OpenRocket simulation drifts far when wind is 0 mph?

[Dan Kusmer]

I set the average windspeed to 0 mph. The simulation has the apogee at 4531 ft where the ejection charge fires. The ascent is near vertical. I delay chute decent deployment until 500 ft (Jolly Logic). The simulation has the rocket landing 6265 ft away from the launch site in 97 seconds after launch. That calculates into a horizontal drift velocity of 44 mph as if a 44 mph horizontal wind velocity is pushing against the rocket. Where is this coming from?

 

[dhbarr]

Try setting a couple of those values to "1" and see if it stabilizes? Just a wild guess, not even a hunch really.

 

[Dan Kusmer]

Thanks. I tried your suggestion and although it did effect the results slightly I'm still getting huge horizontal velocities (around 44 mph).

 

[dhbarr]

It's very strange. If I delete your fins and substitute tubefins, I land within a couple of feet of the pad. But no matter how else I simplify the sim, no substantial improvement.

 

[UhClem]

I played around with it and found a large lateral acceleration just after apogee. Then I looked and saw that you have no deployment at all until 500 feet. When I changed that to apogee...

 

[Dan Kusmer]

Thanks for both of your replies. I think I see what's going on. I set the ejector charge on the motor to trigger at apogee, thinking that the nose cone, shock chord, and the Jolly Logic with the chute would eject at apogee and that the Jolly Logic would keep the chute bound by a rubber band and not allow it to open until 500 ft.

Since there is no deployment at apogee (only an ejection charge), OR must treat the nose cone as still attached to the rocket with the shock cord, Jolly Logic, and chute still inside the rocket. OR must then treat the rocket like a glider and it glides very fast (44 mph horizontally).

To test this I added a very small lightweight streamer so as not to effect the rocket weight or cg, assigned a very high cd (drag coefficient), and deployed it at apogee. The steamer deployed at apogee and the chute at 500 ft. With 0 mph wind velocity the rocket landed only several feet from the launch site.

 

[dhbarr]

Thanks for both of your replies. I think I see what's going on. I set the ejector charge on the motor to trigger at apogee, thinking that the nose cone, shock chord, and the Jolly Logic with the chute would eject at apogee and that the Jolly Logic would keep the chute bound by a rubber band and not allow it to open until 500 ft.

Since there is no deployment at apogee (only an ejection charge), OR must treat the nose cone as still attached to the rocket with the shock cord, Jolly Logic, and chute still inside the rocket. OR must then treat the rocket like a glider and it glides very fast (44 mph horizontally).

To test this I added a very small lightweight streamer so as not to effect the rocket weight or cg, assigned a very high cd (drag coefficient), and deployed it at apogee. The steamer deployed at apogee and the chute at 500 ft. With 0 mph wind velocity the rocket landed only several feet from the launch site.

Ahh yes.
https://www.rocketryforum.com/threads/open-rocket-and-jolly-logic-chute-release.132114/#post-1550457and
https://jollylogic.com/chute-release-for-rocketry-ready-for-testing/cover adding a destabilizer of some kind, but I had forgotten. Thanks for the reminder!

 

[Dan Kusmer]

Wow, great links. Just as the links suggested, I adjusted the cd of my massless streamer to match the decent tumble rate (74 fps) from previous launches.