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Effect of Base Drag On Stability

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GrossApproximator

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Someone (or maybe several people, I don't remember) told me that on wide cone rockets (and their multi-sided kin), the base drag is so enormous that it moves the Cp aft and even off of the actual airframe. Is there any way to predict this behavior (or at least get some kind of approximation) without using a wind tunnel?
 

MarkII

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Someone (or maybe several people, I don't remember) told me that on wide cone rockets (and their multi-sided kin), the base drag is so enormous that it moves the Cp aft and even off of the actual airframe. Is there any way to predict this behavior (or at least get some kind of approximation) without using a wind tunnel?
Build a prototype (scaled down if the design is large) and try flying it?

Mark K.
 

rstaff3

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Bruce Levinson wrote a series areticles for the Apogee newsletter. Here are some references:

Bruce Levison's has a series of articles in the Apogee Newletters on the simulation of short/fat rockets. Part 1, The Simulation of Short Wide Rockets using RockSim Version 8: An Additional Base Drag Consideration for Rockets With Less Than a 10:1 Length to Diameter Ratio, appears in Issue #154. In Newsletter #158, he extends his method to rockets with rear transitions (3FNC with a tail cone and even 'Sputnik'-style rockets). As a bonus, he also tells how to simulate tumble recovery rockets. The third installment is in Newsletter #162. In this one, he digs into pyramids, cones, saucers, and 'Borg cubes'. The upshot is that these designs are more stable than is prediced by static CP estimates such as those discussed in items 6 and 7, above.
OOPS! No linkies moved over so look at item #14 in this post
http://rocketdungeon.blogspot.com/2005/11/i-may-be-unstable-but-i-dont-like-my.html
 

GrossApproximator

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I've never built anything like this before. I imagine I'd just build a mock up and give it a swing test. I'm planning to attach a Frisbee or something to the back of a short piece of body tube. I guess my real problem is that I don't know how stable my rocket needs to be: the 1-caliber stability thing is just a rule of thumb and doesn't really apply to really wierd rockets like the one I want to make. Or at least the rule has to be applied differently.
 

ScrapDaddy

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I've never built anything like this before. I imagine I'd just build a mock up and give it a swing test. I'm planning to attach a Frisbee or something to the back of a short piece of body tube. I guess my real problem is that I don't know how stable my rocket needs to be: the 1-caliber stability thing is just a rule of thumb and doesn't really apply to really wierd rockets like the one I want to make. Or at least the rule has to be applied differently.
If you are new to this, then I dsuggest the sunward khufu's pyrimid, starting scratch is hard, expecaly with odditys like this
 

MarkII

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As far as I know this is the only *static* way of testing, have you built a pyrimid model before?
You have got to be kidding again, right? Pyramids, cubes, cones and saucers were practically the only types of rockets that I built during my first couple of years as a BAR. The total number of that type that I have built and flown is several times the number of rockets in your entire fleet. The cardboard cut-out method has some usefulness (and a whole lot of limitations) as a way to locate the center of pressure of typical 3FNC rockets, but balancing a cardboard profile of a pyramid or a saucer on the edge of a ruler will tell you absolutely nothing about the effect of base drag on the CP's location for those shapes. I think that wind tunnel testing might be the only way to determine or predict it for them. Flying a scale prototype of the rocket can be a form of wind tunnel testing of it.

Mark K.
 
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ScrapDaddy

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Was asking gross apProximator mark :D sorry I shoudve specifyed
 
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ScrapDaddy

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But mark seriously, everyone respects your knowlage, I wouldn't ever (again :D) question you about stuff like that..... I only know stuff I don't need to know
 

MarkII

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:roll: Sorry.
GA: assemble a smaller version of the design that you want to built and launch it a few times on either 13mm 1/2A3T or A3T motors. The flights should provide you with good information about the stability of your design. This is a classic aerodynamic testing method.

Mark K.
 

MarkII

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But mark seriously, everyone respects your knowlage, I wouldn't ever (again :D) question you about stuff like that..... I only know stuff I don't need to know
Well, since you had included a quote of my post, I inferred that you were addressing your entire response to me.

Mark K.
 

GrossApproximator

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GA: assemble a smaller version of the design that you want to built and launch it a few times on either 13mm 1/2A3T or A3T motors. The flights should provide you with good information about the stability of your design. This is a classic aerodynamic testing method.

Mark K.
Good idea. I'll try it with a mock up and swing test first, then goto micro motors.
 

ScrapDaddy

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Well, since you had included a quote of my post, I inferred that you were addressing your entire response to me.

