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Stability at mach+ speeds

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TRFfan

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Would a rocket be fine with a 1.15 stability margin at mach 1.6? That's as fast as it'll go during the flight but i wanted to know if it would weathercock or not with that level of stability.
 

AlphaHybrids

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Would a rocket be fine with a 1.15 stability margin at mach 1.6? That's as fast as it'll go during the flight but i wanted to know if it would weathercock or not with that level of stability.
All rockets weathercock. Some more than others. Without more information it's hard to give a more concise answer. You should consider getting RasAero and running some simulations of stability vs. velocity to make sure that your rocket is stable over the entire flight profile. CP moves forward the faster you go.

Edward
 

manixFan

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Would a rocket be fine with a 1.15 stability margin at mach 1.6? That's as fast as it'll go during the flight but i wanted to know if it would weathercock or not with that level of stability.
You can run a CG vs. CP plot in Open Rocket or RASAreo to check stability at Mach+. Really depends on the parameters of your particular rocket. You want a chart like below.


Tony

Mach-stability-check.png
 

SpaceManMat

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You can run a CG vs. CP plot in Open Rocket or RASAreo to check stability at Mach+. Really depends on the parameters of your particular rocket. You want a chart like below.


Tony

View attachment 313277
Absolutly. The quest is not is it stable, the question is "is it stable at all speeds during flight"
 

TRFfan

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It is, (i checked RASAaero and openrocket) and they both gave the same results (1.17 cal at top speed, which is mach 1.6). I wanted to know whether or not it would weathercock at these speeds.
 

Buckeye

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Would a rocket be fine with a 1.15 stability margin at mach 1.6? That's as fast as it'll go during the flight but i wanted to know if it would weathercock or not with that level of stability.
More information needed. Stability margin depends on several things, not just speed. Plot stability vs. time.

The link in your sig says you created OpenRocket and RASAero simulations. So, what did they say?
 

mpitfield

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It is, (i checked RASAaero and openrocket) and they both gave the same results (1.17 cal at top speed, which is mach 1.6). I wanted to know whether or not it would weathercock at these speeds.
In order to predict that you would need to supply the motor that you are using and you would have to make some assumption about the weather conditions, specifically wind.
 

TRFfan

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More information needed. Stability margin depends on several things, not just speed. Plot stability vs. time.

The link in your sig says you created OpenRocket and RASAero simulations. So, what did they say?
They said that stability at top speed wouls be 1.17 cal.
 

TRFfan

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In order to predict that you would need to supply the motor that you are using and you would have to make some assumption about the weather conditions, specifically wind.
Wind would probably be around 10 to 20 mph, motor is a CTI G150.
 

AlphaHybrids

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It is, (i checked RASAaero and openrocket) and they both gave the same results (1.17 cal at top speed, which is mach 1.6). I wanted to know whether or not it would weathercock at these speeds.
Yes, your rocket will weathercock. You cannot stop it weathercocking unless you implement some sort of active controls. Now, the magnitude to which it weathercocks is dependent on the stability due to the length of the moment arm.

https://spaceflightsystems.grc.nasa.gov/education/rocket/rktcock.html

Edward
 

mpitfield

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Wind would probably be around 10 to 20 mph, motor is a CTI G150.
Well with 10-20 mph winds you are likely to have some weather cocking. The G150 seems to be a nice high thrust motor so you should be up and stable pretty quickly, mitigating your weather cocking considerably for the first boost phase of the flight. As with most flights it is very likely to turn into the wind once the velocity drops off during your coast phase. However I wouldn't be concerned with the stability at that stage as it will just improve.
 

markkoelsch

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You would likely be better with a bit more margin. More like 2. Some will argue that, and are entitled to.
 

watheyak

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All rockets will weathercock in the wind. Rockets with higher stability margins weathercock more than ones with lower stability margins.
 

TRFfan

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Well with 10-20 mph winds you are likely to have some weather cocking. The G150 seems to be a nice high thrust motor so you should be up and stable pretty quickly, mitigating your weather cocking considerably for the first boost phase of the flight. As with most flights it is very likely to turn into the wind once the velocity drops off during your coast phase. However I wouldn't be concerned with the stability at that stage as it will just improve.
Yeah that is what i thought for the G150.

You would likely be better with a bit more margin. More like 2. Some will argue that, and are entitled to.
A stability margin of 2 at top speed or at 0 speed?

Get a tracker ;)
Yeah i chose the telemini for the avionics.

All rockets will weathercock in the wind. Rockets with higher stability margins weathercock more than ones with lower stability margins.
So the rocket should be fine then? At top speed the stability isn't that high.
 

mpitfield

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A stability margin of 2 at top speed or at 0 speed?
Mark's advice mirrors what I have been told and read many times, Generally you should try to maintain at least 2 calibers during the entire flight of a Mach 1-2ish range flight. It provides a nice margin and unless you are trying to push the boundaries there is no down-side.

On a related matter I have read a few posts that in the higher Mach 2+ flights, that some recommend 3 calibers. Obviously not something you or I need to worry about at this stage, but once you get the Mach bug you tend to want more, so learning more on the topic is definitely in my future.
 

TRFfan

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Mark's advice mirrors what I have been told and read many times, Generally you should try to maintain at least 2 calibers during the entire flight of a Mach 1-2ish range flight. It provides a nice margin and unless you are trying to push the boundaries there is no down-side.

