Base Drag Stability?

[accooper]

I am designing a 4 inch stubby rocket. As it sits now it has .53 caliber stability. How do I take base drag into consideration before I add a lot of nose weight?

Andrew

 

[Tonimus]

Go out in the middle of nowhere and try it out. BrainSim tells me that .75 caliber stability would be enough... I wouldn't mind knowing this as well, as I'm building a 1/4 scale 4" Patriot. Little fins are not conducive to stable flight...

 

[DizWolf]

faster is better, so it'll depend on the motor used

 

[les]

Take a look at this

https://www.apogeerockets.com/education/downloads/Newsletter154.pdf

 

[Mightyrocketman]

https://www.apogeerockets.com/education/downloads/Newsletter325.pdf

Check this out

https://www.rocketryforum.com/archive/index.php/t-10450.html

 

[codysmith]

Thanks for the links!

 

[Mightyrocketman]

https://www.rocketryforum.com/archive/index.php/t-66538.html

How to configure in rocksim

 

[Ted Cochran]

Direct links to Bruce Levison's work:
https://www.apogeerockets.com/education/downloads/Newsletter154.pdf
https://www.apogeerockets.com/education/downloads/Newsletter158.pdf

 

[bill_s]

My own experiments say there is no such thing as base drag stabilization. The rocket in question had no fins and Open Rocket said it was highly stable, which it was not, and OR doesn't even factor in base drag, which would have made it much more stable as the base was large. On the other hand, the rocket was not short and stubby.

I haven't seen any claims that a short and stubby rocket that would otherwise be unstable becomes stable due to base drag. What is claimed is that a rocket 2-3 times as fat compared to its length is plenty stable even when the stability margin is maybe 1/2 normal, in units of its own diameter. What this says to me is that diameter is not the sole determiner of needed stability margin. Length is also a factor.

Edit add: messing around in Open Rocket, I found short rockets more stable at angles of attack. One was .415 cal stable and would not go below .015 at any angle. Another design had 2.55 cal in normal long config. decreasing to -.223 at 59 degrees AoA. Reducing body tube to minimum and increasing diameter by 1" (to 3.6") reduced stability to .6 cal but again I couldn't find an angle where it went negative. One possible reason for the difference is that long rockets tend to have their CP and CG a larger portion of their length aft of midpoint.

There are cases where drag and shape need be considered regarding stability, spools and saucers for example, but if the above standard methods show stability, there is no need for more complex ones, which may be misleading.

 

[ksaves2]

Once saw a triangular rocket made out of a USPS shipping tube disintegrate on a K motor. Nothing but a puff of confetti/cardboard and hearty laughs all around.
A minute or so later, a tube connected to a parachute is coming in ever so nicely. The triangular plywood bulkheads were still there. So it went from a rocket form
to a flying spool and flew arrow straight. It turned to paper powder while it was going very fast so that likely helped with it staying stable along with the intact bulkheads. A long 10 foot launch rail can sometimes help too. Kurt

 

[boomtube-mk2]

So we have a short stubby rocket, such as the LOC Precision “Mini-Magg” that relies on “Base drag” for much of its stability; or so conventional wisdom leads us to believe.

Now adding a tail-cone is said to reduce base drag.

So if you were to add a tail-cone to a short stubby rocket (“Mini-Magg&#8221 could that potentially destabilized said rocket?

 

[SCrocketfan]

So we have a short stubby rocket, such as the LOC Precision “Mini-Magg” that relies on “Base drag” for much of its stability; or so conventional wisdom leads us to believe.

Now adding a tail-cone is said to reduce base drag.

So if you were to add a tail-cone to a short stubby rocket (“Mini-Magg&#8221 could that potentially destabilized said rocket?

Potentially. The Magg does need some weight to keep the CP far forward, or it tends to fly like this:

 

[bill_s]

Adding a tail cone always reduces stability, not just on short and stubby rockets. It is the tail cone itself, not just the reduction of base drag.

 

[G_T]

One is typically looking for that extra bit of margin in stability at launch, when the motor at the back end still has its load of propellant. Now high performance rockets and sounding rockets may have the propellant mass close to centered around the CG so this isn't universal...

When the motor is thrusting, the area one can use for base drag is essentially decreased by the diameter of the high speed gas plume of the motor. Take a look at drag during thrust and coast in RasAero for instance. It makes a difference.

In essence, if one is looking for base drag to save the stability margin under boost, don't expect much help...

Gerald

 

[T-Rex]

Dang I sure hope the nay sayers are wrong, otherwise this rocket is gonna do cartwheels....

View attachment Fat Daddy.ork

 

[bill_s]

^^-- what nay sayers? Checking that file, it has .52 cal stability or better at any angle of attack. That makes it one of the most stable designs out there. The .82 static stability doesn't sound high but again, that's in units of its own diameter, and it is large diameter and short length. No base drag simulation or guesswork required.

The bad part is it weighs nearly 10 pounds and has nearly 1 pound of nose weight attached to a length of all thread in the nose. Therefore it is not a rocket to experiment with! However, comparing to the Big Daddy rocksim file on the 'net, that shows with a D12 in it, .323 cals and critical stability reached at 35 degrees AoA (apparently the old version, as the motor spacer and stability does not allow E motors). I cannot verify that but if true, the experiments have been done. You can also fly first with a lighter motor and look for weathercocking into the wind, indicating overstability.