OpenRocket and supersonic CP shift

[SolarYellow]

Was playing with some supersonic sims in OpenRocket at lunch today. Just plugging in the CTI 29mm 4-grain loads. Saw some weirdness. Max speeds ranged from ~Mach 1.1 to ~Mach 1.6.

With the H255, the stability factor reported in the main display panel was dramatically less than with any of the other motors. Looked them up on Pro38.com and the loaded weights are all within a few grams. I exported the sim and looked at the file in detail. With nose weight that was giving me 1.49 calibers in the display panel, it came off the rod with more stability factor than that and went up, approaching 3 calibers at max velocity. It only dipped anywhere close to the 1.49 a couple of tenths of a second before apogee, which I'm guessing was caused by AOA change as it nosed over. Haven't exported data for the lower-average-thrust motors, but I suspect the much lower value of stability reported on the display panel is an anomaly related to that weird row, rather than something actually systemic.

My overall concern I was investigating was with the forward shift and the standard rule of thumb to ensure at least 2.0 calibers of stability so that when the supersonic forward shift of CP is in full effect, you're left with one. What I saw in the detailed sim output was the reported stability factor increasing dramatically with speed, as speed went both up and down.

I took a quick look at the tech doc white paper discussing the physics behind the sim, and it discussed an interpolation method to account for the supersonic forward shift of CP. So it's logical to assume that that is included in the sim and when it says something like 2.8 calibers at Mach 1.6, that really is 2.8 calibers at Mach 1.6 (leaving wiggle room for precision in that it's not a perfect sim, of course). Is that a correct assumption? Or could it be closer to, say, 2.2 calibers in reality?

@neil_w

 

[neil_w]

This is outside my area of expertise... calling @JoePfeiffer.

 

[rocket_troy]

Dunno. From my understanding, the CP shift of (typical geometry) rockets is generally forward with >M2 velocities. That's (typically) the opposite to high speed aircraft that generally sees a shift aft.
If you look at the shift for the Tomahawk sounding rocket & the Aerobee 150A, there is a shift *aft* around the transonic regime and just beyond; it only then starts shifting in the forward direction beyond M1.5ish.

Like Neil, aero is certainly not my area expertise either. I find much of it counterintuitive.

TP

 

[mikec]

If you really care, use RASAero, though in my experience OR does an adequate job, at least up to mach 2.6 or so which is as fast as I've flown.

 

[SolarYellow]

Will be looking into RASAero.

Also digested a few of the "Similar threads" shown at the bottom of this page.

https://www.rocketryforum.com/threads/cp-shift-of-supersonic-rocket.163381/
https://www.rocketryforum.com/threads/super-sonic-cp-shift.138812/
https://www.rocketryforum.com/threads/affect-of-mach-on-cp.130690/
In general, it appears the rocket becomes more stable in the transonic range, then the CP moves forward from low supersonic, reaching its subsonic position around M2.

The 2-caliber recommendation seems to be best-founded when considering AOA-related loss of stability in the beginning of the flight if there is wind when the rocket leaves the launch guidance. It's a balance between keeping things stable at the instantaneous AOA and avoiding excessive weathercocking. I still think that's probably going to be substantially absorbed in the discussion of calibers vs. percentage of airframe length (denominator selection) as a basis for reporting stability factor.

 

[JoePfeiffer]

This is outside my area of expertise... calling @JoePfeiffer.As

As I understand it (and I'm not really an aerodynamicist either) this is expected behavior. CP shifts back as you go supersonic, and then shifts forward again as you get even faster.

 

[SolarYellow]

Will be looking into RASAero.

Downloaded, installed, first sim built and run.

You won't be able to do it without consulting the manual. As if you wouldn't, download that, too.

Another thing to burn time instead of just building and flying rockets. But learning may occur.

 

[watheyak]

The forward shift doesn't really happen belor about M2.5. And as injnderstand it, it's due to an exponential increase in nosecone drag at high mach.

If you're going fast enough for it to make a difference, I really only trust RASAero.

 

[rocket_troy]

The forward shift doesn't really happen belor about M2.5. And as injnderstand it, it's due to an exponential increase in nosecone drag at high mach.

I don't think there's "exponential" increase... well... maybe there is, but the exponent would be <1 as Cd decreases beyond transonic. Of the 2 examples I listed earlier, the "lift" or "normal force" curves appear to have striking resemblances with Cd and Cp shift. Again, looking at those charts, the primary cause of the Cp shift appears to be a change in aerodynamic lift. I would speculate as a more fundamental consequence of the turbulent wake from the primary shock, but... dunno... maybe you're right.

TP

 

[user 30299]

I'm still waiting for someone to develop an onboard electronics package to collect real data during a flight . . .

 

[Buckeye]

I'm still waiting for someone to develop an onboard electronics package to collect real data during a flight . . .

You will wait a long time, because I don't believe it is possible to measure CP during flight. CP comes from a force and moment calculation based on the exterior shape of the rocket.

CFD simulation would be the next step up from the models used today by OR and RASAero.

 

[user 30299]

You will wait a long time, because I don't believe it is possible to measure CP during flight. CP comes from a force and moment calculation based on the exterior shape of the rocket.

CFD simulation would be the next step up from the models used today by OR and RASAero.

Oh ye of little faith . . . .