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China successfully launches its first robot space truck

Anonymous Coward

"fast-dock tech that will reduce coupling time to just six hours."

Paging any resident Register space nerds: What on Earth* takes so long that 6 hours is the fast version of docking? I would imagine the process of maneuvering the supply craft to the station is slow and careful due to the risks involved (no space body shops to buff out any dings from a fender bender). The article kind of implies it's more to do with the actual coupling, not the flying part.

I'm clueless on the process, but I'm guessing someone can fill us in.

*or generally within its gravity well

Silver badge

Re: "fast-dock tech that will reduce coupling time to just six hours."

1) A big issue is matching vectors exactly.

2) Exact orientation

3) Then with no friction, actually joining vessel to the small station without changing the orbit is a difficult operation.

It's not like DS9 or Babylon 5.


Re: "fast-dock tech that will reduce coupling time to just six hours."

Based on my experience*, it works as follows:

Launch spacecraft, try to launch directly into an intercepting orbit.

After several minutes realise that this is most certainly not an intercepting orbit, even if I leave it for several months.


Swear profusely after looking up orbital mechanics and discovering inclination changes should be performed before circularisation.

Match orbital inclination, wasting lots of fuel.

Check target location and relative orbital average speeds.

If ahead of target and travelling faster, or behind target and travelling slower, wait.

Otherwise, burn prograde to increase orbital speed to let target catch up, or burn retrograde to reduce orbital speed to catch up to target (yes, really), wait.

Grossly overshoot/undershoot, swear profusely, repeat above steps but with less difference in spacecraft/target speeds.

After many orbits and adjustment burns, approach target while travelling at approximately the same orbital velocity (i.e. pretty much same direction/speed).

Start moving slowly towards target, preferably using only RCS (if this was even remembered and set up correctly during spacecraft design/build).

Get impatient, increase speed.

Panic when realising closing speed is too high.

If lucky, overshoot, repeat last three steps. If unlucky, decide to use main engines to slow down, grossly mismatch orbital velocities, repeat last ten steps. If unluckier, expose target to close range 100% power rocket exhaust while trying to slow down, destroy/damage target, mission failed. Or, simply, collide with target, spacecraft/target damaged/destroyed, mission failed.

If exceptionally lucky, achieve holding position within tens of metres of target, negligible relative speed.

Swear profusely when realising target's docking port is on the other side.

Manoeuvre accordingly.

Set up spacecraft controls relative to docking port, target other docking port.

Try to remember all RCS translation/rotation controls, hope spacecraft CofG is where it should be.

Move slowly toward target.

Wait for ports to engage, if they're too far away, or at an angle, back off and try again.

* disclaimer - this is entirely from Kerbal Space Program.


Re: "fast-dock tech that will reduce coupling time to just six hours."

Alternatively, request docking permission and match rotation to the station entrance or switch on the Auto-docking and listen to the 'Blue Danube'......'s the flying suit with the name tag ' Jameson'...

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Re: "fast-dock tech that will reduce coupling time to just six hours."

Don't forget to accelerate time, or you'll never actually get there. ;)

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Re: "fast-dock tech that will reduce coupling time to just six hours."

Overview with .gifs and videos. Wow, Business Insider was useful for once.

The post that explains the Kerbal process isn't far off the mark, NASA just puts some more time and effort into pre-planning the process so there's fewer surprises and swearing. Even then, there's a lot of corrections.

Launching takes about 8 minutes to reach orbital velocity. However, even "direct insertion" paths used by the shuttle require a circularization burn after 45 minutes to turn an elliptical orbit with a perigee of "launch pad" into something that mostly stays above the atmosphere. For a space station intercept, this circularization burn should put you into an orbit similar to the station.

The shuttle went through several burns just to settle into its non-intercepting orbits. Besides the circularization burn, which used the orbital maneuvering system (OMS), many earlier flights used an earlier OMS burn to set up the initial elliptical orbit. The advent of "direct insertion" flights with STS-41C cut that down to one burn since the elliptical orbit was established with the main engines.

In between main engine and OMS flights, the shuttle had a variety of burns performed with its reaction control system (RCS). It did a little RCS burn to get away from the external tank after separating and to orient the shuttle for the upcoming OMS burn. After each OMS burn, there were planned opportunities to cancel out errors from the burns with other RCS burns.

Other spacecraft go through similar processes on their way to orbit: big burn to a very elliptical orbit, then one or more burns to turn that into a circular orbit.

You want to get all that basic "getting to orbit" stuff completed before trying to dock with a space station.

After you're in a happy orbit, then you need to make your orbit match the station's. If all the planning went well in advance, the process of adopting a circular orbit put you close to the space station in terms of distance. Over the remaining 5-ish hours, spacecraft going to the station make 2 to 3 small burns to cancel out small differences in orbital inclination and circularity, then spend about an hour drifting closer and closer to the station over the last couple of miles. Given the ISS is the most expensive vehicle built in human history, NASA-n-pals are very cautious with it.

General overview of orbital interceptions and techniques.

Simple summary of 6-hour intercepts

Anonymous Coward

Docking timelines and fast docking procedures

Without some additional information, it is hard to guess what exactly is meant by the "fast-dock" procedure. On the other hand, the rendez-vous and docking procedures for the ISS workhorse Soyuz are very extensively documented, so there are some hints there.

The "standard" Soyuz arrival timeline is here:

Soyuz docking timeline

It is a rather unhurried procedure, with the final automated docking procedure taking five orbits (seven hours or so) from start to finish. With no crew on-board the vessel (ie Progress cargo version) I assume this would be down to four orbits - roughly the same duration as the Tianzhou's "fast-dock" process.

Most of the time on the "standard" trip profile is however taken by the orbital chase part - so whenever possible, Russians are now trying to use the "fast rendezvous" procedure, discussed here:

Soyuz/Progress fast rendezvous

The fast procedure takes only four orbits from launch to the start of the automated docking procure. The downside are the much tighter launch and maneuvering windows, with the possibility of having to burn considerably more fuel if any of the windows are missed.

Anonymous Coward

Re: Docking timelines and fast docking procedures

I vaguely remember hearing about the Soyuz fast rendezvous procedure. Part of the idea was that astronauts tend to be happier in the space station versus a cramped capsule.

Still, if the Tianzhou-1 is going to be backing away from the station and attempting to re-dock, it seems like more of a "docking" time saver versus a "rendezvous" procedure. Seems like backing off from and re-approaching a space station is a far different maneuver than the Soyuz.

It's all well above my head (no pun intended). Thanks to the posters who have helped impart a bit more knowledge.



I for one, never knew it took so long, but reading all you above post I can now understand why.

I assume they are using some form of electromagnetic docking clamp with variable polarity to match positioning, as that would make sense and reduce timing to reposition etc.

I love space stuff, there are always new variables to take into consideration.

Silver badge

Re: Interesting

Apparently the Chinese are using something similar to (probably functionally identical, no point in re-inventing the wheel) the APAS docking ports which were first developed by the US and USSR for the Apollo-Soyuz Test Project.

These are basically entirely mechanical, using clamps that grab each other and are drawn together, pulling both craft together.


Dragon capsule capacity and capability

The SpaceX Dragon capsule used for cargo launches to the ISS usually carries about three tonnes or so of supplies on each flight, not six. It can't transfer fuel and other liquids to the station via the docking port adapter and it can't dock automatically, it's captured by a robotic arm under manual control. It can return scientific samples, equipment and other materiel to Earth which no other unmanned capsule can.


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