Cone finned rocket - how to?

[Balltip]

I am looking to build a cone finned rocket. I am aiming at making it from BT50 or -55 and am now wondering if there is some sort of nifty tool or rule of thumb as to how wide the cone needs to be to make the rocket stable.
As I am thinking along some yet "secret" *cough cough* lines of design, I am also looking for a cone shape that preferably would not be more than the diameter of a BT70...

 

[BobH48]

You could take this plan and scale the cone to your body tube size.

https://www.dars.org/jimz/finless.htm

Edit: Oops, I just noticed that there wasn't any dimensions for the cone on that plan. It was a paper transition that was a standard Centuri part so you would have to have one to get the sizes.

 

[skippy-2]

I made the Centuri finless rocket this year using the shroud that comes with the Semroc Mercury Capsule kit. The shroud tapers from 14mm at the narrow end, to 45mm at the wide end. Shroud depth is 47mm. (I'm taking these measurements off the model in front of me, so they may not be 100% accurate).

Semroc part # for the shroud is IKM-2S, so buy one!

OR

Plug the numbers into the shroud calculator here:

https://www.rocketreviews.com/tool_shroud.shtml

BTW, the Centuri Finless design is a great park flier on B motors.

 

[Balltip]

Thanks, now I got ideas!

Skippy, what BT size is that rocket of yours? If I understand you correctly then that rocket of yours is based on BT5.

 

[skippy-2]

[Breaks out the vernier...]

Sorry Balltip, that's my bad eyeballing, what we down here call 'bloke merasuring'! It is actually a BT-20 airframe, so the shroud is a 19 mm to 46 mm taper, with a depth of 56.5 mm.

It is amazing how fast these cone fin rockets go together. This one was built, ready for paint in less than an hour!

 

[uncle_vanya]

I have a 3" cardboard tube rocket that has an 8.5" plastic automotive funnel used as a skirt fin. The rocket has flown well on H and I motors but had a bad flight on a J420 in higher than normal winds (approaching 20mph).

There are several funnel fin rockets on EMRR. You should search there for funnel and see what tickles your fancy.

In general the funnels I have seen used are about 2.5-3x the body diameter.

 

[Balltip]

Great! Thanks to all for your replies.

(And I'll try to remember that is is "funnel fin" )

What I have in mind is making a two stage rocket where the second stage looks like a bit like a mercury capsule. The idea is to have the second stage look "finless" (and have the entire set up look like a one stager) as to totally surpise the audience when the second stage suddenly kicks in!

 

[adrian]

Rocksim and VCP both simulate conical transitions. Declare a transition right at the aft end of the rocket and they should simulate your cone-tailed rocket. VCP will even print the cone template for you.

As for using a cone-finned upper stage on a multi-stage rocket, why limit yourself to two stages? Somewhere on my build queue is the Jaguar RTV...

 

[uncle_vanya]

Great! Thanks to all for your replies.

(And I'll try to remember that is is "funnel fin" )

I have been told that the true technical term is skirt fin but I have not been able to find references that way. Typically if I use the words 'funnel fin' in searches I find these rockets.

 

[shockwaveriderz]

in the manual " Design of Aerodynamically Stabilized Free Rockets" they are called a Flare. Check out Chapter 5.

hth

terry dean
nar 16158

 

[Balltip]

...As for using a cone-finned upper stage on a multi-stage rocket, why limit yourself to two stages? Somewhere on my build queue is the Jaguar RTV...

There is this thing that is called "getting the rocket back" :lol:
Got a great launch field, but I have still lost rockets going up on single C's...
On the other hand I have plans for building something I will call "One +Click High" as I intend to send it up way above 1000 meters. Three stages with E's in the second and third ought to send it way out of sight.
Recovery? Streamer and address label...


As a side note: It seems that there are some questions about the correct term for "funnel fins". Might this be a case where there are several correct answers? Might a "skirt fin" be a fin where the motor sits further up the cone so that the cone actually creates a skirt? I am merely speculating here...