Air-augmented rockets

[Wjolcz]

An air-augmented rocket is a rocket which uses atmospheric gases as working mass. They are (as far as I understand) pretty much ramjets with oxidizer and fuel, except more efficient. There used to be an ICBM, called GNOM, with this technology but it never got beyond that point because the lead designer died and there was no interest to continue the project.

Now, are they more efficient? As far as I understand they need some sort of cowling/intake around the body which redirects the air towards the exhaust. I see two problems here: while the TWR would probably be higher the cowling/intake part adds drag, mass and as the air flows into the exhaust it probably lowers the ISP of the engine (because of raised pressure).

Am I imagining this correctly? What if an aerospike was used instead of a reguar nozzle? Or maybe I'm overthinking this? Maybe it never was considered because of the complexity for something that would be expendable anyway? Maybe now that the reusability is more and more popular in the aerospace industry this neat piece of tech could finally be utilized?

Edited March 26, 2017 by Veeltch

[sevenperforce]

Au contraire. It vastly increases specific impulse.

[KerikBalm]

it slows the rocket exhaust, but the air around it heats and expands in the ramrocket nozzle, producing greater thrust for the same amount of fuel.

Drag is increased, but the thrust is increased more. The problem is simply, as with any air-breathing design, that the added equipment is just dry mass in space.

One needs to get to 8km/second, but its rather hard to to get more than 2km/sec in atmosphere of any reasonable thickness. Also the simplest ramrocket design, like a ramjet, needs to reach about mach 0.5 before theres any real thrust augmentation. So...it should help from about mach 0.5 to mach 5 (maybe as much as mach 10 depending on ascent profile? I dont know), but beyond that... its added dry mass. For a suborbtial ballistic missile, that should be fine. For an orbital rocket... I dont know, maybe on one stage, but perhaps the added cost makes it not worth it?

[Zeiss Ikon]

Augmentors are cheap and lightweight, and do not need ramjet speeds to operate.  Look at YouTube videos of pulsejets; they get significant thrust increase from very, very simple augmentors even in static conditions.  I've also seen model rockets (Estes engines) built with augmentors, with experimental proof that the augmentor increased thrust as shown by increased acceleration at launch.

As noted, the exhaust velocity is greatly reduced from any sort of well designed rocket engine, but the increase in exhaust mass (adding the mass of the air flow through the duct) keeps the effective Isp as measured with the rocket engine's fuel/oxidizer close to constant while increasing thrust by anywhere from 10% to 200% (strongly dependent on details of the duct design and operating conditions).  Generally, I'd consider an augmentor to be a significant improvement for that part of a rocket's flight that's within the troposphere and stratosphere, up to whatever Mach number you can manage to keep the inlet duct from choking on its own shockwaves.

Unlike a turbine engine's inlet, you won't want to slow the intake air below sonic speed; the faster the air is going when it mixes with the rocket exhaust, the faster the exhaust will be (and we all know faster exhaust is better, other factors equal).  Also worth noting that if the flow through the augmentor is subsonic, it would be possible (at the cost of some complexity and weight) to inject fuel to make the augmentor function as an afterburner, which would increase both thrust and Isp.

[sevenperforce]

Specific impulse is inversely proportional to thrust-specific fuel consumption. Thus, if you can increase thrust without increasing propellant consumption, your specific impulse goes up. 

An air augmentation system increases the thrust without costing more fuel over a selected range of speeds and altitudes, but reduces the TWR of your engine since you're carrying extra weight. 

The range of speeds in which adding an afterburner increases specific impulse is rather low. At low speeds, the airstream isn't compressed enough by ram effect to burn efficiently; at high speeds, the airstream is moving so fast that trying to slow it down and burn it makes it super draggy. Thrust is increased in both cases though.  

In the SSTO galore thread, I show that a Raptor-derived VTVL SSTO using basic air augmentation could achieve a payload fraction over 3% with full and rapid reusability, with a GLOW of only 100 metric tonnes. 

Adding some hot-gas methalox landing thrusters that could also boost launch thrust would push GLOW up to about 120 tonnes and increase payload fraction to 4.5% or so. 

With a PSTO configuration you can get payload fractions as high as 8-10% for under 200 tonnes GLOW.

[Finox]

15 hours ago, Veeltch said:

Maybe it never was considered because of the complexity for something that would be expendable anyway?

Air-augmented rockets were considered back in the sixties, they went so far as developing a few prototypes and test firing them, but they never got off the test stand and into anything flight ready.   As far as I know the reason it wasn't developed further had nothing to do with the technology, the problem was when it was first developed.    Air-augmented rockets were first developed in the middle of the Cold War and the Space Race, proven technologies like traditional rocket engines were more useful for military and civilian applications given the time constraints.

