Downsides of overstable design?

[JackMatt]

Greetings - new to this Forum, but have been active in my local club / forum for about three years. Got my L2 a few months ago - but mostly have used PML and other kits. Newly retired and trying my hand at my first serious scratch-build. The design I have (and modeled in RockSim) is long (92"), with a 2.5" diameter air frame. I've already built the fin can, so the fin size/shape is fixed - the rest is still 'flexible' and am willing to modify, if necessary.

Question - other than severe wind cocking (I'd launch only on the calmest of days and with motors that provide a quick start off the pad), what are the other downsides of an overstable design (on the order of 8.5)? One reason for the length is that I wanted to get the tracking hardware as far away from the Av-bay (dual deploy) as possible,,, but I'm sure it's due in equal measure to my inexperience in design.

Bottom line - are the downsides significant enough for me to re-work the design,, or can I build this 'pencil' and only launch in suitable conditions with appropriate motors?

Jack Matthews
NAR 90834
TRA 13150

 

[BABAR]

I'm sure those with more expertise will eventuallly chime in.
You've tapped the major issue, weather cocking. "Suitable conditions" for this rocket are probably going to be essentially zero wind.
I suspect with that degree of stability you have a lot more finnage than you need, so you probably have a lot more drag than you want, but assuming you have enough engine power that is surmountable.
Sounds like a pretty long rocket, is it structurally sound enough to handle the thrust? This is an issue for "SuperRocs", where when you get the rocket long enough it tends to flex.
Good luck!

 

[JackMatt]

Thanks for the input, Tom,

The story on the fins - I had a different project in mind that got more complicated than I anticipated, so they were already cut and airfoiled (0.093 G10), so I used 'em - not ideal for this design, but have more unused parts and pieces around than I know what to do with, so trying to clean up the left overs, so to speak.

Good point on the airframe flex - the airframe is fiberglass wrapped phenolic, so I think it should be good, but something I'll look into in more detail.

Jack

 

[rcktnut]

Your design should be fine, look at the Estes Mean Machine. Our last launch of the season we had a guy put an F50 in one, stock built accept for the 29mm. motor mount, flew great. I thought for sure it was going to fold up on him, but it did not. I wouldn't be worried about flying it in some wind, just up the thrust a bit when you have wind. There are not to many dead calm days, especially when a launch is scheduled.

 

[Micromeister]

Agree with most of what has already been posted, Really weather cocking is the only down size to overstable designs. your rocket seems more along the line of a SuperRoc which by the way generally have fairly tiny fins.

I Have and fly BT-20 (.736") C super Rocs that are 98-7/8" long, and D SuperRocs That are BT-50(.976"dia) by 117-1/8" long without problem in breezes up to 20mph without much of a problem.
Also have a 2stage BT-70 to BT-60 x 120" long Grumpy-Dog flying on D12-0/D12-5 that doesn't like any wind at all but flys just fine in calm to near calm conditions. In winds over 10 it weathercocks badly but still contiues gaining altitude just on a more horizontal bearing

Bottom line build you model based on the motors you plan on flying, just keep in mind the higher the launch time breeze the longer the recovery walk will be



View attachment 133-sm_SuperRoc D (297.5cm-117.125in)_10-10-92.jpg

 

[CZ Brat]

Jack,

If you decide you don't want to be so overstable, one option is to shorten the fins in place. I did this for a rocket last year. What I did was make a jig to cut the tip of the fins off. I was dealing with 1/4" plywood, so I may have had it easier. I took two straight edge pieces of scrap and glued a 3rd piece the same thickness as the fin in the middle of the two, creating a 3 layer stack. The middle was glued half an inch from the straight edges (cause I wanted to remove 1/2"). I then slid it onto the tip edge of the fin and used the straight edges as saw guides. Then repeated for the other fins.

 

[JackMatt]

Jack,

I took two straight edge pieces of scrap and glued a 3rd piece the same thickness as the fin in the middle of the two, creating a 3 layer stack. The middle was glued half an inch from the straight edges (cause I wanted to remove 1/2"). I then slid it onto the tip edge of the fin and used the straight edges as saw guides. Then repeated for the other fins.

I like it, CZ - I'll thunk it out and may try this... make sure I get a clean cut on the G10. I've got some G10 scraps laying around I'll practice on. Thanks, Jack

 

[Buckeye]

In my mind, overstable is not a problem at all. Long, skinny rockets can have much less dynamic stability and more angle of attack coming off the rail (with some wind present). Play around with OR to see this. Make sure you have plenty of launch rail and a motor with enough oomph, and your pencil will fly just fine.