Short fat rockets

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cwbullet

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Thanks. I will work on the portapotty next week.
 

ThirstyBarbarian

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View attachment 406931

There was a lot of weight in the nose to get it stable
Is that not the key to short fat rockets?
If there’s a coyote strapped to the outside, then you probably need a lot! But many short, fat rockets don’t need much nose weight, if any.

The algorithms used by most sim programs don’t give a good idea of the real stability of the rocket. They don’t account for “base drag” or other stabilizing forces. You can use the zero-mass cone trick to get a better idea of the stability of a short, fat rocket. My rockets often show a seriously low stability margin of .3 cal or sometimes even less, but when you do the cone trick, it increases quite a lot. I’ve never added any weight to my Warlock, foam rocket, or other shorties, and they fly great.
 

BABAR

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cwbullet

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If there’s a coyote strapped to the outside, then you probably need a lot! But many short, fat rockets don’t need much nose weight, if any.

The algorithms used by most sim programs don’t give a good idea of the real stability of the rocket. They don’t account for “base drag” or other stabilizing forces. You can use the zero-mass cone trick to get a better idea of the stability of a short, fat rocket. My rockets often show a seriously low stability margin of .3 cal or sometimes even less, but when you do the cone trick, it increases quite a lot. I’ve never added any weight to my Warlock, foam rocket, or other shorties, and they fly great.
It depends on the length. the shorter the fins and tube, the more weight is needed.
 

ThirstyBarbarian

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I got a PM with a question about the zero-mass cone trick I mentioned earlier, so I thought I could expand on it here.

Sim programs like Open Rocket and Rocksim calculate rocket stability by calculating the center of pressure and comparing that to the center of gravity. The general rule of thumb is that the center of gravity should be about one body diameter (1.0 calibers) forward of the center of pressure. If the stability margin is less than one caliber, usually the recommendation is that you add some nose weight to increase the margin to about 1.0.

That’s a guideline that is pretty good for rockets that are not short and fat. But the shorter and fatter the rocket, the harder it is to achieve a one caliber stability margin. A single caliber is a larger percentage of the length of a short rocket. And because of the short length, any added mass in the nose doesn’t have as much lever arm to move the the center of gravity as far as it does in longer rocket. The more mass you add up front, the more massive a motor you need in back, and you get into a vicious cycle. If a rocket is short enough, it become impossible to get 1 caliber of stability. Think of a saucer — it’s entire length might not even be one caliber.

But we know saucers and spools fly fine. And we know from experience that short, fat rockets like the Warlock fly fine with no nose weight, even though they have stability margins of far less than 1 caliber. These kinds of rockets are a half step between longer rockets and saucer-type rockets, and there are forces that keep them stable that aren’t part of the CG/CP relationship as it’s calculated by our sim programs. Sometimes that force is referred to as “base drag”, and the idea is that the fat base of the rocket helps to stabilize it in flight.

There is a trick you can use to kind of get the sim programs to give a little credit for base drag when calculating the center of pressure on a fat rocket and can move the CP back a bit, increasing the calculated stability margin. To do the trick, you add a cone with zero mass to the base of the rocket. In OpenRocket, I’ve done this trick by adding a transition component to the base of the rocket. The forward end at the base of the rocket has zero diameter, the length of the transition is pi times the rocket diameter, the aft end of the transition is the same size as the rocket diameter, and the mass of the transition is zero. After you add the cone to the simulation, check the stability margin with the motors you want to use, and it should be a lot closer to 1.

This trick is mostly to reassure yourself that the rocket will be stable. Maybe the rocket will still need a bit of nose weight, but this trick can help you avoid adding way more than you need. After you’ve done the trick and feel confident in the rocket’s stability, you need to remove the cone from the design sim for your flight sims. If you leave it there, your flight sims will be off. The cone will make your altitudes and optimal delays sim too short.

If you still feel a bit wary about the stability of your short fat rocket, you can minimize the chance of problems by easing into the motors you use. Don’t pick a motor that weighs a lot and has a long burn. Get something small with a lot of thrust, but a very short burn, and low total impulse. Small size keeps the weight aft to a minimum for better stability. High thrust gets the rocket off the rail at high speed for better stability. And if there is a stability problem, a short burn means less skywriting under thrust. If the flight goes well, start working up in motor size and burn time.

Here’s some more info about simulating base drag:
https://www.apogeerockets.com/education/downloads/Newsletter154.pdf
https://www.apogeerockets.com/education/downloads/Newsletter158.pdf
https://www.apogeerockets.com/education/downloads/Newsletter162.pdf
 

MikeyDSlagle

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Another school of thought that is generally accepted and also debatable:
The standard of 1 CAL stability is derived from "normal" rockets in the neighborhood of a 1:10 diameter:length ratio. Meaning a 4" rocket that is 40" long will need 4", or 1 CAL stability. Shrink the rocket down to 20" and it will only need .5 CAL to be reliably stable. Stretch it out to 80" and and now you need 2 CAL.
The longer a rocket is relative to it's diameter the more CAL of stability it needs.
Like many things, it's a rule of thumb.

I regularly fly short stubbies well below 1 CAL as well, many below .5.
 

ThirstyBarbarian

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Another school of thought that is generally accepted and also debatable:
The standard of 1 CAL stability is derived from "normal" rockets in the neighborhood of a 1:10 diameter:length ratio. Meaning a 4" rocket that is 40" long will need 4", or 1 CAL stability. Shrink the rocket down to 20" and it will only need .5 CAL to be reliably stable. Stretch it out to 80" and and now you need 2 CAL.
The longer a rocket is relative to it's diameter the more CAL of stability it needs.
Like many things, it's a rule of thumb.

I regularly fly short stubbies well below 1 CAL as well, many below .5.
Right. Instead of taking the difference between the CP and CG, and dividing by the diameter to get calibers, you can divide the difference by the length of the rocket. The rule of thumb in that case is that the CG should be ahead of the CP by about 10% of the body length.

Like you said, these ratios are rules of thumb. They can often be fudged a bit if you do it for a reason that makes sense. When you do fudge it a bit, you can test the design with a small, fast, short-burn motor. And use the longest rail available.
 

o1d_dude

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Ha ha! Yup!

Inspired by this (thus the German...):

View attachment 425689
Reminds me of the time my dad and I drove around the Nurburgring in our VW microbus in 1962-63? at the 1000km race.

We watched the race from the long straightaway that came out of the trees down into a gentle valley and up another hill on the other side...at least a mile long. When the first car broke out of the trees, everyone started shouting “Moss! Moss!”

Stirling Moss had gotten the jump on everyone and was in the lead...

Good times.
 
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