Is a two finned rocket stable?

Is a two finned rocket stable?

Post by Bob » Thu, 22 Jan 1998 04:00:00



I have just been given a large cardboard tube (not sure of the measurements, but
at least 3" diameter) by a friend and am going to scratch build some sort of rocket.
The thing is, it's got two wee slits about 5 mm wide, one on either side of one end
of the tube. I was originally going to make a 3 fined design, but I will now probably
make a 4 fined one.

However, the real question is, would this thing be stable with just two fins? One fin
in each of the slits (with a bit of modification to make them longer). Or are two fined
rockets inherently unstable?

Bob.
---------------------------------------------------------------------
"There are theories at the bottom of my jargon"

 
 
 

Is a two finned rocket stable?

Post by Ted Cochr » Thu, 22 Jan 1998 04:00:00


Quote:

>However, the real question is, would this thing be stable with just two
fins? One fin
>in each of the slits (with a bit of modification to make them longer). Or
are two fined
>rockets inherently unstable?

No need to modify the slits--the tabs that go through the slits need not
be as long as the fin root.

Two fin rockets can be stable if the fins are T shaped, otherwise not--you
need a stability North, South, East, West--not just North and South.

|       _       |
|     /   \     |
|_____|   |_____|
|     \   /     |
|       _       |

--tc

My opinions only.

 
 
 

Is a two finned rocket stable?

Post by The Silent Observe » Thu, 22 Jan 1998 04:00:00


Quote:

> I have just been given a large cardboard tube (not sure of the measurements, but
> at least 3" diameter) by a friend and am going to scratch build some sort of rocket.
> The thing is, it's got two wee slits about 5 mm wide, one on either side of one end
> of the tube. I was originally going to make a 3 fined design, but I will now probably
> make a 4 fined one.

> However, the real question is, would this thing be stable with just two fins? One fin
> in each of the slits (with a bit of modification to make them longer). Or are two fined
> rockets inherently unstable?

Assuming the rocket doesn't roll at all, two diametrically opposed fins
won't produce stability in the plane of the fins -- that is, if you see
the fins as "wings" the rocket would be stable in the pitch axis, but
unstable in yaw.

The solution to that is to put opposing tabs on the fins (I don't know
if opposing airfoils would be enough) to cause the rocket to roll; that
will ensure that by the time a disturbance can cause the rocket to
tumble in that unstable yaw axis, it's rolled enough to put the nice
stable pitch axis into the disturbed plane.  Effectively, you can then
make two fins do the work of three or four.

You may or may not gain any performance over a three-fin design -- it'll
likely depend on how much drag you add with the roll inducers -- but
you'll certainly have a rocket that doesn't resemble anything else at
the field that day!  B)

--
Better freedom and a chilly road than a warm hearth and servitude.
                                                              -- Fafhrd

Donald Qualls, aka The Silent Observer           NAR # 70141-SR Insured
Rocket Pages             http://members.aol.com/silntobsvr/launches.htm

Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.

 
 
 

Is a two finned rocket stable?

Post by planetwalde » Fri, 23 Jan 1998 04:00:00


Hey, it should work.  Just don't point it to fly North or South.

Chuck

 
 
 

Is a two finned rocket stable?

Post by Dennis Bisho » Fri, 23 Jan 1998 04:00:00


Estes used to have a 2 fined rocket, called the Gyroc I think, it went
up ok and came down kind of like a chopter.
--
"You are not ready for Immortality" Kosh on Babylon 5

 
 
 

Is a two finned rocket stable?

Post by Bob » Fri, 23 Jan 1998 04:00:00


I quite like the idea of a two fined rocket like the ones mentioned above. So
it seems like an option is the JayHawk target drone. Does anyone have a
source of scale data for this thing? I tried doing a web search and kept on
finding an on-line book!

Apart from making up a rocket design in my head are there any other rockets,
drones, etc. that use a similar fin desing to the JayHawk?

mince,

Bob.
---------------------------------------------------------------------
"There are theories at the bottom of my jargon"

 
 
 

Is a two finned rocket stable?

Post by Len Haz » Fri, 23 Jan 1998 04:00:00


Quote:

> I quite like the idea of a two fined rocket like the ones mentioned above. So
> it seems like an option is the JayHawk target drone. Does anyone have a
> source of scale data for this thing? I tried doing a web search and kept on
> finding an on-line book!

Bob, there was a post here on rmr a few day's ago.  I don't if it's been removed,

but to make it easier here it is:

Quote:
>>Special thanks to Chris Timm.  He scanned (and sent) me some JayHawk
>>blue prints.

>>They aren't perfect, but the are available to those that need them.

>>Check out:
>>Ftp://www.bomarc.com/ROCKET/JAYHAWK/

>>If anyone else needs to post scans, drawings etc., let me know.

>>Dan French
>>HTTP://WWW.Bomarc.com


Hope this helps,
Len Haze
 
 
 

Is a two finned rocket stable?

Post by Pokey7 » Fri, 23 Jan 1998 04:00:00


About the 'jayhawk' type design, bear in mind  that the two little 'tips' on
the large main fins will be farther from the lateral rocket CP, and therefor
more effective.  This 'moment arm' effect may or may not show up on Vcp, but it
does work, and you dont need very large fins at all....they may even be about
the size of the BT diameter.  These would then be 'invisible' when viewed from
that side, but they do work if placed out at least 2 calibers.

Have done this sort of thing with gliders, but dont have a version currently
kitted, but if youd like, that will change.

AstronMike (yea, under Moms name)

 
 
 

Is a two finned rocket stable?

Post by Pokey7 » Fri, 23 Jan 1998 04:00:00


Quote:
>Estes used to have a 2 fined rocket, called the Gyroc

This is/was a cool functional rocket, but it did have the 'rotor stops' which
were outboard fins like a Jayhawk.  So, it didnt really have just two flat
fins.

If you had just two large flat fins, you could spin it up like a previous
poster mentioned, at the expense of added drag.  Of course, such a model would
fall under the 'test yourself before flying at a NIRA launch' criteria.

Quote:
>up ok and came down kind of like a chopter.

Well, back in 83 my freind made one that spun like heck going up and hit a
power tranformer that the electric co. was working on in the bucket truck.
Direct hit, really cool.
 
 
 

Is a two finned rocket stable?

Post by jordan soldier valle » Fri, 23 Jan 1998 04:00:00


Quote:
>Well, back in 83 my freind made one that spun like heck going up and hit a
>power tranformer that the electric co. was working on in the bucket truck.
>Direct hit, really cool.

Learn any new four letter words that day?

-DGH-

flying flexia's website....
http://www.netins.net/showcase/flexia