Selecting motors question

[ghp3]

Hello:

I've been predominantly a low power rocketeer (BAR since last year) and am starting to move to mid-power, having launched a couple of G35 econojet powered rockets recently. One of the challenges that I'm having is selecting which engine to mate with a particular flight, particularly in the F and G ranges. Whereas low power motors are have average thrust ratings that are pretty standard (B4 or B6, C6 or C11, etc.), with mid powers there is a fairly wide range (e.g. G25, G35, G40, up to G125). I understand the basic concept of average thrust (a lower average thrust motor delivers a lower thrust but over a longer time, whereas a higher average thrust delivers high thrust for a short time), but I'm not quite sure how to decide what motor to use for a partucular rocket or flight. Any advice for a budding MPR?

Thanks!

George

ghp3

 

[rbeckey]

George,
Perhaps a eval copy of RocSim would help. There is a datbase with many fully "assembled" simulations on EMRR.

https://www.apogeerockets.com/rocksim_demo.asp

After a while you start to get a feel for what goes with what. The only thing I have been told and have observed is that RocSim sometimes makes delays a little too long. I have the full version and it is a great tool. Don't know what I'd do without it.

 

[n3tjm]

General rule, the short burn/high thrust motors provide a more stable flight, especially in wind, but since you are accellerating faster, the force of drag is also higher, so you generally would get an lower altitude than with a longer burn, low thrust motor with the same total impulse.

 

[loopy]

Generally, if the rocket is built properly, it should have no problem withstanding the forces involved in the range of mid-power impulses. It really comes down to whether or not the delays match the rockets characteristics well enough to provide a safe deployment. Beyond that, higher initial thrust (as Doug said) will provide better speed off the pad, making for a more stable flight as wind conditions increase. Also, it comes down to personal preference. The different average impulses are brought on by different propellant types - White Lightning versus Blue Thunder versus Black Jack versus Econojet. It comes down to personal preference - do you like slower liftoffs, or do you want your rocket to scream off the pad.