# Effect of Base Drag On Stability

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#### GrossApproximator

##### Well-Known Member
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?

##### Well-Known Member
Have you tried the cardboard cutout method?

#### MarkII

##### Well-Known Member
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.

#### MarkII

##### Well-Known Member
Have you tried the cardboard cutout method?
Useless for calculating base drag.

Mark K.

#### rstaff3

##### Oddroc-eteer
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

##### Well-Known Member
Useless for calculating base drag.

Mark K.
As far as I know this is the only *static* way of testing, have you built a pyrimid model before?

#### GrossApproximator

##### Well-Known Member
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.

#### GrossApproximator

##### Well-Known Member
Bruce Levinson wrote a series areticles for the Apogee newsletter. Here are some references:
Cool

##### Well-Known Member
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

##### Well-Known Member
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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##### Well-Known Member
Was asking gross apProximator mark sorry I shoudve specifyed

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#### GrossApproximator

:roll: Sorry.

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

#### MarkII

##### Well-Known Member
: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

##### Well-Known Member
But mark seriously, everyone respects your knowlage, I wouldn't ever (again ) 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

##### Well-Known Member
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.

##### Well-Known Member
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

##### Well-Known Member
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

##### Micro Craftman/ClusterNut
TRF Supporter
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

##### Well-Known Member

Try my 13 mm Freebie

#### Ironnerd88

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 ##### Well-Known Member 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

##### Well-Known Member
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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