Super Big Bertha fails spin test

[lakeroadster]

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?

At some point it becomes a function of rocket length vs usable string length. A 4ft rocket length maximum works for me and most folks that are swing testing rockets.​

What about HPR less than 4 feet?

Sure, go ahead. Wear your helmet. ​

 

[lakeroadster]

These are simulation assumptions, just like any others.

Nope, those are questions for the OP, who did a swing test.​

 

[lakeroadster]

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?

Nope, my comment was in reference to swing tests.​

 

[Capt. Eric]

If simulations are so perfect, why do we bother building and flying the things? Why do we hold our breath when launching a new rocket for the first time?

Sometimes people put all their faith into whatever a computer tells them. It may be a pretty good predictor, but it's not real world. I never understood the propensity to trust computers and their programs so completely.

 

[techrat]

If simulations are so perfect, why do we bother building and flying the things? Why do we hold our breath when launching a new rocket for the first time?

To be fair.... some people only play Kerbal.

But yes, computers are marvelously imperfect, because at the end of the day, they are only as good as their software, which is written by fallible humans.

 

[rcktnut]

The problem with our hobby is that you can put a rocket motor on/in anything and it will fly one way or another if the motor has enough power.

 

[SolarYellow]

Thrust: The solution to, and cause of, all of rocketry's problems.

 

[neil_w]

If simulations are so perfect, why do we bother building and flying the things? Why do we hold our breath when launching a new rocket for the first time?

This is a bizarre argument.

If the reason you build rockets and hold your breath when they launch is that you’re unsure if the simulations are accurate, then we are apparently in two completely different hobbies.

 

[Capt. Eric]

This is a bizarre argument.

If the reason you build rockets and hold your breath when they launch is that you’re unsure if the simulations are accurate, then we are apparently in two completely different hobbies.

Swing and a miss!

 

[JoePfeiffer]

If simulations are so perfect, why do we bother building and flying the things? Why do we hold our breath when launching a new rocket for the first time?

Sometimes people put all their faith into whatever a computer tells them. It may be a pretty good predictor, but it's not real world. I never understood the propensity to trust computers and their programs so completely.

How do you get from anything said in this thread to simulations being perfect, or people putting all their faith into a computer program? All I'm seeing from the people advocating sims is that they are much better predictors than the swing test. That's certainly the only point I've been trying to make.

 

[lakeroadster]

All I'm seeing from the people advocating sims is that they are much better predictors than the swing test. That's certainly the only point I've been trying to make.

Well, that and bashing swing tests...​

The best thing that be said about the swing test is it isn't as hopelessly awful as the "cardboard cutout" method.

 

[JoePfeiffer]

Well, that and bashing swing tests...​

How bad swing tests are is part of why sims are better.

 

[lakeroadster]

How bad swing tests are is part of why sims are better.

How good / bad swing tests are, is in large part, based on the knowledge of the guy on the end of the string.​

How good / bad sims are, is in large part, based on the knowledge of the guy on the keyboard.​

​

Do stupid crap... win stupid prizes.​

 

[Art Upton]

Thrust: The solution to, and cause of, all of rocketry's problems.

Or F4 Jet Fighters

 

[bad_idea]

Oddroc that won't sim: swing it. As @OzHybrid says, if it swings stable, it should fly stable.

Normal rocket: sim it.

Normal rocket that you're worried sims with too little stability for whatever rule of thumb you favor*: swing it, but understand you're getting a very conservative result for the reasons @SolarYellow explained early in the thread. Or shoot it alone in an empty field and record the results for better understanding.

* Personally, @Buckeye 's CFD analysis of cp shift of a Fat Boy has me thinking the 8% of OAL rule of thumb I usually use between cg and cp is probably overly conservative.

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.

I always bring a laptop to launches to rerun my sims because my memory is quite feeble when it comes to numbers.

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.​

No one in this thread is saying that a swing test will not demonstrate a stable rocket if it is passed. What is being said is that failing a swing test does not indicate an unstable rocket (and some, though not all. are therefore terming it a bad test). In other words, it is no surprise at all that people using the swing test over the decades have met with success.

We now have more precise tools than the cardboard cutout and the swing test though, and these tools allow us to expand the envelope of what our rockets can be and do. All of aerospace history has been about making better calculations and using better tools and methods in order to expand the envelope. I don't know why we would turn our backs on that tradition now, nor do I know why we would turn our backs on known-good - yet known to be conservative - tools like the swing test when delving into areas those more recent tools can't handle.

 

[Cape Byron]

Oddroc that won't sim: swing it. As @OzHybrid says, if it swings stable, it should fly stable.

Normal rocket: sim it.

I must say that’s how I work.

 

[Daddyisabar]

I must say that’s how I work.

Hush little RSO don't say a word, Daddy's gonna buy you a Whooshing bird. If that Whooshing bird won't fly, Daddy's gonna go with the Police bye bye!

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Oddroc won't sim.
Oddroc don't swing, can't get it on the string.
Oddroc cardboard cut outs are all asymmetrical. It has four claw fins, two wings and a freaking tail!

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Yes Mr. RSO. Since it has no removable nose cone I had to determine nose weight and install at the beginning of the build, then attach cockpit to fuselage. It's an eight motor, sports scale Avro Lancaster, with canted tractors and widely spaced pods. I know where the CP was on the real airplane. So just need to weight it to fly vertical like a model rocket should. Easy peasy lemon squeezy. Didn't need a sim, swing or cut out tools. I used another tool that has always served me well. Thank you for the far far away pad assignment. It's just an ignition issue!



Golly, I think the Top Men are acting like they are at NASCAR, they just want to see a crash! That is the only reason they let me on the far far away pad...right? It's gonna be fun, again and again!

Gosh darn, let them down again! Better go argue sim vs. swing vs. cardboard vs. Homemade wind tunnel. That is soo much fun. Today I am TEAM SWING, but tomorrow? Leaning toward TEAM HOMEMADE WIND TUNNEL. Never used any of those flawed tools because they just don't work on my birds and I lack the knowledge/skills/ patience to use them. Someday my good boy rocket science dreams will come true. Until then, just scum and villainy.