Following a highly successful four month mission, Europe’s Automated Transfer Vehicle-2 (ATV-2) spacecraft has undocked from the International Space Station (ISS) on Monday, for a destructive re-entry into Earth’s atmosphere on Tuesday. ATV-2’s undocking will clear the way for the launch of Russia’s Progress M-11M/43P spacecraft, which took place on Tuesday, with a docking to the ISS set for Thursday.
ATV-2 mission summary:
The European Space Agency (ESA) owned ATV-2 launched to the ISS from the Kourou space center in French Guiana atop an Ariane 5 rocket on 16th February this year. Following an eight day free-flight, the uncrewed resupply vessel docked autonomously to the ISS at the Service Module (SM) Aft port on 24th February, just hours prior to the launch of Space Shuttle Discovery on the STS-133 mission.
The reboost fuel was used to perform numerous ISS orbit corrections and attitude control assistances during ATV-2’s stay at the station, notably on 2nd April when the spacecraft maneuvered the station to avoid a collision with some orbital debris. But by far the biggest use of reboost fuel was the four “big boosts” conducted in stages throughout this past week.
The goal of the “big boosts” was to raise the ISS to its new operating altitude for the post-Shuttle era. With the US Segment of the ISS now complete, Space Shuttles no longer need to bring large, heavy components up to the ISS, and so the ISS can now be operated at a higher altitude where less atmospheric particles exist, resulting in less drag being placed on the station.
This reduction in drag will reduce the amount of ISS reboosts that will be needed in future, meaning that Visiting Vehicles (VVs) will be able to carry less reboost propellant, and thus more dry cargo to the station.
It is estimated that the higher operating altitude of the ISS will reduce the requirements for reboost propellant from 19,000 pounds per year down to 8,000 pounds per year, resulting in 11,000 pounds of extra VV payload capacity each year, although this amount may not directly translate into an increase in dry cargo capacity since VVs will also need to use slightly more fuel to rendezvous with the ISS at its higher altitude.
Overall though, the higher operating altitude will result in an increased amount of dry cargo capacity for ISS VVs, which will further help to keep the ISS resupplied in the post-Shuttle era. Although one Space Shuttle mission still remains on the manifest (STS-135 in July), the higher operating altitude of the ISS will not be an issue, and any reduction in Shuttle payload capacity is negligible.
The first of the “big boosts” kicked off last Sunday (12th June), which saw two reboosts occur on the same day. The first reboost lasted 36 minutes 6 seconds, which resulted in a change in velocity (delta-V) of 5.2 m/s, and a mean altitude gain of 9.2 km.
The second reboost occurred roughly 3 hours 30 minutes later, and lasted 40 minutes 12 seconds, which resulted in a delta-V of 5.8 m/s and a mean altitude gain of 10.1 km. Combined together, the total reboost duration was 1 hour 16 minutes 18 seconds, which gave a delta-V of 11 m/s, and a mean altitude gain of 19.3 km.
Instead of conducting one large reboost, two reboosts were necessary with a pause in between, due to “how ATV’s fuel system pumps propellant”, and to let the reboost engines cool down. However, the two reboosts caused an issue with power balances, since the two reboosts drew more power from the batteries than expected.
The high solar beta angle (where the sun hits the ISS side-on) that was in effect at the time caused shadowing of the station’s and ATV-2’s solar arrays, and prevented the batteries from being recharged at their normal rate. Thus, lower than expected battery charge levels were incurred due to the higher power draw and lower recharge rate.
Following a meeting of the ISS Mission Management Team (IMMT), the original plan to conduct another two reboosts on Wednesday 15th June was changed. Under the new plan, only one reboost would be conducted on Wednesday 15th June, with the second being performed on Friday 17th June, and a third back-up opportunity being available on Saturday 18th June.
To aid battery recharge rates in the high beta angle period, the 4B and 2B BGAs (Beta Gimbal Assemblies), which control the beta rotation of the station’s solar arrays, would be kept in Autotrack, allowing them to track the Sun.
Wednesday’s reboost was performed nominally, with a duration of 39 minutes 40 seconds, a delta-V of 5.84 m/s, and a mean altitude gain of 10.2 km. Friday’s fourth and final reboost also went off without a hitch, with a duration of 26 minutes 53 seconds, a delta-V of 3.96 m/s, and a mean altitude gain of 6.9 km.
