CG vs. CP for Supersonic Flights

[TMJ]

I understand that the CG needs to be moved more forward, for supersonic flights, however. Just how far north should I go from the CP? Is there a 'rule of thumb' concerning the cal of stability required for supersonic flight, like there is for subsonic flight stability?

 

[rharshberger]

Use OR and RASAeroII to simulate your flight, both programs will allow you to plot the CG/CP relationship over time. Rocsim will probably do the same. Make sure that the rocket remains stable during the entire flight profile.

 

[markkoelsch]

I would recommend a minimum static margin of 2. This needs to be simulated, preferably with Rasaero.

Going to more static margin is not a bad idea.

SIM it to make sure you keep a margin of 2 across the range of velocity.

 

[ksaves2]

RAS Aero will give you decent results and it's fun to input stupid fast Mach numbers and see how far forward the CP goes. Really moves forward with stupid high velocities. Low Mach "ish" and it's not so bad. There's an exception for long necked rockets mentioned here: www.argoshpr.ch/j3/articles/pdf/sentinel39-galejs.pdf. It's better to have the CG moved a little bit farther forward with long necked rockets for any flight. That's the same Galejs that designed the
MAD unit that Aerocon used to sell. Really nice person. I dorked one that I built with an early homemade e-match and he fixed it for me 10 years ago.
Kurt

 

[TMJ]

Thanks for the input and advice. I've not yet had a chance to look into Rasaero, yet, but I will... for sure! Two simulated heads are definitely better than just one...
In the meantime... OpenRocket shows my stability, with a Loki M-900 to be at 2.74 cal. Max velocity estimated at mach 1.4
The only warning that OpenRocket tosses my way, during flight sims, is that 'calculations might not be exactly correct at supersonic speeds' thingy. No other warnings. Everything else checks good.
Do these numbers, regarding the cal of stability seem to be within a good ballpark of safety and stability, as best you folks can tell, via your previous experience?

 

[SpaceManMat]

Thanks for the input and advice. I've not yet had a chance to look into Rasaero, yet, but I will... for sure! Two simulated heads are definitely better than just one...
In the meantime... OpenRocket shows my stability, with a Loki M-900 to be at 2.74 cal. Max velocity estimated at mach 1.4
The only warning that OpenRocket tosses my way, during flight sims, is that 'calculations might not be exactly correct at supersonic speeds' thingy. No other warnings. Everything else checks good.
Do these numbers, regarding the cal of stability seem to be within a good ballpark of safety and stability, as best you folks can tell, via your previous experience?

You need to graph your CG vs CP, it's not a single figure. Stability is dynamic, it changes over time. CP generally moves forward as you go faster, CG moves forward as you burn fuel. The relation ship between the two may or may not provide stable flight across the entire burn. Pretty sure a lot of people that tried to fly the N5800 got caught out on this, the motor would get the rocket up to around Mach 2 where there is a big CP shift without the motor loosing enough weight.

 

[Chuck Rogers]

You need to graph your CG vs CP, it's not a single figure. Stability is dynamic, it changes over time. CP generally moves forward as you go faster, CG moves forward as you burn fuel. The relation ship between the two may or may not provide stable flight across the entire burn.

Example plots from RASAero II are shown below for a two stage rocket predicted to reach Mach 5. The RASAero II minimum recommended stability margin at transonic and supersonic Mach numbers is 2.0 calibers.

The Mach number change with time, and the CP and CG shifts with time and Mach number illustrate the effects described above.


Chuck Rogers
Rogers Aeroscience


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

 

[rstaff3]

Wow, when the CP changes, it seems to change abruptly.

 

[Chuck Rogers]

Wow, when the CP changes, it seems to change abruptly.

The big changes (vertical lines) are staging, the lines to the left are Stage-1, the lines to the right are Stage-2. There is a short coast phase (horizontal lines) where Stage-2 is coasting after separating from Stage-1, and then Stage-2 is ignited.

The forward movement of the CP with increasing Mach number is significant. If the rocket (in this case Stage-2) is rapidly increasing Mach number during the motor burn, then the CP can be moving forward quite rapidly.


Chuck Rogers
Rogers Aeroscience

 

[TMJ]

Example plots from RASAero II are shown below for a two stage rocket predicted to reach Mach 5. The RASAero II minimum recommended stability margin at transonic and supersonic Mach numbers is 2.0 calibers.

The Mach number change with time, and the CP and CG shifts with time and Mach number illustrate the effects described above.


Chuck Rogers
Rogers Aeroscience


View attachment 301537

Okay Chuck... This is one that I can wrap my head around, crunch numbers on a calculator with... and somewhat verify, blindly, via flight sims. 'Your' rocket design is substantially different than mine, however. There are also a lot of mathematical 'clues' to be extracted, for reference, from your examples. Is this an 'Empirical' design, that has yet to be launched and actually proven, or has it been proven via flight?

 

[Chuck Rogers]

Okay Chuck... This is one that I can wrap my head around, crunch numbers on a calculator with... and somewhat verify, blindly, via flight sims. 'Your' rocket design is substantially different than mine, however. There are also a lot of mathematical 'clues' to be extracted, for reference, from your examples. Is this an 'Empirical' design, that has yet to be launched and actually proven, or has it been proven via flight?

It's just an example, it hasn't been built yet.

The RASAero II Users Manual, and the Example Files included in the RASAero II download, include a two stage rocket that went 104,659 ft. The error in the RASAero II predicted altitude was +6.33% (the predicted altitude was 6.33% high). Pretty good prediction. Two stagers are tougher to predict than single stagers, there are twice as many things to mispredict (two sets of engine data, two sets of aero data, etc.).


Chuck Rogers
Rogers Aeroscience

 

[SpaceManMat]

This is from a rocket I'm currently building, Max velocity is Mach 1.8

 

[SpaceManMat]

Take a look at this discussion on the DDT build...

https://www.ausrocketry.com/forum/viewtopic.php?p=44319#p44319