TARC Rocket Simulation Difficulty

Ez2cDave

Well-Known Member
TRF Supporter
Thanks for the informative responses. I learned a lot.

One question I have is how to quantify the effects of temperature and elevation on the final altitude achieved. The Northville team minimized drag on their design to limit these variables but did not detail how to model them in their mass-altitude plot. Are temperature, elevation, and humidity effects low enough to ignore?

Thanks,
Walter
Altitude will have the greatest effect, in my opinion . . .

As Altitude increases, Air Pressure decreases . . . As Air Pressure decreases, Drag decreases . . . As Drag decreases, Altitude INCREASES.

As Altitude increases, Air Pressure decreases and Descent Rate INCREASES . . . As Descent Rate increases, Duration decreases . . . As Duration decreases, the Area of the Parachute must INCREASE to offset the change.

An extreme example would be launching a model from Death Valley ( 122 deg F and 282.2 feet BELOW Sea Level ) and then launching the SAME model from the top of Pikes Peak, in Colorado ( 22.5 deg F and 14,115 feet ABOVE Sea Level ) . . . The difference in performance would be quite large !

Dave F.

Steve Shannon

Well-Known Member
TRF Supporter
Thanks for the informative responses. I learned a lot.

One question I have is how to quantify the effects of temperature and elevation on the final altitude achieved. The Northville team minimized drag on their design to limit these variables but did not detail how to model them in their mass-altitude plot. Are temperature, elevation, and humidity effects low enough to ignore?

Thanks,
Walter
No. Look into “density altitude” to get an idea how much effect they can have.

cls

Well-Known Member
Are temperature, elevation, and humidity effects low enough to ignore?
No, those account for the final 5% to 8% of the accuracy of the simulation. Assuming the sim weights, sizes, surface prep, etc is all accurate.

Hopefully you took notes of meteorological conditions for each of the test launches. Then, you can back out the actual Cd to make each simulation flight match the actual...

Then you'll be ready to adjust the ballast and chute choke to perfectly nail the altitude and time for the qualifying flight, and with different atmospherics on the day of the TARC finals. Good luck!

diyaerospace

Well-Known Member
Hello,

I looked into density altitude but don't understand a way to convert density altitude into apogee height.

I previously used the simulation to find how much effect weather conditions have on the rocket's flight but as everyone mentioned this doesn't represent reality.

In this presentation, Trip gives some numbers regarding how much these variables affect drag. (slide 19)

I don't know a formula to find the exact effect of drag on the rocket's flight other than comparing simulation results.

Thanks,
Walter

diyaerospace

Well-Known Member
Does anyone know a way to convert atmospheric conditions into altitude lost or gained in a rocket flight?

Steve Shannon

Well-Known Member
TRF Supporter
Does anyone know a way to convert atmospheric conditions into altitude lost or gained in a rocket flight?
In your simulations put in the atmospheric pressure that you will be flying at for nationals. Simulate until you fly to the target altitude.
Then, without changing anything else, put in the actual air pressure for the location where you are doing your practice flights. Make note of the change in altitude and compensate. Now here's where it gets tricky. Air pressure at airports is normalized to 29 in. Hg. So you are not always getting the actual air pressure. Right now I looked up the "aviation weather" for my local airport, Bert Mooney Airport (BTM). It says the air pressure is 29.80 in. Hg. But, I have a handy little app on my phone called My Altitude that uses the built in sensor in my iPhone to report the actual pressure. Guess what, it says 24.3346 in. Hg. That's a big difference and my air density is proportional to it.

So, I'm at 5511 feet, give or take. My air density will almost always be less than the finals at Manassas. Therefore, flights at my altitude will fly higher because there's less drag. I have two problems.
Problem 1. I have to fly a good qualifying flight at my elevation to 800 feet. I concentrate on that first, because if I don't have a good qualification flight I won't fly at finals. For Problem 1, my air pressure relative to Manassas is meaningless. Actually, worse than meaningless; it's a distraction.

Assume I am successful in getting a good score and I'm pretty confident I'm going to finals. Now I have to solve the second problem. I have to be ready when I get to finals to make that same rocket fly to 800' at Manassas, which typically has denser air. So, everyday I fly a few practice flights. Because atmospheric conditions change frequently I get the actual air pressure for Manassas and the actual air pressure for my location and I use RockSim or Open Rocket to compare simulations to determine how high I have to fly at my elevation in order for my rocket to fly to 800 feet at finals.

diyaerospace

Well-Known Member
In your simulations put in the atmospheric pressure that you will be flying at for nationals. Simulate until you fly to the target altitude.
Then, without changing anything else, put in the actual air pressure for the location where you are doing your practice flights. Make note of the change in altitude and compensate. Now here's where it gets tricky. Air pressure at airports is normalized to 29 in. Hg. So you are not always getting the actual air pressure. Right now I looked up the "aviation weather" for my local airport, Bert Mooney Airport (BTM). It says the air pressure is 29.80 in. Hg. But, I have a handy little app on my phone called My Altitude that uses the built in sensor in my iPhone to report the actual pressure. Guess what, it says 24.3346 in. Hg. That's a big difference and my air density is proportional to it.

So, I'm at 5511 feet, give or take. My air density will almost always be less than the finals at Manassas. Therefore, flights at my altitude will fly higher because there's less drag. I have two problems.
Problem 1. I have to fly a good qualifying flight at my elevation to 800 feet. I concentrate on that first, because if I don't have a good qualification flight I won't fly at finals. For Problem 1, my air pressure relative to Manassas is meaningless. Actually, worse than meaningless; it's a distraction.

Assume I am successful in getting a good score and I'm pretty confident I'm going to finals. Now I have to solve the second problem. I have to be ready when I get to finals to make that same rocket fly to 800' at Manassas, which typically has denser air. So, everyday I fly a few practice flights. Because atmospheric conditions change frequently I get the actual air pressure for Manassas and the actual air pressure for my location and I use RockSim or Open Rocket to compare simulations to determine how high I have to fly at my elevation in order for my rocket to fly to 800 feet at finals.

Thank you so much,
This is what I was looking for.

diyaerospace

Well-Known Member
Hey everyone!

I know it has been a while since my last post but I thought I would tell you how our qualification flights went.

Because of Southern California's large amounts of rain in spring, we couldn't do much more test flying.

We attempted our qualifications at Santa Fe Dam on April 3rd. It was very windy (around 10-15 mph). Our first flight had a score of 42. Our second flight was disqualified because the parachute on the altimeter section got tangled which led to our egg breaking. The altitude achieved on this flight was 844 ft.
We were able to fix the damage and fly again. Our score for this flight was 160. The wind was now very strong and caused severe weathercocking but everything was recovered successfully.

Unsurprisingly we did not make it to finals. However, I learned a lot and am interested in competing next year.

Thank you so much to everyone that offered their advice and help on this project.
I made a lot of mistakes in the design and testing of my rocket and I hope others can learn from this thread.

Walter