[X] My Assistant

[SkyeLab]

Pushing out of Earth orbit now...........

[Greg Hullender]

That makes sense -- I'd wondered why the projected path didn't seem to show much change after the assist -- but it still seems you could just as easily change inclination on an outbound flyby as an inbound one.

Again, I'm sure there's a reason why this works out to be superior -- I'm just not seeing what it is. Why is this outer-planet flyby different from all others?

--Greg

[3488]

Also as Doug before you said, it is about changing inclination & also speed. The Mars encounter IIRC also prevents DAWN from coming much closer to the Sun again.

Andrew Brown.

[Greg Hullender]

Also as Doug before you said, it is about changing inclination & also speed. The Mars encounter IIRC also prevents DAWN from coming much closer to the Sun again.

I wasn't wondering "why is there a Mars flyby." I was wondering "why is this an 'inbound' (toward the Sun) flyby not an 'outbound' one."

Given the way ion propulsion works (long-term but weak), I'm actually surprised Dawn can do this maneuver at all. I'd have thought the thrust would always be close to parallel to the velocity vector and that the orbit (if they turned the engines off) would be nearly circular at any given point. But Dawn is actually falling back towards the Sun (a little) in order to make this flyby.

Maybe that explains it; the angle between the velocity vector of the spacecraft and the velocity vector of the planet needs to get smaller during the flyby or else energy will be lost, not gained. Perhaps it's easier to lift Dawn above the orbit of Mars and then drop it than it would be to get it to rise past it at any significant angle. In that case, though, I do wonder why they quit thrusting entirely for a few months before and after the assist. It'd seem you could get a steeper angle that way.

Of course I know you plan your orbits with the planets you have -- not the planets you'd like to have. :-) At a certain point, I suppose the answer is just "Yeah, you'd go faster, but you wouldn't get to Vesta."

--Greg

[BrianJ]

I wasn't wondering "why is there a Mars flyby." I was wondering "why is this an 'inbound' (toward the Sun) flyby not an 'outbound' one."

Hi, just wanted to point out that for earlier launch windows, the flyby was indeed "outbound", as per this image http://upload.wikimedia.org/wikipedia/comm..._as_of_2006.jpg

Also, just out of interest, here's a graphic comparing the orbit of Dawn to the orbit of Mars, before and after the (current trajectory) flyby.
http://img169.imageshack.us/img169/3606/dawn1copyzs2.gif

Regards,
Brian

[Greg Hullender]

Hi, just wanted to point out that for earlier launch windows, the flyby was indeed "outbound", as per this image http://upload.wikimedia.org/wikipedia/comm..._as_of_2006.jpg

Also, just out of interest, here's a graphic comparing the orbit of Dawn to the orbit of Mars, before and after the (current trajectory) flyby.
http://img169.imageshack.us/img169/3606/dawn1copyzs2.gif

Well, I think the first graphic actually shows an inbound, not an outbound flyby. Did you mean to give a different link?

The second graphic is really cool, though. Does look like a pretty good bang for the buck from this flyby!

dmuller: On the question of the orbit always being circular, I didn't mean to imply that I thought that was the case with Dawn -- it's that it seemed to me that that would be the optimal way to use a low-thrust engine, and I'm curious why that's not the case in practice. Is it solely to make the Mars flyby work?

EDIT

Should have checked Vallado (Fundamentals of Astrodynamics and Applications) first. In Chapter 6.7 (Continuous Thrust Transfers) he works out that this type of transfer returns to circular only on integral numbers of orbits.

--Greg

[dmuller]

On the question of the orbit always being circular, I didn't mean to imply that I thought that was the case with Dawn -- it's that it seemed to me that that would be the optimal way to use a low-thrust engine, and I'm curious why that's not the case in practice. Is it solely to make the Mars flyby work?

Greg, if you have the Orbiter simulator (http://www.orbitersim.com) you can simulate this in Earth orbit. Get a scenario where your ship is docked to the ISS, set your Orbit display to PRJ ship and DST altitude (will look more or less like a circle), undock, hit "prograde", get your engines on the lowest thrust setting (i.e. press and hold "Ctrl" and then literally hit your "+" button on the Numpad as short as possible), speed up the simulation to 100x (unless you have a lot of time - but dont go over 100x), and watch the PeA (perigee altitude) and ApA (apogee altitude) increase ... everytime you nearly get a circle (circle: PeA = ApA), youl'll find the ApA running away, half an orbit later the PeA starts to catch up.