On Mon, 18 Apr 2005 14:42:20 +0000, Jorge R. Frank wrote:

Craig Fink wrote in
news
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.

That's how it turned out. You know that wasn't the intent.

Like Ray said, the Soviet system didn't work right the first time,
either.

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".

I've read the accident reports. There are indeed a lot of stupid things
the Russians did that set up the Progress-Mir collision that we would
never do, period.

lol, so we'll do something different to cause a collision, or complain a
lot when someone else does it. Having a large spacecraft maneuvering up to
and dock with a huge spacecraft may be glamorous, but it's inherently
risky and dangerous.


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.

Nope. It's just moving the problem. For docking you have a set of
contact condition limits to engage the docking mechanism, and for
tethering you have a set of capture conditions to catch the tether. You
still need translational thrusters and all the rest in order to achieve
those conditions. And as TSS taught us, tethers can be very dynamic,
even when you try your best to keep then taut along the gravity
gradient. The visiting vehicle can wind up hosing prop chasing the
tether. All you really buy with this scheme is reduced RCS plume
impingement on the station, while buying into a whole bunch of other
headaches.

I'll provide more detail.

The early portion of the rendezvous is the same. Approach along the Vbar
with a bunch of loops to get closer. On the last loop, the one that loops
under the Space Station, when the Shuttle approaches the Rbar the Hook on
the end of the line flys down the RBar, maneuvering in the x-y plane and
attaching to the Shuttle. The Shuttle could be in a very loose attitude
hold, or even a short period of free drift so as not to disturb the Hooks
approach and Capture. At the Rbar or close to it, the Hook attachers, then
the Shuttle does a small Translation burn to match the Space Station
velocity, and a small attitude adjustment to align the Shuttle's cg thru
the tether. Stable long term Capture has just been achieved.

Rendezvous and Capture are now complete and everyone can sit back and
monitor the reel in, Arm Capture, and mating. As you pointed out, with the
current docking mechanism the Shuttle may have to apply a little bump to
get the latches to catch. Future docking mechanisms wouldn't require this.

If the Capture is missed, the Shuttle can do a small translational burn to
enter a circular orbit around the Space Station for additional Capture
attempts. The Shuttle essentially in a stable circular orbit around the
Space Station were capture can be attempted every 90 minutes. Or, 45
minutes if they are willing to capture up along the Rbar.

This sounds like a much simpler way to approach and dock many different
spacecraft to the Space Station than putting all the requirements on the
individual and different spacecraft. It has a lot of good attributes.

1) All Captures and Dockings are operationally the same, no matter what
vehicle is coming to visit. The same for the US, Russian, European,
Chinese, Private Enterprise, robotic, or manned. The requirements and all
that they entail for the visiting vehicle are vastly reduced. Training,
Maneuverability, Redundancy, Software, Fuel (actual and reserve)
requirements are all reduced.

2) Safety, no large vehicles freely maneuvering close to each other. Since
it's always best to have the lightest vehicle doing the maneuvering, with
the larger vehicle station keeping or in free drift. The Hook on the
tether does the maneuvering. It's much much lighter and therefore would do
much less damage in a mishap.

3) I'm sure there are some more attributes to be added.

The maneuvering Hook at the end of the tether is a much simpler rendezvous
vehicle (if you could call it that) than any other vehicle built. A
relatively simple concept. Since the Hook is at the end of the tether, the
reel on the Space Station controls the radial (Rbar) direction. Since
tension in the tether is important, the Hook needs to be thrusting all
the time, once it leaves the reel. A very small amount of thrust would be
necessary, and throttling would be nice but not necessary. The tether
cable attaching to a rod the goes right up aft end of the nozzle.
Attaching to the inside of the small cold (or hot?) gas engine that will
be running during the Capture maneuver. This aligns the engine thrust to
the tether cable. X-Y maneuvering is just as simple with the Hook's single
engine design. Since the tether bar is coming out the nozzle of the
engine, simply pitching and yawing the bar would maneuver the Hook in X
and Y. That with the reel Z axis give the Hook 5 axis control with one
tether and one small continuously thrusting engine. The last axis being
the roll axis about the tether. This would probably require some roll
control fins in the nozzle.

So the Hook has 6 axis control with a reel, tether, one engine and roll
control fins. It requires no throttling (but would be nice), just a
continuous small amount of thrust to keep a small amount of tension in the
line. Since, the critical z direction (R-bar) is under positive control of the tether
and reel, much faster closing rates would be acceptable during much of the
cast of the Hook.

As the maneuvering Hook approaches it's coasting target, the reel slows
the closing rate. The Hook's only engine is thrusting towards the target,
but the exhaust is away from the target and towards the Space Station, no
plume impingement problems with the coasting target. Soft capture could be
achieved with a magnet on the Hook and a corresponding magnet on the
target vehicle. The magnets automatically align the Hook to the targets
capture plate and the reel lets out a little more line until they come
into contact. Capture complete.

Other considerations:

A plug on the reel matches the shape of the inside of the nozzle. The reel
could be attached to the fishing pole, in this case the robotic arm. When
the Hook is reeled in, and the reel's plug inserted into the Hook's
nozzle, Arm capture is complete.

Video cameras, range finders, radio control, add a computer with all the
nice bells and whistles. Hook propellant recharging, battery recharging.
Redundancy, a couple of extra fishing rods (Reel, Line and Hook).
Emergency cable cutters. Now it's finally beginning to sound like a
project NASA could love.

I'm sure there's more, but I would think all the positive aspects would
make it worth it when compared to all the current requirements to do a
rendezvous.

I think it will happen at some point, maybe not with this Space Station,
maybe when it becomes a space town, or a space city.

Need some pictures to go with this long winded description?

--
Craig Fink
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