Ram Air Induction -ducted rocket propulsion augmentation

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shockwaveriderz

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I was looking at some old jetex designs and I noticed that they used to sell "thrust augmenter" tubes. These were of a larger diameter than the Jetex unit itself and it was supposed to "augment" the thrust...somehow.

Then I was looking at the Lil Augie and Augie, 2 ducted propulsion designs from the 60's....


So that got me to thinking; I am fully aware of the Krushnic effect... but what if you had a model rocket motor thrusting into a slighlty larger diameter tube, such as air was "rammed" into a scoop such that this slowed down air was directed right along side the exhaust plume.

would this work to increase the actual thrust of the motor?

What I'm thinking of is some kind of ram air induction system ..I know that model rockets generaly stay subsonic, but would some air still be compressed some?

I did some patent seraches and other resarch and there are air breathing solid propellant rocket motors.... but they direct the rammed air directly into the top of the solid propellant rocket motor....

I'll try and get a rough drawing up soon


thanks

terry dean
nar 16158

ram air ducted rocket.jpg
 
Thrust augmenters seem to work well
on pulse jet engines, thou they have to
be rather precisely engineered.

The problems in model rocket application most
likely will include fireproofing and additional
tail weight plus drag. Off the cuff methinks the possible
gains will not likely offset the handicaps, perhaps there
could be something to gain on large HPR designs...

Would prolly look cool as s**t thou...
:D
 
Thats definately a curious proposal you make.

I would assume that ALL the dimensions would need to be rather precise as Susan said.

I'm not sure if the same physics principals apply, but I would think that your ramjet would need to act much like NASA's X43-a scramjet. The scramjet was only able to reach sustainable operating speed after it reached well above mach on a solid rocket booster.

https://www.youtube.com/watch?v=-MLQqCFwl6g
 
It works....the math is somewhat intense to get it right and reap full benefits. However I suggest experimenting some to see. Main nozzle design will be different than a solid rocket motor. Very simple graphite expansion nozzle will suffice. Check out the Bomarc, very simple design and notice the nozzle port is much larger diameter for ramjet than standard solid rocket motor. Thrust augmentation can result in dramatic increase on thrust if done right. Good Luck!:)

Retro

TRA #279
 
Ever seen a rocket with tube fins? Will imagine a large central solid booster motor with three or four ramjet tubes around it. Nestled inside the tubes are small solid rocket motors with over expanded nozzle ports so they burn slow and rich (gas generators) Add a 4:1 diffuser section ahead of the motor, Phenolic lined tube serves as a combustion chamber with slight converge/divergent nozzle at the end. If you want add three standard fins for extra stability. Think about it.....

Retro

TRA #279
 
Several of the above posts are right on the mark, it will be tricky to make an augmenter that actually augments. It will be especially difficult for our type of motors because they tend not to operate at a constant thrust (or mass flow), and that adds to the complexity of trying to select which part of the motor's thrust curve to augment. When the motor is operating away from the selected thrust condition, the augmenter will probably not be helping. IIRC, a Jetex motor had a longer burn at a fairly constant thrust level, so an augmenter had a better chance to be useful in that case. Jetex exhaust was also much less intense, less pressure, less everything, and the flying models that used Jetex motors flew at much lower speeds, so they could get away with a very light augmenter (we can't).

It is also going to be a bit tricky to design and build a proper inlet and exhaust system for this. Yes, Jetex had a simple one, but I have never seen any data on whether those things actually worked (that is, if the nuisance factor was really outweighed by the improved performance).

I am certainly not saying it would not work because this device really is known to work...when done right. Give it a try!
 
I was thinking the outer ram air intake tube would be formed from say a hi-temp thermoset plastic and nomex paper under pressure.... use it a few times and then throw it away...design it to be disposable


I wonder if the ram air that mixes with the exhaust plume would heat up or cool down? Remember that air is about 78% nitrogen and 21% oxygen..... and it would be mixed at subsonic speeds with the heated exhaust plume which is what??

Does anybody have any idea how thick the boundary layers are at low to mid subsonic flight reimes for model rcokets size craft?


I can see making a model where it separtes right at the line where the ram air intake housing is.... that would allow the bundary layer to be laminar and attached as it reached the ram air intake......

