Air-Augmented Rocket Without Oxygen?

[JMBuilder]

Since an air-augmented rocket relies on air pressure alone as opposed to oxygen, would it work in an atmosphere with no oxygen? If it does, it would be an excellent candidate as the main engine of an atmospheric lander. Mars missions would be made a lot easier.

[QuesoExplosivo]

The atmosphere of Mars would almost certainly be too thin to make a difference.

[Tommygun]

An Air-Augmented engine only works if there is a significant amount of oxidizer or fuel in a planet's atmosphere.

There is a type of jet engine that is called an arcjet that can heat almost any type of atmosphere as reaction mass.

[Red Iron Crown]

An Air-Augmented engine only works if there is a significant amount of oxidizer or fuel in a planet's atmosphere.

That's only one type of air-augmented rocket. It is possible to use the air purely as reaction mass accelerated by conventional rocket fuel mixtures.

That said, I'm not sure Mars' thin atmosphere would make such a thing worthwhile.

[JMBuilder]

Might work on Titan. Heck, it might even be useful for reusable first stages.

And yes, I was referring to the fuel-and-oxidizer rocket that uses air pressure as reaction mass.

[shynung]

It'll work. Given a stoichiometric O/F ratio to maximize energy output, and a large enough airspeed, it'll work similar to a ramjet. Though, one would need some sort of compressor turbines if you plan to use it in low-speed crafts, such as landers.

Edited November 4, 2014 by shynung

[K^2]

An Air-Augmented engine only works if there is a significant amount of oxidizer or fuel in a planet's atmosphere.

That is absolutely not true. Air augmented rocket relies on atmosphere primarily for added reaction mass. It can be completely inert. You'll just have to inject a bit more oxidizer into combustion chamber. I think, you might be confusing air-augmented rocket with scramjet.

The low pressure of Martian atmosphere is another matter, though. At high enough speeds, you'll still get a decent flow through the engine, but you'll also have heating losses at that point. Add to that extra weight of an air-augmented rocket over conventional biprop, and I'm not sure it'd be worth the trouble.

Venus, on the other hand...

(Edit: Or Titan. There, you might not need to bring the fuel. Just oxidizer. That should be fun.)

[KerikBalm]

Yes, air augmented rockets can work in atmospheres without oxygen.

I doubt Mars' atmosphere would be thick enough to matter, but Titan and Venus's atmosphere certainly would (as well as the gas giants, but I'm not sure what you'd plan to do in those atmospheres)

[magnemoe]

Yes, air augmented rockets can work in atmospheres without oxygen.

I doubt Mars' atmosphere would be thick enough to matter, but Titan and Venus's atmosphere certainly would (as well as the gas giants, but I'm not sure what you'd plan to do in those atmospheres)

How will this work? you add more inert gas, this get heated but cool the rocket flame, yes you get some effect as the atmosphere molecules are lighter than water and get higher speed but this effect should not be large unless its hydrogen.

[K^2]

How will this work? you add more inert gas, this get heated but cool the rocket flame, yes you get some effect as the atmosphere molecules are lighter than water and get higher speed but this effect should not be large unless its hydrogen.

Lets say that atmosphere is also water vapor, for simplicity, so that molecular weight is exactly the same. Burning hydrogen with oxygen produces enough energy to accelerate the exhaust to about 4km/s. (It's not vacuum I_SP, after all.)

Now, imagine that for each kg of fuel/ox, we pull in 3kg of atmosphere. So now the same energy is pushing 4kg. What will the exhaust velocity be? That's right. 2km/s. What happened to the thrust? It doubled. Consequently, your I_SP has doubled as well.

It is true that if you are carrying all of the propellant with you, you want to have as much energy going into it as possible. All you care about is getting maximum exhaust velocity. But augmented rocket is way more efficient for the same reason that the jet engine is. You simply don't carry all of the propellant with you. You use atmosphere for extra reaction mass. That gives you way better efficiency.

[KerikBalm]

It works because of the formula for kinetic energy.

TO accelerate 1KG of mass to 400 m/s, you could accelerate 4kg of mass to 200 m/s because KE = 1/2 MV^2

Accelerating the 4x the mass to half the velocity produces 2x the thrust for the same amount of energy.

or accelerate 16 kgs (ie 1 kg of fuel+Ox, and 15 KG of atmosphere) to 100 m/s for 4x the thrust from the same amount of fuel

This is the same reason a jet engine is more efficient (well the primary reason, the other is that the atmosphere supplies the oxidizer).

in the 16kg and 100 m/s exhaust velocity case, basically, your ISP is 4x higher than a pure rocket, assuming 100% efficiency (which you won't actually acheive, but it will still be much higher).

