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  #1  
Old 02-24-2019, 01:50 PM
DaveRKP DaveRKP is offline
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Default RockSim Simulation Guidelines

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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  #2  
Old 02-25-2019, 11:03 AM
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teflonrocketry1 teflonrocketry1 is offline
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Quote:
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...


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.
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Last edited by teflonrocketry1 : 02-25-2019 at 12:14 PM.
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  #3  
Old 02-25-2019, 04:19 PM
DaveRKP DaveRKP is offline
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Bruce, Thanks for the prompt response with much helpful information - appreciate the effort to craft the details in the response with the type of info I was lacking! I will chase some of the references you indicated and continue to dig into the simulations.

I recognized that you have been a great assist to a number of people over the years with their RockSim questions and novel extensions in using the app. If you're still "at large" committing your acts of said cruelty, I may have some additional questions to send your way. Is there a more appropriate thread out there or do we just continue here? Alas, still learning forum etiquitte...

Thanks again on the real-world input to assist with simulations!
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  #4  
Old 02-25-2019, 07:53 PM
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teflonrocketry1 teflonrocketry1 is offline
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Quote:
Originally Posted by DaveRKP
Bruce, Thanks for the prompt response with much helpful information - appreciate the effort to craft the details in the response with the type of info I was lacking! I will chase some of the references you indicated and continue to dig into the simulations.

I recognized that you have been a great assist to a number of people over the years with their RockSim questions and novel extensions in using the app. If you're still "at large" committing your acts of said cruelty, I may have some additional questions to send your way. Is there a more appropriate thread out there or do we just continue here? Alas, still learning forum etiquitte...

Thanks again on the real-world input to assist with simulations!


Dave,

Feel free to post things here so everyone gets a chance to learn, or you can always PM me through this forum.

Simulation of the Estes Orbital transport is a formidable task. I don't personally have a kit that I can measure, but I will try to create a simulation file from the plans at the JimZ site: http://www.spacemodeling.org/jimz/estes/k-42.pdf

To get this simulation to work I will need to use concepts from the following Apogee Peak of Flight Newsletters:
Pop Pod Boost glider simulation: https://www.apogeerockets.com/educa...wsletter117.pdf
Boosted Dart simulation: https://www.apogeerockets.com/educa...wsletter172.pdf
Derivation of the Apparent Drag Coefficient by David Flanagan: https://www.apogeerockets.com/educa...wsletter489.pdf

I will get back to you when I have something.
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  #5  
Old 02-26-2019, 03:29 PM
DaveRKP DaveRKP is offline
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Default Upscale RockSim File

Bruce, Please take a look at the file attached - this is my reworked version of an original file to upscale everything up to 1.68x.

You will also not that I made a crude attempt at simulating rail guides - I did see your previous posts, but was trying an alternate approach. In addition to the rail guide variants, I also made provisions for a 29mm motor tube that can take an Estes 24mm to 29mm Engine adapter, added a rear shuttle stabilizer that I've used on a 1x version, and added a motor retainer in place of an engine hook.

Take a look and let me know.
Attached Files
File Type: rkt Orbital_Transport_168_Upscale.rkt (212.0 KB, 13 views)
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Old 02-26-2019, 10:13 PM
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teflonrocketry1 teflonrocketry1 is offline
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Quote:
Originally Posted by DaveRKP
Bruce, Please take a look at the file attached - this is my reworked version of an original file to upscale everything up to 1.68x.

You will also not that I made a crude attempt at simulating rail guides - I did see your previous posts, but was trying an alternate approach. In addition to the rail guide variants, I also made provisions for a 29mm motor tube that can take an Estes 24mm to 29mm Engine adapter, added a rear shuttle stabilizer that I've used on a 1x version, and added a motor retainer in place of an engine hook.

Take a look and let me know.

