Comment by NikolaeVarius | Hacker News Reader

Comment by NikolaeVarius | Hacker News Reader

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> My body has a thrust to weight ratio of 1, sitting in this chair

No it doesn't. Its 0

> For instance, my body is using the whole earth as its reaction mass, and is using zero energy pushing it downwards at zero speed

That is not how gravity works. There is nothing happening.

> A jet is much more impressive because it gets that high thrust to weight ratio while using a small reaction mass ejected at a high speed. That is what is required to push an aircraft up to supersonic speeds

I wouldn't call it a small reaction mass. A shit ton air is needed to feed the compressor. A jet sacrifices efficiency to get as much mass thrown out the back as fast as possible.

> if it might be possible to fly supersonic much higher than jets, because there is no reduction in engine power.

Why would there be no loss in engine power. If you are using standard propellers, you are limited by air density. If you are using some electric engine to turn a turbine, you have the exact same issue. What magical engine is this electricity powering?

>"I wouldn't call it a small reaction mass. A shit ton air is needed to feed the compressor. A jet sacrifices efficiency to get as much mass thrown out the back as fast as possible."

It's the other way round - a Jet gains efficiency the more air mass it can 'process', because larger mass means you need to throw it out at a lower velocity to get the same impulse from it. If you compare formulas for momentum and kinetic energy, which are, respectively:

p = m * v and e = 0.5 * m * v ^2

You will see that energy needed for higher velocities grows quadratically, while the momentum you get from it grows linearly.

basically that's why we have Turbofan engines with high bypass rations, to increase efficiency of passenger Jets

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

I know how a turbofans/jets work. My comment had nothing to do with the comparison with turbofans, and only that large amounts of air is being moved.

> It's the other way round - a Jet gains efficiency the more air mass it can 'process', because larger mass means you need to throw it out at a lower velocity to get the same impulse from it.

How is it the other way around, I explicitly said a Turbojet sacrifices efficiency to increase exit velocity.

Firstly, if you are gonna poke holes in other people's arguments, don't get defensive when someone corrects you, especially when you make such precise and well qualified claims as "Shitton of air". Neither were also not spesific about Turbofan vs Turbojet.

Regardles of which one it is, the point of the previous poster stull stands. His point of comparison is how reaction mass compares to the weight of the vehicles.

All jets move less air, by mass, than helicopters do, when measured as a fraction of the vehicle's mass.

> Why would there be no loss in engine power. If you are using standard propellers, you are limited by air density. If you are using some electric engine to turn a turbine, you have the exact same issue. What magical engine is this electricity powering?

Parent is assuming that combustion engines run out of oxygen to burn before the propeller runs out of atmosphere to push against.

This is a valid assumption. https://en.wikipedia.org/wiki/Helios_Prototype

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