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Motor exhaust "suck-back"

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vjp

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I've noticed in a few of my rockets, a tendency for the hot exhaust gases to char the bottom of the rocket, sometimes severely.

The rockets which have this problem share some common design characteristics:

1. Wide, flat base (larger than the motor diameter), with no boattail. Baby Bertha, Fat Boy, etc.

2. Motor fits flush with the bottom of the rocket, or sticks out very little.

I think what is happening is that a vacuum is being created in the wake of the rocket, and the hot motor exhaust is accumulating in that wake, even being sucked back into the tail section of the rocket (the gap between the MMT and BT). In a Baby Bertha I just built, with a "D" motor it got so hot the innermost layer of the BT actually began to peel.

Anyone ever do any empirical research on this? I'm building a 5x cluster Saturn V with engine bells in place for flight, and I'm wondering how much the motors should protrude below the bells to prevent plastic meltdown.
 

hokkyokusei

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What sort of blast deflector do you use? I used to suffer from this problem when I used a standard flat blast deflector. Fixed it by using a blast deflector with an inverted 'V' shape. You can make one cheaply by cutting a line down a can and then folding it back upon itself. Use a steel rather than an aluminium can.
 

Micromeister

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Vince:
Your flat bottomed models are suffering from Bernoulli Lock when placed to close to the flat blast defectors commonly used on most of our club equipment. To avoid this porblem use the cloths pins provided to keep the model bottom about 1 body dia. ABOVE the blast deflector. Don Brown flys a 1/!44th Saturn-V PMC on 5 C6 motors without damage this way. OBTW it's usually a good idea to have a least 1/4" of motor extending beyond the motor bell for ease of removal, but flush with the end of the bells will not result in melting if the model space off the deflector as detailed above.
 

wwattles

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Originally posted by Micromister
To avoid this porblem use the cloths pins provided to keep the model bottom about 1 body dia. ABOVE the blast deflector.
In a pinch, you can also use expended SU casings. Just core it out and slide it down the launch rod. It'll provide a good standoff from the deflector. Another good use for spent casings!

WW
 

vjp

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I should clarify----

I'm not having problems getting stuck to the pad - and the flights I'm referring to are being supported well above the deflector plate.

What I'm referring to, is the rear end of the rocket getting burned by a buildup of hot exhaust gasses, probably as a result of the partial vacuum created in the wake of the rocket at high speed. So to eliminate this from happening on my next Saturn V, I intend to have the ends of the motors "stick out" to reduce this phenomenon.

Most rockets don't have this problem, the only ones I've noticed it on are when the motor is flush, or nearly flush, with the base of the rocket - and the base of the rocket is wider than the motor itself. So to prevent this from melting my engine bells, I think the motors will have to stick out a bit - I think 10mm (about 0.4") ought to do it (crossing fingers).

I guess what I'm wondering, is - if anyone has ever done a R&D project, or written a technical bulletin researching this phenomenon.
 

rstaff3

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I believe your general explanation is accurate. The wider the base, the more likely this is to occur. This is easily seen with saucer, pyramid, and similar designs. I have seen several articles on this, but the one link I have is dead. I'm not sure the explanations were substantiated with any research per-se, but rather derived from a general understanding of aerodynamics. I don't remember any discussion about having the motors extend past a flat base. However this makes sense since having them inset into the base of a rocket makes the vacuum effect more pronounced. I wonder what Stine has to say about this? If I find a link I'll post it.

BTW, I think I have the NARHAMS record for Bernoulli lock :) You orf micro remember my Archer BBQ?
 

Ryan S.

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I think this could be the beggining of the Krushnik effect.
 

bobkrech

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vjp

What you are observing is the result of the recirculation and mixing of the hot exhaust gases with air filling in the base vacuum. This recirculation heats up the underside of the rocket, and has nothing to do with "getting stuck" on the launch pad.

It is particularly bad in wide flat bottommed rockets has have a lot of drag and thus a large amount of exhaust gas recirculation, and is virtually nonexistant on a boat-tailed rocket.

Basically the rocket engine is a source of hot pressurized gas with a great deal of energy and momentum flowing away from the rocket nozzzle. The gas expands outward rapidly and slows down as it moves away from the nozzle. The motion of the rocket through the air creates a lower than atmospheric pressure under the base of the rocket which pulls in air behind the rocket. Some of the hot exhaust gases get caught up in this flow and actually move forward and go under the base of the rocket, heating and possibly charring the underside of the rocket.

This base-heating effect is observed in rockets of all sizes, and methods to duct air into the base area of the rocket to prevent exhaust recirculation are incorporated into big rockets.

Bob Krech
 

shockwaveriderz

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this isn't the Krushnic effect, its a low pressure area at the base of the rocket which results in the sucking of the exhaust gases forward..... boattails are used to reduce and alleviate this low pressure sucking area at the base of rockets...

soak the last 1/4" or so of the bigger body tube and then sand it down into a angled shape like a boattail ...it may help or it may not....

