How to dock and undock with the ISS

Written By Magnum

The International Space Station (ISS) is the world’s biggest space station and has been continuously inhabited since the year 2000. Each year, more than 10 spacecrafts, carrying cargo and crew, reach, dock and undock with the ISS. Here we’re going to explore everything about that procedure, from the moment they’re launched to the moment they dock and, subsequently, undock.

Launch

Crew-9 lifting off from SLC-40. Credit: SpaceX

The first step is obviously the launch, but there are some constraints nonetheless: the spaceport must be within 51.6 degrees N and 51.6 degrees S of latitude; this is because the ISS’ orbital inclination is 51.6 degrees so it means that it crosses every latitude up to, and including, these latitudes; a spacecraft launching from a spaceport outside of this range would need an incredible amount of fuel to reach the ISS. The spaceports used to launch spacecrafts towards the ISS are the Kennedy Space Center or Cape Canaveral Space Force Station, both at 28.4 degrees N, and the Baikonur Cosmodrome at 46.5 degrees N. Then, the launch window must be instantaneous to allow the best use of orbital mechanics to reach the ISS: this means there’s only a roughly 1-second interval during which the spacecraft has to launch in order to reach the ISS. Once these criteria are met, the rocket launches and puts the spacecraft into an initial parking orbit, with separation shortly after the last stage engine cut off. The rocket can be a Falcon 9, Soyuz 2.1a, or in the future, Vulcan and Antares 330. While the launch phase is basically the same for every spacecraft, the approach and docking procedures differ slightly given the spacecraft, as we’re going to see.


Approach and docking

Crew-2 docking. Credit: SpaceX

Dragon, Cygnus, and Starliner

The American spacecrafts Dragon, Cygnus, and Starliner follow similar docking maneuvers and burns, but only Dragon’s specific timeline is publicly known, which means the following burns and maneuvers are performed by Dragon.

  • Separation and nosecone opening: after being put into an initial parking orbit, the Dragon spacecraft (which can be both crewed or cargo) separates from the second stage just a few minutes after SECO (Secondary Engine Cut-Off) and is checked out by the crew onboard or by the ground teams; a few minutes after separation, the nosecone opens, exposing the forward bulkhead’s Draco thrusters (there are 16 Draco thrusters in total, 4 of which are in the nosecone). The checks continue in preparation for the first of a few major burns.

  • Phase burn: this is the first major burn conducted by the Dragon spacecraft: the thrusters fire for several minutes at the apogee of the first orbit to raise the perigee of the orbit. Several hours later, a Phase Adjust Burn is conducted: a roughly 30-s burn that adjusts the orbital parameters in preparation for the next burn.

  • Boost burn: it’s conducted several hours after the Phase Adjust Burn and it lasts about 2.5 minutes. This important maneuver brings the apogee of the orbit 10 km lower than the ISS’ orbit and the perigee 20-180 km lower than the ISS’ orbit.

  • Close coelliptic burn: less than an hour from the Boost Burn, the Close Burn is conducted, which is a long (about 10 minutes in the case of Crew-8) burn that circularizes the orbit so that Dragon will continue to orbit about 10 km lower than the ISS’ orbit.

  • Transfer burn: this burn lasts less than 1 minute and it brings the apogee of the orbit just 2.5 km below the ISS’ orbit.

  • Final coelliptic burn: this 30s burn is conducted roughly 20-80 km in horizontal distance from the ISS, and it circularizes the orbit to 2.5 km below the ISS’. At this point, the spacecraft is just a few hours from docking. The spacecraft also conducts the Approach OOP (Out-Of Plain) burn, which adjusts the trajectory.

  • Approach Initiation: this is the last major burn, and it requires a GO/NO-GO poll before its execution: it lasts one and a half minutes and is conducted at 7.5 km from the ISS (horizontally), to put Dragon in a trajectory for docking.

    After these, there aren’t any major burns as the capsule gets closer to the ISS, but the RCS are used for control in short firings. So now the capsule is in course with the ISS, and a few checkpoints need to be passed:

  • Approach Ellipsoid: this is an imaginary 3D-ellipsoid that measures 4X2X2 km from the ISS’ center of mass. Before receiving permission to enter this checkpoint, Dragon needs to be on the so-called “24-hour safe trajectory”, meaning that if the spacecraft lost all of its thrusters and thrust control, it’d take 24 hours before it entered the Approach Ellipsoid. After entering the AE, Dragon can also perform the AI-Midcourse Burn, to adjust its trajectory if needed.

  • Waypoint 0: Waypoint 0 (or WP-0) is a checkpoint that is at a distance of 400 m from the ISS. Dragon can hold here if necessary, but if all systems check out, it can go without stopping. From WP-0 it starts shifting around the station to position itself for the next checkpoint. Then, a GO/NO-GO poll is conducted for the approach to WP-1.

  • Waypoint 1: WP–1 is a checkpoint at 220 m of distance from the ISS, and it’s located exactly in front of the docking port. Here, Dragon aligns so that it’ll just need to approach the docking port. Then, a GO/NO-GO poll is conducted for the approach to WP-2.

