Where?
Super Big Bertha fails spin test
- Thread starter Jim McNabb
- Start date
Help Support The Rocketry Forum:
Shoot, I just make sure the CG is right with the loaded rocket and will be fine. If I gotta add nose weight, no big deal. Just add a bigger motor if one wants to fly higher. Swing testing is for modrocs. Certainly can't be done with larger projects easily at all! Invest in a good sim program and/or follow the manufacturers recommendations that come with a kit rocket. Kurt
John Boren said something to that effect back when he was active on the forums.
lakeroadster
When in doubt... build hell-for-stout!
And the sim accurately reflects the as built rocket.
Last edited:
There are plenty of scratch builds turning up these days. 3/4fnc where the builder has not done a sim and there's no-one with a laptop at the launch. A SUCCESFUL swing test gets it past the RSO and allows them to launch. So it's more useful than for just oddrocs.
- Joined
- Jul 18, 2022
- Messages
- 1,751
- Reaction score
- 1,764
I have a slightly modified SBB (6" taller due to a small Payload bay), I quasi-glassed it, and overbuilt the fins (balsa) so they would survive. It's considerably heavier than stock, but it's one of my best flying rockets and is aways a crowd pleaser with an E35 and up. I don't think a Bertha can be unstable, because those fins are just so large and extend well beyond the body. Just stuff a motor in there and fly it. It'll be fine!
Just don't do really stupid modifications to your Big Bertha and you will be fine. Common sense and a little mindsiming is fine. The club RSO will tell you if you are too wacky. You can always put in some "stinking" nose weight and bigger motor. Usually that solves about 90% of your problems.
For an old NARAM Big Bertha competition I glued a "GI Joe" type action figure pickelhaube to the nose cone. It flew successfully and certified for the competition. I flew it successfully a few other times and unfortunately the little pickelhaube was lost. Those plastic doll helmets are now OOP. Despirate, I replaced it with a Jager version that turned out to be made of metal.
My inner historian was pleased because the Jager helmet was much better for my egg head Kaiser cone. My inner RSO didn't mind the additional weight but the asymmetrical birdie on top was much worse than the enlisted man's "spike" on the original one.
Now I am scared to launch it. More nose weight needs more power and the aerodynamics up front are now all jacked up. How do I know it will be OK? Should I follow my own advice and just make it a shelf queen? Should I pack in a Quest D22-4W and swing test?...But that might mess up the finish and Vern's Sharpe signature and comments.
So confused and scared now. What would Big Bertha herself do? She would have a massive party and the Kaiser would come!
For an old NARAM Big Bertha competition I glued a "GI Joe" type action figure pickelhaube to the nose cone. It flew successfully and certified for the competition. I flew it successfully a few other times and unfortunately the little pickelhaube was lost. Those plastic doll helmets are now OOP. Despirate, I replaced it with a Jager version that turned out to be made of metal.
My inner historian was pleased because the Jager helmet was much better for my egg head Kaiser cone. My inner RSO didn't mind the additional weight but the asymmetrical birdie on top was much worse than the enlisted man's "spike" on the original one.
Now I am scared to launch it. More nose weight needs more power and the aerodynamics up front are now all jacked up. How do I know it will be OK? Should I follow my own advice and just make it a shelf queen? Should I pack in a Quest D22-4W and swing test?...But that might mess up the finish and Vern's Sharpe signature and comments.
So confused and scared now. What would Big Bertha herself do? She would have a massive party and the Kaiser would come!
- Joined
- Aug 6, 2022
- Messages
- 3,269
- Reaction score
- 2,969
I'm tempted to go buy more Baby Berthas at Hobby Lobby to build sorta-clones of BB, BBB, and Ranger. They are sooo cheap right now.
Ah, the rope test. I've seen a college athlete wind up the heavy hammer.
None to be had at any of the Hobby Lobbys around here... they discontinued carrying it. I've looked at 5+ in the area, since we're using them as the bases of a "kit bash" competition at our clubs 25th anniversary.
