The ISS is equipped with a set attitude control jets and 220-pound flywheels - Control Moment Gyros (CMG) - that rotate 6,600 times per minute. At least two of these are needed to produce the torque that keeps the station holding the proper attitude without wasting the jets propellant. These attitude control mechanisms allow the ISS to maintain a LVLH attitude suitable for docking, undocking and fly-around and also cover the ISS particular attitude envelope that it can fly dictated by thermal and power generation requirements.During docking and undocking the ISS is in FREE mode (sort of an uncontrolled Inertial mode) , thus avoiding any attitude incompatibilities with the Shuttle and allowing for a smooth maneuver. After docking, the ISS + Shuttle assembly attitude is under the Shuttle DAP control.
During the period between docking and undocking, the Shuttle crew adjusts this attitude depending on thermal, power generation and of course EVA human requirements. Some of the concerns during EVA is how radiation, temperatures, light intensity coming from the Sun affect the Astros performing EVA, and Astros Spatial Situation Awareness. The ISS+Shuttle attitude is adjusted so that it offers the best protection and best working conditions to EVAs within the ISS+Shuttle assembly attitude envelope.
The next Service Pack due to be released towards the end of August, will implement a realistic Line Of Sight Indicator (LOS) required for true rendezvous, docking, undocking and fly-around procedures. The way we've been doing rendezvous and docking until now was much easier to understand and perform by novices since the "target", in this case the ISS, was right in front of the flight deck and visible from the CDR and PLT positions. Now the target is visible from the upper-Aft windows and approach and docking are performed from this view alone.
So basically what we're trying to say is that among other things Service Pack 4.00 implements the long-planned and long-requested R-BAR, TORVA, V-BAR and Fly-Around procedures.
These major changes to the rendezvous and docking procedures we've all been used to until now, will have you going back to the NASA SCOM and other Shuttle-ISS and Shuttle-Satellite rendezvous-related material in preparation for the release of Service pack 4.00. As you read the documents you will realize that these procedures are not so hard to understand, but a successful execution requires practice.
Another addition is differential braking after touchdown. This will allow virtual Astros to hold the center line more easily, just as per real procedures.
More announcements will come as we get closer to release date.
A sneak preview (videos etc.) can be seen here, on the unofficial SSM2007 Fans Portal: http://portal.ssm-fans.info/blog/sp4-sneak-preview.
ENJOY!
//The SSM Ground Crew//
