Quantcast

Stability question

The Rocketry Forum

Help Support The Rocketry Forum:

ScrapDaddy

Well-Known Member
Joined
Jan 26, 2010
Messages
2,081
Reaction score
1
My rocket is .68 staticly stable but a few inches on pad the rocket is at a "safe take off speed" so once that speed is reached that the static stability no longer applys...... I think that's how it works the faster the more stable it is right? Because the fins are more effective?
 

rocketlaw

Well-Known Member
Joined
Jan 29, 2009
Messages
237
Reaction score
0
Not exactly. Speed does have an effect on stability, but it is kind of already built into the stability margin. That is why a stable rocket that does not achieve the required speed will be unstable.

The truth is that these values are just estimates. Also, the actual stability margin changes during flight depending on many factors, including speed and crosswind component. That is the reason for using the 2 caliber rule. Some are trying to get this rule changed based on percentage of body length, but that is a different thread all together. So 0.16 stability margin might result in a stable flight on a no wind day. However, as wind or other factors come into play, your rocket has a bigger chance of becoming unstable.
 

luke strawwalker

Well-Known Member
Joined
Jan 18, 2009
Messages
9,139
Reaction score
14
My rocket is .68 staticly stable but a few inches on pad the rocket is at a "safe take off speed" so once that speed is reached that the static stability no longer applys...... I think that's how it works the faster the more stable it is right? Because the fins are more effective?
Stability is a VERY complicated and thorny issue, because essentially it is a MOVING target that greatly depends on the rocket, the flight parameters, and the environmental parameters of the atmosphere through which the rocket is moving.

Static stability is the easiest concept to grasp and to define, because dynamic stability, that is to say, stability in flight basically is constantly changing to some degree or other. Therefore static stability seeks to make these changes moot by defining the stability of the rocket "at rest", hopefully in a close approximation of typical conditions the rocket will be exposed to in flight, and conditions which the rocket is unlikely to exceed in flight.

CG, center of gravity, is of course rather easily obtained by balancing the launch-ready rocket (without ignitor, since that is ejected at ignition and does not fly with the rocket (usually, HOPEFULLY!). CG gets a bit more complicated on "non-symmetrical rockets" like the space shuttle stack, multistage rockets which eject series stages or parallel stages in flight, rockets carrying off-center payloads (cameras, etc.) and the like. Even in a "typical" three-fin and nosecone (3FNC) rocket, CG moves SOME, as the propellant is burned in engine, the CG moves forward, since the nose weight (and hopefully the parachute weight and location) remains the same. Other engine types (liquid, predominantly, though hybrids are a different case too) have their CG locations change in different ways, usually aft, as the engine burns. BUT for 99% of hobby rocketry, it can be said the CG moves FORWARD as the engine burns.

CP, center of pressure, is a MUCH more dynamic and difficult to measure property. CP is affected by the design, fin size, span, shape, drag, the rocket's overall shape, size (Reynolds numbers come into play at certain size thresholds), angle of attack, weight distribution (moments of inertia, length, oscillation characteristics, etc.) wind speed and direction, flight speed, static stability margin, "apparent" angle of attack from wind, air density, air turbulence, and probably a hundred other factors. Some have extremely subtle effects that are barely measurable. Some have significant effects that can have OVERT effects, and some are subtle at times, and overt at others, and combine and compound with other effects to create greater CP changes than at other times, and sometimes certain factors can cancel each other out or work together beneficially to enhance stability. It's EXTREMELY complicated (that's a part of why rocket science is HARD:))

SO, how do we get a handle on it?? One way is to look at the 'worst case scenario'... if we design the rocket to be stable with the WORST conditions it's likely to ever see, we can be pretty darn sure it'll be stable under virtually every flight condition we're likely to come up against. Fortunately this is a rather simple matter. By determining the center of lateral area we can determine the *rough* CP point, which, interestingly enough, is the point at which the CP would approximately be if the rocket were ninety degrees to the airflow-- essentially 'flying sideways'... So if the rocket could stabilize from THAT, we can be pretty darn sure it's stable. This is called the cardboard cutout method. Draw an exact outline of the rocket on cardboard, cut it out, and balance that "profile" on a ruler-- the balance point is the center of lateral area. Rocksim can be reset under the 'calculate stability by' box by clicking it and choosing "Cardboard cutout method" from the drop down menu.