  • Keep Out Sphere: this is the second safety zone around the ISS, after the AE, and it’s a sphere 200 m in diameter from the center of mass of the ISS. If Dragon were to lose all its thrusters, at this point, it would take 6 hours before the spacecraft came in contact with the ISS. 

  • Waypoint 2: WP-2 is only at 20 m from the ISS, and nothing much happens here except the Dragon capsule can hold here if needed. As the capsule gets closer, at less than 5 meters of distance and about 25 seconds from docking, there’s the CHOP (Crew Hands Off Point): if an abort is needed from now on, the capsule will take care of that.

  • Soft contact: at this point, the capsule’s soft capture ring (made of electromechanical actuators) enters and makes contact with the IDA (International Docking Adapter), which is the universal adapter for American visiting vehicles to the ISS. Soft capture means that the active soft capture ring on Dragon is pushed and intersected with the passive soft capture ring on the IDA; latches attach for soft capture, so that the spacecraft can align its hooks for the Hard Capture: here, Dragon’s actuators retract bringing the capsule and 12 hooks closer, where these attach, so that the capsule firmly secures itself to the ISS, about 10 minutes after the soft capture. Now the capsule is docked!




Approach sequence of Crew-8. Credit: SpaceX

CRS-21 docking sequence. Credit: SpaceX

Soyuz and Progress
Soyuz and Progress, Russia’s crew and cargo spacecraft respectively, follow different trajectories from the American spacecraft: they usually take just 6 hours from launch to docking, but some have taken as little as 3 hours! Why? Because they’re usually launched into an orbit that makes a fast approach possible and makes the best use of orbital mechanics. Both Soyuz and Progress have an automated system named Kurs, which is an autonomous docking system that allows for remote control from the ISS if needed, something the Cygnus doesn’t have. Soyuz and Progress don’t have the IDA, but they use a Russian universal version called the SSVP (Sistema Stykovki i Vnutrennego Perekhoda, or System for Docking and Internal Transfer): it has 2 components called an active probe, on the spacecraft, and a passive drogue, on the ISS: during docking, the probe enters the drogue(which is of conical shape) and once it’s in the narrow part of the cone, it’s softly captured by latches; these latches are then retracted using electrically driven motors as 8 hooks extend and make a hard capture. 



Undocking
After the spacecraft has completed its stay, it has to undock. After all the pre-undocking procedures, such as loading of cargo and eventual crew, hatch closing and pressurization, the undocking procedure can start, and we’ll start by analyzing Dragon: so, first of all, the umbilicals that supplied power during Dragon’s stay, disconnect, and then the 12 hooks retract leaving the capsule in a soft capture-like state. After the latches are disconnected as well, Dragon uses the 12 Draco thrusters on its body to execute “Burn Zero”, a short burn that pushes Dragon away from the docking port, eliminating any “stiction” between Dragon and the IDA. Just after that, another burn called “Departure Burn One” is executed to increase the relative velocity between Dragon and ISS and therefore increase their distance. Dragon can also execute short firings of its thrusters to adjust its trajectory; after that, it takes about 20 minutes for Dragon to exit the Approach Ellipsoid and be in the so-called free-drift trajectory, ready for its deorbit burn, which can take place a few hours later or more than a day later, ready to bring crew or cargo home. Soyuz performs a similar thing: the umbilicals and hooks disconnect, then the spacecraft is pushed away from the docking port at 0.12-0.15 m/s by pushers instead of thrusters, and then it performs 2 short burns to maneuver itself away from the ISS: the first burn in particular is conducted to position the spacecraft in the correct inclination for the second burn, which lasts 15 seconds and increases the Soyuz’s relative speed to the ISS to 33 m/s.  Soyuz’s journey takes just a few hours, usually 3-6, so less than most Dragon returns. Progress and Cygnus’ undockings are a little bit different because they’re not supposed to survive reentry in the atmosphere: Progress autonomously undocks and then uses its thrusters to move farther from the ISS, while the Cygnus does the same thing but needs to be undocked with the help of the CanadArm2. Then they both perform deorbit burn and burn up during reentry in the atmosphere.


What about the future?
Currently, the spacecrafts that visit the ISS are Dragon, Starliner, Cygnus, Soyuz, and Progress: Dragon can be launched as a cargo or crew capsule, and it’s the only one that carries regularly crew to the ISS; Starliner has also carried crew during its CFT and is designed to carry crew alternating with Dragon, although that won’t be a reality until 2026 at least due to its problematic CFT. Soyuz is another spacecraft that carries crew, while Cygnus and Progress only carry cargo. In the future, we’re going to see many of these spacecraft launch multiple times a year (except Starliner since its fate is uncertain), but we’re also going to see new spacecraft dock with the ISS! The best example is Dream Chaser, a spaceplane under development from Sierra Space that aims to carry cargo and maybe, in the future, even crew to the ISS. But for now the plans are just cargo, and it aims to debut in May 2025 launching aboard a Vulcan rocket. Many spacecrafts are in development for the future, but none of these have plans of carrying cargo or crew to the ISS, but rather to commercial space stations. And since future space stations will use the IDA, every spacecraft with that design will be able to dock with them!

Render of Dream Chaser docked to ISS. Credit: Sierra Space Corporation

Magnum
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