IPB

Welcome Guest ( Log In | Register )

20 Pages V  « < 4 5 6 7 8 > »   
Reply to this topicStart new topic
Dawn Cruise
djellison
post Aug 27 2008, 04:20 PM
Post #76


Administrator
****

Group: Chairman
Posts: 14153
Joined: 8-February 04
Member No.: 1



The Mars flyby is mainly about changing the orbital inclination of Dawn iirc.
Go to the top of the page
 
+Quote Post
Greg Hullender
post Aug 27 2008, 04:44 PM
Post #77


Senior Member
****

Group: Members
Posts: 1017
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
Go to the top of the page
 
+Quote Post
3488
post Aug 27 2008, 07:25 PM
Post #78


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.
Go to the top of the page
 
+Quote Post
Greg Hullender
post Aug 29 2008, 06:02 PM
Post #79


Senior Member
****

Group: Members
Posts: 1017
Joined: 29-November 05
From: Seattle, WA, USA
Member No.: 590



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
Go to the top of the page
 
+Quote Post
BrianJ
post Aug 29 2008, 09:15 PM
Post #80


Newbie
*

Group: Members
Posts: 17
Joined: 5-January 06
Member No.: 636



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
Go to the top of the page
 
+Quote Post
dmuller
post Aug 30 2008, 01:36 AM
Post #81


Member
***

Group: Members
Posts: 339
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


--------------------
Go to the top of the page
 
+Quote Post
tasp
post Aug 30 2008, 06:04 AM
Post #82


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 . . . )


blink.gif



Go to the top of the page
 
+Quote Post
dmuller
post Aug 30 2008, 09:37 AM
Post #83


Member
***

Group: Members
Posts: 339
Joined: 11-April 08
From: Sydney, Australia
Member No.: 4093



QUOTE (Greg Hullender @ Aug 30 2008, 04:02 AM) *
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


--------------------
Go to the top of the page
 
+Quote Post
siravan
post Aug 30 2008, 03:19 PM
Post #84


Member
***

Group: Members
Posts: 125
Joined: 10-December 06
From: Atlanta
Member No.: 1472



QUOTE (tasp @ Aug 30 2008, 01:04 AM) *
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.
Go to the top of the page
 
+Quote Post
djellison
post Aug 30 2008, 03:36 PM
Post #85


Administrator
****

Group: Chairman
Posts: 14153
Joined: 8-February 04
Member No.: 1



QUOTE (siravan @ Aug 30 2008, 04:19 PM) *
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.
Go to the top of the page
 
+Quote Post
Hungry4info
post Aug 30 2008, 04:12 PM
Post #86


Senior Member
****

Group: Members
Posts: 1170
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)
Go to the top of the page
 
+Quote Post
vjkane
post Aug 30 2008, 04:58 PM
Post #87


Member
***

Group: Members
Posts: 517
Joined: 22-April 05
Member No.: 351



QUOTE (Hungry4info @ Aug 30 2008, 05:12 PM) *
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.


--------------------
Go to the top of the page
 
+Quote Post
Greg Hullender
post Aug 30 2008, 06:01 PM
Post #88


Senior Member
****

Group: Members
Posts: 1017
Joined: 29-November 05
From: Seattle, WA, USA
Member No.: 590



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
Go to the top of the page
 
+Quote Post
tasp
post Aug 30 2008, 10:40 PM
Post #89


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 . . .




Go to the top of the page
 
+Quote Post
BrianJ
post Aug 30 2008, 11:09 PM
Post #90


Newbie
*

Group: Members
Posts: 17
Joined: 5-January 06
Member No.: 636



QUOTE (Greg Hullender @ Aug 30 2008, 06:01 PM) *
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 smile.gif )

Best regards,
Brian
Go to the top of the page
 
+Quote Post

20 Pages V  « < 4 5 6 7 8 > » 
Reply to this topicStart new topic

 



RSS Lo-Fi Version Time is now: 26th March 2019 - 08:37 AM
RULES AND GUIDELINES
Please read the Forum Rules and Guidelines before posting.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted. Do not reproduce without permission. Read here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators.
SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society and is funded by donations from visitors and members. Help keep this forum up and running by contributing here.