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The Wikipedia Article on Soyuz MS-04 says:

It is the first of the Soyuz MS series to rendezvous with the Station in approximately 6 hours, instead of the 2 day orbital rendezvous used for the previous launches.

I have several ideas (like optimal launch window or more delta-V due to only two crew members at launch, which means more flexibility for orbital maneuvers), but I am not sure at all.

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On one of the Russian web sites there is an interview with Rafail Murtazin, deputy head of ballistics department of the Energia corporation, who is described as the developer of the expedited rendezvous scheme. In brief, they have achieved this as follows (I apologize for possibly incorrect terms, please feel free to edit).

Firstly, the ISS orbit is adjusted in advance to ensure that on the launch date the phase angle between the two crafts is within 30-35 degrees.

Secondly, Soyuz makes its first orbit transfer on the very first orbit, using theoretical orbit parameters computed before launch. This is in contrast with the two-day rendezvous scheme, where they measure actual orbital parameters before the transfer, which takes them two extra orbits.

At the bottom of the interview page is a link to the presentation on the topic (PowerPoint, in Russian) Murtazin gave at a conference at the Bauman State Technical University in 2013; it has all sorts of diagrams showing in detail the entire launch and rendezvous process.

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  • $\begingroup$ As I understand it, 30 degrees is the optimal phase angle, and 35 degrees is the width of the range of the angles. So the range is approximately from 12.5 to 47.5 degrees. $\endgroup$
    – Litho
    Jun 7, 2017 at 8:46
  • $\begingroup$ I don't see that in the presentation, and that seems too wide a range for comfort (safety). On page 8 they show two schemes: 5-orbit with the starting angle of 35° and 4-orbit with the starting angle of 33°. On page 16 they suggest an improved, 3-orbit scheme with the 20° starting phase angle given a launcher that can ensure better accuracy. $\endgroup$
    – mustaccio
    Jun 7, 2017 at 11:15
  • $\begingroup$ I took this from the interview, not from the presentation. "Фазовый диапазон включает в себя все допустимые начальные фазовые углы. [...] У 5-витковой схемы этот диапазон составляет порядка 35 градусов.": "Phase range includes all the acceptable starting phase angles. For the 5-orbit scheme, this range is about 35 degrees." And later he says that even in the case of unforseen course correction of the ISS due to space debris avoidance, proper planning would get it within the range, which is "not zero at all, but ±15 degrees". $\endgroup$
    – Litho
    Jun 7, 2017 at 12:04
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    $\begingroup$ About the presentation: it seems to me that they use ΔΦ to denote both the starting angle AND the phase range. The graph on page 10 seems to show how ΔV for course correction depends on the starting angle, denoted as ΔΦ on the x-axis. But ΔΦ_5 and ΔΦ_4 are used to show the ranges where ΔV is within the nominal, and these ranges are 35 and 33 degrees wide. $\endgroup$
    – Litho
    Jun 7, 2017 at 12:13
  • $\begingroup$ I'm wondering when/if direct launch to rendezvous - less than an hour or two inside Soyuz - will become a thing. It should be possible in theory. It may have been done in KSP already, but you don't get a quick save slot in real life. $\endgroup$
    – John Dvorak
    Jun 8, 2017 at 12:21
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Russia has been using expedited rendezvous since 2012 with the uncrewed Progress and since 2013 with the crewed Soyuz. Expedited rendezvous is a huge bonus for a crewed vehicle. This is particularly the case with the Soyuz capsule, which is a bit cramped when it carries two people and very cramped when it carries three.

The newest revision of the Soyuz fell back to the longer two day rendezvous because there is very little margin for error in the expedited rendezvous. Roscosmos needed to make sure that there wasn't a misfeature hiding amongst the many upgrades made to the Soyuz. The first Soyuz MS flight was planned as a two day rendezvous to allow time for on-orbit checkout. The second flight experienced some difficulties. The third flight used a two day rendezvous for safety. Everything went well (very well) with that flight, giving Roscosmos confidence that they could go back to using an expedited rendezvous for the fourth flight of the Soyuz MS.

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    $\begingroup$ This answers why (and does so quite well), but not how. What's done differently in the expedited rendezvous and standard rendezvous? $\endgroup$
    – Carl Kevinson
    Jun 6, 2017 at 13:36
  • $\begingroup$ @CarlKevinson I'm guessing fewer, but more aggressive Hohmann transfer burns to the ISS. That would explain why if there was some error in the burn it would take longer for ISS rendezvous. $\endgroup$
    – Jake Blocker
    Jun 6, 2017 at 16:31
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    $\begingroup$ They probably do not use Hohmann transfers given that they need to perform both an inclination change and an apogee change. Hohmann transfers only work between coplanar circular orbits. $\endgroup$
    – ChrisR
    Jun 6, 2017 at 16:57
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    $\begingroup$ During the Shuttle Program, Hohmann transfers were rarely (if ever) used. High energy transfer orbits were used, largely because, during manned missions, time is a more important consideration than prop conservation... $\endgroup$
    – Digger
    Jun 6, 2017 at 18:31

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