V2: The laws of physics were suspended yesterday (swing test)

[Blast it Tom!]

I had a hard time figuring out where to put this thread. I built an Estes V2 for my brother-in-law, and courtesy @James Duffy used the smaller scale fins - it looks so much nicer. But of course there are stability concerns, especially when you shove a black powder E motor up the tailpipe. So I've been super-busy and, while I have a very good and reasonable calculation of the center of pressure, I haven't had time to add the CG calc to the spreadsheet. Aw, what the heck - I'll just do a swing test! Added wadding, packed the 'chute, stuffed the E motor in, now I'm in full light trim.

I was immediately concerned when I saw where the rocket balanced - I knew it was behind my calculated CP. Well, it's on a string, what could go wrong? I'd kinda like to see how an unstable rocket acts on a swing test... Head out to the yard...
...
Oh! Just fine! Flew in beautiful, stable circles about me (nose forward, BTW... ). And I'm like, "What the heck?" So I double-checked my CP calcualtion and it's fine - CP about 2.26" forward of the aft end of the body tube - and it "moves" correctly - back with stock fins, forward with no fins. So I wonder how this happened? A successful string SWING test with the CG about 3/4" behind the CP?

 

[MidOH]

[Bruce Lee voice: On]
"Strings don't fly.''

All of these tests for CP have error. Sting test puts the CP rearward. Cardboard test puts it waaaaay forward. Sim tests just lock up my Smartphone and try to buy a tank of jet A in Albania.

 

[lakeroadster]

I had a hard time figuring out where to put this thread. I built an Estes V2 for my brother-in-law, and courtesy @James Duffy used the smaller scale fins - it looks so much nicer. But of course there are stability concerns, especially when you shove a black powder E motor up the tailpipe. So I've been super-busy and, while I have a very good and reasonable calculation of the cenert of pressure, I haven't had time to add the CG calc to the spreadsheet. Aw, what the heck - I'll just do a swing test! Added wadding, packed the 'chute, stuffed the E motor in, now I'm in full light trim.

I was immediately concerned when I saw where the rocket balanced - I knew it was behind my calculated CP. Well, it's on a string, what could go wrong? I'd kinda like to see how an unstable rocket acts on a swing test... Head out to the yard...
...
Oh! Just fine! Flew in beautiful, stable circles about me (nose forward, BTW... ). And I'm like, "What the heck?" So I double-checked my CP calcualtion and it's fine - CP about 2.26" forward of the aft end of the body tube - and it "moves" correctly - back with stock fins, forward with no fins. So I wonder how this happened? A successful string SWING test with the CG about 3/4" behind the CP?

Did you do an Open Rocket simulation on it?

 

[Blast it Tom!]

Did you do an Open Rocket simulation on it?

No, I was going old-school (moments and centroids), but I have been so busy I've never had time to do the CG calculation, and as old Joe Shigley said, "A good engineer never went wrong with good test data." i.e. test trumps our finely-crafted calculations. I was actually just going to do the swing test Saturday night when I had to drop everything and race down to my in-laws house - my mother-in-law fell trying to keep the cat in the house, and both dislocated and fractured her hip.

I'm resisting the simulators because when I get retired here, I want to work through the math myself. I have an original copy of "Topics in Advanced Model Rocketry". I didn't have the chops to understand it when I bought it as a freshman in college... but now...

 

[Blast it Tom!]

[Bruce Lee voice: On]
"Strings don't fly.''

All of these tests for CP have error. Sting test puts the CP rearward. Cardboard test puts it waaaaay forward. Sim tests just lock up my Smartphone and try to buy a tank of jet A in Albania.

All the more reason to crack the books on the real stability criteria! Yeah, my spreadsheet is basically a motorized cardboard test. But that's the first time I've heard that the string test puts it rearward. Are you saying the fact that it "passed" the string test is not necessarily proof that it will not become a skywriter?

 

[tsmith1315]

I'm resisting the simulators because when I get retired here, I want to work through the math myself. I have an original copy of "Topics in Advanced Model Rocketry". I didn't have the chops to understand it when I bought it as a freshman in college... but now...

I'm very sorry to hear about your mother-in-law, that's a bad injury. But this part of your message is wonderful!

 

[MidOH]

It's way better than nothing. But yeah, string test, cardboard test, and the Albanian theft programs, all put the CP in a different spot. But at least you did your due diligence and should be ''safe'' to launch.

We used to just watch our rockets fly. They'll wobble as you go up in engine sizes, then add nose weight. If they weather vane, consider less weight, more motor. I prefer old school. If the rocket just sucks, boot stomp it, and build another. Don't get hung up on something way off.

