On Mon, 18 Apr 2005 06:37:15 +0000, Jorge R. Frank wrote:

"Ray S" wrote in
m:

Glad to hear that NASA's DART mission was a near 100% success. Now on
to a full blown autonomous rendezvous and docking demo of two unmanned
NASA spacecraft. Hope NASA can pull this off soon.

The Soviets, of course, pioneered autonomous rendezvous (and docking)
procedures and have used them to support their Salyut and Mir space
stations.

Not quite. The Soviets/Russians use *automated* rendezvous and
autonomous prox ops, while DART is fully autonomous in both rendezvous
and prox ops.

The distinction? With the Russian system, prior to Kurs acquisition the
maneuver plan is generated in mission control, and the Russian
equivalent of the FDO uplinks burn solutions to the spacecraft, which
dutifully executes them. The spacecraft has no "big picture" of the
overall plan. Once given a target vector, DART internally generates and
executes its own maneuver plan. The Soviets/Russians have never done
anything like that.

Ahhh, so the Russians use "automated" rendezvous and "autonomous" prox
ops, while the DART uses "autonomous" rendezvous and "no" prox ops. Maybe
the next one will have prox ops.

Rendezvous: During rendezvous translational thruster firings are much more
important that rotational thruster firings and are orders of magnitude
larger. Any translation caused by a rotational burn to maintain attitude
is so small compared to the much larger translational burns that are
required, the cross coupling can essentially be ignored and it's effects
treated like other perturbations and taken out during the next
translational burn.

Proximity Operations: Both rotational thruster firings and translation
thruster firings are equally important, and of the same order of
magnitude. Any cross coupling can quickly lead to excessive thruster
firing and fuel usage. Need to translate, oops, now I'm rotating, oops,
now I'm translating, oops, ...

Prox Ops and Docking is one area were both the Russians and United States,
and soon the Europeans won't have learned the lessons of the past. The
first hand experience of the Russian, when the Progress slammed into Mir
has essentially been ignored by the Russian. The US being a bit arrogant,
thinking, "we would never do that". And, the Europeans, "well that's how
it's always been done by everybody else, it must be right."

At some point in the future it's going to take another accident for people
to realize there are better ways to dock with large space structures like
the Space Station. Tethered Capture seem to me to make more sense, and
would vastly reduce the maneuvering capability/requirements of a delivery
vehicle to the Space Station. The delivery vehicle could have fewer
thrusters, use less fuel, and improve overall safety. Tethered Capture,
then reel the delivery vehicle into a robotic arm capture, which moves it
to the proper docking port.

With such a system, the Shuttle could be captured then dock with the Space
Station using only the vernier jets for attitude control. RCS jets would
still be needed for translation, but only to get the Shuttle within a
couple of hundred feet (or is that yards). No RCS jet firings would be
required close to the Space Station. Do it along the Z-axis were orbital
mechanics will keep the tether taught and the vehicles separated.
Essentially, all proximity operations would be handled by the Space
Station, not by every other vehicle that wants to visit. Simplifying the
design and certification of those vehicles.

--
Craig Fink
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