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Computing Center of Pressure

The document discusses computing the center of pressure (CP) of a rocket using the Barrowman equations to determine stability. The CP is the distance from the nose tip to the balance point, and for stability should be aft of the center of gravity (CG). The CG can be found by balancing the rocket. The calculated CP should be at least one rocket diameter aft of the measured CG for "one caliber stability". Terms used in the equations to calculate the CP are defined.

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José
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222 views3 pages

Computing Center of Pressure

The document discusses computing the center of pressure (CP) of a rocket using the Barrowman equations to determine stability. The CP is the distance from the nose tip to the balance point, and for stability should be aft of the center of gravity (CG). The CG can be found by balancing the rocket. The calculated CP should be at least one rocket diameter aft of the measured CG for "one caliber stability". Terms used in the equations to calculate the CP are defined.

Uploaded by

José
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Computing Center of

Pressure
The Barrowman equations permit you
to determine the stability of your
rocket by finding the location of the
center of pressure (CP). The value
computed is the distance from the tip
of the rocket's nose to the CP. In order
for your rocket to be stable, you would
like the CP to be aft of the center of
gravity (CG).

The computation of CP isn't as hard


as it looks at first. Check out the
spreadsheet example at the bottom of
this page.

You can find the CG of your rocket by


simply finding the balance point after
loading recovery system and motor.
(Literally - balance the rocket on your
hand - or finger - and that's the CG).
You can then measure from the tip of
the rocket's nose to the CG. The
calculated CP distance should be
greater than the measured CG
distance by one rocket diameter. This
is called "one caliber stability".

Terms in the equations are defined


below (and in the diagram):

LN = length of nose
d = diameter at base of nose
dF = diameter at front of transition

dR = diameter at rear of transition

LT = length of transition

XP = distance from tip of nose to front of transition


CR = fin root chord

CT = fin tip chord

S = fin semispan

LF = length of fin mid-chord line


R = radius of body at aft end

XR = distance between fin root leading edge and fin tip leading edge parallel to body

XB = distance from nose tip to fin root chord leading edge

N = number of fins

Nose Cone Terms


(C N) N = 2
For Cone: X N = 0.666LN
For Ogive: X N = 0.466LN

Conical Transition Terms


Fin Terms

Finding the Center of Pressure


Sum up coefficients: (C N)R = (C N) N + (C N) T + (C N)F

Find CP Distance from Nose Tip:

Who the Heck is Barrowman?


In March, 1967, James S. Barrowman of the National Aeronautics and Space
Administration's Sounding Rocket Branch submitted a document entitled 'The Practical
Calculation of the Aerodynamic Characteristics of Slender Finned Vehicles' as his
Master's thesis to the School of Engineering and Architecture of the Catholic University
of America. The document included, among other things, the simple algebraic method
described above, capable of determining the center of pressure of a rocket flying
subsonically and at small angles of attack to a high order of accuracy.

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