The Krushnic effect is when you have a motor up inside a body tube instead of outside a body tube.....and the actual thrust of the rocket motor is reduced by it being enclosed and not out in the open....

everything you every wanted to know about the Krushnic Effect:

http://www.nar.org/pdf/TCR1.pdf
 

powderburner

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In the sense that these problems are all various forms of gas recirculation, I believe that the so-called Krushnik effect is a more severe form of the problem that vjp is having.

With a wide base and a more-or-less flush-fitted nozzle, you can easily get recirculation of part of the exhaust gas. bobkrech gives you a pretty good top-level explanation.

With a wide base and an inset nozzle, the recirculation gobbles up more of the energy of the motor's exhaust, trapping more of the high temperature gas and roasting the rear of the rocket more severely.

With the Krushnik effect (motor nozzle inset by one body diameter or greater), almost all of the energy of the motor's exhaust is trapped and wasted in this swirling flow.

You can effectively defeat this recirculation effect by building the motor mount with a solid disk (to direct the ejection gas forward) at the mount's front end only-----build the rear motor mount supports out of struts. Put a few perforations or small scoops in the outer BT between these motor mount supports so that external airflow is admitted to the rocket 'base' area, and the motor exhaust will function more normally.
 

shrox

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If the end of the rocket is a thin disk, you could drill a small hole at an angle thru the body tube about 3/4" to 1" from the end, Drill another hole thru the end disk about 1/2" from the edge aligned with the side hole, then slip a launch lug thru the holes and glue in place. The tube should allow a small bit of air thru into the low pressure area. I would place one on opposite sides.
Mind you I haven't tried this, but it should work from what you describe.



shrox
 

powderburner

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This is purely intuitive, with absolutely no analysis or hard data to back it up.
But I think you are going to need to introduce quite a bit more bypass air, something more like 25 to 50 percent of the rocket's base area, before you will put a dent in the losses caused by recirculation.
I don't think a launch-lug-sized auxiliary flow path would have a noticeable effect.
 

shockwaveriderz

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I think both powderburner and shrox have both hit upon the correct solution to this problem....place some small diameter holes of slots near the base of the outer large body tube to alleivate the low-high pressure..... see you learn something everyday in TRF........

Multi holes/slots/slits around the circumference should suffice..

This would make a n excellent R&D report for NARAM 46 for B division modlers who might want to win $1000 USD....

Build a model with a 13mm inner and say a bt60 outer.....use the same engines from flight to flight......have an aft bt 60 body tube skirt with a coupler such that you could tape it on the inside to the upper body tube.... this would allow the model to be the same from test to test and you change out the skirts....vary the number and size of the holes.....photograph the results.......

you could repeat the experiments with 18mm-bt60 too.....to see if you get different results
 

shrox

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powderburner said "25 to 50 percent of the rocket's base area, before you will put a dent in the losses caused by recirculation.
I don't think a launch-lug-sized auxiliary flow path would have a noticeable effect."

I was thinking a thin cusion of moving air might be created, at least enough to keep the scorching away.

shrox
 

wwattles

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Just guessing here, but wouldn't an external scoop help to direct more airflow into the ducts? Yes, it would disrupt airflow outside, but the effect would 1) pull the CP aft (good!) and 2) increase the airflow to reduce the exhaust gas swirling (good!).

I had an idea like this a while back to ram air past a 4x24mm cluster inside a BT-80. Might try it on my FrankenPhoenix.:rolleyes:

WW
 

powderburner

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Originally posted by wwattles
Just guessing here, but wouldn't an external scoop help to direct more airflow into the ducts?
I think this is the way to proceed, as noted in the previous post:
"Put a few perforations or small scoops in the outer BT between these motor mount supports so that external airflow is admitted to the rocket 'base' area, and the motor exhaust will function more normally."
 

powderburner

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Originally posted by shrox:
I was thinking a thin cusion of moving air might be created, at least enough to keep the scorching away.
Perhaps, but I am thinking that what you really need is a substantial blast of ventilation air to break up the recirculation. Then the exhaust will go where it is supposed to go (out the rear) and the base structure will no longer need thermal protection?
 

rstaff3

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I don't know how much flow would be required, that would be an interesting research subject. I do know people have successfully defeated the Krushnik effect by ducting.

I think vjp was looking for a simpler solution for your run-of-the-mill rocket.
 

shockwaveriderz

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I will make an educated guess that you would find a semi-optimal number and placement of ventilation holes depending on egine and body tube inner and outer diameters......
 

vjp

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Thanks all for the discussion & enlightening info.

The vent hole idea is interesting, I am going to try that on the D/E powered Baby Bertha to see if it helps. The first flight really charred the inside of the BT, I doubt it would last many more flights like that. Normally, I would have let the motor protrude about 1/2" from the bottom, but I was trying to keep the CG forward to eliminate the need for nose weight.