I assume the flow would be turbulent inside external ram air intake housing as it hit and mixed with the exhaust plume?


terry dean
nar 16158
 
For this concept to work, wouldn't the air have to be moving slowly right next to the opening of the nozzle to work? It wouldn't work if the slower moving, but compressed air, was a few inches below the nozzle, right?


Someone should test this...:D
 
and it would be mixed at subsonic speeds with the heated exhaust plume which is what

First get acquainted with the Brayton Cycle. Your motor manufacture might be of some assistance as well. If not then you can ball park it (exhaust temp) with some formulas. Propellant chemical composition, ratio's, and nozzle expansion data would be helpful. Otherwise you should have ISP and thrust information which is a start point. Derive the heat of combustion based on average solid motor F/O composition and ratio, then subtract the cooling from nozzle expansion should get you in the ball park.

I’m working on my ducted ram rocket design code (excel spread sheet). I’ll post some basic formulas for you in a couple days. In the meantime do your research and perhaps you will discover the answer on your own. Besides if you’re serious about this then you will have to get your head into it.

If you plan to go supersonic you will need to establish an oblique and normal shock structure at the throat of your inlet to slow the free stream to subsonic. Many ways to do that - external, internal and mixed compression inlets.

Couple of terms and concepts I’m struggling with right now. Inlet “choking” seems to relate to sonic (speed of sound) mass flow in the diffuser section. And “inlet start” they both seem to be related not 100% sure I have full grasp on their significance.
 
It wouldn't work if the slower moving, but compressed air, was a few inches below the nozzle, right?

That would be an interesting experiment. In pure thrust augmentation you’re trying to achieve an afterburner effect. I would think closer to the motor exit would be better to allow some axial distance to achieve good mixing. That probably depends to some degree on where one locates the motor in the duct. Or distance from the afterburner exit (body tube) to the rocket (ejector) nozzle. I've seen SFERJ examples where captured air is introduced just below the motor nozzle and also further down near the combustion/afterburner/body tube exit. This is called bypass air and is used to create a throttling effect by varying pressures and F/O ratio (burn or regression rate) of the solid fuel core.

Retro
 
I'm really happy to discover there are other people looking at this possibility.

I'm not 100% this will even work at less than supersonic speeds.... I was hoping to get a small thrust augmentation that outweighs the added weight and drag that this design brings forth.

So can anybody explain why the thrust augmenters on the old jetex worked? I can only see one major difference: the Jetex had a low and long thrust, unlike typical model rocket motors.

SO maybe I need to consider a composite motor like? an E6? or F10? It seems to me that the longer the motor generates gases, the greater opportunity for mixing and thrust augmentation. Myabe these newer Chinese made longer buring C6 motors would work?


Just for your information, "back in the day" experiments were run with both water and freon , being injected into the exhaust plume, and some additional thrust augmentation was noted; water of course was way to heavy to carry "on-board"; freon was used more for directional control......sorta like the graphite vanes and the V2.

Tominator: That was a great link!. I think what I want though is what is called a ducted rocket or ram rocket..... versus scram and ramjets. Close to the same but slighlty different.


Another potential advantage of ram air induction is not only thrust augmentation but also stability control; sorta like the old Forest Mim's ram air control system.....

https://en.wikipedia.org/wiki/Air-augmented_rocket

well back to the drawing board!


terry dean
nar 16158
 
I would think long burn motors would be best to take advantage of the augmentation effect.

Don't be misled by the naysayers who claim inlet weight and drag will offset any benefits gained from thrust augmentation. At very low or high supersonic velocities, off design conditions or poorly implemented designs that may be true. If done correctly it can provide a substantial increase in overall performance. Keep in mind a rocket accelerates very quickly so the low speed effects only impact performance for a fraction of a second. Nose inlet drag was actually less than with a standard pointed nose cone. In the case of the KATANA ducted cone rocket the inlets are very simple and actually act as fins to aid in stabilization. In the case of "base burning" the pressure difference can actually offset base drag on designs with blunt aft sections.

