Limits of air augmentation in rocket engine design

[sevenperforce]

First post here; joined to discuss rocket designs, SSTO, and reusability.

Air-augmented rockets (also known as ejector jets or ducted rockets) are a sort of cross between turbofan jet engines, ramjets, and pure rockets. Ramjets depend on a ram-compressed flow of atmospheric air for combustion, making them useless for launch and for orbital insertion. An air-augmented rocket uses a stream of atmospheric air only as added reaction mass, greatly increasing specific impulse in a fashion similar to a turbofan bypass. They can afford to use fuels with greater energy density, because so much of the reaction mass is external. Best of all, because their core is more or less an ordinary rocket, they have no problem functioning from a standstill or in a vacuum.

(For reference, there was a prior forum post on air-augmented rockets here, though it didn't go into many details.)

The most efficient turbofan engines are built with extremely high bypass ratios, exceeding 10 kg of bypass air for every 1 kg of airflow through the central turbojet. Of course, the primary difference between a turbofan and an air-augmented rocket is that the power is delivered to the air mechanically in the former case, but thermally in the latter case.

Air-augmented rockets have not historically been very successful. In most cases, adding a shroud around the outside of an existing rocket was a large weight cost in exchange for only a modest increase in thrust specific fuel consumption, and because they weren't optimized for using the air as reaction mass, most of the added thrust was the result of secondary combustion between the fuel-rich rocket exhaust and the atmospheric air, making them essentially very inefficient ramjets.

If, however, an air-augmented rocket engine were designed in an inside-out configuration with a central bypass rather than an external bypass, you'd end up with a much simpler, more compact, potentially much more efficient design:

Such a design could allow a really, really high bypass ratio, causing thrust specific fuel consumption to drop ridiculously low. The combination of really high thrust and really high specific impulse is pretty nice. I wonder whether this could be made large enough that the thrust augmentation more than overcomes additional drag.

Thoughts?

[Elthy]

Not usefull for normal launches. After 1min you are out of the dense atmosphere and loose all benefits while still hauling more weight than necessary. If you try to accelerate horizontaly while still in the atmosphere you are limited by compression-heating, your rocket would burn up before it safed much fuel.

[K^2]

3 minutes ago, Elthy said:

Not usefull for normal launches. After 1min you are out of the dense atmosphere and loose all benefits while still hauling more weight than necessary. If you try to accelerate horizontaly while still in the atmosphere you are limited by compression-heating, your rocket would burn up before it safed much fuel.

You use almost half of your fuel during that first minute, as you combat aerodynamic drag.We are talking about 1.5-2km/s of extra dV accumulated as you traverse that layer. A rocket that could cut that overhead in half is nothing to sneeze at. Especially for SSTO design.

[sevenperforce]

And if we're going with a reusable SSTO design, you can afford to use some more expensive and slightly heavier materials to dissipate heat from compression, especially if you can manage to get your thrust augmentation high enough. Not to mention that re-entry of an SSTO with so many surfaces would be an aerospace engineer's dream.

Using atmospheric air seems to be the only way of augmenting thrust and augmentation simultaneously. We already use teh atmosphere as the primary reaction mass for re-entry; why not try to do the same thing for launch as well? Beats trying to coax lift out of a hypersonic vehicle, that's for sure.

I'm interested in figuring out the limits of bypass ratio in a central design like this. I can imagine massive ring-shaped engines significantly wider in diameter than their axial thickness....

 

 

[fredinno]

1 hour ago, sevenperforce said:

And if we're going with a reusable SSTO design, you can afford to use some more expensive and slightly heavier materials to dissipate heat from compression, especially if you can manage to get your thrust augmentation high enough. Not to mention that re-entry of an SSTO with so many surfaces would be an aerospace engineer's dream.

Using atmospheric air seems to be the only way of augmenting thrust and augmentation simultaneously. We already use teh atmosphere as the primary reaction mass for re-entry; why not try to do the same thing for launch as well? Beats trying to coax lift out of a hypersonic vehicle, that's for sure.

I'm interested in figuring out the limits of bypass ratio in a central design like this. I can imagine massive ring-shaped engines significantly wider in diameter than their axial thickness....

 

 

And it would be good for landing burns.... Would it be economical for expendables, too, or is it too expensive for that?

