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Comment by lmm | Hacker News Reader

Most rocket launches start accelerating horizontally either as soon as they've cleared the thickest part of the atmosphere, or right from the start. If you can energy you can impart to a satellite with an air-breathing stage, you can save an exponential amount of fuel on the final rocket stage.

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So far nothing could beat just increasing the size of the first stage. Simplicity and weight as any air breathing hardware brings in weight and complexity. Even the simplest and my favorite approach - air augmentation - could not be made so far (beside a small USSR surface-to-air missile).

Also with the size of the rocket the proportional air breathing machinery power scales between linear and square while rocket mass cubically, so a useful air breathing on say a Starship is hard to imagine.

The reason that there are plenty of rocket first stages and basically no air-breathing first stages that go to orbit is that, essentially, this isn't true. OSC Pegasus and Virgin LauncherOne are the only exceptions, and both are only good for very tiny payloads and accordingly have very limited flight numbers- compare the less than 50 total across both launchers to the 150 for just the already retired Delta II rocket (which entered service at basically the same time as Pegasus, is why I chose it). The only real use case that I can see for air launch is that they can be fully launch azimuth independent for launches from within US territory, so there can be some sort of responsive launch capability (read: military wants to launch something immediately). Other than that one use case, it's hard to see what air-breathing engines bring to the table for space-flight.

Payload size is clearly a massive problem because airplanes are so volume limited. But there is another issue here: heating. In order for an air-breathing engine to provide useful thrust, you have to have enough air for it to breathe. That pretty much requires you to now have to deal with aerodynamic heating, which is a thing that most rockets try very very hard to ignore on launch.

The most prominent air-breathing to orbit attempt going right now, Skylon, seems to be viewed generally pretty skeptically by most people in the space community, from what I can tell. And even then, the log-line is different: instead of separate jet engines and rocket engines, the idea is to have a rocket engine that can make a mediocre jet engine for some portion of the flight. Most people outside RE don't think this idea will work very well (I recall people who ran the numbers pointing out that HOTOL- the program that Skylon grew out of- would actually have significantly more payload if it had just rocket engines) but Bond and co. continue on, with very small budgets, the sort of budgets suitable for long-shot technology development. Thus, it appears that the people who control the purse strings generally agree that this has a low chance of working out, even if it is worth trying.

This is a common theory, but if you look at the math of the rocket equation, a very large percentage is for pure horizontal velocity, with a small percentage of gravity losses for getting from the surface to the orbital altitude, an even smaller percentage are losses due to aerodynamic drag.

I haven't done the math, but in general, the benefit of something like stratolaunch is much less than you'd expect

Here's a great (but complicated) reddit post on this topic:

https://www.reddit.com/r/KerbalAcademy/comments/63nzq6/how_d...

"But getting to space is easy. The problem is staying there." - that's been an real eye opening read for me from fantastic "What If":

https://what-if.xkcd.com/58/

The point isn't reduced drag. The point is to be able to do more horizontal acceleration with less propellant, because you're getting your oxidizer for free.

Getting something "for free" is hardly an issue since liquid oxygen is quite cheap as a percentage of launch costs (for example in case of Falcon 9, it's less than $100k out of ~$28M of launch costs for a reused Falcon 9). Now you might argue that the resulting vehicle that needs to be carrying needs to be bigger, but then again, in practice we found it less complicated than hypersonic air-breathing propulsion, so whether the vehicle capable of usefully consuming it would actually be cheaper is pure speculation. So far nobody has been able to build even just one, much less mass-produce it.

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