Affect of mach on CP

[les]

Sorry folks - another thread asking questions on a mach breaking min diameter rocket...

I recall seeing various threads (although I couldn't locate any right now) that A rocket that is going to break mach needs a higher level of stability because the CP will move forward

I've modeled my rocket in Rocksim, OR, and RasAero (although the RasAero was a rougher model with some numbers from memory - it needs to be cleaned up)

I grabbed a plot from OR. It shows the CP moves back, then it moves forward again to its original position, then back again, then slowly forward again.
Meanwhile, the CG moves forward as the motor burns.
Note - I also looked at a similar plot from RasAero which showed similar changes

So am I missing something? I can get more performance if I tighten up the CP/CG closer to 1 caliber

 

[bobkrech]

You really don't want to cut the Cg/Cp too close. The CP moves around as a function of Mach number, and the Cg moves as you burn propellant.

The Cg/Cp ratio is really a measure of the twitchiness of the rocket. Rockets that react too fast to disturbances (such as wind gusts) will not be stable, while rockets that are extremely stable will weathercock into the wind and be relatively immune to gusts. IMO if you do not want an unstable rocket you should run you sim and design the rocket so the Cg/Cp does not drop below 2 at any Mach number. YMMV.

Bob

 

[les]

I thought the "rule of thumb" was to have at least 1 caliber between the CG & CP.

More is better, until you are subject to extreme weathercocking.

But again, I've read in other threads that for going mach you should have 2 or more calibers.

Where I am confused is the plot shows the CP moves back and the CG move forward making the rocket more stable.
By the time the CP starts moving forward again, the CG has moved way up due to the weight loss of the burnt propellant.

It appears that the worst case CP/CG is right at launch....

I forgot to mention in my original thread that this is a minimum diameter 38mm rocket, so 1.6"

I'm just trying to understand the standard rule of thumb for over-stability for mach flights versus what the simulations show....
Thanks

 

[REK]

I thought the "rule of thumb" was to have at least 1 caliber between the CG & CP.

More is better, until you are subject to extreme weathercocking.

But again, I've read in other threads that for going mach you should have 2 or more calibers.

Where I am confused is the plot shows the CP moves back and the CG move forward making the rocket more stable.
By the time the CP starts moving forward again, the CG has moved way up due to the weight loss of the burnt propellant.

It appears that the worst case CP/CG is right at launch....

I forgot to mention in my original thread that this is a minimum diameter 38mm rocket, so 1.6"

I'm just trying to understand the standard rule of thumb for over-stability for mach flights versus what the simulations show....
Thanks

Let me explain a few things and include my opinion.

First off the CP drops due to the change of air pressure dropping at the event the rocket breaks the sound barrier. After that the pressure begins to increase, hence why the CP moves forward.

Now from what I've read and what makes sense to me, the Center of Pressure moves in accordence to the surface area of your rocket.

Your Center of pressure will always move forward. This effect, however is minimized by the fins. If you have big fins the CP will move less forward. If you have small fins, the CP will move more forward.

I think having stability at 2 calibers is more suspectible to weathercocking. Especially once all fuel has been spent, the stability will rise so much that winds in higher elevation can easily make it weathercock.

Open Rocket, RasAero, and Rocksim all have a good CP shift plot graphs. By using that you can determine the stability change vs time and keep the rocket stable at one caliber or just slightly over one caliber.

Hope this makes sense and is some help for you.

 

[Buckeye]

It appears that the worst case CP/CG is right at launch....

Correct. More specifically, at launch guide clearance. Now, add some wind to your simulation, and it gets worse. I don't get all this Mach shift discussion if the rocket can't even get off the rail correctly.

See my very similar thread here:

https://www.rocketryforum.com/showt...ive-stability-at-launch-guide-clearance-in-OR

The rocket described here came off the tower a little squirrely and at a downwind angle I was not comfortable with. For the second flight, I increased the launch guide from 5' to 8', added nose weight, and used a higher-thrust motor, all of which contributed to eliminating the negative stability off the tower in the sim. The actual flight was much straighter and with some weathercocking, which I can tolerate. 13,589' on a J453

 

[Chuck Rogers]

I recommend at least 2.0 calibers of stability margin at Supersonic Mach numbers. Why is this greater than the 1.0 calibers of stability margin typically used with the Barrowman Method for Subsonic? At Supersonic Mach numbers, especially Mach 2-3, if the stability margin falls below 1.0 calibers, there may be coning, and/or pitch-roll coupling, and other undesirable effects. The additional 1.0 calibers is additional stability margin, to make sure the total stability margin never falls below 1.0 calibers. As an example, a rocket might have a stability margin at Mach 3 of 1.20 calibers, but there might be an error in the CP prediction of 0.25 calibers, lowering the stability margin below 1.0 calibers and into the coning/pitch-roll coupling potential danger area.

