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Comment by marze | Hacker News Reader

Andrew Cote gives the missing details here:

https://twitter.com/Andercot/status/1763063321857757210

If it can truly "act as the first stage of a rocket," that's impressive--the most powerful jet engine ever built. If, instead, it will power a plane that will carry a rocket, that's a well-trodden path whose engineering problem is not particularly related to the carrier's efficiency.

[1] https://en.wikipedia.org/wiki/Geared_turbofan

GTF is exactly what it is. The fact that Andrew Cote missed this is a testament to his lack of familiarity with anything in aerospace.

Don't geared turbofans typically have a fixed gear ratio? This is more like a continuously variable transmission for a turbofan. Seems novel at least.

> Don't geared turbofans typically have a fixed gear ratio? This is more like a continuously variable transmission for a turbofan

Yes and no. An electric compressor doesn't mean the turbine doesn't drive the fan. (It might make pre-cooling easier.)

I just saw an interview with more detail and it sounded to me like they were going to have the turbine drive a generator powering an electric motor to drive the fan. Like a diesel electric locomotive. And this is supposed to allow it to transition from turbofan to turbojet to reach mach 2.7 to allow starting up a ramjet which launches the second stage to mach 5 before lighting its rocket engine. Not sure why none of that detail was mentioned here.

https://youtu.be/Detvd2EqWWU

> the turbine drive a generator powering an electric motor to drive the fan

Maybe the transmission losses aren't relevant if you're thrusting for a short interval?

> transition from turbofan to turbojet to reach mach 2.7 to allow starting up a ramjet

Oh. Cool. Sounds like an air turboramjet [1]. Plus a fan?

Not sure how you ditch the fan, nor why they'd describe an electric motor driving the fan as an electric compressor. Granted, I skimmed the video.

[1] https://en.wikipedia.org/wiki/Air_turborocket

The ramjet is on the second stage, separate from the turbofan/jet on the first stage.

The loss in the electrical system should be pretty small, and they claim that they gain a lot of efficiency from being able to optimize the engine over the whole range from 0 to mach 2.7.

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