OpenRocket and the thrust-to-weight ratio

Discussion in 'Rocketry Electronics and Software' started by Funkworks, Sep 2, 2019.

Help Support The Rocketry Forum by donating:

  1. Sep 2, 2019 #1

    Funkworks

    Funkworks

    Funkworks

    Well-Known Member

    Joined:
    Jul 28, 2018
    Messages:
    162
    Likes Received:
    22
    Gender:
    Male
    I’ve been relying on OpenRocket for flight safety, but I realized just last weekend that it doesn’t display the thrust-to-weight ratio, and it doesn’t flag the simulation if the value is less than 5:1 (or 4:1, or 3:1).

    So to ensure safety, can I rely on OpenRocket, or should I calculate the T-to-W ratio independently?

    I have a few low- and mid-power systems (<1 kg and <600 ft) that would like to fly near a 4:1 ratio, or 12 m/s off the pad, which is fine according to OR, but now I understand RSO's may not approve of them.
     
  2. Sep 3, 2019 #2

    Voyager1

    Voyager1

    Voyager1

    Forum Supporter TRF Supporter

    Joined:
    Oct 23, 2015
    Messages:
    221
    Likes Received:
    34
    OpenRocket does display T/W ratio, but you need to generate the simulation summary sheet PDF to see it.

    File > Print/Export PDF > Then select Design Report

    A PDF is generated showing all the critical simulation parameters, including T/W on the top line over on the right.
     
    Funkworks likes this.
  3. Sep 3, 2019 #3

    SpaceManMat

    SpaceManMat

    SpaceManMat

    Space Nut

    Joined:
    Dec 20, 2013
    Messages:
    627
    Likes Received:
    38
    Thrust to Weight ratio is a simple “rule of thumb” method for determining in the field if a safe launch speed will be achieved.
    When you go to the bother of actually doing a simulation you should use the simulated speed to determine if the launch will be safe.
     
    ihbarddx, Funkworks and Buckeye like this.
  4. Sep 3, 2019 #4

    Buckeye

    Buckeye

    Buckeye

    Well-Known Member

    Joined:
    Sep 6, 2009
    Messages:
    2,138
    Likes Received:
    143
    OR will not tell you if the flight is fine or not. It will simulate 1 m/s off the pad just fine and not raise any warnings.

    50 ft/s or 15 m/s off the rail (no wind) usually is enough speed for the fins to be effective and stabilize the rocket.
     
    Funkworks likes this.
  5. Sep 3, 2019 #5

    Funkworks

    Funkworks

    Funkworks

    Well-Known Member

    Joined:
    Jul 28, 2018
    Messages:
    162
    Likes Received:
    22
    Gender:
    Male
    Yes, this resolves my issue.

    I agree, but the local RSO's want to know the T:W ratio as a (standardized) condition for launch.
     
    Last edited: Sep 3, 2019
  6. Sep 3, 2019 #6

    ihbarddx

    ihbarddx

    ihbarddx

    Active Member

    Joined:
    Jan 18, 2019
    Messages:
    35
    Likes Received:
    3
    Gender:
    Male
    Location:
    Pittsburgh
    I agree with this in principle; however, when a vertical launch is at stake, I would advise also simulating a 10 degree off-vertical launch to make sure you have some leeway in the design. If the trajectory degrades of powerprangs you have some design work to do.
     
  7. Sep 4, 2019 #7

    UhClem

    UhClem

    UhClem

    Well-Known Member

    Joined:
    Feb 6, 2009
    Messages:
    1,435
    Likes Received:
    57
    The NFPA 1127 requirement (4.9.1) is based on rocket liftoff weight and certified average thrust. No simulation required. Just make sure the ratio is greater than 3/1. But you are going to need a long rail to reach a safe velocity with a 2G liftoff.
     
    Steve Shannon and Funkworks like this.
  8. Sep 4, 2019 #8

    Nytrunner

    Nytrunner

    Nytrunner

    Master of Rivets

    Joined:
    Oct 15, 2016
    Messages:
    4,996
    Likes Received:
    1,011
    Gender:
    Male
    Location:
    Huntsville AL
    I've always dislike the 3:1 rule. It ignores progressive motors that build up thrust and overly restricts hardhitters or dual-thrust motors that have large thrust events at the beginning of burn.
     
    Titan II likes this.
  9. Sep 8, 2019 #9

    ihbarddx

    ihbarddx

    ihbarddx

    Active Member

    Joined:
    Jan 18, 2019
    Messages:
    35
    Likes Received:
    3
    Gender:
    Male
    Location:
    Pittsburgh
    In summary, if you just want to get past the RSO, you need F/T and you don't need a simulation. If you want a successful flight, OTOH, you need a simulation, and you still have to get past the RSO :)
     

Share This Page