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<\/a>An early image of ISS, Zarya module in center and Zvezda at far right.<\/p>\nVisiting spacecraft attached to the Russian side of the ISS can transfer propellant into these combined tanks, and they\u2019ve been topped off regularly over the years. In fact, the NASA paper A Review of In-Space Propellant Transfer Capabilities and Challenges for Missions Involving Propellant Resupply<\/a>, notes this as one of the most significant examples of practical propellant transfer between orbital vehicles<\/a>, with more than 40,000 kgs of propellants pumped into the ISS as of 2019.<\/p>\n But while the thrusters on Zvezda are still available for use, it turns out there\u2019s an easier way to accelerate the Station; visiting spacecraft can literally push the orbital complex with their own maneuvering thrusters. Of course this is somewhat easier said than done, and not all vehicles have been able to accomplish the feat, but over the decades several craft have taken on the burden of lifting the ISS into a higher orbit.<\/p>\n Earlier this month<\/a>, a specially modified SpaceX Cargo Dragon became the newest addition to the list of spacecraft that can perform a reboost. The craft will boost the Station several times over the rest of the year, which will provide valuable data for when it comes time to reverse the process and de-orbit the ISS in the future.<\/p>\n Reboosting the Russian Way<\/p>\n By far the easiest way for a visiting spacecraft to reboost the ISS is to dock with the rear of the Zvezda module. This not only places the docked spacecraft at what would be considered the \u201crear\u201d of the Station given its normal flight orientation, but puts the craft as close as possible to the Station\u2019s own thrusters. This makes it relatively easy to compute the necessary parameters for the thruster burn.<\/p>\n Historically, reboosts from this position have been performed by the Russian Progress spacecraft. Introduced in 1978, Progress is essentially an uncrewed version of the Soyuz spacecraft, and like most of Russia\u2019s space hardware, has received various upgrades and changes over the decades. Progress vehicles are designed specifically for serving long-duration space stations, and were used to bring food, water, propellants, and cargo to the Salyut and Mir stations long before the ISS was even on the drawing board.<\/p>\n Reboosts could also be performed by the Automated Transfer Vehicle (ATV). Built by the European Space Agency (ESA), the ATV was essentially the European counterpart to Progress, and flew similar resupply missions. The ATV had considerably greater cargo capacity, with the ability to bring approximately 7,500 kg of materials to the ISS compared to 2,400 kg for Progress.<\/p>\n Only five ATVs were flown, from 2008 to 2014. There were several proposals to build more ATVs, including modified versions that could potentially even carry crew. None of these versions ever materialized, although it should be noted that\u00a0 the design of the Orion spacecraft\u2019s Service Module is based on the ATV.<\/p>\n American Muscle<\/p>\n Reboosting the ISS from the American side of the Station is possible, but involves a bit more work. For one thing, the entire Station needs to flip over, as the complex\u2019s normal orientation would have the American docking ports facing fowards. Of course, there\u2019s really no such thing as up or down in space, so this maneuver doesn\u2019t impact the astronauts\u2019 work. There are however various experiments and devices aboard the Station that are designed to point down towards Earth, so this reorientation can still be disruptive.<\/p>\n As described in the \u201cAUTO REBOOST\u201d section of the STS-129 Orbit Operations Checklist<\/a>, the Shuttle\u2019s computer would actually be given control of the maneuvering systems of the ISS so the entire linked structure can be rotated into the correct position. A diagram in the Checklist even shows the approximate angle the vehicle\u2019s should be at for the Shuttle\u2019s maneuvering thrusters to line up properly.<\/p>\n With the retirement of the Space Shuttle in 2011, maintaining the Station\u2019s orbit became the sole domain of the Russians until 2018, when the Cygnus became the first commercial spacecraft to perform a reboost. The cargo spacecraft had a swiveling engine which helped get the direction of thrust aligned, but the Station did still need to rotate to get into the proper position.<\/p>\n After performing a second reboost in 2022, the Cygnus spacecraft was retired. It\u2019s replacement, the upgraded Cygnus XL \u2014 is currently scheduled to launch its first mission to the ISS no earlier than September 14th<\/a>.<\/p>\n Preparing for the Final Push<\/p>\n That brings us to the present day, and the Cargo Dragon. SpaceX had never designed the spacecraft to perform a reboost, and indeed, it would at first seem uniquely unsuited for the task as its \u201cDraco\u201d maneuvering thrusters are actually located on the front and sides of the capsule. When docked, the primary thrusters used for raising and lowering the Dragon\u2019s own orbit are essentially pressed up against the structure of the ISS, and obviously can\u2019t be activated.<\/p>\n To make reboosting with the Dragon possible, SpaceX added additional propellant tanks and a pair of rear-firing Draco thrusters within the spacecraft\u2019s un-pressurized \u201ctrunk\u201d module. This hollow structure is usually empty, but occasionally will hold large or bulky cargo that can\u2019t fit inside the spacecraft itself. It\u2019s also occasionally been used to deliver components destined to be mounted to the outside of the ISS, such as the for the outside of the ISS, such as the International Docking Adapter (IDA)<\/a> and the roll-out solar panels<\/a>.<\/p>\n<\/a>Progress 72 in 2019<\/p>\n<\/a>Depending on the spacecraft, simply flipping the Station over might not be sufficient. In the case of the Space Shuttle, which of the American vehicles performed the most reboost maneuvers by far, the entire complex had to be rotated into just the right position so that the thrusters on the spaceplane would be properly aligned with the Stations\u2019 center of mass.<\/p>\n<\/a>Crew Dragon approaching the ISS, note four Draco thrusters around docking port.<\/p>\n