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Dawn Cruise
SkyeLab
post Sep 27 2007, 12:31 PM
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Pushing out of Earth orbit now...........

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"There are 10 types of people in the world - those who understand binary code, and those who don't."
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Greg Hullender
post Aug 27 2008, 04:44 PM
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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
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3488
post Aug 27 2008, 07:25 PM
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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.


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"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.
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Greg Hullender
post Aug 29 2008, 06:02 PM
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QUOTE (3488 @ Aug 27 2008, 12:25 PM) *
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
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BrianJ
post Aug 29 2008, 09:15 PM
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QUOTE (Greg Hullender @ Aug 29 2008, 06:02 PM) *
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
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Greg Hullender
post Aug 30 2008, 06:01 PM
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QUOTE (BrianJ @ Aug 29 2008, 01:15 PM) *
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
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Posts in this topic
- SkyeLab   Dawn Cruise   Sep 27 2007, 12:31 PM
- - elakdawalla   loon reports from the Cape that AOS happened about...   Sep 27 2007, 02:39 PM
|- - ustrax   Horst Uwe Keller, Dawn's FC Team Leader report...   Sep 27 2007, 02:55 PM
- - climber   May be little OT...and that's the reason of th...   Sep 27 2007, 04:10 PM
|- - punkboi   Godspeed, Dawn!   Sep 27 2007, 04:18 PM
|- - Rakhir   QUOTE (climber @ Sep 27 2007, 04:10 PM) W...   Sep 27 2007, 08:49 PM
- - dvandorn   Well -- I missed the launch (6:34 am is a little e...   Sep 27 2007, 05:18 PM
- - stevesliva   Amazing! Given the incredible complexity of t...   Sep 27 2007, 05:39 PM
- - Toma B   Just as I thought...nobody in the press conference...   Sep 28 2007, 07:33 AM
|- - ugordan   QUOTE (Toma B @ Sep 28 2007, 09:33 AM) I ...   Sep 28 2007, 07:36 AM
||- - stevesliva   QUOTE (ugordan @ Sep 28 2007, 03:36 AM) I...   Sep 28 2007, 04:57 PM
|- - elakdawalla   QUOTE (Toma B @ Sep 28 2007, 12:33 AM) I ...   Sep 28 2007, 04:35 PM
- - Greg Hullender   jabe: No, that's not it, but this has already ...   Sep 29 2007, 05:46 AM
|- - jabe   QUOTE (Greg Hullender @ Sep 29 2007, 05:4...   Sep 29 2007, 11:56 AM
- - Del Palmer   Latest Dawn journal: http://dawn.jpl.nasa.gov/mis...   Oct 2 2007, 05:14 PM
|- - punkboi   Interesting journal. Hopefully, a "Where is ...   Oct 2 2007, 05:41 PM
|- - Stephen   QUOTE (punkboi @ Oct 3 2007, 03:41 AM) In...   Oct 3 2007, 02:38 AM
- - djellison   It doesn't need a subforum until it starts doi...   Oct 3 2007, 06:59 AM
|- - Stephen   QUOTE (djellison @ Oct 3 2007, 04:59 PM) ...   Oct 4 2007, 02:03 AM
|- - jamescanvin   QUOTE (Stephen @ Oct 4 2007, 03:03 AM) I ...   Oct 4 2007, 08:50 AM
|- - djellison   QUOTE (Stephen @ Oct 4 2007, 03:03 AM) ...   Oct 4 2007, 10:54 AM
|- - Stephen   QUOTE (djellison @ Oct 4 2007, 08:54 PM) ...   Oct 5 2007, 01:49 AM
- - volcanopele   I could see the creation of a new thread though   Oct 3 2007, 07:09 AM
|- - ugordan   QUOTE (volcanopele @ Oct 3 2007, 09:09 AM...   Oct 3 2007, 08:47 AM
- - Greg Hullender   I think it's simpler than that; it's becau...   Oct 4 2007, 04:57 AM
