Over Stable?

Over Stable?

Post by Marty Paul » Tue, 06 Jul 1999 04:00:00



I've been using RockSim for a while now to design a couple of 4" rockets.
When the static margin exceeds 2.5, RockSim reports that the rocket is "Over
Stable".  Any one care to elaborate on exactly what this means and what the
outcome of an overstable rocket is?

Marty Paulk
remove the obvious

http://hypercon.com/martinp/index.htm

 
 
 

Over Stable?

Post by John ODonne » Tue, 06 Jul 1999 04:00:00


Quote:

>I've been using RockSim for a while now to design a couple of 4" rockets.
>When the static margin exceeds 2.5, RockSim reports that the rocket is "Over
>Stable".  Any one care to elaborate on exactly what this means and what the
>outcome of an overstable rocket is?

>Marty Paulk
>remove the obvious

>http://hypercon.com/martinp/index.htm

Generally, it means that it will be very sensitive to even light winds.  It
will be more likely to weathercock far upwind, and possibly at a very shallow
angle relative to the ground.  What this means is that if you would use
something like a ten-second delay for a perfectly vertical flight, for a badly
weathercocked flight, that ten seconds may be a few seconds too long.

John O'Donnell  NAR 60742  Sr. L2  
Visit my web page at http://members.aol.com/jon3854/science3/index.htm

"The last man to leave here was never heard from again, he won't be back this
way 'til 2010" - Planet P

 
 
 

Over Stable?

Post by Jerry O'Sulliva » Tue, 06 Jul 1999 04:00:00


Hello John, Marty,

In response to Johns comments, which I think he's exactly correct about, I
don't imagine  weathercocking will be an issue with higher initial thrust
motors (Blue Thunder).

And Marty, I'm assuming you mean the program reports Over Stable when the
motor is loaded?

Jerry

Quote:

>Generally, it means that it will be very sensitive to even light winds.  It
>will be more likely to weathercock far upwind, and possibly at a very
shallow
>angle relative to the ground.  What this means is that if you would use
>something like a ten-second delay for a perfectly vertical flight, for a
badly
>weathercocked flight, that ten seconds may be a few seconds too long.

>"The last man to leave here was never heard from again, he won't be back
this
>way 'til 2010" - Planet P

 
 
 

Over Stable?

Post by Andrew Waddel » Tue, 06 Jul 1999 04:00:00


Substantial overstability can also cause coning, which is when the tail
tends to swing in circles (when viewed from above) around the heavy nose.

--
Andrew D. Waddell
PML Online Support Rep
TRA 2043 L2/NAR 52875 L2

PML: www.publicmissiles.com

 
 
 

Over Stable?

Post by Andy En » Wed, 07 Jul 1999 04:00:00


"Marty Paulk"

Quote:
> I've been using RockSim for a while now to design a couple of 4"

rockets. When the static margin exceeds 2.5, RockSim reports that the
rocket is "Over Stable".  Any one care to elaborate on exactly what this
means and what the outcome of an overstable rocket.......>>>>

RockSim appears to declare any bird with Cg/Cp margin over 2.5 calibers
as overstable with little in additional clarification. Software is funny
about that sometimes, eh?  8^)

In addition to the other valid responses, *nothing* may happen other
than a very straight verticle flight. Peter Olivola described this best
a few months ago but take a long slender model like a super roc or my 7
foot 4" bird, both reported by RockSim as being overstable.  These long
slender birds have high moments of inertia and assuming you scoot them
along fast enough without buckling, they'll climb nice and straight
regardless of the wind.

Since HPR is involved, tell us a bit more about your project and maybe
we can figure if coning and weathercocking needs to be addressed.

Regards,
Andy

Sent via Deja.com http://www.deja.com/
Share what you know. Learn what you don't.

 
 
 

Over Stable?

Post by Marty Paul » Wed, 07 Jul 1999 04:00:00


Thanks for the info guys.  I've seen a few tidbits here and there that led
me to believe what you guys have said about weather***ing.  I'm curious
about the high moments of inertia concept and why RockSim doesn't include
this.  Is there any method for including this in a stability calc, or do you
guys just have a feel for results of an overstable bird - I'm an engineer so
I like modeling before I fly.

I've posted a couple of screen captures from RockSim for one of the rockets
on my website at the following url
http://www.FoundCollection.com/;As you can see, this
particular configuration is dual staged, is about 280 oz., and 114" long.
The rocket is a custom made from 4" PML tubing that has been glassed with
first 4oz and then 2oz cloth, with g10 fins, and an ogive fiberglass
Scotglass nosecone.  The pics on the web site show the sustainer loaded with
a J350, the booster with a K550. With the booster, the rocket has a static
margin of 3.53.  The sustainer alone has a static margin of 3.19.