Ideally one wants to compress the captured free stream air in a diffuser (expansion) section just ahead of the combustion or afterburning section. One question to answer is how much fuel is present in the rocket (ejector) motor exhaust. If the motor mixing and combustion cycles are very efficient then there maybe little fuel reaming in the exhaust to mix and burn with the captured and compressed free stream air (afterburner). If one knew the chemical mix of the motor the potential energy could then be estimated. Then compare that to the thrust and ISP to see how much energy was converted into useful work. That remaining energy is then available for conversion to heat and pressure (work) in the afterburner section. Keep in mind the enthalpy of the system, heat and pressure (conservation of energy).


Here are some declassified resources to search for most are avaliable for free download through AREADE or NASA Tech Report Server.

NACA RM No. E7105 "Supersonic Ducted Rocket", 1948

NACA RM L52J23

NACA RM L52119

NACA RM L56D24

NACA RM L52K03

Check out the references page on those reports for more sources.
 
I've been working on a similar problem for the last few days...
If you want to get this thing producing thrust you need to do a few things:

1) Treat the flow in the duct as Fanno flow. This means you have constant area and friction. Friction, believe it or not can lead to increased flow velocity in certain conditions. I know, strange, right? Take a look on wikipedia at Fanno flow and the Fanno line.

2) Once you have established the conditions at the end of the constant area duct (mach number, temp, pressure, etc), you can use model the next segment as Rayleigh flow. This model is valid for constant (or near constant) area with heat transfer. It is quite amazing... you can take subsonic flow and add heat to it to increase the velocity...or you can get sonic or supersonic flow and pull heat out of it to increase velocity!!! Amazing. (In fact, at a certain point on the Rayleigh line, adding heat decreases the temperature!)

3) Of course, you have to worry about shock effects if you are flying your rocket in the supersonic regime. Normal and oblique shocks are always fun to deal with.

I guess my point is that there is no free lunch in the thermodynamics world. You need to add energy to that flow to produce more thrust. Sometimes this energy is easily available (in the form of excess heat from the rocket engine) but sometimes it requires quite ingenious methods of heating/cooling.

P.S. My technical sources are not Wikipedia so don't worry...it's legit.
 
Graham,

Very interesting concept I will have to research this further. If I follow correctly
combustion of exhaust fuel is not required. This seems to be based more on heat transfer.
 
Tominator: That was a great link!. I think what I want though is what is called a ducted rocket or ram rocket..... versus scram and ramjets. Close to the same but slighlty different.



https://en.wikipedia.org/wiki/Air-augmented_rocket

Shockwaveriderz,
Yeah i know what the ducted rockets are and i saw another website that was in French but i cant seem to find it yet...just thought it was a cool website. Glad you liked it!
-Tom
 
Their was a OOP kit called the Ram-Jet.

I saw a 4" shorta version of that in the late 80s with the motor moved up higher into the body tube so it would mix with the air being ramed into the 4" ram tube.

IE a 4" body tube has a 2.56" body tube and nose cone inside it up top and tapers down to 38mm motor mount tube that stop short near the rear.

The fake internal static tubine fins hold the internal tubes to the 4" body/ram tube.

It makes a nice Kewl looking smoke trail as the Krushnic effect mixes with the air stream.
 
Thanks Art,

Wish I knew someone that flown one of those OOPs kits to find out more.

Fanno Flow..... I think I'm starting to grasp it. As a fluid moves through a constant duct diameter friction heats the fluid, the fluid expands. To maintain equal mass flow the heated less dense fluid must increase its velocity.
 
"Fanno Flow..... I think I'm starting to grasp it. As a fluid moves through a constant duct diameter friction heats the fluid, the fluid expands. To maintain equal mass flow the heated less dense fluid must increase its velocity."

Yep. :) It is a method of manipulating flow to get the conditions you want. Since the drag forces (integral of shear stress on wall) are greater than the force that is produced energy is lost. However, it is effective in increasing your temperature...
 
Graham,

Fanno Flow... I ran this concept by a couple of former aerospace engineers. While not going into detail they confirm you’re barking up the right alley for "air augmentation". While yielding less ISP and thrust gain than the ram rocket concept (burning of fuel rich entrained air referenced in my earlier thread "motor ID") it appears more suited to general HPR applications as discussed in this thread. The ram rocket concept would perhaps be more suited to EX HPR applications. I plan to pursue both concepts as they are very similar.

