What do i believe

[sgirard]

After retiring I found Openrocket and have been enjoying rocket design and scratch building. I have learned more doing this and reading the Rocketry Handbook than I ever did building kits. There is more to the story but I didn’t want my post to look like the dead sea scrolls, so I will get to my question. My friend built the Estes super big bertha but he weighed it and it came out 2 ounces heavy. I told him I would check it out in Openrocket to see how it looked. I downloaded the file for the Estes super big bertha (a rocksim file) and opened it in Openrocket. I played with the Mass override until I got the C/G and weight correct. It showed the CP at 26.18” with the stability down to .797 after adding weight and getting the C/G correct. I then looked on some web sites and found the listed CP was listed at 28”, that’s almost 2” difference. I know this could get into a lot of questions but given weight and CG will effect the rockets stability it won’t change the CP. So simply put do I believe Openrocket or data listed on the spec in ads (assumed it was provided by Estes) as far at the CP location. All my designs are not kits and I hope I can still trust openrockets CP location. I’m in danger of rambling on so I will quit here. So, what do I believe?

Thanks

steve

 

[Back_at_it]

OR is only as accurate as the information you feed into it. Very few of those SIM files that are floating around out there are accurate or even close. My guess is that the fins are not the correct size or shape even if they look close. I've downloaded SIM files from a reputable site and seen odd results. Best thing to do is to print the fin pattern out of OR then compare it to the actual fins.

On a rocket with fins as large as a SBB, a small change in the fins will make a large difference in the CP.

 

[neil_w]

Yes, it is axiomatic that you must not blindly trust ORK or RKT files found online. In this case, the file is *not* provided by Estes, and likewise the 28" CP number is almost certainly also not provided by Estes. Where did it come from?

It is true that the CP output from Rocksim is sometimes more optimistic than what you get from OR, depending on which set of CP calcuations are selected. I don't know if it would normally be that large of a difference for a simple rocket like this, though... Need someone to take the RKT file you started with and see what Rocksim has to say. I don't have a Rocksim license so I can't do that.

For a simple 4FNC like the SBB the OR calculations should be solid. As @Back_at_it says, small changes to the fins can swing the CP around quite a bit, so measure carefully and ensure that the fin shape is exactly correct.

Finally: stability around .8 will usually fly just fine.