Comment by PaulHoule | Hacker News Reader

Comment by PaulHoule | Hacker News Reader

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It's a toughie.

Circa 1960 they imagined that supersonic military aircraft would "dash" when the mission required it but otherwise fly more slowly, then they realized the fuel economy of supersonic aircraft peaks around Mach 3. Something SR-71 class has good fuel economy at speed, but has horrible problems when it comes to (1) materials, and (2) taking off. The SR-71 was of course made of Titanium and dealt with (2) by taking off with as little fuel as possible and then refueling once it was up in the air -- an answer which isn't suitable for civilian use.

Also circa 1965 it was widely thought that SSTs would make airliners like the 747 obsolete quickly because they could make more flights in a given amount of time thus being able to pay the capital cost quickly. The US aimed to develop Mach 3 class airliners

https://en.wikipedia.org/wiki/Boeing_2707

and failed but the Europeans developed a slower airliner

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

which was "successful" in terms of getting into the air but probably doomed in economics from the very beginning. The Concorde couldn't actually fly more flights per day than the 747 on the routes it flew on so it didn't get the capital cost amortization advantage that a faster plane might have gotten, and fuel economy was worse.

A more ambitious plane might require some crazy ideas like

https://venturebeat.com/offbeat/nasa-sideways-supersonic-pla...

to solve the takeoff problem.

An issue with SR-71 materials and fuel: it leaked fuel (by design) when on the ground, because the panels needed room for expansion when they heated up during flight.

https://theaviationgeekclub.com/heres-why-the-sr-71-blackbir...

> then they realized the fuel economy of supersonic aircraft peaks around Mach 3

As I noted in a below comment, there isn't a way for this statement to be accurate without a lot more qualifications. For "equivalent technology level", Mach 2 cruise will beat Mach 3 cruise in passenger seat-miles per unit fuel burn, and below Mach 2 will be better still.

The extremely questionable economics of supersonic commercial flight were realized by technical people far before the 1960s US programs were terminated. Two excellent sources on this topic are by aviation historian Richard K. Smith,

https://www.jstor.org/stable/26802349 , with text freely available at: https://www.thefreelibrary.com/THE+SUPERSONIC+AIRLINER+FIASC...

and the book "High Speed Dreams" which also covers the 1990s HSCT era: https://press.jhu.edu/books/title/8516/high-speed-dreams

There's significantly more to the problem than just managing heat and "solving" takeoff.

"The SR-71 was of course made of Titanium and dealt with (2) by taking off with as little fuel as possible and then refueling once it was up in the air -- an answer which isn't suitable for civilian use."

Couldn't we, in theory, slingshot the planes into the air to achieve the same relative fuel burn rates ?

I think this only exists on aircraft carriers ?

> Couldn't we, in theory, slingshot the planes into the air to achieve the same relative fuel burn rates ?

The required release velocity would need to be so high that drag from air resistance (which increases with the square of velocity) would slow down the vehicle such that there would be no benefit.

See: https://en.wikipedia.org/wiki/Max_q

Thank you - that's interesting.

However, it then raises the question: What are they doing on aircraft carriers and how is that different ?

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

... I assumed those had some fuel-use benefit, but perhaps their only benefit is shortening the runway ?

Carrier catapults violently accelerate aircraft to the point they are going fast enough to lift their own weight, within the takeoff length on the deck.

You could likely lessen fuel burn of a supersonic aircraft taking off (as you could with a subsonic aircraft) by accelerating it via a catapult of sorts, but you have to do so at acceleration levels tolerable to all passengers, which would make for a long, unwieldy catapult. Release speed is also obviously going to be very subsonic.

The fuel needed to get to take-off speed is a tiny fraction of the fuel needed to get from take-off to altitude. In practice, the benefit is for runway length only.

The closest thing to a Mach 3 airliner that has ever flown is the XB-70 Valkyrie, which is a totally awesome plane: https://en.wikipedia.org/wiki/North_American_XB-70_Valkyrie

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