Open Rocket and "Cg"

[BLKKROW]

Recently I finished up my Blackhawk 38.

So I filled her up with all the laundry and some weight to simulate the motor. Then tested the CG.

Now when I enter it into OR, it says "Its from the top of the stage", so does that mean nosecone or from the top of the body tube?

Cause no matter how I enter it, it is either directly on the CP, or way to far in the front.

Thanks

 

[kruland]

BLKKROW,

The Blackhawk 38 is a single stage rocket. I presume you modeled it in OpenRocket as a single stage - you can tell this from the component tree. It starts with "rocket" then has each of the stages under it. If it's only one stage, it's most likely named "sustainer".

Measure the location of the Cg from the tip of the nose cone. Then click on the "sustainer", press the Edit button, and enter the Cg & mass override information in the dialog.

If you have any doubts, post the ork file you're using and I can help more.

Kevin

 

[BLKKROW]

BLKKROW,

The Blackhawk 38 is a single stage rocket. I presume you modeled it in OpenRocket as a single stage - you can tell this from the component tree. It starts with "rocket" then has each of the stages under it. If it's only one stage, it's most likely named "sustainer".

Measure the location of the Cg from the tip of the nose cone. Then click on the "sustainer", press the Edit button, and enter the Cg & mass override information in the dialog.

If you have any doubts, post the ork file you're using and I can help more.

Kevin

I will look into that once I get home, I am currently at work for another 6 hours.

Thanks for the reply.

 

[BLKKROW]

So here is the OR file.

I entered in the CG, and the mass. Everything on this file is perfectly measured with digital calipers.

I just am getting the CG directly over the CP.


View attachment Blackhawk 38, Stock.rkt.ork

 

[T-Rex]

You have a lot of individual components with overrides. Did you create this file weighing each piece, or download it? When I clear the stage mass and cg overides, it has 3.7 calibers of stability.

 

[duckhead]

Keep in mind that in OR when you override the entire stage mass and Cg, this is *without* the motor and casing mass. This allows OR to adjust the total flight mass when different motor configurations are used, without you having to go tweak the overrides. Based on your initial comment that you weighed it including dummy motor mass, then your override value is now much too high. Remove the dummy motor mass, reweigh, and enter that value and the new Cg in the stage override.

Doug

 

[BLKKROW]

I am yet at work again, so I cannot pull up the file.

But the override mass option, is the correct mass for the rocket minus the motor. When I weighed the rocket also, it did not include any of the recovery items, it was just the basic rocket is this an issue?

I will have to look into it a little further when time allows.

 

[CarVac]

Maybe you went overkill on fillets and it indeed is unstable?

 

[BLKKROW]

Maybe you went overkill on fillets and it indeed is unstable?

I used the fillet technique that was in the instructions.

The only other thing I can think of us, I used Thurstcurve for the weight of the motor since I do not have it. So I assumed that weight is of the reload itself and not reload+casing. Did I assume correctly?

 

[kruland]

Weigh your rocket and measure the CG without the motor but with recovery gear. Use these measurements in the mass & cg override in OpenRocket. When you simulate in OpenRocket the correct (or mostly correct depending on cases, adapters, etc) motor mass will be added.

Kevin

 

[BLKKROW]

Weigh your rocket and measure the CG without the motor but with recovery gear. Use these measurements in the mass & cg override in OpenRocket. When you simulate in OpenRocket the correct (or mostly correct depending on cases, adapters, etc) motor mass will be added.

Kevin

Thats what I will do next, most likely will be done Sunday, maybe tomorrow inbetween classes.