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Originally Posted by DaveRKP
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:
(1) What is an optimal Static Margin (besides just "Stable")? When a rocket is indicated as "Overstable" or "Unstable", how much is too much?
(2) 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?
(3) 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?
(4) What is the maximum velocity that is low enough for a chute release (e.g., for a ripstop nylon chute)?
(5) 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...
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Please realize I and not the programmer who developed RockSim that distinction goes to Paul L. Fossey. I am just a program "hacker" that Paul once threated to report to the "Society for Prevention of Cruelty to Software"! My geometric equivalence hacks of the RockSim software have lead to the current state of the software which is capable of handling tube fins, ring fins, fins on fins and gliders among others. We haven't seen much changes in the software for at least the past 15 years.
(1) A static margin of 1 caliber works good for rockets with at least a 10:1 length to diameter ratio. If the CG is below the CP you will definitely have an unstable body providing an erratic flight path. The one caliber of margin of stability takes into account things like recovery devices and other internals being pushed rearward during the initial high g's of acceleration from the launch, heavier motors (weight can vary as much as 20%) and crosswind forces at the top of the launch rod. This is just a general guideline that seems to have worked well over the years.
Rockets with a length to diameter ratio of less than 10:1 benefit from base drag stabilization which allows designs like the Estes Fat Boy to fly stable with much less than 1 caliber margin of stability see:
https://www.apogeerockets.com/educa...wsletter154.pdf and
https://www.apogeerockets.com/educa...wsletter158.pdf .
(2) 35 mph is the suggested speed at the top of the launch guide. Unless you are flying in a dead calm situation (an extremely rare occurrence) there will be lateral forces from the ambient winds acing upon your rocket at the top of the launch guide. You are not supposed to fly rockets in winds over 20mph, so look at the 35 mph at the top of the launch guide as an approximately two times the force the ambient winds can exert on the model when it is traveling in the vertical direction. This should allow the rockets fins to exert enough restoring force to right the rockets flight path in high winds.
(3) The glues and adhesives we use to build model rockets have much more strength than the actual materials they hold together. I recall carpenters and white glues having a strength rating of around 4000 psi and epoxy's are around 6000 psi, while balsa wood has a strength around 150 psi. So if the balsa fins experience a force of over about 150 psi they will likely shred. There is a separate software program AeroFinSim
http://www.aerorocket.com/finsim.html that can be used to look at this problem which is more pronounced at speeds higher than most model rockets will ever encounter. Fin flutter is also discussed in these 2 Apogee Peak of Flight Newsletters:
https://www.apogeerockets.com/educa...wsletter291.pdf and
https://www.apogeerockets.com/educa...wsletter411.pdf .
For a fin flutter calculator to use with RockSim visit;
https://www.apogeerockets.com/downl...er_Velocity.xls
(4) This maximum velocity for deployment value will be highly dependent on the rockets materials of construction as cardboard tubes are more prone to denting and zippering than phenolic or fiberglass airframes and stretched Kevlar has more cutting and knife like properties than elastic. Ideally you want the deployment velocity to be as low as possible to avoid excessive forces on any part of the rocket. Again you could look at the strength of the materials in the parachute, shroud lines and shock cord to get some idea of what the system is capable of. However the bottom line may come down to how much stress the body tubes and associated recovery device mounts are capable of handling. You could always test this empirically on some scrap tubing and mounts with a weight set to see how much force you have to apply to a mount or tubing wall before it fails or deforms.
(5) Again, as in the answer above ,this will be highly dependent on the rockets materials of construction. A fiberglass or phenolic airframe can handle much higher landing forces than a balsa and cardboard construction. My RocketVision Mach Buster Rugged Rocket (ABS nosecone, phenolic airframe and G-10 fins) took several ballistic recoveries into the ground (buried itself up to the fins) and is no worse for wear! Keep in mind the 150 psi strength of balsa wood, if that is what you are using for fins then plan the recovery velocity accordingly. Perhaps use a 2:1 safety margin which would be about 75 psi maximum force.