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
From what you describe I'd say it's making use of the Venturi effect... same principle as old style carburators and water well jet pumps. Fluid (air or water) is drawn in from an open port, through a narrowed passage, which increases the velocity (and lowers pressure per the Bernoulli principle) and increases the mass going out the back.
https://www.aymcdonald.com/GenInfo/Pumps/Pump Basic-Sizing.pdf Carburators use the pressure drop to draw a liquid into the airstream and atomize it; a water well jet is a far better example as it uses a high pressure high velocity stream injected into the fluid exiting through an enlarged nozzle, drawing the surrounding fluid along with the stream and increasing the mass of pumped fluid. The tradeoff is a pressure drop and while increasing the MASS of moved fluid, the VELOCITY of the combined stream is much lower. These types of things are used in jet agitators in spray tanks for agricultural spraying as well. Use high pressure/velocity pump overflow to induce high volume circulation of the spray mixture in the tank to keep the chemicals properly suspended or mixed.
https://www.deindustrial.com/category-s/788.htm
Now, the problem is, in a Jetex motor the thrust is low, and you WANT low thrust... G. Harry Stine in the 'Handbook of Model Rocketry' described some of the spectacular failures that occurred when folks tried to use high thrust model rocket motors in place of low thrust Jetex's. Folks who tried Jetex's in rockets commonly found they wouldn't lift off the pad or would be unstable because the thrust was too low to build sufficient speed for stabilization. If you try this principle on a rocket, you'll probably see much the same results as with the Krushnik effect. Probably not as pronounced, due to the Krushnik effect commonly occurring on rockets where the engine is recessed too deeply in the body tube, and the exhaust plume of the rocket engine over expands and becomes turbulent and thrust drops precipitously. With the open ports to the surrounding air, some air should be drawn in to minimize the turbulence, but it will be reducing the effective exhaust jet velocity of the engine by creating the 'suction' to draw in the surrounding air and propel it out the back, which will show up as reduced thrust.
The closest I've seen to this idea is something John Pursley is working on, on 'large diameter' (say BT-60 or above), using ports on the side of the rocket near the bottom between the fins to allow the air to enter the cavity surrounding the motor mount, drawn in by the base drag 'vaccuum', to reduce base drag. He isn't using scoops though, and I bet the reason why is that scoops projecting out into the slipstream would create as much or more drag than they would reduce, and the venturi effect will draw in air from flat ports as the base drag vaccuum increases in intensity as velocity increases after liftoff. In discussing the project, he said (and I completely agree) that much work remains to be done to prove the concept, in the shape, size, location, and number of ports, and their actual effect on the flight compared to the rocket flown with it's ports taped over.
It's a fascinating subject and would make a really good NARAM R&D project.
My own experience tells me that while it's an intriguing idea to reduce base drag on large diameter models, it has yet to be verified with measured performance data, let alone the research on the effect of size/shape/number of ports. As far as 'thrust augmentation' goes, I would seriously doubt any positive effect could be gained, due to trading lowered velocity of the motor exhaust stream for increased mass of the combined air+motor exhaust stream, and any 'scoops' used to 'ram air' into the system would create more drag than they produced in thrust (second law of thermodynamics). Basic physics...
Good luck and let us know of your results if you experiment with this! OL JR
