Help Support RocketryForum by donating using the link above or becoming a Supporting Member.


Results 1 to 9 of 9
  1. #1
    Join Date
    15th December 2013
    Location
    Westminster, CA
    Posts
    345

    Open Rocket Sim error

    Folks
    I have this dandy Interceptor-E modified to 29mm. I have flown it once on an Aerotech G79R. I was wanting to sim it with motors bigger than a G80. I tried an H115 and an H135 and got screwy results.
    I am using this model
    estes_interceptor_e (2).rkt
    and I get these results for the G135W-14 example
    Click image for larger version. 

Name:	H135W-14.jpg 
Views:	72 
Size:	82.6 KB 
ID:	331730
    It shows good results for motors G80 and below.

    What gives?

    Thanks
    Tom

    Tom Smith
    Real rockets aren't enough. I need toy rockets too.

  2. #2
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    1,145
    Looks like it is going unstable. The G135 motor is quoted as having a length of 20.6cm in thrustcurve, and the G79 a length of 15cm. The longer motor will shift the CG forward a little, perhaps pushing it into unstable territory? I suspect the design is marginally stable according to the sim and only starts skywriting when the AoA gets too far out of whack (any oscillations building up in the sims?), then the fins lose lift and everything goes wild.

    Usually things starting stable and going unstable during burn are hybrids. Solids usually start off unstable and then snap into stability at a random direction

    Anybody else have any good ideas on this one? Marginal stability is the only thing that comes to mind for me currently.

    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  3. #3
    Join Date
    30th January 2016
    Location
    US > OK > NE
    Posts
    3,227
    I think plotting stability-vs-time will reveal an increasing oscillation off the rod tip. On phone, can't peek.

  4. #4
    Join Date
    15th October 2015
    Location
    Minnesota
    Posts
    1,245
    Quote Originally Posted by TomSmith58 View Post
    Folks
    I have this dandy Interceptor-E modified to 29mm. I have flown it once on an Aerotech G79R. I was wanting to sim it with motors bigger than a G80. I tried an H115 and an H135 and got screwy results.
    I am using this model
    estes_interceptor_e (2).rkt
    and I get these results for the G135W-14 example
    Click image for larger version. 

Name:	H135W-14.jpg 
Views:	72 
Size:	82.6 KB 
ID:	331730
    It shows good results for motors G80 and below.

    What gives?

    Thanks
    Tom
    Double check the weight and CG override for the entire model, that they match yours. I'm getting negative stability on everything.

    Could also be that the overly complex fin shapes are throwing off the CP calculation for those fins(wings), giving a CP result that is too far forward, resulting in negative stability for the sim.

    As a workaround, since the G79 is a known stable motor, add an extra generic fin set to the back. Large enough to pull the CP back enough so that stability is comfortably positive. Then load up the G135/H135 and see if that works for the sim. Should give you a ballpark figure.

    Anyone who has it, what does RockSim give for the CP?
    NASA success rate since 2000: 94.7%
    SpaceX Falcon 9 success rate: 95.5%
    My personal success rate: 91.1%
    My YouTube channel

  5. #5
    Join Date
    27th March 2013
    Location
    Has Changed
    Posts
    9,277
    I'm no help on this one Tom... I can create the appearances of rockets, but when it comes to something as complicated as the Interceptor, the OR version I use doesn't give me anything on flight performance. Sorry
    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

  6. #6
    Join Date
    5th December 2013
    Location
    MD
    Posts
    6,683
    How do you have the wind speed/direction set for the sim? Launch rod/rail length?

    John S. ---- NAR #96911 ---- TRA #15253 ---- MDRA #067 ---- BARC #028
    L1, 3/15/14: Aerotech Sumo, CTI H133BS
    L2, 6/21/14: Giant Leap Vertical Assault, CTI J240RL
    L3, 3/12/16: MAC Performance Radial Flyer, CTI M1101WH
    Altitude: 13,028', L3 flight; Speed: Mach ???, L3 flight

  7. #7
    Join Date
    18th January 2009
    Location
    Maryland
    Posts
    11,538
    Quote Originally Posted by TomSmith58 View Post
    Folks
    I have this dandy Interceptor-E modified to 29mm. I have flown it once on an Aerotech G79R. I was wanting to sim it with motors bigger than a G80. I tried an H115 and an H135 and got screwy results.
    I am using this model
    estes_interceptor_e (2).rkt
    and I get these results for the G135W-14 example
    Click image for larger version. 

Name:	H135W-14.jpg 
Views:	72 
Size:	82.6 KB 
ID:	331730
    It shows good results for motors G80 and below.

    What gives?

    Thanks
    Tom
    I loaded your file into RockSim and I do not get those crazy results. Are you using RockSim? The graph format isn't the same as mine. Anyway, the margin is only at .22 so maybe it is statically unstable on your system?
    Dick Stafford
    The member formerly known as the Pointy-Haired Moderator.
    The Original Rocket Dungeon
    Volunteer compiler of product news for ROCKETS Magazine

  8. #8
    Join Date
    15th October 2015
    Location
    Minnesota
    Posts
    1,245
    Quote Originally Posted by rstaff3 View Post
    I loaded your file into RockSim and I do not get those crazy results. Are you using RockSim? The graph format isn't the same as mine. Anyway, the margin is only at .22 so maybe it is statically unstable on your system?
    So if the margin in RS is .22 with the CG overridden to the same location, then that would only leave a difference in calculated CP between OR and RS.

    This is one area that RockSim shines. Simming complicated models.
    NASA success rate since 2000: 94.7%
    SpaceX Falcon 9 success rate: 95.5%
    My personal success rate: 91.1%
    My YouTube channel

  9. #9
    Join Date
    15th December 2013
    Location
    Westminster, CA
    Posts
    345
    Ok, who has an Interceptor E Model for Rocksim? Last I looked I didnít find one.

    Tom Smith
    Real rockets aren't enough. I need toy rockets too.

Similar Threads

  1. open rocket error
    By bobby_hamill in forum Rocketry Electronics and Software
    Replies: 15
    Last Post: 3rd January 2017, 07:16 PM
  2. Open Rocket Lateral Distance Error
    By TomSmith58 in forum Rocketry Electronics and Software
    Replies: 15
    Last Post: 30th December 2015, 11:22 PM
  3. Open rocket help?! Error
    By soxfan121 in forum The Watering Hole
    Replies: 4
    Last Post: 4th June 2013, 09:41 AM
  4. Open rocket help?! Error
    By soxfan121 in forum Rocketry Electronics and Software
    Replies: 6
    Last Post: 23rd May 2013, 12:15 PM
  5. Open Rocket Error
    By Lentamental in forum Rocketry Electronics and Software
    Replies: 4
    Last Post: 17th October 2011, 02:51 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •