Dawn Cruise |
Dawn Cruise |
Aug 27 2008, 04:20 PM
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#76
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Founder Group: Chairman Posts: 14434 Joined: 8-February 04 Member No.: 1 |
The Mars flyby is mainly about changing the orbital inclination of Dawn iirc.
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Aug 27 2008, 04:44 PM
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#77
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
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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Aug 27 2008, 07:25 PM
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#78
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Junior Member Group: Members Posts: 47 Joined: 27-June 08 From: Ashford, Kent, United Kingdom. Member No.: 4244 |
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. -------------------- "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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Aug 29 2008, 06:02 PM
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#79
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
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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Aug 29 2008, 09:15 PM
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#80
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Newbie Group: Members Posts: 17 Joined: 5-January 06 Member No.: 636 |
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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Aug 30 2008, 01:36 AM
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#81
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Member Group: Members Posts: 340 Joined: 11-April 08 From: Sydney, Australia Member No.: 4093 |
My two cents worth:
Dawn is picking up speed from the flyby. As this happens halfway between periapsis (closest point to sun) and apoapsis (farthest point to sun), both are raised. It also rotates the apsises clockwise, so Dawn will be at periapsis shortly after the Mars encounter. By thrusting then, it efficiently raises the apoapsis further. It also increases the inclination from 1.9 to 6.5 degrees, which I think would be costly to do without a gravity assist. I dont think it makes a difference whether the flyby is inbound or outbound, you gain speed w.r.t. sun as long as you fly behind Mars. Horizons gives the following orbital elements w.r.t. solar system barycenter (Mars flyby as inferred from Horizons data is around Feb 12): CODE 1 Feb 2009 28 Feb 2009 Change
Eccentricity 0.160521 0.132016 orbit gets rounder Periapsis (km) 180,738,278 203,950,288 +23 million km Apoapsis (km) 249,858,224 265,989,955 +16 million km Inclination 1.9249 6.5018 gets steeper -------------------- |
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Aug 30 2008, 06:04 AM
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#82
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Member Group: Members Posts: 903 Joined: 30-January 05 Member No.: 162 |
I will reiterate my ignorance about such matters, but will still post a question.
Let's say the Dawn ion drive accels the craft nominally at .001 G right now (neglecting slow increase in accel with fuel mass depletion) and let's say every one in the front office monitoring the engine is happy to sign off at running it in the range of .00085 to .00115 G. And the office crew is ok with adjusting the throttle from time to time, so long as Mars encounter and Vesta arrival (or was Ceres first ? no matter) happen on schedule, and the average accel is .001 for optimal fuel utilization. So, by tweaking the throttle only, in the narrow approved range, with a minimum impact to our amazing flight of discovery to Vesta and Ceres, how far off the 'nominal track' can the probe get, and does this resulting 'lens' shaped area in the graph (assuming frame of reference w/ Dawn) take us near anything known and interesting ?? For example, from Mars to Vesta, we accel at lower approved G limit 1/2 way and then go rest of the way at higher approved rate (allowing for the trajectory smiths to approve limits, of course) or we accel 1/2 way at higher G limit, then at lower limit the rest of the way. This gives us an area (instead of a line) along the way to Vesta we can explore for interesting objects to encounter for 'free'. (other than the brain wear and tear it takes to figure this out) So is this already obvious to everyone and has been found to be undesirable for some reason I haven't figured out yet, or does this put a toe in the door for a free extra object to look at ?? ( I am assuming the area potentially available with this technique compared to the known number of asteroids yields a figure of >1 for # of objects on average expected to be in an area of that size, but if it is more like .001, then it looks like I have my answer . . . ) |
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Aug 30 2008, 09:37 AM
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#83
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Member Group: Members Posts: 340 Joined: 11-April 08 From: Sydney, Australia Member No.: 4093 |
that the orbit (if they turned the engines off) would be nearly circular at any given point. Greg, that does not seem to be the case. To start with, the original post-launch (pre-ion thrusting) was nowhere near a circle (150m km x 246m km). And the Mars flyby boosts both the periapsis and apoapsis quite a lot (as compared to the thrusting) Again from Horizons: CODE Date Periapsis Apoapsis
1-Oct-07 150,019,330 246,071,890 1-Nov-07 150,128,927 244,454,659 1-Jan-08 151,655,060 246,166,277 1-Jul-08 165,446,119 251,548,401 1-Jan-09 180,583,781 250,621,283 1-Jul-09 204,715,862 269,844,671 1-Jan-10 224,351,731 293,297,300 1-Jul-10 260,729,882 309,582,209 1-Jan-11 299,787,522 326,511,305 1-Jul-11 317,551,998 368,001,819 1-Jan-12 320,554,935 382,753,035 1-Jul-12 319,854,770 382,797,552 1-Jan-13 338,984,080 382,275,315 1-Jul-13 363,921,049 408,392,742 1-Jan-14 364,440,390 457,493,646 1-Jul-14 364,604,289 449,257,439 1-Jan-15 378,673,307 445,436,420 1-Feb-15 382,279,514 445,557,925 -------------------- |
