Is this new? I remember reading an article a long time ago (maybe 15 years?) discussing how growth in aircraft speed was overall exponential, but linear for each new technology that was introduced.
It would be a change to macro models of economic growth, if accepted. The default is that TFP grows exponentially. There's also a useful comment by Marginal Revolution here (https://marginalrevolution.com/marginalrevolution/2022/04/ad...) where he points out that TFP itself is really just a residual, and perhaps not a very well-defined concept. That is, if you regress
Y = AK^beta L^(1-beta)
with K being the amount of capital and L the amount of labour, A is just the unexplained "scaling factor" and it gets called TFP. But what TFP actually is... is a bit of a whatever-you-like.
Why is TFP assumed exponential by default? What's the theory behind that? A time lag in the transfer between exponential input growth (e.g. population) and exponential output growth?
No. The inputs are labour and capital, so that's already taken care of.
One answer might be "TFP reflects technical knowledge; the more knowledge there is, the easier it is to generate new knowledge". But you'd have to get into growth macro for the details. Indeed, Romer is the person.
It's not always assumed to be exponential. It was assumed exponential by Paul Romer because long-term economic growth is pretty exponential. Much subsequent work has followed that model, but not all of it.
A justification for maybe why this is the right model is that the previous n technologies all help make the next technology possible in time ~1/n because they (n existing technologies) all contribute to innovating the next technology.
Aircraft speed maxed out a long time ago. "All the aircraft which went significantly over Mach 3 are now in museums." - Scott Crossfield, former X-15 test pilot.
Well any exponential growth curve in the real world is actually a logistic growth curve in disguise. A civilization would have to go from Kardashev II to Kardashev III in about the same time as it went from I to II to maintain exponential energy growth.
As an aside, the Falcon 9 flew 30 times last year and goes significantly over Mach 3.
they're not? it just turns out that the most efficient cruising airspeed for passenger jets is in the upper end of the subsonic range. most people can't afford supersonic travel, and the ones that can overwhelming prefer to spend the extra money on a more comfortable subsonic flight.
interestingly, extremely high speeds turn out not to be that useful for military aircraft either. the fastest production fighter jet of all time, the mig-25, was introduced in 1970. the f-22 has a considerably lower top speed, and it's not because they lacked the funding to make it go faster.
Also, we don't really build dedicated interceptors anymore; the F-106 is the last US interceptor I can think of.
The F-15 was designed primarily as an air-superiority fighter (and in fact misinterpretation of the Mig-25 design was motivation for increasing its maneuverability; the Mig-25 was a dedicated interceptor).
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In general the Soviets seemed to really like interceptors, the MiG-31 is arguably faster than the Mig-25; altitude and willingness to destroy the engines affect the calculation.
This is largely just military doctrine. Stealth was determined to be a better bet than speed against SAMs and other jets. With improved signal processing techniques and better standoff weapon, speed may be the better bet.
Adding detail to leetcrew's good reply: the energy of an aircraft in flight is proportional to the square of its velocity. Given steady progress in plane technology you'd expect practical top speed to be limited by cost of energy before too long. That happened more than thirty years ago.
Y = AK^beta L^(1-beta)
with K being the amount of capital and L the amount of labour, A is just the unexplained "scaling factor" and it gets called TFP. But what TFP actually is... is a bit of a whatever-you-like.