# Thread: Optimum Mass Calcs - 2.60" OD w/ AT G64

1. ## Optimum Mass Calcs - 2.60" OD w/ AT G64

I was curious to see what the optimal mass for a rocket with following givens:

Airframe OD: 2.60"
Cd: 0.7
Motor: AT RMS G64

Using a spreadsheet, and tweaking the weight until I got the highest value, the calculation came to 430 grams (15.17 oz) for the launch weight with an altitude of 535.9 meters (1,758 ft).

I have two questions:

1. Does this sound about right?
2. Is this the easiest way to calculate such things (does OR do this?)

Greg

2. Member
Join Date
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Location
SE Michigan
Posts
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OpenRocket does this.
On the toolbar, Select 'Analyze' and then 'Rocket optimization'

3. Thanks. I give that a go.

Greg

4. Greg- I trust your methodology better- there are no 'hidden' calcs and you are not depending on 'unseen code' for your parameters. Of course, I'm really old school and prefer it this way-so YMMV. Airfoiling and losing the lug may bring down your Cd a bit, maybe even to .65 ( I use an avg of .72 for my junk). Don't get me wrong- I luv OR and RockSim, but still like the long hand version better.

5. Thank you fyrwrxz. I am somewhat "old school" in this approach. I did some tests with wRASP and the optimal came in about 450 grams, and nearly the same altitude and seconds to apogee. So I know I am "in the ballpark" for the calcs, so I think I can proceed.

On the drawing board: a dual deployment rocket that is optimized for the G64. So the goal for the build is that the "without motor" weight will come in at about 300 grams.

Greg

6. Using the Simulate -> Optimal Mass -> for Max Altitude in WRASP32, it calcualted 10.655 oz. and esitmates an altitude of 1796 ft. That would be without the motor.

If you get the Cd down to .65, the weight drops to 10.408 and altitude goes up to 1882 ft.

7. Originally Posted by Handeman
Using the Simulate -> Optimal Mass -> for Max Altitude in WRASP32, it calcualted 10.655 oz. and esitmates an altitude of 1796 ft. That would be without the motor.

If you get the Cd down to .65, the weight drops to 10.408 and altitude goes up to 1882 ft.
I don't have that option in my wRASP32 (v2.1). Where did you get your version?

Greg

8. Originally Posted by GregGleason
I don't have that option in my wRASP32 (v2.1). Where did you get your version?

Greg
Mine is version 2.2.1. I don't know where I got it, but you can get it here http://quarkers.net/index.php?option...ents&Itemid=90

9. Member
Join Date
5th March 2009
Posts
273
Originally Posted by GregGleason
I was curious to see what the optimal mass for a rocket with following givens:

Airframe OD: 2.60"
Cd: 0.7
Motor: AT RMS G64

Using a spreadsheet, and tweaking the weight until I got the highest value, the calculation came to 430 grams (15.17 oz) for the launch weight with an altitude of 535.9 meters (1,758 ft).

I have two questions:

1. Does this sound about right?
2. Is this the easiest way to calculate such things (does OR do this?)

Greg

Try a credible range of drag coefficients to see the sensitivity of the optimal mass to the Cd. Normally the optimal mass has a significant range, but the corresponding altitude range is minor.

It's also helpful to look at how the figures change when you launch at 85 an 80 degrees.