Alain Fournier wrote:

wrote:

On Apr 9, 12:21 am, Craig Fink wrote:

A planetary probe should be able to fly in and sample Saturn's ring
material and analysis it and/or return a sample to Earth. The relative
velocity between the probe and ring material would be almost zero if the
probe is in-plane near circular.


Yes -- it's getting the probe into this circular orbit which is the
problem. Escape velocity at the outer edge of the rings (120,000 km
above Saturn's equator) is about 20 km/sec. Circular orbital velocity
is 14 km/sec. So a return probe needs 6 km/sec delta-v to escape
Saturn, at least the same amount to transfer from an interplanetary
trajectory to circular Saturn orbit in the first place (repeated Titan
flybys could help a little with either), then AT LEAST 5.5 km/sec
delta-v to get back to Earth. Flat-out impossible with chemical
rockets, and doable but very expensive with nuclear-powered ion
propulsion.

I wouldn't go so far as saying flat-out impossible with chemical
rockets. You can use multiple gravity assists with multiple
Saturnian moons. But it would be a very complex mission.

With chemical rockets and without using gravity assists it would be
a ridiculously monstrous mission. But with careful planing you can
lose and gain almost all the delta v needed to get in and out of
the Saturnian system with multiple gravity assists. Still it would
be a very complex mission. Using, Venus, Earth, Mars, Jupiter,
Titan, Rhea, Tethys and Mimas, you can do the trip with little
more energy than that needed to reach Venus. Unless you are lucky
with the planetary alignment it would take a lot of time, but it
could be done.

A chemical rocket would complement a nuclear/ion engine quite well for a
Saturn Ring Sample return. If the third stage of the Earth ascent vehicle
is the Aerobrake Sample Return vehicle, a much larger sample of Ring
material could be returned to Earth. Probable larger than the payload taken
to Saturn.

After orbit insertion at Earth, the Return vehicle would have almost empty
tanks. The Nuclear/Ion Engine could be used to climb out of the Earth's
Gravity well. At the edge of the Earth's gravity well, very little delta-V
is required to lower perigee back down to within several hundred km above
the Earth. The rest of the fuel in the Third Stage/Return Vehicle/Aerobrake
vehicle could be used to send it on it's way to Saturn, or the gravity
assists.

Chemical rocket burns deep in gravity well are very efficient and a much
larger delta-V above Earth escape can be attained by a small burn deep in
the gravity well. Essentially, this is why the injection burns at Saturn or
Jupiter are reasonable.

Another way to think of it, is the that the Ion engine vastly increases the
potential energy of the Chemical propellant. This potential energy isn't
lost, but is converted to kinetic energy by the burn deep in the gravity
well. Gravity is a time function, the time to fall from apogee to perigee
is much longer than the time to escape Saturn's (or Earth's) influence
after the perigee burn.

In-situ propellent for both the Nuclear/Ion Engine and the Return Vehicle
can be used at Saturn. Aerobraking to the inner ring of Saturn eliminates
the need for propellant at Saturn for the injection burn and the huge burn
to get down to the bottom of Saturns gravity well.

Using the nuclear/Ion Engine to maneuver around, collect samples, refill the
Return Vehicle tanks, refill the Ion Engine's tanks, and slowly climb out
of Saturns gravity well. Even visit and orbit each Saturnian Moon. Once at
the upper edge of Saturns gravity well, very little delta-V is required to
lower perigee back down to Saturn.

During the plunge back down Saturn's gravity well, the large Sample return
vehicle would be separated from the nuclear/ion powered Orbiter vehicle
that contains all the science packages. Both going there separate ways. The
Orbital probe could spend decades visiting interesting things, until the
nuclear power plant is depleted.

A Chemical rocket burn from this highly eccentric orbit at perigee is
reasonably small to send the Return Vehicle on it's way directly back to
Earth, or use some gravity assists (more payload).

I would think only one launch would be required for the mission, and the
Returned Sample would probably be larger than the Saturn Orbital Probe, as
the Return vehicle would have tanks that were sized for the third stage of
Earth ascent. Several thousand kilograms of Saturn's rings, maybe even some
small asteroids if some can be found in the rings.

--
Craig Fink
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