Problem with Finite Burn Mission

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Problem with Finite Burn Mission

Postby AsteroidX » Wed Feb 24, 2016 4:43 am

I have been able to get a finite burn mission to mars to converge when their is no fuel mass depletion, but when the thrusters are set to deplete mass, GMAT throws the error "Hardware Exception Thrown: The value of "-12700.53" for field 'FuelMass' on object 'MainTank' is not an allowed value. The allowed values are: [Real Number >= 0.0].

I understand that the fuel mass can't be negative, but when I set the "MainTank" to "Allow Negative Fuel Mass", it actually completes the maneuver it wasn't before, and the maneuver doesn't use up all the fuel(I checked with a report on fuelmass), so I don't understand why GMAT is throwing the error.

My only guess is that it has to do with the solver using way more fuel then is needed in the first few iterations, so it throws the error, even though the maneuver wouldn't actually use that much fuel if the iterations would continue towards the point that is actually required for the fuel mass depletion.

Am I correct in the assumption that the problem lies somewhere within the solver or propagator? If not, does anyone have any other ideas? I have included the script so it can be run and others can see the issue. Notice that I have included a report in the mission, which will need to be changed to a different save path before the script can be run.
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finite.2tw.withdecrementMass.script
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Re: Problem with Finite Burn Mission

Postby Torr919 » Wed Feb 24, 2016 10:01 pm

I'd say that you're most likely correct in your assumption. It does seem like the error is thrown from an early, less efficient iteration resulting in negative fuel mass. When you select to allow negative fuel mass, it ignores the flag thrown and continues to iterate, eventually finding a much more efficient solution that doesn't completely deplete the fuel. I've had to deal with this on another project of mine where the first few iterations completely miss the target and cause the spacecraft to use way too much fuel.

I haven't had a chance to view or run your script yet. When you get the final solution for convergence with negative fuel mass allowed, do you then apply those corrections to the maneuver, and does the error persist if you disable the negative fuel mass option?
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Re: Problem with Finite Burn Mission

Postby 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.
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Re: Problem with Finite Burn Mission

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