I can't really put vent holes in the Saturn V I'm building, so I'm just going to let the motors protrude from the scale nozzes about 10mm or so.
 

bobkrech

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vjp

The real Saturn C-1 and C-5 had airscoups to reduce the base heating.

Bob Krech
 

powderburner

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Bob,
That is interesting to me, I never heard that the real Saturn had that feature. Where were the inlets, and where was the air delivered to the aft end? Any further info, or drawings, or pix?
 

rocketkid88

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i just read this thread... this is interesting, i'm working on a FB with the stock 18mm and two 13mm mounts. instead of having the engine stick out like it says to in the instructions i put them all flush with the BT... since its mostly done i think i'll fly it stock once (no outboards) and i'll see what happens... if i need to i'll try putting small scoups on it and re-inforcing the rear CR with balsa struts and cutting out some 'vents'... i'll be sure to yet you guys know what happens!
 

vjp

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Originally posted by rocketkid88
i just read this thread... this is interesting, i'm working on a FB with the stock 18mm and two 13mm mounts. instead of having the engine stick out like it says to in the instructions i put them all flush with the BT... since its mostly done i think i'll fly it stock once (no outboards) and i'll see what happens... if i need to i'll try putting small scoups on it and re-inforcing the rear CR with balsa struts and cutting out some 'vents'... i'll be sure to yet you guys know what happens!
I have flown a 4x18mm clustered Fat Boy, with the motors sticking out about 1/4" from the base of the main BT, and have gotten some mild charring. I coated the whole rear end of the rocket (rear CR, and the parts of the BT that protrude) with epoxy which has helped resist it.

With one 18mm and two 13mm, it probably won't be as bad.
 

rocketkid88

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Posibly, but i moved the mottors forward so they are even with the end of the BT. I did this so that the CG would be as far forward as posible, so i don't (hopfuelly) have to add any nose weight, for a single 18mm flight any way. i'll see how it goes b4 i modify it. Thanx for the advice:cool: !
 

powderburner

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Before we all go making vent holes, scoops, inlets, ducts, and the like, and a bunch of tunnels through the insides of our rockets to connect them, let's not lose sight of some other options.

Try sticking the motor out the back like God intended. Spend an ounce on nose ballast.

Try substituting an aft CR made of something more 'roast-resistant' like fiberglass (maybe some of that G10 stuff that some guys make fins out of), or using some high-temp paints.

Try covering the exposed rear face with aluminum foil, or aluminum tape, to shield against the heat.

My real point is this: making air passages through the inside of a rocket is more tricky than it looks, at least to deliver a quantity of air with minimal pressure/velocity losses, and with the least possible drag increment on the overall vehicle. For example, a constant-diameter tube from 'up there' to 'back here' will cause choking of the flow within that tube, spillage drag at the inlet when the tube flow backs up, and complicates the heck out of how you make the rest of the rocket work.

Keep it simple; simple to understand, simple to build, simple to operate.
 

wwattles

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I tend to agree with Powderburner on this one... the old KISS principle (Keep it simple, Stupid!). I was considering the ducting only because the Phoenix has got such a deep exhaust nozzle on the stock build. I used foil tape and that worked pretty well. The high-temp paint should also do the trick nicely, and if necessary, can be reapplied between launch days!

WW
 

rocketkid88

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so far as protecting the rkt that is fine... but i am wondering more about the resulted thrust eficency loss... in a relativly marginal rocket that can barely lift itself and maintain sufficent speed to be stable, wouldn't this effect cause it to slow down and posibly become unstable? i know that trying the air duct method would also incur allot of drag; which would be worse and slow it down more i duno. I've heard that sometimes FBs don't fly that well, and since mine'll be a little heavier than stock and the motor is flush with the BT when i fly a single mottor in it i don't want it to go unstable.
any thoughts?
 

DynaSoar

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Originally posted by shrox
If the end of the rocket is a thin disk, you could drill a small hole at an angle thru the body tube about 3/4" to 1" from the end, Drill another hole thru the end disk about 1/2" from the edge aligned with the side hole, then slip a launch lug thru the holes and glue in place. The tube should allow a small bit of air thru into the low pressure area. I would place one on opposite sides.
Mind you I haven't tried this, but it should work from what you describe.



shrox
I'd be wary of this particular arrangement without testing it. It could backfire. The opening in the side of the body is larger than the one on the bottom. The airflow past the opening on the side is going to create a vacuum. The differences in the amount of vacuum on the side and at the bottom, modulated by the different size of the holes, could result in the opposite effect than intended. If it does, it's going to suck hot exhaust up into the tube. The possible results aren't pretty. To work properly, the intake should be ducted; that is, it should force the air in and not let it slide by.

"Spoiler" ducts near the base of the body, that grab airflow and toss it under the base, should accomplish the same thing without going through the engine mount. For that matter, if the fins hang below the base, small vanes near the bottom of the fin root which grab some of the flow along the body and drive it under the base might be easier to impliment. See attached.
 
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