Now this method, as you can tell, is VERY VERY conservative. It can lead to EXCESSIVE stability, where the rocket will gyrate and oscillate excessively, weathercock excessively in windy conditions, and other such undesirable effects that rob energy and therefore altitude. That's where the Barrowman Method comes in, which makes certain assumptions about the shape of the rocket and their aerodynamic affects on the rocket's flight. Those assumptions simplified the calculations back in the pre-computer era so that rocketeers with sufficient math skills who wanted to take the time and effort to crunch the numbers by hand could actually sit down and calculate the CP to greater accuracy than the cardboard cutout method would allow. This would allow greater precision and optimization of the rocket design. But it was a VERY labor-intensive process taking several hours at best, and STILL had quite a few limitations because of the ASSUMPTIONS made in creating and simplifying the mathematical formulas. For instance, the Barrowman Method couldn't handle but either 3 or 4 fins-- odd numbers of fins were out. Complicated shapes beyond the basic tubular rocket with radial symmetry (transitions, nosecones, tailcones, evenly spaced fins) were out-- so calculating CP with side pods, boosters on the sides, gliders, etc. was out. Basically, at the end of the day, you STILL had an APPROXIMATION.

The "Rocksim Method" used in the Rocksim program is based on the Barrowman Method and uses it's calculations as the basis of it's function, with some tweaks to the assumptions and some more complex mathematical modelling to get 'closer' to the "real" CP than the basic Barrowman Method allowed. Complex math isn't much of a problem anymore with the massive computing power available now (your PC is more powerful than the Apollo flight computers that got us to the MOON!). Still, there are underlying ASSUMPTIONS in the math models that may or may not accurately reflect or predict the CP location in every instance-- some parameters may be inaccurately modelled, some may be ignored, some may be 'over conservative' in the math models, etc.

Basically, to know the TRUE CP, you'd have to measure all the influences on the rocket at EVERY GIVEN MOMENT IN FLIGHT and plug those measurements into equations to determine the EXACT CP AT THAT PRECISE MOMENT, because as the parameters change from moment to moment, the CP changes as well.

Fortunately, the approximations we get from computer programs, hand crunched numbers, and even cardboard cutouts are enough 99% of the time. The real ART of stability determination is in contest rocketry, where balancing all those forces, and making certain design tradeoffs combine to either give you a winning design or leave you in everyone else's dust...

SO, summing up, static stability is the mathematically calculated stability points (CG/CP relationship) of the rocket. As the rocket takes off and speed increases, the fins DO have more stabilizing effect. This is important to know in relation to launch guide length and motor selection, because the rocket needs to get sufficient speed before leaving the launcher for the fins to stabilize it. The static stability doesn't 'go away', but it DOES change (subtley we hope!) For heavy rockets, this means either A) longer launch guides, or B) a "bigger motor", either in a higher impulse class (switching from an "A" to a "B" motor, or choosing a motor with a higher 'peak thrust', say a A8 instead of an A3, which will accelerate the heavier rocket faster off the pad, reaching fin-stabilizing speed faster.

Hope this helps! OL JR :)
 

ScrapDaddy

Well-Known Member
Joined
Jan 26, 2010
Messages
2,081
Reaction score
1
Ummm..... Luke? How long have you had that sitting in your brain maybe you should do somthing to get your head of rocketry......:dark: you could write a book it helps a bit..... I think I'm wondering because my rocket will be supersonic withen 300 feet and an unstable rocket at Mach 1.12 is a VerY VERY bad thing:y: I'm not quite sure if I should launch....
 

luke strawwalker

Well-Known Member
Joined
Jan 18, 2009
Messages
9,139
Reaction score
14
Ummm..... Luke? How long have you had that sitting in your brain maybe you should do somthing to get your head of rocketry......:dark: you could write a book it helps a bit..... I think I'm wondering because my rocket will be supersonic withen 300 feet and an unstable rocket at Mach 1.12 is a VerY VERY bad thing:y: I'm not quite sure if I should launch....
Challenger should have taught the world "if in doubt, DON'T LAUNCH!"

