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Just never heard of base drag.
And since your an engineering student, who hasn't ever heard of base drag. That makes it pixie dust... Wow... Your a bright one. When someone mentions something you haven't heard about. How about turning on a computer and asking Google what it is.

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Andrew,
Base drag is something even professional aerodynamicists haven’t been able to fully define yet. Most of us believe that it’s a low pressure region aft of large diameter square tailed shapes which has the equivalent effect of shifting center of pressure rearward as velocity increases, up to a point. Because short fat rockets in atmosphere are almost never able to fly supersonically, the shift forward of the Center of Pressure at supersonic speeds has no relevance in their discussion.
But more simply, as David mentioned anytime the center of mass is ahead of the center of pressure there’s a restorative force that tries to make the rocket face directly into any airstream. Because Center of Pressure is only an estimated location the rule of thumb is to exceed 1 caliber stability margin.


Steve Shannon
 
Sim ninjas always dismiss real world experience. Anyone who has been flying for a bit knows that short fat rockets have base drag. Flying cones, spools and all other odrocs rely on base drag. OP, if you built your kit as directed and did not go nuts with glue in the wrong place, the thing will fly within the motor range recommended. Add larger motors as you see how it flies in various conditions and don't worry about over stability in extreme crosswinds. First of all, any ground wind is a cross wind to a rocket. Secondly, you won't be flying in extreme anything if you fly with a club.


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Boatgeek, you outlined the steps I was considering. Thanks for letting me know the ropes. Re 6 cals, maybe that's the answer to David's "I don't want to dissuade you about the BH, but..." comment. Small fins, 5 cal overstable, could that maybe work out?

I know about angle of attack from gliders and GA aircraft. Here's it's like the vector dot product of the rocket axis and the direction of travel. More fun rocket science to learn!

There's been enough crosstalk on the thread that I lost which BH you're talking about. If it's the Wildman Blackhawk, then you should be fine with lots of stability. Wildman has the motto "If it fits, it flies" which works pretty well as long as you're not using lead dust as a sanding filler for your fin fillets. (Please don't do that)

Vector dot products are a thing I left behind a long time ago, but you are definitely on the right track. I think of it as an apparent wind thing, where there's a difference between the vector of the rocket's direction of travel as it leaves the rod/rail and the apparent wind, which is the sum of that vector and the wind speed. As long as that angular difference is acceptable, everyone's happy. I think the rule of thumb is rocket speed 5x wind speed, but that can be different for different rockets. Experience with individual rockets counts for a lot.

And since you were asking about motor choices, a sparky anything is a wonderfully stylish way to get a cert.
 
Base drag. I wondered how those curled-up foam pool noodles flew! I'm not an aero student - or maybe I'm a student of any subject - but I love learning things here.

From cambridgerocket, a fabulous PDF: The aerodynamic forces are complex phenomena and there are no simple analytical solutions for the coefficients. In the incompressible flow regime the forces can be divided into pressure force and viscous force. Pressure force arises through the stagnation of fluid on the rocket forebody, fins, and any other protrusions, and also through a suction force created by a low pressure region at the base of the rocket where boundary layer separation occurs. Viscous force is due to skin friction between the rocket and the air. ... The base drag on the rocket is the drag due to the low pressure region at the base of the rocket that is caused by boundary layer separation. This drag is estimated using equation (42):

BaseDrag1.png
db = max body diameter
dd = base diameter
Cd(fb) = Cd of rocket forebody


From UiO in Norway:

BaseDrag4.png



BaseDrag2.png

UiO also had this super shock wave photo!

ShockWave.jpg

From the Flight Dynamics Laboratory, Wright-Patterson AFB, July 1984:

BaseDrag5.png

The last formula is based primarily on aircraft, but I thought it was interesting to compare the two equations. I couldn't find a definition of Sref, but S in this document is surface area. Clearly a noodle rocket's Sbase must approach Sref, explaining its high base drag and stability.

Andrew: one day you will be applying for a job. Maybe it's time to learn that HR departments these days are very good a combing the net for posts and breaking through anonymity using outsourced big-data correlations. What company wants to hire someone who's posted pix of themselves snorting coke? Every year the search algos get better, so it's only going to get more transparent. You're leaving embarrassing breadcrumbs, unless you're planning a career in bowling ball design.

So imagine yourself applying at Lockheed or Boeing someday, then after a good interview HR calls and asks "Are you Andrew_ASC?" Whatcha gonna say about your posts? That you were too lazy or arrogant to search the net? That you thought you knew everything in your early 20's? For goodness sake, in less than an hour I learned how noodle rockets have stability and why boat tails are so good (which I use for benchrest target shooting)... and it's all about base drag, not pixie dust.
 
Here’s a fun wrinkle on base drag as it relates to high altitude attempts...

If the drag is caused by a low pressure zone behind the rocket, how much of that drag can be reduced by having a smoke grain burning after motor burnout?
 
