Simulating short, wide rockets in OpenRocket

[DigBaddy]

Did you run a simulation with no cone at all, but where you moved the cg to obtain the same stability margin as shown when the simulation has the cone?

That's what @neil_w and others are proposing as the correct method.

View attachment 546514

That adjustment made no difference in my case; the result is the same as with the cone removed and CG left as-is since there's enough margin. But, thank you for pointing it out since I did not do that initially. I suspect that since mine aren't going that high, the difference in delay is not statistically significant; but if I started pushing things higher, then the cone-based sim may make the delay too short. It also makes me wonder who else could be ending up with short delays due to Thrustcurve sims (or are they not?) I will definitely need a better altimeter and more testing. Who wants to donate a pile of 38/240-480 motors to my testing?

 

[Starfire73]

I apologize if I missed it in the conversation, but how does one determine the size of dimensions to use? It seems to me like the cone is still a guess and isn't based on actual calculations - am I wrong?

I have a LOC Warlock which I kit bashed into a four fin rocket. Knowing it was a short/fat rocket my goal was to come as close to a stability margin of 1.0 without going over (which normally is not enough margin). I thought that might account for some of the base drag issues a short/fat rocket has. But that was just guessing. I launched it on a J270 and it did weather cock quite a bit (indicating possibly overstable); however, it was pretty windy and there's not a lot of initial thrust in the motor for a rocket of that weight.

 

[dhbarr]

I apologize if I missed it in the conversation, but how does one determine the size of dimensions to use? It seems to me like the cone is still a guess and isn't based on actual calculations - am I wrong?

I have a LOC Warlock which I kit bashed into a four fin rocket. Knowing it was a short/fat rocket my goal was to come as close to a stability margin of 1.0 without going over (which normally is not enough margin). I thought that might account for some of the base drag issues a short/fat rocket has. But that was just guessing. I launched it on a J270 and it did weather cock quite a bit (indicating possibly overstable); however, it was pretty windy and there's not a lot of initial thrust in the motor for a rocket of that weight.

It's pi-to-one length to body base OD, but I do three-to-one because lazy

 

[Banzai88]

I apologize if I missed it in the conversation, but how does one determine the size of dimensions to use? It seems to me like the cone is still a guess and isn't based on actual calculations - am I wrong?

I have a LOC Warlock which I kit bashed into a four fin rocket. Knowing it was a short/fat rocket my goal was to come as close to a stability margin of 1.0 without going over (which normally is not enough margin). I thought that might account for some of the base drag issues a short/fat rocket has. But that was just guessing. I launched it on a J270 and it did weather cock quite a bit (indicating possibly overstable); however, it was pretty windy and there's not a lot of initial thrust in the motor for a rocket of that weight.

The references are in post 10 from Apogee's Peak of Flight newsletter, links about half way through the post

https://www.rocketryforum.com/threa...de-rockets-in-openrocket.175924/#post-2345438

 

[bobbyg23]

I apologize if I missed it in the conversation, but how does one determine the size of dimensions to use? It seems to me like the cone is still a guess and isn't based on actual calculations - am I wrong?

I have a LOC Warlock which I kit bashed into a four fin rocket. Knowing it was a short/fat rocket my goal was to come as close to a stability margin of 1.0 without going over (which normally is not enough margin). I thought that might account for some of the base drag issues a short/fat rocket has. But that was just guessing. I launched it on a J270 and it did weather cock quite a bit (indicating possibly overstable); however, it was pretty windy and there's not a lot of initial thrust in the motor for a rocket of that weight.

pi x body tube diameter. so for a 7.6" rocket like the warlock, 3.14x7.6=23.864". 4" rocket would be 3.14x4=12.56"

 

[bobbyg23]

I apologize if I missed it in the conversation, but how does one determine the size of dimensions to use? It seems to me like the cone is still a guess and isn't based on actual calculations - am I wrong?

I have a LOC Warlock which I kit bashed into a four fin rocket. Knowing it was a short/fat rocket my goal was to come as close to a stability margin of 1.0 without going over (which normally is not enough margin). I thought that might account for some of the base drag issues a short/fat rocket has. But that was just guessing. I launched it on a J270 and it did weather cock quite a bit (indicating possibly overstable); however, it was pretty windy and there's not a lot of initial thrust in the motor for a rocket of that weight.

Wind + low initial thrust motor = weathercock.

 

[lakeroadster]

Wind + low initial thrust motor = weathercock.

Aggravated by excessive fin area.