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  #11  
Old 10-28-2006, 08:03 PM
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Default I am going to build a few of the Augies myself

The Augie looks very interesting. I will have to build a few myself.
I saw the plans JimZ Rocket Plans. Very interesting.
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  #12  
Old 08-07-2010, 10:34 PM
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Quote:
Originally Posted by Rocketflyer
Thanks Scott, that's the one, Li'l Augie. I'm gonna build another one of those. For some reason I thought it used the old BT-40 tube instead of the BT-30. No matter, both tubes were strong. Thanks again Scott!
The Soviet Gnom ("Gnome") air-augmented propulsion ICBM (see: http://www.astronautix.com/lvs/gnom.htm ) would have used a solid-fuel ramjet second stage, whose high specific impulse would have made this road-mobile ICBM less than half the mass of competing all-rocket designs. A sub-scale version of the Gnom's solid-fuel ramjet motor was tested successfully in the PR-90 SRBM (Short-Range Ballistic Missile, see: http://www.astronautix.com/lvs/pr90.htm ). A carefully-designed ducted rocket/ramjet system can increase the performance of a model rocket, but without using a rocket with a fuel-rich exhaust (which will burn with the air in the ramjet combustion chamber), the performance gain is rather small.
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  #13  
Old 08-08-2010, 12:47 AM
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This sounds like air induction. Like the "bladeless fans" that Dyson is now selling. These are fairly common in agriculture. We use them in our water wells as jets, or ejectors, where a surface jet pump pumps water down the well casing to the jet, either through a second pipe, or more usually, through the casing surrounding the "rot line" (the water line coming up from the jet to the pump inlet). This water travelling down the casing from the pump goes through a passage in the packer jet (which is sealed with leather O-rings to the casing to prevent the water going past it down the well) and squirts through an orifice, up into the rot line. Surrounding the orifice is a venturi tube with water drawn up from the foot valve below. The high velocity jet of water coming from the orifice nozzle 'entrains' the surrounding water into a lower velocity, but higher VOLUME, stream going up the rot line back to the pump inlet. The pump has a throttling valve (can be a simple gate valve) which you adjust so that the the "extra" entrained water is pumped out to the tank and plumbing. If you open the throttle valve too much, the jet gets insufficient flow and the pump will quit pumping.

Similarly, these things are used in sprayers for jet agitators. A nozzle is attached by four 'fins' to a larger venturi section above it. When the spray pump is activated, the excess spray solution returning from the valves squirts through the nozzle, which then draws more liquid off the bottom of the tank surrounding the nozzle, draws it in past the four fins supporting the venturi, and it's all drawn through the venturi and squirts out the top at a lower velocity but at a higher volume.

Now, note, that these things are NOT going to create "more thrust". Quite the opposite, in fact. They take a high-velocity, high pressure gas stream and use it to entrain (induce motion) into a larger volume of working fluid, moving the larger volume at a lower velocity and pressure than the nozzle pressure. The energy to do this has to come from SOMEWHERE, which is, the flow coming out of the nozzle.

Now, if the rocket motor is recessed up into the back of the rocket by a considerable amount, the Krushnik Effect will occur, which is, the rocket exhaust will be trapped in a confined space. Due to it's velocity, it creates lower pressure surrounding the nozzle (which is what draws the working fluid into the stream in the water jets) which tends to draw surrounding air or gases into the flow around the exhaust stream coming out of the motor nozzle. In a rocket suffering from the Krushnik effect, this creates a partial vacuum around the nozzle, causing two things-- the motor's exhaust over-expands into this vaccuum, which lowers thrust considerably, and it sets up an exhaust plume recirculation, which draws part of the exhaust gases back up into this vaccuum, which creates a vortex that robs energy. These two effects conspire together to rob most all the energy from the exhaust stream coming out of the motor.

Exhaust gas plume recirculation is often easy to see on larger rockets with very flat bottoms-- as it accelerates through the air, the base drag increases due to the rocket leaving a substantial vaccuum wake behind it. Hot exhaust gases from the nozzle exhaust of the rocket get drawn back up into this partial vaccuum and move back toward the more intense vaccuum created by the high exhaust velocity at the motor nozzle (remember that a moving gas exerts less pressure than a stationary gas, which creates the vaccuum-- like the Saturn V's engines that sucked all the flame and smoke from engine start back down into the flame bucket as they came up to full thrust!) This hot gas stains the bottom of our rockets from this plume recirculation. ALL of this turbulence, drag, and recirculation takes energy to create-- energy it ROBS from the exhaust stream of the motor, which is part of the total energy the motor is capable of producing.