In total, the four ATV-2 “big boosts” lasted 2 hours 22 minutes 11 seconds, and resulted in a delta-V of 20.8 m/s, and a staggering mean altitude gain of 36.4 km. The ISS is now at a mean altitude of 381 km, with a 384 km apogee (highest point) and 379.1 km perigee (lowest point). In 12 years of operation, the ISS has never been this high above the Earth.
ATVs are the only vehicles capable of performing such massive reboosts due to their large propellant capability and their four mighty Orbit Correction System (OCS) engines, which consumed 1,400 kg of propellant during the four reboosts.
According to NASA, ATV-2 performed the reboosts “with an outstanding precision that has never been reached for such a maneuver since the Apollo TLI (Trans-Lunar Injection) burns by the Saturn V S-IVB stage”.
The required amount of propellants were successfully transferred to the ISS’ fuel tanks during ATV-2’s stay at the station, and 78 kg of the 100kg of oxygen (O2) delivered by ATV-2 was also released into the ISS’ atmosphere.
However, the remaining 22 kg of O2 was not able to be transferred to the ISS, due to a failed fan inside ATV-2 which caused safety concerns since it was not able to cool equipment used in O2 transfers.
This will not adversely affect the O2 supplies aboard the ISS, despite recent concerns relating to the failure of Russia’s Elektron O2 generator, since during Space Shuttle Endeavour’s recent visit to the station, 10 pounds of O2 was transferred to the ISS from Endeavour, and the US Segment Oxygen Generation System (OGS) was successfully repaired. Also, the Elektron has since been successfully repaired by the Russian crewmembers.
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ATV-2 undocking and de-orbit:
Now depleted of all useful dry cargo, propellants and oxygen, ATV-2 was filled with 1,200 kg of trash, which according to an ISS stowage status document obtained by L2, includes 6 RFTAs (Recycle Filter Tank Assemblies), the ESA flywheel experiment, ATV-2 and STS-134/ULF-6 foam, common trash, and payload trash.
Hatch closure between ATV-2 and the ISS occurred Sunday, and ATV-2 undocked from the ISS at 2:48:21 PM GMT on Monday. ATV-2 conducted a separation burn using its thrusters shortly after undocking, at 2:51 PM GMT.
Following a free-flight for just over one day, an orbit-lowering burn was conducted on Tuesday 21st June at 5:07 PM GMT. The de-orbit burn followed roughly three hours later at 8:05 PM GMT, whereupon the ten tonne ATV-2 plummeted through Earth’s atmosphere as a giant fireball, eventually disintegrating under the intense heat.
Surviving parts of ATV-2 splashed down in a remote area of the Pacific Ocean around 8:52 PM GMT. A no-fly zone was in place above the debris fallout zone, and sea traffic was warned to keep out of the area.
Aboard ATV-2 during re-entry was the second Re-Entry Breakup Recorder (REBR), the spacecraft equivalent of a black box, designed to record data on the physics of re-entry disintegrations and send the data to a satellite prior to anticipated destruction upon splashdown in the Ocean.
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The first REBR flew on Japan’s HTV-2 (H-II Transfer Vehicle-2) during its re-entry in March this year, and performed well above expectations, managing to survive re-entry and splashdown intact and still send signals while floating in the Ocean.
Progress M-11M launch and docking:
Roughly 5 hours 25 minutes prior to ATV-2’s de-orbit burn, Russia’s uncrewed Progress M-11M/43P spacecraft blasted off atop a Soyuz-U rocket from the Baikonur Cosmodrome in Kazakhstan, on a resupply mission to the ISS. Liftoff was at 2:38:18 PM GMT on Tuesday.
In the short 5 hour 15 minute period between Progress M-11M’s launch and ATV-2’s de-orbit burn, two VVs will be free-flying in space together, albeit in different orbits.
Following a two day free flight, Progress M-11M will autonomously dock to ATV-2’s recently vacated docking port, Service Module (SM) Aft, on Thursday 23rd June at 4:35 PM GMT. Progress M-11M will remain attached to SM Aft until 29th August, whereupon it will undock and head towards the same fiery demise as ATV-2.
(Images via ESA and L2 ATV documentation). For other ATV-2 content, visit the excellent ATV-2 blog by ESA: http://blogs.esa.int/atv