[sevenperforce]

The SSTO design probably wouldn't be economical for expendables, but having expendable air-augmented central-bypass engines mounted around an expendable tank to serve as the first stage would almost certainly be. At the very least, you could have a design similar to the Atlas, with a conventional rocket engine at the bottom of your stage and a pair of central-bypass engines mounted on the sides that fall away once their thrust is no longer needed (which would also be planned as the point at which you'd be far enough out of the atmosphere that their advantage wasn't there either).

Landing burns would be...beautiful. Complete thrust vectoring, throttling, and feather-light hover capability.

[magnemoe]

Main use for this would be missiles, you stay in the atmosphere all the time, better range than an rocket and more speed while cheaper than an full jet engine.
Issue might be variable attitude it will be used at. 
Know Russia had ramjets on some of their older surface to air missiles but looks like they dropped it for solid rockets. 

[sevenperforce]

Air-augmentation is already used to great effect in solid-fueled air-to-air missiles. I'm thinking specifically for SSTO concepts.

Without moving parts, what maximum volume of air (as a cross-section ratio) can reasonably be entrained and mixed with the exhaust plume? Even slight increases in airflow mean large boosts to thrust and fantastic increases to specific impulse.

[kunok]

Talking about the two possible designs the first one has the advantage that the full rocket can be inside, and I'm thinking: the afterburner nozzle, could be used as a vacuum nozzle extension? with a mechanism closing the intake part of course. Will be the geometry adequate? It's too late here to run numbers...

Edited January 27, 2016 by kunok

[Nothalogh]

I love air augmented rockets, the misbegotten spawn of ramjets and rockets.

The soviets nearly put a compact road mobile ICBM into service with one as the booster stage, but the designer, and brains behind it all died, and the project was shelved.

http://www.astronautix.com/lvs/gnom.htm

 

[Nothalogh]

7 hours ago, sevenperforce said:

First post here; joined to discuss rocket designs, SSTO, and reusability.

Air-augmented rockets (also known as ejector jets or ducted rockets) are a sort of cross between turbofan jet engines, ramjets, and pure rockets. Ramjets depend on a ram-compressed flow of atmospheric air for combustion, making them useless for launch and for orbital insertion. An air-augmented rocket uses a stream of atmospheric air only as added reaction mass, greatly increasing specific impulse in a fashion similar to a turbofan bypass. They can afford to use fuels with greater energy density, because so much of the reaction mass is external. Best of all, because their core is more or less an ordinary rocket, they have no problem functioning from a standstill or in a vacuum.

(For reference, there was a prior forum post on air-augmented rockets here, though it didn't go into many details.)

The most efficient turbofan engines are built with extremely high bypass ratios, exceeding 10 kg of bypass air for every 1 kg of airflow through the central turbojet. Of course, the primary difference between a turbofan and an air-augmented rocket is that the power is delivered to the air mechanically in the former case, but thermally in the latter case.

Air-augmented rockets have not historically been very successful. In most cases, adding a shroud around the outside of an existing rocket was a large weight cost in exchange for only a modest increase in thrust specific fuel consumption, and because they weren't optimized for using the air as reaction mass, most of the added thrust was the result of secondary combustion between the fuel-rich rocket exhaust and the atmospheric air, making them essentially very inefficient ramjets.

If, however, an air-augmented rocket engine were designed in an inside-out configuration with a central bypass rather than an external bypass, you'd end up with a much simpler, more compact, potentially much more efficient design:

Such a design could allow a really, really high bypass ratio, causing thrust specific fuel consumption to drop ridiculously low. The combination of really high thrust and really high specific impulse is pretty nice. I wonder whether this could be made large enough that the thrust augmentation more than overcomes additional drag.

Thoughts?

So, you want to use the incoming air as the spike of an aerospike.

Holy smokes, it would likely work at supersonic inlet velocities too, this is like a turbofan and a scramjet had a baby.

[DerekL1963]

9 hours ago, K^2 said:

You use almost half of your fuel during that first minute, as you combat aerodynamic drag.We are talking about 1.5-2km/s of extra dV accumulated as you traverse that layer. A rocket that could cut that overhead in half is nothing to sneeze at. Especially for SSTO design.

So long as you're not going for a reuseable - because on landing the airflow is in precisely the wrong direction.   (Unless you're going for a HTHL RLV, in which case you need professional help.)

And either way the second engine, the high bypass one, is going to have some very serious cooling issues to deal with...  and once it gets to altitude some very, very serious plume expansion/recirculation problems without some heavy, complicated, and very heat resistant mechanism for sealing off the inlet.

