ATV-2 successfully docks with International Space Station

by Chris Gebhardt and Pete Harding

After an eight-day (8 day) orbital chase, Europe’s Automated Transfer Vehicle-2 (ATV-2) has conducted a picture perfect automated docking to the aft end of the International Space Station’s (ISS’s) “Zvezda” Service Module (SM). ATV-2 was launched atop an Ariane V booster from the Kourou Space Center in French Guiana last Wednesday (16th February) evening, and has been chasing down the ISS ever since.

ATV-2 Docking Sequence:

In preparation for the docking manoeuvre, ATV-2 controllers commanded spacecraft’s thrusters into a programmed firing sequence to move the spacecraft from its initial phasing orbit into a “transfer to vicinity of ISS” orbit. This was then followed by an integration of the phasing and rendezvous portions of the orbital approach timelines.

These manoeuvres will, in turn, bring ATV-2 to within 30 kilometers of the ISS, at which point direct Space-to-Space comm coverage will begin, and the ISS crew will be able to monitor ATV-2’s systems. At this time, the rendezvous phase of ATV-2’s approach will commence, with rendezvous taking place along the V-bar (Velocity bar).

At a distance of 4.5km, RGPS (Relative Global Positioning System) navigation will begin, taking over from AGPS (Absolute GPS) navigation. RGPS detects ATV’s position in space using sensors on the ATV’s and station’s exterior, as opposed to AGPS which uses GPS satellites for navigation.

New rendezvous equipment will be used during the rendezvous and docking sequence – the Proximity Communication Equipment (PCE). PCE was installed in the SM last October, as was successfully checked out in order to ensure proper functionality. The PCE is designed to transmit signals between the WAL3 and WAS2 antennas in the SM, which receive data from the ATV, and ground stations on Earth.

The ISS crew will begin taking official distance measurements on ATV-2 once the vehicle passes inside the 20 meter mark. At 1 meter distance, the ISS crew will go “hands off” and let the ATV-2 perform its final docking sequence.

This final docking sequence will include four steps: contact/capture by Probe Head and Latches, Docking start/Probe retraction by the Probe and Docking Mechanism (at which point interface alignment will be verified), ATV hooks closure (at which time the interface seal will be confirmed), and ISS hooks closure.

During the rendezvous and docking sequence, that Stations’ solar arrays will be placed into a new configuration to protect for RGPS multipathing.

As noted by the MOD FRR (Mission Operations Directorate Flight Readiness Review), available for download on L2, “New configuration [will] allow Fwd BGAs  (Beta Gimbal Assemblies) to be positioned at their best power generating angles, or even remain in autotrack, while the aft BGAs [will be] constrained for loads/erosion”.

Longeron shadowing concerns on ATV-2 will also be mitigated during rendezvous and docking ops.

Previous In-Flight Anomalies and Lessons Learned from ATV-1:

In all, the European Space Agency’s (ESA’s) flight of ATV-1 in 2008 was remarkably clean, with the MOD FRR only noting one IFA (In-Flight Anomaly): Structural Dynamic Measurement System Late Record Stop Command.

As the MOD FRR document relates, “During Structural Dynamic Measurement System (SDMS) operations to record accelerometer/strain gauge data during ATV-1 Docking, the command to stop recording data was not sent in time to prevent overwrite of the 632 second memory buffer.”

This led to the overwrite of the first two (2) minutes of SDMS sensor data, including information on initial contact. To prevent this problem from occurring again, the OSO has scheduled dedicated personnel to perform SDMS operations during “high intensity times.”

Additionally, several lessons learned from ATV-1’s mission were initially planned for implementation during ATV-2 but have since been deferred to ATV-3 and subsequent ATV flights in the interest of time and resources.

One such lesson resulted in the request by the ATV Control Center for more responsibilities in ATV crew time planning. As it currently stands, the ATV is considered a USOS vehicle and, as such, the USOS is responsible for planning all ATV activities.

Under the agreement to defer most ATV Control Center planning to ATV-3+, including the planning of all ATV crew tasks not covered by cargo and docking/undocking operations, some planning activities are being phased in during ATV-2.

These include the planning of all waste, urine, and/or condensate transfers by MCC Moscow. All waste, urine, and/or condensate transfer planning and control activities for ATV-3+ are open for discussion.

However, two lessons learned from ATV-1 have already been implemented for ATV-2.

During ATV-1, the requirement to perform bladder integrity testing prior to loading the empty ATV was a constraint that was not documented in any flight rule. For ATV-2, the requirement has been included in the Flight Rules.

Likewise, during ATV-1, crew time was allotted weekly to manage the Center of Gravity (CG) of the spacecraft; however, CG adjustments were only performed once over the entire docked mission since propellant is the major contributor to CG with cargo second.

For ATV-2, clearly defined CG limits and uncertainties have been defined (which allows for uncertainties in mass distribution), as have minimum undock mass limits.

ESA will monitor ATV-2’s CG and “notify ATV ISO when approaching CG limit. If CG adjustment needed, ATV ISO will put the task on the crew’s to do list.”

Another anomaly that was seen during ATV-1 related to Multi Layer Insulation (MLI) coverings on the exterior of the ATV. MLI serves to prevent the pressure shell of the ATV from becoming too hot or cold in the vacuum of space.

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During the ATV-1 launch in April 2008, trapped air underneath the MLI rapidly expanded once the vehicle reached the upper atmosphere. The result was a “ballooning” effect on the MLI, as the trapped air beneath the MLI ripped it from its attachment points as it escaped into the lower air pressure of the upper atmosphere.