Since my rockets have nearly always been ''normal''. I can set the CG in a normal range and usually get good results. No payload, and the fins need to be bigger. Dual deployment screamers, make the fins smaller.

 

[Blast it Tom!]

It's way better than nothing. But yeah, string test, cardboard test, and the Albanian theft programs, all put the CP in a different spot. But at least you did your due diligence and should be ''safe'' to launch.

We used to just watch our rockets fly. They'll wobble as you go up in engine sizes, then add nose weight. If they weather vane, consider less weight, more motor. I prefer old school. If the rocket just sucks, boot stomp it, and build another. Don't get hung up on something way off.

Since my rockets have nearly always been ''normal''. I can set the CG in a normal range and usually get good results. No payload, and the fins need to be bigger. Dual deployment screamers, make the fins smaller.

Much thanks! Even as a kid, I didn't get to fly much, but when I read that Estes report that I could design my own, pal, I was gone! Doing what's essentially a spreadsheet with a slide rule, pencil, and paper - I credit that with pushing me toward being an engineer. I tried to program the Monroe-Lytton programmable calculator to do the heavy lifting and even optimization in my last two years of high school, but it had all went to blazes when I was about 16 (bad, bad family breakup), and though I continued to learn and attempt to design, I was essentially out of the flying business.

Ah, Mid-Ohio! Flat ground and fewer rocket-eating trees! Perhaps someday...

 

[neil_w]

I'm resisting the simulators because when I get retired here, I want to work through the math myself. I have an original copy of "Topics in Advanced Model Rocketry". I didn't have the chops to understand it when I bought it as a freshman in college... but now...

I see no reason why they would be mutually exclusive. Until you work through the math (and even then), a simulator is a valuable tool. Why avoid it? I see this often on TRF and I confess I don't understand it.

 

[lakeroadster]

I'm resisting the simulators because when I get retired here, I want to work through the math myself. I have an original copy of "Topics in Advanced Model Rocketry". I didn't have the chops to understand it when I bought it as a freshman in college... but now...

Trust, but Verify....

I'm a huge spreadsheet fanatic, I've got a spreadsheet for nearly every project I do, so I get where you are coming from. But why re-create the wheel? Open Rocket is free and it even allows you to download the data to put into your own spreadsheet. I've done that for my scratch build rockets. If nothing else it would be a great free tool for you to use to double check your spreadsheet.

I'm sure once you try Open Rocket you'll be asking yourself "Why is it that I wanted to re-create the wheel?"

 

[tsmith1315]

It's not always about the destination!

 

[boatgeek]

Best wishes for a quick and complete recovery for your MIL! That's a rough injury.

I've seen recommendations to also try doing the swing test with the rocket starting out backwards. If it flips around to stable flight, it's likely to be fine in practice.

I feel like simulations are like FEA or CFD work. I wouldn't try to do either of those by hand or spreadsheet, but I also know how to look at them skeptically to figure out of they're lying to me.

 

[Blast it Tom!]

I see no reason why they would be mutually exclusive. Until you work through the math (and even then), a simulator is a valuable tool. Why avoid it? I see this often on TRF and I confess I don't understand it.

Trust, but Verify....

I'm a huge spreadsheet fanatic, I've got a spreadsheet for nearly every project I do, so I get where you are coming from. But why re-create the wheel? Open Rocket is free and it even allows you to download the data to put into your own spreadsheet. I've done that for my scratch build rockets. If nothing else it would be a great free tool for you to use to double check your spreadsheet.

I'm sure once you try Open Rocket you'll be asking yourself "Why is it that I wanted to re-create the wheel?"

It's not always about the destination!

Yes, but another aspect is time. And I'm concerned that if I start with OR or Rocksim, I might never wean myself from them. But of course they could be another point of data... it'd actually be interesting to see if any differences manifested between the two programs. And further (in this case) if they predict stability or a skywriter, where they find the CP/CG, etc.

Tim, thanks for your sympathy. I could divert us into a rant about the health care system that allowed her to lay for about 40 hours prior to surgery, or the idiotic Wuhan-flu related restrictions imposed on us as we tried to take care of the two of them (my father-in-law is 92, using a walker, and can't go that far with that, even), but... wrong thread!

 

[Blast it Tom!]

Best wishes for a quick and complete recovery for your MIL! That's a rough injury.

I've seen recommendations to also try doing the swing test with the rocket starting out backwards. If it flips around to stable flight, it's likely to be fine in practice.

