How do you check stability in large rockets

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aviator81

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Ive been wondering a lot on this but how do you check stability in a large rocket. i know in model rocketry there's always the swing test and things like that but how to you check stability on a rocket that is too big to be able to do that
 
Most Kits will have A Rocksim or OpenRocket simulator file that provides this information.

If building from scratch....you enter the parts dimensions into the simulator and it will provide them. OpenRocket is free...google it.

Then there is always the old fashion method....Barrowman equations.
 
Ahhh... the barrowman equations.....

Worked 'em out one time for fun. :p
 
17170618022_a71ecc50aa_b.jpg


The gray dot is the calculated center of pressure. As long as the rocket is moving "forward/up/etc" it is the focal point of the aerodynamic forces on the rocket. The blue dot is the center of gravity. As long as the CG is ahead of the CP, you're good to go.... Typically you want the separation to be at least one body tube distance to be able to absorb any changes during flight and not lose stability.

At least thats what I recall off the top of my head....If I've messed any of that up I'm sure someone will chime in.
 
Most Kits will have A Rocksim or OpenRocket simulator file that provides this information..

that's good for the CP. for thr CG, I hang the rocket with a rope making a loop around the rocket on the ceilling of my shop , I slide the rope until the rocket is in balance. I do it also hang to the canopy on the field.
 
When we RSO, we balance the rocket on the scale, two birds, one stone.
 
For a down-and-dirty: I hold the rocket near each end and slide my hands toward each other until my fingers touch. That's the CG. Have a helper --like my girlfriend or one of the kids-- measure from nosecone tip to fingers, and update Rocksim with this info. Use Rocksim's calculations for CP. Biggest I've done with this is 7', 12Kg. I may have to reconsider if there's something I can't lift --like why am I building so far outside my budget, because if I can't lift it, I'm fairly certain I can't afford the motor...


Later!

--Coop
 
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Don't use a rope, you will throw our your rotator cuff.

I use a saw horse.
 
Big thing I would say is trust the CP calculation, but not CG.
 
You train the horse to trot in a circle whilst reciting "The Jabberwocky." If the rope vibrates like the gyre and gimble in the wabe, your borrogroves are not mimsy. This is of utmost concern, as if they're not mimsy, then, by default, they're outgrabe, which means the rocket cannot fly in that configuration, of course. The mome raths have to be outgrabe... never the borrogroves.



Later!

--Coop
 
You train the horse to trot in a circle whilst reciting "The Jabberwocky." If the rope vibrates like the gyre and gimble in the wabe, your borrogroves are not mimsy. This is of utmost concern, as if they're not mimsy, then, by default, they're outgrabe, which means the rocket cannot fly in that configuration, of course. The mome raths have to be outgrabe... never the borrogroves.



Later!

--Coop


:rofl:
 
that's good for the CP. for thr CG, I hang the rocket with a rope making a loop around the rocket on the ceilling of my shop , I slide the rope until the rocket is in balance. I do it also hang to the canopy on the field.

...and if it's a *really* big rocket that you can't hang from the rope, you can get the CG by 'moments' (force (or weight) x distance) -- like this:

Balance each individual part (i.e. find the CG of just that part) and measure that CG location from one, common datum (i.e. the tip of the nose cone). For intermediate parts, you'll have to measure from the forward edge and then add to that the additional distance up to the nose cone tip. Multiply the individual weight by that 'moment arm' (which is the 'moment' of that part -- don't worry about the units of this (in-lb), as it'll all resolve back to "inches" when we're finished). Do this for each individual part (again, remember, the 'moment arm' of ALL parts *has* to be referenced back to ONE, COMMON DATUM (the nose cone tip) - that's the only way this works). Once you have the moments of all the pieces, ADD them up and then DIVIDE by the total weight of all components. The answer you get will then be the moment arm of the total (from the nose cone tip) - which is the location of the overall CG.

-- john.
 
Big thing I would say is trust the CP calculation, but not CG.

Best post in a long time! In my humble opinion, sim/CP programs are there to provide a CP location and offer some reasonable flight simulations.

CG is a physical property of a loaded rocket....not something that should be calculated in a hypothetical world. I recognize that input of proper masses of components should result in an accurate CG location, but oftentimes people fail to input the masses and/or the CG still comes out wrong.

In my idealistic world, sim programs would eliminate the phrase "your rocket is stable" (in reality it may not be!), or at least eliminate the mass from the program...it has trained people to rely upon the sim's interpretation of the CP-CG relationship instead of performing a CG check on the actual model.

Off my soap box now:cool:.

-Eric-
(who is a broken record, but has witnessed the following sequence of events many, many times: unstable rocket followed by the owner saying "....but Rocksim said it was stable")
 
I measure the CG by basically balancing the completed rocket on a simple balancing beam (less the motor) and then input that number into Open Rocket.
So here's a question - say my stability is great than 1.0 with no motor but when I select a certain motor, the stability drops to 0.6. Would you add nose weight?
 
I measure the CG by basically balancing the completed rocket on a simple balancing beam (less the motor) and then input that number into Open Rocket.
So here's a question - say my stability is great than 1.0 with no motor but when I select a certain motor, the stability drops to 0.6. Would you add nose weight?

Absolutely. 0.6 is an unstable rocket. If nose weight is impossible in the given situation, go with a smaller motor.

I mark my CP with a piece of tape, and then for each motor, I rebalance it and mark my CG. If it isn't 1 to 1.5 cal., then some adjustment needs to be made.
 
Absolutely. 0.6 is an unstable rocket. If nose weight is impossible in the given situation, go with a smaller motor.

I mark my CP with a piece of tape, and then for each motor, I rebalance it and mark my CG. If it isn't 1 to 1.5 cal., then some adjustment needs to be made.

Thanks!
 
Depends. My Minie-Maggs, for example, are close to 0.6 with most motors, and stable. But there's a fair amount of base drag on them and other rockets similarly-configured (i.e: short, fat).


Later!

--Coop
 
Depends. My Minie-Maggs, for example, are close to 0.6 with most motors, and stable. But there's a fair amount of base drag on them and other rockets similarly-configured (i.e: short, fat).


Later!

--Coop

Got it. Rocket in question definitely wasn't short/fat!
 
Build a smaller scale model and do flight tests.

CG won't be the same. Seven-foot long rocket with a 54mm MMT won't balance the same as a seven-inch long rocket with an 18mm MMT. CP will be the same if it is scale, but CG won't.
 
CG won't be the same. Seven-foot long rocket with a 54mm MMT won't balance the same as a seven-inch long rocket with an 18mm MMT. CP will be the same if it is scale, but CG won't.
A rocket scientist would find a work-around for that.
 
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