As usual the devil, and why this hasn't actually been done, lies in the details.

[K^2]

13 minutes ago, DerekL1963 said:

So long as you're not going for a reuseable - because on landing the airflow is in precisely the wrong direction.

I'm picturing a shuttle-like design that goes up nose-first, and comes down on the belly. That should create enough of the shock to protect the intakes.

[Hary R]

15 minutes ago, K^2 said:

I'm picturing a shuttle-like design that goes up nose-first, and comes down on the belly. That should create enough of the shock to protect the intakes.

or in other word, this Nasa concept

 

[K^2]

That looks like it would re-enter like a lawn dart, in which case, that sharp nose makes me worried. Actually, it makes me worried even on ascent. It'd need to be really large to be viable. I was picturing something way smaller and way flatter.

[Shpaget]

According to wikipedia article
"Many modern solid fueled 'ramjet' powered missiles, such as the MBDA meteor, may in fact be air augmented rockets,^{[citation needed]} and the distinction between a ramjet and an air augmented missile is rather blurred. Many solid fueled ramjet missiles seem to be solid fueled ramrockets in all but name."

So, if the principle is so widespread among the military rockets carrying weapons, why has there been so little* research into, at least, sounding rockets that would be using the principle?

*So little that this is the first time I hear about this.

Also, from the picture of the MBDA meteor the air intakes seem to be quite unbalanced. Interesting.

 

[DerekL1963]

41 minutes ago, Shpaget said:

According to wikipedia article
"Many modern solid fueled 'ramjet' powered missiles, such as the MBDA meteor, may in fact be air augmented rockets,^{[citation needed]} and the distinction between a ramjet and an air augmented missile is rather blurred. Many solid fueled ramjet missiles seem to be solid fueled ramrockets in all but name."

So, if the principle is so widespread among the military rockets carrying weapons, why has there been so little* research into, at least, sounding rockets that would be using the principle?

If the "may" isn't enough, the [citation needed] should tell you why there has been so little research.   That such ramjets are actually air augmented rockets is pure unsupported speculation on that part of whoever wrote that line.

 

3 hours ago, K^2 said:

I'm picturing a shuttle-like design that goes up nose-first, and comes down on the belly. That should create enough of the shock to protect the intakes.

That's the HL in "HTHL" - Horizontal Takeoff Horizontal Landing.   (Though my original post is in error, it should have said "HTHL (which is silly) or VTHL".)

[11of10]

I actually wondered the same thing myself, at the time I didn't know about air augmented rockets, so in my mind I dubbed it High-bypass rocket engine.

Now, in turbojet, you have compressor in the front, turbine in the rear that gives the motive power for the compressor, and the combustion process of fuel inside the incoming airstream to add energy to the entire thing.

Would open flames be a good way to add energy to the incoming supersonic (or hypersonic) airstream?

Maybe some sort of heat exchanger might be better?

Also, what exactly forces the air to go inside? Pressure differential?

[RizzoTheRat]

The usual method for supersonic inlets is to use the shockwave to compress the air at the front.  a really good example being the SR71 which uses what KSP'ers will recognise as a shock cone inlet, but the cone can move forwards and backwards to adjust the position of the shockwave.  It also had bypass doors to allow a lot of air to bypass the compressor at high speeds, leading to it being referred to as a turbo-ramjet.

In a ramjet you still need to get the airflow down to subsonic speeds to be able to support a flame (flame's move through the mix relatively slowly, in the order of 20-30m/s), you do this by expanding the channel the air's flowing through as it enters the combustion chamber. 

In a scram jet the airflow stays supersonic, so you need some way to keep igniting the gas as it comes in, which means having some kind of flame holder, which can be done aerodynamically with the shocks rather than being a lump of metal like you get in a reheat duct.

Edited January 28, 2016 by RizzoTheRat

[sevenperforce]

22 hours ago, kunok said:

Talking about the two possible designs the first one has the advantage that the full rocket can be inside, and I'm thinking: the afterburner nozzle, could be used as a vacuum nozzle extension? with a mechanism closing the intake part of course. Will be the geometry adequate? It's too late here to run numbers...

It would likely be possible to design an intake/nozzle shroud that can be simply slid forward or backward relative to the central rocket so that it is perfectly matched to airspeed, air density, and ambient pressure. But I'm primarily looking at the central-bypass approach anyway, simply because it offers a higher bypass ratio and intrinsic altitude/pressure compensation.

