The General Grant and OpenRocket Weirdness...?

[DAllen]

This is a "is it me or is this right?" thread...

I have a 3 tube fin rocket where OR seems to simming it super low for the motors I have picked out. Years ago I had a 4" Solar Warrior that was 6# and about 1/3 again as long as this one and usually flew to about 2k on a J350. This rocket is 3.5# and sims to 1370' on the same motor which to me seems ridiculously low. Am I missing something in my OR file here? Or are tube fin rockets that draggy? For comforts sake, I put an alt in it for the first few flights just because I am dubious of the optimal delays I am getting here.

 

[SolarYellow]

I'm not a tube fin guy so I haven't bothered to remember all the details, but I recall some discussion about OR doing some weird things with tube fins. Try adding three more of them and see how it changes.

Then read backward from this post: https://www.rocketryforum.com/threa...lable-for-download.177835/page-2#post-2391961

 

[Buckeye]

I am not a tube guy either, but 1370' ft seems awfully low. Some quick runs in Thrustcurve indicates that an average Cd = 2.5 is needed for that altitude. Normal fin rockets use Cd = 0.6 for a reasonable simulation.

Good idea for altimeter recovery at this point.

 

[DAllen]

Going to start with a H242 I am picking up at Airfest...we'll see what happens and adjust the sim from there. If I can get a decent flight or two I will likely remove the alt I have in there. I just intend for this to be single deploy with a chute release.

 

[Tractionengines]

YES there IS an issue with Tubefins In. 22.2... it was working well in a Bata version. BUT when released a gremlin snuck into the code that inversely changes CD compared to what it should. (It's much more involved, and not a quick fix.)

The estimates in 15.03 is reasonably close, if you still have that... Or I can tell you the Bata version where it was working if you can find that...

 

[neil_w]

YES there IS an issue with Tubefins In. 22.2... it was working well in a late Bata version. BUT when released a gremlin snuck into the code that inversely changes CD compared to what it should. (It's much more involved, and not a quick fix.)

The estimates in 15.03 is reasonably close, if you still have that... Or I can tell you the Bata version where it was working if you can find that...

This is correct. Known bug that will be fixed in the next release. Sorry about that.

 

[Tractionengines]

I played with your SIM in 15.05 & an older 22.2 bata were the logic went in the right direction with changes. And seemed reasonable...my guess is you'll be about 2/3 of the way from 22.2 (under estimated) to 15.3 (over estimated). NO guarantees of anything, but see if it matches your altimiter readings...

H242
15.05, Altitude: 1357ft
15.05, Optimum delay: 7.27sec
22.2 Beta, Altitude: 1003ft
22.2 Beta, Optimum delay: 5.82sec

I211
15.05, Altitude: 2347ft
15.05, Optimum delay: 8.67sec
22.2 Beta, Altitude: 1599ft
22.2 Beta, Optimum delay: 6.92sec

J350
15.05, Altitude: 3188ft
15.05, Optimum delay: 9.77sec
22.2 Beta, Altitude: 1997ft
22.2 Beta, Optimum delay: 7.39sec

Have fun. Nice looking rocket.

 

[DAllen]

Thank you!!

That’s very helpful

 

[Tractionengines]

Did General Grant fly yet? Curious how actual apogee matches up to any of the sims...

 

[DAllen]

Did General Grant fly yet? Curious how actual apogee matches up to any of the sims...

It did! I flew a I218 and it went to 1517 which I think matches up nicely.

 

[Tractionengines]

Wow... just threw this into the 23.09 Beta with updated Tubefin calcs.... the I218 SIMS to 1503ft where your actual was 1517... that is Very Promising.

 

[rfjustin]

It did! I flew a I218 and it went to 1517 which I think matches up nicely.