In article ,
Remy Villeneuve wrote:
I was wondering how they're considering the attitude control
systems... One of the limitations for Galileo was the fuel, same goes
for Cassini...

It's not that big a limitation, if you're using it for nothing else. The
Voyagers use RCS jets for attitude control, and they'll die due to power
shortage before they run out of fuel. Galileo went through its fuel
relatively quickly because it was using fuel steadily for *maneuvering*,
setting up for one moon encounter after another. Cassini will have the
same problem.

Note that Galileo was spin-stabilized, eliminating the need to expend very
much fuel on attitude control. And if I recall correctly, Cassini has
reaction wheels for the same purpose.

I was reading the other day about the attitude control
on Hubble, which works with a combination of momentum wheels and
magnetic torquers for CMG desaturation manoeuvers, since there's a
risk for contaminating the optics with normal thrusters.

It's actually a fairly standard system for operation in LEO. The lack of
fuel is an advantage in several ways.

(By the way, reaction wheels, momentum wheels, and CMGs [control moment
gyros, aka gyrodynes] are three different things, although they're used
for much the same purposes and the terminology is sometimes mixed up.)

Would Jupiter's or Jovian moon's magnetic fields be powerful enough to
enable the use of magnetic torquers?

The fields of the moons are insignificant. Jupiter is another story,
but it depends heavily on how close to Jupiter you are. Might work.

Of course, one would bring along chemical-based thrusters for
emergencies or manoeuvers where CMGs would just not cut it...

You definitely need thrusters for maneuvers; you can't do those with
wheels.

Given that, it's a non-trivial question whether a wheels+torquer system is
worth including. Bear in mind that the operating lifetime of a Jupiter
orbiter is going to be limited by radiation dose, so it may not be worth
the trouble. Even if it would save some mass, it would add complexity.

It would depend somewhat on what the orbiter was doing. Magellan had
reaction wheels because its mission design required it to do a *lot* of
turning back and forth. On the other hand, people who want to do
scientific observations of magnetic fields really don't like things on
the spacecraft that deliberately generate time-varying fields.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |