How does delaying Soyuz TMA-15M crew return help with ISS schedule slip due to failed Progress M-27M resupply vehicle?

Question

After the failed Progress M-27M / Progress 59 resupply mission in late April 2015, Roscosmos and NASA have been working on ISS schedule changes to accommodate loss of supplies to the station, while an investigation into Progress M-27M failure is still ongoing. Yesterday, May 12, 2015, the following schedule changes were announced:

• Return of Soyuz TMA-15M (Virts, Cristoforetti, Shkaplerov) in early June instead of on May 13,
• Progress M-28M launch in early July instead of August 6,
• Launch of Soyuz TMA-17M with Kjell Lindgren (NASA), Kimiya Yui (Japan Aerospace Exploration Agency) and Oleg Kononenko (Roscosmos) in late July instead of on May 26.

Not surprisingly, this includes changes to the crew rotation scheme. But it also delays Soyuz TMA-15M crew return for at least 18 days, possibly more. And that is somewhat surprising to me, considering that Progress M-27M supplies weren't delivered. While the station isn't running out of supplies, having a crew of six instead of a crew of three aboard for an extended period should thin them out faster. So to my question:

Given slightly extraordinary circumstances, is keeping a crew of six aboard the station for longer and maintaining tight crew rotation scheme better than extending the period during which the station only hosts a single Soyuz TMA crew of three?

In other words, I'm asking why wouldn't the Soyuz TMA-15M crew return as previously scheduled, and then launch Soyuz TMA-17M later, keeping only the Soyuz TMA-16M crew of three on the station for a few weeks longer? Is Soyuz TMA-17M launch still considered too tentative, pending conclusion to the Progress M-27M failure investigation, or something else prompted Soyuz TMA-15M crew return delay?

Answer 1

The reason for swapping the Progress and Soyuz flights is to ensure that the third stage (which is suspect at this time in the Progress failure) is working properly before launching a manned crew on the same basic booster.

By doing that, it means the manned launch is delayed and if TMA-15M crew returned, the station would be down to a crew of three.

The problem with the ISS is that it is fairly maintenance intensive. The crew of three ends up doing very little science as between sleep, rest, and station maintenance there is very little time for science.

With a crew of 6 they get more accomplished. The US believes that once they get the Commercial Crew vehicles flying 4 astronauts at a time, and thus a crew of 7, that they will effectively DOUBLE the science time available. So with a crew of 6, they get the equivalent of a single persons work on Science each day. Adding one more person is sufficient to double that amount which tells you how little science really gets done.

The purpose of the ISS is to do science, and if you drop the crew down to three, you basically get zero science time, until July and the TMA-17M crew arriving,

In terms of supplies, the station has reserves for many months. (It is complicated, since some consumables are in better supply than others). I saw a table showing the various consumables and their state somewhere, but cannot find it to link in at the moment.

There is a Dragon flight coming in June (CRS-7 with the IDA! Yay! International Docking Adapter for Commercial Crew docking) with supplies. (Alas poor CRS-7 and IDA, we knew ye well. (CRS-7 was lost when the upper stage burst due to a strut failing, which dropped a COPV, which burst releasing high pressure helium, midflight)).

There is an HTV flight (which has a fairly hefty payload capacity compared to Dragon or Progress) due this summer as well. So the supply situation is not close to critical, and there are two more strings of backup levels on the schedule.

Thus the station managers are trading off the science value of a crew of 6 against possible failures in Dragon/HTV/Progress such that they run out of supplies in the near future.