Guess I need to break down and order online.
cbrarick
Wildman CT
- Joined
- Jan 23, 2009
- Messages
- 3,155
- Reaction score
- 1,019
i'd say put down your strings and get with the 2000's.....
seriously, swing tests are notoriously flawed. All those variables, some of which are completely out of control.
We can control string length
we can, in an extremely rough way, control speed
we can't control any cross winds because you can't control mother nature
We can't control our perception of what's happening while we worry about not smashing the rocket (no human can do two things well simultaneously)
we're not certain what other variables are present, and what their effect size is.
Yes, sims aren't perfect, but they give you a starting place.
there is some limitations. Know them and understand them.
I use Rocsim for most rockets
For high performance rockets, RS Aero II because it's just more accurate at speeds well above mach 1.
Odd Rocs will always create a challenge. the best solution is to get them far enough away that they can't come back to the flight line. Anyone who went to URRF in 2022 will tell you I flew my J powered 3 inch diameter superrocc (27.75 feet long) from the away cell..... it was never gonna get anyone from there!
seriously, swing tests are notoriously flawed. All those variables, some of which are completely out of control.
We can control string length
we can, in an extremely rough way, control speed
we can't control any cross winds because you can't control mother nature
We can't control our perception of what's happening while we worry about not smashing the rocket (no human can do two things well simultaneously)
we're not certain what other variables are present, and what their effect size is.
Yes, sims aren't perfect, but they give you a starting place.
there is some limitations. Know them and understand them.
I use Rocsim for most rockets
For high performance rockets, RS Aero II because it's just more accurate at speeds well above mach 1.
Odd Rocs will always create a challenge. the best solution is to get them far enough away that they can't come back to the flight line. Anyone who went to URRF in 2022 will tell you I flew my J powered 3 inch diameter superrocc (27.75 feet long) from the away cell..... it was never gonna get anyone from there!
I'm not seeing how a swing test is seriously flawed. If it passes the swing test, it's going to be stable if the motor thrust to weight is sufficient to get it off the rod at an appropriate flying speed. Pretty simple really.
If it doesn't pass the swing test, find another method to validate its stability. (swap these methods around depending on your access to technology)
If no stability validation method is available, launch it at LDRS at a pad specified by the RSO/LCO/Launch Director.
I don't think anyone has done a RASAero file for a Super Big Bertha. (I might be wrong, but those fins are not likely to survive Mach+)
- Joined
- Sep 5, 2009
- Messages
- 3,538
- Reaction score
- 1,675
lakeroadster
When in doubt... build hell-for-stout!
Last edited:
- Joined
- Aug 6, 2022
- Messages
- 3,269
- Reaction score
- 2,969
Well, when I wrote the text you quoted, OzHybrid had not yet posted in this thread. So it's kinda silly for you to suggest that I should have read something that hadn't been written yet.
However, to the point, I've read the whole thread up until now and I don't think OzHybrid and I have any significant disagreement. My post was an explanation of some of the whys behind the whats that OH has expressed.
lakeroadster
When in doubt... build hell-for-stout!
And the "long rocket" is defined as...
- Joined
- Aug 6, 2022
- Messages
- 3,269
- Reaction score
- 2,969
A rocket that appears to be unstable in the swing test when in reality, it is stable with an appropriate safety margin.
Yes, the argument is completely circular. Use your judgement. Or a sim, if your rocket is one that will be simmed reasonably accurately.
Examples:
But, as I explained, a long rocket will be made to appear less stable than it actually is in the swing test, so I have no disagreement with OzHybrid's statement that a reasonably conventional rocket that does well in the swing test will more likely than not fly just fine.
Yes, the argument is completely circular. Use your judgement. Or a sim, if your rocket is one that will be simmed reasonably accurately.
Examples:
- Estes Star Orbiter, two BT-60 tubes. Its swing test result might be marginal.
- An Estes Star Orbiter built with just one of the tubes is probably going to be reasonably evaluated if the string in the swing test is reasonably long. However long that is. As long as you can make it and actually perform the test. (It's better than a two-tube Star Orbiter for a bunch of other reasons, too, some related to this thread.)
