Would a recessed motor improve aerodynamics?

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I'm playing around with some low altitude designs, while I wait for them to retract the launch prohibition.
I was designing a couple rockets with BT-80 bodies and 18mm motors. No boat tail.
I was wondering if the aerodynamics would be better if I had the aft centering ring right near the back edge of the body tube or would it be better to recess the engine so that the exhaust cone from the motor nearly fills up the void? Recessing the motor would also move the CG forward, allowing a shorter body tube which also reduces drag. Obviously, if I recessed it too far, I'd burn the back edge of the body tube.
With a BT-80 body tube I should still have room to reach in and install motors.
 

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I think it's probably safe to recess an 18/24mm engine into a bt-80 body tube.... perhaps 1 motor diameter at most......I think the Krushnik effect only rears it's ugly head when the engine and body tube are both near the same diameter, then it will happen past 1 body tube diameter recess...

Recessing engines into a bigger body tube just makes it more difficult to get the engine in and out and to hook up your ignition cables , Plus the exhaust plume from the rocket engine is going to expand 2-3x the motor diameter (at least with some BP engines) which is going to charboil the larger body tube.
But this can be ameliorated by using adhesive backed aluminum foil...

I don't know if anybody has seen this before, but here's what the plume looks like from a simple 1/2A3-2t using Schlieren_photography



Now imagine that exhausting into a larger diameter tube x2-x4 diameters up inside it.

A ring of small diameter hole or slots around the bottom circumference of the outer body tube would help, but it increases drag.... although the laminar to turbulent boundary has already been tripped by the time it gets to the bottom of the rocket

much better description of and analysis of the Krushnik effect by Gordon Mandell here:

https://www.ninfinger.org/rockets/ModelRocketry/Model_Rocketry_v02n02_11-69.pdf
 
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The rule of thumb (shockie mentioned) is to never recess a nozzle more than one body tube diameter. So it would be wise to keep it under a half a diameter to be on the safe side.

Filling the back of the BT with the expanding plume is the very thing you don't want to do, as that is exactly how the Krushnic effect works (as well as scorching the body tube or worse).

I can't say with authority, but I rather doubt recessing the motor mount will make any noticeable difference in drag. A motor hook extending out behind a minimum or nearly minimum diameter rocket might be significant contributor to drag, but in the back of a BT-80 it would probably not be in the air stream and would no effect worth mentioning. I think.

A ring of small diameter hole or slots around the bottom circumference of the outer body tube would help.
I recall reading somewhere (but not where) that adding vents just behind the aft centering ring lets air in to the cavity and combats Krushnic. Since that air comes out with the exhaust and thus increases the mass flow at the body tube exit, if you do it just right it might increase your thrust. I'm sure that doing it just right requires a lot of research, calculation, and/or experimentation. And doing it wrong could mean that Krushnic still manages to bite you.
 
I always picture a 45deg danger cone -- not exactly scientific but it seems to do the trick for, e.g. my toploading Big Daddy.
 
The rule of thumb (shockie mentioned) is to never recess a nozzle more than one body tube diameter. So it would be wise to keep it under a half a diameter to be on the safe side.

Filling the back of the BT with the expanding plume is the very thing you don't want to do, as that is exactly how the Krushnic effect works (as well as scorching the body tube or worse).

I can't say with authority, but I rather doubt recessing the motor mount will make any noticeable difference in drag. A motor hook extending out behind a minimum or nearly minimum diameter rocket might be significant contributor to drag, but in the back of a BT-80 it would probably not be in the air stream and would no effect worth mentioning. I think.

I recall reading somewhere (but not where) that adding vents just behind the aft centering ring lets air in to the cavity and combats Krushnic. Since that air comes out with the exhaust and thus increases the mass flow at the body tube exit, if you do it just right it might increase your thrust. I'm sure that doing it just right requires a lot of research, calculation, and/or experimentation. And doing it wrong could mean that Krushnic still manages to bite you. The link below will bring you to JimZ plans site
Can and has been done, although the modifications in the design required almost certainly negated any increase in performance. This is one of the only production model rockets designed for CHEap And Dirty (CHAD) staging I know of, but I am far from an expert.

