Acceptable Stability

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dhkaiser

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The Handbook of Model Rocketry recommends a stability between 1 and 2. And yet here on TRF I have seen people flying everything from 1 to 9. I am preparing to launch my Madcow DX3 SD. In OpenRocket it comes out with a stability of .85. To reach a stability of 1 or more I would need to add weight to the nosecone.

Would you launch on a stability of .85? How about a stability of 9? What range of stability is acceptable to you?
 

burkefj

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It depends on wind and rail/rod exit velocity and stability margin, I tend to stick to 1 to 1.5 caliber of stability and ensure at least 4x to 5x rail exit velocity compared to crosswind velocity in fps.
You can run a higher thrust motor, longer rod/rail if you are marginal, or wait for less wind, or move the CG forward. AS mark says below it also depends on how fast you are going to fly, I tend to not get very close to mach in my flights.

Frank
 
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markkoelsch

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That is very marginal in my opinion. If this rocket goes fast the Cp moves forward and this could be problematic. Frankly, not sure if I would design a rocket much under a margin of 2.
 

Banzai88

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I have a RW Mouse, and in as delivered form CG and CP are coincident with any 1grain 29mm motor. 4 ounces of weight in the nose later, it's .85 calibers stable with any 1grain motor CTI makes. Flies just fine.
 

markkoelsch

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I have a RW Mouse, and in as delivered form CG and CP are coincident with any 1grain 29mm motor. 4 ounces of weight in the nose later, it's .85 calibers stable with any 1grain motor CTI makes. Flies just fine.
I believe the Mouse qualifies as short and stubby, which relies on base drag to add to the stability of the rocket.
 

Bat-mite

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Before reading below, keep in mind that simulations rarely have an accurate CG location, unless you maticulously put in the weight distribution of all your expoxy, machine screws, shear pins, etc. Most people don't do that.

What they give you is a pretty good starting CP. Assemble your rocket in ready-to-fly configuration, just as if you were on your way to the pad. Measure your actually CG by balancing the rocket on your hand or a sawhorse, etc. Use your calculated CP and measure CG to check the actual stability. If you still come up with less than one caliber, read below.

To further elaborate on Frank's post, the CG moves forward as the propellant burns away. So if you start with .85 calibers, you need to give yourself a long enough time on the rod/rail, so that by the time the rocket leaves the rod/rail, the CG has moved sufficiently forward to achieve a one cal. or better stability.

Less than one cal. means that gravity might not be able to overcome wind rotation, which is why the rocket will start to "sky write" (spin in the air). Greater than one cal. leads to weathercocking, which is unavoidable in many cases anyway, but should be minimized.

Supersonic rockets experience a sudden forward leap of the CP at Mach transition, and thus need to start with two or more calibers.

As stated above, short and stubby rockets are more stable by design, and thus don't need a full caliber of stability. And long, skinny rockets (think "Estes Mean Machine," "Fliskits Richter Recker") are inherently overstable and risk bending in the air.

For a DX3, add nose weight to get to one cal.
 

tfish

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Sorry about this post I was trying to reply to another stability thread and posted here by mistake. It's still kinda on the right subject matter.

I'll remove it if the Op wants...

I've see a bunch of record attempts out at BR and have seen many of them not fly as planned. These rockets are right on the "edge" and the smallest thing sets them off. So, for the past few years I’ve been building my rockets with fins that are slightly more then 1 caliber of span. “slightly” is a relative term, for a 3″ rocket I go about 3.375″ spans. I also have been making sure they have at least 2.5 calibers of stability. I’ve been calling my type of rockets ‘AeroPac Sport Flyers”. You get decent altitudes, and they track very well. At Aeronaut 2014 I had a motor mishap, where I lost..broke off ..one side of my exit cone. This happened about 3 seconds into a 5 second burn. Jeremy Nelson happened to catch the flight on video. It’s pretty interesting slow mo. Amazing how it was able to correct itself [video=youtube;y5wpiyQfMO0]https://www.youtube.com/watch?v=y5wpiyQfMO0[/video] . Still went a tad under 25K…same rocket same load without that issue went a tad under 30K.

Tony
 
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CORZERO

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.85? Sure, upon conditions.

9? Never been there. Maybe I'll try someday.

DX3 is a long, skinny, 3FNC. With minimal to [no] wind and a standard rail height I would fly at .85 with no concern even with a slow burn. Although they may not have been approved by an RSO at a sanctioned launch, I've flown a handful of sub 1cal flights independently with no issues, however many factors were considered (see above posts) before I pulled the trigger. It is as possible to successfully fly and maintain stability with a sub 1cal vehicle as it is for loopdy-loop flights to occur with marginally over-stable vehicles. Just consider the factors and do the math.
 

mpitfield

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Sorry about this post I was trying to reply to another stability thread and posted here by mistake. It's still kinda on the right subject matter.

I'll remove it if the Op wants...

I've see a bunch of record attempts out at BR and have seen many of them not fly as planned. These rockets are right on the "edge" and the smallest thing sets them off. So, for the past few years I’ve been building my rockets with fins that are slightly more then 1 caliber of span. “slightly” is a relative term, for a 3″ rocket I go about 3.375″ spans. I also have been making sure they have at least 2.5 calibers of stability. I’ve been calling my type of rockets ‘AeroPac Sport Flyers”. You get decent altitudes, and they track very well. At Aeronaut 2014 I had a motor mishap, where I lost..broke off ..one side of my exit cone. This happened about 3 seconds into a 5 second burn. Jeremy Nelson happened to catch the flight on video. It’s pretty interesting slow mo. Amazing how it was able to correct itself [video=youtube;y5wpiyQfMO0]https://www.youtube.com/watch?v=y5wpiyQfMO0[/video] . Still went a tad under 25K…same rocket same load without that issue went a tad under 30K.

Tony
Wow that was a pretty spectacular self correction, I would never of imagined that it would of held together and I think it speaks to your building skills.
 

Tonimus

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I unintentionally launched a Super DX3 with about .6 calibers of stability. I forgot to include the aeropack tailcone in my CP calculations... In virtually no wind, it managed to go up. Mostly.

With one caliber, it flies great. Still not something I would do in wind. One thing to keep in mind is that for the size of the body, the fins are on the small side.
 

Winston

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