(Thoughts after internet searches.) Androgynous docking ports (IDSS) - oops, since shuttles have gone and Canadarm & CBM have come.

[kerbiloid]

Boeing  may not hurry too much with CST-100 launch, as I took an archaeological attempt on NASA forums and found out that the drama with the docking adaptor replacement from iLIDS to the mystic Boeing SIMAC has been finished between IDSS Rev B and C, so the current Rev E is already SIMACed.  All changes have been brought in and implemented in the installed IDA adaptors.

Just wiki and other sources describe this as a then-future event in ~2012 and don't mention it later. and it's described at once as a return to APAS-95 compatible standard and as attempt to stop making the adaptor excessively wide by decreasing the inter-petal clearance from 800 mm down to 684 mm to adapt the adaptor to the capsules (rather than to huge spaceplanes).

So, Crew Dragon, CST-100, Orion, etc are going to use the current adaptors (current IDA-compatible), they won't replace them and carry some another docking adaptor.

So, Being may feel free with their launch schedule, as it's now clear with their planned docking adaptor and (absence of) toilet cabin.

Also the SIMAC does not mean a proprietary hardware to be being bought from Boeing by others. The standard stays free, SIMAC was a payed research, not an actual commercial product.

***

At once I realised a really terrific picture of the androgynous docking adaptor actual androgyny...
My world will never be the same...

***

As I could understand the picture in whole (from that forum and other sources).

To pass through the airlock tunnel a thin military man in a rescue suit needs actually 550 mm of clearance (TKS emergency holes between VA and FGB and on top of VA, under chutes), but fat civil crews prefer 684 mm to be guaranteed. 
So, 684 mm of permanently existing tunnel clearance is enough to pass from vessel to vessel and bring bags.

But as both Apollo and Soyuz were originally lunar ships and supply ships for orbital stations, without any wider cargo ships, they required a tunnel allowing to pass through in a heavy lunar/EVA suits.
So, both Apollo and Soyuz used same 800 mm as a docking tunnel  clearance standard. 
(Maybe, the Soyuz's 800 mm was copypasted from early Apollo one, so they are equal, but who knows, and who cares. They are both 800).

Almaz's docking port clearance was a bit wider, to let the 850 mm film capsule pass through, but it has absolutely different design, not in use now.

When Apollo-Soyuz required a universal docking adaptor, they created APAS-75 as a Soyuz 800 m wide docking port with external ring with petals tilted outwards.

***

When Buran required a docking port to dock with Mir and to receive a rescue ship Zarya, they took the APAS-75 and inverted it, swapping the HCS (hard-capture sys.) and SCS (soft-capture sys.) rings.
So, no the ring with petal appeared to be inside the pressurized volume, and the petals were tilted inwards, and called it APAS-89.

This would make the passage narrower, but they made the hull and the HCS ring wider, so the clearance between the petals stayed same 800 mm.
The soft-capture ring with petals was thin (like in IDSS Rev.E), the petals were fixed.
Hypothetically they presumed future ability to totally unmount the soft-capture system from inside the docking tunnel, getting a fully clean tunnel 1250 mm wide.  But not to be then-currently implemented.

When Shuttle began wanting to fly to Mir, they updated APAS-89 to APAS-95, bringing minor changes.

When Shuttle began wanting to build ISS and fly to it, they took APAS-95 as a de-facto standard for both.
I.e. pressurized 1250 mm wide tunnel, with an extendable soft-capture mechanism inside, with thin wide ring with three fixed petals with 800  mm clearance between the petal tips, with mechanical contact strikers (thin pins on this thin ring).

***

APAS-95 became a de-facto standard prototype of what de-jure IDSS standard is.

***

Then they began wanting to implement a standard wide berthing port to perth station modules and trusses, and wide-mouthed cargo ships.

So, they made CBM (Common Berthing Mechanism) which holds together the ISS non-Russian modules 
and receives the non-Russian cargo ships (except ATV), such as. HTV, Cygnus, Cargo Dragon. 
(ATV were equipped with Russian docking adaptors and not refer to IDSS or CBM).

The CBM consists of two parts: active and passive.
All mechanics is contained in the active part which is installed on the station side (like in Gemini project, unlike in Soyuz/Apollo ones).
The passive part of CBM is a simple cheap ring with latch holes to receive the latch from the active parts. (Like in Gemini, partially like in Almaz).
So when a an expendable cargo burns, just an aluminium rings burns. When (Cargo Dragon lands, they can use it as hoola-hoop).

As originally they planned to move cargo boxes through both IDSS and CBM, the CBM presumes a squared opening of 1270 mm wide behind.
This matches the IDSS theoretical possibility to unmount the soft-capture and turn IDSS in an empty wide tunnel. Then any cargo of 1250 mm can pass through coaxial IDSS and CBM ports.

