Open Rocket Lateral Distance Error

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TomSmith58

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Hi folks,
Long time no hear. I was trying out open rocket on a design that is much like a Wildman Blackhawk 38 and I find that Open Rocket isn't simulating the lateral distance correctly. It shows that there is very little drift during ascent, 20,000 ft drift during descent up to parachute deploy and then very little drift after deployment. Is it something in my file or is it a bug?
Thanks
Tom

View attachment Wild Aspirations 38.ork
 
Did you set the simulation parameters to your local field? I don't believe there is a way to set the winds aloft for sims, but you can set your location, average windspeed, rail length etc.
 
I did. I set the local field lat and long, the altitude, picked 4 mph wind speed. This doesn't explain why I get next to no drift during ascent, 4 miles drift during descent with no chute, and then next to no drift with the chute from 800 ft on down.
Tom
 
4 miles drift during descent with no chute

This is the part that is messed up. This drift should be nearly zero, like the ascent. Looks like OR can't deal with a ballistic descent trajectory???

I changed the wind speed to zero. Same result.

I changed your parachute deploy to apogee instead of 800 ft. That looks better.

If you add another parachute to simulate dual deploy, that should work fine.

I don't see anything else out of the ordinary in your design, other than your launch site is in China :). 177 West (not East) puts you in California.
 
I did. I set the local field lat and long, the altitude, picked 4 mph wind speed. This doesn't explain why I get next to no drift during ascent, 4 miles drift during descent with no chute, and then next to no drift with the chute from 800 ft on down.
Tom
Watch you units. Your wind speed was 4.something meters/per second or ~10 mph....

This still does not explain the drift. It actually does correspond to the inverse of the distance from apogee....

Could be a programing error.

Bob
 
On further review of the angle of attack and your rocket orientation, I believe that since you do not break up the symmetry of the rocket for the descent, the rocket is gliding down at a stable angle so it has considerable horizontal velocity! Do a proper apogee deployment to break up the symmetry and you rocket should not move so far sideways......

Bob
 
On further review of the angle of attack and your rocket orientation, I believe that since you do not break up the symmetry of the rocket for the descent, the rocket is gliding down at a stable angle so it has considerable horizontal velocity! Do a proper apogee deployment to break up the symmetry and you rocket should not move so far sideways......

Bob

Yes, I mentioned this above. An apogee parachute deploy looks normal.

Gliding at a stable angle for 4 miles??? Is that even possible? This is a basic, 3FNC coming in ballistic, not a glider.

This also looks suspicious to me. The peak vertical velocity after apogee is greater than the terminal velocity of the ballistic rocket:

Capture3.jpg
 
This is a basic, 3FNC coming in ballistic, not a glider.
OR's results with an undeployed descent have always been weird. Put in a tiny drogue chute if you want to simulate drogueless.
 
Yes, I mentioned this above. An apogee parachute deploy looks normal.

Gliding at a stable angle for 4 miles??? Is that even possible? This is a basic, 3FNC coming in ballistic, not a glider.

This also looks suspicious to me. The peak vertical velocity after apogee is greater than the terminal velocity of the ballistic rocket:

View attachment 278622

Anything is possible in a sim where the rocket is idealized. I think the ballistic velocity of the rocket should be much higher. My guess is that after reaching some velocity after apogee, a "gust" of wind changed the attitude enough to cause the rocket to glide in a stable configuration converting some of the vertical velocity into horizontal velocity hence the large cross range velocity. When the main was deployed, the chute took over. The mass distribution in a real rocket may not give such nice gliding behavior.

In reality, you would not come in ballistically from several mile up and pop a main at 800' as that would surely strip the chute. You would have an apogee event to destroy the symmetry via a drogueless or drogue deployment to reduce the descent velocity to the 100 fps range or so.

Bob
 
Anything is possible in a sim where the rocket is idealized. I think the ballistic velocity of the rocket should be much higher. My guess is that after reaching some velocity after apogee, a "gust" of wind changed the attitude enough to cause the rocket to glide in a stable configuration converting some of the vertical velocity into horizontal velocity hence the large cross range velocity. When the main was deployed, the chute took over. The mass distribution in a real rocket may not give such nice gliding behavior.

Bob

Nope, even with no wind in the simulation, the result is nearly the same.

No need to try to rationalize the simulation result any further. It is simply a programming error. OR expects a deployment of some kind around apogee and doesn't properly calculate a ballistic trajectory. Seems like this has been a known problem as noted by mikec.
 
The sim's can definitely get ... interesting. I had one where the rocket drifted twice its altitude, even in no wind. Adding .2 oz. of noseweight changed that to 100 feet in the other direction from the launch point. I think there was even one where the rocket did a loop, or certainly a bunch of S curves. Some of this is even possible in real life -- really interesting behavior in the sim certainly points to the possibility of somewhat interesting actual behavior.
 
Some of this is even possible in real life -- really interesting behavior in the sim certainly points to the possibility of somewhat interesting actual behavior.

Not in this case.

I have similar WM BH38 and studied it extensively in Rocksim, RASAero, and Open Rocket. I think OR tries to be too fancy with AOA and 3D flight dynamics, and simple scenarios like this one can quickly get out of whack.
 
The sim's can definitely get ... interesting. I had one where the rocket drifted twice its altitude, even in no wind.

This scenario is easily possible, if the parachute does not evenly spill the air from beneath or in combination with a spill hole the gliding ability can actually be improved.
 
This scenario is easily possible, if the parachute does not evenly spill the air from beneath or in combination with a spill hole the gliding ability can actually be improved.

In real life, yes. In simulation, no. I am pretty sure the hobby rocket softwares do not attempt to model parachute gliding dynamics.
 
In real life, yes. In simulation, no. I am pretty sure the hobby rocket softwares do not attempt to model parachute gliding dynamics.

Personally the main things that concern me with sim programs like OpenRocket and Rocksim are the A) Stability , B) Descent rate under drogue, and C) Descent rate under main. Most everything else is just too variable due to flying field conditions, and above ground wind variables and without flight data to compare to the sims there is little if any way to adjust and correct. So far I have found OpenRockets sims when set to the average conditions of my flying field to be within acceptable limits.
 
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