177C8299-AE10-41C2-82A4-A95E0B7EE1B9.png


The link below takes you to JimZ plans site
https://www.spacemodeling.org/jimz/eirp_10.htm
 
I found his example of accidentally confronting the Krushnic effect very relatable. I had an engine block fail in a BT-80 rocket with a baffle. The rocket was obviously struggling to gain altitude and the nose cone never came out. It lawn darted. On inspection, I realized the motor was between the mount and the baffle and it was lucky the whole thing didn't start on fire. The body tube was very hot.
Not sure I understood everything in the article, but I get the thrust (pun intended) of the article. Don't recess more than a body tube. But on a BT-80, that's quite a bit. More than I would have.
On two stroke racing motors, they use an exhaust pipe that is tuned to take advantage of the shock waves coming from the engine. I believe they called it an extractor exhaust, but my memory may have failed me. I might see if I can come up with an image for you.
 
Open Rocket doesn't know about the Krushnic effect. Let's assume that recessing the motor less than one body tube diameter has no negative effect.
I did a couple rough drafts. Identical BT-80 rockets except the motor is recessed in one and both have had their body tube lengths adjusted to get the same stability.
In rocket one, with the motor recessed 6 cm and a stability factor of 1.54, I get a projected apogee of 60.1 meters.
In rocket two, with the motor not recessed and a stability factor of 1.55 I get a projected apogee of 56.5 meters.Screenshot 2020-11-20 07.05.17.pngScreenshot 2020-11-20 07.12.41.png
 
I don't know, but I'd bet that the difference in apogee is due to something else about the recession of the engine. The altered moment of inertia and resulting flight dynamics perhaps. I see that the non-recessed engine does not stick out below the base of the rocket, so I expect it will make no difference in drag at all. But one would need more expertise than I have and/or CFD to be sure.
 
Can and has been done, although the modifications in the design required almost certainly negated any increase in performance. This is one of the only production model rockets designed for CHEap And Dirty (CHAD) staging I know of, but I am far from an expert.

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The link below takes you to JimZ plans site
https://www.spacemodeling.org/jimz/eirp_10.htm
and the Augie II:

https://www.spacemodeling.org/jimz/eirp_28.htm
The major difference between these 2 Augies's is they are examples of ducted propulsion.....there is no Krushnik effect involved.

Lindsay Audin, who in the mid 60's did additional work on the subject , after the Krushnik effect was discovered in 1958, pioneered these ducted rockets which overcame the krushnik effects.
 

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I used a C5 for the example, but I would actually use as much as a D16. Even with a C5, there is a noticeable difference in apogee. I guess it's all relative.
Yes,but... it’s still a big BT and a nose cone with a lot of frontal area.
And you know what’s a few percent base drag. Are you filling the spirals, using thin
sealed basswood or FG fins , super smooth paint job. etc. after all, it is the sum of the parts! Lol!

BT80 begs for 2 D cluster haha !!!
 
I think a motor that sticks out of the bottom of the airframe would improve the aerodynamics of the rocket. May cut down on the base drag a bit.
Hmmm. Your typical cardboard mount minimum diameter rocket with the motor sticking out is going to have some step off regardless of how FAR the motor sticks out ( if it sticks out at all), that “bump” I think would cost you a bit of drag.

All other things being equal (they rarely are, but we are theorizing) the further back the motor, the further back the CG. To compensate, you either need to add nose weight (which may or may not decrease performance, but with optimal fin size likely will) or add more fin surface to adjust CP. So to me, it seems if you can avoid Krushnic effect without some fancy ducting (which will likely cause MORE drag) recessing the motor 1/2 to 1 body diameter can HELP you go with smaller fins.

From a practical sport flying perspective, it’s a PITB to get a recessed motor out after the flight!
 
and the Augie II:

https://www.spacemodeling.org/jimz/eirp_28.htm
The major difference between these 2 Augies's is they are examples of ducted propulsion.....there is no Krushnik effect involved.

Lindsay Audin, who in the mid 60's did additional work on the subject , after the Krushnik effect was discovered in 1958, pioneered these ducted rockets which overcame the krushnik effects.
Excellent, I didn’t know there were two versions of this. Am I wrong or is are the main differences a larger outer tube and 4 fins on the II, vs smaller tube and 3 fins on 1?

