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Comment by japanuspus | Hacker News Reader

Everyday Astronaut has a recent video on "why don't we launch rockets from jets"[0]. The video obviously does not include reference to the engine mentioned here -- but I think most of the points still stand.

In particular - orbit is mainly about going fast, not so much about getting up high - your second stage will pay a structural price for allowing air-launch loads, and this structure needs to be carried to orbit - rockets are *big*

[0]: https://www.youtube.com/watch?v=AAt9WDQEMoA

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I think what’s missing from Everyday Astronauts discussion is: what if we could make the plane/rocket go much faster.

The cruise speed of a jet plane of 800-1000km/h may not be a high enough fraction of orbital velocity to be worth it.

But what if you could fly even higher and faster, say 3000km/h, before you do stage separation?

Now you’ve actually achieved a significant fraction of orbital velocity.

Obviously that would require new technology. And in particular the main challenge is making an engine that can operate efficiently at a wide range of speeds. So this engine could change the equation.

But I suspect it’d only make sense for smaller vehicles delivering personnel/cargo to arbitrary orbits on short notice.

I also wonder how they can get enough electrical power without a big weight penalty.

Keep in mind that Electron is already using electric motors for their pumps. So the question is kind of whether you could also use a motor to capture and compress air to exploit the oxygen in the air rather than having to carry it as payload.

Minimal orbital speed is somewhere around 25,000-28,000 kph. The fastest air breathing plane every built (SR-71) had a top speed of about 3,500 kph. So for the added weight and complexity of adding an air breathing first stage (wings, air breathing engines, landing gear etc) you are only getting about 14% of the way to the minimal orbital velocity needed, and that would be if your air breathing first stage could match the performance of the fastest plane ever built, keep in mind the SR-71 carried almost no payload. It is far more likely that any realistic air breathing first stage is going to get to less than 10% of orbital velocity, and that would likely be for launching a fairly small rocket. On the other hand the Falcon 9 first stage usually doesn't separate until 6,000-9,000 kph which is closer to 25-35% of orbital velocity, and that's while carrying almost 23,000 kg of payload plus the fully fueled second stage.

Thank you, that context was illuminating.

To make matters worse, even 14% of the speed is still just 2% of the energy which is possibly more relevant here.

After watching the video I'm left wondering how k2pilot would respond to the analysis. Is the idea that you could use these jet engines as part of a vertical takeoff?

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