Soyuz – Docking Onto The ISS

space station
Image Credit – NASA / International Space Station after undocking of STS-132(Inter)

The International Space Station (ISS) is the largest single structure humans have ever sent out into space. It is the world’s largest international cooperative programme in science and technology till date. It was built by sending parts–one by one–on separate missions, and connecting them together in space. Building the ISS was not the only difficult part. To transport humans safely to the space station, and back, was a mighty task as well.

After the retirement of the Space Shuttle program, the Soyuz spacecraft has single-handedly shouldered the responsibility of carrying humans to the ISS and back until recent developments.

You may be wondering– How exactly does the Soyuz accomplish this? How does this gigantic 6800 kg machine track down and attach itself to another machine the size of a football field, travelling at 27000 km/h in an orbit 420 km above the Earth’s surface?

The answer lies in a complicated series of orbital manoeuvres and engineering feats that are a testament to human intelligence.

Like all human expeditions in history, this journey, too, begins on land. The Soyuz spacecraft is launched into space from the Baikonur Cosmodrome in Kazakhstan. The latest version of the spacecraft called Soyuz MS (MS- Modified Systems) has three Modules:

I) The Service Module carries all the life support systems, Solar panels, steering engines etc.

II) The Orbital module, as the name suggests, is where the crew stays, while in orbit.

III) The Descent Module, equipped with heat shields and a parachute, facilitates return from the ISS, back to the Earth.

esa soyuz
Image Credit – ESA

The Soyuz Capsule is mounted atop the three stage Soyuz FG Rocket that will carry the capsule into orbit around the Earth.

With all preparations in order, the astronauts / cosmonauts enter the Orbital Module (tip of the spacecraft) through a side hatch and prepare for launch. During the ascent, the Orbital Module is sealed off from the Descent Module with the inner latch.

rocket launch
Image Credit – NASA

The first stage of the rocket consists of four boosters that aid the lift off. They burn for about two minutes after which, they are jettisoned (i. e.- separated) and eventually, the rest of the stages take over to continue the journey upward. Only about nine and a half minutes into flight and the Soyuz leaves behind the atmosphere to reach space.

The third stage of the Rocket is cut off at this point and the crew is no longer in powered flight. The rocket carries the spacecraft into a safe orbit, called the Insertion Orbit, where it wouldn’t fall back down to the Earth.

The real challenge comes in now. The Insertion Orbit lies at an altitude of about 220 kms from the surface of the Earth, while the ISS orbits at an altitude of 420 kms. Now, the Soyuz has to first manoeuvre to higher altitudes to align itself with the ISS and then begin the docking procedure.

However, there’s a catch. The exact altitude of the Insertion orbit cannot be calculated in advance due to the unpredictability of the rocket launch. That’s why, the the task of reaching the ISS is divided into three stages:

  • 1. Hohmann Transfer –

To account for the unpredictability of the exact altitude of the Insertion Orbit and to decrease the phasing angle (angle between the spacecraft and the ISS in their respective orbits), the spacecraft is first manoeuvred into an intermediate orbit called the Phasing Orbit.

The Phasing Orbit is located in between the ISS Orbit and the Insertion Orbit, about 100 kms below the ISS orbit. The transfer to the Phasing Orbit from the Insertion Orbit is done using an orbital manoeuvre called the Hohmann Transfer.

Named after the German scientist Walter Hohmann, the Hohmann transfer is used to transport a satellite or a spacecraft from one circular orbit into another circular orbit in the same plane. Since the process requires only two burns, it is considered the most fuel efficient way to move a spacecraft. It is a fairly slow process and is used mostly for transferring spacecraft covering shorter distances.

To enter the outer orbit, the spacecraft needs to speed up. The engines are fired once with accuracy to speed up the spacecraft enough to achieve an elliptical orbit. This elliptical orbit, called the Hohmann Transfer Orbit, has its perigee at the inner orbit and an apogee at the outer orbit.