David,

You did an excellent job on this simulation! I have been trying to get a simulation of the Orbital Transport where the glider separates and comes down separately from the booster but I can't seem to make this happen when the glider is on the back of the booster. What you did for the rail guides looks appropriate (perhaps even overkill) and should work. If you want to stay true to the original Orbital Transport design the wings and canards are not at + and - 90 degrees from the vertical they are mounted at approximately + and - 82 degrees (higher up on the body tube), this makes for a much more difficult simulation. Also in the original kit the Spacecraft or glider wings are even with the top of the body tube, in your case instead of -15.2 degrees it should be -17.6 for the starboard side glider wing. I did not see any extra aft tubing that takes into account the motor retainer you are using; this will not make much difference in the simulation results since you took the weight into account with a mass object. You might want to make the fake tubes you used to position fins exceedingly thin so their weight does not enter into the flight calculations. I also suggest using a user reference diameter of 2.1060 for the static margin reference which is the diameter of a circle that has the equivalent frontal surface area of both the booster plus the glider's.

I attached the only simulation file for the Estes Orbital Transport where I was able to have the glider and booster come down separately from one another. To do this I had to have them stacked vertically, glider on top of booster in a two stage configuration and then used a dart motor (attached) in the glider. I also attached my simulation for the Estes Orbital Transport (K-42) with the spacecraft or glider on the back of the booster .
Attached Files
File Type: rkt oop_estes_Orbital_Transport_full_stack.rkt (178.6 KB, 14 views)
File Type: rse Dart motor.rse (496 Bytes, 11 views)
File Type: rkt oop_estes_Orbital_Transport_BG.rkt (173.7 KB, 16 views)
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Last edited by teflonrocketry1 : 02-26-2019 at 11:02 PM.
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Old 03-01-2019, 07:46 AM
DaveRKP DaveRKP is offline
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Bruce, Thanks for the additional details and the sim files - I'll be looking to try them out and get my head around them over the weekend.

I was not aware of the original OT config - my understanding was based on a Semroc repro edition (KV-66) that I built last year. Just wondering if either of the wing orientations provide better stability.

As for the motor tube config, the file I sent you was a 29mm config with the mass object representing the engine retainer. I have a variant of the file where the 29mm tube is unchecked as a motor mount then I added another component, an Estes 29mm to 24mm Engine adapter.

One additional question in my current mix - the shuttle is typically build with elevons fixed upward at an (approx 30 degree angle). The default RockSim depictions have them incorporated into the wing with no elevation. Is there a way to define elevons angled to the shuttle wing. I could not come up with a novice way to configure elevons in RockSim (see pic of the Semroc 1x version for reference).
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  #8  
Old 03-02-2019, 07:45 PM
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teflonrocketry1 teflonrocketry1 is offline
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Quote:
Originally Posted by DaveRKP
Bruce, Thanks for the additional details and the sim files - I'll be looking to try them out and get my head around them over the weekend.

I was not aware of the original OT config - my understanding was based on a Semroc repro edition (KV-66) that I built last year. Just wondering if either of the wing orientations provide better stability.

As for the motor tube config, the file I sent you was a 29mm config with the mass object representing the engine retainer. I have a variant of the file where the 29mm tube is unchecked as a motor mount then I added another component, an Estes 29mm to 24mm Engine adapter.

One additional question in my current mix - the shuttle is typically build with elevons fixed upward at an (approx 30 degree angle). The default RockSim depictions have them incorporated into the wing with no elevation. Is there a way to define elevons angled to the shuttle wing. I could not come up with a novice way to configure elevons in RockSim (see pic of the Semroc 1x version for reference).


David,

After many attempts, I have been unable to position the elevons at an upward angle. I have even tried to edit the RockSim file and force the software to do so! I attached a RockSim file for the Orbital Transport with the wings mounted at 90 degrees from the vertical as you mentioned they are in the Semroc kit. As per RockSim, this doesn't make any significant difference in the calculated center of pressure CP 15.5361 in the original design versus a CP of 19.5368 in the Semroc configuration, the CG's are nearly identical off by one in the last digit.

By the way, nice work on the Spacecapsule/Glider in the picture you provided!
Attached Files
File Type: rkt oop_estes_Orbital_Transport_Semroc.rkt (173.7 KB, 12 views)
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Last edited by teflonrocketry1 : 03-02-2019 at 08:38 PM.
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