I feel like simulations are like FEA or CFD work. I wouldn't try to do either of those by hand or spreadsheet, but I also know how to look at them skeptically to figure out of they're lying to me.

Yeah, I've been pretty active with FEA throughout my career, and a smattering of CFD as well, and seen some of the wonderful ways that you can fool yourself! I always tell the "kids" if you can't do a simple hand calc or calcs to verify your methods, you could very easily be doing something dangerous. I once caught a verification calc of ring buckling case in a major software program that was using the wrong correlation for verification and had to point out to them that their mistake overstated the buckling load by 33% - a very dangerous situation.

Much thanks for the tip! I knew there was some reason that I didn't take the string off! I'll try it later.
....
To be continued!

 

[neil_w]

And I'm concerned that if I start with OR or Rocksim, I might never wean myself from them.

I would humbly submit that this is a poor reason to shun a useful tool. But ultimately it's up to you.

 

[lakeroadster]

Since you're a spreadsheet guy... I'll take one last stab at getting you excited about Open Rocket... this is the spreadsheet I created from the Open Rocket data download.

 

[Blast it Tom!]

I would humbly submit that this is a poor reason to shun a useful tool. But ultimately it's up to you.

Well, I ain't completely shunning it - just kind of a "not yet". I don't have much time anyway... 24 scheduled working days left and that will change! Meanwhile, they're picking as much from my pea-brain as they can!

@lakeroadster - now that's what I'm talking about! I've enjoyed watching some of your builds.

Best to you both!

 

[lakeroadster]

Thanks @Blast it Tom!

Are you saying the fact that it "passed" the string test is not necessarily proof that it will not become a skywriter?

Consider the actual speed of the rocket during a swing test... vs actual flight. And things such as wind cocking aren't evaluated during a swing test.

Is it good that it passed the swing test? Sure.

But Marvin and Tammi can elaborate..

 

[Blast it Tom!]

Well, yeah, I've fiddled with things like that in my head. Now you have me thinking of counting the rpms, measuring the swing radius, and figuring what the linear velocity of the rocket was! <nerd>

And, as you say, the response to a side force. Ain't nothing like the real thing, baby! But I don't want to wreck a good rocket, one of the prettiest finishes I've ever achieved (ain't saying much, but, hey...), And so my quest to assure stability will continue! I'll try a tail-first as soon as I can and see if it flips around. But I suspect I'll need more nose weight.

<walks away singing softly> "well, it don't mean a thing if it don't pass that swing.... doo-wha - doo-wah

 

[lakeroadster]

@rharshberger posted up a .ork file here for a BT-80 based Estes V-2. Maybe he can shed some light on this by making the fins smaller?

I looked at modifying his V-2 model by making the fins smaller, and, of course, that made the model unstable. The correction is to add nose weight. But his model has so many over rides in regard to each components and sub-assemblies mass that it's probably best if he modifies the model.

 

[OverTheTop]

Yes, but another aspect is time. And I'm concerned that if I start with OR or Rocksim, I might never wean myself from them.

Why would you want to wean yourself off them? Simulations are a great tool for learning performance and design sensitivities, and "proving" designs. Also refer to my signature for a relevant comment regarding simulations .

 

[Blast it Tom!]

Why would you want to wean yourself off them? Simulations are a great tool for learning performance and design sensitivities, and "proving" designs. Also refer to my signature for a relevant comment regarding simulations .

Well, like I said upthread, I want to re-awaken some analytical brain cells and work through some of the math in "Topics in Advanced Model Rocketry", a book I bought as a freshman in college - and I couldn't understand it! But NOW... ! My work has mostly been stress, strain, fatigue, fracture mechanics, FEA and vibration/structural modal analysis relating to large motors and generators. I told elsewhere on here a couple of months back when I did some "recreational calculus" and estimated the speed and time to clear the tower for a typical Saturn V launch. I'm by no means a very good mathematician, but I've always favored the design/analytical/test/diagnostic side of my profession.

Just the fact that the venerable swing test seems to show that a rocket I calculate to be unstable somehow is stable (maybe) is enough to get me digging through books to find out why! Oh, Mr. Barrowman, where are you?!

And yes, I agree heartily with your signature! See my reply at #14.

 

[Blast it Tom!]

Best wishes for a quick and complete recovery for your MIL! That's a rough injury.

I've seen recommendations to also try doing the swing test with the rocket starting out backwards. If it flips around to stable flight, it's likely to be fine in practice.

