docking

Docking approach procedures (spacecraft)

            Occasionally a space craft will need to hard attach itself to some space structure, such as a space station or another space vehicle, and in these special circumstances special modes of operation have to be in use when docking.

            Engines

            Because of the potentially destructive nature of the gravitational propulsion most ships use, these flight systems must be replaced by ‘docking soft’ systems that do not cause the risk of damaging the other vehicle. Most spacecraft have a number of secondary drives, mostly fusion flame or cold gas, the later is preferred for docking, as there is less danger associated with these drives. A sophisticated and large vessel will begin exchanging to these systems a good deal from the target (unless fuel conservation is critical), and will have already decelerated a great deal. Smaller ships can use their normal propulsion drives until within about 50km of the docking target in which case they will either have to exchange to the docking soft systems, or, if the vehicle has no soft systems, use very mild spatial curvature which will not damage the target.

            After switching from primary drives down to an appropriate docking drive the ship will decelerate to an almost stop as it prepares to finally contact, in all ships the final velocity of the craft is rarely greater than 10cm/s, approaches harder than this probably wont damage the docking devices, but may impart enough energy to greatly destabilize or spin the two vehicles. At very close ranges to docking, using fusion flame is strictly prohibited, since these drives work by reaction, slowing down to dock should not require toasting the target in fusion flame, the cut off for this kind of drive is absolute within 50m (pulses less than 0.5 in 5 seconds) and again at 200m (continuous drive but at flared reaction to minimize heat focus).

            Release after docking is usually assisted by some mechanical thrust from the docking collar which starts to push the craft apart, a ship without cold gas drives will have to drift away until drives it can use can be activated, ships equipped with cold gas drives (nitrogen, helium etc.) can beginning using these drives immediately. As with docking certain drives can not be used in close proximity though when suitably distant from the docking target primary drives can be used again for proper flight.

            Navigation and alignment

            Docking requires some very accurate fixes to ensure that the ship is properly aligned with the docking cradle, in certain ways the alignment has to be better than the ship would normally use when flying interstellar. The space craft will already have a comprehensive suite of sensors which although used for other purposes can be used in accordance with flight software for ‘line-up’. Larger ships may have small specialized docking subsystems which run effectively autonomously of the rest of the flight systems and are invoked only during docking. Usually both vehicles exchange flight information and both flight computers calculate the fine adjustments. Occasionally the docking process is different, such as when manual docking is required (massive computer system failure), or when docking with a derelict or non-active vehicle.

            The two most important processes that need to be considered in a final docking line-up are range-finding and orientation alignment. Range-finding can be measured from laser or radar altimetry devices that most vehicles will possess, other systems can also be used for range finding such as the use of stereographic telescopic measurement (measuring target distance through twin telescopes) or by rapid laser beam splitting range finding (using a laser to ‘read’ the docking surfaces topology). Orientation alignment is achieved by acquiring docking markers, such as navigation light or surface markings, these are called passive alignment features, more sophisticated alignment can be based on laser beam methods, or from measuring plane polarized light emitted from a docking source.

As a general rule it takes longer for a large ship to dock than a smaller one, as with the increase in size a greater problem with alignment and also approach velocity (final approaches are often in mere mm/s, this needs to be small as a large carries much more inertia than say a small shuttle).