A nuclear thermal rocket is limited by reaction mass.

A chemical rocket generally is limited by a product of reaction mass and energy (as the fuel/energy storage medium, and the propellant are the same thing)

A jet engine is generally limited by stored energy (in the form of chemical energy)

Basically, you use the atmosphere as reaction mass, and your rocket fuel just supplies the chemical energy.

A jet on Earth only carries the LF, but not the oxidizer.

You could run a jet in an alien atmosphere by carrying both, and it would still be more efficient than a pure rocket.

Likewise, you could run a ramjet carrying both, and in this case, it is called an air-augmented Rocket.

Edited November 4, 2014 by KerikBalm

[Red Iron Crown]

Lets say that atmosphere is also water vapor, for simplicity, so that molecular weight is exactly the same. Burning hydrogen with oxygen produces enough energy to accelerate the exhaust to about 4km/s. (It's not vacuum I_SP, after all.)

Now, imagine that for each kg of fuel/ox, we pull in 3kg of atmosphere. So now the same energy is pushing 4kg. What will the exhaust velocity be? That's right. 2km/s. What happened to the thrust? It doubled. Consequently, your I_SP has doubled as well.

It is true that if you are carrying all of the propellant with you, you want to have as much energy going into it as possible. All you care about is getting maximum exhaust velocity. But augmented rocket is way more efficient for the same reason that the jet engine is. You simply don't carry all of the propellant with you. You use atmosphere for extra reaction mass. That gives you way better efficiency.

Slower exhaust velocity and higher Isp seems like a contradiction. Is this one of the cases where effective exhaust velocity is much higher than actual exhaust velocity?

[KerikBalm]

Yes, effective exhaust velocity.

ISP being measured in units of thrust per unit of fuel+ox, not including the mass of the air.

A Turboprop has a lower exhaust velocity than a turbo fan (which is lower than a turbojet, which is lower than a ramjet, which is lower than a scramjet) but a higher effective ISP (and the same holds true for the turbojet -> ramjet -> scramjet progression)

[K^2]

Slower exhaust velocity and higher Isp seems like a contradiction. Is this one of the cases where effective exhaust velocity is much higher than actual exhaust velocity?

Depends on what you call an effective exhaust velocity, I suppose, but it has more to do with definition of I_SP. It is defined as impulse you get per weight of fuel you have to expend. If all of your reaction mass comes from expended fuel, then I_SP and exhaust velocity are proportional. v_p = g I_SP. But if you have access to external reaction mass, relationship becomes more complicated, and you can increase I_SP even as exhaust velocity goes down.

And yeah, so long as you aren't traveling too fast, progression KerikBalm describes applies. The more air you push, the slower you are going to push it, and the more fuel-efficient your engine will be. Unfortunately, at high speeds, energy losses on collecting the air tend to outweigh benefits.

[magnemoe]

Lets say that atmosphere is also water vapor, for simplicity, so that molecular weight is exactly the same. Burning hydrogen with oxygen produces enough energy to accelerate the exhaust to about 4km/s. (It's not vacuum I_SP, after all.)

Now, imagine that for each kg of fuel/ox, we pull in 3kg of atmosphere. So now the same energy is pushing 4kg. What will the exhaust velocity be? That's right. 2km/s. What happened to the thrust? It doubled. Consequently, your I_SP has doubled as well.

It is true that if you are carrying all of the propellant with you, you want to have as much energy going into it as possible. All you care about is getting maximum exhaust velocity. But augmented rocket is way more efficient for the same reason that the jet engine is. You simply don't carry all of the propellant with you. You use atmosphere for extra reaction mass. That gives you way better efficiency.

Understand, I was just thinking of it as an rocket/ ramjet hybrid or rather an rocket with afterburner.

How hard will the intakes be to manage here?

[Excalibur]

There was a real-life example (however crude) of using the atmosphere for thrust augmentation on the N-1 rocket first-stage;

To achieve the required amount of thrust, it was proposed that a large number of NK-15s would be used in a clustered configuration around the outer rim of the lower-stage booster. The "inside" of the ring of engines would be open, with air piped into the hole via inlets near the top of the booster stage. The air would be mixed with the exhaust in order to provide thrust augmentation, as well as additional combustion with the deliberately fuel-rich exhaust..

Source: http://en.wikipedia.org/wiki/N1_(rocket)#Moon_missions (paragraph 7).