Now, this Augie design is allowing ambient air surrounding the rocket to be drawn into this low pressure zone (partial vaccuum) at the rear of the rocket, and especially surrounding the motor nozzle which it's high velocity exhaust stream, so that it mostly eliminates these drag effects and energy robbing turbulence from plume recirculation. The entrained air will increase the VOLUME of the exhaust gases, but REDUCE THE VELOCITY. It still takes energy to entrain this ambient air and accelerate it out the back, which is energy robbed from the exhaust, but that happens anyway with ALL rockets operating in an atmosphere, even with minimum diameter ones, so basically, it's minimizing the base drag losses that would otherwise occur from the partial vaccuum created in the rocket's "wake", and eliminates the exhaust plume recirculation which would otherwise be robbing additional power from the high velocity exhaust stream, since part of the exhaust gases would have to otherwise be drawn BACK up to the nozzle area by the high-velocity exhaust induced partial vaccuum and then entrained into the gases surrounding the high velocity exhaust stream.

These things CANNOT break the second law of thermodynamics and "create more thrust"-- they can only reduce the losses occuring from base drag and plume recirculation. If they could, you'd NEVER have to build a bigger rocket engine-- you could just put a bunch of air venturi nozzles surrounding the exhaust nozzle and use that to create more thrust! The ONLY energy available for flight is the total energy in the propellant exhaust stream, and much of that is wasted. Anything we can do to REDUCE that waste, shows up as a performance improvement. Hence, the reason these things work.

NOW, if, as blackshire mentioned, you use the incoming air as the oxidizer for more injected fuel or a fuel-rich exhaust stream that then RE-COMBUSTS in a secondary expansion nozzle, THEN you will multiply your thrust and therefore power, but of course you're expending more fuel (propellant) in the process, so this doesn't violate the laws of thermodynamics.

In fact, that is EXACTLY how a jet engine afterburner works...

LateR! OL JR
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  #14  
Old 08-08-2010, 04:05 AM
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I'm not sure if the simple ducting (*without* additional combustion of the exhaust with the duct air) *doesn't* increase the system's thrust (at the cost of a lower specific impulse). Here's why I suspect this may be so:

A turbojet and a turbofan that both burn the same quantity of fuel per minute will not produce the same thrust or have the same specific impulse. The turbojet moves a relatively small volume of air to a very high ejection velocity from its exhaust nozzle (low thrust/high specific impulse). A turbofan is the opposite (high thrust/low specific impulse)--for the same quantity of fuel burned, it moves a larger volume of air at a lower velocity from its exhaust nozzle, which produces higher thrust.

The turbojet is analogous to a liquid hydrogen/liquid oxygen rocket engine (high ISP/low thrust), while the turbofan is more like a kerosene/liquid oxygen rocket engine (high thrust/low ISP). The non-afterburning ducted rocket seems like a turbofan, moving a larger volume of gas (a mixture of rocket exhaust and duct air) more slowly to produce more thrust with a lower specific impulse.
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  #15  
Old 08-08-2010, 09:35 PM
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I built the Augie II as a kid in the 60s. A fun two-stager, and no booster to recover. I drew up plans for an upscale to 24mm a few years ago, and was thinking the other day about building, since I've got a couple of mini 2-stagers under construction. Funny that this thread go resurrected from several years back. I remember painting the insode of the big tube with Estes hi-temperature paint. Now it might be BBQ grill paint ...
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  #16  
Old 08-09-2010, 12:10 AM
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Well, yall can always go straight to the horse's mouth (sorry blackshire, couldn't resist the pun!) and ask John Pursley at accur8.com-- he was experimenting with inlets cut into the sides of his large-base diameter models a couple years back, with the hope of increasing performance measurably. He might have some interesting information to share on the subject. IIRC, he wasn't planning on using 'scoops', as they would tend to create turbulence and anything sticking out from the rocket into the airstream would create a lot of drag, but he WAS working on the best shape/size of "port" to put near the back of the rocket to admit ambient air into the low pressure zone around the nozzle to reduce recirculation and base drag.

