3 ms·
When a jet engine burns fuel, some energy goes into turning the fan, and some energy goes into creating thrust. At low-mach, the fuel burn is primarily produci
by etwigg 10y ago
When a jet engine burns fuel, some energy goes into turning the fan, and some energy goes into creating thrust.
At low-mach, the fuel burn is primarily producing torque which turns the fan. This fan could be turned by electricity.
At high-mach, the fuel burn is primarily producing pure thrust, and the fan portion hurts the engine's performance.
Supersonic turbofans are a compromise that have to produce thrust from takeoff through to high speed. I may be incorrect, but I don't believe we have ever built a ducted fan which can use pure torque to generate supersonic thrust, although I guess I don't see why it wouldn't be theoretically possible. We can put megawatts of energy into a fast-moving flow using thermal energy and a brayton cycle. In order to put megawatts of energy into fast-moving flow using rotors would require a totally new design - nothing like the supersonic turbofans currently in production.
- nickff 10y agoYou have to distinguish the conventional (high efficiency) cycle of a jet engine at low Mach, from the reheat (low efficiency) operation usually used for high Mach. No engine (of which I am aware) has air flowing past the fan blades at supersonic speeds, because the trans-sonic transitions tear engines apart. There is one prototype aircraft (a naval derivative of the F-84) which did have a supersonic propeller, but it was very inefficient and loud. All jet engines compress sub-Mach 1 flows, then expand them at the very back of the engine to produce supersonic exhaust.
- etwigg 10y ago> You have to distinguish the conventional (high efficiency) cycle of a jet engine at low Mach, from the reheat (low efficiency) operation usually used for high Mach. Exactly. At high mach, almost all of the energy being put into the flow is from expanding combustion gases - not the compressor stage. If you wanted to go at supersonic speeds using only spinning fans, you will need a heck of a lot of fans. Maybe we could do it! But we haven't so far.
- hx87 10y agoI suppose one way to do so is to build an electro-thermal equivalent of a low-bypass turbofan, with a jet section to deliver high-velocity thrust and a fan section to deliver low-velocity thrust.