There are enough 'gotchas' in the world that we DON'T know about to take excessive risks with the ones we DO know about! What do they say at NASA, "it's the UNKNOWN unknowns that get you!"...

Book... nah. I've just read a LOT and put it all together with practical experience. I screw up enough to still remain an enthusiastic yet rank amateur... :)

Later! OL JR :)
 

Fred22

Well-Known Member
Joined
Jan 6, 2009
Messages
2,458
Reaction score
3
Stability is a VERY complicated and thorny issue, because essentially it is a MOVING target that greatly depends on the rocket, the flight parameters, and the environmental parameters of the atmosphere through which the rocket is moving.

Static stability is the easiest concept to grasp and to define, because dynamic stability, that is to say, stability in flight basically is constantly changing to some degree or other. Therefore static stability seeks to make these changes moot by defining the stability of the rocket "at rest", hopefully in a close approximation of typical conditions the rocket will be exposed to in flight, and conditions which the rocket is unlikely to exceed in flight.

CG, center of gravity, is of course rather easily obtained by balancing the launch-ready rocket (without ignitor, since that is ejected at ignition and does not fly with the rocket (usually, HOPEFULLY!). CG gets a bit more complicated on "non-symmetrical rockets" like the space shuttle stack, multistage rockets which eject series stages or parallel stages in flight, rockets carrying off-center payloads (cameras, etc.) and the like. Even in a "typical" three-fin and nosecone (3FNC) rocket, CG moves SOME, as the propellant is burned in engine, the CG moves forward, since the nose weight (and hopefully the parachute weight and location) remains the same. Other engine types (liquid, predominantly, though hybrids are a different case too) have their CG locations change in different ways, usually aft, as the engine burns. BUT for 99% of hobby rocketry, it can be said the CG moves FORWARD as the engine burns.

CP, center of pressure, is a MUCH more dynamic and difficult to measure property. CP is affected by the design, fin size, span, shape, drag, the rocket's overall shape, size (Reynolds numbers come into play at certain size thresholds), angle of attack, weight distribution (moments of inertia, length, oscillation characteristics, etc.) wind speed and direction, flight speed, static stability margin, "apparent" angle of attack from wind, air density, air turbulence, and probably a hundred other factors. Some have extremely subtle effects that are barely measurable. Some have significant effects that can have OVERT effects, and some are subtle at times, and overt at others, and combine and compound with other effects to create greater CP changes than at other times, and sometimes certain factors can cancel each other out or work together beneficially to enhance stability. It's EXTREMELY complicated (that's a part of why rocket science is HARD:))

SO, how do we get a handle on it?? One way is to look at the 'worst case scenario'... if we design the rocket to be stable with the WORST conditions it's likely to ever see, we can be pretty darn sure it'll be stable under virtually every flight condition we're likely to come up against. Fortunately this is a rather simple matter. By determining the center of lateral area we can determine the *rough* CP point, which, interestingly enough, is the point at which the CP would approximately be if the rocket were ninety degrees to the airflow-- essentially 'flying sideways'... So if the rocket could stabilize from THAT, we can be pretty darn sure it's stable. This is called the cardboard cutout method. Draw an exact outline of the rocket on cardboard, cut it out, and balance that "profile" on a ruler-- the balance point is the center of lateral area. Rocksim can be reset under the 'calculate stability by' box by clicking it and choosing "Cardboard cutout method" from the drop down menu.

