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Mongoose 75 calipers of stability

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ColumbiaNX01

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I have a Mongoose 75. I did a stability check. It has come in just over 1 caliper of stability. I have always had the rule of thumb anywhere between 1 and 1.5 calipers was ideal. I have never flown a min diameter. My sim softwear Rasareo says it wants at least 2 calipers of stability because of the speeds it will go through.

What are your guys thoughts? Should I go with it or should I add some weight to the nose cone. Really, adding nose weight is no big deal.
 

Mightyrocketman

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Need at least 2 or more calipers because your CP will move up with high speeds.
 

markkoelsch

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Get it to two or more. Check the Cp location shift with increases in velocity. It can move a fair amount.
 

manixFan

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Run the plot in Rasaero where it shows your CP/CG relationship as it goes through Mach and beyond. Both the GC and CP move forward but the distance between them is dynamic so it's best to look at the plot over time. That will help you out.


Tony
 

tfish

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I've see a bunch of record attempts out at BR and have seen many of them not fly as planned. These rockets are right on the "edge" and the smallest thing sets them off. So, for the past few years I’ve been building my rockets with fins that are slightly more then 1 caliber of span. “slightly” is a relative term, for a 3″ rocket I go about 3.375″ spans. I also have been making sure they have at least 2.5 calibers of stability. I’ve been calling my type of rockets ‘AeroPac Sport Flyers”. You get decent altitudes, and they track very well. At Aeronaut 2014 I had a motor mishap, where I lost..broke off ..one side of my exit cone. This happened about 3 seconds into a 5 second burn. Jeremy Nelson happened to catch the flight on video. It’s pretty interesting slow mo. Amazing how it was able to correct itself . Still went a tad under 25K…same rocket same load without that issue went a tad under 30K.

[video=youtube;y5wpiyQfMO0]https://www.youtube.com/watch?v=y5wpiyQfMO0[/video]

Tony
 

ColumbiaNX01

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What you guys are saying is that with the speeds I am expecting I should start out on the pad roughly between 2 and 2.5 calipers. As it accelerates the CG and CP change so fast that it has to have enough calipers to stay stable?

As for rockets that fly less then mach it is okay to have only 1 - 1.5 calipers because at slower speeds the CG and CP dont change as much.
 

watheyak

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Gents,
The word is caliber. Which you might be able to measure with calipers. :wink:

For high Mach numbers there has been some evidence that OpenRocket is better at predicting CP than RASAero. I'm not sure that a "rule if thumb" is prudent in this situation. Like others, I highly recommend plotting CP and CG over time and making sure that they stay in the proper relationship.
 
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Chuck Rogers

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For high Mach numbers there has been some evidence that OpenRocket is better at predicting CP than RASAero.
I propose that since the new supersonic CP models were included in RASAero II starting with the initial RASAero II release in September 2015, RASAero II has the most accurate supersonic CP predictions.

Attached below are RASAero II predictions for Subsonic, Transonic, and Supersonic Center of Pressure (CP) and Drag Coefficient (CD) compared to wind tunnel data for two configurations of the ARCAS sounding rocket (ARCAS Short and ARCAS Long). The Supersonic wind tunnel data is up to Mach 4.63, approaching Hypersonic (Mach 5). The wind tunnel data is from NASA TN D-4013 and TN D-4014.

Slides 2-4 present the wind tunnel model configurations, and Slide 5-6 show the two ARCAS configurations entered into RASAero II.

Note for the ARCAS Long configuration wind tunnel data on Slide 9 the unusual forward movement of the CP at Transonic, and then the CP moves aft again going into low Supersonic. The ARCAS Short configuration wind tunnel data on Slide 8 shows a more typical looking CP curve with Mach number, where the CP moves aft Transonic, and then for Supersonic the CP starts moving forward with increasing Mach number. This illustrates why you want to have an additional 1.0 calibers stability margin on top of the bare minimum 1.0 calibers stability margin, for a total stability margin of 2.0 calibers, at Supersonic Mach numbers to cover for possible CP mispredictions like shown on Slide 9.

Slides 10 and 11 show the RASAero II predictions for the Supersonic CP compared to the wind tunnel data for the ARCAS Short and ARCAS Long configurations. RASAero II produced very accurate predictions for the Supersonic CP from Mach 1.5 to Mach 3, the area of interest for the forward movement of the CP for high power rockets.


Chuck Rogers
Rogers Aeroscience

View attachment RASAero II Comparisons with ARCAS CP and CD Data.pdf
 
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markkoelsch

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What you guys are saying is that with the speeds I am expecting I should start out on the pad roughly between 2 and 2.5 calipers. As it accelerates the CG and CP change so fast that it has to have enough calipers to stay stable?

As for rockets that fly less then mach it is okay to have only 1 - 1.5 calipers because at slower speeds the CG and CP dont change as much.
In essence that is the idea. You need to have enough stability throughout the flight. I would sim it and plot your calibers of stability. I would also consider running the sim with wind- call me crazy.
 

watheyak

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I propose that since the new supersonic CP models were included in RASAero II starting with the initial RASAero II release in September 2015, RASAero II has the most accurate supersonic CP predictions.

