LOI Strangeness

General help that does not fall into a more specific category

LOI Strangeness

Postby JackCrenshaw » Thu Sep 12, 2013 1:13 pm

Well, I still don't seem to know how to use Vary/Achieve, but I still managed to get a very nice trajectory, just by tweaking the initial conditions. That said, there's still something strange going on. I'm attaching the script. At periselene of the incoming hyperbola, I'm applying a retrograde delta-v for LOI.

But when you look at the MoonInertial view, you can see a definite and easily visible right turn, just at the delta v. As a result, the apoapsis goes high, the periapsis goes low, and the S/C crashes into the surface.

Tinkering with the delta-v, I was able to get a nice, nearly circular orbit by adding thrust along the radial axis. But that's crazy.

Unless there's something I don't understand about Newtonian physics, this shouldn't happen. When the S/C is at periselene, it's moving horizontally, relative to the center of the Moon, right? That is, after all, the very definition of periapsis.

So if I apply an impulsive delta-v along that same velocity vector, there is NO WAY the S/C should turn right.

Now look: I don't consider for a second, the fact that GMAT has a bug in its gravity model, that little ole me managed to find. You guys have had many people working on this thing for a long time, and an error in the dynamic model is inconceivable. I have no doubt the problem must be pilot error.

But I'm danged if I can see where I went wrong. I'm hoping you can. Thanks.

Jack
Attachments
CrenshawScript_2.m
(10.72 KiB) Downloaded 119 times
JackCrenshaw
 
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Re: LOI Strangeness

Postby shughes » Wed Sep 18, 2013 3:22 pm

Hi Jack,

The coordinate system for your maneuver is Moon Inertial and you are applying the maneuver in the x-direction in that coordinate system. So, how that maneuver relates to the radial and velocity direction will depend on where in the orbit the spacecraft is located. For this reason, we usually work in a Lunar LVLH or Lunar VNB (Velocity, Normal, Binormal) system.

I changed your maneuver from this

Code: Select all
Create ImpulsiveBurn LOI;
GMAT LOI.CoordinateSystem = MoonInertial;
GMAT LOI.Element1 = -0.86;
GMAT LOI.Element2 = 0;
GMAT LOI.Element3 = 0;


to this

Code: Select all
Create ImpulsiveBurn LOI;
GMAT LOI.CoordinateSystem = Local;
GMAT LOI.Origin = Luna;
GMAT LOI.Axes = VNB;
GMAT LOI.Element1 = -0.86;
GMAT LOI.Element2 = 0;
GMAT LOI.Element3 = 0;


This configuration applies the capture maneuver in the (negative) orbit velocity direction, regardless of where in the orbit the spacecraft is located.

hope this helps
Attachments
CrenshawScript_2_SPHMod.script
(10.77 KiB) Downloaded 148 times
Capture.PNG
shughes
 
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