OpenRocket Mach 2+ design stability

I'm in the process of designing a 5" dia rocket that will be powered by a CTI N5800. As you know, and I have recently learned, there can be a serious dip in stability just prior to motor burnout. If my design starts off above the 1 cal of stab. but dips to .4 cal of stability just before motor burn-out (for about a full second), will it be stable in flight at that dip? At that dip in stab. the velocity will be well above Mach 2.

Thanks for the help!

You might be better off running the sim through RASAero to get a more accurate result. Personally I'd keep at least 1 caliber of stability.

Walldiver7:

A new version of RASAero will be released soon where the supersonic CP will move forward 1.0 caliber, especially for short fin span rockets. Running the current release of RASAero, for the supersonic CP predictions for both zero degrees angle of attack and 0-4 degrees angle of attack, move the CP curve forward by 1.0 caliber, and this will be close to the new CP predictions from the new version of RASAero.

I recommend not letting the stability margin fall below 2.0 calibers at supersonic Mach numbers. So with the current RASAero, once you shift the supersonic CP curve forward 1.0 caliber, I would have at least 2.0 calibers stability margin relative to that curve at all supersonic Mach numbers.

As noted in other threads, once the supersonic stability margin falls below 1.0 caliber, even though the rocket is technically stable, coning, pitch-roll coupling, and other difficult to predict effects can become present.


Chuck Rogers
Rogers Aeroscience

I have reason to believe from looking at actual flight experience that Openrocket is overconservative about predicting CP position at high mach, at least for some fin shapes (that is, it predicts instability when none exists.) But on the other hand it's hard to say how lucky some successful flights have been. I haven't done a comparison of how Openrocket's prediction compares with RASAero's.

Thanks, I take a look at it.

COrocket said:

You might be better off running the sim through RASAero to get a more accurate result. Personally I'd keep at least 1 caliber of stability.

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I would love for someone to explain this sudden drop in stability as I have never heard of this before. Is it only when going mach+? What causes it? I have always thought the rocket becomes more and more stable as the motor burns because there is less and less weight in the aft end.

Thanks for the response!

mikec said:

I have reason to believe from looking at actual flight experience that Openrocket is overconservative about predicting CP position at high mach, at least for some fin shapes (that is, it predicts instability when none exists.) But on the other hand it's hard to say how lucky some successful flights have been. I haven't done a comparison of how Openrocket's prediction compares with RASAero's.

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Yes, Sir, that's exactly how I thought, prior to working on designs that would approach Mach 3.

Bat-mite said:

I would love for someone to explain this sudden drop in stability as I have never heard of this before. Is it only when going mach+? What causes it? I have always thought the rocket becomes more and more stable as the motor burns because there is less and less weight in the aft end.

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So is there an explanation...?

Bat-mite said:

So is there an explanation...?

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Not a simple explanation that I understand well enough to pass on. It has to do, fundamentally, with changing fluid-dynamic properties of the air at higher mach numbers. Read Barrowman's original thesis and the Openrocket document https://openrocket.sourceforge.net/techdoc.pdf for starters.

It would be a long explanation, but here are a few key things happening at supersonic to hypersonic Mach numbers. I'd add "approximate" in front of each of these statements as there are a lot of additional details going on.

1) Subsonic the fin center of pressure is approximately at the 1/4 chord point, supersonic it is approximately at the 1/2 chord point. So initially the rocket center of pressure moves aft as the rocket goes through Mach 1.

2) As the Mach number increases, the CNalpha of the nose cone increases and the nose cone CP moves aft towards the nose cone/bodytube junction. Eventually the nose cone CNalpha decreases and the nose cone CP moves forward, both back towards their subsonic values. Eventually at hypersonic Mach numbers, based on linear theory, the nose cone CNalpha and CP return to close to their subsonic values. The increase in the nose cone CNalpha at supersonic Mach numbers up to Mach 3-4 moves the rocket CP forward.

3) At supersonic Mach numbers, once the fin leading edge goes supersonic, the fins constantly lose lift. Based on linearized theory for a thin airfoil of arbitrary shape, planform (delta, swept) independent, the lift coefficient, shown as a small cl here, is determined from the equation below. At small angles of attack normal force coefficient is approximately equal to lift coefficient, so take the derivative of the equation with respect to angle of attack, and you get CNalpha. Note that as the Mach number increases, the CNalpha of the fins is decreasing via the one over square root of Mach number squared minus 1 term. This is a major contributor to the rocket CP moving forward at supersonic Mach numbers. (Once it has done its initial move aft due to the 1/4 chord to 1/2 cord fin CP shift.) Note wing sweep (not included in this equation) has an important effect here, the max supersonic CNalpha of the fin occurs when the fin leading edge goes supersonic, and then decreases past that point.


Again, put "approximate" in front of the above statements, and there are many more additional details, fin leading edge sweep being a big one, and fin planform shape (delta, swept, tip chord to root chord ratio, etc.) having other important effects.


Chuck Rogers
Rogers Aeroscience

Bat-mite said:

I would love for someone to explain this sudden drop in stability as I have never heard of this before. Is it only when going mach+? What causes it? I have always thought the rocket becomes more and more stable as the motor burns because there is less and less weight in the aft end.

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This is a normal deal. The Cp will move forward the faster you go.