Cassini's Extended Mission, July 2008 to June 2010 Options

[jsheff]

The January PSG meeting is now in progress and the tour for extended mission is scheduled to be chosen on Thursday (Feb 1). There are 13 tours being considered OF4a, PF-3, PF-4, PF-6, PF-6h9, PF-7, PF-8, PF-9, PF-10, PF-11, PF-12 & PF-13 --plus 'tweaks' on these tours e.g. there is a PF-8a, PF-9a

S-S-So ... has anybody heard anything?

- John Sheff
Cambridge, MA

[john_s]

The winner is (drum roll please) PF6h9. Officially adopted on Thursday. I haven't sifted through all the details yet, but from my parochial point of view, I know it includes seven close Enceladus flybys, so that's good. Most of the science groups (Titan, Rings, Magnetosphere, Saturn, and Icy Satellites) were pretty happy with this choice- it packs in an amazing number of science opportunities.

John.

[Guest_Analyst_*]

I have done a limited analysis of the propellant usage so far.

Sources are:
[1] Cassini Mission Plan
[2] Cassini Significant Event Reports
[3] Propellant Remaining per October 2005 given by AlexBlackwell
[4] Cassini Tour Redesign for the Huygens Mission
[5] Initial Cassini Propulsion System In-Flight Characterization
[6] Cassini Maneuver Experience: Launch and Early Cruise
[7] Cassini Maneuver Experience: Finishing Inner Cruise

The description of the txt file follows:

In head are four blocks, from left to right:

- specific impulse and thrust of the main engine from source [5], the thrust I used is slighly lower than the nominal 445 N; there are several reportings of a small engine underperformance (less than 1%, resulting in slighly longer burn times), 441 N helps to match the numbers, but the errrors using 445 N is very small too
- masses from source [5]
- propellant used until specific dates from sources [3] and [5]; these numbers are a little iffy: in source [5] the numbers are given for EOY 2001, not 01.04.2002; in source [3] for “late October”, these data are used only to guess the monopropellant usage
- monopropellant remaining as per specific dates; the number for 30.06.2004 (SOI) is computed as is the usage per day for cruise (about 6 g/d) and tour (27 g/d)

The 11 columns in the table are:

(1) TCM or OTM number
(2) the maneuver date, can be off be one day because times were given in UTC and PST and I didn’t care to check
(3) maneuver name/event
(4) maneuver duration; for TCM 1 to 17 (including) this has been computed using the rocket equation and the delta v given in column (5), for all other maneuvers these are actual numbers from source [2]
(5) actual delta v using biprop main engine; source [5] for TCM 1 to 17 (including), source [2] for TCM 18 and later
(6) actual delta v using monoprop thrusters; sources like column (5)
(7) and (8) predicted delta v for the tour from source [4]
(9) the computed delta v using the rocket equation, the given maneuver duration (4) and the propellant usage from coloum (13)
(10) the difference between (5) and (9), is of course zero for TCM 1 to 17 (including)
(11) the remaining monoprop using the numbers from above (6 or 27 g/d)
(12) the biprop remaining before the maneuver
(13) the biprop used during the maneuver computed using the duration (4), isp and thrust

I only care about biprop, the monoprop usage is assumed to be linar and monoprop delta v maneuvers are discounted. The monoprop tour delta v has only been 3.4 m/s so far.

The numbers match very good. For instance the computed DSM duration is off by about 20 s (less than half of a percent), the computed bipropellant remaing in late October 2005 is 493 kg vs. 499 kg given by [3] (about one percent error). The delta v difference for SOI is a little large (5 m/s, still less than one percent), I don’t have the exact burn time and used 96 minutes.

The biprop delta v after OTM 92 with 437 kg is 540 m/s, the mean of the remaining prime tour is 202 m/s ([4], old source), coming nicely to the 340 m/s after the prime tour given in this thread.

Any suggestions? I have a .xls file if someone cares.

Analyst

Attached File(s)

 Cassini.txt ( 23.27K ) Number of downloads: 461