Erroneous readings where seen on ATV-1’s temperature sensors, which gave ground control teams a good idea of what had occurred. The theory was confirmed once ATV-1 rendezvoused with the ISS, with its MLI visibly protruding in places.

Although the temperatures of ATV-1 were never an issue and the loose MLI was deemed to be not a serious failure, ATV-2 teams worked hard to ensure that the same incident doesn’t occur on ATV-2.  An upgraded MLI was developed with increased numbers of holes for air ventilation – which will allow the air to escape without having to push the MLI aside.

The number of MLI attachment points was also increased, and the way the MLI blankets overlap each other was improved. The new designs underwent rigorous testing in a vacuum chamber. LINK BLOG

ATV-2 Payload:

The ATV’s pressurised cargo is carried inside the Integrated Cargo Carrier (ICC), which contains two types of cargo – dry and fluid.

Dry cargo is contained in the pressurised segment of the ICC, which accounts for 90% of the volume of the ICC. The dry cargo area is capable of launching eight space station racks. However, the hatch on the Russian probe and drogue docking system, which the ATV uses, is not large enough to facilitate the transfer of racks. Therefore, only rack-sized frames are installed in the ATV, which are used to house Cargo Transfer Bags (CTBs).

Fluid cargo is contained in the unpressurised segment of the ICC, which accounts for 10% of the volume of the ICC. ATV-2’s fluid cargo will not include any water, as enough water already exists on board the ISS. However, ATV-2 will carry 220 pounds of oxygen for the station. ATV-2’s ICC has an increased cargo capacity than it did on the ATV-1 mission, due to the use of lighter weight racks, and modifications that have been made to the Ariane V booster.

In total, the ATV-2 mission will carry 1,600kg of dry cargo to the ISS. None of this cargo will be unloaded until Expedition 27 begins in March, due to the fact that the PMM being launched on STS-133 will not be ready to accept cargo until this time. As soon as a CTB is removed from ATV-2, a piece of trash will be loaded in its place, meaning that ATV-2 will slowly fill up with trash over the course of a few months.

Although ATV can carry eight (8) racks in total, ATV-2 will only carry six (6). Once docked to the ISS, two soft racks made from fabric will be assembled in the two (2) empty racks bays, in order to provide the ISS crew with some temporary stowage space. The reason for the decreased amount of dry cargo is because ATV-2 is carrying an increased amount of wet cargo.

Wet cargo is contained in the ATV’s Service Module (SM), which houses all propellant tanks, avionics and electrical power systems.

Two types of propellants are contained in the SM – propulsive support propellant, which is used for ATV rendezvous burns and ISS reboosts, and refuelling propellant, which is transferred to the ISS’s fuel tanks. The SM can contain up to 4 tonnes of propulsive support propellant, and up to 860kg of refuelling propellant. ATV-2 will carry slightly less propulsive support propellant than ATV-1 did, due to the fact that ATV-2 will not perform any demonstration manoeuvres.

ATV-2 ISS Reboost Plan:

ATV-2 will reboost the ISS’s orbit by a massive 40km, and will hold the world record for the largest reboost ever performed over the shortest amount of time.

The ATV is the only vehicle with enough propulsive might to perform such a massive reboost – Russia’s Progress vehicles do not possess enough propellant to perform the reboost on a single flight. In order to perform the 40km reboost, nearly 10,000 pounds of propellants will be consumed by ATV.

The purpose of such a massive reboost is to set the ISS up for operations in the post-Shuttle era. From the ISS’s launch in 1998 until now, it has operated at an altitude of approximately 250km, in order to allow Space Shuttles to visit carrying the maximum amount of payload. This is because, if the ISS orbits at a lower altitude, then vehicles need less propellant to reach the ISS – meaning that more payload can be carried.

However, operating the ISS in a lower orbit increases drag on the complex due to increased amounts of atmospheric particles hitting the station’s exterior. Operating at a lower altitude also makes the station more susceptible to solar activity, which increases Earth’s atmospheric pressure, which further increases drag on the station. Over time, this drag causes the ISS’s orbit to lower – meaning that the station has to be periodically reboosted, which requires propellant.

Raising the ISS’s orbit by 40km will result in less drag being placed on the complex, meaning fewer reboosts will be required. Due to the decreased requirement for reboosts, VVs will be able to carry less propellant to the ISS. At its current altitude of 250km, the ISS requires around 19,000 pounds of propellant a year for reboosts. In its new, raised orbit (290km), it is estimated that the ISS will only require 8,000 pounds of propellant a year for reboosts.

The decreased requirement for propellant in VVs will be slightly offset by the fact that VVs will also be able to deliver less dry cargo to the ISS, due to its higher orbit. This means that the upmass gained by carrying less propellant won’t translate into an equal increase in dry cargo upmass.

ATV-2 is currently scheduled to undock from the station on 4th June. However, an extension to the ATV-2 mission is currently being considered by ESA in order to ensure that all of ATV’s propellants will be consumed.

It is not yet known whether the ATV-2 mission will be extended to its maximum duration of six months, in order to allow the 40km reboost to be performed after the STS-135 mission currently scheduled for June. Doing so would allow STS-135 to gain the maximum upmass by rendezvousing with the ISS in its lower orbit.

On Friday 25th February, the day following the ATV-2 docking and the day prior to the planned STS-133 docking, a test reboost of the ISS will be performed by ATV-2. The reboost is scheduled to begin at 10:33 AM GMT, with a Delta V of 0.5 meters per second.

(Images via ESA and L2 ATV documentation). For other ATV-2 content, visit the excellent ATV-2 blog by ESA:

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