# Minimum Diameter, Extreme Performance and Supersonic Speeds...

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#### TMJ

##### "Where the heck'd it go!"
It's time for me to venture higher, further and faster, however. I don't trust my simulation software when it comes to stability at supersonic speeds. I know that the CP will shift, but just how 'much' will it shift under variable circumstances? There 'must' be a good way to determine a good starting point without simply 'shooting from the hip', crossing fingers... and hoping for the best.
Does anyone know of any proven 'Rules of Thumb', ratios, etc. to help me successfully determine a safe/satisfactory cal of stability prior to all maiden supersonic launches?

#### dhbarr

##### Amateur Professional
I could be very wrong, but I thought the rule was you'd lose about a caliber with the peak at around mach 1.2

CG of course shifts forward as mass gets ejected as thrust, but that part's pretty straightforward.

#### Dwatkins

##### Well-Known Member
Download RASAERO II! It will look for 2 cal through all Mach numbers and warn you if it goes out. Best software for these kind of flights there is IMO.

Dennis

#### tfish

##### Well-Known Member
My builds for the fast few years have had fin spans of 1 caliber+ (a little) and a stability margin of 2.5 calibers on the pad. I've been calling these rockets "AeroPac Sport Flyers". They are not the best for altitude records, but they're not glass slippers either! Attached is a video of a 3" version of this type of rocket. Shortly into the burn...1.5 sec.. It lost one side of the exit cone of the nozzle. The rocket was able to get corrected and still flew to 24,449'.

Chuck Rogers has added some very good info....

Tony

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#### Buckeye

TRF Supporter
The questions you ask can only be answered with a flight simulation, unless you have access to a supersonic wind tunnel, have many prototypes constructed, and hundreds of hours of test time which run about $1000/hr. The "good starting point" is the simulation. Be a rocket scientist. Use the software. It's free and pretty easy (OR and RAII). You can study all the "variable circumstances" to your heart's content. In the end, the simulations are usually pretty conservative for stability. #### TMJ ##### "Where the heck'd it go!" The questions you ask can only be answered with a flight simulation, unless you have access to a supersonic wind tunnel, have many prototypes constructed, and hundreds of hours of test time which run about$1000/hr.

The "good starting point" is the simulation. Be a rocket scientist. Use the software. It's free and pretty easy (OR and RAII). You can study all the "variable circumstances" to your heart's content. In the end, the simulations are usually pretty conservative for stability.
Yep, I use OR religiously. It always treats me well with subsonic designs. When going 'supersonic' I can't get rid of that OR warning stating "Calculations might not be accurate at supersonic speeds!" No matter how I adjust the CG in relation to the CP... I still get that same pesky warning. I stop adjusting things when performance goes to the wrong side of the curve. Always stuck with that warning. That bugs me, LOL. That's why I asked the question looking for formulas, rules of thumb, etc..
I was 'thinking' that a minimum of 2 cal, up to about mach 1.5 would be sufficient (gut feeling), with my 38mm designs. That 2 cal thing seems to be supported by other folks inputs, however. Wouldn't airframe diameter and length of airframe, etc. become variables that could easily change the equations/gut feelings when the design parameters change? This is fascinating stuff. I wish I'd have stayed in school longer!

#### Incongruent

##### Well-Known Member
Yep, I use OR religiously. It always treats me well with subsonic designs. When going 'supersonic' I can't get rid of that OR warning stating "Calculations might not be accurate at supersonic speeds!" No matter how I adjust the CG in relation to the CP... I still get that same pesky warning. I stop adjusting things when performance goes to the wrong side of the curve. Always stuck with that warning. That bugs me, LOL. That's why I asked the question looking for formulas, rules of thumb, etc..
I was 'thinking' that a minimum of 2 cal, up to about mach 1.5 would be sufficient (gut feeling), with my 38mm designs. That 2 cal thing seems to be supported by other folks inputs, however. Wouldn't airframe diameter and length of airframe, etc. become variables that could easily change the equations/gut feelings when the design parameters change? This is fascinating stuff. I wish I'd have stayed in school longer!
NASA has some graphs of how the CP (measured from tail end) moves forwards with speed on the Nike Apache. Along with other cool information.

It's in this document: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670015760.pdf

It moves quite a lot, though considering the speeds it flies at...

#### SpaceManMat

##### Space Nut
From following extreme flight builds of fliers much more accomplished than myself:
Minimum stability of 1.5 at all speeds in OR works for flights up to atleast Mach 3.5
Watch the stability closely as the rocket approaches Mach 2, the CP can really shift around.

#### Buckeye

##### Well-Known Member
TRF Supporter
Yep, I use OR religiously. It always treats me well with subsonic designs. When going 'supersonic' I can't get rid of that OR warning stating "Calculations might not be accurate at supersonic speeds!" No matter how I adjust the CG in relation to the CP... I still get that same pesky warning. I stop adjusting things when performance goes to the wrong side of the curve. Always stuck with that warning. That bugs me, LOL. That's why I asked the question looking for formulas, rules of thumb, etc..
I was 'thinking' that a minimum of 2 cal, up to about mach 1.5 would be sufficient (gut feeling), with my 38mm designs. That 2 cal thing seems to be supported by other folks inputs, however. Wouldn't airframe diameter and length of airframe, etc. become variables that could easily change the equations/gut feelings when the design parameters change? This is fascinating stuff. I wish I'd have stayed in school longer!
That warning is just a disclaimer for any flight over Mach 1. RAII gives an equally-pesky warning. Since the Mach CP modeling is tricky stuff, the authors throw in some safety margin.

The Cp shifts at Mach 2-3 is cool stuff and all, but most of us never get there. For us Mildmen, the critical point in time is when the rocket leaves the rail. This is is where your design will be at its lowest stability. Plot it and see. I had a similar Mach 1.5 38mm MD (long + skinny + big motor = stability concern) that showed brief negative stability off the rail in the simulation. In real life, it indeed was a bit squirrely off the pad. I addressed that and got a better flight next time. FWIW.

#### manixFan

##### Not a rocket scientist
Here's the chart I found useful when simming my CF Mongoose 54 which went to Mach 2.3 and 23,500'. It's from Open Rocket. Just select the appropriate parameters and choose plot. RASAero has a similar function. This plot shows CP, CG, and stability margin vs. Mach number. The Open Rocket altitude was off a bit but the speed was on the money. The green line is the important one - it shows the stability margin throughout the flight.

Tony