question about OR 23.09

[sgirard]

Thought I posted this but don’t know where it went so I’ll try again.

I came back into the model rocket world after a 20 year break. I am retired now and Open Rocket has improved my enjoyment by 10 fold. I have learned so much about rocket design by using the program. I new nothing about CG and CP when kit building. Just put them together and fly. I design and fly all my rockets now by build from scratch using open rocket. Thanks for giving this old man something to enjoy again. I do have a question I would like some information on. I live about 100 miles from any club and fly by myself so this forum and my rocketry handbook by Stine is my only source in information.

I just undated my Open Rocket to 23.09 release. Can someone explain what the percentage number for stability means. I understand the caliber number fine but don’t know what the percentage is telling me. Percentage of what and is there a range I should be looking for?

Thanks

Steve

 

[Raythain]

Thought I posted this but don’t know where it went so I’ll try again.

I came back into the model rocket world after a 20 year break. I am retired now and Open Rocket has improved my enjoyment by 10 fold. I have learned so much about rocket design by using the program. I new nothing about CG and CP when kit building. Just put them together and fly. I design and fly all my rockets now by build from scratch using open rocket. Thanks for giving this old man something to enjoy again. I do have a question I would like some information on. I live about 100 miles from any club and fly by myself so this forum and my rocketry handbook by Stine is my only source in information.

I just undated my Open Rocket to 23.09 release. Can someone explain what the percentage number for stability means. I understand the caliber number fine but don’t know what the percentage is telling me. Percentage of what and is there a range I should be looking for?

Thanks

Steve

Steve,
The percentage number is the percent of total rocket length between the CG and CP. Or mathematically, (CG-CP)/length * 100. It's useful for long, thin rockets or short, fat rockets, where the aspect ratio makes calibers a less useful measure of stability.

Might want to take a look at threads like this (https://www.rocketryforum.com/threads/low-mass-high-drag-designs.132002/page-4#post-1607769) if you're interested in more.

Not sure I can give you a good number, but I've seen 10-18% tossed around. Might be worth looking at some stable rockets and see what they stability percentage numbers give you. Or look at some of your own designs and see what the percentage number ends up at and use that to anchor your acceptable range.

 

[neil_w]

For the canonical 10:1 rocket, 10% = 1 caliber. So in general you can think of it that way: 10% is an equivalent rule-of-thumb to 1 caliber, except (as @Raythain points out) it is more broadly applicable to rockets of different aspect ratios.

So if you normally would way 1.5 calibers, then if using percentages you should shoot for 15%. Etc.

Note that with 23.09 you can show both in the rocket figure if you like, e.g.:

 

[sgirard]

Uh-Oh, another can of worms, but that’s OK. It’s what I like about this phase of my rocketry, I’m learning something. I understand the jest of what you guys are saying but won’t tell you I’ve got it down pat. What I’m going to do now is play with some math on my rockets that I have flown and have flown well. It would explain why one of my rockets had a less the 1 caliber number and I worried about it but it flew straight (it was short fat rocket).

Thank you both for the information. I will play with what you have told me and see what I can grasp.

Thanks again

 

[bad_idea]

Not sure I can give you a good number, but I've seen 10-18% tossed around.

I routinely fly rockets under 10:1 L/D down to 8% stability margin. I suspect I'm being conservative with some of them, but the last thing I want to do is give an RSO qualms about my flight (I have enough other ways I do that unfortunately). I've not yet had a problem getting cleared for flight after explaining why stability is less than one caliber on one of my 8% birds, but I wouldn't want to cause anyone a headache by pushing things much further. Maybe if I used the base drag cone hack more, I'd have a better feel for flying under 8%.

 

[sgirard]

I have a scratch-built model of the rocket used in the 50's movie "destination moon". it has a bullet shaped fuselage from one end to the other. It's more like a space plane. There is not a straight section on it. i don't know how OR handles that but it has an aspect of 8 to 1. I was nervous about flying it because OR show a caliber number of .554. but it flew great, very straight. I like low and slow and that's what I got. I flew it on a E26-4W to 410 feet at 110 MPH. After updating OR to 23.09 I looked at my design again and it showed 6.74%. So it looks to me like there is room for some experimenting.