This happens often with space technologies where something that's promising isn't developed further because it might take too long to make it flight ready.

[sevenperforce]

Thusfar, no launch companies have elected to throw a shroud on their expendable rockets because it is cheaper and simpler to use existing aerodynamics models and simply make the fuel tanks a little bigger.

I haven't really ever seen any developed launch vehicles which used a shroud for more than just one purpose, though. That could be useful.

[magnemoe]

On 26.3.2017 at 6:39 PM, Finox said:

Air-augmented rockets were considered back in the sixties, they went so far as developing a few prototypes and test firing them, but they never got off the test stand and into anything flight ready.   As far as I know the reason it wasn't developed further had nothing to do with the technology, the problem was when it was first developed.    Air-augmented rockets were first developed in the middle of the Cold War and the Space Race, proven technologies like traditional rocket engines were more useful for military and civilian applications given the time constraints.

This happens often with space technologies where something that's promising isn't developed further because it might take too long to make it flight ready.

Air-augmented rockets sounds an better idea for sea skimming missiles, perhaps air to air too. 
You stay in the atmosphere and speed is fairly constant and it works well with solid fuel rockets, you would tune fuel so the first layer who burn bring rocket up to speed. Afterward you have fuel rich mixture so part of the fuel burn with the air. 
 

[Nothalogh]

1 hour ago, magnemoe said:

Air-augmented rockets sounds an better idea for sea skimming missiles, perhaps air to air too. 
You stay in the atmosphere and speed is fairly constant and it works well with solid fuel rockets, you would tune fuel so the first layer who burn bring rocket up to speed. Afterward you have fuel rich mixture so part of the fuel burn with the air. 
 

The MDBA Meteor AAM uses what is roughly an air augmented rocket, although they call it a solid fuel ramjet.

The problem is these devices all exist within a very blurry zone 

[monophonic]

4 hours ago, magnemoe said:

Air-augmented rockets sounds an better idea for sea skimming missiles, perhaps air to air too. 
You stay in the atmosphere and speed is fairly constant and it works well with solid fuel rockets, you would tune fuel so the first layer who burn bring rocket up to speed. Afterward you have fuel rich mixture so part of the fuel burn with the air. 
 

As @Nothalogh noted, that is exactly what the MBDA Meteor does. It can even control thrust by varying the size of the exhaust from the solid fuel chamber thus controlling pressure therein. The pressure changes the burn rate and thus amount of (fuel-rich) gas entering the ramjet stage. Although I think the boost stage is a separate solid fuel rocket housed inside the ramjet and ejected once it has burnt out.

[Nothalogh]

Just now, monophonic said:

As @Nothalogh noted, that is exactly what the MBDA Meteor does. It can even control thrust by varying the size of the exhaust from the solid fuel chamber thus controlling pressure therein. The pressure changes the burn rate and thus amount of (fuel-rich) gas entering the ramjet stage. Although I think the boost stage is a separate solid fuel rocket housed inside the ramjet and ejected once it has burnt out.

Yeah, the Meteor is a really neat mongrel of a design.

It's closest design relative is the Air-Turborocket

[kurja]

On 26.3.2017 at 5:26 PM, sevenperforce said:

Specific impulse is inversely proportional to thrust-specific fuel consumption. Thus, if you can increase thrust without increasing propellant consumption, your specific impulse goes up. 

 

Did you mean to say that delta-v is inversely proportional to thrust-specific fuel consumption? Because specific impulse = exhaust velocity, as far as I understand..???

Edit, augmentation adds reaction mass, which at same or even lowered Isp, adds thrust. Which will mean higher total delta-v. Am I wrong?

Edited March 29, 2017 by kurja

[Wjolcz]

1 hour ago, kurja said:

Did you mean to say that delta-v is inversely proportional to thrust-specific fuel consumption? Because specific impulse = exhaust velocity, as far as I understand..???

Edit, augmentation adds reaction mass, which at same or even lowered Isp, adds thrust. Which will mean higher total delta-v. Am I wrong?

More isp means more delta-v.

It's like having a balloon full of air. You can pop it and see a piece accelerate fast but not going very far (high thrust, low isp) or simply release it and see it flying for much longer because the air isn't coming out as fast as when after popping it (low thrust, high isp). At least that's how I imagine isp. Sbd correct me if I'm wrong. 

Edited March 29, 2017 by Veeltch

[kurja]

3 hours ago, Veeltch said:

More isp means more delta-v.

It's like having a balloon full of air. You can pop it and see a piece accelerate fast but not going very far (high thrust, low isp) or simply release it and see it flying for much longer because the air isn't coming out as fast as when after popping it (low thrust, high isp). At least that's how I imagine isp. Sbd correct me if I'm wrong. 