Once the spacecraft reaches the apogee, the engines are fired once more, to speed up the spacecraft and change its trajectory to enter the Phasing Orbit from the Transfer Orbit. A few correction burns may be performed to achieve the precise speed and the altitude of the Phasing Orbit.

phasing orbit
Image Credit –  Wikipedia Commons (1. Blue- Insertion Orbit, 2. Yellow- Hohmann Transfer Orbit, 3. Red- Phasing Orbit)
  • 2. Bi-Elliptic Transfer –

Now that the Soyuz is in the Phasing Orbit, the next course of action is to enter the ISS Orbit and finally rendezvous with the ISS. The Phasing Orbit is smaller than the ISS orbit. So even though the speed of the spacecraft is less than the speed of the ISS, the distance it has to travel to go around the Earth is shorter. This means that the spacecraft is orbiting the Earth faster than the ISS.

The crew has to wait for the ISS to be in the correct position, with respect to the spacecraft, before making a transfer to the ISS Orbit from the Phasing Orbit. However, this could take a while and so, until the right moment, the Soyuz continues to drift in the Phasing Orbit. For this reason, this orbit is often dubbed as the Parking Orbit. During this phase, the crew can get out of their suits and perform routine checkups as well as enjoy some leisure time to themselves.

When the ISS is finally in the ideal position, an orbital manoeuvre called Bi-elliptic transfer is performed by the spacecraft to enter the ISS Orbit. This transfer consists of two half ellipses and requires three burns, instead of two.

A Hohmann transfer can also be used to shuffle between orbits in this stage. In fact, this method was previously in use for older missions. However, to increase efficiency, the Soyuz now uses a Bi-elliptic Transfer instead.

Image Credit – Wikipedia Commons (1. Blue- Phasing Orbit, 2. Green – First-half Elliptical Orbit, & Yellow- Second-half Elliptical Orbit, 3. Red- ISS Orbit)

The first burn takes the Soyuz into an elliptical orbit, which has its perigee at the inner orbit (Phasing Orbit). The apogee of the ellipse is much farther away than the ISS Orbit. When the spacecraft reaches the apogee, the second burn is performed, which takes the Soyuz into another elliptical orbit which has its perigee at the ISS Orbit.

The spacecraft now travels along this elliptical orbit until it reaches the perigee. This is when the third and final burn is performed to put the Soyuz in the ISS orbit. This transfer is planned meticulously so that when the Soyuz enters the ISS orbit, it will be right in front of the ISS.

  • 3. Docking –

Now at last, the Soyuz is in the same orbit as the ISS, and at the right speed, thanks to the Bi-elliptic transfer. However, it still has to align itself properly or the entire trip could end in a total disaster.

Once the Soyuz is in the vicinity of the Space Station, an automated docking system called KURS takes over and begins the docking procedure. However, this doesn’t mean that the crew can relax now. Instead, they have to continuously monitor the process and ensure that the computerised operations are running smoothly. Incase something goes wrong with the automated system, the crew has to take over and manually dock the Soyuz, using the control system aboard.

The relative speed of the Soyuz continues to decrease in the final stage. To get the alignment right, the Soyuz performs a two-step fly-around the ISS, which decreases the distance between the spacecraft and ISS, and then aligns the spacecraft with the correct docking port.

Now, everything is finally in order for the final approach to begin. The probe of the spacecraft touches the entrance cone of the port and the engines give one final thrust to ensure the probe has entered the docking port. As the Soyuz gets closer to the port, its probe retracts and eight mechanical hooks secure the spacecraft to the space station.

With the docking procedure over, the spacecraft is checked for leaks and the pressure inside the Soyuz is equalized with the pressure of the Space Station. Now, the crew can finally open the hatch and begin their life aboard the ISS.

soyuz iss docking
Image Credits- NASA / Soyuz Approaching ISS for Docking

As if the entire procedure wasn’t complex enough, it is further complicated by various factors along the way.

For eg. – The rendezvous point with the ISS should be where the ISS is illuminated by the sunlight so the crew can see the space station and ensure smooth docking. However, the sun should not be at an angle where the sunlight proves to be blinding to the crew.

Despite facing such odds, the Soyuz has been highly successful in its missions. The Soyuz program is the longest operational human spacecraft program in the history of space exploration. Along with transferring crew members, it also serves as a means to transport food and water to the Space Station. Over the years, it has proved to be crucial in the development of human space exploration. Years of research, experimentation and efforts of countless human beings have made this ambitious feat a reality.

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