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The Northrop Grumman OA-9E Cygnus spacecraft has departed the ISS at the concluding phase of her mission. The departure comes five days after a unique test objective. The cargo resupply vehicle provided a reboost to the Station with a short 50 second burn of its main engine on the aft of the vehicle, raising the Station’s altitude by 295 feet. This test will pave the way for future, longer burns, removing some of the orbital stationkeeping strain from the Russian assets.

Cygnus concluded her stay at the ISS on Sunday, released from the grip of the Space Station Remote Manipulator System (SSRMS).

Two tons of trash had been loaded into the since-emptied vehicle, following its delivery of 3,350 kilograms (7,385 lb) of cargo to the International Space Station.

This included 800 kilograms (1,764 lb) of propellant. The cargo included 1,191 kilograms (2,626 lb) of hardware for the US and international segments of the outpost and 13 kilograms (29 lb) for the Russian segment.  The payload also included 132 kilograms (291 lb) of hardware to support spacewalks, 100 kilograms (220 lb) of computer equipment and 811 kilograms (1,788 lb) of supplies and provisions for the crew.

The vehicle was then refilled with trash, set to burn up along with the vehicle during its destructive re-entry. That will occur on July 30, following the deployment of six cubesats.

However, the vehicle still had one final role to conduct on the ISS, a first-ever reboost of the Station by a commercial vehicle.

Cygnus Reboost firing – as envisioned by Nathan Koga for NSF/L2

This was the first time since the retirement of the Space Shuttle fleet in July 2011, a US spacecraft had performed a reboost of the ISS.

Although the Station is high above the heavens, there is still a very thin amount of air in the 220 mile orbit of the ISS, enough to provide a tiny amount of atmospheric drag that results – over time – in the Station losing some altitude while increasing velocity.

Reboosts of the Station’s altitude are required to counter the natural decay of the ISS’ orbit as it races around the planet.

Reboost events were commonplace during the Shuttle era, conducted via firings of the Reaction Control System (RCS) thrusters on the docked orbiters, providing a thank you present to the Station that was protecting and – in later years – feeding the orbiter during her stay.

Shuttle docked to the ISS timelapse

With reboosts required every couple of months, the Russian visiting vehicles would also chip in with the occasional push, a practice that has continued to this day, in tandem with Europe’s since-retired ATV craft.

The Station also has a set of thrusters on the Zvezda module can be employed. However, they are mainly reserved for when a Visiting Vehicle can’t conduct the task, as the requirement of protecting the Station’s propellant stores is paramount.

Future Visiting Vehicles, such as the Commercial Crew vehicles, are also being evaluated for potential reboost capabilities and now a Commercial Resupply Services (CRS) vehicle has shown it is capable of helping take up some of the load.

Dragon 2, Starliner and CRS2 Dream Chaser all at the ISS – via Nathan Koga for NSF L2

“We actually started engaging NASA on this topic probably the fall of last year,” noted Frank DeMauro, Vice President and General Manager of the Advanced Programs Division for Northrop Grumman Innovation Systems.

The reboost was small and negligible, classed officially as a Detailed Test Objective in which Cygnus fires its main engine for just a few seconds to demonstrate its capability to perform more robust ISS orbit raising maneuvers in the future.

“We have a large engine on the back of the spacecraft that puts out a lot more thrust [than the 32 maneuvering thrusters on Cygnus], and this is the engine we use for orbit raising burns,” noted Mr. DeMauro.

The S.S. J.R. Thompson at its 10 m capture point – as the Station arm reaches out to grapple Cygnus. (Credit: Nathan Koga for NSF/L2)

“So we started talking with NASA at the program office about the possibility of Cygnus providing some form of orbit raising capability using that engine.  And one of the things we decided to do earlier this year is to put this Detailed Test Objective in place and at least work through the process of seeing if we could get that approved by NASA and of course specifically the safety review panel.”

The NASA program office showed great interest in this potential capability from Cygnus, and NASA and its safety office have been moving through the process of performing the various analyses needed to ensure that using Cygnus while berthed to the Node-1 nadir port to reboost the ISS does not impart dangerous thrust loads onto the structure of the Station.

“If we’re going to be imparting thrust or forces on the ISS by thrusting our engine, [NASA] has to do work on their side, and they’ve done that,” noted Mr. DeMauro.

“As far as if it’s going forward, we expect it to go forward. We are waiting for the final sort of dot the Is and cross the Ts with the safety panel, but we don’t expect any issues closing that all out.”

Cygnus berthed to Node-1 nadir, where it will hopefully perform the first U.S. craft orbit raising of the Space Station since the Space Shuttle fleet’s retirement. (Credit: Nathan Koga for NSF/L2).

One last hurdle for the Tuesday test had to be passed, namely the arrival of the Progress MS-09 craft during its record-breaking ascent to the Station.

Had the two orbit attempt suffered issues, the test would have been delayed as the Progress needed to be firmly docked to the ISS for the reboost to be carried out.

Assuming this test is declared successful on the review that will now be taking place, Northrop Grumman hopes to offer this capability to NASA on future Cygnus missions both as part of the extended CRS1 and upcoming CRS2 contracts for cargo resupply of the orbital lab.

Northrop Grumman has the option to repeat the test on the following flight of Cygnus later this year.

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