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Hit mach in sims for L2 cert. Need advice.

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Ccolvin968

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I was running sims for my L2 certification next year.
The rocket would be the fiberglass Formula 75 that I just got my L1 this past month.
In the sims, I hit Mach 1.01 for speed and 7,579 ft altitude. Well below our 20,000 ft. waiver.
My concern is this... Did I need to perform certain construction techniques for the rocket to be able to withstand the speed?
They're slotted fins, used 45 minute epoxy on the edge of the fin that contacts the mmt and used JB Weld to run fillets along the edges.
I didn't use any support inside of the rocket around the fins like dowl rods like I saw on some posts after I had already completed construction.
Should I be okay? Or just build a more sturdy L2 rocket and be extra sure it can withstand the stresses of that kind of acceleration all the way up to mach?
Thanks for any thoughts.
 

mpitfield

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My guess is that you're okay for your construction and materials used.

Having said that it is always a good exercise to validate your simulations, regardless of your concerns. What did you use to run your simulations, RockSim or OR, and have you validated your simulations against actual flight results. For example have you launched your rocket using a previous sim, and then adjusted the simulation to match your results? The first things I look at are the specs to weight of the rocket and CG, if that is correct then you can adjust the CD by manipulating the finish or leading edges of your fins etc. Once I have an accurate sim to flight I then feel more confident on the next sim. Keep in mind these simulations are considered accurate at +-10%. Also if you used RockSim then your Mach flight will likely be off, so you're better off to use OR.
 

Ccolvin968

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I used Openrocket. I love it so far.
Yes, I have had two flights to adjust the model in OR to meet the exact flight characteristics within 2% either direction for both speed and altitude.
I am overall happy with how close the predictions were to begin with. Little to no tweaking the rocket at all in the sim.
I'll keep this in mind.
Thank you for your time.
 

OverTheTop

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like dowl rods like I saw on some posts
I think that was me on my Velociraptor!

Given that the fins are through the wall and assuming you got the epoxy about right and stuck to the suitably sanded tubes and fins, my thoughts are that it will not be a problem. Lots of variables, but the kit looks like it has a bit of safety margin. Likely outcome would be a successful flight IMHO.
 

rharshberger

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I concur with the above evaluations based on the information given. It has been much debated as to whether internal fillets are even necessary. Using Crazy Jims "double buttering" technique creates small fillets on the root edge to mmt joint. The JB wmWeld external fillets should be plenty strong.
 

rc dude

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It should be plenty strong enough, but if you were looking for a reason to perhaps convince your wife you need a new rocket...
 

firesalto

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Your rocket should definetly be able to handle the force flew my formula 75 on a J510 to 8,000 ft and Mach 1.6 yours should do just fine. I'm actually planning on staging two of these with 54mm mounts to around 20,000 ft and Mach 2 hopefully this will work and they won't fall apart but I'll have to wait for launch to get a waiver for that height
 

Incongruent

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only words of advice...

put a tracker on that thing...

you will not regret it

fm
Unless the rocket Catos on the pad and the tracker is destroyed in addition to the rocket.

Otherwise, I agree with my zero personal experience.

You actually should put one though, the statistical probability of the rocket blowing up with a tracker is less than that of it getting lost without a tracker.
 

Ccolvin968

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I do not have a tracker, but I am in the process of doing an avionics bay retrofit.
It'll be a priority to add a tracker now since it was pointed out.
On top of that, I plan on using a drogue chute, with a main chute with the Jolly Logic Chute Release.
Still have to sim all of that. Still trying to decide on where/how to place my avionics bay. (Note to self... add one right away and don't do a retrofit next time...)
OverTheTop, that was your post! Very helpful for ideas and steps for my first go at a HP and FG build. Thanks a million!
In the mean time, I have at least 6 months to plan, simulate, and construct anything I would like to do.
Thanks for the advice everyone.
I simulated it with an Aerotech J350W (38mm).
Should be a good flight.
Who knows... I may buy a rocket for a winter project! :D
 

firesalto

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I do not have a tracker, but I am in the process of doing an avionics bay retrofit.
It'll be a priority to add a tracker now since it was pointed out.
On top of that, I plan on using a drogue chute, with a main chute with the Jolly Logic Chute Release.
Still have to sim all of that. Still trying to decide on where/how to place my avionics bay. (Note to self... add one right away and don't do a retrofit next time...)
OverTheTop, that was your post! Very helpful for ideas and steps for my first go at a HP and FG build. Thanks a million!
In the mean time, I have at least 6 months to plan, simulate, and construct anything I would like to do.
Thanks for the advice everyone.
I simulated it with an Aerotech J350W (38mm).
Should be a good flight.
Who knows... I may buy a rocket for a winter project! :D
Oh yes you will definetly need a tracker no matter what for that flight if you don't have one and can't get one borrow one. You will not see the rocket when it's 7,000 in the air. I have the BRB 900 and it's great really recommend it for a first tracker.
 