Chuck Rogers
Rogers Aeroscience
My apologies, Chuck. I wasn't aware that RASAero ll was out. Pretty exciting!
 

manixFan

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My apologies, Chuck. I wasn't aware that RASAero ll was out. Pretty exciting!
Wow, I didn't realize it was available either until I downloaded the manual and realized it didn't match my version! What you really want to do is plot the CG/CP relationship. A sample is below.

There is almost a 2K' difference in altitude between Openrocket and RASAero. I believe that RA is probably a lot closer to the truth but I'll know for sure at BALLS.


Tony

This is an example of the CG/CP chart from RASAero
Stability margin.gif
 
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BDB

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So if I'm interpreting Tony's plot correctly, it means that the rocket is becoming more overstable during the flight, presumably because the loss of mass from the motor burn is more significant than the aerodynamic forces that cause the forward movement of CP.

Is that right? And if so, is this an exception to the rule, or a general trend for all Mach-breaking rockets?
 
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ChrisAttebery

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I've been dealing with a similar issue on my 4" Punisher. The fins are small for the airframe and it needs nose weight to make it stable.

On the first iteration I added 54oz to the nose to achieve 1.5 calibers of stability. It had an early deployment due to the way I designed the camera housing within the altimeter bay but the flight was laser straight until the deployment.

For the second iteration I had to cut 3" off the booster and then added a 22" payload section to get the camera away from the altimeters, but it still needs nose weight. I've been leaning towards 28 oz. which again will give me 1.5 calibers at launch. By the time the rocket hits 700 mph the CG has moved forward enough to give me 2.2 calibers. Both my OR and RASII sims show that the CP moves backward ~1 caliber in the 700-1000mph range.
 

Chuck Rogers

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Attached below is a typical Center of Pressure (CP) shift and Center of Gravity (CG) shift for a high Mach number high power rocket. This two stage rocket is a little unusual in that it was going to reach Mach 5.

Note that as the second stage burns, the CG moves forward. The second stage starts supersonic, so you don’t see the CP move aft just over Mach 1, but the CP moving forward with increasing Mach number on the second stage is clearly apparent. As others have noted, both the CG and the CP are moving forward during the second stage burn, the CG and CP plot is very informative showing the “race” between CG and CP as the motor burns and the Mach number increases.

RASAero II includes a Stability Margin plot (also included in the attachment) showing that at Mach 5 the minimum stability margin for supersonic (and hypersonic) Mach numbers of 2.0 is met (barely) at Mach 5.

The second stage becomes very stable after burnout, because the CG is fixed at the burnout CG position, but the CP moves aft as the Mach number decreases (moving back from its forward CP position at high Mach number), eventually of course returning to its subsonic CP position.

6-7 calibers stability margin during the later phases of coast and near apogee seems excessive, but that is the result of sizing the fins and nose weight to maintain the minimum 2.0 calibers stability margin for the supersonic (for this rocket hypersonic) Mach numbers at burnout. The flight experience for these types of rockets is that 6-7 calibers stability margin during the later phases of coast, especially near apogee, doesn’t cause any problems. Letting the stability margin become marginal (less than 2.0 calibers) at high Mach numbers near burnout will definitely cause problems; coning, angle of attack and sideslip oscillations, resulting in break-up of the rocket.


Chuck Rogers
Rogers Aeroscience

View attachment Typical CP and CG Shift at High Mach Number.pdf
 

manixFan

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So if I'm interpreting Tony's plot correctly, it means that the rocket is becoming more overstable during the flight, presumably because the loss of mass from the motor burn is more significant than the aerodynamic forces that cause the forward movement of CP.

Is that right? And if so, is this an exception to the rule, or a general trend for all Mach-breaking rockets?
That particular plot is a of a rocket that is MD and the propellant weight is nearly the same as the empty rocket. It is also a very long motor (CTI 6XL) relative to most motors. It is similar to other rockets that typically break Mach.

If you sim a more typical rocket you'll find the CG change isn't as dramatic as the motor accounts for a much smaller percentage of the overall weight. Think of a G80 (2.2 oz propellant) in a typical 1 lb. mid-power rocket. Or any non minimum diameter rocket on a typical motor. The CP also doesn't move as much since the speed stays below Mach.

As an aside, being able to import a Rocksim file directly into RASAero II is a huge benefit.


Tony
 

ColumbiaNX01

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Its all good. My mongoose comes in at 11.3125 pounds with everything except motor. Then with the M1297 it weighs 22.625 pounds. The CP is 65.06 from nose cone. The CG is 58.5 from nose cone. The difference in that is 6.56 inches. That comes to 2.1867 Calibers. Altitude is simulated to 20,585 ft. Mach is 1.39. It enters Mach at 2.21 seconds after lift off. It leaves mach at 7.70 seconds after liftoff. It is only in Mach for 6.31 seconds. At the end of the 8ft launch tower the speed is 92.9 Feet per second. Thrust to weight ratio is 12.88/1
 
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