This bird utilizes the motor/fin can from my extended Nike Smoke (see
detailed info at http://www.FoundCollection.com/) for the sustainer.
I've got numerous 4" components between these two rockets that I can swap
around for different motor configs.  That's another problem I've got - how
to save assemblies in RockSim so that I can pull them up in any 4" rocket
config without having to rebuild them everytime <---what a pain.

Marty Paulk
TRA7194 L1
remove the obvious

Quote:

> "Marty Paulk"

> > I've been using RockSim for a while now to design a couple of 4"
> rockets. When the static margin exceeds 2.5, RockSim reports that the
> rocket is "Over Stable".  Any one care to elaborate on exactly what this
> means and what the outcome of an overstable rocket.......>>>>

> RockSim appears to declare any bird with Cg/Cp margin over 2.5 calibers
> as overstable with little in additional clarification. Software is funny
> about that sometimes, eh?  8^)

> In addition to the other valid responses, *nothing* may happen other
> than a very straight verticle flight. Peter Olivola described this best
> a few months ago but take a long slender model like a super roc or my 7
> foot 4" bird, both reported by RockSim as being overstable.  These long
> slender birds have high moments of inertia and assuming you scoot them
> along fast enough without buckling, they'll climb nice and straight
> regardless of the wind.

> Since HPR is involved, tell us a bit more about your project and maybe
> we can figure if coning and weathercocking needs to be addressed.

> Regards,
> Andy

> Sent via Deja.com http://www.FoundCollection.com/
> Share what you know. Learn what you don't.

 
 
 

Over Stable?

Post by Marty Paul » Wed, 07 Jul 1999 04:00:00


What is considered substantial overstability?


Quote:
> Substantial overstability can also cause coning, which is when the tail
> tends to swing in circles (when viewed from above) around the heavy nose.

> --
> Andrew D. Waddell
> PML Online Support Rep
> TRA 2043 L2/NAR 52875 L2

> PML: www.publicmissiles.com

 
 
 

Over Stable?

Post by Andrew Waddel » Thu, 08 Jul 1999 04:00:00


My personal rule of thumb is anything 2.5 calibers or above.

--
Andrew D. Waddell
PML Online Support Rep
TRA 2043 L2/NAR 52875 L2

PML: www.publicmissiles.com


Quote:
> What is considered substantial overstability?



> > Substantial overstability can also cause coning, which is when the tail
> > tends to swing in circles (when viewed from above) around the heavy
nose.

> > --
> > Andrew D. Waddell
> > PML Online Support Rep
> > TRA 2043 L2/NAR 52875 L2

> > PML: www.publicmissiles.com

 
 
 

Over Stable?

Post by Robert Galej » Thu, 08 Jul 1999 04:00:00


Quote:

> My personal rule of thumb is anything 2.5 calibers or above.



> > What is considered substantial overstability?

It's not so simple...

Rockets with a relatively large ratio of body planform area to
fin area may need much greater CP-CG margins since the
CP moves froward substantially on these rockets, especially
if they are of the long-and-skinny type.  Look at my article
in the latest Sport Rocketry, "What Barrowman Left Out"
It shows a rocket with over 10 calibers of stability margin
that went unstable at a CMASS launch and explains why
this could happen.

I have a PDF version of the article at the CMASS website
as well..

http://www.cmass.org/uploads/Robert.Galejs/galejs.html

For "normal" rockets your rule of thumb is probably a
good one.

- Robert Galejs

 
 
 

Over Stable?

Post by Andrew Waddel » Thu, 08 Jul 1999 04:00:00


You're absolutely right that it's not that simple, which is why it's a "rule
of thumb". Your article was one of the reasons I called it a rule of thumb,
and was hoping that someone who knew more about it than me would reference
it in this thread.

--
Andrew D. Waddell
PML Online Support Rep
TRA 2043 L2/NAR 52875 L2

PML: www.publicmissiles.com


Quote:

> > My personal rule of thumb is anything 2.5 calibers or above.



> > > What is considered substantial overstability?

> It's not so simple...

> Rockets with a relatively large ratio of body planform area to
> fin area may need much greater CP-CG margins since the
> CP moves froward substantially on these rockets, especially
> if they are of the long-and-skinny type.  Look at my article
> in the latest Sport Rocketry, "What Barrowman Left Out"
> It shows a rocket with over 10 calibers of stability margin
> that went unstable at a CMASS launch and explains why
> this could happen.