Ccolvin968

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Oh yes you will definetly need a tracker no matter what for that flight if you don't have one and can't get one borrow one. You will not see the rocket when it's 7,000 in the air. I have the BRB 900 and it's great really recommend it for a first tracker.
Do you have a link for it or similar trackers?
 

OverTheTop

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only words of advice...

put a tracker on that thing...

you will not regret it
You haven't experienced life until you fly a rocket completely out of sight, and then walk up to it using GPS telemetry. I can't explain the feeling. I flew a 54mm to 21k' and it landed 3 mile downrange. Drove strait to it in the Jeep :) :). Priceless!
 

dshmel

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I concur with the above evaluations based on the information given. It has been much debated as to whether internal fillets are even necessary. Using Crazy Jims "double buttering" technique creates small fillets on the root edge to mmt joint. The JB wmWeld external fillets should be plenty strong.
I built my WM Demon 98 using only Proline 4500 on the root edges. I "triple" buttered them to create a sizable fillet. I did not fillet the fin to the inside of the boat tail. On the outside, I used Proline for a very small, mostly cosmetic, fillet. Launching it today at MWP14 on an AT K990 DM. So I will see if that was enough epoxy or not.
 

Ccolvin968

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Dshmel, looking forward to your report!
Thanks for the link firesalto. I'll look into it shortly.
 

Ccolvin968

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Firesalto, I'll probably spring for the 900mhz one so it comes as a complete kit and also because I don't have the time to pursue my HAM license. Seems like a really good price for what I get out of it.
 

EeebeeE

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I've seen several people break Mach 1 with that rocket. The JB Weld Fillets are going to be plenty strong enough. With a filament-wound NC and airframe along with G-10 fins, your materials are plenty strong enough. If you got the waiver to do it and a really good tracker, heck I' certify with a CTI L1030 or an AT L1000. Go for it all.

Seriously, it would be better to put a motor in it that would punch through mach 1 to about mach 1.4 than to hang around transonic speeds of Mach 0.9-Mach 1.1 where there is a ton of turbulence. Your rocket has the strength for it. Certify with a K2045.
 

firesalto

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Firesalto, I'll probably spring for the 900mhz one so it comes as a complete kit and also because I don't have the time to pursue my HAM license. Seems like a really good price for what I get out of it.
Good choice I got mine for a good deal since I bought it used from somebody. I've found that these trackers are REALLY durable too. Your reasons are pretty much the same as mine for why I bought it.
 

Nytrunner

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Also if you used RockSim then your Mach flight will likely be off, so you're better off to use OR.
Are you saying that Openrocket actually has better Mach approximations than Rocksim? I haven't built a Mach-rocket yet, but I'm very curious to know for the future.
 

mpitfield

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Firesalto, I'll probably spring for the 900mhz one so it comes as a complete kit and also because I don't have the time to pursue my HAM license. Seems like a really good price for what I get out of it.
I would contact your local club, assuming you belong to one. Some clubs have club receivers or if not there is very likely someone in the club that would be more than happy to let you use theirs. This way you can ease into the technology before you make a bigger commitment financially.

Are you saying that Openrocket actually has better Mach approximations than Rocksim? I haven't built a Mach-rocket yet, but I'm very curious to know for the future.
Yes it is generally known that RS has issues with any simulation that approaches Mach. My general rue of thumb is to use RS for launches up to transonic, but above that I use OR and RASAero.

I cannot recall the thread but there was a good one that discussed the topic a while back, which included pretty graphs for guys like me who are not aeronautical engineers but wannabees.
 

EeebeeE

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Yes it is generally known that RS has issues with any simulation that approaches Mach. My general rue of thumb is to use RS for launches up to transonic, but above that I use OR and RASAero.

I cannot recall the thread but there was a good one that discussed the topic a while back, which included pretty graphs for guys like me who are not aeronautical engineers but wannabees.
I second that. Nice thing about OR is that it can open up your Rocksim file, so you don't have to start from square one.
 

Nytrunner

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That's good to know. So Rocksim has more precise CP and oscillation models, but it freaks out going supersonic. I'll have to keep that in mind. Or just convert to RASaero. Does RA have better CP definition than just Barrowman(OR)?
 

markkoelsch

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That's good to know. So Rocksim has more precise CP and oscillation models, but it freaks out going supersonic. I'll have to keep that in mind. Or just convert to RASaero. Does RA have better CP definition than just Barrowman(OR)?
I would say that is an exaggeration. Rocksim is ok until Mach 1.2 or so. Faster and it does get a bit off.

I think both Rocksim and OR have issue with Cp calculation at Mach. They are based on Barrowman, which has a number of assumption namely staying below Mach.

The above said, I use a combination of Rocksim and Rasaero.
 

mpitfield

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That's good to know. So Rocksim has more precise CP and oscillation models, but it freaks out going supersonic. I'll have to keep that in mind. Or just convert to RASaero. Does RA have better CP definition than just Barrowman(OR)?
I am not aware that RS has more precise CP and oscillation models, that is the first I have ever heard about this. I know that RSPro has some nice features like 6DoF and dispersion for Class 3 flights, but I have no hands on RSPro because I live in Canada, so I only own the basic RS. RASAero is a no frills windows only application and the parameters you input for the model are very basic. You don't build a rocket like you do in OR and RS you simply input measurements, weights, CG, etc., however there seems to be a general consensus among the higher velocity crowd that it is a better tool in the higher Mach launches. As for RA vs. OR for better CP I am not aware, maybe someone else will pop in on it.