On a related matter I have read a few posts that in the higher Mach 2+ flights, that some recommend 3 calibers. Obviously not something you or I need to worry about at this stage, but once you get the Mach bug you tend to want more, so learning more on the topic is definitely in my future.
I'm trying to get as much altitude as i possibly can, and what I'm actually worried about is if the rocket will go unstable at these high speeds. I also want a decent safety margin of stability at top speed (for example if the stability at top speed is 1.1, i would want at least 0.3 cal of stability over that, so 1.4 cal). Would it go unstable with 1.4 cal at top speeds?

And I don't think I'll be going into the higher range of flights anytime soon. I'm only 14 and there still is a couple of years before i can get my L1 cert. With MPR motors i suppose i could push a G150 to about mach 1.9, but that would be it.
 

Exactimator

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Weathercocking and instability are actually two different things. Weathercocking is when the rocket turns into the wind and continues to fly stable. Rockets that are overstable tend to weathercock more.

Unstable is when there's not enough margin between the CG and CP and the rocket tumbles or sky-writes.
 
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TRFfan

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Weathercocking and instability are actually two different things. Weathercocking is when the rocket turns into the wind and continues to fly stable. Rockets that are overstable tend to weathercock more.

Unstable is when there's not enough margin between the CG and CP and the rocket loses stability and tumbles or sky-writes.
So i shouldnt worry about weathercocking but i should worry about going unstable?
 

Exactimator

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So i shouldnt worry about weathercocking but i should worry about going unstable?
Since your goal is highest altitude, you'll want to worry about weathercocking a bit. Weathercocking turns the rocket so it's not flying straight up, and you'll lose some altitude. Not much you can do about it other than launch on a windless day.

You always want to worry about stability. With an unstable rocket, best case you lose the rocket, worst case you damage property or injure someone.

[video=youtube;U7kYO5a42HY]https://www.youtube.com/watch?v=U7kYO5a42HY[/video]
 

TRFfan

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Since your goal is highest altitude, you'll want to worry about weathercocking a bit. Weathercocking turns the rocket so it's not flying straight up, and you'll lose some altitude. Not much you can do about it other than launch on a windless day.

You always want to worry about stability. With an unstable rocket, best case you lose the rocket, worst case you damage property or injure someone.

[video=youtube;U7kYO5a42HY]https://www.youtube.com/watch?v=U7kYO5a42HY[/video]
Yeah i know thats why i want at least 1.3 cal at top speed but i dont know if it'll go unstable.

View attachment 313328
 

Bat-mite

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Launch your rocket 180* away from the wind. The weathercocking will then pull it straight.
 

Bat-mite

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Launch your rocket 180* away from the wind. The weathercocking will then pull it straight.
Sorry, not 180* angle! I just mean "with the wind," as opposed to "into the wind." Your angle will depend on your sim. Try different angles and see what gets you the most altitude.
 

TRFfan

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Sorry, not 180* angle! I just mean "with the wind," as opposed to "into the wind." Your angle will depend on your sim. Try different angles and see what gets you the most altitude.
Can you do that with openrocket?
 

mpitfield

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I'm only 14
Okay fair enough that helps. I assumed you were a bit older and had a better base of knowledge. It may help you using OR to try to the rocket optimization tool, select tools, rocket optimization add the nosecone's mass to the parameter to optimize window, then under optimization options select your motor, the optimized value of "Stability (cal)", and the optimization goal "Seek value of" run the sim with your motor and see what it says. You can also use the min/max values as well as set the "Required stability" min max values, lots of tools in OR to help you figure this out. Once you get some results then post your findings with your OR file for others to review and provide a sanity check.
 

TRFfan

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Okay fair enough that helps. I assumed you were a bit older and had a better base of knowledge. It may help you using OR to try to the rocket optimization tool, select tools, rocket optimization add the nosecone's mass to the parameter to optimize window, then under optimization options select your motor, the optimized value of "Stability (cal)", and the optimization goal "Seek value of" run the sim with your motor and see what it says. You can also use the min/max values as well as set the "Required stability" min max values, lots of tools in OR to help you figure this out. Once you get some results then post your findings with your OR file for others to review and provide a sanity check.
Yeah i tried that now and either the results didnt get any better than the sim i had now or it was just impractical.

I'm pretty sure. I don't have it in front of me, but in your simulation options, you can set your wind direction and launch angle.
Ok i see that now. Thanks!
 

markkoelsch

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Sim it and plot stability vs time. Do not let the stability go under two, or at least not by much.

If you fly with such a slight margin as you are planning you are flirting with going unstable and losing your rocket.

Transonic is hard to predict due to compressibility issues, and you are spending a chunk of the flight there.

Your altitude will not be drastically different.

As you get to Mach 1.5 plus the Cp can move a bunch.
 

ActingLikeAKid

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Another thing to consider (that I've experienced myself, though on 100% subsonic launches):

Weathercocking goes into the wind; parachute drift goes with the wind. Using something like OpenRocket (with as much information as you have - wind speed, wind variance, altitude, humidity, etc etc) you should be able to play around with chute size (and if you have something like the chute release, opening altitude) so that you can get your rocket back close to where you launched. I have an Estes Argent that's overstable (around 4cal loaded, I think); I launched it on a moderately windy day. It weathercocked (into the wind) to the point that it was almost at the edge of the field at apogee, but then the wind brought it about halfway back under parachute.
 
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