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Aug 30 2008, 03:19 PM
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#84
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Member Group: Members Posts: 128 Joined: 10-December 06 From: Atlanta Member No.: 1472 |
So, by tweaking the throttle only, in the narrow approved range, with a minimum impact to our amazing flight of discovery to Vesta and Ceres, how far off the 'nominal track' can the probe get, and does this resulting 'lens' shaped area in the graph (assuming frame of reference w/ Dawn) take us near anything known and interesting ?? IIRC, the ion engines of Dawn are not exactly throttlable. In fact, they have such a low thrust that the main way to control them is to simply turn them on and off. Dawn has periods of "powered flight", separated by periods of cruise. I think each powered segment (days or weeks in a row) has essentially a fixed attitude. Then, then turn off the engine, determine the exact location, turn Dawn to a new attitude and turn on the engine again... But, back to your main question. Essentially, Dawn does not have one fixed route from where it is to Vesta and in theory it possible to target it to some interesting object en route. However , there are two complications: the low thrust means that everything should be planned way ahead of time and Mars encounter. The exact time and location of the Mars encounter is still not determined, so there is still lots of uncertainty in the post encounter orbit. I believe that it is only after the encounter that Dawn will have a constrained orbit and the team can start planning for any possible extra targeting. |
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Aug 30 2008, 03:36 PM
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#85
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Founder Group: Chairman Posts: 14434 Joined: 8-February 04 Member No.: 1 |
IIRC, the ion engines of Dawn are not exactly throttlable. They are - dramatically. http://dawn.jpl.nasa.gov/mission/journal_10_07_07.asp - lots of detail of the 112 different throttle levels they can use. |
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Aug 30 2008, 04:12 PM
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#86
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Senior Member Group: Members Posts: 1452 Joined: 26-July 08 Member No.: 4270 |
@ tasp
They may not due that so as to conserve fuel. According to Wikipedia (which I realise may not be accurate), if the Vesta and Ceres investigations are successful, DAWN may go on to Pallas -- something I would really love to see. -------------------- -- Hungry4info (Sirius_Alpha)
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Aug 30 2008, 04:58 PM
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#87
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Member Group: Members Posts: 715 Joined: 22-April 05 Member No.: 351 |
They may not due that so as to conserve fuel. According to Wikipedia (which I realise may not be accurate), if the Vesta and Ceres investigations are successful, DAWN may go on to Pallas -- something I would really love to see. Per the Dawn Q&A page: "Q: Will there be opportunities to visit other asteroids, either en route to Ceres or as part of an extended mission? Answer: Unlikely, because there is greater return by spending more of our resources on Vesta and Ceres." http://dawn.jpl.nasa.gov/mission/faqs.asp# I personally have my doubts about this answer. If at the end of the *extended* Ceres investigations, there is still meaningful amounts of fuel left, I strongly suspect that Dawn will either 1) be sent to orbit another asteroid (a fairly expensive option because of mission ops) or 2) be sent to flyby one or more other bodies in similar solar orbits as Ceres (a less expensive option). NASA is really good at getting the most bang out of a working spacecraft. On the other hand, I don't think that anyone is working really hard on the question of what to do after Ceres -- that event is 7 years away and lots of things can happen to fuel levels, spacecraft and instrument health, and NASA budgets. -------------------- |
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Aug 30 2008, 06:01 PM
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#88
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
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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Aug 30 2008, 10:40 PM
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#89
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Member Group: Members Posts: 903 Joined: 30-January 05 Member No.: 162 |
Just wanted to comment how much fun it is to have a new 'toy' (ion drive) to 'play' with on the Dawn mission (I wasn't paying attention during Deep Space 1).
I think for folks brought up during the space age we all mostly have a 'feel' for Hohmann transfer orbits and their permutations, but 'learning the ropes' with an ion drive is new and exciting. No shortage of ideas on things to try with it, and probably some good caution being exercised by the mission planners. I really appreciate all the interest in this mission. And of course, future applications of this drive technology seem to be promising all kinds of exciting possibilities. I'd love to wire up an ion drive to an RTG and see what we could get going . . . |
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Aug 30 2008, 11:09 PM
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#90
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Newbie Group: Members Posts: 17 Joined: 5-January 06 Member No.: 636 |
Well, I think the first graphic actually shows an inbound, not an outbound flyby. Did you mean to give a different link? Erm...I don't think so. I would call it "outbound" since Dawn approaches the flyby from within the orbit of Mars (assuming the link I gave shows you the same as it shows me!) I only comment since I spent quite some time trying to figure out the flyby trajectory for the earlier June launch window (as an "interested layman" who finds these things fascinating )Best regards, Brian |
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