Which stability method do you prefer?

[TeenRocketNerd]

I was working on Rocsim tonight, and got to wondering:

Which stability method do you look at/trust/believe the most?

Oh, and "all three" is not an option.... I'm curious as to which one you prefer.

This also applies directly to rocsim users; which method do you look at the most on rocsim, considering each method gives you different (sometimes drastically different) stability margins.

 

[ben]

rocsim!!! When I have it if not then borrowman comes in at a STRONG 2nd (I use RS mainly just cause I am lazy with the figures sometimes )

thanx, Ben

 

[jetra2]

I'll go with Barrowman for 500, Alex.

Wait a second...

I've worked the Barrowman Equation before. It's kinda fun. It's algebra, but easy, because you don't have to figure any numbers. They're all given to you on your rocket!

Jason

 

[cjl]

I generally use Barrowman (as calculated by Rocksim) because it tends to be more conservative than the rocksim method, and therefore is more of a guarantee of stability, IMO. I also swing test anything that I can, though some are ludicrously impractical (I didn't swing test the big deuce for example...).

 

[skippy-2]

What about C+D, cut outs and swing tests? That's my usual on scratch built stuff.

But if it is most trusted, I have to go with swing tests. Mind you, I don't have Rocsim...

 

[BlueNinja]

Originally posted by jetra2
I'll go with Barrowman for 500, Alex.

Wait a second...

I've worked the Barrowman Equation before. It's kinda fun. It's algebra, but easy, because you don't have to figure any numbers. They're all given to you on your rocket!

Jason

Another vote for Barrowman, because it's simple to work out. I wrote a program to do it for me, though, since I can't be bothered to buy rocksim

Cardboard cutout... Nah... I've heard of the inaccuracies it can have and would rather not deal with it.

Swing test... My one and only time trying this, I managed to *krunch* my Deuce against the clothesline pole... Dumb, I know, but I don't want to do it again.

 

[Stymye]

Rocksim.

It provides a more accurate (dynamic) simulation of the entire flight,by taking alot of data into consideration
(if the input is accurate)

2nd choice is Barrowman, mainly because it only provides me with a (static) simulation of a perfect windless day without any dynamic changes such as propellant weight, wind effect, and CD changes during the flight. however it is a proven means and works well enough in most cases.

Rocksim is more realworld so I use it.

 

[powderburner]

I like the CCO and Barrowman methods. They are quick, easy, and probably slightly conservative (which is OK with me). They have their quirks and foibles, but at least they don't cost me $95 to use.

 

[Stymye]

considering what I spend on rocketry,(tho I cringe to admit it)
$95 is not that much.

 

[teflonrocketry1]

Loose five scratch designs to unstability or ruin them during swing testing and you will understand how RockSim can pay for itself! RockSim is an extension of the Barrowman equations and puts in corrections that were impractical for the methods of calculation that were available in the late 1950's and early 1960's (the slide rule era).

Bruce S. Levison, NAR #69055

 

[PunkRocketScience]

I voted "swing test", but I use Rocksim too. There is a point when a swing test just isn't practical...say the Tripoli San Diego Little Joe at Plaster Blaster last year!

Often my designs don't easily go into rocksim, so I try and get them as close as possible in the software and then back it up with a swing test before making the rocket "pretty"....

 

[hokkyokusei]

They're just all tools in the tool box. They all work within their own limitations. There's no more point having a favourite stability method than there is favouring a hammer over a screwdriver. It depends on the job in hand.

 

[adrian]

Originally posted by TeenRocketNerd
I was working on Rocsim tonight, and got to wondering:

Which stability method do you look at/trust/believe the most?

Oh, and "all three" is not an option.... I'm curious as to which one you prefer.

"All three" is not an option because there are four choices.

I don't have Rocksim, being a cheapskate. If the rocket can be handled by VCP, I use its version of extended Barrowman. If not, I'll try a swing test. But I've built some rockets which VCP says are stable, swing test says are unstable, and in flight are actually stable; and I've even built a couple of rockets which VCP says are unstable, swing test says are stable, and in flight are actually stable. On the other side, I've also built one or two which VCP says are stable, swing test says are stable, and in flight are actually unstable.

So my preferred method is a combination of Barrowman, swing test, gut instinct, and just putting a motor in and yelling "Heads up".

 

[roadkill]

I'm for RockSim,

Works great in scratch building oddrocks
like my Vipers and that rather complex Griffin...

Thou I wish there'd be an easier way to do some
stuff than editing the RockSim files on WordPad...
(Hint hint, Bruce...)

 

[Micromeister]

A & d,
with or without a computer program. but flight testing in isolation is the only way to know for sure weather your project is flight worthy or not.