Here’s a fun wrinkle on base drag as it relates to high altitude attempts...

If the drag is caused by a low pressure zone behind the rocket, how much of that drag can be reduced by having a smoke grain burning after motor burnout?
Now we start talking base drag vs. Base bleed systems...the rabbit hole keeps getting deeper.
 
Because Center of Pressure is only an estimated location the rule of thumb is to exceed 1 caliber stability margin.

Steve, thank you for jumping in. I had been wondering about the calculation of CP and how exact our programs could be in computing it. Thinking about NASA and human lives at stake, I supposed that they must have done some wind tunnel testing to (first) come up with analytical means, then (later) verify the accuracy of their calculations.

And, sure enough, there is a NASA publication on it. While I had learned on TRF that CP moves forward >Mach, I just learned that CP can also move based on angle of attack. There's also a cute You Tube video in a wind tunnel. It's a car, not a rocket, but it shows CG/CP weathervaning quite well.

[video=youtube;WLm4wkTBbuc]https://www.youtube.com/watch?v=WLm4wkTBbuc[/video]​

Colleges only exist to put structure around learning and issue diplomas. These days, all the information you could ever absorb is available by asking. Learning is a choice (that incidentally helps grow more neurons - so we can learn more).

I hope we can shake hands at LDRS.
 
OMG, gentlemen, as usual... the more we learn, the more we realize there is much more to learn. It's very pleasant to know that what we are doing is real rocket science with small rockets. "Model" understates the effort.

I'm gonna go build a research rocket for a very sparkly motor.
 
Thee you go! Research with a skid. Skids are the redneck wonderkind of the rocket world. Everyone loves them but few admit it in public! Now cluster a few together in a short fat rocket and get some! LOVE ME SOME SKIDS. LOVE ME SOME BASE DRAG PIXIE DUST. Now that I think about it, that would be a great name for my next build....Base Drag Pixie Dust.
 
Sounds reasonable. My parents bought a station wagon in the early 70's that had a "spoiler" on the roof to direct air down the back and it really did keep the back window clean(er).

I'm "All about that base, no trouble."
 
I just had this funny thought about base drag while assembling the MM. Given this equation:

BaseDrag1.png

...we can reduce base drag in the coast phase by "extending the tail cone". I'm visualizing an iris diaphragm as an example, but something conical that a servo could extend when a G meter detects MECO. Triangular "teeth" hidden within a typical tail cone, retracted until coasting begins, then extended until they mesh with each other. If they converged to zero diameter, according to the equation we'd have zero base drag (in a perfect world). Remember Dd is base diameter, Db is Diameter of Body.

It should work and maybe help set altitude records. The longer the coast time, the greater the benefit. If someone likes this idea and tries it, please let me know. I won't be in that build space for a while.
 
You sure about *any* 29mm L1 motor? I bet an H399 White Thunder might give it some trouble... It would be quite the show though!

I strongly recommend an H90 to certify on, and for your second flight.. I highly recommend the CTI H410..

You'll fall in love in under a second.. just saying...
 
Tommy, check out the G's for various motors. In this rocket, the H90 is very strong, over 16G's on the cardboard tube. The H410 is a bone crusher at almost 59G's. Maybe someday when I want to make a hole in the air, but now the challenge is smooth application of long-duration thrust. This is not the BH38.

I like the shower of sparks a lot. The CTI H123-SK is on the list.

Thust1.png
Thrust1.png
 
LUNAR members - is it OK to launch the Smoke with an H motor on a 1/4" rod? If I need rail guides, now is the time to decide.
 
I strongly recommend an H90 to certify on, and for your second flight.. I highly recommend the CTI H410..

You'll fall in love in under a second.. just saying...
Funny, that... Guess what I did last Sunday!0320181744.jpg

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LUNAR members - is it OK to launch the Smoke with an H motor on a 1/4" rod? If I need rail guides, now is the time to decide.
Yes, the rails on the high power pads are easily swapped with 1/4" rods. I did it for my Mammoth at the last launch. If you need some 1010 buttons, I always have a stash in my launch kit.
PS- SARG launch this weekend was just cancelled due to the Tough Mudder race at the Ranch the same time. :(
 
While I had learned on TRF that CP moves forward >Mach, I just learned that CP can also move based on angle of attack. .

Yes. Everybody wants to be a rocket scientist and thinks their model rocket is gonna have some cool Mach effects on CP, but that doesn't happen much unless you are pushing Mach 3 and above. Most of us pedestrian model rocketeers should be more concerned with cross winds increasing angle of attack, especially at the moment of launch rod clearance. Long, skinny rockets are susceptible to this and can easily have negative stability and wiggles off the rail.
 
Here’s a fun wrinkle on base drag as it relates to high altitude attempts...

If the drag is caused by a low pressure zone behind the rocket, how much of that drag can be reduced by having a smoke grain burning after motor burnout?

Probably very minor. The thrust on/thrust off difference is already pretty small in the simulations.

Capture.PNG
 
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