I'll be posting in the near future comparisons of RASAero II Supersonic Center of Pressure (CP) predictions with wind tunnel data out to Mach 4.5. RASAero II really does an excellent job predicting the Supersonic CP. But there are some interesting variations in the wind tunnel CP data for one of the configurations at Transonic. There can always be mispredictions in aero data. Add the extra 1.0 calibers stability margin (for a total Supersonic stability margin of 2.0 calibers) and don't risk it.

RASAero II has the feature that it will display an error message, shown below, if the stability margin falls below 2.0 calibers at any point in the flight (at any Mach number). If the stability margin falls below 2.0 calibers, the Flight Simulation still runs to completion, it just notifies the User with the warning message below. If the rocket goes unstable, RASAero II stops the Flight Simulation at the point the rocket went unstable, and plots the trajectory data up to that point.

Once you get the less than 2.0 calibers stability margin warning message, you can pull up the CG and CP plot shown below (note the plot below is for a two stage rocket), and look for where you're violating the 2.0 calibers stability margin limit, and then decide how to address the issue (larger fins, change the fin shape, add nose weight).


Chuck Rogers
Rogers Aeroscience

 

[Chuck Rogers]

Correct. More, specifically at launch guide clearance. Now, add some wind to your simulation, and it gets worse. I don't get all this Mach shift discussion if the rocket can't even get off the rail correctly.

By using the 2.0 calibers stability margin limit for all Mach numbers, RASAero II is also applying this limit for all Subsonic Mach numbers. The rationale is 1.0 calibers stability margin for the Barrowman Method, and then an additional 1.0 calibers stability margin for wind induced instability. The angle of attack present when the rocket clears the launch rail in a wind moves the CP forward.

The Rogers Modified Barrowman Method option included in RASAero II includes this forward movement of the CP with angle of attack due to viscous cross-flow on the rocket body. Of course if the 2.0 calibers stability margin limit is applied to the Rogers Modified Barrowman Method Subsonic CP rather than the Pure Barrowman Method Subsonic CP (which does not include the forward movement of CP with angle of attack), then this may be too conservative. This is why the RASAero II User gets the warning message, and then should look at the CP plot and the CG and CP plot and use their own judgment on whether there is an issue or not. (The rocket may have reduced stability at the peak angle of attack just off the launch rail for just fractions of a second before the angle of attack starts to decrease, and jet damping, also included in RASAero II, will keep the dynamic stability angle of attack oscillations at reasonable values.)


Chuck Rogers
Rogers Aeroscience

 

[dixontj93060]

Man I need to upgrade to RASAero II!

 

[les]

I need to spend some time to clean up my RASAero model, but this is what I have from OR.

The rocket is a Madcow 1.6" Tomach modified to minimum diameter

Here is a screen shot of the OR simulation (note - I did start with the file from Madcow - thanks)



And here is a plot of the CP/CG calibers and Mach number



Some things I noticed. The first figure on the right states I have around 2 calibers at Mach .30, but the CP/CG curve doesn't show 2 calibers until Mach 1

It does show that I end up with almost 5 calibers after motor burnout!! Will probably have lots of weathercocking at altitude.

NOTE - I ran the simulations with a 5' rail. Velocity off the rail is 128 fps. And I believe the club I fly at actually has 6' and 8' rails
The simulation was also run with 11mph winds.
Recovery shows high speed but I will use electronics for apogee and cable cutter for lower altitude
I will be using fly away guides for the launch


Finally, the ORK file

View attachment fgtomach16dd_Short1.ork

 

[markkoelsch]

Man I need to upgrade to RASAero II!

It is free, and appears to work really well.

 

[Chuck Rogers]

I need to spend some time to clean up my RASAero model, but this is what I have from OR.