 

[kgholloway]

My preference is to use both the Barrowman and RockSim CP calculations in RockSim when designing a rocket. Barrowman is more conservative and I use that to determine my 1+ caliber of stability. When RockSim and Barrowman do not agree within a reasonable margin, say about 1/2 a caliber, then I start looking closely at my design to see what elements may be affecting stability.

Ken Holloway
NAR #78336, L-II

 

[Art Upton]

Originally posted by stymye
Rocksim.
..
It provides a more accurate (dynamic) simulation of the entire flight,by taking alot of data into consideration
(if the input is accurate)

..

Hi Andy,

I didn't know the rocksim' CP was a dynamic sim.

Now ya going to make me look tonight and see if it plots CP and moves it around as the rocket's flight progresses.

Does it do this? it might save me some lookin' and rootin' about tonight.

Good news if it does, thanks.

I've been diggin' in that military manual of the dynamic stability of free rockets tryin' to get a grip on that, but I don't have enough time;as I have some other electronics work I need to finish up on first.

 

[shockwaveriderz]

I'm old school I guess.. I like barrowman with extensions..... is this what rocksim does? HEY TIM.... I know rocksim uses USAF Datcom but does it also use barrowman extensions?

I have a .pdf copy of the orginal report if I can find it......

terry dean
nar 16158

 

[Hospital_Rocket]

I prefer a launch pad 200' away and a pile of sandbags!










(Actually I use RocSim)

 

[RevKeith]

Rocksim all the way!

 

[wwattles]

Originally posted by PunkRocketScience
I voted "swing test", but I use Rocksim too. There is a point when a swing test just isn't practical...say the Tripoli San Diego Little Joe at Plaster Blaster last year!

I suddenly had a mental image of Andy with a giant crane swinging the LJ out in the desert on a 200-foot long steel cable...

Personally, I use all of them, and swing-test whenever practical just to make sure. After all, as I like to say, "There's more than one way to skin a cat. Personally, I've found several!"

 

[apogee]

Originally posted by shockwaveriderz
I'm old school I guess.. I like barrowman with extensions..... is this what rocksim does? HEY TIM.... I know rocksim uses USAF Datcom but does it also use barrowman extensions?

I have a .pdf copy of the orginal report if I can find it......

terry dean
nar 16158

Just a slight correction: Datcom is used to estimate the drag coefficient.

For CP prediction, the RockSim method actually is the classic Barrowman equations without all the assumptions (what Bruce mentioned previously). So I trust the RockSim Method as much as the classic Barrowman equations.

Dynamic stability plays an important roll too. We're finding this out more and more as we get into the new RS-PRO development (https://www.apogeerockets.com/rs-pro.asp).

 

[Stymye]

Originally posted by Art Upton
Hi Andy,

I didn't know the rocksim' CP was a dynamic sim.

Now ya going to make me look tonight and see if it plots CP and moves it around as the rocket's flight progresses.

Does it do this? it might save me some lookin' and rootin' about tonight.

Good news if it does, thanks.

I've been diggin' in that military manual of the dynamic stability of free rockets tryin' to get a grip on that, but I don't have enough time;as I have some other electronics work I need to finish up on first.

yep,you can plot and the cd and will see it change.

 

[Pantherjon]

I use T.L.A.R. - That Looks About Right


So far batting .500...

Seriously I use Rocksim..tho admittedly I have only barely scratched the surface..

 

[adrian]

Update to my previous reply:

I do now have Rocksim. Its own calculations always seem to put the CP further back than the Barrowman version. So if I were designing for maximum altitude, I might accept the Rocksim version as evidence for an RSO and put a bit less weight in the nose cone. However, I'm generally not that bothered about maximum altitude, so I stick with Barrowman, and even then tend to include enough nose weight for a bit more than the customary 1 calibre between CG and CP.

My preferred method is therefore still a combination of Barrowman, swing test, gut instinct, and just putting a motor in and yelling "Heads up".

 

[dave carver]

VCP, the writer of which is a member of this forum. It's free online and is as accurate as any you can buy.

 

[uncle_vanya]

VCP, the writer of which is a member of this forum. It's free online and is as accurate as any you can buy.

What is the underlying math used by VCP? Is it relying on Barrowman or another method? RockSIM offers Barrowman and it's own method - I think the discussion in this case was geared to the math accuracy not the tool per say.

But I have been wrong before.

 

[cjl]

VCP is Barrowman based, IIRC.

 

[shreadvector]

VCP is Barrowman based, IIRC.

Yes. I clicked mine open and you can choose "Standard Barrowman" or "Extended Barrowman".

 

[boris katan]

I use the RockSim method. As noted above, it's the Barrowman method with improvements. Have to go with approximate modelling for more complicated rocket shapes.