Below is my attempt to recreate what the differences are between OR and RS when it comes to transonic and supersonic flights. I just ran the same simulation, same rocket, same motor choices, same RS file, etc. on a rocket and a flight profile I have launched twice. These graphs represent CD vs. Mach number the top one is from OR and the bottom one is from RS. As you can see there is a big difference between the two with RS having a pessimistic sharp increase as it approaches and moves through Mach. This sharp curve in RS seems to be the norm for any simulation I have seen. Obviously a different algorithm is being applied and from the threads I have read it seems to be a common thought that OR is more realistic.



 
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Nytrunner

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I am not aware that RS has more precise CP and oscillation models, that is the first I have ever heard about this. I know that RSPro has some nice features like 6DoF and dispersion for Class 3 flights, but I have no hands on RSPro because I live in Canada, so I only own the basic RS. RASAero is a no frills windows only application and the parameters you input for the model are very basic. You don't build a rocket like you do in OR and RS you simply input measurements, weights, CG, etc., however there seems to be a general consensus among the higher velocity crowd that it is a better tool in the higher Mach launches. As for RA vs. OR for better CP I am not aware, maybe someone else will pop in on it.

Below is my attempt to recreate what the differences are between OR and RS when it comes to transonic and supersonic flights. I just ran the same simulation, same rocket, same motor choices, same RS file, etc. on a rocket and a flight profile I have launched twice. These graphs represent CD vs. Mach number the top one is from OR and the bottom one is from RS. As you can see there is a big difference between the two with RS having a pessimistic sharp increase as it approaches and moves through Mach. This sharp curve in RS seems to be the norm for any simulation I have seen. Obviously a different algorithm is being applied and from the threads I have read it seems to be a common thought that OR is more realistic.
I haven't bought Rocksim to try it past the demo (thus my hijacking of this thread with questions to determine if its worth the investment. Sorry Ccolvin!)
Your real flight vs sim data is great. Scientific method doing its job again! Which one do you believe more accurately matched your flights?

I remember a Peak of Flight article where Tim van Mil discussed how the Rocksim equations were formulated. If I recall correctly, they took the Pressure/Sheer integrations (that Barrowman severely simplifies for generic geometries) and simplified them to a lesser extent with fewer assumptions while coding in the ability to model more body geometry options in order to leverage modern computing power. This calculates a more accurate (less conservative) CP for more complex geometries. And I believe I heard OR doesn't account for base drag, where RS does.

A fellow flyer clued me in to the oscillations analysis; I believe he said it generates a "damping" factor of how quickly the rocket responds to aerodynamic stabilization (ie, returning to 0 degree departure from flight path). If that factor is too low, the rocket is in danger of corkscrew or spiral behavior that could destabilize the flight (if anyone's read on vibration mechanics, think about behavior similar to slowly approaching a resonance condition). This is more pronounced in stubbier rockets.
 

rharshberger

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I haven't bought Rocksim to try it past the demo (thus my hijacking of this thread with questions to determine if its worth the investment. Sorry Ccolvin!)
Your real flight vs sim data is great. Scientific method doing its job again! Which one do you believe more accurately matched your flights?

I remember a Peak of Flight article where Tim van Mil discussed how the Rocksim equations were formulated. If I recall correctly, they took the Pressure/Sheer integrations (that Barrowman severely simplifies for generic geometries) and simplified them to a lesser extent with fewer assumptions while coding in the ability to model more body geometry options in order to leverage modern computing power. This calculates a more accurate (less conservative) CP for more complex geometries. And I believe I heard OR doesn't account for base drag, where RS does.

A fellow flyer clued me in to the oscillations analysis; I believe he said it generates a "damping" factor of how quickly the rocket responds to aerodynamic stabilization (ie, returning to 0 degree departure from flight path). If that factor is too low, the rocket is in danger of corkscrew or spiral behavior that could destabilize the flight (if anyone's read on vibration mechanics, think about behavior similar to slowly approaching a resonance condition). This is more pronounced in stubbier rockets.
Short and Stubby rockets still require a work around in RS and OR both, RS has a few more functions and the ability to do pods, ring fins and some other things but if you don't absolutely have to have RS then OR is the better deal FREE! FWIW my entire L3 project was primarily designed and simmed on OpenRocket, the only reason I used RS at all was because my L3CC asked for the design in RS, the other sim I use frequently is RASAeroII. Both OR (still being developed) and RASAeroII are still supported and the designers and programmers frequent TRF, whereas RocSim (not the Pro version) seems to be a withering away product and no longer seems to be supported by the developer (read as no new updates in while or responses to known bugs).
 
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