- - CAP-Team   Here's where Dawn is now: Earth as seen fro...   Oct 4 2007, 10:00 AM
|- - punkboi   QUOTE (CAP-Team @ Oct 4 2007, 03:00 ...   Oct 4 2007, 05:27 PM
|- - ugordan   QUOTE (punkboi @ Oct 4 2007, 07:27 PM) I ...   Oct 4 2007, 05:56 PM
- - CAP-Team   QUOTE (punkboi @ Oct 4 2007, 07:27 PM) Co...   Oct 4 2007, 09:20 PM
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- - peter59   Second Ion Thruster Checkout Completed Successfull...   Oct 27 2007, 06:36 AM
- - nprev   Thanks for the update, Peter. Green bird, baby, gr...   Oct 27 2007, 01:32 PM
|- - brellis   QUOTE (nprev @ Oct 27 2007, 06:32 AM) has...   Oct 27 2007, 02:06 PM
|- - punkboi   New journal up: http://dawn.jpl.nasa.gov/mission/...   Oct 27 2007, 06:45 PM
- - peter59   More Ion Thrusting Tests Completed November 12 - 1...   Nov 20 2007, 08:59 PM
|- - punkboi   A week late, but whatever: http://dawn.jpl.nasa.g...   Nov 20 2007, 09:04 PM
- - peter59   Main Antenna Checked Out and New Software Uploaded...   Nov 24 2007, 07:35 AM
|- - punkboi   "Where is Dawn now?" page finally up: h...   Nov 29 2007, 06:48 PM
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|- - punkboi   http://www.planetary.org/blog/article/00001266/ N...   Dec 20 2007, 09:15 AM
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|- - ugordan   QUOTE (Holder of the Two Leashes @ Feb 14 200...   Feb 14 2008, 07:31 PM
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- - peter59   I nearly forgot about these cancelled missions. P...   Mar 20 2008, 04:34 PM
|- - dmuller   QUOTE (peter59 @ Mar 21 2008, 02:34 AM) P...   Jun 30 2008, 06:41 AM
|- - 3488   QUOTE (jasedm @ Jul 1 2008, 11:30 AM) I...   Jul 1 2008, 03:42 PM
|- - ElkGroveDan   QUOTE (3488 @ Jul 1 2008, 07:42 AM) No de...   Jul 1 2008, 04:55 PM
||- - Del Palmer   QUOTE (ElkGroveDan @ Jul 1 2008, 05:55 PM...   Jul 1 2008, 05:01 PM
|- - Del Palmer   QUOTE (3488 @ Jul 1 2008, 04:42 PM) No de...   Jul 1 2008, 04:56 PM
|- - 3488   QUOTE (ElkGroveDan @ Jul 1 2008, 05:55 PM...   Jul 1 2008, 06:47 PM
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- - Holder of the Two Leashes   Yes, the diagrams are much better now, especially ...   May 1 2008, 04:26 PM
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|- - punkboi   New Dawn journal up: http://dawn.jpl.nasa.gov/mis...   Jun 30 2008, 03:17 AM
- - nprev   ...Dr. Rayman is a hoot! He sure can write an ...   Jun 30 2008, 11:11 AM
|- - Greg Hullender   QUOTE (nprev @ Jun 30 2008, 04:11 AM) .....   Jun 30 2008, 12:41 PM
|- - ugordan   QUOTE (Greg Hullender @ Jun 30 2008, 02:4...   Jun 30 2008, 12:48 PM
|- - MahFL   He sounds like a typical slightly nutty scientist ...   Jun 30 2008, 01:01 PM
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- - Greg Hullender   That makes sense -- I'd wondered why the proje...   Aug 27 2008, 04:44 PM
|- - 3488   Also as Doug before you said, it is about changing...   Aug 27 2008, 07:25 PM
|- - Greg Hullender   QUOTE (3488 @ Aug 27 2008, 12:25 PM) Also...   Aug 29 2008, 06:02 PM
|- - BrianJ   QUOTE (Greg Hullender @ Aug 29 2008, 06:0...   Aug 29 2008, 09:15 PM
||- - Greg Hullender   QUOTE (BrianJ @ Aug 29 2008, 01:15 PM) Hi...   Aug 30 2008, 06:01 PM
||- - BrianJ   QUOTE (Greg Hullender @ Aug 30 2008, 06:0...   Aug 30 2008, 11:09 PM
|||- - Greg Hullender   QUOTE (BrianJ @ Aug 30 2008, 04:09 PM) Er...   Sep 1 2008, 05:57 PM
||- - dmuller   QUOTE (Greg Hullender @ Aug 31 2008, 04:0...   Aug 31 2008, 02:08 AM
|- - dmuller   QUOTE (Greg Hullender @ Aug 30 2008, 04:0...   Aug 30 2008, 09:37 AM
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|- - Marz   QUOTE (kwp @ Sep 1 2008, 11:01 PM) I aske...   Oct 3 2008, 06:20 PM
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