Design Questions

[rainyday101]

This is my first design and I have a couple of questions. The rocket is a BT-60 upper body and nose cone with 3 BT-20's at mid section and a section of BT-60 three inches long at the tail section. The three BT-20's are exposed between the upper and lower BT-60 sections. RocSim shows a marginal stability of 5.69 with three C6-7's loaded. Total rocket length is 38" long. Three fins on back with an individula fin span of 4" and they sweep back extending about 2" past the back of the rocket. Is my loaded stability good enough? With the engines unloaded the stability is 9.87. Is 3/32 balsa good enough for the fins or should I use 1/8" with a paper and glue coat? Any advice/tips would be appreciated.

 

[luke strawwalker]

This is my first design and I have a couple of questions. The rocket is a BT-60 upper body and nose cone with 3 BT-20's at mid section and a section of BT-60 three inches long at the tail section. The three BT-20's are exposed between the upper and lower BT-60 sections. RocSim shows a marginal stability of 5.69 with three C6-7's loaded. Total rocket length is 38" long. Three fins on back with an individula fin span of 4" and they sweep back extending about 2" past the back of the rocket. Is my loaded stability good enough? With the engines unloaded the stability is 9.87. Is 3/32 balsa good enough for the fins or should I use 1/8" with a paper and glue coat? Any advice/tips would be appreciated.

Hmmm... I'm a bit confused by Rocksim telling you it's "marginally stable" with 5.69 (calibers I presume)?? This doesn't really make any sense... to be 'marginally stable' would imply less than 1 caliber of stability (less than 1.6 inches with a BT-60 tube. Are you sure your settings are right?? The "frontal area" that Rocksim uses to calculate stability might be set to an erroneous reading-- make sure it's set to 'use maximum frontal diameter' in the drop-down menu.

If the fins are 4 inches wide from root to tip, individually, with that rocket length, and swept back 2 inches, and presuming they're placed at the rear of the rear BT-60 body tube, it sure seems like it would pass the TLAR test (that looks about right) especially with a 38 inch rocket length...

Maybe try adding a mass object to the rocket in the nose-- simulate some noseweight and see what that does...

Otherwise, I'd say it sounds about right... if you truly have 5.69 calibers of stability, that is actually OVERSTABLE and will weathercock badly in any wind...

3/32 balsa MIGHT be ok for the fins-- but landings might be kinda rough on them, especially with the fins swept back behind the rocket. I would CERTAINLY paper them for strength though! 1/8 is only 1/32 inch thicker than the 3/32 (1/8 is 4/32) but that slightly thicker fin increases the strength quite a bit. For this size, weight, and power of a rocket, you would GREATLY benefit from papered fins, in either thickness. Actually I'd probably go with 3/16 balsa for extra strength for this large/heavy/powerful of a rocket.

Good luck and hope this helps! OL JR

 

[rainyday101]

Correction that is 5.69 overstable. What amount of stability will give me the best flight without the weathercocking? Is there a general rule of thumb?

 

[Boosterdude]

Generally your looking for 1 caliber stability, any thing over 2 to 3 calibers may contribute to excessive weathercocking.

 

[rainyday101]

Thanks boosterdude. I found that in my Model Rocket Design and Construction book also. I have it down to 2.93 calibers and am working on getting down more. Changing fin shapes has a big effect. I have had to make the fins much smaller in area to get down to 2.93. Any suggestions for getting my stability down?

 

[MarkII]

"Overstable" doesn't meant that it absolutely will weathercock, just that it will have a stronger than average tendency to do that if there is a decent cross-wind. You can adjust for that tendency by giving the rocket a longer guide length on the pad, and even better by giving it a stronger boost with a punchier motor. You also won't see any weathercocking if you launch it in still air.

MarkII

 

[powderburner]

Any suggestions for getting my stability down?

It feels weird trying to come up with ways to DE-stabilize a rocket, but you can always add fin area to the front end or weight to the back end (and now I am going to go wash my mouth out with soap)

I wouldn't worry too much if rocsim says it is "overstable," you might want to go ahead and test-fly to see how it behaves as is.

 

[adrian]

Personal experience says Rocksim usually gets the CG wrong, so its estimate of stability is also likely to be wrong. The estimate of CP is probably right, though. You can find the real CG by loading the rocket up as if for a flight, then see where it balances on your finger.

The rule about CP being 1 body diameter behind the CG is valid for "normal" proportion rockets. 38" long, 1.6" diameter is a relatively thin rocket and may need a larger gap. I'd leave the stability at 2.93 (and check by finding the real CG whether the stability margin really is that great). Load it, launch it, see what it does, and if it does weathercock badly then clip the fins a little more. (Do this in Rocksim first to see what it does to the CP before actually cutting bits off the rocket - it is a lot easier to cut bits off the fins than to put bits back on. )

 

[rainyday101]

I added a small set of front fins and got the stability down to 1.93 without sacrificing much altitude. I am going to get building it this weekend. Thanks for all the tips and help.

 

[shreadvector]

The TARC people went nuts over allegedly overstable designs (the simulation software told them it was overstable) so they were adding forward fins and cutting away the aft fins. What a disaster!

As Trip Barber explained: Over stable is FINE unless the rocket is moving very slowly as it leaves the launch rod relative to the cross wind at the time of launch. If the rocket is moving fast or there is no wind, there will be ZERO (or nearly zero) weathercocking.

 

[gpoehlein]

One other thing to remember here as well - Rocksim underreports stability on short fat rockets (ie the Fat Boy) and overreports it in long skinny (superrocs) rockets. From what I understand (I'm not the expert), you WANT several calibers of stability for a superroc.

 

[rainyday101]

Great info thanks, I am going to go back to my original design with the slightly larger rear fins and lose the front fins. This will give me about 3 calibers overstable. I will try it and see what I get!