Only a few weeks ago NASA announced its decision to pick SpaceX to provide a spacecraft capable of deorbiting the International Space Station. However, considering both the unique requirements as well as the necessary power to complete such an important task, at the time it wasn’t clear what vehicle design the company would use.
That was until today when we were provided both graphics along with new details about its design. Here I will go more in-depth into this vehicle’s design, what we know so far, its upgrades from a standard Dragon spacecraft, and more.
New Deorbit Vehicle
Earlier today NASA held a nearly hour-and-a-half-long media teleconference providing updates on the deorbit plan, vehicle, selection, etc. In addition to this, SpaceX posted some information and even graphics. For example, they tweeted saying, “With 6x more propellant and 4x the power of today’s Dragon spacecraft, SpaceX was selected to design and develop the U.S. Deorbit Vehicle for a precise, controlled deorbit of the Space Station.” With this, they included a render of what the vehicle is expected to look like docked to the ISS. This tweet alone has quite a lot to unpack.
First off, the comment about 6 times the propellant and 4 times the power of today’s Dragon spacecraft is very significant. When they were officially picked many thought that SpaceX would upgrade a Dragon as more power would be needed thanks to the size of the station along with the duration of its stay. This now has been confirmed with a vehicle that in many ways resembles a Dragon spacecraft but with some important upgrades. Looking at the image some things that stand out include its extended length, large solar panels, and a significant number of thrusters at the back of the spacecraft.
In the teleconference, they provided more context into exactly what the vehicle design includes. In a quote, the manager of NASA’s International Space Station Program said “The USDB (US Deorbit Vehicle) has such a critical function for us, obviously its got to continue to perform critical burns, even if it encounters anomalies. So we require high reliability and a two-fault tolerance approach to the vehicle design. So in the RFP one of the things we asked for is maximizing the use of heritage flight-proven hardware so we could increase the reliability.” She then went on to say, “Of course, there is no existing vehicle that meets the high propulsive needs of the USDB. But we certainly can leverage hardware and systems that have been flown and tested in space already. The SpaceX concept leverages the Dragon vehicle but about half of the vehicle will be new design new development. And 100% of the new deorbit functionality is new to the spacecraft” she said. With this in mind, it’s clear that the company will be using as much hardware as they can from the Dragon, however, at the end of the day, a lot of the capabilities are brand new and as such, will require new hardware from SpaceX.
In regards to timing and when this vehicle will be ready, during the media teleconference they were quoted saying, “Ideally we would have the US Deorbit vehicle delivered well in advance of the station’s end of life as Ken mentioned. So the contract has what we call a dwell in storage requirement in it that allows us to deliver the vehicle early and then just store and do just periodic maintenance on it until we are ready to launch it. When we do make the decision to deorbit the station we’ll launch the USDB about 1 and a half years before the final reentry burn. We’ll dock it to the Node 2 Forward Port. Well do a series of checkouts, and then once we’re convinced that everything looks healthy and we’re ready, we’ll allow the ISS to be lowered down and begin drifting down. It takes a while to drift down, we think that’ll be about a year to a year and a half in total of drifting down” they said. This is an interesting comment as it means this vehicle will be in space for around 3 years. This definitely has impacts on its design and factors such as power generation and necessary propellant.
Going even further in-depth, Sarah Walker, the director of Dragon Mission Management at SpaceX said, “So the concept of operations for the deorbit vehicle is incredibly complex as Dana described. It requires substantial development to build a vehicle capable of such a mission. So it will ultimately take driving control of the International Space Station so to speak and propel the entire station into a precise deorbit trajectory that terminates in an unpopulated ocean area. So that any elements that could survive atmospheric reentry pose no risk to the public. So to achieve this the deorbit vehicle will need 6 times the usable propellant and 3 to 4 times the power generation and storage of today’s Dragon spacecraft, just for scale. It needs enough fuel on board not just to complete the primary mission but also to operate on orbit in partnership with the space station for about 18 months. She then clarified, “First the deorbit vehicle will perform orbit shaping burns to put the station in the low elliptical orbit, and then eventually it will perform a final reentry burn to lower the perigee to intersect with Earth at the intended location. So the thing that I think is most complex and challenging is that this burn must be powerful enough to fly the entire space station, all the while resisting the torques and forces caused by increasing atmospheric drag on the space station to ensure that it ultimately terminates in the intended location” she said.
