Variable mass and CG in OR?

[Peter Bech Svalgaard]

The total mass of our rocket is 67 kg with fuel. 49 kg without fuel. The fuel is burnt linearly with time. Is their a way to take into account this variability in mass and CG in the simulations? it would be pretty significant i think.

 

[Bat-mite]

OR already does that for you. Run a sim and plot stability over time. You can also include CP and CG values during flight.

 

[Peter Bech Svalgaard]

Ah ok so you include the mass of fuel in the engine file? but how does it know where the fuel is located in order to calculate CG?

 

[Bat-mite]

Are you using an EX motor or a commercial motor? Are you making your own .eng file? Commercial motor files contain the burnout information, and OR can plot your CG over the length of the flight, from ignition to touchdown.

If you are making your own .eng file, then you need to use something like BurnSim.

 

[ProjectG]

18 kg is a lot of propellant! You'll want a good idea of how the motor mass changes over time, and how that affects your CG. I don't know if there are existing mass properties for the motors in OR, or if it assumes a linear mass change over the duration of the motor burn. That would be worth looking into if you have a significantly progressive or regressive burn in your motor. You can get a rough idea of your change in mass over time from the thrust curve of your motor, higher thrust represents a proportionally higher mass flux.

 

[mikec]

I'm pretty sure that OR assumes the mass goes down and the location of the CG of the motor is constant over the burn, which is valid for BATES grains and moonburners, not valid for endburners, and definitely not valid for hybrids. But I haven't looked at the code so I'm not certain.

 

[Peter Bech Svalgaard]

Bat-mite, I'm making my own .eng file. Im looking into using ThrustCurveTool to generate the right .eng file