I plan to study the fanno flow calculations and adapt them into my initial air augmented design. See what happens
 
Okay, okay...So like I've said, I've been working on something similar but with Rayleigh flow. It is a gas-breathing engine that uses heat (from chemical or electrical sources) to provide energy for propulsion.

The theoretical design has a gas inlet (converging section), a constant area duct (where ∆Q happens) and an outlet (diverging section). I've also done calculations with no outlet divergence (see graphs below).

Basically, this thing is pretty sweet. If you had 100% conversion of gasoline to heat you would get an equivalent Isp of about 2000.

I've plotting the performance of the engine verses different mach numbers of the vehicle to which the engine is propelling. This is a gas-breather so you'd expect performance to vary (significantly) in different regimes. (It appears that this design is not good from about mach 0.7-2 ...)

delVperQ.jpg

delPperQ.jpg
 
Basically, this thing is pretty sweet. If you had 100% conversion of gasoline to heat you would get an equivalent Isp of about 2000.

Yeap. . . that would be sweet! Is your converging inlet 3D axisymmetric? Are you transferring heat energy to electrical via a magnetic field to bring about a chemical change in the fuel prior to injecting into combustion chamber? Justhad to ask....

ejector ramjets

SERJ Delta X-15 I'm surprised you came across that design. I just happen to be designing an all composite RC rocket/EDF glider based on that exact design. Mr. W. Escher was kind enough to share some aspects and papers of that design with me. I hope I can do it justice. The SERJ was an early RBCC concept. I believe Marquardt built one. An aerospace engineer recently came across it, purchased it, and is contemplating resurrecting it!
 
Yeap. . . that would be sweet! Is your converging inlet 3D axisymmetric? Are you transferring heat energy to electrical via a magnetic field to bring about a chemical change in the fuel prior to injecting into combustion chamber? Justhad to ask....

The inlet, so far, is axisymmetric. It gives the best adherence to the isentropic model for that part.

The idea would be to "scoop" up the air and converge it to a particular mach number (which i cannot say) through a supersonic diffuser (aka a subsonic nozzle) and then use a N2-tuned microwave emitter to heat the fluid passing through the constant area duct until it is thermally choked. At that point, you can diverge the duct to act like the last half of a De Laval nozzle...giving you higher exit mach numbers. This, however, is only effective in higher inlet velocities...so it would actually be more efficient to not diverge the flow after the constant area duct for M<1.2, assuming gamma = 1.4.
 
hey guys.... what about if you placed a "tubular/ring PVC or parafin ring inside the lower RAM unit? Then you would have the modle rocket supersonic jet mixing with compressed air and ablating the Parafin or PVC plastic creating a higher mass flow rate?

terry dean
nar 16158
 
Similar perhaps to an "ELECTROTHERMAL RAMJET"? Sounds like a fascinating concept. I must admit my inlet knowledge/and brief experience is obviously somewhat rusty and dated by your standards. Beyond basic MHD concepts I fade out quickly. Glad to hear others still share some affection for airbreathing propulsion. My hope is that rocketeers like shockwaveriderz and me can generate some airbreather interest and awareness in the model rocketry hobby.
 
hey guys.... what about if you placed a "tubular/ring PVC or parafin ring inside the lower RAM unit? Then you would have the modle rocket supersonic jet mixing with compressed air and ablating the Parafin or PVC plastic creating a higher mass flow rate?

Judging by my calcs, this would have the effect of adding thrust but at the expense of lower efficiency. It'd be like an afterburner...which can prove useful especially if you have unreacted O2 in the flow.
 
hey guys.... what about if you placed a "tubular/ring PVC or parafin ring inside the lower RAM unit? Then you would have the modle rocket supersonic jet mixing with compressed air and ablating the Parafin or PVC plastic creating a higher mass flow rate?

terry dean
nar 16158

My thoughts exactly. Though PVC, poly and parafin all have shortcomings. Polymetric based fuel cores have slower regression rates (less thrust). Parafin burns 3-5 faster than polymetric fuels however it has poor (weak) mechanical properties. A poly/parafin mix might be ideal. One would have to experiment to find the best mix ratio. So far I have not been able to find published results of such research. What your suggesting is an ejector ramrocket. The Saab Scania Viggen utilzed this concept as an ejector nozzle.
 
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