posted 08-01-2012 01:16 PM               

This thread is intended for members' comments and questions about the Progress M-16M mission and the updates published under the topic: Progress M-16M (48P) ISS resupply craft.

Russia's Progress M-16M (48P) cargo craft will lift off atop a Soyuz-U booster from the Baikonur Cosmodrome, Kazakhstan on Wednesday, Aug. 1 and arrived at the International Space Station six hours later.

With this launch, Russian mission controllers are testing a modified rendezvous plan designed to reduce the typical two-day flight to the ISS. If applied to the crewed Soyuz vehicles, this would increase crew comfort and provide for additional contingency time at the end of the spacecraft’s mission.

posted 08-02-2012 02:56 AM               

The low fuel consumption trajectories used by the Progress craft starting with Salyut 6 were first tested on the Soyuz T craft if I remember correctly and the Soviets opted to go with that slower rendezvous path for the Progress modules since it had less chance for error and burned less fuel (which would allow for more fuel to be pumped into the fuel tanks of the Salyut for reboost operations).

Just remember that at the time Progress started flying (early 1980s) automated dockings were still a relatively new thing. The US systems used for Apollo and Skylab (plus Gemini) had helped to automate part of the rendezvous process with mission control on the ground updating things in near realtime. But it was still up to the astronauts to acquire the target visually and fly the last part of the docking. The same was true with the Soyuz/Salyut dockings in those early days.

The Soyuz craft began utilizing the longer approaches as well for similar reasons (more fuel for docking attempts), since they weren't as limited in their on orbit life as the early ferries. The TM series introduced a new Argon computer that could fly a completely automated rendezvous approach and docking with no input from the crew or the ground if everything worked properly. But the designers preferred to have it flying a Progress path as opposed to something closer to what the early approaches utilized, hence two day flights to stations instead of one.

I believe part of the reason why the slower rendezvous trajectories with Progress continued to be used had to do with a lack of funding. The Igla was the first rendezvous system and then it was replaced by the first Kurs system introduced on Mir. We might have seen a more advanced Kurs come about sooner if the Soviet Union had not fallen since the factory that made the original system is based in the Ukraine. After Ukraine became independent, the price they were charging for Kurs went up.

posted 08-02-2012 03:44 AM               

...but there is a second, less well-known launch window that has been the primary driver behind a standard two-day (sometimes three-day) rendezvous. It is connected to the awkward fact that at the precise moment the spacecraft must launch to hit the right orbital plane, the target satellite can be anywhere along its near-circular, approximately 92-minute-long orbit around Earth. The difference in the position of the spacecraft and target along the same track is called the phase angle.

You make up for the phase angle by launching into a lower orbit, which has a speed slightly greater than that of the target. As the chasing vehicle climbs into the target’s orbit, it closes the distance between them. The lower the initial orbit, the faster the chaser catches its target.

With two days to catch the space station, you’ve got a lot of choices for your initial altitude, a wide range of possible overtaking speeds, and therefore a big choice of phase angles to work with. According to Murtazin's recent paper, Soyuz can catch the ISS as long as it gets anywhere within 42 percent of the station's entire orbital arc (about 150 degrees). This gave Russia's space agency opportunities to launch as often as every other day.

But for the fast rendezvous, there is a greatly diminished capability to overtake the station. Even with the most recent tweaks to the plan, less than 6 percent of the orbital arc is accessible.

Waiting for the right slot is frustrating — weeks could go by between opportunities. So throughout July, the station's orbit has been gently tweaked, with rocket firings to nudge it into the sweet slot for Kazakhstan launches...