Converting impulsive burn to finite burn

Post here for assistance in troubleshooting a problem using GMAT

Converting impulsive burn to finite burn

I've created a script which calculates the deltaV requirements for a mission to Saturn with a Jupiter flyby. It's current using impulsive burns, but I want to change to finite ones. What's the best way to do this? Everything I've tried so far doesn't converge.

Currently I have:
Code: Select all
`Vary Impulsive Burn element 1Vary Impulsive Burn element 2Vary Impulsive Burn element 3Maneuver (Impulsive Burn)`

I've tried something along the lines of:
Code: Select all
`Vary Thruster 1 burnDurationBegin Finite Burn (Thruster 1)Prop burnDuration secsEnd Finite Burn (Thruster 1)Vary Thruster 2 burnDurationBegin Finite Burn (Thruster 2)etc...`

Do I need three thrusters, one in each direction?
Do I need to vary three burnDuration variables?
Should I (can I?) propagate them all at once, or sequentially?

Any help would be very much appreciated.

Joseph
jdudley1123

Posts: 1
Joined: Tue May 12, 2015 5:52 pm

Re: Converting impulsive burn to finite burn

You can replace a single ImpulsiveBurn resource with a single FiniteBurn resource, but you'll need to vary an additional parameter.

With ImpulsiveBurn, you vary `Element1`, `Element2`, `Element3` (magnitude is implicit).

With FiniteBurn, you vary `Thruster.ThrustDirection1`, `Thruster.ThrustDirection2`, `Thruster.ThrustDirection3`, and propagation duration.

Also, remember to set your thrust & isp correctly: `Thruster.C1`, `Thruster.K1`.
Joel J. K. Parker
Flight dynamics engineer, GMAT team
http://gmatcentral.org
jjkparker

Posts: 617
Joined: Thu Jan 07, 2010 9:48 pm
Location: NASA Goddard Space Flight Center, Greenbelt, MD

Re: Converting impulsive burn to finite burn

jjkparker wrote:You can replace a single ImpulsiveBurn resource with a single FiniteBurn resource, but you'll need to vary an additional parameter.

With ImpulsiveBurn, you vary `Element1`, `Element2`, `Element3` (magnitude is implicit).

With FiniteBurn, you vary `Thruster.ThrustDirection1`, `Thruster.ThrustDirection2`, `Thruster.ThrustDirection3`, and propagation duration.

Also, remember to set your thrust & isp correctly: `Thruster.C1`, `Thruster.K1`.

I have an additional question for this. How would one view the delta-V after converting an impulsive burn to a finite burn? That information isn't displayed in the mission summary and there is no longer a single maneuver entity in the mission tab showing the delta-V. Would I need to add an additional entity to the mission structure or have it produce a separate report once convergence has been reached?

Edit: I think I've answered my own question. When I calculate the magnitude of the three thrust directions, I get reasonable numbers matching what I'd expect for the delta-V in the given scenario I'm working with.

delta_V = sqrt(Thruster.ThrustDirection1^2 + Thruster.ThrustDirection2^2 + Thruster.ThrustDirection3^2)
Torr919

Posts: 8
Joined: Fri Feb 12, 2016 2:45 am