Square Rocket ... Analysis vs. Measured Altitude

[KerryQuinn]

I built the "4 Square" rocket from Newway Spacemodels.
This rocket uses a 1.375"x1.375" square body tube, a pyramid nose "cone", and four square tube fins. I don't know if this is something that can be anaylzed in Rocksim or not...? (I don't have Rocksim.)

Instead, I tried to use WRASP 2.1... but you have to enter a body tube "diameter" and a drag coefficient.

The square tube "width" is 1.375"
The square tube diagonal is 1.94"
I also used the average of these two which is 1.66"
The rocket weighs 2.93 oz without an engine.
I also flew it with the QuEST how-high altimeter which adds 0.25 oz.

With the altimeter, I measured an altitude of 194 feet.

I ran simulations between drag coeff of 0.75 and 1.25.

Analysis using a QuEST B6-4 engine showed best agreement at:

WITH altimeter:
drag coeff of 0.9 and 1.66" diameter gives a prediction of 199 feet

Without altimeter it would have been lighter and gone to...
With NO altimeter:
drag coeff of 0.9 and 1.375" diameter gives a prediction of 243 feet
drag coeff of 0.9 and 1.66" diameter gives a prediction of 220 feet
drag coeff of 0.9 and 1.94" diameter gives a prediction of 197 feet

I don't have a wind tunnel so I don't know if the estimate of 0.9 is good or not for this rocket. It makes sense to me that the average diameter would give the answer closest to my measured altitude. I have also looked at drag coefficients for standard nose cones vs pyramids with an aspect ration near the "4 square" - seems that they are not that different.


Anyone out there able to shed more "science" on my method?

 

[bobkrech]

First your rocket is square not round so you need to get an equivalent diameter. This can be pased on either one of the following assumptions.

0.) Simply use L = D or find the equivalent diameter based on either

1.) Perimeter = cicumference; or 2.) Area of square = area of circle.

for 0.) L = D = 1.375"

for 1.) P = 4*L = Pi * D: D = 4*L/Pi = 1.75"

for 2.) A = L^2 = Pi*R^2: D = 2*R = 2* sqrt (L^2/Pi) = 1.96"

Now for each assumption, vary Cd until you calculate the observed apogee. You will need a higher Cd for 0.) than either 1.) or 2.) to match the data.

Cd is an average drag coefficient and most sims use a very simplified drag model. I don't have wrasp running on this computer, but 1.375" with a Cd of ~0.9 probably works for this rocket assuming the altimeter is accurate, and at 192' this can be questionable with some altimeters (the high altitude variety.)

Bob