Cause of complexity of Cassini's orbit round Saturn? 3

[RPstress]

See the orbital sample tour at

http://en.wikipedia.org/wiki/Cassini–Huygens#Trajectory

.

I assume there was a significant burn by Cassini for orbital insertion (and maybe a small one when Huygens departed), but what has given the complexity it its orbital path since? Is it gravitational interactions with the moons?

 

[btrueblood]

Dunno, but I would guess the primary perturbation is JPL commanding minor thruster burns to correct the orbit, or target other moons. They don't depict it in the diagrams your post shows, but one feature of the orbits is likely that they spend the majority of their time on the sun-facing side of the planet, so imaging cameras can take their pictures. As the orbits precess to the dark side, a few squirts of thrust at the right times will put the bird into a new orbit, esp. if a flyby of a moon is involved.

 

[RPstress]

Thanks. I didn't think they'd have taken enough fuel for the amount of changes that the diagram looks like it shows, but I have no feel for how much fuel that sort of messing takes.

WP says the orbital insertion needed a speed change of 662 m/s. Maybe the amount of changes since is well within their means.

Apropos of which, does anyone have a guess as to the mass of fuel needed for a 600+ m/s change on something weighing a couple of tons?

http://saturn.jpl.nasa.gov/news/cassiniinsider/insider20090128/

has some info on the propellant used and

http://saturn.jpl.nasa.gov/mission/saturntourdates/saturntourdates2014/

to ~2017/ show the number of small burns they do; looks like quite a few.

A few other websites look informative (maybe

http://www.spacekb.com/Uwe/Forum.aspx/space-flight/.../Cassini-s-engine

) but my network nanny won't give me access.

 

[Compositepro]

The specific impulse of that propellant is about 280 sec. (pound force thrust x second per pound mass of fuel).

http://www.braeunig.us/space/propel.htm

 

[btrueblood]

Once captured into Saturn's system, the remaining orbit changes were probably done with less than 1/10 of the fuel used for insertion. Most of the maneuvers were likely done using gravity assist from the various moons, and lots of calculations back at mission control.

 

[IRstuff]

see:

http://saturn.jpl.nasa.gov/news/cassiniinsider/insider20090903/

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[RPstress]

280 sec (pound force thrust x second per pound mass of fuel)...

http://www.astrobio.net/pressrelease/1044/cassini-saturn-orbit-insertion-timeline

gives the burn duration for orbit insertion as 96 minutes = 5760 s.

Say half Cassini's fuel load of three tons was left before insertion, that gives a total mass of 3500 kg (was three tons of fuel at launch + two ton spacecraft).

The velocity change was 662 m/s (WP), v = at => a = 662/5760 = 0.128 m/s^2.

f = ma => f = 3500*0.128 = 448 N = 101 lbf.

With a specific impulse (?) of 280 lbf/lbm * s => lbm = 101*5760/280 = 2080 lb of fuel used...? (Ignores mass change as fuel is used; it'd probably be nearer 1750 lbm.)


Other websites make it pretty clear that most of the orbital changes are done by moons' gravity, with nudges from motors to get in the right place, particularly

http://www.planetary.org/explore/topics/cassini_huygens/tour.html

. The last three lines on that webpage make quite sad reading.

Thx.

 

[btrueblood]

"End of mission, atmospheric entry into Saturn (1.0 Saturn radii)"

Um, dunno why that's all that sad, the bird will be pretty much out of fuel, and batteries/solar cells quite degraded by that time, so useful data return from the spacecraft will be fairly minimal. The data on Saturn's atmosphere from aero entry maneuver will be quite valuable. Similar operation was done with Magellan probe to Venus, and gave a lot of surprising info. regarding that planet's atmosphere. Enough that future missions will likely use the data to plan aerobraking maneuvers, which will help save fuel and allow extending mission durations. Aerobraking was looked at for Cassini, but I believe rejected because of uncertainties in Titan and Saturn atmospheres.

 

[RPstress]

I just thought it was pity to go all that way (1b klicks away) and poff, burn up! Shame it couldn't be put into a safe orbit as some sort of a future monument. (It would also stop all these Internet nutters who think it's all a plot to use fission (caused by high pressure in the depths of Saturn) of the plutonium thermoelectric generator fuel to in turn cause sustainable fusion (yes, really!) of the plant's basic hydrogen). They're out there...)

 

[btrueblood]

I'd forgotten the bird had RTG's. That'd be kinda cool, to have Saturn lit up at night...

 

[RPstress]

It would be awesome! (Most of) said nutters think it would be a bad thing!