***

Since Shuttles the ISS was equipped with Canadarm with zero-impact end effector which strangles a grappling fixture stick under chin with three wires, so the mechanical contact doesn't push a drifting object away.
This allows to effectively catch a cargo ship drifting around with manipulator and not equip it with a real docking node like IDSS, just with this passive CBM ring.

Theoretically, CBM cargo ships should have 1250 mm wide square doors. Actually, the square is much smaller, closer to the old 800 mm.

Also, Shuttle made things worse.
While Buran was equipped with extendable docking port, Shuttle had it fixed, deep inside the cargo bay (because no other Shuttle would dock to Shuttle).
Also the cargo bay should stay clear from anything above.
So they made those ugly curved PMA adaptors, to let the Shuttle dock to ISS.

The ugky PMA adaptor of course doesn't have a straight passage from IDSS to CBM on its ends, and even a 800 mm wide passage is a problem.

This made the implemented CBM/IDSS passage compatibility almost fictious. 
No cargo ships are/will-ever-be equipped with IDSS.
No available IDSS & CBM couples have a straight passage way.
No need in large cargo delivery with IDSS-compatible passenger ships since all large cargos are berthed to the CBM.

***

On the other hand, all coming spaceships are capsules with docking adaptors, typically IDSS-compatible.
The IDSS adaptor is more expensive, so it should be reusable. So, it should be placed on top of the capsule.

But the top of the capsule is narrow, twice as narrow as the bottom.

And it should also contain chutes, antennas, sensors, etc. The room is expensive.

So, they wanted a narrower docking adaptor to save as much room on top as possible.
This could be achieved by decreasing the permanent docking passage way clearance from 800 mm to 684 mm. Enough for rescue suits and postal bags. 
This became the early IDSS standard.

At the same time they wanted to keep the existing 800 mm clearance as an option.
So, the early IDSS standard made the petals removabl/foldable to quickly make the passage 800 mm wide if needed.

Also they wanted a softer dokcing contact system.
APAS-95 uses mechanical strikers. They wanted magnetic strikers to get in sensoric contact before the mechanical one, to adjust the petal rings before actual touching, and make the contact softer.
The magnetic strikers appeared to be rather big. (Twice three circles every 60° on the scheme).

The removable/foldable petals and the big magnetic strikers made to make the soft capture ring twice thicker.
So, the external, hard-capture ring stayed same, with 1250 full clearance.

Also they probably wanted to keep it compatible with APAS-95, and were pleasantly forced by the thicker inner ring to do this.

Finally they called this iLIDS  (low-impact docking system, because the magnets do not impact, and the rings would match each other's position before contact).

As a result, the total width of the docking adaptor stayed same, nothing was gained for the capsule top free room.

As a result, the whole iLIDS / IDSS Rev B efforts resulted into a docking adaptor standard incompatible with previous APAS-95, but same wide, with more complicated petals system, just with magnetic strikers never really tested, unlike more archaic but tested many times by Shuttles mechanical strikers of APAS-95.

***

This looked strange, "they" became unhappy with that and ordered Boeing a research on "how should we optimise the IDSS thing?"

The reseatch was made under conditions of "no proprietary things or processes should be used, everything should be available for everybody being downloaded from internet known"

Boeing named this study SIMAC. 
SIMAC is not an actual hardware for sale, it's a study resulted into a hardware prototype.

***

As a result, they stated that:
Magnetic strikers are big and not tested, (un)screw them.
Mechanized petals are complicated and not tested, (un)screw them.
The hull width stayed same, so no need to decrease the passage way, ... guess what?... yes, screw this, too.

So, almost all significant improvements have been (un)screwed, the final SIMAC thing was smiling and mocking "I'm APAS-95! I'm APAS-95!"
So, the summary of the SIMAC was "What the hexx all those improvements did here at all? Let's just use upgraded APAS-95."

As a result, the IDSS Rev C was adopted, with current thin ring, fixed petals, permanent 800 mm wide passage way, and hypothetically presumed but who-needs-it-now future possibility of making it 1250 wide by unmounting the soft-capture. All major improvements have been hearthlessly flushed.
Currently it evolved into IDSS Rev E without significant changes.

As unlikely anybody will ever care of this unmountable soft-capture mechanism, the current ring is solid, with fixed petals, thin pins of mechanical strikers, and the 1250 mm compatibility with CBM means actually nothing anymore.

On the other hand, this makes the CBM dimensions being caused by obsolete reasons, but who cares.

***

So, the modified APAS-95 was put on the IDA adaptors which are installed used  on both ugly curved PMA of ISS.

