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Comment by dexen | Hacker News Reader

>Are supersonic aircraft more or less efficient

Surprisingly enough, supersonic planes have good fuel economy while supercruising - thanks to thin air at the high altitude, and speeds around Mach 2 ... 3 being good for turbojet engines.

Expect worse taxiing, take-off and climb fuel economy, due to dense air and also suboptimal (high) angle of attack. This means supersonic planes are best suited for longer routes.

Even the grandpa Concorde had good fuel economy in cruise, however it was overshadowed by atrocious fuel economy at low speeds, and the use of afterburners in transsonic region[1]. The designed but never built Concorde "B" would have notably better economy thanks to more advanced engines doing away with the afterburners.

[1] https://www.quora.com/How-fuel-efficient-was-the-Concorde-co...

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The taxi phase, at least, can be mitigated with an electric tug motor built into nose gear. Engines can be off or idle until needed.

https://en.wikipedia.org/wiki/WheelTug

Great point. Or motors in the rear wheels could probably get it up to 100MPH on the runway before the jets had to take over.

I'm not sure if those fuel savings would be worth getting rid of the integrated pre-takeoff engine thrust test, though.

Yea what is generating the power for those motors?

APU, normally. A smaller turbine generator, just enough to run electrical and hydraulic things, usually, and maybe to start the main engines. I think they shut off APUs after the mains come up. They use almost nothing compared to, say, an Olympus engine on the Concorde at idle.

An apu doesn't generate nearly enough power to taxi the aircraft. Since a large APU is essentially another turbine, you're back to using a turbine to propel the aircraft. If this really were a place to save money then the simplest solution would just have the tug pull it all the way to the runway. That way the aircraft doesn't need to take on additional weight for motors/apus which will sap fuel economy during flight.

To give some perspective, the hourly fuel consumption for Concorde is almost identical at Mach 2 than at max subsonic speed...

And Concorde powerplant had at some point the highest overall thermal efficiency at max speed:

https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593

The overall thermal efficiency of the engine in supersonic cruising flight (supercruise) was about 43%, which at the time was the highest figure recorded for any normal thermodynamic machine.

> Expect worse taxiing, take-off and climb fuel economy, due to dense air and also suboptimal (high) angle of attack.

This reminds me, in the book "Skunk Works", SR-71 Blackbird pilots mentioned their schedule of mid-air refueling 20-30min after takeoff. After that they could make it 3-5hrs at high altitude before refueling again.

This is because the Blackbird took off with only partially filled tanks, to lower the weight and therefore the takeoff speed. It was not designed to take off or land with full tanks.

Also the tanks leaked untill thermal expansion at supersonic speed sealed them.

Wow, shouldn’t that be a safety concern? Fuel leaking all over the airbase prior to takeoff?

The fuel was so resistant to ignition that it required an external source (TEB in this case) to even get alight.

The SR-71 did take off with much less than full fuel load; IIRC to keep the mass low enough for safe T/O abort or maybe for a safe go-around.

As for books, I wholeheartedly recommend the excellent https://www.barnesandnoble.com/w/sled-driver-brian-shul/1123... written by one of the pilots.

Yea the SR-71 has very poor lift at slow speeds because of its lack of wing hence why it needs the chines on the forward section of the fuselage.

Yes, because of the second point, it makes sense to build new runways just to accommodate SSTs outside of normal landing procedures to let SST land while gliding down from their cruising altitude.

Concordes were wasting double digit of their fuel reserve while flying in the holding pattern if runways were not available for some reason.

> Concordes were wasting double digit of their fuel reserve while flying in the holding pattern if runways were not available for some reason.

Though note this was uncommon; they were typically cleared for a direct approach all the way from cruise to touchdown. Part of the reason why they rarely got put in the holding pattern was the fact that it would quickly turn into a low fuel emergency, and part was giving the SST preferential treatment.

Is it accurate to say that high speed efficiency matters much more for any flight a supersonic jet would reasonably take

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