Ya know, I read an interesting book a few years ago, about the design of jet engine intakes. In the early days when jets were subsonic, the inlet design was pretty straightforward, but when transonic and supersonic sustained flight became practical, one of the largest hurdles to overcome was in jet intake design... For most turbojets, turbofans, etc. the air MUST be subsonic at the compressor face. This led to a lot of early designs simply having inlets that were sized small enough that the supersonic airstream the jet was flying through would enter the small inlet, expand (and thus slow to subsonic speeds) and then enter the compressor fans of the jet engine properly. Problem is, on the ground and at low airspeeds, the inlet is undersized and leads to significant power loss in the jet engine, and efficiency hits, because the engine is basically 'starved for air' and pulling a fairly strong vaccuum at the compressor face.

When Mach 2 designs were coming about, it was pretty obvious more sophisticated methods would be required. The Soviets favored the "sliding cone" inlet restrictor, which could slide back, enlarging the opening at low flight speeds and allowing the compressor unrestricted airflow, while moving forward to close the opening down at mach speeds to restrict the airflow coming in to the plenum ahead of the compressor face, allowing it to slow, expand, and cool down before entering the compressor. Additionally, the correct shape of the cone at high supersonic speeds could put the "Mach shockwave" outside the inlet altogether, since the air flowing over the aircraft behind the Mach shockwave is subsonic, which meant that the opening has to change size to accomodate the differing flow rates/pressures/inlet velocities. The US, on the other hand, stood their intakes off the side of the jets (like the F-4 Phantom) so they'd be in "clean air" outside the shockwaves and surface slipstream effects of the skin of the aircraft, and then employed swivelling "gates" that could be tilted outward from the front of the intake to modify the incoming airflow, putting it in a Mach shockwave and lowering the airflow to subsonic as it entered the compressor inlet while reducing supersonic drag at the same time. Of course, the speed and angle of attack and other things also came into play and had to be accounted for as well. Some of the Migs that had side inlets instead of nose inlets used half-cones that slid forward or backward depending on the airspeed to accomplish the same thing.

Later, with newer jet designs, the US used the 'variable droop' rear-slanted inlets like those on the F-14 Tomcat and the F-15 Eagle. The degree of "droop" was controlled by actuators to control the intake air coming into the duct and put it into or out of the mach shock to reduce the speeds to subsonic at the compressor face. The Russians also adopted this on their Foxbats and some other high performance designs.

When high-bypass ratio turbofans came into use, the problem was exacerbated by the especially large intake sizes for the enlarged bypass compressor fans. Many of the same design problems cropped up again-- inlets large enough for good low-speed and takeoff performance, when maximum performance is needed most, created excessive compressor-face airspeeds and high drag at high speeds, whereas smaller inlet diameters that had good performance characteristics at high speeds were too small and created problems at low speeds by restricting airflow into the compressors too much. This was not as much of a problem on airliners staying significantly subsonic, but on high-performance aircraft using high-bypass turbofans, like the Harrier, the problem was much more acute. The problem was ultimately solved by ADDING SMALL SPRING-LOADED DOORS SURROUNDING THE LEADING EDGE OF THE ENGINE INLET(S) WHICH, AT HIGH SPEEDS, CLOSED UNDER SPRING PRESSURE WHEN THE INCOMING AIRFLOW WAS SUFFICIENT EVEN WITH THE SMALL ENGINE INLET SIZE, BUT WHEN THE AIRCRAFT SLOWED TO LANDING/TAKEOFF SPEEDS (or hovered in the case of the Harrier!) THE SPRING LOADED INLETS WOULD BE 'SUCKED' OPEN BY THE HIGHER VACCUUM CREATED BEHIND THE SMALL INLET BY THE HUNGRY TURBOFAN COMPRESSOR FACE, AND ALLOW MORE AIR INTO THE ENGINE. These small 'doors' were TOTALLY self-operating and idiot-proof-- they responded only to the pressure environment inside the engine inlet at the compressor face-- closing by spring pressure when incoming air was at ambient pressure or higher in high speed flight, and self-opening when the compressor was drawing more air in than the inlet could deliver at low speeds, creating a vaccuum behind the doors, so the ambient air outside the doors forced them open to relieve the partial vaccuum behind the doors, essentially increasing the inlet size.