***

***

Another funny/sad thing is about the androgyny of these androgynous docking ports.

***

According to the IDSS standard, an IDSS port should have a centerline docking target inside the docking tunnel for visual and finally automatic docking, and four reflective targets around the hull.

Actually, there's no room for the latter on the capsule top, so at least Crew Dragon, but probably CST-100 and Orion as well, do not have these targets.
So, first of all this makes unlikely possible an automatic ship-to-ship approaching and docking. Only manual and mostly visual.
But no problem, real spacemen like the manual dockings.

***

Theoretically, the IDSS adaptors are androgynous, and any adaptor should dock to another adaptor,

Practically, there is no need to have an extendable soft-capture ring on the station side, as the station never docks to something.

So, the originally active-passive androgynous adaptor receives a purely passive version without telescopiv trusses and extendable ring. The petal ring becomes fixed.
All ISS docking "androgynous" adaptors are such. 

***

But do the vessels dock anything but station?
Theoretically they can dock each other, and that was the main sense of the androgynous adaptors.

But nowadays the flights are rare, and there are no rescue ships ready-to-launch.

So, this means that a ship can dock only a station, but not another ship.
It will not dock an active-passive adaptor, only a passive one.

***

As the androgynous hard-capture system (as well as the Soyuz one, which is its prototype in APAS-75/89 times)  uses "hooks-hooks" system.
(Unlike "pins-holes" of Almaz, Canadarm, and afaik CBM, or "hooks-holes" of Gemini and Apollo).

On the hard-capture it has 12 active hooks and 12 passive hooks to be hooked by the active hooks.

Normally, when two full-featured IDSS adaptors connect, there are 2x12 = 24 passive hooks grabbed by 24 active hooks., which hold the whole thing together.

But the IDSS standard declares that just 12 hooked hooks are required to consider the attachment successful.

So, if have a close look at the docking ships with IDSS adaptors, there is only one hook instead of two inside every of 12 slots.
Because on the ships they don't install passive hooks. 

So, any IDSS spaceship can dock to the station, but not to another ship (there is no counterpart passive hooks on another other ship, as well).

As anyway nothing can't dock to a ship, the ships again doesn't need the targets, which it anyway has no room for.

***

But maybe the ships will be able to dock later?

But the only purpose of the capsule ship is to deliver the crew to the station.
So, its autonomous flight is normally limited with 2-3 days, and they anyway have no purpose except rescue, to dock each other.

But can a capsule ship be rescued?
If the rocket failed before it has gotten to orbit, it will just reenter.
If the rocket failed but put it in orbit, the orbit will be randomly elliptic and probably low (as the rocket fuel amount is calculated to be as low as possible).
So, in these cases no rescure ship can arrive in time or even dock. The ship should deorbit itself.

If the ship approached the station but can't brake, even the docked ship can't intercept it due to limited fuel and velocity difference.
The same if the ship has started the deorbit burn.

So, the only case when a capsule ship can dock another capsule ship is when both were docked, then one of the, after the proper system check, undocked and wants to deorbit but can't move.
But in this case the docking is not necessary, too, as they can get the crew to the rescue ship through the door, by drifting aside.

It was asctual for large and vulnerable spaceplanes, to let a rescue capsule ship to dock.
But currently there are no spaceplanes, and even if they were, they anyway need some rescue pod.

***

But IDSS has another advantage above the probe-drogue ports.
It is larger, so stronger. It can hold up to 350 tonnes.

But actually large things are attached by CBM ports, and the IDSS payloads are not greater than 30 tonnes.
So, old adaptors of Mir or Almaz or Apollo look still enough strong for this purpose. Nobody actually docks now even a 100-t Shuttle with it.

***

So, the real IDSS-based "androgynous" docking ports now and in future are not more androgynous than the old probe-drogue adaptors of Soyuz and Apollo.

All of them are "active" and "passive", no bimodal really are or probably ever be, until tens km/s delta-V propulsion systems making free orbital maneuvers possible.

***

So, now it looks like the IDSS and the androgynous ports became useless once the Shuttles disappeared.

Actually, the 1970s-1980s adaptors in addition to CBM should be used instead, to receive all kinds of spaceships except the CBM-berthable ones.

Also they are narrower, and that was required by the capsule developers.

***

So , the only reason why they keep using IDSS pseudo-androgynous adaptors is "it's already in production and it's hard to change."

***

Upd.
That "IBDM" design (International Berthing-Docking Mechanism), which can be found in several places of the internet, is an old an totally obsolete European study of pre-IDSS Rev.C epoch, that's why it uses a thick soft-capture ring wth 684 mm clearance.
It has been totally replaced with IDA, it was an early study of IDA.

 

Edited February 18, 2021 by kerbiloid