This would be an excellent Rocket for horizontal spin recovery, with no outside fins and a larger outer body tube. I think you would HAVE to eject the nose cone to get the CG right for recovery, though.
 
Consensus seams to be that the recession it's self doesn't improve aerodynamics, but I still contend that the resultant improvement in stability allows changes to the design that do. Anytime you can reduce fin size or body length, it helps.
Unless you were going for an altitude record, practical considerations might outweigh the slight apogee increase.
As pointed out, there are other ways to improve apogee that might be better.
 
There is, in some cases, another reason to recess the engine slightly: the rocket looks better that way. As a matter opinion, the back end of the engine, and even more the engine hook, are just unsightly. My long stalled L2 build has a 4 inch tube and a 54 mm motor mount. I added a 4 inch to 75 mm tail cone that the engine retainer fits into, recessed about an inch. I'm sure it won't reduce drag is much as a tail cone going down to 54 mm with the motor sticking out, but it looks terrific.
 
e resultant improvement in stability

so exactly what improvements?
What changes in the natural frequency and dampening factor?
 
Bob Parks many moons ago did a study where it seemed to indicate that a slightly recessed motor , no more than 1 body diameter, with small slots or holes around the periphery actually increased altitude.

At 1 body diameter recessed, there is no Krushnik effect, but the way the plume interacts with the blunt base , the air holes seemed to act as an additive to the thrust or at least changed the aerodynamics at the base ........

a good R&D report would be to fly a model rocket with and without the slots or holes around the base of the body tube......use an altimeter, do say 10 flights each with and without and see if the results are outside the margin of error.

Now I'm specifically talking about BP model rocket motors, not LDRS composites....

Trip Barber also back in the day did a study where he attached nozzle extensions onto the rear of model rocket engines.....it resulted in much lower thrust.....

although everybody knows (or should know and will know now) than the Estes nozzles are bad at best..... although better than the simple cylindrical nozzle that Orv Carlisle used in his Original Rock-A-Chutes..... funny thing is I've only ever seen 1 Thrust-Time curve from one of these ( A-4.4 ?) and it looks just like Estes T-T curves...hmmmm

Jonathan Dunbar(RIP) also did a more recent investigation of the Krushnik effect .

SO the bottom line is if you recessed a model rocket motor more than 1 body diameter place some small air holes around the base and it should migitate the Krushnik effect somewhat. The holes should be on the same plane as the bottom of the engine itself
 
I have recessed my motor mounts / tubes, so that when the rocket is empty & sitting on the floor, it has a larger contact with the floor. (So it don't tip over)

I generally like my motors to be flush or slightly proud of the 'flame end' of the rocket. but not always practical
 
Reading this all with great interest. Experimented a long time ago with recessed motors and found a few of my old notes. You can end up with some interesting smoke trail generation with slightly recessed motors. My Gooney Vostok has the motor 15mm from the end of the 47mm main tube and consistently has a 'fat' smoke trail (ambient temp? air density?). Have to find the rest of my notes and try to get some video of the next flight.
 
Apogee's Peak of Flight #379 - December 2 2014 has a nice work up on recessing of motors into the body tube for purposes of gas dynamic stabilization of the rocket. The article also provides basis for sizing of the body tube vents needed to let sufficient mix air into the body tube to avoid the Krushnik effect. If the recessed cavity is not vented, there will be a substantial reduction in net thrust due to the low pressure zone around the exhaust plume recirculating back into the base of the rocket.

If the motor is recessed deep enough (about 4x the motor diameter), the gas circulation at the base of the rocket will stabilize the rocket to the point fins are not required (at least as long as the motor is burning...). I've flow little finless 18 mm rockets about 5 inches long with the mini Estes A10 motors. Little more than a BT, the motor, and a nose cone, with the motor recessed about 1.5 inches. These will zip straight up and perfectly stable to maybe 400 feet, then the moment the motor burns out, they spin around in a tight little tumbling ball suspended in mid air until the ejection charge goes off.

The Peak of Flight article includes examples of larger rockets, and various scale rockets, all flow without fins. Some of them are apparently self-stable upon lift off, no rod required.
 
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