Quantcast

RockSim questions

The Rocketry Forum

Help Support The Rocketry Forum:

ascastil

Well-Known Member
Joined
Jan 2, 2013
Messages
278
Reaction score
11
Just downloaded it. I found the Loc IV and did a launch with a G64. All I really found out is that it will fly on a G64. But wont my Cg and cp be different than whats already on the program? Im completely new to this, Ive always just built them and flown. I think my cg is different than whats listed. How do I change anything in the program and am I supposed to? Completely new to me and I have no idea really what Im doing, what to look for, and how to use that program I guess.

Im trying to learn things to get my L1 cert with an h123 and the LocIV.
 

Madison Alum

Well-Known Member
Joined
May 1, 2011
Messages
695
Reaction score
0
You can manually enter measured mass and cg using the mass override portion. Cp should be correct unless you have made some significant mods to the shape and size of the fins and body.
 

EXPjawa

Well-Known Member
Joined
Jan 27, 2015
Messages
2,205
Reaction score
57
Location
Finger Lakes, NY
The CP should be consistent regardless, unless your model is built non-standard. Thats really were a program like Rocksim or OR are useful - predicting the CP. The CG, on the other hand, can vary wildly. There are a few reasons for that - there are variations in materials like wood and cardboard from piece to piece. Other factors like glue and paint add mass - not necessarily evenly distributed - that wouldn't be in the simulation unless you or someone specifically put it there. And if you downloaded a file someone else made, you'll probably have to correct it to some extent. A lot of times, these files have the wrong materials selected or wrong surface finishes, etc. You can override the mass and CG position of individual parts, and you can override that for the entire assembly. As you start adjusting things, make sure these overrides aren't checked unless you want them to be. If you have correct mass and CG location for every part (either by adjusting them till they're correct or override), then the entire assembly should pretty accurate. But again, you need to account for the mass of glue and paint.
 

ascastil

Well-Known Member
Joined
Jan 2, 2013
Messages
278
Reaction score
11
Do I measure the cg with a string at home with parachute and motor and then change them on the program.
 

Rex R

LV2
Joined
Apr 21, 2010
Messages
6,101
Reaction score
144
parachute and wadding, no motor. the program will adjust things for different size motors.
Rex
 

ascastil

Well-Known Member
Joined
Jan 2, 2013
Messages
278
Reaction score
11
Ok, thanks. Ive been reading and cant find where to change the cg location on the program, any tips?
 

Buckeye

Well-Known Member
TRF Supporter
Joined
Sep 6, 2009
Messages
2,634
Reaction score
484
Ok, thanks. Ive been reading and cant find where to change the cg location on the program, any tips?
It's called "mass override."

Lots of good articles, tutorials, and instructions on Apogee's website.
 

markkoelsch

Well-Known Member
Joined
Mar 18, 2009
Messages
4,364
Reaction score
148
The Cp is not exactly a stable location. The faster the rocket moves the more it moves. For a Loc 4 on the motors you are talking that effect is negligible though.

To have an accurate model you need to weight your parts and accurately place them within the model. This includes adhesives and paint.

I would build the sim as you do the build. Get a dry weight and Cg location. Then compare this to the sim. If they jive then you are golden. Paint it, reweigh it, and add the additional mass of the paint at the Cg.
 

Worsaer

Amateur Propulsionist
TRF Lifetime Supporter
Joined
Jan 11, 2012
Messages
1,769
Reaction score
82
Location
Central Virginia
As Mark explained, assemble your rocket as though it will be ready to fly, with chute, harness, etc., without the motor installed. Weigh it, and locate the CG.

Compare your 'as-built' measurements to the ones in the Rocksim file. Assuming your actual rocket is heavier, add a Mass object for the additional weight, and move its location forward or aft until the CG of your simulation matches your actual rocket.
 

Buckeye

Well-Known Member
TRF Supporter
Joined
Sep 6, 2009
Messages
2,634
Reaction score
484
As Mark explained, assemble your rocket as though it will be ready to fly, with chute, harness, etc., without the motor installed. Weigh it, and locate the CG.

Compare your 'as-built' measurements to the ones in the Rocksim file. Assuming your actual rocket is heavier, add a Mass object for the additional weight, and move its location forward or aft until the CG of your simulation matches your actual rocket.
No need for a mass object. Just use the overall rocket mass and CG override.
 

markkoelsch

Well-Known Member
Joined
Mar 18, 2009
Messages
4,364
Reaction score
148
No need for a mass object. Just use the overall rocket mass and CG override.
I like my method better. Yours over rides the total mass including motors if I am not mistaken. Mine is an add and forget assuming your sim is accurate.
 

rstaff3

Oddroc-eteer
Joined
Jan 18, 2009
Messages
11,759
Reaction score
14
No, mass and CP overrides are before motors. I sometimes use mass objects to get the required noseweight, for example, but generally plug in actual measured values in the end as a final check. This method works fine, except maybe if the mass of your rocket exceeds your scale's range.
 

Buckeye

Well-Known Member
TRF Supporter
Joined
Sep 6, 2009
Messages
2,634
Reaction score
484
I like my method better. Yours over rides the total mass including motors if I am not mistaken. Mine is an add and forget assuming your sim is accurate.
Nope. Mass overrides are sustainer dry weights. This is true for RS and OR. Total launch weight is then calculated by the software when you choose a motor.

Only RasAero II requires you to input complete launch-ready mass and CG with motor installed. In order to use RAII in the build phase, you need to have a comparable model opened in RS/OR or be ready with a calculator. This really sucks and makes no sense to me, given that RAII includes a motor database from which to draw upon.
 
Top