Changes in apogee due to changes in launch site levation

I am team mentor for a TARC team. This is our first year to participate in TARC. We had a really good day when we flew our qualification flights and are hopeful for an invitation to the finals. In that hope my team is preparing to the best of their ability.

They have created a mass budget and are continuing to learn to manage the apogee of their rocket by manipulating mass and drag. They are testing the effects of wind velocity and temperature changes. Their goal is to gather empirical evidence through experience that they then compare to theoretical and simulated results. They have about 25 flights under their belt already and will be flying more.

The one metric we are unable to test is the change in apogee due to changes in launch elevation. We have two fields we launch from. One has an elevation of 1900 feet and the other is at 1700 feet. We found a .pdf file posted on the NAR TARC Yahoo forum regarding flight testing that advises a change of 3.4% per 1000 feet of elevation change. By my best estimate the launch elevation of the field in VA is 600 feet. This is a net change of 1300 feet from our home field.

3.4% * 1.3K feet = 4.42% expected change in elevation. 850 feet * .0442 = 37.57 feet of elevation change due to the change in air density. Our RockSim file estimates a change of 14 feet. My dilemma is which is more accurate. Our current approach is to split the difference and assume an elevation change of about -25 feet.


Is there any advice anyone can offer the team beyond what we are doing? Is there any test the team can conduct that might help them make a more informed choice in managing their rocket?

I misread the original file. The change due to launch elevation is 3.4% per1000 to the AIR DENSITY - not apogee.

With that said, I would still appreciate anyone's comments on this issue.

The correspondence between density and apogee is not 1-1. Density will proportionally scale your drag force at a given speed, which will slow you down faster. But as you slow down, drag goes down too (and drag is proportional to the square of speed, i.e. twice the speed is 4x the drag). Also, drag will be the main force at high speed but gravity will dominate at lower speeds.

So anyway I'd expect a 3% density change to result in a less than 3% apogee change - the Rocksim result seems reasonable.

Also, keep in mind that altitude is not the only factor in air density - humidity and temperature also play an important role. I'll leave it to your kids to do the Googling on that one