18 hours ago, Nothalogh said:

So, you want to use the incoming air as the spike of an aerospike.

Holy smokes, it would likely work at supersonic inlet velocities too, this is like a turbofan and a scramjet had a baby.

Precisely. With an open central bypass, it's a lot easier to use shockwaves to your advantage. If necessary, there could be an internal asymmetry so that the position of the peak shockwave was independent of airspeed.

15 hours ago, DerekL1963 said:

On landing the airflow is in precisely the wrong direction.

On re-entry, you can pick your attitude to produce the best possible airflow characteristics. During vertical propulsive landing, backwards airflow will be a source of additional aerodynamic drag; I don't think it would hurt the engine performance.

The small rocket engine bells underneath the lip would be independently throttleable and be capable of gimbaling radially about 100 degrees with respect to the engine bypass, further allowing complete control over the airflow:

Depending on the angle of each of the engine bells, compensation for an extremely wide range of conditions would be possible.

Looking at that wide expanse of intake surface, I almost want to contemplate adding some kind of exoskeletal turbine to enhance low-speed airflow (basically a drum-shaped inside-out turbine, where the blades are mounted on the inner surface rather than on a central spine) which can be shuttered at high speeds. Might be an unacceptable weight cost, though.

15 hours ago, DerekL1963 said:

The second engine, the high bypass one, is going to have some very serious cooling issues to deal with...  and once it gets to altitude some very, very serious plume expansion/recirculation problems without some heavy, complicated, and very heat resistant mechanism for sealing off the inlet.

Oh, the inlet wouldn't be sealed.

Since the thrusters can be angled straight down (and out, actually), vacuum operation would be totally normal. The angling would have an effect comparable to an expansion-deflection spike. 

And yes, cooling issues are going to be present...but that's why you pick a launch trajectory that optimizes airflow to give the best possible performance. Drag, compression heating, and mass flow are nonlinear; there will be an ideal trajectory which balances all of these things to get the lowest possible gravity drag and fuel consumption numbers.

Moreover, while this engine need not depend on atmospheric oxygen for combustion, it can use atmospheric air for combustion. There will likely only be a very narrow speed range where this offers any advantage at all, but because it requires no modification to do it, that's an added bonus.

A prograde re-entry will allow the same heat shielding used for launch to be used for re-entry. You'd need to flip around to do a retrograde burn, of course, but that's to be expected.

9 hours ago, 11of10 said:

Would open flames be a good way to add energy to the incoming supersonic (or hypersonic) airstream?

Maybe some sort of heat exchanger might be better?

Not enough time for a heat exchanger to function.

As the airspeed increases, the compressive heating of the airflow is going to increase, until you can no longer heat up the airflow enough to get meaningful expansion or net thrust (especially with a nozzle which has become very short in comparison to how fast you're moving forward). You can also think about it in terms of momentum; if you slow down air as it comes into your engine, you have to speed it back up to the same speed to even break even, and eject it out the back at a far greater speed to get net thrust. That's the problem that scramjets have encountered; they can get combustion to work just fine, but the body of the craft is absorbing so much kinetic energy from the onrushing air that they can't get net thrust above a certain point.

The whole reason for having a very large central bypass is that you can slow down the incoming air as little or as much as you want. At really high speeds, you can let the air flow through with as little drag as possible and only get a modest augmentation. With the right design shape, this can happen automatically as a consequence of the airflow speed itself.

9 hours ago, 11of10 said:

Also, what exactly forces the air to go inside? Pressure differential?

 At a standstill, you have some induced flow due to the low pressure adjacent to the rocket exhaust flows; this generally causes air to be pulled through. Of course, as you pick up speed, forward momentum forces the air inside, first due to pitot pressure and then due to a ram effect.

8 hours ago, RizzoTheRat said:

In a ramjet you still need to get the airflow down to subsonic speeds to be able to support a flame (flame's move through the mix relatively slowly, in the order of 20-30m/s), you do this by expanding the channel the air's flowing through as it enters the combustion chamber. 

In a scram jet the airflow stays supersonic, so you need some way to keep igniting the gas as it comes in, which means having some kind of flame holder, which can be done aerodynamically with the shocks rather than being a lump of metal like you get in a reheat duct.

The nice thing about an air-augmented rocket is that you don't have to worry about having enough time for combustion to take place; you just need to make sure you have enough time for expansion of the exhaust gases within the compressed airflow.

