supersonic flight. The engine of the F-22 raptor doesn’t need to eject supersonic exhaust, it just needs to prevent the supersonic slipstream from reaching the combustors and interfering with the burning fuel.
[“Wet” takeoff of a KC-135 aircraft using water/alcohol injection to boost the peak thrust of the comparatively primitive J-55 turbojet engines. The B-52 in the background used the same system.]
Write: Adam Daymude
