Air is mostly nitrogen. And they need a whole lot of 02 to mix with the fuel in the right ratio in order to get the proper thrust. It's just not that feasible in the few minutes they're even in the atmosphere

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Skylon is a British company trying to construct an air breathing SSTO. Apparently that's an open question.

Not enough air, and it's full of nitrogen.

LOX is 1141g/L at its condensation point (~54K) and I think rockets make it even colder to increase the density a bit higher, while atmospheric air is only ~1.3g/L.

Compressing it at ~1000:1 would take some pretty epic equipment, and then it'd be way too hot.

Even if you somehow manage to sort that out while still having a launch TWR > 1, air is still only ~19% oxygen or so, meaning the fuel wouldn't be able to burn effectively.

Furthermore, the atmosphere gets even thinner within a few dozen seconds of lift-off, so you'd still need to carry oxidizer anyway - and the little bit extra it takes to get up to that point is far lighter and simpler than having the rocket itself run on atmospheric air for half a minute.

It's much more sensible to process it as much as possible on the ground, and load LOX into the rocket.

Having said that, companies that strap their small rockets to aeroplanes are technically already using atmospheric air for that phase of the flight - but the type of engine required is radically different, and those rockets have to be relatively tiny because aeroplanes can only carry so much mass.

Well... Sometimes they do, if you think about air launch (pegasus and launcher 1) as jet powered first stages.

Also, the hybrid approach has been studied extensively, and usually it's just not practical. You can read up on the SABRE project.

Jet engines mix the incoming air in a flow that feeds into the burning and compression for thrust. This loses efficiency quickly as you go past Mach2. Rockets use a combustion chamber that is sealed off and has a steady and controlled inflow of oxidiser that is mixed in an injector plate.

They are two very different processes. The additional weight to have different engines means you do not gain in terms of mass.

Super complex systems that can burn both ways are on drawing boards. And have been on drawing boards for 70s years.

There is a concept to do something along these lines, the Sabre rocket engine. https://en.m.wikipedia.org/wiki/SABRE_(rocket_engine)

That was the Skylon idea, but it's not efficient and very complex.

Rockets do not need "air", they need oxygen.

A rocket motor needs liquid propellant at best, so no. So like a jet engine first stage? A Falcon 9 is already 40 miles up in less than two minutes. Your jet first stage would maybe be able to get the rocket to 40,000 feet before rockets have to take over, so maybe some tens of seconds of flight. And then we have size/weight issues. Our most powerful jet engine is the GE9X at 110,000 lbs thrust. You'd need several of these to replace rockets, and they're huge and heavy.

The best idea we have so far is to haul a rocket up high on a jet airplane and then launch it, but that only works for smaller rockets.

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Vertically launching rockets try to get out of the dense atmosphere as quickly as possible, so an air breathing engine wouldn't work for very long - Falcon 9 is above 30000 ft about 1 minute after launch, and above 50000 ft 20 seconds later. The added cost and complexity of another stage for such a short period of the flight aren't really worth it.

It's called A Jet and they've been doing it since the 80s.

50’s

It would seem to be so easy to take advantage of all the oxygen in the air but it turns out to generally be extremely not worth the trouble.

The first major problem is just getting the air and making use of it, which is insanely complicated and requires lots of complex machinery. We're talking turbojet, ramjet, or scramjet engines. And while it may seem that a rocket engine is insanely complex, they are actually a lot simpler than jet engines in many regards. The first liquid fueled rocket propelled vehicle flew about 20 years before the first jet powered aircraft, for example. Even more so it's incredibly difficult to design air breathing engines which operate over wide ranges of speeds and altitudes. Consider the SR-71 and the ridiculous level of engineering that went into that and how those speeds and altitudes are just baby steps compared to orbital launch. It takes about a minute and 45 seconds for a Falcon 9 to surpass the speed and altitude of the SR-71. It is very challenging to build an airbreathing engine that would be worth its weight.

Once you add airbreathing to a launch vehicle you would want to spend more time in denser atmosphere in order to maximize its usefulness. However, that's very problematic for several reasons. For one it creates much more drag to spend so much time in denser atmosphere, which saps efficiency. For another it adds more aerodynamic heating and strong aerodynamic forces forcing you to add more heat protection and increase the strength of the vehicle. All of which adds weight, complexity, and potential failure modes.

In contrast, if you just say no to airbreathing at all you end up with a simpler vehicle that only has rocket engines (saving weight and complexity) and you have a much simpler optimization problem for launch. You can climb out of the thick atmosphere early and do the majority of acceleration in vacuum or near vacuum.