> I have a PDF version of the article at the CMASS website
> as well..

> http://www.cmass.org/uploads/Robert.Galejs/galejs.html

> For "normal" rockets your rule of thumb is probably a
> good one.

> - Robert Galejs

 
 
 

Over Stable?

Post by Jerry O'Sulliva » Thu, 08 Jul 1999 04:00:00


Quote:
>> Rockets with a relatively large ratio of body planform area to
>> fin area may need much greater CP-CG margins since the
>> CP moves froward substantially on these rockets, especially
>> if they are of the long-and-skinny type

The CP moves forward??

I thought that was a fixed point determined by fin geometry.

Jerry

 
 
 

Over Stable?

Post by Robert Galej » Thu, 08 Jul 1999 04:00:00


Quote:

> >> Rockets with a relatively large ratio of body planform area to
> >> fin area may need much greater CP-CG margins since the
> >> CP moves froward substantially on these rockets, especially
> >> if they are of the long-and-skinny type

> The CP moves forward??

> I thought that was a fixed point determined by fin geometry.

> Jerry

The CP location is a function of the rocket angle-of-attack (AOA).
The CP moves forward as the AOA increases from zero.  (There are
strange cases where the CP moves backwards but you will likely
not get these past your RSO).
When a rocket leaves the launch rod in the presence of wind, the
rocket is not moving straight ahead relative to the air.  This is where
the AOA is usually the greatest and you want the CG to be ahead of the
CP at this point.  For "normal" rockets, a 1 caliber zero AOA CP-CG
margin is usually sufficient to keep the CG ahead of the CP.
Long/skinny
rockets need a greater margin.

- Robert Galejs

 
 
 

Over Stable?

Post by Joe Pfeiffe » Thu, 08 Jul 1999 04:00:00


Quote:

> The CP moves forward??

> I thought that was a fixed point determined by fin geometry.

It is determined by geometry (not just fin geometry), but it's not a
fixed point.  It's an artifact of the air flowing past the rocket, so
the angle of attack makes a huge difference in the CP.

Barrowman was very careful to note that his derivations assumed a very
shallow angle of attack (which is, hopefully, what matters!).

Planform area approximates CP for a rocket with a 90 deg angle of
attack.  Since CP normally moves forward as angle of attack increases,
it will give a result further forward than Barrowman -- and so, is the
more conservative technique.
--
Joseph J. Pfeiffer, Jr., Ph.D.       Phone -- (505) 646-1605
Department of Computer Science       FAX   -- (505) 646-1002
New Mexico State University          http://www.cs.nmsu.edu/~pfeiffer

 
 
 

Over Stable?

Post by Andy En » Fri, 09 Jul 1999 04:00:00


Hello Folks,

We were chatting about Marty's rocket and at this point, I've got to
punt.  If it were a model rocket, I'd bet a pack of D12 motors that it
would weathercock but all the HPR two stagers I've seen, on video, went
up nicely (although I'm curious about the K550 booster).  Can somebody
else take a look at this?

I'll sit back and listen 8^)
Andy


Quote:
> Thanks for the info guys.  I've seen a few tidbits here and there that

led me to believe what you guys have said about weather***ing.  I'm
curious about the high moments of inertia concept and why RockSim
doesn't include this.

Go to Dejanews.com and do a Power Search -> rec.models.rockets ->author
= "Peter Olivola" -> search for (inertia )    This should get you
started.

Quote:
> I've posted a couple of screen captures from RockSim for one of the

rockets on my website at the following url
http://www.FoundCollection.com/
As you can see, this particular configuration is dual staged, is about
280 oz., and 114" long.

Quote:
> The rocket is a custom made from 4" PML tubing that has been glassed

with first 4oz and then 2oz cloth, with g10 fins, and an ogive
fiberglass Scotglass nosecone.  The pics on the web site show the
sustainer loaded with a J350, the booster with a K550. With the booster,
the rocket has a static margin of 3.53.  The sustainer alone has a
static margin of 3.19.

Quote:
> This bird utilizes the motor/fin can from my extended Nike Smoke (see

detailed info at http://www.FoundCollection.com/) for the
sustainer. I've got numerous 4" components between these two rockets
that I can swap around for different motor configs.  That's another
problem I've got - how to save assemblies in RockSim so that I can pull
them up in any 4" rocket config without having to rebuild them everytime

Marty...  Were you the one that Kent needed the dremel for at Bomber
field?

Sent via Deja.com http://www.FoundCollection.com/
Share what you know. Learn what you don't.