reloadable motors in OR or Rocksim

[Powder Junky]

How do you account for different case/spacer combinations in Open Rocket, when using a reloadable motor. That is, what do they use for the default. I am trying to figure out which motor system to buy so I don't yet have the hardware to measure.

Thanks

[stantonjtroy]

In OR there is no accounting. When you pull down the motor selection list it has wheather it's a reload or not and adjusts the mass as needed. Any additional mass from spacers is neglegable. I've been getting remarkably accurate results. If it really matters to you you can add the weight of the spacer in the motor tube as a generic mass object. Remember, most sims only come out 85-95% accurate (My experience and those I fly with). They get you into a workable ballpark but won't give you the homerun. FWIW

[Powder Junky]

So, on a 1lb rocket a 1 grain case is in the same "ballpark" as a 3 grain case and 2 spacers? Interesting! Then, if I size my fins for a 3 grain motor they should be OK for anything in that 3 grain case?

Thanks!

[stantonjtroy]

So, on a 1lb rocket a 1 grain case is in the same "ballpark" as a 3 grain case and 2 spacers? Interesting! Then, if I size my fins for a 3 grain motor they should be OK for anything in that 3 grain case?

Thanks!

Based on my experience with OR, yea. I use the CTI ProX29mm and 38mm systems with and without spacers as needed and have had no surprises. Do keep in mind though, the sim will adjust the motor weight based on the addition (or removal) of propellant grains based on the motor selection made from the pull down menue. I suspect if the motor chosen calls for a single grain it defaults weight to a single grain case with no reguard as to weather you use that or a three grain case w/ 2 spacers. I sim each flight to get a sense of my needed delay times and stability and, so far, no issues. If you're working with a marginally stable design you can add the weight of the spacers and/or pull it out as a free mass object to see if the difference is noticable or critical. That's the nice thing about sims, you haven't cut or glued anything yet so a test or change is just a couple of keystrokes away. Obviously the more accurate your data, the more accurate the resultant sim.

[bobkrech]

How do you account for different case/spacer combinations in Open Rocket, when using a reloadable motor. That is, what do they use for the default. I am trying to figure out which motor system to buy so I don't yet have the hardware to measure.

Thanks

A sim is on as accurate as your data input. The engine file contains the propellant mass and the loaded motor mass as the last 2 numbers on the first row of data.

The last number on the first row of the .eng file is the loaded motor mass in kg. for the standard motor casing. For a Pro38 motor using a 1G spacer, you have to add 0.050 to this value and for a 2G spacer you have to add 0.100 to the loaded mass value to account for the additional casing and grain spacer mass.

Bob