Another great thing is that the drag equation actually works for you. Drag is proportional to cross-sectional area, which for a cylinder of constant thickness is linear with respect to the radius. At the same time, mass flow through the center of a cylinder is proportional to the square of the radius. So doubling the radius of your bypass will double your drag but quadruple your potential thrust augmentation.

Edited January 28, 2016 by sevenperforce

[wumpus]

I've been mucking about in KSP wondering if the new physics bubble and wondering if it would "feel" similar to this engine.

Er, what?

KSP recently enlarged the physics bubble in the atmosphere.  It looks like you can recover an unmodded stage 1 as long as it goes under 500m/s (I might have this wrong, a lot depends on mucking with deployment altitudes of chutes.  The 500m/s limit comes from the limit of a drogue chute) and lands before the rocket hits 20,000m.  Since Kerbin has a delta-v requirement to LKO of ~3000 and Earth has a delta-v requirement of ~9000m/s to LEO, a KSP first stage that stops at 500m/s (500m/s delta-v plus most of your gravity losses) would correspond to about 1500m/s on Earth (about mach 5).  While I'd expect such a beast to go quite a bit faster (don't ask me how to survive the compressive heating, I just launch kerbals), that isn't far off what you might expect.  [Please ignore that a lot of such first stages are going to be SRBs or other high-thrust engines, if only to get to the correct speed at a low enough altitude to land before the physics bubble deletes them.  The effect to the rest of the rocket was supposed to be similar, not the actual air-breathing stage.]

Such an engine could certainly save a lot of fuel.  Could it ever be cost effective?  Even with the number of launches of the delta family?  Can't tell you that.  But trying out low-delta-v "free recovery" systems like this at least gives you an idea of what such a boost saves you.

[DerekL1963]

1 hour ago, sevenperforce said:

The small rocket engine bells underneath the lip would be independently throttleable and be capable of gimbaling radially about 100 degrees with respect to the engine bypass, further allowing complete control over the airflow:

If you have to keep making your engine more complex and heavier (and costing performance as the individual nozzles aren't as efficient as a single aerospike type combustor) in order to magically overcome real engineering concerns...  that's usually prima facie evidence that you're headed off in the wrong direction entirely if your first goal was to increase efficiency.  Not to mention that (multiple swiveling motors) engine will have even worse heating problems and will still have plume recirculation issues.  And also not to mention, airflow is also a function of speed and altitude, not just of the gimbal position of the internal motors.

 

1 hour ago, sevenperforce said:

And yes, cooling issues are going to be present...but that's why you pick a launch trajectory that optimizes airflow to give the best possible performance.

Um...  No, no matter what airflow you choose, you're going to have massive heating problems.   Airflow alone won't cool those nozzles, nor cool the exhaust sufficiently that you won't cook the downstream portion of your shroud - you'll need active cooling.   Especially for the period when you'll have insufficient or zero airflow, E.G. once you're at sufficiently high altitude or in vacuum.
 

2 hours ago, sevenperforce said:

A prograde re-entry will allow the same heat shielding used for launch to be used for re-entry.

Umm... no.   A few cm of cork or foam is sufficient for launch, but will be a feather in a windstorm during re-entry.   (Not to mention, one of the reasons the DC-Y wasn't followed up on was the impact the TPS would have on the design, and concerns about weight and structural strength during the dive-and-swoop maneuver.)

[wumpus]

1 hour ago, DerekL1963 said:

If you have to keep making your engine more complex and heavier (and costing performance as the individual nozzles aren't as efficient as a single aerospike type combustor) in order to magically overcome real engineering concerns...  that's usually prima facie evidence that you're headed off in the wrong direction entirely if your first goal was to increase efficiency.  

One thing I've learned from KSP is that engine weight for the lowest stages is surprisingly irrelevant.  The complexity will probably keep this idea on the drawing board for a long, long, time.  I expect it will take a real attempt to colonize space or other need for *lots* of lifters to get to the point that fuel efficient rockets matter.

I'm also wondering if you could get just the same effect with a cheap SRB?  Note that while KSP may have exponentially increased my understanding of orbital mechanics, I think it has increased my ignorance of SRBs (hint, they don't appear to be *anything* like KSP SRBs).  My guess is that you might be able to get away with a single stage SRB, with high thrust and a short burn (removing the O-ring issues that doomed Challenger, and lowering transportation issues).  Connecting it to the rocket might be a hoot (presumably some sturdy thing that would make sure all the thrust was evenly centered across the cross-section of the rocket).  SRBs might not be cheap, but the NRE needed to make your aerospike would be astronomical.