- A Mean Machine can be expected to not reflect its actual flight performance in a swing test.
But, as I explained, a long rocket will be made to appear less stable than it actually is in the swing test, so I have no disagreement with OzHybrid's statement that a reasonably conventional rocket that does well in the swing test will more likely than not fly just fine.
It's real world, but it's not a real flight, and the differences (which others have cited) are much more significant the differences between reality and a simulation.
The best thing that be said about the swing test is it isn't as hopelessly awful as the "cardboard cutout" method.
Last edited:
lakeroadster
When in doubt... build hell-for-stout!
It would seem that there is nothing on this site that can't be trampled on and discarded. Folks have been using Vern Estes' swing test for decades, successfully.
Absolutely. But that doesn't mean it's the best solution -today- for most rocket designs. The swing test certainly has its place for edge cases.
lakeroadster
When in doubt... build hell-for-stout!
If you define "edge cases" as any rocket less than 4 ft in length that are LP or MP... I totally agree.
The best thing about the swing test is that if the RSO refuses to let to let you fly over stability concerns, you can demonstrate the swing test and if you pass he will let you fly.
Sorry - you're making the assertion that 'any rocket less than 4 ft in length that are LP or MP' don't mesh well with simulation?
- Joined
- Jul 18, 2022
- Messages
- 1,751
- Reaction score
- 1,764
I'm sorry, but I need to chime in here. A simulation on a computer that has nothing to do with the actual rocket in question is a poor way to test anything IMHO. It doesn't take into account how you built that rocket. I can start with a fine kit that has tested stable before it was manufactured. But if I slather 32 ounces of Epoxy into the fin-can because I don't know what I am doing, I am going to throw off the CG by a considerable amount, no matter what the "Sim" says.
And I say this as someone with a great amount of experience at screwing things up.
And I say this as someone with a great amount of experience at screwing things up.
Sure, but that's operator error in both the assembly and simulation.I'm sorry, but I need to chime in here. A simulation on a computer that has nothing to do with the actual rocket in question is a poor way to test anything IMHO. It doesn't take into account how you built that rocket. I can start with a fine kit that has tested stable before it was manufactured. But if I slather 32 ounces of Epoxy into the fin-can because I don't know what I am doing, I am going to throw off the CG by a considerable amount, no matter what the "Sim" says.
And I say this as someone with a great amount of experience at screwing things up.
Garbage in, garbage out. Mass and CG of the fully assembled rocket should *always* be measured and overridden in the sim before final analysis is done. Always.I'm sorry, but I need to chime in here. A simulation on a computer that has nothing to do with the actual rocket in question is a poor way to test anything IMHO. It doesn't take into account how you built that rocket. I can start with a fine kit that has tested stable before it was manufactured. But if I slather 32 ounces of Epoxy into the fin-can because I don't know what I am doing, I am going to throw off the CG by a considerable amount, no matter what the "Sim" says.
And I say this as someone with a great amount of experience at screwing things up.
lakeroadster
When in doubt... build hell-for-stout!
Both... do both. Run the sim and do the swing test. On LP or MP rockets, less than 4 ft in length.
- Joined
- Sep 5, 2009
- Messages
- 3,538
- Reaction score
- 1,675
These are simulation assumptions, just like any others.
rcktnut
Well-Known Member
Bingo!! Kit or scratch built, mass and CG should always be determined after the build and then entered into the sim. I've never weighed separate components on a scratch built rocket and entered it into the sim. I let the sim determine the mass according to the material used in that component. After built, weigh and determine CG. Adjust CG if needed.
I guess a string test is better than nothing for a light, small design that is impossible to enter into a sim.
- Joined
- Sep 5, 2009
- Messages
- 3,538
- Reaction score
- 1,675
You sure like black and white rules for rather nuanced engineering approaches.
If less than 10:1, always do Base Drag Hack, else don't do it.
If LPR or MPR less than 4 ft, then do swing test.
What is the significance of 4 feet? What about HPR less than 4 feet?
Last edited:
Similar threads
