LOC-IV gets a stability margin of 0.56 with motor included, is that a problem?

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tibbe

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I just finished my LOC-IV (following https://www.jcrocket.com/loc-iv.shtml) and got a final weight and CM measurement. Now OpenRocket is telling me that my stability margin is 0.56 calibers with the motor is included. On one hand that seems low (i.e. <1) but on the other I'd be surprised if LOC/Precision designed a rocket that's unstable for the motor class it's design for. Is this an acceptable stability margin for this rocket, assuming a reasonable amount of wind (e.g. 3-9 fps) and velocity of the rail (e.g. 30fps or 48 fps, depending on the motor)?
 
Not all designs are inherently stable right out of the box. Have you included all the recovery gear and electronics in your sim? When you weighed everything, did you make sure to override the mass and CG in the sim at the highest level in the tree so it's not double-dipping any internally calculated weights? If so, then it sounds like you'll need to add a generic mass component to the nose cone and increase the weight until you get a good stability margin in the sim, then add that amount of lead to your nose cone and rebalance.
 
In the numbers I gave above I had weighted the rocket with all the recover gear inside and overridden the mass and CG in OpenRocket at the top level stage. For comparison I looked at the stability of of the original RockSim file (i.e. without the mass/CG override caused by my painting, etc) and it's similarly (un)stable for the same motor i.e. for a H123 my rocket is at 0.76 cal while the original RockSim file is at 0.78 cal. My modification have only moved the CG 0.39" backwards.
 
What motor are you using? I built my LOC-IV stock & flew it motor eject only. With no motor loaded, stability was 2.22 calibers. What is your stability with no motor? Have flown it with about 7 different motors and stability never went below 1.0 caliber in any case.
 
I'm planning to use a small H for my L1. An AeroTech H123 or maybe an H78. Even the original RockSim file (from Apogee Components) only has 1.75 cal without any motor (unlike your 2.22 cal). Every single 38mm AeroTech motor simulates with <1 cal on the original RockSim file. Perhaps the RockSim file is just not quite accurate enough. I will try to measure the fins etc. when I get home to see if I get any other values.
 
Only difference I can think of is that the new owners of LOC made some changes to the kit but I can't imagine them doing so. It is a pretty simple kit.
Care to share your OR file?
 
I did my L1 with the Loc IV and the H123 motor too. I just used wood glue so I don&#8217;t have heavy epoxy fillets. I have a heavy eye bolt on the forward centering ring, a good amount of nylon shock cord and I still added some nose weight. I made mine stable with the motor in the 360 casing. Ill get the details from my sim file tonight.

Remember to upsize the parachute. Those fins are problematic. On my 4th flight I developed a crack above the fillet.
 
Interesting. The CP on my sim is 35.62" and yours is 33.98". Not sure why the difference. Dimensions are almost identical & don't think there is any way to over-ride CP.
 
Tim, did you explicitly check the fin dimensions between your and my sim?
 
Probably being redundant, here, but use RockSim or OR to get the CP. Then get the rocket flight-ready and get the actual CG by balancing it. If it is less than one caliber, then it should be considered unstable.

Just because the manufacturer says it is stable for certain motors doesn't mean it is. Maybe the manufacturer used heavier recovery gear, or less epoxy in the aft end, etc.
 
Probably being redundant, here, but use RockSim or OR to get the CP. Then get the rocket flight-ready and get the actual CG by balancing it.

THANK YOU, JOHN! You rock!!! Been preaching that for years...virtual world simulation will only get you half way there!

If it is less than one caliber, then it should be considered unstable.

I've always understood it as between 0 and 1 calibers you have marginal stability. IE, the rocket has some level of stability, but a small safety margin. Rocket may wander but is not likely to do loopty-loops. Many rockets and rocketeers are marginally stable and fall in this range, believe it or not!:wink:

CG on top of CP would be neutral stability; the rocket will fly unpredictably.

CG aft of CP: Rocket is unstable: will perform loops, land shark, hunt down the innocent, and so on.

Disclaimer: I'm not advising less than 1 caliber of stability; just noting that having less than 1 caliber of stability does not always mean rockets exhibiting unstable flight characteristics.

P.S. Loc IV is a pretty stable design with those decent sized and spanning aft swept fins. I've flown a pile of flights on IV's and Heavy Duty Beauty's with motors ranging from C11's (okay, as outboards), to I200's. No noseweight, no problem... but we all build different so take John's advice to mark the CP on the rocket and check ACTUAL CG of the loaded rocket!

Cheers!
 
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Interesting. The CP on my sim is 35.62" and yours is 33.98". Not sure why the difference. Dimensions are almost identical & don't think there is any way to over-ride CP.

You can choose which method of calculating the CP to use. The Rocksim method generally shows better stability than Barrowman. (Openrocket uses Barrowman.)

I see about a 3" difference in locations for the Apogee LOC IV file.
 
Just downloaded the demo version of Rocksim and, as David said, I get a 3.7" difference between Rocksim (which has a "better" CP) and OpenRocket. That's a huge difference (almost 1 cal)! How am I going to rely on simulation results when one piece of software says a rocket is just fine and another says it's only marginally stable?
 
