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Docking standard for the International Space Station (internationaldockingstandard.com)
75 points by forza on May 25, 2012 | hide | past | web | favorite | 28 comments

This is not actually the interface that is used on the Dragon. This is a docking mechanism, which allows autonomous connections between two spacecraft. The Dragon uses the Common Berthing Mechanism (CBM) which requires assistance from a manipulator arm to mate the two spacecraft.

One of the reasons the CBM was chosen for the resupply ships (including Dragon) is that the CBM has a larger inside diameter and so larger pieces of equipment (such as standard equipment racks) can be delivered through it, while the docking connectors cannot pass them.

Correct. But the manned Dragon spacecraft, cleverly named DragonRider, will use this interface.

Clearly, section 3.2.1 is the most important part:

All dimensions are in millimeters. All angular dimensions are in degrees

Implementors in non-metric countries, take note. Nothing is more annoying than arriving at a place where you might want to use your docking interface, only to find out it doesn't work. :)

>Nothing is more annoying than arriving at a place (...) to find out it doesn't work. :)

Which has sort-of happened already in history of space exploration, when Mars Climate Orbiter mis-handled Mars orbitial insertion and was lost. The problem was traced down to mis-match of measurement units used in software (Newtons vs. Pound-force).


Yep happened once in 50 years of space travel.

Ugh, we get it, everyone thinks they're being cute and its a way to mock US units. Yes, yes, this isn't getting incredibly tiresome.

The most famous smallest but most significant human error was Mariner I:

"The error had occurred when a symbol was being transcribed by hand in the specification for the guidance program. The writer missed the superscript bar (or overline) in [the formula] by which was meant "the nth smoothed value of the time derivative of a radius R". Without the smoothing function indicated by the bar, the program treated normal minor variations of velocity as if they were serious, causing spurious corrections that sent the rocket off course. It was then destroyed by the Range Safety Officer."


But you have to admit the "guy insists on doing things unnecessarily complicated and falls on his face" trope has a strangely entertaining aspect to it :)

As an American, I would prefer that the stubbornness continue to be mocked in a hope that someday we might get rid of this silly system.

IIRC, it was an accumulation of errors when converting between units, not a unit mis-match.

Not exactly, ultimately it was just that. The problem was that there was a spec for a file format for data from the spacecraft that was supposed to be in metric units, but some of the data was in imperial units, against the spec. This resulted in a course deviation which ultimately doomed the spacecraft. That said, there were warning signs that something was wrong which the ops team didn't pick up on but should have.

Root cause is still a unit mis-match though.

Implementers in metric countries need to take note as well, since the degree is not the SI unit for angular dimension.

I think you would notice if you built a dock confusing inches for millimeters.

Is it just me or does this docking design look overly complex and easily damaged by a docking miss? I am partial to designs with as few moving parts as possible. Maybe I am nieve to the needs of space docking too.

Actually, this is intentionally a low impact design, to avoid the following problems with the old "ram into each other" method:

* require substantial force and velocity for docking alignment and capture

* create critical operations

* affect structure fatigue life

* disturb zero-g environments

* difficult for small mass vehicles to use

* support a limited range of vehicle performance capabilities

So the miss you're thinking of isn't really where this is used. Also, this is an androgynous design (it can dock to itself) which makes things more complex.

(From this PDF: http://dockingstandard.nasa.gov/Meetings/TIM_%28Nov-17-2010%... )

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

I've never seen "K kg" stand for metric tonnes before. Two Ks of different cases both standing for "kilo-" therefore meaning "kilokilogram"? At first I thought "five kilograms is quite small for a spaceship" and then I had to re-read it. To say nothing of the fact that "K" means Kelvin, not kilo-.

On the other hand, what are the other options? "Mg" for "megagram"?

There are (several) guidelines that abolish certain abbreviations because misinterpretation can kill, for example http://www.ismp.org/Tools/errorproneabbreviations.pdf

I cannot find a guideline w.r.t. Mg, but it can easily be confused with milligram and microgram. That may be the reason to use this weird construct ('ton' seems a better term, but it is ambiguous, too)

"Ton" would be ambiguous, but "tonne" is unambiguously the metric tonne (1000 kilograms).

Good spotting, I assumed a typo and skimmed on. I was imagining some kind of unknown-to-me efficiency in super light space ships..

Mating Interface Definition.. Androgynous Docking System.. Soft Capturing..Hard Capturing.. Someone in NASA was watching porn when they wrote this.

Definitely: Ā»The IDSS docking interface is fully androgynous about one axis, meaning the interface configuration is capable of mating to an identical configurationĀ«

Is that an official Nasa chat-up line ?

That's refreshing -- we're sending things into space from a readable 31-page spec (with pictures!). RFC authors, take note.

31-page spec for the door;)

It'll be fun watching the Dragon docking live while following along with this document.

I think this is the protocol that Dragon is following on the current mission:


We're living the future, guys.

I don't want to be the immature one here -- but I can't help but feel there is a "yo momma" joke in there somewhere

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