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  1. #1
    Join Date
    13th October 2014
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    Bridgeton, NJ
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    497

    Question Component Mass Override - OR vs. Rocksim

    I use Rocksim for my rocket builds - much less experience with OR. However, a situation recently came up that requires me to identify potential OR model design "oversights" and I'd appreciate some clarification/ confirmation about what I think I'm seeing in this OR model. For various reasons I do not wish to post the ORK file so I'll try to be as clear as possible with my observations.

    The majority of my concern is that I found what appear to be inconsistencies in the use of the OR component mass override.
    - First issue: Some components have the Override Mass box checked but the Override CG box is not checked. Doing this in RockSim places the component CG at the extreme forward position of the component, which is not realistic. How does OR react under this condition?

    - Second: Some components have the Override Mass box checked but there is a zero value entered for the mass. In this model the fins and the MMT

    So the model has some component masses calculated (with the appropriate component CG, I assume), other components have mass overrides with zero mass values entered, and finally many of the components with overridden masses have no associated CG override values entered.

    So which of these situations do the seasoned OR users say are problematic & why?

    Thank you

    NAR 98451 L2; MDRA 024; BARC 001 (President)
    L1 - Madcow Honest John 2.6, CTI H163
    L2 - Madcow Level 2, CTI J335
    Mile High Club 5,650 ft (07/16/16)

  2. #2
    Join Date
    19th June 2014
    Posts
    49
    Its pretty easy to get a feel for OpenRockets mass override behavior. Just create a new rocket, place a body tube(or whatever component) and override the mass and watch the CG behavior. Doing this it looks like when you override the mass but not the CG OpenRocket leave the CG in the same location it would of been before the override.
    As to why someone would override the mass of the fins to zero, im not sure but i don't think there is any practical reason to do that. Maybe the rocket was already complete and they where going to weigh the rocket and measure the CG and override the simulated specs.

    -Stephen A.

  3. #3
    Join Date
    27th March 2013
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    Quote Originally Posted by slaak View Post
    Its pretty easy to get a feel for OpenRockets mass override behavior. Just create a new rocket, place a body tube(or whatever component) and override the mass and watch the CG behavior. Doing this it looks like when you override the mass but not the CG OpenRocket leave the CG in the same location it would of been before the override.
    As to why someone would override the mass of the fins to zero, im not sure but i don't think there is any practical reason to do that. Maybe the rocket was already complete and they where going to weigh the rocket and measure the CG and override the simulated specs.
    Since my sims try to recreate classic designs with the markings that came with the kits, I often have to create fins that have the decal's image reversed to make it legible. I will make one set of fins using the correct materials, and another out of "air" (thus zero mass) that are slightly offset, streamlined, and polished (lately attached to a phantom body tube for easy removal for flight characteristics).
    Dreaming of making the rockets I dreamed of as a kid (and then some).


    NAR L1 Cert flight: Sheridan, Oregon, USA. Sept. 19, 2015. Flew Deep Space OFFl on an I357T-14A Blue Thunder

  4. #4
    Join Date
    13th October 2014
    Location
    Bridgeton, NJ
    Posts
    497
    Thanks for the feedback... Actually I did created a basic rocket to test how OR treated not entering a CG value when the component's mass is overridden. I did find it is different than how RockSim treats it. OR assumes the CG did not change just because the component's mass differs from the calculated value (inherent in that assumption is that component's material density is uniform and unchanged), whereas RockSim does not make that assumption. If a value for the CG is not entered in RS it will move the CG to the forward end of the component.

    Actually I prefer how OR handles the mass override. It makes it easier to handle complex geometry parts (fins & NCs) when the as-found masses differ from the software's computed value.
    NAR 98451 L2; MDRA 024; BARC 001 (President)
    L1 - Madcow Honest John 2.6, CTI H163
    L2 - Madcow Level 2, CTI J335
    Mile High Club 5,650 ft (07/16/16)

  5. #5
    Join Date
    28th November 2014
    Location
    Indiana
    Posts
    354
    With a finished rocket chose the stage and override mass and CG. Be sure to check override subcomponents and then it does not matter what the other settings are set to.
    TRA #16452
    L1 Wildman Drago

  6. #6
    Join Date
    22nd September 2017
    Location
    TN
    Posts
    1,211
    I have rarely ever overridden a Openrocket CG value for component, ever. I have however used mass override on scratch built competition multistages from solidworks mass evaluation of components before rockets were even constructed and from actually weighing the real physical components. If you use mass override I will say the OR automatic CG is relatively accurate for flight by simply using mass overrides itself. Airfoils were pickier than the OR cg positon but those masses were nearly so slight. If you have a real forward heavy nosecone with a lot of weight at tip you may need an or CG override. I have had two scratch built multistages fly with Stability fine by only using OR mass overrides. I have used OR mass override for an L-1 Kit build. I don't know Rocksim behavior from a hole in ground. But mass override in OR seems mostly sufficient for most tasks, unless your looking at unique parts with different moments of inertia that do not balance within reason.

    On an RB-05A sport kit I added a mass override for a nosecone. Then I positioned two long slender rods with mass overrides as payloads into the forward cavity of a nosecone for the one pound of lead shot and rocketpoxy I added. This approximates a CG override if you do not have a way to get CG of component easily.

    I worry about overriding CG as it overrides for all subcomponents let's say pieces inside NC. So if you are stuffing a component full of masses let's say chutes or whatever hardware the CG is overwrite to the position you described vs OR determining CG via statics of all the hardware masses at described accurate dimension lengths in the nosecone at positions from the datum plane with the nosecone CG position and mass override too by using just mass overrides and proper lengths. So if it's a solid component awkwArd geometry like airfoil it might deserve a CG override vs a hollow component your filling with many different masses unless you fill that component then get CG. I'm still a noob myself with about seven rockets entered.


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