Is there a "Minimum" Cals for safe flight?

I've got a design that I'm working on (downscale of a larger rocket) but Open Rocket says that w/o payload, or additional nose weight, it has a stability of 0.669 cal with a "D" and 0.544 cal with an "E" engine.

I know that 1.0 cal is ideal, and 3.0 or more cal is prone to weathercocking. But what about less than 1.0 cal? As long as the CP is behind the CG it's stable... right?

I don't want to build this, as is, and find that I need additional weight, nor increase the fins much beyond the scaled version of the original. However, I won't launch it if I think that it's going to be unsafe either.

[EDIT] I re-built the rocket (from scratch) in OR, and I have a 10 oz discrepancy than from the original downscaled version. The re-built version is 6oz w/ "E" motor, the original is 17.5 with "D" motor, and 0.75oz noseweight [/EDIT]

Thanks for the help!
Jim
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The calibers of stability concept is sometimes confusing--which is why there are rules-of-thumb. And for this, the rule-of-thumb is 1.0cal. BUT... short, fat rockets with appreciable base drag (think of this phrase as: "The way saucers and pyramids are stable in flight... with similar altitude limitations due to drag") are just fine with 0.66 or, even, 0.54cal. Skinnier, longer rockets do fine on 3 or more.

Also, keep in mind that what you're measuring is a static snapshot of a dynamic process--that is, worst case scenario. As the motor burns, the CG will move forward. How much? Well... that depends on the motor you're using and the rocket's configuration. You can get an idea of where it will move to by simming the rocket with a mass object equal to the burn-out weight of the motor you're planning on using.

Not all D or E motors are the same, remember, so they won't have all uniform weights. Try other lighter motors with the thrust range you need--see where they put you.

Also, posting the sim file would be helpful... Heheh.


Later!

--Coop

K'Tesh said:

I know that 1.0 cal is ideal, and 3.0 or more cal is prone to weathercocking. But what about less than 1.0 cal? As long as the CP is behind the CG it's stable... right?
.

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1.0 calibers is not ideal; it provides a very small margin of safety against things like weight shifts or crosswinds. After some bad experiences with smaller margins of stability, I like to have the CG ahead of the CP by at least 10% of the length of the airframe. If the rocket is going supersonic, I try to have 15%. A rule of thumb normalized by the rocket diameter doesn't work very well for long, skinny rockets or short, fat rockets.

There's a safe margin but you get to find out what it is .

Coop is right about rules of thumb and CG, but also CP needs to be considered. It can move as a result of angle of attack of the airflow across the rocket. Originally a method was used to calculate CP that was very conservative, worst case, resulting in unnecessary overstability. Barrowman proposed a method of calculation that is valid for low angles of attack and generally valid for flight prior to apogee. Therefore, needing some margin of stability anyway, one is left with estimating the needed extra margin from an optimistic CP rather than estimating how much you can go negative from a worst case scenario. However this worst case, often considered to be that determined by the "cardboard cutout" method, is also a useful guideline. Barrowman gives extra weight to effect of the fins and ignores body tubes, so comparitively most long rockets will have more CP shift.

Stabilty margin is often expressed vs. diameter (cals) but I also agree the needed margin seems to have more to do with a % of length.

I also use wind testing to verify stability.

If you are going to intentionally try for such a small stability margin, especially on a high-power rocket, I would strongly recommend that you follow up with very careful c.g. checks after you complete construction. It is very easy to have a few extra grams of weight in the aft end if you deviate even the slightest bit from the kit instructions and this can quickly eat up your stability margin. If you use a larger (heavier) motor without re-checking, you can easily completely erase an initial margin as small as 1 caliber.

Until you get a little more experience/confidence with different designs and a range of motor sizes, I would suggest that you aim for intentional stability margins in the range of 2 to 3. My two cents....

Upon a 2nd OR rebuild from the ground up, the weight remains nearly constant between 8.5oz and 10oz with motors installed (D/E, E/E) and the cals are now hovering around 2.3 and 5.3 (downscaled rocket is 33" long). This is compared to the original simulation which was scaled down from the .rkt found on Rocketreviews.com. I suspect that despite my best efforts, the scaling didn't cut down on some of the weights of original materials.

This is just my 2¢, but when I design a scratch-built I plan on having a minimum of 0.95 CAL at the end of the launch rod. It usually sacrifices some speed and altitude, but so far I have not had a spinner or a bad flight doing this.

I found that, depending on the rocket, the CAL goes up considerably as the rocket continues and it can (at least on my scratch-builts) hit around 2.5 to 3 just after motor burnout. With the CAL off the launch rod and the plenty high enough CAL in flight you're pretty much guaranteed a good flight unless you hit a bird (almost never happens) or have a CATO.

So in a nutshell, add a touch of nose weight if you can and try to get the CAL as close to 1.0 when the rocket leaves the launch rod.

GDJ said:

With the CAL off the launch rod and the plenty high enough CAL in flight you're pretty much guaranteed a good flight unless you hit a bird (almost never happens)

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Ummm... You ever wonder why my tag says "Seagulls Beware..."?

My rocket barely missed it... My kites didn't.

K'Tesh said:

Ummm... You ever wonder why my tag says "Seagulls Beware..."?

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I just figured you were being funny.....which it was.