You can choose which method of calculating the CP to use. The Rocksim method generally shows better stability than Barrowman. (Openrocket uses Barrowman.)

I see about a 3" difference in locations for the Apogee LOC IV file.

'Tis true. Another forum member and I each bought the same rocket around the same time. He simmed his in RS, I mine in OR. We compared dimensions and they were exactly the same. However, Rocksim's CP was considerably favorable to OR's.
 
I've always understood it as between 0 and 1 calibers you have marginal stability. IE, the rocket has some level of stability, but a small safety margin. Rocket may wander but is not likely to do loopty-loops.

While technically anything >1 is &#8220;stable&#8221;, numbers close to zero can result in squirrelly flights, cause the distance between CP and CG is the lever arm on which the aero forces act to keep your rocket pointed straight. Just like with a wrench, a longer arm gives more leverage and makes it easier to turn the rocket into the airstream.

This is partly why stubby rockets can often get away with a smaller margin - it&#8217;s the absolute distance rather than the relation between margin and rocket diameter that really matters. Likewise, a long, dense rocket, with a significant moment of inertia, probably needs need more than one caliber.
 
That's what I was going to ask, is the stability margin really dependent on the rocket diameter?
 
Likewise, a long, dense rocket, with a significant moment of inertia, probably needs need more than one caliber.

A rocket with a high L/D will experience a much greater shift in CP location for a given wind angle of attack. This is easy to see by plotting the CP location in Rocksim or Openrocket. Be sure to dial in plenty of wind first and use the lowest thrust motor you plain on flying.

The classic high L/D rocket is something used for the NAR superroc event. CP shifts of 25 calibers or more are possible.
 
Just downloaded the demo version of Rocksim and, as David said, I get a 3.7" difference between Rocksim (which has a "better" CP) and OpenRocket. That's a huge difference (almost 1 cal)! How am I going to rely on simulation results when one piece of software says a rocket is just fine and another says it's only marginally stable?

There is no one correct answer for aerodynamics using analytical approximations in simple hobby software. This is why one software is free and the other costs $120. Rocksim includes the proprietary "Rocksim stability method" in addition to the widely available Barrowman equations. Your choice.

In my experience, OR is overly pessimistic. RS is more liberal, but probably gives a safe answer 95% of the time. Split the difference and use the RASAero II result, which claims to be the most accurate.
 
That's what I was going to ask, is the stability margin really dependent on the rocket diameter?

You've hit upon a misconception that has spectacular longevity.

The 1 caliber "rule" is an approximation intended for use as an easy design point for building *standard* model rockets. Track this back and most model rocket guides that I've read suggest that "a 10:1 Length/diameter ratio is a good place to begin when designing a model rocket from scratch".

Interestingly enough, early NASA reports on sounding rockets use a stability margin that's 8-15% of the vehicle's length for subsonic flights, higher values for supersonic flights, and no mention of vehicle diameter whatsoever.

So 1 caliber on a 10:1 rocket is 10% of the rocket's length which falls nicely in that range.

There are many model and high-power rockets that are far from a 10:1 L/d ratio. Like Oberon mentioned above superrocs (It had 2 calibers, why did it Skywrite?) and short-fat stubbies (It only had a .9 caliber and it STILL weathercocked.....?) need more design consideration than the All Holy 1 caliber.

Since shifting my design basis, I've had 99 rocket problems, but stability isn't one of them!

Analyze don't Assume!
 
Simulations are great and I wouldn't fly without doing it, but they do vary as someone has pointed out earlier. Depends on how realistic the model reflects reality. Refer to my signature at the bottom of the post for further clarification. :)
 
That's what I was going to ask, is the stability margin really dependent on the rocket diameter?

No. But it is a half decent rule of thumb, especially given the relatively primitive tools and understanding we have as hobby builders and amateurs.

Designs like the LOC-IV are incredibly versatile when it comes to which motors they can fly on. Try simulating with longer and shorter motors. I would be a little surprised if it is unstable on a motor that LOC recommends, but it is definitely possible. It never hurts to measure and check on your own!
 
Any idea what Rocksim does beyond the Barrowman calculations? The Barrowman calcs have limitations, but they are known limitations, so you can factor them in. It's great if Rocksim is more accurate, but harder to judge what its limitations / edge cases might be without knowing.
 
The LOC-IV hasn't had any changes at all since we took over. I'm also glad to see people ask when they see stability issues vs. just flying it and potentially hurting someone. However as mentioned many times above, software can be very misleading as well. The best method I've learned is to use sims to determine your CP. Then balance your flight ready rocket to see if adjustments need to be made. The 1x body diameter rule is a great one, but it's not a Law... I would be confident with your .56 margin. I personally have flown down to .29 but of course would never recommend it! :wink: And I also do prefer Rocksim vs. Open Rocket and using the RS calculations for CP. They seem to be way more accurate. Paid software vs. free.... fwiw
 
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