danieldyer's comments

It's worth pointing out that Concorde didn't cruise using its afterburners; it only lit them briefly to punch through the transonic flight regime.

She's certainly far less well known than many of her contemporaries, but Noether's theorem is an incredibly important tool in theoretical physics, and has been described as "certainly one of the most important mathematical theorems ever proved in guiding the development of modern physics, possibly on a par with the Pythagorean theorem". [1]

[1]: http://en.wikipedia.org/wiki/Emmy_Noether#cite_note-FOOTNOTE...

Don't forget Emmy Noether: http://en.wikipedia.org/wiki/Emmy_Noether

"In the judgment of the most competent living mathematicians, Fräulein Noether was the most significant creative mathematical genius thus far produced since the higher education of women began. In the realm of algebra, in which the most gifted mathematicians have been busy for centuries, she discovered methods which have proved of enormous importance in the development of the present-day younger generation of mathematicians."

- Albert Einstein

The issue is that people confuse getting into space with getting into LEO. Virgin Galactic are not even close to getting into LEO – their plan is to fly a ballistic trajectory which takes them just above 100km before falling back to Earth.

Since they don't need to achieve anywhere near the required velocity to enter LEO, they can use a much smaller solid-fuel rocket engine and launch from a jet.

Yes, my Kurzweil K2000 (http://en.wikipedia.org/wiki/Kurzweil_K2000) supports fully customisable temperaments – you can even tune it to the Bohlen-Pierce (http://en.wikipedia.org/wiki/Bohlen%E2%80%93Pierce_scale) scale if you want!

> Regarding the Square-Cube law, I'm not sure it's that relevant here. It's pretty clear you could put two of those engines in a AMA sized model, so not exceeding 55 lbf. That'd give you a thrust-to-weight ratio of greater than 1.0.

Yes, a small turbine-powered RC aircraft can easily have a higher thrust-to-weight ratio than a full-sized aircraft, but it's not thrust-to-weight ratio that determines top speed; it's thrust-to-drag ratio.

This is where the square-cube law comes into it. If we take a full-sized delta-winged high performance aircraft like the Eurofighter Typhoon, we could very roughly approximate the difference in drag between that and the model to be proportional to the difference between the square of their wingspans.

I'm guessing the aircraft in the video has a wingspan of 1 m, and the Typhoon has a wingspan of 11 m, so the drag should be greater by something like a factor of 121.

However, the Typhoon's engines provide a combined 180 kN of thrust, which is greater than the 160 N thrust of the Jetcat P160 by a factor of 1125.

So, you can see that the thrust-to-drag ratio of a full-sized jet fighter is something like an order of magnitude larger than for a model aircraft like this, which is the main reason why model aircraft are unable to attain supersonic speeds.

Of course, supersonic flight for model aircraft would pose all the same problems it poses for full-sized aircraft; onset of compressibility affecting control surface response, engine inlet geometry and so on.

Roughly speaking, the drag is proportional to cross-sectional area, whereas engine thrust required is proportional to mass, which in turn is proportional to volume. So, you end up running into a square-cube law situation: http://en.wikipedia.org/wiki/Square-cube_law

Also, the Reynolds number will be completely different for a model aircraft compared to a full-sized aircraft, so they will behave quite differently aerodynamically.

It's also worth pointing out that even the fastest full-size jet aircraft can only just break the sound barrier at sea level (the F-111 did Mach 1.2 at sea level) – going supersonic typically requires flying at a high altitude, which obviously isn't practical or legal for a radio-controlled model aircraft.

Exactly. It's just a radio-controlled model aircraft, which have been around for a long time.

Also, it's worth pointing out that it's not a model of a fighter jet, and that this video seems to be a repost of one that's been up for five years: https://www.youtube.com/watch?v=dTHWBSluUjU

Only if it were rocket powered. The aerodynamics simply don't scale well enough for something that small to become supersonic using any currently available small turbine engine.

> Sounds shockingly good, so I have to wonder if the compressor is basically an ablative heat shield and this is good for only a few runs.

The compressor isn't the part of the engine which gets really hot here; that would be the turbine. In these kinds of engines the turbine is typically made out of Inconel, which has no problems handling the ~800C exhaust gas temperature seen at maximum thrust. Preventing the EGT from exceeding a specified limit is the main job of the engine computer, so this is not a common failure mode.

The service interval on Jetcat turbines is 25 hours (some manufacturers specify 50-hour intervals, or even longer), for which the significant portion the work involved is a bearing replacement.

While the thrust-to-weight ratio does seem very good, keep in mind that the quoted weight of 3.1 lbs is likely to not include ancillaries (engine computer, battery and mounting hardware). Also, lubrication is provided by oil mixed in with the fuel rather than a closed-loop oiling system.

A thrust-to-weight ratio of 10:1 is impressive, but not out of line with what you'd expect to see from a full-sized turbojet. On the other hand, the thrust-specific fuel consumption for these engines is much worse than a full-sized engine due to the much lower pressure ratio.