Mark K.
I was only quoting that for my static* responce
 

MattieShoes

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I've never built anything like this before. I imagine I'd just build a mock up and give it a swing test. I'm planning to attach a Frisbee or something to the back of a short piece of body tube. I guess my real problem is that I don't know how stable my rocket needs to be: the 1-caliber stability thing is just a rule of thumb and doesn't really apply to really wierd rockets like the one I want to make. Or at least the rule has to be applied differently.
There's a kit along those lines... It'd give you a place to start anyway.

Reviews of it here and here
 

Micromeister

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Well, since you had included a quote of my post, I inferred that you were addressing your entire response to me.

Mark K.
Mark:
Remember what we talked about earlier.....source;)
 

artapplewhite

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Try my 13 mm Freebie

or my 18 mm pyramid kit, it's only $6.

I doubt very seriously that the swing test will work well on a pyramid. Accurately determining the CG and then attaching a string there will be a challenge to begin with. The swing test doesn't even work every time for a stable conventional rocket like the Estes Alpha. If you start it with the Alpha pointing backwards it is stable then too.
The swing test doesn't work properly if you start with a high angle of attack, i.e. the rocket not pointing froward. Some normally stable rockets will tumble or point backwards sometimes giving you a false fail result.

Art Applewhite
www.artapplewhite.com
 

Ironnerd88

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I build a lot of cones based upon the Centuri X-24. Mine, however have no fins. They kinda glide, and are great for smaller fields, plus they are easy and cheap, and make people go "huh?"

I have found through swing tests, and launches, that the CG can be well behind the Barrowman calculated CP (2/3 back from the nose). In fact, with the "proper" CP/CG relationship, the cones tend to weathercock into a light breeze.

The articles from APOGEE's E-mag definately support my real world results (which is always nice). I have yet to launch a cone with a full-aft CG, but I am currently working on the mock-up for just such a rocket. The plan is to launch on a 13mm A10 ballasted to the weight of a C6-7, since this is pretty much a worst case scenario (high weight, aft CG, low thrust). If it survives that test, then I know the cone is stable.

On a side note, this was a good article:
PAGE 4

x24bug.jpg
 

Micromeister

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I've used base drag in several other type coversions as well. it's a great way to get LEM's to fly:)

LEM-d3-sm_all 4 2pic complete(86dpi)_04-16-03..jpg
 

Solomoriah

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On a side note... I downscaled the X-24 to 13mm, and it just won't glide. I gave up, removed the clay and let it fall. It weighs almost nothing and has a ton of base drag, so it definitely qualifies as "featherweight."

I got a big pile of A10-3T's at Wal-Mart for $1.00 a pack. A quarter a flight, with nothing to rig but the igniter, makes for a lot of fun in the back yard.
 

Ironnerd88

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I got a big pile of A10-3T's at Wal-Mart for $1.00 a pack. A quarter a flight, with nothing to rig but the igniter, makes for a lot of fun in the back yard.
Hmmm... gonna have to hit Wal-mart...
 

Solomoriah

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Sorry... 2 years ago.

I'm watching one local Wal-Mart which is clearing rocket engines. Currently $5.00 per pack, but I'm waiting until they go lower.

If I miss them... no problem, really. I have 85 packs of engines, mostly Estes C6-x, mostly bought for $1.00 to $3.50 a pack. Probably more than half are $1.00 a pack.

Y'see, one day I walked into the Wal-Mart in Quincy, IL and saw 12 packs of A10-3T engines in clearance for $1.00 each. When I scanned a pack, the scanner said "Model Rocket Engines." Suspicious, I went to the toy section and grabbed some of the C6-x engines there... and they scanned as "Model Rocket Engines" for $1.00 a pack.

So I picked up the entire display and bought it. Something like 45 packs of engines.

Then, about 6 months later, I did the same thing in Hannibal, MO. Only about 15 packs that time, $1.00 each.

Unfortunately, in both cases they were all C6-x and A10-3T. The ones I'm watching now are about half C6-x, half A8/B4/B6. If they make it to $3.00 I'll clean out the lower impulse engines; for $1.00 a pack I'll buy them all.
 

Ironnerd88

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On a side note... I downscaled the X-24 to 13mm, and it just won't glide. I gave up, removed the clay and let it fall. It weighs almost nothing and has a ton of base drag, so it definitely qualifies as "featherweight."
Probably the same problem as the old GYROC - they are tricky to scale. Still a really cool idea.

ALTHOUGH... There is room inside and X-24 Bug for a Cluster of 13mm motors. That could be a hoot.

A bit of quick openrocket simulation and it looks as though this would be a stable configuration. (the butt-end of the motors would be in teh same place as the butt-end of the 18mm motor the kit was designed for).
 
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