Rocksim/Design question

[Peartree]

I am designing a semi-secret rocket for the EMRR challenge. it looks pretty good on paper but I have approximated it in Rocksim and it does something sorta strange. In zero wind it does okay, its draggy and doesn't get a lot of altitude, but it should do what I want it to do. In even light breeze it weathercocks so that it goes as far downrange as it does vertically. It will be difficult to deploy a chute above ground if it does this for real.

I have tried bigger fins, more or less nose weight, more thrust and a longer mid-section and all the results are about the same (although longer fins helped a little). The real thing will have fin pods but Rocksim can't deal with that at all.

Here are my questions:

1) Is it normal for rockets to weathercock this badly in Rocksim or is it just some goofy part of my design (I really don't want to show everyone the design since it IS a part of a contest)?

2) What else can I try to get a more vertical flight?

3) I expect that the fin pods will help some but how much help can I expect?

This is going to be challenging to build in any case, so I would like to have some confidence that it isn't going to self destruct before I spend a lot of time building it.

I know it's hard to give an opinion without seeing the thing but I welcome all of your suggestions.

Thanks a ton...

 

[fastfreddie]

I'm guessing it's short and fat with fairly large fins? If so, try adding length and reducing the fins a bit. Short rockets with a lot of fin weathercock badly even in light winds. Take the Fat Boy for instance. Adding more fin just makes it worse. However, rocksim tends to overdo the unstability. It might actually fly better than rocksim says it will.

 

[Peartree]

Short and fat is a reasonably accurate description. A lot of fin? Not especially. Any thoughts on the proper stabilty margin for short, fat rockets? Even those variations labelled as marginally stable by Rocksim still did as well as vastly overstable models.

 

[fastfreddie]

I've been told by folks way more experienced than me that even though Rocksim shows short rockets unstable or marginally stable, they'll still fly just fine. I'm doing a 7.5" Big Daddy upscale for my L3 and I've been assured it will fly OK without a bunch of nose weight. Rocksim says it's marginally stable, but hey, the real one flies great.

 

[teflonrocketry1]

Short fat rockets have additional stability that RockSim doesn't account for:
https://www.apogeerockets.com/education/downloads/Newsletter154.pdf
https://www.apogeerockets.com/education/downloads/Newsletter158.pdf
https://www.apogeerockets.com/education/downloads/Newsletter162.pdf

Fin pods can be approximated using these tricks for RockSim:
https://www.apogeerockets.com/education/downloads/newsletter119.pdf

If you need help with your design you can PM me the details, I will keep your design information confidential. Additionally, since I can't seem to view or get to EMRR site directly; you won't have to worry about me competing against you.

You will have to wait a few days for my reply since I am really busy lately,

Bruce S. Levison, NAR #69055

 

[jj94]

You can change the angle the launch rod is at. The rocket also sounds a bit too much overstable. Try to give it a margin of 1-2 calibers.

 

[Peartree]

Thanks Bruce, I have sent you an email with Rocksim files and other explanations.

jj94: 1-2 calibers makes it far worse. In fact two calibers is probably not possible given the length of the rocket. The sims with the best stability and resistance to weathercocking so far have stability margins of about .25 to .5. It goes unstable with negative margins of course...

 

[jj94]

Oh, I didn't realize that. If you can, you can make some test flights with a heads up warning.