Question about vehicle mass for L3 chute selection calculation....

[tammaraki]

I am completing my L3 construction/certification project. I scaled up a Quest High-Q to 5.5" and used G10 for the airframe. I selected this design because every time I fly my High-Q it flies so straight and comes right back to the pad.

I have a question about my vehicle mass for chute selection. How do I account for the residual mass of the expended motor? Is that weight published somewhere? Or do I estimate/guess? Or just use the unloaded mass of the rocket?

Thanks for any insight on this.

 

[rharshberger]

I am completing my L3 construction/certification project. I scaled up a Quest High-Q to 5.5" and used G10 for the airframe. I selected this design because every time I fly my High-Q it flies so straight and comes right back to the pad.

I have a question about my vehicle mass for chute selection. How do I account for the residual mass of the expended motor? Is that weight published somewhere? Or do I estimate/guess? Or just use the unloaded mass of the rocket?

Thanks for any insight on this.

Your sim program SHOULD be accounting for that, unless you are calculating it manually.

 

[tammaraki]

I was planning to show the calculation in my project doc, is that not required? If not I can just add a screenshot of the data from RockSim.

 

[rharshberger]

I was planning to show the calculation in my project doc, is that not required? If not I can just add a screenshot of the data from RockSim.

If using an AT motor, the propellant mass is shown in their catalog (or was at one time), subtracting the propellant mass if you know the loaded motors weight will be really close.

It really depends on what your L3CC or TAPs want to see in the doc.

 

[Steve Shannon]

I am completing my L3 construction/certification project. I scaled up a Quest High-Q to 5.5" and used G10 for the airframe. I selected this design because every time I fly my High-Q it flies so straight and comes right back to the pad.

I have a question about my vehicle mass for chute selection. How do I account for the residual mass of the expended motor? Is that weight published somewhere? Or do I estimate/guess? Or just use the unloaded mass of the rocket?

Thanks for any insight on this.

The remaining mass of the motor as is burns is part of the calculation done by either RockSim or Open Rocket.

 

[mikec]

How do I account for the residual mass of the expended motor? Is that weight published somewhere? Or do I estimate/guess? Or just use the unloaded mass of the rocket?

You certainly can't just use the unloaded mass of the rocket because the expended motor is pretty heavy for all L3 motors.

The RASP file has the loaded mass of the motor and the mass of the propellant and to get the mass of the expended motor subtract the second from the first. As noted, your simulator will do this automatically but it's easy to do the calculation and confirm. See https://www.thrustcurve.org for all of these files.

 

[tammaraki]

I did a check on RockSim and it is definitely accounting for the residual mass of the motor.

Thanks all!

 

[teepot]

I printed the AT catalog and all that info is there. ThrustCurve will tell you too. Click on the icon to the right of the motor delay.

 

[Handeman]

Don't get in the weeds unless your TAPS require it. Just add about 4 lbs. to your empty rocket weight and go with that. The chute calculations are usually ±2 ft/sec. so there is a lot of tolerance in there, especially if you are selecting a chute size in 6" or 12" increments. Much more tolerance than a couple of lbs. of empty motor mass that will affect a L3 rocket. It's kind of like worrying about cooking your turkey for 3:00 hours or 3:04 because it's 18 1/8 lbs. instead of just 18 lbs. It just doesn't make enough difference to worry about. Kind of horseshoes and hand grenades.

 

[Titan II]

If using CTI, the info is on their website.