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Comment by block_dagger | Hacker News Reader

It's NOT rocket science?

You can reach space using air breathing jets. You can’t stay in space using air breathing jets.

I don't think there are any physics reasons why it'd be impossible, but certainly we can't do it with existing technology. You'd need an air breathing jet that could get a vehicle to go about five or six times faster than any current such engine has ever achieved (i.e. around mach 20-30), which is perhaps ridiculous, but I don't think it's necessarily impossible, just something we don't know how to do. There have been some (failed) efforts to get there, like the X-30.

Can an air breathing jet actually attain those velocities? I thought most supersonic aircraft use rockets after a certain point

"Most" supersonic aircraft are fighter jets and other military aircraft that use jet engines, not rockets. They may have afterburners that are much like a rocket that just injects jet fuel in the exhaust stream, but that's still using atmospheric oxygen.

The issue, I think, is more about balancing drag and air intake at appropriate atmospheric densities for different speeds. An SR-71 Blackbird could fly at 85,000 feet continuously, and a MiG-25 set what I believe is still the air-breathing record max altitude by pulling a "zoom climb" (accelerating in higher-density air that the engines could use effectively, then pulling the stick back and coasting up through rarefied air too thin for the engines) to 38km or 123,000 feet.

Most experimental hypersonic aircraft use rockets because that's what works.

> Can an air breathing jet actually attain those velocities?

There's no theoretical limitation on how fast an air breathing jet can move. You just have to redesign everything every few mach numbers, and deal with the atmospheric drag.

Basically when you cut thrust you must pass through that altitude again or escape orbit.

So either fire a rocket in space to circularize the orbit or reach more than Earth’s escape velocity 25,020 mph (11.186 km/s, 40,270 km/h) ~ Mach 32.6, due to some drag in air to thin for any kind of air breathing engine to work.

X-30 was aiming far lower ~Mach 20. Nuclear could make it more realistic than any form of chemical combustion. It might be physically possible using Hydrogen but you’re talking generating extreme thrust at vastly more extreme conditions than the space shuttle’s retry.

Or go high enough to let the moon alter your orbit into one that doesn't hit the atmosphere.

Yea thus ‘Basically’ you can also escape earth’s orbit slightly more easily using the sun. However, none of this really helps much you’re still looking at more than escape velocity in atmosphere with a purely air breathing engine due to drag.

Well you can't reach a high orbit using air breathing engines because your impulse must be given within the atmosphere, and then your trajectory inevitably re-intercepts the atmosphere (unless you achieve an escape trajectory) and would decay quickly. You can get around this by packing a small rocket engine and circularizing on apogee!

There isn't enough air at high altitudes for jets to reach space even if you count 100km as space.

The highest jet record is 37km in MiG-25. The scramjet record is 33km. I found source that says the limit is 40km at Mach 15.

The MiG didn’t reach 37km in level flight, which is probably where Mach 15 @40km comes from. Instead the MiG was doing a nearly parabolic trajectory starting with Mach ~3 worth of kinetic energy.

The NASA X-43A hit Mach 9.6 as an air breathing engine (test used a rocket at lower speeds for cost reasons) which in theory should be capable of ~65 km assuming it could survive a similar maneuver. Actual limits are heavily influenced by how much thrust you can generate while slowing down etc not just max velocity.

So yea no actually built air breathing aircraft can hit space, but it’s within the realm of possibility.

Nobody has been able to build one, but I'm not aware of any proof that it's impossible. You need some way to build an engine that doesn't appreciably slow the air that's passing through it.

Didn’t Chuck Yeager burn his face off doing that?

Nearly so, yeah. But a stock starfighter couldn't do that, the one Yeager was flying was outfitted with a rocket engine.

You can build a decent sounding rocket with just trial and error.

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