Now this method, as you can tell, is VERY VERY conservative. It can lead to EXCESSIVE stability, where the rocket will gyrate and oscillate excessively, weathercock excessively in windy conditions, and other such undesirable effects that rob energy and therefore altitude. That's where the Barrowman Method comes in, which makes certain assumptions about the shape of the rocket and their aerodynamic affects on the rocket's flight. Those assumptions simplified the calculations back in the pre-computer era so that rocketeers with sufficient math skills who wanted to take the time and effort to crunch the numbers by hand could actually sit down and calculate the CP to greater accuracy than the cardboard cutout method would allow. This would allow greater precision and optimization of the rocket design. But it was a VERY labor-intensive process taking several hours at best, and STILL had quite a few limitations because of the ASSUMPTIONS made in creating and simplifying the mathematical formulas. For instance, the Barrowman Method couldn't handle but either 3 or 4 fins-- odd numbers of fins were out. Complicated shapes beyond the basic tubular rocket with radial symmetry (transitions, nosecones, tailcones, evenly spaced fins) were out-- so calculating CP with side pods, boosters on the sides, gliders, etc. was out. Basically, at the end of the day, you STILL had an APPROXIMATION.

The "Rocksim Method" used in the Rocksim program is based on the Barrowman Method and uses it's calculations as the basis of it's function, with some tweaks to the assumptions and some more complex mathematical modelling to get 'closer' to the "real" CP than the basic Barrowman Method allowed. Complex math isn't much of a problem anymore with the massive computing power available now (your PC is more powerful than the Apollo flight computers that got us to the MOON!). Still, there are underlying ASSUMPTIONS in the math models that may or may not accurately reflect or predict the CP location in every instance-- some parameters may be inaccurately modelled, some may be ignored, some may be 'over conservative' in the math models, etc.

Basically, to know the TRUE CP, you'd have to measure all the influences on the rocket at EVERY GIVEN MOMENT IN FLIGHT and plug those measurements into equations to determine the EXACT CP AT THAT PRECISE MOMENT, because as the parameters change from moment to moment, the CP changes as well.

Fortunately, the approximations we get from computer programs, hand crunched numbers, and even cardboard cutouts are enough 99% of the time. The real ART of stability determination is in contest rocketry, where balancing all those forces, and making certain design tradeoffs combine to either give you a winning design or leave you in everyone else's dust...

SO, summing up, static stability is the mathematically calculated stability points (CG/CP relationship) of the rocket. As the rocket takes off and speed increases, the fins DO have more stabilizing effect. This is important to know in relation to launch guide length and motor selection, because the rocket needs to get sufficient speed before leaving the launcher for the fins to stabilize it. The static stability doesn't 'go away', but it DOES change (subtley we hope!) For heavy rockets, this means either A) longer launch guides, or B) a "bigger motor", either in a higher impulse class (switching from an "A" to a "B" motor, or choosing a motor with a higher 'peak thrust', say a A8 instead of an A3, which will accelerate the heavier rocket faster off the pad, reaching fin-stabilizing speed faster.

Hope this helps! OL JR :)
A great post on stability :)
Thanks
Fred
 

MarkII

Well-Known Member
Joined
Jan 18, 2009
Messages
8,212
Reaction score
9
Ummm..... Luke? How long have you had that sitting in your brain maybe you should do somthing to get your head of rocketry......:dark: you could write a book it helps a bit..... I think I'm wondering because my rocket will be supersonic withen 300 feet and an unstable rocket at Mach 1.12 is a VerY VERY bad thing:y: I'm not quite sure if I should launch....
Did you try cutting down the fins like I suggested? I think that your fin shape with its forward strakes is contributing to the problem by bringing the CP further forward than you would like.

LS expressed it very well, but many rocketeers gain these insights over time, as they acquire launch experience and, just as important, as they learn from other rocketeers. It is always good to be reminded of the fundamentals of stability in flight from time to time. Harry Stine described them many years ago. LS was carrying on a long tradition of passing on knowledge and insights to newer rocketeers. Paying it forward is a guiding principle in this hobby, which is one of the reasons why it is so rewarding.

You are right to be concerned about the stability. A model rocket that is veering and yawing during its flight will never achieve supersonic speed.

