Thursday, January 12, 2006

Ion the Prize - Orbiter and Trajectories

That last post about ion engines got me thinking about trajectories and Orbiter. With continous or long-duration thrust, as would be typical with ion engines (even higher thrust, next-gen ion engines will be relatively low thrust compared to chemical rocket engines), things are not so simple in terms of trajectory planning. If you do a Google search on "orbital mechanics low thrust" you will find references to "trajectory optimization" and many other things related to ion engines as well as solar sails (here's a paper [PDF] that uses an evolutionary algorithm to optimize low-thrust orbits).

Many common maneuvers (such as raising the apoapsis by thrusting pro-grade at perapsis) are based on the assumption that the thrust is applied instantaneously (or at least for a short finite time). A simple Hohmann transfer orbit is essentially this, and ignoring perturbations and mid-course corrections, requires only two "burns." If you apply thrust continuously, the energy of the orbit (and the orbit itself) changes continuously, and you end up with some sort of a spiral trajectory. Here's a page that discusses some of these issues and has some charts that could be useful for planning.

I wondered if there are ion-engine-powered spacecraft for Orbiter, and I found one, Soltech v1.1 by "spacingbluefrog." This looks pretty cool, and it includes a Moon base and 2D cockpit panels, a pretty rare thing in spacecraft add-ons. I haven't tried it yet, and the author warns that the low-thrust behavior is tricky to manage in Orbiter. This looks like a promising area for a future MFD add-on (trajectory planning/control MFD for low-thrust propulsion), but I don't know how much demand there is for this right now.

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