Overstable rocket and how to fix it

I am thinking about trying dual deploy for the first time and will be buying an aerotech mirage. I will also buying the madcow ebay for this rocket. Now when I sim the rocket in rocsim it says a margin of 7.06 with an I200w. Now how in the heck do I bring that margin down? It seems to me I would have to add weight to the rear of the rocket. I always read the people are putting weight in the nose cone but that wont work as it seems that it would make the margin bigger not smaller.

thanks for your help
Highflyer1968

Use a different rocket. Why choose a rocket that is does not meet your design goals?

Or cut the body tube shorter. The long body tube will cause the extra weight of the dual deployment gear to be way out in front the fins leading to the big over stable margin.

Or you could cut the tube just forward of the fins and put the dual deployment there, which keeps the weight closer to the rear which will help the stability move rearward.


Tony

Apply a several-wrap ring of blue tape just below your nosecone / supported from the inside by your shoulder. Massive increase in nose drag for a minor, reversible mass cost.

highflyer1968 said:

It seems to me I would have to add weight to the rear of the rocket.

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Add bigger motors!

If you have a long rail (or a still day) you can put up with some over-stability, as long as it is guided in the right direction for long enough.

I fly rockets built to use hybrid motors and they all employ dual deployment. This results in a long body. Typically, these rockets have stability margins of 6 - 10 cal, depending on which hybrid motor I’m using at the time. In spite of the over-stability, I have never had an issue with significant wind cocking.

There are a few reasons why I don’t have issues with the over-stability with these rockets:
1. The thrust/weight ratio is generally high (no less than 10:1)
2. The rail is long enough (at least 3 m) to ensure a respectable launch velocity (at least 20-25 m/s)
3. Low ground level, mid and upper winds. This requires patience and planning.

Additionally, the Mirage has a L/D ratio of 33(!)

If I was flying it, I'd try to keep the CG 9-10 inches above the CP

Nytrunner said:

Additionally, the Mirage has a L/D ratio of 33(!)

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Cripes! There's yer problem!

As Highflyer1968 said, if you must use the Mirage then cut it short, or better yet build and fly it stock and add DD to something else. Tony's suggestion to cut the tube and install the e-bay and laundry lower should work too.

Still, the best choice would be just to start with something with a "normal" L/D. There are a good number of LOC 2.26" and 3.1" models that would probably do nicely; Forte, Iris, Lil Nuke, and (at 4") EZI-65 for example. (I'm not trying to push LOC; a lot of folks here like them and they were quick examples to find.)

There's a thread somewhere in this forum that explains why overstability is not as big a problem as is sometimes feared. I've flown rockets with 7 cal stability without issue. Just don't fly an overstable rocket on a windy day, and use a motor with high thrust. Wind + low thrust + overstability = likely weathercocking.

My main point in my previous post was to suggest that you don't need to get too hung up on your rocket being over-stable. As an example, my hybrid rocket in the image below has a stability margin of nearly 11 cal at launch on a 36" Contrail J246 motor. Additionally, I relaxed the T/W to about 8.5:1, because it simmed to leave the 3 m rail at over 30 m/s, and into an almost windless sky. I was able to retrieve it after a flight to about 4,500' with a walk of about 250 m.

I've had a Mirage for about 18 years now, built stock, launches off a 6 foot 1/4" rod, never had more than a G motor in it because built stock, it has flown great every launch. On an I200 I'd worry more about folding it in half than stability issues!

Just as I said for extremely short rockets, calibers are a useful unit of stability in the field but not a good gauge of stability margin. The actual needed stability margin is more related to the length of the rocket than its diameter, which is often irrelevant. Think about it, if you increased diameter only and the CP and CG stayed put, the margin in calibers would be a lower number, but would it be any less stable? Barely.

Part of simulation should always include angle of attack analysis. In Open Rocket, Tools>Component Analysis> vary angle of attack. I've flown rockets that showed loss of stability at barely over 10 degrees, but while this will probably be higher you'll be surprised how fast stability degrades. I'd also trust OR to simulate weathercocking.

You can also see how much the actual rocket weathercocks. Really long rockets with a marginal margin can anti-weathercock, head downwind at high angles of attack in technical instability predictably becoming stable as their speed increases. I believe it is also advantageous to have the masses concentrated towards the ends of the rocket and have a low mass to windage ratio, so the rocket rapidly accelerates sideways to match the wind.

I have a Mirage - photo below. It was a gift from my Uncle Gus

I recommend "The handbook of model rocketry (seventh edition)" by Stine -It's the defacto standard reference manual for all things Hobby-Rocketry related.

You can find a good used copy online starting at $1.50 (Really!)

Understanding stability and how to adjust it is a really good research topic, and Stine covers stability extensively in the Handbook.

Heres a hint for you - The Mirage is really long... one way to reduce over stability is to shorten the rocket. (just remove one of the three body tubes)