A heat engine is a device that converts heat into kinetic energy. The amount of heat that is converted into movement as a proportion of the total heat is the th

A heat engine is a device that converts heat into kinetic energy. The amount of heat that is converted into movement as a proportion of the total heat is the thermal efficiency. With a jet engine it's the amount of energy in the burnt fuel that's converted to both rotation in the shaft and the speed of the expanding burnt gasses exiting the back. Propulsive efficiency is how good the engine is a converting this energy into thrust. The Concorde's engines were a type where all the propulsion was from the fast gasses exiting at the back, i.e. a turbojet. Efficiency is less than 100% as energy is taken from the gasses to power the engine and not all the gasses exiting the rear usefully push the aircraft forwards. I don't have the exact number for the Concorde to hand but looking at [1] from my previous post, we can see it was typically around 60% in 1970, but these numbers are likely for (low bypass) turbo fans. These work in a slightly different way where they have more energy in the rotation of the shaft and less in the gasses going out the back. The energy in the shaft drives the big fan at the front which pushes a large volume of air more slowly out of the back. This is to address the problem of the gasses which exit the back but don't usefully push the aircraft forward. To get the total efficiency of the jet you have to multiply the thermal efficiency by the propulsive efficiency. edit: changed division to multiplication