Such systems are in common use today on high-bypass turbofans. High mach-speed aircraft still generally use some kind of 'controlled opening' type of system due to the more demanding and radical changes needed to maintain proper airflow into the engine regardless of speed of the aircraft.

Sound familiar?? In the same way that these "small trap-doors" relieve the partial vaccuum in front of the turbofan compressor face, these small ports will allow ambient air surrounding the rocket to be drawn in near the base from the surrounding slipstream, in this case, reducing base drag and exhaust recirculation at the rocket base, removing the parasitic drag losses incurred by the turbulence created with those ports being absent.

Later! OL JR
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Old 08-09-2010, 01:04 AM
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Quote:
Originally Posted by luke strawwalker
Well, yall can always go straight to the horse's mouth (sorry blackshire, couldn't resist the pun!) and ask John Pursley at accur8.com-- he was experimenting with inlets cut into the sides of his large-base diameter models a couple years back, with the hope of increasing performance measurably. -SNIP-
I don't at all mind being (rhetorically speaking) ridden thusly, OL JR! :-) Thank you for providing the synopsis on subsonic, transonic, and supersonic jet engine inlet duct design considerations; I did not know about the spring-loaded inlet doors ahead of the fan of the AV-8 Harrier's Pegasus turbofan (given the engine's name, I should have...). It sounds like John might have been experimenting with NACA ducts. For the velocities at which most model rockets fly, the NACA flush intake ducts (see: http://en.wikipedia.org/wiki/NACA_duct and http://minijets.org/typo3/index.php?id=105 ) might be the most efficient, lowest-drag, and easiest-to-build inlet ducts to use. They are used in circular aircraft fuselages and engine cowlings as well as in flat aircraft fuselage sides and flat automobile body panels.
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Last edited by blackshire : 08-09-2010 at 01:06 AM. Reason: This ol' hoss done forgot somethin'.
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Old 08-09-2010, 01:24 AM
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Quote:
Originally Posted by LeeR
I built the Augie II as a kid in the 60s. A fun two-stager, and no booster to recover. I drew up plans for an upscale to 24mm a few years ago, and was thinking the other day about building, since I've got a couple of mini 2-stagers under construction. Funny that this thread go resurrected from several years back. I remember painting the insode of the big tube with Estes hi-temperature paint. Now it might be BBQ grill paint ...
In the heyday of the English Jetex-powered F/F model jet planes, modelers painted the inner surfaces of rolled-paper and rolled-balsa jet exhaust ducts in the models with "waterglass" (a solution of sodium silicate, if memory serves) to protect the ducts from the hot Jetex exhaust. The spun-aluminum Jetex Augmentor Tubes made this largely unnecessary when they became available. A convergent nozzle added to the rear of the Augie II's duct tube should increase its thrust.
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  #19  
Old 12-23-2012, 02:19 PM
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It works...fanno-flo.... very well in many forms. However balancing inlet area/combustion area and nozzle size makes it work all the better. The "Krusnik effect" is way over dramatized....supply an inlet, diffuser section, a slightly restricted basic nozzle be it subsonic or supersonic (depending on max velocity) and it can work very well indeed. Be warned going supersonic and one can ignite the unused fuel in the exhaust stream which then requires a combustion chamber designed to withstand the pressures and heat. For subsonic use a phenolic liner in the ejector tube, paint or coat with Hi-temp....I coat my phenolic liners with Zicrotech.
http://www.youtube.com/watch?v=JYlG...eature=youtu.be

or KDT,inc for more conventional application of coating like spray-on or brush on.

Going supersonic with augmented combustion use an aluminum/phenolic tube and liner and flame holder mixer.
http://www.freepatentsonline.com/2926613.pdf

http://www.freepatentsonline.com/2547936.pdf

SFDR having multi-grain for boost and fuel rich burn phases....
http://www.freepatentsonline.com/4729317.pdf

I use graphite for my nozzles...graphite tube then turned in a lathe. Use a step overlap to seal between phenolic liner and nozzle.

For fuel rich supersonic use "smokey sam" fuel to achieve true augmented burn.

Basics:
http://www.freepatentsonline.com/4133173.pdf
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  #20  
Old 12-23-2012, 03:29 PM
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With the advent of modern electronics it is now possible (even on a model rocket) to adjust inlet holes dynamically with airspeed to maximize efficiency.

Jerry
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