Higher Isp with same propellant mass = moar dv,

more dv by adding reaction mass != higher Isp.

There are other factors at play in launch of a balloon rocket, however afaik specific impulse is (by definition) the exhaust velocity of said reaction mass.

[sevenperforce]

4 hours ago, kurja said:

Did you mean to say that delta-v is inversely proportional to thrust-specific fuel consumption? Because specific impulse = exhaust velocity, as far as I understand..???

Edit, augmentation adds reaction mass, which at same or even lowered Isp, adds thrust. Which will mean higher total delta-v. Am I wrong?

The specific impulse is a measure of how much impulse (momentum) is provided by an engine per unit mass of propellant. For rockets, this can be found simply as the exhaust velocity, for obvious reasons: ejecting one kilogram of propellant at 1000 m/s provides 1000 kg*m/s of momentum. Now the time derivative of momentum is force (in this case, thrust), and the time derivative of unit mass is mass flow rate (in this case, fuel consumption), so we can say that a given engine provides a certain amount of thrust at a given fuel consumption rate.

The inverse of thrust at consumption rate is consumption rate for a given amount of thrust, also known as thrust-specific fuel consumption. If you know your engine's thrust-specific fuel consumption, you can derive exhaust velocity and thus find specific impulse.

For airbreathers, however, the measure of impulse per propellant unit mass is different. Because the engine is using the atmosphere as part (or even most) of its reaction mass, the actual exhaust velocity is unimportant. Instead, you find out how much fuel is consumed every second for a given amount of thrust, and then you derive effective specific impulse from that, even though it is no longer tied in any way to the actual exhaust velocity.

An air-augmented rocket uses ram pressure to collect and channel airflow. The engine exhaust is then used to compress and accelerate the collected air. The actual exhaust velocity is lower, but since you are working with more reaction mass, you get much more thrust for the same fuel consumption, so the effective specific impulse is higher. 

This is the case even if you are not actually using the oxygen in the air to burn with fuel. It's just free reaction mass.

The only problem is that as you move faster and faster, the drag produced by collecting that air makes it less and less "free". So you can only use air augmentation up to a point. This point is typically higher than what could be used by a scramjet, though, since you don't actually have to combust the air.

3 hours ago, Veeltch said:

More isp means more delta-v.

It's like having a balloon full of air. You can pop it and see a piece accelerate fast but not going very far (high thrust, low isp) or simply release it and see it flying for much longer because the air isn't coming out as fast as when after popping it (low thrust, high isp). At least that's how I imagine isp. Sbd correct me if I'm wrong. 

Eh, not quite. With a balloon the actual thing you're dealing with is the air resistance against the balloon. Releasing a balloon allows the thrust to continue for a while, pushing through the air. Popping the balloon has the same specific impulse, more or less, but the thrust is not sustained and so the air will immediately slow down the balloon fragments.

The confusion arises because when you're dealing with chemical fuels, higher-specific-energy fuels like liquid hydrogen are not as dense as lower-specific-energy fuels like kerosene, and so most rocket engines with higher specific impulses have lower thrust-to-weight ratios. SpaceX has circumvented this rule of thumb by using staged combustion to make a very high TWR engine that uses methalox.

That's one reason I like variable-mixture tripropellant engines. If you can push kerosene through the engine for high thrust and low specific impulse at launch, then gradually switch to liquid hydrogen for higher specific impulse when your thrust requirements aren't so high, you have a great system. Of course, designing an engine and turbopump that can run on different fuel types at different temperatures is no easy task.

I'd love to see an aerospike engine using staged-combustion HTP to run the turbopumps and variable-mixture LH/RP1 as fuel.

[sevenperforce]

Some more thoughts...

Air augmentation by itself doesn't necessarily give enough added performance to easily enable SSTO, but it could be really useful if combined with other concepts. Like, air augmentation on the first stage would greatly decrease the GLOW while increasing payload. The trouble there is that if you want to recover your first stage, the air augmentation shroud would tend to be very, very draggy and place a center of pressure precisely where you don't want it.

Another option is to use a jettisoned shroud, just like you have a jettisonable payload fairing on your payload. This would work quite well. If the shroud doubled as a drop tank, then you really start to get an advantage. For recovery purposes, the shroud can either be chuted down or have its own landing thrusters for flyback.

If you can use a shroud for an SSTO, it can double as a heat-shield for re-entry.

One other thing: if you use a linear aerospike engine, then the shroud can be straight rather than curved, and can therefore be easily actuated to modify the intake shape.

[kurja]

Right, looks like I overlooked that impulse and specific impulse are not the same thing. Thanks