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'[OT]: Rocket advice'
2001\02\10@150237 by Bob Blick

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Hi Everyone,

Does anyone know what the rule of thumb regarding center of gravity on a
model rocket? Specifically, if you want to add telemetry to a model rocket,
is it OK to put it in the nose, or is it better to keep the weight low in
the fuselage?

Thanks,

Bob

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2001\02\10@152324 by Sean H. Breheny

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Hi Bob,

Interesting that you should ask such a question  relatively soon after I
posted the URL for my rocket altimeter project (
http://www.rocket-roar.com/rap/alt.html )

The rule of thumb is that you want the center of gravity forward of the
center of pressure by at least 1, preferably 3, body tube diameters. So,
adding weight to the nose (without changing the shape or size of the
rocket) should actually make it more stable, not less. So, if you are
working with a rocket which can safely be assumed to be stable, you do not
need to do any further tests.

If you are designing from scratch, one method that is said to work well for
small rockets (less than 5 feet long or 4 inches diameter or so, would be
my guess) is to use a paper cutout of the silhouette of the rocket
(including fins), and find the cutouts's center of gravity. This usually is
a good approximation to the real rocket's center of pressure.

For more information, you can try searching Barrowman Equations, or center
of pressure. One link I got was:

http://www.execpc.com/~culp/rockets/Barrowman.html

Sean


At 12:02 PM 2/10/01 -0800, you wrote:
{Quote hidden}

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2001\02\10@170023 by Andrew Warren

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Bob Blick <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU> wrote:

> Does anyone know what the rule of thumb regarding center of
> gravity on a model rocket? Specifically, if you want to add
> telemetry to a model rocket, is it OK to put it in the nose, or is
> it better to keep the weight low in the fuselage?

Bob:

A crosswind (or any airflow with a lateral component, relative to the
rocket) is going to act at the rocket's center of pressure.  That
force, though, will make the rocket pivot about its center of
gravity.

If the CP is behind (i.e., lower than) the CG, the rocket self-
corrects:  It tends to point itself into the wind.  If, on the other
hand, the CP is in front of of the CG, the effect of a crosswind is
amplified and the rocket tumbles.

It's for precisely this reason that modern mid-engine sports cars
have most of their body area (as viewed from the side) toward the
rear.  In the old days, when engines were mounted up front, it was
easy to keep the CP behind the CG; in fact, it was almost impossible
NOT to.  Now that the CG has moved so far back, designers must go to
a lot of trouble to maximize the area at the rear of the car.

The integrated wing at the rear of an NSX, for example, performs
practically no function in terms of aerodynamic downforce.  What it
DOES do, however, is provide an excuse for the vertical wing SUPPORTS
on the fenders, which add a fair amount of area to the place where it
does the most good: as far behind the CG as possible.

The C5-and-newer Corvettes do the same thing, although (to my eye, at
least) the huge ass on a new Corvette looks substantially less
elegant than the NSX wing.

Anyway... The answer is:  Put the weight at the nose.

-Andy


=== Andrew Warren - EraseMEfastfwdspam_OUTspamTakeThisOuTix.netcom.com
=== Fast Forward Engineering - San Diego, California
=== http://www.geocities.com/SiliconValley/2499

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2001\02\10@170651 by David VanHorn

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>
>The integrated wing at the rear of an NSX, for example, performs
>practically no function in terms of aerodynamic downforce.  What it
>DOES do, however, is provide an excuse for the vertical wing SUPPORTS
>on the fenders, which add a fair amount of area to the place where it
>does the most good: as far behind the CG as possible.


BRING BACK TAILFINS! :)
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2001\02\10@172147 by trm

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AMEN, I really liked my '61 Hawk.

Ted

David VanHorn wrote:

{Quote hidden}

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2001\02\10@185623 by Bob Ammerman

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For a rocket to be stable the center of gravity must be forward of the
center of the aerodynamic forces (which has a name but it escapes me).

Note that luckily, as the engine burns, the tail of the rocket gets lighter
and stability improves.

However, this isn't always lucky: if the rocket starts off unstable and
wobbles all over and then turns stable as the engine lightens who knows
where it is pointing when it does go stable.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\10@234332 by Bob Blick

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Thanks for the tips, if it survives(scary picture of the one that blew
apart, Sean) I'll post the results!

-Bob

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2001\02\11@002504 by Sean H. Breheny

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Hi Bob,

Thanks, I'll consider that a compliment :-) I think it will survive, just
don't take your engines out into the cold if they have been stored at a
much warmer temperature (at least, that seems to be the consensus of what
caused my problem). What kind of instrumentation will you be flying?

Sean

At 08:41 PM 2/10/01 -0800, you wrote:
>Thanks for the tips, if it survives(scary picture of the one that blew
>apart, Sean) I'll post the results!
>
>-Bob
>
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2001\02\11@120456 by Bob Blick

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> What kind of instrumentation will you be flying?
It's not my project, just one I'm helping with. First(and hopefully not
last) flight will have a camera.

-Bob

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2001\02\11@121325 by David VanHorn

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At 09:04 AM 2/11/01 -0800, Bob Blick wrote:
> > What kind of instrumentation will you be flying?
>  It's not my project, just one I'm helping with. First(and hopefully not
>last) flight will have a camera.

What size motors are you running?

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2001\02\12@085402 by Wynn Rostek

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Bob,

Keep the center of gravity 1.5 to 2.0 body diameters below the center of
pressure.

Wynn Rostek


> Hi Everyone,
>
> Does anyone know what the rule of thumb regarding center of gravity on a
> model rocket? Specifically, if you want to add telemetry to a model
rocket,
> is it OK to put it in the nose, or is it better to keep the weight low in
> the fuselage?
>

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2001\02\12@175914 by Russell McMahon

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> Keep the center of gravity 1.5 to 2.0 body diameters below the center of
> pressure.


No, above! :-)

RM

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2001\02\12@180910 by David VanHorn

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At 11:49 AM 2/13/01 +1300, Russell McMahon wrote:
> > Keep the center of gravity 1.5 to 2.0 body diameters below the center of
> > pressure.
>
>No, above! :-)

Definitely above! (As in twoard the pointy end)

That would have been amusing.
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2001\02\14@081853 by Wynn Rostek

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> > Keep the center of gravity 1.5 to 2.0 body diameters below the center of
> > pressure.
>
>
> No, above! :-)
>
> RM
>
Si, Too early in the morning!

WR

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2001\02\14@093157 by Tom Handley

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  Sean, when I was a kid (about 100 years ago ;-) and just starting out
with Estes rockets, to test for stability and find the CP, we used a coat
hanger. We bent it in the shape of a caliper and sharpened the edges. This
was clamped to the rocket using rubber bands. Then we got on our bicycles
and went to a nearby steep hill and rode down real fast. While not a wind
tunnel, it worked fairly well ;-)

  BTW, I recall from the old Estes tech notes, that the CP should be at
least 1.5 times the body diameter behind the CG. We tried for at least 2-3x.

  - Tom

At 03:23 PM 2/10/01 -0500, Sean H. Breheny wrote:
{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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