I feel like simulations are like FEA or CFD work. I wouldn't try to do either of those by hand or spreadsheet, but I also know how to look at them skeptically to figure out of they're lying to me.

Thanks for the well wishes!

And it seemed to pass the reverse swing. It was tough to even get it going backwards. At any sort of speed it turned nose into the wind. It's time to start reading! I'll still probably add some nose weight. It's not like I have a whole desert playa, my longest open field dimension is maybe 800 ft.

 

[Sandy H.]

First off, I agree with the well wishes for your mother-in-law (and possibly the cat. . . ).

Second, if I read correctly, you're in your final month before retirement. If that's correct, congratulations and best wishes!

So, to the third point: something you know for fact due to your career thus far - garbage-in = garbage-out. That is why you want to work through the math on your own based on published data as well as observed data and draw your own conclusions. Having done that, you'll be able to use simulations effectively and sanity check everything as you go.

Sounds to me like you aren't retiring, you're just realigning your daily tasks to match your interests and using the tools and methods you've used your whole life to do so.

If so, congratulations^2. Well done.

Sandy.

 

[Blast it Tom!]

You've got it! Much thanks for everything, Sandy!

The cat will probably be "re-homed". It's a very nice, friendly cat. It'd be great for a family with kids, but it's much too active and ready to dash outside for my aged in-laws. We're hoping my daughter takes it, but her hubby has allergy issues - and they have canaries! "I tawt I taw a puddy cat!"

 

[Sandy H.]

Well, I hope the 'puddy cat' does find a fitting home. Our boy cat (15-ish) has tried to kill me twice a day at least by tripping, falling in front of me or darting between my legs, since we got him. Our girl cat stays 6ft away from any shoe at all times. He's been kicked, stepped on and tripped over a ton of times, but she's never been while in our care. Both were adopted, so no clue of their background. Either way, I hope the feline finds a good home. They do have personalities for sure.

From my personal experience (machine design), I know you're busy passing along decades of knowledge to the guys who are in their 20's-40's. I hope they are lapping up everything you can provide. I'm in the 40-50's range of that scale, but my company lost a lot of senior engineers due to real (not forced) retirement in the last few years. I'd worked with these guys for 20 years, so I knew the loss of their knowledge was very serious and as we had been friends for years I'd learned a bunch, but not nearly the full spectrum. I still call a few of the guys on occasion for work stuff, but most often talk to them about real life

I hope you find retirement to be similar, as I think it is very fitting to the engineers that came before us. I wish the younger guys would have learned the version of engineering that involved paper and a calculator vs. FEA/CFD/3D Kinematics, but they'll find their way as well, probably making the world a better place too. But, man, the knowledge of the slide-rule guys to a young engineer is so important, but often disregarded. I'm so glad to be able to talk both technically and personally with my mentors. I still gain respect for them every day I work.

Sandy.

 

[Blast it Tom!]

Absolutely! And I've worked with some real giants. The owner, a dear gentleman of 85 years, is a man of sterling character and, dear glory he's probably forgotten more that I know about designing and building large electric machines. He has 7 patents now; back 10 years or so he did the repowering study on the crawler to be sure it had enough power to get the new Space Launch System up to the pad. His retired partner, now in his 90's, taught me so much about methods, and both of them knew their way around their slide rules. One fellow, a Dr, Lee Kilgore, I think he could see magnetic flux! And he had a slide rule tie clip and he'd use his thumbnail for the cursor. I know the felllow who designed the large Westinghouse hydro generators at Grand Couleee, the largest in the world at the time - he taught me a lot. Another British fellow, long retired but still collaborating... These guys knew how to avoid "analysis paralysis" and get things built. Just amazing people. I still remember the owner destroying my first 3-day finite element modeling effort with his 6 inch slide rule in about 2 minutes. I have some truly wonderful "war stories"!

(As I side note, I keep a working slide rule application on my computer desktop. I troll new hires with it. "What are you doing there?" "What is that thing?" )

 

[Greg Furtman]

I still have my slide rules from college & still know how to use them. I graduated from college in 1973.

 

[Blast it Tom!]

Me as well. I started college in 1973. Should've graduated in 1977, but managed to put it off until 1987... The boss I mentioned above gave us all 6" plastic ones as well. He said, "In case the power goes out!"

I think that any discussion on TRF involving calculations will devolve into reminiscing about our slide rules!

 

[OverTheTop]

I just bought a 12" slide rule on eBay and it is sitting on my desk next to me. I learned how to use it (dad was an engineer) back in primary school. Unfortunately I lost the original one in a house move somewhere.