Rocket Simulation Guidelines

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DaveRKP

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I am relatively new to the rocket simulation world and I am looking to leverage the knowledge and experience of those that may be further down the road. I'm currently using RockSim as I've been simulating various part and engine configurations for a 1.68x Orbital Transport booster / glider upscale. I've enjoyed coming up to speed by stepping myself though a "piece by piece" rebuild of a 1x OT RockSim file and adding some design tweaks along the way.

As part of my learning, I've reviewed many RockSim / OpenRocket threads on this forum, watched many video tutorials, etc., etc., but have not yet found discussions or guidance on a number of questions that I've raised in my efforts to this point, specifically:
  • What is an optimal Static Margin (besides just "Stable")? When a rocket is indicated as "Overstable" or "Unstable", how much is too much?
  • Should a rocket reach stable flight minimum velocity before reaching the end of a launch guide? If not necessarily, how soon should it or how long is too long?
  • What is the maximum velocity (of force) can be handled for a typical low power rocket (e.g., well-constructed paper tube / balsa fin construction) without risk of shredding?
  • What is the maximum velocity that is low enough for a chute release (e.g., for a ripstop nylon chute)?
  • What is the maximum landing velocity (or force) that can tolerated with little to no impact damage for that typical rocket?
I know the simulation software essentially lets you know if your "within limits", but I just would like to have a better understanding of how some of the various limits are determined or the ranges that are acceptable / optimal. I know some answers may yield a lot of "it depends" - it is "rocket science" after all, but I would appreciate any info that others can provide.

Looking forward to the conversation...
 

SpaceManMat

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Optimal stability really depends on what you are trying to achieve. If you just want a stable rocket then probably 2 is what you should aim for, but generally 1.5 - 3 is fine. If your going for max speed or altitude, then your probably going to want to go in the region of 1 - 1.5, but you’ll definitely want to make sure that your rocket meets your desired minimum stability over its entire speed envelope.

Yes, the velocity off the rod is critical. This is where you’ll run into issues if you are too slow, with fins being close to stall speed. Gets worse if there is a wind.

A very well constructed LPR should be able to go supersonic. However for the most part it is recommended to stay below transonic speeds (Mach 0.8). If you go faster parts of the airframe will experience supersonic flow and if there is a weakness with your build, it will show itself.

Keep in mind, OR and RS are not FEA tools, they cannot tell you about how materials will perform under stress.
 

gtg738w

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If CP is aft of the CG, it is stable, the fins will produce a force that will act to bring the rocket back to neutral flight. A margin of >0.0. The key is that this must be satisfied in flight. Cp will shift with angle of attack and airspeed. The rule of thumb of 1.0 static (zero airspeed, zero AoA) margin is usually enough to remain stable at a reasonable angle of attack ~10-20 degrees. You can have a static margin >2.0 and still go negative in flight with enough wind. This will vary considerably with vehicle shape, long & skinny will require more. If you can test stability at an AoA and airspeed, that is the best. The speed off the rod just helps to reduce the max AoA it will see. Yes, you generally want to be stable before you leave the guide.

My usual rule of thumb is deploy at <75 ft/s and land at <25ft/s. This also depends a lot on mass and materials and construction. It is really the ratio between deploy & landing (terminal velocity) that matters. If the chute is making a drag force equal to the total weight at 25 ft/s and you deploy at 75 ft/s the system will see a 9g load (3x^2). If you have a chute that will land at 10ft/s and deploy at 75 ft/s, the system will see ~56g (7.5x^2). Big difference. That of course assumes instant deployment but you get the idea, don't put a big chute out at high speed...
 

DaveRKP

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SpaceManMat / gtg738w - Appreciate the prompt feedback! Both of you have provided the info that will be a great assist.
 

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