Wildman Gizmo... different results in OpenRocket and Thrustcurve.org

[majordude]

I understand that OpenRocket and Rocketsim have challenges with short stubby rockets. So maybe that's what the deal is or I just don't know what the hell I am doing which is entirely possible (and even likely).

In OpenRocket, a Cesaroni J250 will go 3,600' and need a 12 second delay. On Thrustcurve, it will go 2,482' and need an 8 second delay. Is either one more correct than the other? :confused2:

 

[bill2654]

Did you enter the correct drag in thrustcurve?

 

[majordude]

Did you enter the correct drag in thrustcurve?

D'oh!

OpenRocket says the drag is 0.31 (which seems low to me). Updated my rocket and now thrustcurve.org is much closer to OR. 3,628' with a 10 second delay.

Thanks!

 

[bill2654]

My drag coefficient is .25 and my weight is 102 oz. I got 3010ft with 14 sec delay in OR. Thrust curve tells me that the j250 is too slow off the rai

 

[majordude]

My drag coefficient is .25 and my weight is 102 oz. I got 3010ft with 14 sec delay in OR. Thrust curve tells me that the j250 is too slow off the rai

Hmmm. I'm using a file I found in a different thread. The rocket is 56.4 oz. without a motor. Maybe your rocket is sanded smoother than the one in this file.

 

[bill2654]

I just found out that there is a big difference in "regular paint" and "smooth paint" finish, like hundreds of feet difference in altitude. I would like to know the difference. Is rattle can paint considered regular or smooth? I know my finishes are not pollished but they are pretty smooth. Anyone else have any thoughts on OR finish definitions?

 

[WILDMANRS]

D'oh!

OpenRocket says the drag is 0.31 (which seems low to me). Updated my rocket and now thrustcurve.org is much closer to OR. 3,628' with a 10 second delay.

Thanks!

How close is the new nose cone in weight?

 

[majordude]

I just weighed everything this morning and I'll update my OpenRocket file as I build it.

The new cone appears significantly heavier than the old cone. My OpenRocket file (that I downloaded from another thread) indicates the old cone weighs 12.3oz.

The new one (without adhesives) is 488.9g (cone) + 26.8g (metal tip) + 285.9g (shoulder) = 801.6g or 28.27oz.

Hmmm. I was going to add 8oz - 16oz to the cone in order to use 1-3 grain 54mm motors but maybe I don't have to now! (Please, oh please, oh please!) :clap:

Can anyone confirm the weight of the old cone without added weight? (Is is close to 12.3oz.?) And can anyone confirm that the "improved" Gizmo doesn't need added weight for short grain motors?

Update 1: Added OpenRocket file... View attachment Improved Gizmo.rkt

Okay, this isn't the actual rocket as I have not assembled it so it is not the final weight (with all the adhesive and hardware) but it looks like it has 0.715 cal stability with a Cesaroni J250SK-12.

Update 2: Now OpenRocket reports 3,384' on that J250 and thrustcurve says 3,385'! Can't get closer than that! :fly:

Update 3: Grrr! I looked at the first OpenRocket file and the body tube is too long. Whoever did the original file used 17.25". I believe mine is 16". Here is the corrected file: View attachment Gizmo with New Cone.rkt It shows 0.596 cal stability with the J250. Is that good enough without added weight?

 

[bill2654]

You have over ridden the CG on every component, even centering rings. Your fins are showing no weight. Im no expert but I think when your just weighing the parts, you just over ride the mass. I could be wrong.

 

[majordude]

The CG got overridden by default I guess. I unchecked all the boxes on my local file and it doesn't change anything. The fins do indeed have a mass override (494g for the three combined).

 

[CarVac]

D'oh!

OpenRocket says the drag is 0.31 (which seems low to me). Updated my rocket and now thrustcurve.org is much closer to OR. 3,628' with a 10 second delay.

Thanks!

Short rockets tend to have lower drag coefficients. Something like a Mean Machine has a terrible drag coefficient, because drag coefficient has to be multiplied by frontal area to give "drag area", which is the actual dragginess of the rocket.