by **AsteroidX** » Tue Mar 29, 2016 2:58 pm

Update: I was able to get my mission to converge, but it took a little bit of fiddling to understand the problem. I will try to simply describe the solution for anyone else that comes across the same issues with a finite burn mission.

Here where the basic issues:

1. The spacecraft had a TCM maneuver that was not converging.

2. The spacecraft main tank and thrusters were set to decrement mass, and the main tank would run out of fuel during the TCM maneuver.

Solutions:

1. If the initial parameters and vary commands are not close to the converging point, say 355,000,000 km away from converging, the program won't even recognize that the spacecraft is not going to converge(at least in any amount of iterations that you want to wait for), and will try thrusting for longer periods of time. If this is the case, it will thrust much longer than needed, and can cause the vehicle to use all the fuel. To fix this, try to get the initial conditions to allow for a closer approach.

*Conditions that could be changed are the angles that the vehicle leaves orbit from, the epoch date, initial velocities, and of course the TCM variables such as Vary.V, Vary.B, Vary.N. So the problem might not be that the main tank does not have enough fuel, it could just be that the initial conditions are not close enough, so both the first and second issue could be solved above.

2. If the spacecraft runs out of fuel, I found that there are a few parameters that can be varied to help understand why:

*The very first thing that must be checked is if the vehicle just doesn't have enough fuel. Check this through the equations calculating mass flow rate: The equation needed is I_sp=(Force_thrust/m_dot)*(1/g), which gives m_dot=(Force_thrust/I_sp)*(1/g) which has units of (kg/sec). Make sure that the g used is the same g for the other variables calculated (This is because g acts as a conversion between weight and mass).

*If it looks like the vehicle has the correct amount of fuel, it might be that the maneuver iterations for the target is using too much fuel initially. To check this, try checking the "allow negative fuel mass" box for that main tank. Run the mission, and see if that allows the maneuver to converge, and then check the command summary to see if the tank has a positive fuel mass. If it has a positive fuel mass, you know the maneuver worked, and you can proceed.

*Another important parameter to check is the gravitational acceleration constant used with the thruster when decrement mass is allowed. If it is different than the g you used in other calculations, you will probably use to much or too little fuel. It took me a long time to figure this out, so make sure that for each maneuver, if you use different g values in your calculations, the thruster has a matching g value in the decrement mass field.

I wrote this quickly, so if it doenst make sense, just post a comment, and I will try to respond when I see it.