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Looking at the Concorde's fuel usage taxing is only part of the picture. All turbine engines are very inefficient at low power outputs due to the need to use en
by agurk 6y ago
Looking at the Concorde's fuel usage taxing is only part of the picture. All turbine engines are very inefficient at low power outputs due to the need to use energy to create sufficient compression for them to function. On top of this the Olympus engines generated a lot of thrust at idle, so much so that only two were used for taxing and it was still apparently tricky to control.
While cruising at supersonic speeds the engines were actually the most efficient machines of their time achieving 43% thermal efficiency [0]. Despite being very efficient they still had high fuel consumption rates as the total amount of energy required to sustain those speeds is a lot higher than subsonic.
Modern jet are in the 50%s thermally efficient [1] but despite airframe improvements there has been no way to improve fundamental aerodynamics so that supersonic energy costs aren't high.
The Concorde did use reheat to take off and break the supersonic barrier which is very fuel inefficient.
Using a modern airframes and engines that also don't need reheat will reduce fuel consumption but not to a level anywhere similar to a modern jetliner.
[0] https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593 https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593
[1] https://www.nap.edu/read/23490/chapter/6#38 https://www.nap.edu/read/23490/chapter/6#38
- deleted 6y ago[deleted]
- throwaway_pdp09 6y agoI'm a bit suspicious of the qualification 'thermal' in 'thermal efficiency' which sounds like it's hiding something. From the wiki article on thermal efficiency https://en.wikipedia.org/wiki/Thermal_efficiency https://en.wikipedia.org/wiki/Thermal_efficiency, and I'm no wiser for reading it: "For a heat engine, thermal efficiency is the fraction of the energy added by heat (primary energy) that is converted to net work output (secondary energy)." Does thermal efficiency = actual efficiency in the case of jet engines? I find that difficult to believe.
- Neil44 6y agoYes thermal efficiency is a common rating for engines that work by burning fuel to make heat then turn that into useful power. You take the theoretical energy in a litre of fuel and work out how much of that the engine can convert into useful work.
- deleted 6y ago[deleted]
- agurk 6y agoA 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
- gsnedders 6y ago> To get the total efficiency of the jet you have to divide the thermal efficiency by the propulsive efficiency. Also need to consider combustion efficiency (i.e., the percentage of fuel turned into heat as an input to the heat engine), though for gas turbines this is typically ~99.9%.
- shawabawa3 6y ago> divide the thermal efficiency by the propulsive efficiency Multiply surely? Otherwise reducing the propulsive efficiency would increase overall efficiency
- agurk 6y agoYes, you're absolutely right. Edited my comment to correct it.
- C1sc0cat 6y agoThat's why you have tugs to move jets around - back in ww2 the Me226 was towed up to its start point on the runway by a Kitenkrad (and in some cases horses) to save fuel and engine life. I think the industry was thinking of having electric tugs or motors built into the plane to move passenger jets around on the ground to save the fuel used getting from the stand to the start of the runway.
- _0ffh 6y agoYou narrowly missed the opportunity to write "Kittenkrad", which sounds like the cutest form of transport yet conceived. Or, of course, "Kettenkrad", the german half-tracked motorcycle/tractor hybrid from WW2.