In reference to the specific design she mentions, “So the vehicle design will build upon SpaceX’s operational Dragon Cargo spacecraft with an enhanced trunk section that will host propellant tanks, engines, avionics, power generation, and thermal hardware tailored to complete this mission. So almost a spacecraft in and of itself attached there as a new trunk” she said. She finished by highlighting that while there are a lot of new features, they are going to be able to use a lot of certified technology such as docking adapters and even the thrusters used.
This marked the end of the main teleconference portion and it was opened up to questions. The first question asked about the image provided by SapceX and the thruster at the back. Sarah Walker replied, “There are 46 Draco engines total, 16 are on the capsule already for attitude control, and I think 30 for the Delta-V maneuvers in the trunk. Quite a few of those aft-facing Dracos are firing at the same time for that final deorbit burn, somewhere between 22 and 26 of them will be firing at a given time to deliver about 10,000 Newtons of thrust” she said.
All of these comments and answers together help put in perspective the size and power of this upgraded vehicle. Like mentioned in the teleconference, the deorbit vehicle will almost be two spacecraft in one. With the retirement of the station planned for 2030 and the mission goal of launching the spacecraft 1 and a half years early, it should be ready by around 2028. In reality, SpaceX will probably get it done sooner providing NASA more options as the time approaches.
One of the big challenges will be keeping the spacecraft intact during the deorbit. The International Space Station is primarily made up of a combination of truss elements, modules, solar arrays, and radiators. The truss acts as the backbone of the station, providing physical support for the solar arrays, radiators, and modules. The various modules provide pressurized volume for the many microgravity experiments, a habitable area for crew, and ports for visiting spacecraft to dock and undock. The solar arrays and radiators provide power generation and thermal control for station hardware. Related to the burn itself, NASA said, “Based on behavior observed during the re-entry of other large structures such as Mir and Skylab, NASA engineers expect breakup to occur as a sequence of three events: solar array and radiator separation first, followed by breakup and separation of intact modules and the truss segment, and finally individual module fragmentation and loss of structural integrity of the truss.”
At that point, as the debris continues to re-enter the atmosphere, the external skin of the modules is expected to melt away and expose internal hardware to rapid heating and melting. Most station hardware is expected to burn up or vaporize during the intense heating associated with atmospheric re-entry, whereas some denser or heat-resistant components like truss sections are expected to survive re-entry and splash down within an uninhabited region of the ocean. Thanks to the size of the station, some parts are expected to make it through reentry which is why a controlled deorbit is so important.
While SpaceX’s deorbit vehicle will be needed, NASA wants to naturally lower the station over time as well. The chosen approach for safe decommissioning is a combination of natural orbital decay, intentionally lowering the altitude of the station, and then execution of a re-entry maneuver for final targeting and to control the debris footprint. When first announced they highlighted that “This final maneuver is expected to require a new or modified spacecraft using a large amount of propellant.”
Due to the high propellant requirement of this final maneuver, the Earth’s natural atmospheric drag will be used as much as possible to lower the station’s altitude while setting up deorbit. Once all crew has safely returned to Earth, and after performing small maneuvers to line up the final target ground track and debris footprint over an uninhabited region of the ocean, space station operators will command a large re-entry burn, providing the final push to ensure safe atmospheric entry into the target footprint.
Conclusion
We now have a good idea of the vehicle SpaceX is building that will deorbit the International Space Station. Thanks to the size of the station, SpaceX is building a much larger spacecraft than normal with 46 thrusters. We will have to wait and see how it progresses and the impact it has on the space industry.