QUESTION:
I am currently researching a science fiction story and would like to know how the docking procedure between the shuttle and space station is handled - how do the two craft dock?

ANSWER from Jerry Jason on April 15, 1997:
This is not a short answer. I will try to be as detailed as possible without making this a book in itself.

The Orbiter Docking System (ODS) is divided into three sections. The ODS consists of the external airlock, truss assembly, and the androgynous peripheral docking system (APDS).

The external airlock provides an airtight, internal tunnel between the two spacecraft after docking. The external airlock is located inside the payload bay aft of the crew module.

The truss assembly provides a sound structural base within which the components of the docking system are housed. The truss assembly is physically attached to the payload bay and houses rendezvous and docking aids, such as camera/light assemblies and trajectory control systems.

The androgynous peripheral docking system (APDS), manufactured by RSC-Energia in Kaliningrad, Russia, is designed to achieve docking of spacecraft through the use of essentially identical docking mechanisms attached to each vehicle.

The primary components of each docking mechanism are a structural base ring housing 12 pairs of hooks (1 active hook, 1 passive hook per pair), and extendable guide ring with 3 petals, a motor-driven capture latch within each guide petal, 3 ball screw / nut mechanism pairs connected via a common linkage, 3 electromagnetic brakes (dampers), and 5 fixer mechanisms. Two control panels in the aft flight deck and nine avionics boxes in the subfloor of the external airlock provide power and logic control of the mechanical components.

The docking mechanism on the rendezvousing spacecraft (Space Shuttle) is intended to be active, while the corresponding mechanism on the target spacecraft (Space Station) is typically passive.

Operational Sequences

The docking system is launched with the active docking ring fully retracted and aligned in its final position, the structural hooks open, and the capture latches closed.

In preparation for docking, the ring must be extended to its ready-to-dock or ring initial position. This is done in a manual operation. A command activates two dc motors to drive the ring out to its initial position (13 inches from final position) at a rate of approximately 4.3 inches/minute. As the ring is driving, the fixers are also activated to keep the ring in alignment with the base while it drives. When the ring reaches its initial position, the ring motors and fixers are commanded off. The system is than powered down.

The system is powered down when not is use. The electronics can only be on for 2 hours. At that time the unit must be powered off for a 30 minutes, before reactivation.

The ring is usually moved into the initial position two or three days before the actual docking.

During the rendezvous maneuvers the top of the orbiter is pointed at the other spacecraft. This allows the orbiter to use radar or laser to determine the distance to the other spacecraft.

Once the orbiter is on final approach, the docking system is powered back on. In this time frame, it is also necessary to close the inner airlock hatch, check the function of the airlock fan, and turn on the docking lights and cameras.

There are a couple of stations keeping positions before the actual docking. The last position is only 30 feet away. The final rate of closure rate is about 0.1 foot/second.

At initial contact the crew activates the preset postcontact thrusting (PCT) sequence. The PCT is designed to provide the force required to attain capture with the APDS while not exceeding dynamic loads.

Once capture is achieved, the automatic docking sequence is initiated. After a 5 second delay, three electromagnetic brakes (high-energy dampers) are energized for 30 seconds to damp relative motion. Sixty seconds after capture, the ring will start to drive out. The crew will then stop the ring drive. This stops the ring and turns off the fixers, but does not deactivate the auto sequence. The crew will wait up to 8 minutes to allow relative motion to damp. If the ring aligns within that time, the automatic sequence can be picked up again.

The capture latches are only used to bring the vehicles together.

After approximately 3 minutes of ring retraction, the ring will activate the ready-to-hook sensors. The ready signal activates close commands and the hooks begin driving closed. As the hooks drive closed (approximately 3 1/2 minutes) the mating surfaces will compress the pressure seals. Once either set of hooks is closed, the ring is extended slightly to relieve loads on the capture latches. The capture latches are opened and the ring is retracted to its final position. This completes the docking.

The vestibule, the passage between the orbiter and Space Station, is pressurized and leak checked before the hatches are opened to allow crew and payload transfer.

Undocking

The vestibule is depressed. The hooks are commanded open. As they release, four spring plungers compressed between the mating surfaces, with a combined spring force of approximately 700 lbs, impart a small separating velocity on the two vehicles. The separation burns are then accomplished, and the docking system is powered off.