The rocket is a Madcow 1.6" Tomach modified to minimum diameter

Here is a screen shot of the OR simulation (note - I did start with the file from Madcow - thanks)

View attachment 277644

And here is a plot of the CP/CG calibers and Mach number

View attachment 277645

Some things I noticed. The first figure on the right states I have around 2 calibers at Mach .30, but the CP/CG curve doesn't show 2 calibers until Mach 1

Not to speak for the Open Rocket developers, but I'm assuming that the 2.0 calibers stability margin at Mach 0.30 on the right-hand side of the rocket drawing is with no wind.

Again, you can apply the standard stability margin for the Barrowman Method (which with no wind I assume is the method used for the CP shown on the rocket drawing) of 1.0 calibers, and then add an additional 1.0 calibers of stability margin for wind induced instability.

The rocket clears the launch rail with the 11 mph wind, and with wind the method used in Open Rocket (as I remember the Galejs What Barrowman Left Out Tech Article Method) moves the CP forward. Actual stability margin when the rocket leaves the launch rail, 0.30 calibers positive stability margin.

Longer rail of course, higher velocity leaving the rail, for the same wind speed a lower angle of attack and therefore less forward movement of the CP.

The same analysis of course can be done on RASAero II. My viscous crossflow model has improvements over the Galejs model (although I did use the Galejs body planform area equations, thank you Galejs), but it is good that the Galejs model is included in Open Rocket.

So using the Barrowman Method for Subsonic CP, I recommend a minimum stability margin of 2.0 calibers for Subsonic. This example illustrates why the extra 1.0 calibers stability margin is there for wind induced instability.

With the Rogers Modified Barrowman Method, since it already includes the forward movement of CP with angle of attack, applying a 2.0 calibers stability margin to the predicted Subsonic CP is probably too conservative. But RASAero II just uses a minimum stability margin of 2.0 caliber for All Mach numbers and All CP methods, and after the warning message is displayed, the User can then go look at the CG and CP details and further assess the stability situation.


Chuck Rogers
Rogers Aeroscience

 

[Buckeye]

It appears that the worst case CP/CG is right at launch....

This is where OR and RA differ the most, it seems. See below for identical simulations. This is a Blackhawk38, J453, 8' launch guide, 10 mph wind.

OR instantly moved the CP forward dramatically to 38.5" at launch guide clearance (the static M=0.3 Cp shown in the GUI window is 42.4") and lots of oscillation. The RA CP is rock steady at 42.4" until t= 1.0 s when Mach shift starts.

OR is much, much more conservative at launch. My original design in OR showed a negative stability margin off the rail, which lead me to add nose weight, increase the launch guide length, and choose a motor with more oomph. RA probably would have passed it as is.

 

[Chuck Rogers]

The RA CP is rock steady at 42.4" until t= 1.0 s when Mach shift starts.

Buckeye:

For this analysis the Rogers Modified Barrowman Method needed to be checked under the Options Tab. RASAero II uses the Barrowman Method as the default method for Subsonic CP. The Rogers Modified Barrowman Method needs to be selected or the default Barrowman Method will be used.

To be true to the pure Barrowman Method, in RASAero II the forward movement of CP with angle of attack (wind induced instability) is not included when the Barrowman Method is used. The straight line CP with Mach number up to Transonic, with no variation with angle of attack, is exactly what it should look like for a pure implementation of the Barrowman Method. The Rogers Modified Barrowman Method includes this forward movement of CP with angle of attack, and also produces a different CP at zero degrees angle of attack. For most comparisons with wind tunnel data at zero degrees (or +/-2 degrees, +/- 4 degrees) angle of attack it is more accurate, but the results are often very similar to the Barrowman Method results. Basically Barrowman left off additional normal force at the nose and the tail, but since he missed additional normal force at both the nose and tail, often times the Barrowman Method CP is very close to the CP from theoretically more accurate methods. This is why with known things left out of the Barrowman Method, it has still withstood the test of time predicting rocket CP all of these years. (Except for wind induced instability, addressed here.)

I'd check the Rogers Modified Barrowman Method under the Options Tab, and you'll see some interesting results, similar to the Open Rocket results.


Chuck Rogers
Rogers Aeroscience

 

[Buckeye]

Hmm. I thought RMBM was selected, along with all turbulent flow. I will double check later and update. Thanks.

 

[Buckeye]

OK, my bad. I unchecked RMBM above. Now, with it checked, the comparison looks like this:



OR is still more conservative at launch by 5" in CP!

Which to believe? The flight simulated here noticeably weathercocked immediately out of the tower, so I am gonna say I have plenty of stability as shown by RA.