Converting impulsive burn to finite burn

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Converting impulsive burn to finite burn

Postby jdudley1123 » Tue Jun 09, 2015 3:36 pm

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 1
Vary Impulsive Burn element 2
Vary Impulsive Burn element 3
Maneuver (Impulsive Burn)


I've tried something along the lines of:
Code: Select all
Vary Thruster 1 burnDuration
Begin Finite Burn (Thruster 1)
Prop burnDuration secs
End Finite Burn (Thruster 1)

Vary Thruster 2 burnDuration
Begin 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
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Re: Converting impulsive burn to finite burn

Postby jjkparker » Mon Jun 15, 2015 1:44 pm

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
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Re: Converting impulsive burn to finite burn

Postby Torr919 » Wed Feb 24, 2016 11:09 pm

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)
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