On Sun, 17 Apr 2005 00:32:13 +0000, Ray S wrote:

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. And Russian equipment (the Kurs system) was used to autonomously
dock the ISS Service Module to FGB/Unity assembly in July 2000.

The Russkies had to try several times before they worked out the bugs and
performed the world's first automated rendezvous and docking of two unmanned
spacecraft (Cosmos 186 and 188) in October 1967. Gee, that was nearly 40
years ago.


http://www.msnbc.msn.com/id/7516571/

"When we started doing precise maneuvers, we started seeing excessive
propellant consumption. ... It went south pretty quickly," Snoddy said.
"The mission as designed, when it runs out of gas, completes itself."

lol, surely, the the DART community understood that they needed to
calibrate their flight control system?

Sounds like the DART had problems with their attitude control system. Dead
bands too tight and/or thruster to moment coefficients wrong. Sounds like
it hosed out it's remaining fuel when it got up close and tried to do
precision maneuvers.

To me it sounds like they were only a 50% success, and they missed most of
if not all the up close and personal rendezvous testing. The stuff that
comes just prior to docking. And if you judge success by the ability to
"actually" dock, it was a complete failure. Running out of fuel just prior
to docking is not a good thing.

This is not uncommon for a new vehicle. The simulations don't match
reality when the vehicle actually flies. The thrusters give too much or
too little moment, cross-coupling is different. People are overly
optimistic about how precise the vehicle can maneuver, dead band are too
tight. It doesn't take long for a vehicle that is bouncing back and forth
off both sides of a dead band band to hose out a lot of fuel.

Similar thing happened the first few times the Shuttle flew too. Too much
gimbaling of the OMS engines, RCS jet probably banging around in the dead
band. It takes a flight or two to get the control system calibrated.

It's easy to spot a control system that's working correctly or not. The
vehicle should always be bouncing off of just one side of it's dead bands,
with aerodynamic, gravity gradient, or whatever causes the moment slowly
bringing the vehicle back to the same side of the dead band every time.
Cross coupling being a little more difficult to sort out, but when it's
properly tuned, it should be bouncing off only one side of it's dead band,
coming close but never hitting the other side. Hit the other side of the
dead band just one time and now your just hosing out fuel bouncing back
and forth.

If DART had an auto calibration attitude control system onboard, it didn't
work.

--
Craig Fink
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