MarkII
 
Last edited:

ScrapDaddy

Well-Known Member
Joined
Jan 26, 2010
Messages
2,081
Reaction score
1
Ummm iv improved stability to 1 cal but when in doubt don't launch is the rule I'm going to be following from now on and MarkII im not so worryed about the
I wasn't worryed about it not going supersonic I was worryed about it arcing over and killing somone (I'm not kidding)
rocketers are friendly as long as you don't injure them it's nice to have some rocketeers back from the ""golden age""
 

MarkII

Well-Known Member
Joined
Jan 18, 2009
Messages
8,212
Reaction score
9
Ummm iv improved stability to 1 cal but when in doubt don't launch is the rule I'm going to be following from now on and MarkII im not so worryed about the
I wasn't worryed about it not going supersonic I was worryed about it arcing over and killing somone (I'm not kidding)
rocketers are friendly as long as you don't injure them it's nice to have some rocketeers back from the ""golden age""
You apparently didn't appreciate my little bit of dry humor. :rolleyes: ;)

MarkII

P.S. BTW, does Layne even know that you are on his payroll? :confused:

P.P.S. Don't let Fred see that sig of yours. :y:
 
Last edited:

gpoehlein

Well-Known Member
Joined
Jan 18, 2009
Messages
3,555
Reaction score
8
My rocket is .68 staticly stable but a few inches on pad the rocket is at a "safe take off speed" so once that speed is reached that the static stability no longer applys...... I think that's how it works the faster the more stable it is right? Because the fins are more effective?
Actually - to summarize, it's not that the faster the rocket is, the more stable it is, but that the faster the rocket it, the sooner it reaches a stable speed. (We're talking acceleration vs velocity here). As long as your launch rod is long enough to keep the rocket going straight until it reaches that speed, you're fine.

As far as calibers of stability, that depends on what type rocket it is. If it is a short stubby rocket (like the Fat Boy), base drag has an effect and one caliber is more than enough. It will be stable even if the rocket checks out as neutrally stable. On the other hand, a superroc such as the Mean Machine needs at least 4 or 5 calibers of stability if there is even a hint of wind. For rockets with a "normal" proportion (Alpha, etc.) 1 to 2 calibers is usually fine. Remember also that you can either shift the CP back by making the fins longer or moving or sweeping them further back, or you can move the CG forward by adding nose weight. But if you have the stability up to 1 caliber by either Barrowman or Rocksim, you should be fine. That is, by the way, measured WITH an unfired motor in it, right? :p
 

Scotty Dog

Well-Known Member
Joined
Mar 2, 2010
Messages
7,310
Reaction score
95
OK then. Now my brain hurts. hhahaha I remember having a paper on balancing a rocket from estes. Simple thing it seemed. Im sure it was something like-find CG, tie a string around the rocket at that point and swing it around yo head. If it nose dived, you added a little weight to the bottom and if it nosed up you added some weight to the top. So hows that sound??? Scotty Dog
 

gpoehlein

Well-Known Member
Joined
Jan 18, 2009
Messages
3,555
Reaction score
8
OK then. Now my brain hurts. hhahaha I remember having a paper on balancing a rocket from estes. Simple thing it seemed. Im sure it was something like-find CG, tie a string around the rocket at that point and swing it around yo head. If it nose dived, you added a little weight to the bottom and if it nosed up you added some weight to the top. So hows that sound??? Scotty Dog
Yup - the "Swing Test" or "String Test" (I've heard both used). You are close, but it is more simple than that - swing the rocket suspended from the CG - if if "flies" around you nose first, it is considered stable. If it takes any other configuration (rotating around the string, "flying" sideways) it is not stable and will need more nose weight.

A couple of caveats, however. First, it doesn't work well with longer rockets - the string just can't be made long enough to get the proper moment - the nose of the rocket will always be pointing outward from the direction of travel (kind of like having a really strong wind blowing on the nose as it leaves the launch rod). Second, a close to marginal design will sometimes swing tail first - that does not always mean the rocket will be unstable, but it might.
 

ScrapDaddy

Well-Known Member
Joined
Jan 26, 2010
Messages
2,081
Reaction score
1
You apparently didn't appreciate my little bit of dry humor. :rolleyes: ;)

MarkII

P.S. BTW, does Layne even know that you are on his payroll? :confused:

P.P.S. Don't let Fred see that sig of yours. :y:

You Know it was the funniest thing its been over a month and i have yet to receive my paycheck........ :y: i am also wondering if her knows im on his payroll can someone PM him?
 

Latest posts

Top