Dragon is the Latest, and Final, Craft to Reboost ISS
The International Space Station has been in orbit around the Earth, at least in some form, since November of 1998 — but not without help. In the vacuum of space, an object in orbit can generally be counted on to remain zipping around more or less forever, but the Station is low enough to experience a bit of atmospheric drag. It isn’t much, but it saps enough velocity from the Station that without regular “reboosts” to speed it back up , the orbiting complex would eventually come crashing down.
Naturally, the United States and Russia were aware of this when they set out to assemble the Station. That’s why early core modules such as Zarya and Zvezda came equipped with thrusters that could be used to not only rotate the complex about all axes, but accelerate it to counteract the impact of drag. Eventually the thrusters on Zarya were disabled, and its propellant tanks were plumbed into Zvezda’s fuel system to provide additional capacity.
Visiting spacecraft attached to the Russian side of the ISS can transfer propellant into these combined tanks, and they’ve 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, notes this as one of the most significant examples of practical propellant transfer between orbital vehicles, with more than 40,000 kgs of propellants pumped into the ISS as of 2019.
But while the thrusters on Zvezda are still available for use, it turns out there’s 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.
Earlier this month, 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.
Reboosting the Russian Way
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 “rear” of the Station given its normal flight orientation, but puts the craft as close as possible to the Station’s own thrusters. This makes it relatively easy to compute the necessary parameters for the thruster burn.
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’s 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.
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.
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 the design of the Orion spacecraft’s Service Module is based on the ATV.
American Muscle
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’s normal orientation would have the American docking ports facing fowards. Of course, there’s really no such thing as up or down in space, so this maneuver doesn’t impact the astronauts’ 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.
As described in the “AUTO REBOOST” section of the STS-129 Orbit Operations Checklist, the Shuttle’s 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’s should be at for the Shuttle’s maneuvering thrusters to line up properly.
With the retirement of the Space Shuttle in 2011, maintaining the Station’s 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.
After performing a second reboost in 2022, the Cygnus spacecraft was retired. It’s replacement, the upgraded Cygnus XL — is currently scheduled to launch its first mission to the ISS no earlier than September 14th.
Preparing for the Final Push
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 “Draco” 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’s own orbit are essentially pressed up against the structure of the ISS, and obviously can’t be activated.
To make reboosting with the Dragon possible, SpaceX added additional propellant tanks and a pair of rear-firing Draco thrusters within the spacecraft’s un-pressurized “trunk” module. This hollow structure is usually empty, but occasionally will hold large or bulky cargo that can’t fit inside the spacecraft itself. It’s 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) and the roll-out solar panels.
While the ability to have the Dragon raise the orbit of the International Space Station obviously has value to NASA, the implications of this experiment go a bit farther.
SpaceX has already been awarded the contract to develop and operate the “Deorbit Vehicle” which will ultimately be used to slow down the ISS and put it on a targeted reentry trajectory sometime after 2030. Now that the company has demonstrated the ability to add additional thrusters and propellant to a standard Dragon spacecraft via a module installed in the trunk, it’s likely that the Deorbit Vehicle will take a similar form.
So while the development of this new capability is exciting from an operational standpoint, especially given deteriorating relations with Russia, it’s also a reminder that the orbiting laboratory is entering its final days.