Estes engine CGs

kevin:

you are probably going to hav to generate this date yourself from used engines . I once did this for A3-4T's and the CG moved around up to a

1/4".... same goes for loaded CG..although closer as the weight difference was in 10th's of a gram...

terry dean

Terry Thanks for the tip.

I was looking for this information just to get VCP models as accurate as I could and I assumed the Cg wouldn't necessarily be in the middle of a given Estes engine simply by noting that some engines use the full length for fuel and some do not.

As the Cg information doesn't seem to readily available does this mean most people model the engine Cgs in the middle of the engine cases or just at the bottom of body tubes or is there some other rule of thumb typically used to site engine mass in rocket Cg calculations perhaps?

Thanks

Kevin

Kevin, there are enough variables in building a model that it's mass rarely matches what a simulation estimates, and thus the estimated CG is suspect even without the motor.

I'd take a worst case motor CG (put the CG at the nozzle). This will hardly affect the model's CG unless you're modeling a Mosquito-style rocket, in which case the worst-case CG will guide you to design a safe CP.

Then build the model and balance it to find the real CG. :-)

kevin: I don't know how people model the motor CG... I never really thought about it all that much to be honest; I also don't know how any simulation software tried to model the CG for any specific motor.

my experience with motor burnout CG's primarliy has to do with rocket gliders where you have to use a sepnt motor casing to determine the glide cg vs the boost cg....

terry dean

One important thing to keep in mind here, is that in an end-burning BP motor, the CG will only move forward during the burn. Therefore, any effect the motor's CG change will have on the rocket's stability will be positive, except in the case of certain esoteric designs (i.e., tractor rockets).

You may just have to measure the rocket's Cg with an unburnt motor and then with a empty motor case and then interpolate. As mentioned before, since Estes motors are end burning the Cg will progess forward at an initially fast rate then will slow down as it reaches the end of the burn. Core and c-slot engines, like Aerotech, probably won't change Cg much, unless you are using the 29mm case with like a E or F reload.

I think, though, that the Cg of a motor is not going to be as critical as the affect of the loss of propellant during flight on the model's Cg. An end burning motor will move the Cg forward fasted than a core/slot burning motor.

Tim Barr

Thanks to you all for your replies. At least I now know that I was searching for this information in vain and can get on with experimenting rather than searching.

We're new to rocket design and simulation and haven't yet built up enough experience to know which factors really matter, which can be approximated and therefore when we must chase a design factor down rigorously or just go with the flow. So meanwhile, we're assuming we should find all the facts we can.

Having flown Estes motor-based rockets so far we're just beginning to look at RMS too and hence core-burners are on our horizon. And it hadn't occured to me that the motor Cg positions would be affected differently from the end-burners.

From here on we'll start keeping and testing our bunt-out motors to see what happens to the Cgs experimentally.

Many thanks once again.

Kevin