[DerekL1963]

54 minutes ago, wumpus said:

One thing I've learned from KSP is that engine weight for the lowest stages is surprisingly irrelevant.  The complexity will probably keep this idea on the drawing board for a long, long, time.

You need to be careful of extracting lessons from KSP.  It's first and foremost a game, and only in a very distant second place comes being a low fidelity engineering simulator.   That being said, this is being proposed and discussed as a propulsion system for an RLV, where engine weight matters a great deal there as it dominates vehicle weight by a wide margin.    In this case, even if it was an ELV, weight matters because you're spending so much of it for a system that's really only useful for a short period of time.   Dead weight doesn't impact payload on the first stage as much as it does on later stages, but it does have an impact.

 

1 hour ago, wumpus said:

I'm also wondering if you could get just the same effect with a cheap SRB?  Note that while KSP may have exponentially increased my understanding of orbital mechanics, I think it has increased my ignorance of SRBs (hint, they don't appear to be *anything* like KSP SRBs).  My guess is that you might be able to get away with a single stage SRB, with high thrust and a short burn (removing the O-ring issues that doomed Challenger, and lowering transportation issues).

If your SRB is blocking your main engines...  it's probably a waste of time.   Unless it's a smallish rocket and a biggish SRB, you'll lose most if not all of your already modest velocity advantage while your mains are starting up.   And the O-ring issues that doomed Challenger are long since fixed.   No need to avoid joints since doing so only (very) marginally increases safety and (very) greatly impacts total possible performance.

(And it's not 'my' aerospike, I was just pointing out where he was losing a lot of efficiency.)

[fredinno]

On January 27, 2016 at 0:55 PM, magnemoe said:

Main use for this would be missiles, you stay in the atmosphere all the time, better range than an rocket and more speed while cheaper than an full jet engine.
Issue might be variable attitude it will be used at. 
Know Russia had ramjets on some of their older surface to air missiles but looks like they dropped it for solid rockets. 

Russia still uses Liquid for ICBMs. Actually, you'd wonder why no one bothers to have a air-augmented ICBM, especially for rail/submarine transport.

On January 27, 2016 at 0:28 PM, sevenperforce said:

The SSTO design probably wouldn't be economical for expendables, but having expendable air-augmented central-bypass engines mounted around an expendable tank to serve as the first stage would almost certainly be. At the very least, you could have a design similar to the Atlas, with a conventional rocket engine at the bottom of your stage and a pair of central-bypass engines mounted on the sides that fall away once their thrust is no longer needed (which would also be planned as the point at which you'd be far enough out of the atmosphere that their advantage wasn't there either).

Landing burns would be...beautiful. Complete thrust vectoring, throttling, and feather-light hover capability.

How much more expensive are air-augmented? Might be a decent SSME alternative.

On January 28, 2016 at 8:48 PM, DerekL1963 said:

So long as you're not going for a reuseable - because on landing the airflow is in precisely the wrong direction.   (Unless you're going for a HTHL RLV, in which case you need professional help.)

And either way the second engine, the high bypass one, is going to have some very serious cooling issues to deal with...  and once it gets to altitude some very, very serious plume expansion/recirculation problems without some heavy, complicated, and very heat resistant mechanism for sealing off the inlet.

As usual the devil, and why this hasn't actually been done, lies in the details.

Use a second set of engines for high-altitude burns- a conventional engine. This would also be used for steering and landing. The Air-augmented engines only are used in the early boost stage of flight.

On January 28, 2016 at 11:47 PM, Shpaget said:

According to wikipedia article
"Many modern solid fueled 'ramjet' powered missiles, such as the MBDA meteor, may in fact be air augmented rockets,^{[citation needed]} and the distinction between a ramjet and an air augmented missile is rather blurred. Many solid fueled ramjet missiles seem to be solid fueled ramrockets in all but name."

So, if the principle is so widespread among the military rockets carrying weapons, why has there been so little* research into, at least, sounding rockets that would be using the principle?

*So little that this is the first time I hear about this.

Also, from the picture of the MBDA meteor the air intakes seem to be quite unbalanced. Interesting.

 

Sounding rockets need to be cheap, not high-performance, so it's probably not worth it.