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The IDSS ( International Docking System Standard) is a docking standard that every Docking adapter should follow. According to its latest revision:

Vehicles using this interface may include light vehicles in the range of 5-8 tonnes, and medium vehicles in the range of 8-25 tonnes. These vehicles will dock to each other, to large space complexes in the range of 100-375 tonnes, and to large earth departure stages in the range of 33-170 tonnes

Upon reading more into the document I found out that it is mainly the soft capture system (SCS) and hard capture system (HCS) of the adapter that decide these mass ranges. But I'm still a bit confused as to how these systems affect the mass ranges. Also how can we increase these mass ranges?

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  • $\begingroup$ The adaptor would have be respecified, redesigned, and retested to manage the torques of heavier components $\endgroup$ – user20636 Mar 11 '18 at 20:33
  • $\begingroup$ Hi JCRM, well that I'd my doubt. I'm still unclear as to which components if the adapter decide this mass range and hence I am finding it really difficult to redesign the adapter. I would be really thankful if you could help. $\endgroup$ – Sam G. Mar 12 '18 at 1:06
  • $\begingroup$ The latches would need to be stronger, so they didn't bend and the motors driving them would need to be stronger to match the increased work involved in closing. The rings would need to be stronger so they didn't bend under increased load, and the whole structure would need to be stronger. $\endgroup$ – user20636 Mar 12 '18 at 12:31
  • $\begingroup$ On the face of it, you could allow larges mass spacecraft to dock, if they could guarantee lower approach velocities (better alignment, less rotation at the moment of contact, etc.) I guess the IDSS embodies some particular set of design choices. $\endgroup$ – Steve Linton Mar 12 '18 at 13:31
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    $\begingroup$ Sam - that is a whole new question, so should be asked as a separate one, not as a comment! $\endgroup$ – Rory Alsop Mar 12 '18 at 23:29
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Docking starts with two ships moving towards each other. This closure speed has to be reduced to 0 during the docking maneuver, this is done by the Soft Capture System. The larger the docking ship is, the more energy needs to be handled and dissipated by the SCS.

The SCS has a set range of motion. Within that range, the docking object has to come to a standstill. This gives a max. acceleration and a maximum amount of kinetic energy that has to be dissipated, which leads to a mechanical design for the SCS:

  • spring rate if you use springs
  • electrical parameters if you use a linear motor

By setting a mass limit and closing speed limit, they've set an upper bound to this energy. A 1000-ton docking system has to be much larger than a 5-ton docking system, and it makes no sense to specify a 1000-ton docking system when all the modules you need to dock are in the 5 ton range.

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  • $\begingroup$ If a space ship in the 5 ton range docks to a system designed for 1000 ton ships, it might be damaged by the decleration over a too short distance. The soft capture system should adapt to the mass docking. Very soft for 5 ton and very stiff for 1000 ton. $\endgroup$ – Uwe Mar 12 '18 at 11:42
  • $\begingroup$ if you could just tell me which of the following components of SCS and HCS would have to be changed and how in order to allow larger ships to dock to it. SCS: Guide Petal System,Soft Capture ring,Mechanical Capture Latch System,Soft capture sensor Actuation and Soft Capture Sensor Striker. $\endgroup$ – Sam G. Mar 12 '18 at 15:42
  • $\begingroup$ There should be a mechanism that allows the Soft Capture Ring to move up and down. That mechanism needs to be changed. Other components depend on their exact mechanical design (material strength, etc.) - that's not specified in the reference documents. $\endgroup$ – Hobbes Mar 12 '18 at 15:48
  • $\begingroup$ here is the latest revision of the IDSS document: internationaldockingstandard.com/download/… could you specify further why the mechanism that allows the SCS ring to move and down should be added. and what other changes can we do to make it handle heavier shuttles. $\endgroup$ – Sam G. Mar 12 '18 at 15:56
  • $\begingroup$ The Interface Definition Document only specifies what the interface looks like, not how the docking mechanism is designed. It says nothing about the mechanism for the SCS ring. $\endgroup$ – Hobbes Mar 12 '18 at 16:08
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A docking ports job is fundamentally to perform three tasks.

  1. Reduce the relative speed of the craft to zero.
  2. Bring the craft into alignment.
  3. Clamp the two craft together.

The forces involved in this process will depend on the mass of the two craft. In particular with two craft of unequal size it will depend mostly on the mass of the lighter craft.

Too much force is likely to break things. Too little force is likely to result in a failure to properly engage. Restricting the range of masses for the smaller craft restricts the range of forces that the docking mechanism will have to handle.

I'm not sure why they restricted the mass of the larger craft though.

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