how to select the correct motor

[fatdaddynerd]

I have been just getting back into the hobby and am working on my first scratch build in over a decade. As it nears completion I am trying to determine how to select the motor for the first launch in order to maximize the possibility of a successful launch and recovery.
So my question is what pointers do you guys have for me? What factors such weight etc etc do I need consider to chose the correct impulse and does rock SIM or OR do helicopter recovery sims?
Thanks!

 

[Zeus-cat]

I would think you always want to deploy your recovery system as close to apogee (and likely the lowest speed) as possible, so the type of recovery system should be irrelevant to the calculations. I always compare my rockets to ones I have flown before or commercially available ones to estimate the correct motor to use. And then for the first flight I choose the smallest motor that should work safely.

 

[GregGleason]

Since I don't know anything about your situation other than you described, it may be helpful if you provided a pic or description of the rocket and the recovery system.

First consideration for me is the liftoff mass. That is the major determining factor of what motor you should use. Typically you want a 5/1 thrust/weight ratio (e.g., a 1 lb rocket needs 5 lbs of thrust to assure a good velocity off of the rod/rail). But, that's just for light winds. In higher winds you would want it to 6/1 on up to 10/1 for even higher winds. Another important piece of information is where you fly. Is it a baseball field or a 50 acre lot? Smaller fields should use motors that are at the "shallow end of the pool". Sometimes I don't decide what motor I fly until I am at the field and can take all conditions into consideration.

Simulation programs like OpenRocket are helpful for standard type rockets, but for unique geometries then the accuracy can get a bit "fuzzy".

Greg

 

[CRC]

Here is one tool that helps
https://www.thrustcurve.org/guidepage.jsp

 

[g zilla]

Just like picking out a cam for your car go right to the bottom of the page. You allready know its to big but you buy it anyways.

 

[fatdaddynerd]

thanks for the replys. when i get her completely done i will post pics and stats and such. so basically it seems as though, the recovery method has no bearing on the optimum ejection point, it is simply a matter of having it eject as close to apogee as possible, in order to eject at lowest speed correct? So i just have to run simulations with different engine types to try and figure out best impulse/delay combos, and use OR to simulate up to that point. Is this about my best option? Did i miss anything?

 

[GregGleason]

Pretty much "Yes" to your questions.

OR has a column that outputs the optimum delay as part of the output. So you can pick a handful of motors and run the simulation on a single motor or all of them, whichever ones you highlight.

On the pic below of this OR simulation screen shot, the fourth column is "Apogee" and the simulation is sorted on that. So you can pick the altitude you want to fly it to. The sixth column is "Optimum Delay". So you would pick the delay of that motor that more closely matches that value.



Greg

 

[Zebedee]

thanks for the replys. when i get her completely done i will post pics and stats and such. so basically it seems as though, the recovery method has no bearing on the optimum ejection point, it is simply a matter of having it eject as close to apogee as possible, in order to eject at lowest speed correct? So i just have to run simulations with different engine types to try and figure out best impulse/delay combos, and use OR to simulate up to that point. Is this about my best option? Did i miss anything?

Yes - for example if you decide that a C6 motor has the correct thrust (for liftoff) and total impulse to lift your rocket safely but not so high it will drift outside your field on descent then you can choose the best delay between motor burnout and ejection charge from the different flavors of C6 which are available, E.g. C6-3, C6-5, C6-7 the last number being the delay in seconds. Single use AP motors also have different delay times available. Simulating in OR will help decide on a good delay. It doesn't need to be perfect but within a couple of seconds is usually good.

When you go into reloadable motor systems like CTI they usually come with a long delay and then you buy a re-usable tool which allows you to drill out some of that delay to make it shorter.

 

[fatdaddynerd]

excellent thanks!!!