A clown is entertaining, and a caged lion dancing to the trainer's tune is breathtaking. But the acrobats in a circus are the real stars of the show. When a gymnast builds momentum and leaps from a swing, body streamlined and arms extended, calculating the speed and trajectory and aligning perfectly with their partner's reach, the heart stops.We hold our breath as we watch the acrobat,...
A clown is entertaining, and a caged lion dancing to the trainer's tune is breathtaking. But the acrobats in a circus are the real stars of the show. When a gymnast builds momentum and leaps from a swing, body streamlined and arms extended, calculating the speed and trajectory and aligning perfectly with their partner's reach, the heart stops.
We hold our breath as we watch the acrobat, monetarily poised in the exhilarating abyss of mid-air. She spreads her arms, fully trusting in her partner. The second acrobat bracing, poised high on a platform, leans forward, arms spread like a lifeline, establishing the faultless hold in a dazzling display of synchronisation.
ISRO is preparing to perform such gymnastics in the vastness of space, around 470 kilometres above the earth, when two spacecraft approach close together and fasten together in ISRO's first-ever space docking experiment (SpaDeX).
What is the SpaDeX experiment? Why is it tricky? Why is it crucial for ISRO? Here's all you need to know about this experiment and its incredible space docking technique.
What is space docking?
A train consists of several coaches joined together by an engine. In long-distance mail and express trains, passengers can move from one coach to another through the vestibule, a small space or passageway that connects one coach with another.
Consider connecting two spacecraft carrying humans by a short corridor or passageway that allows astronauts to travel from one to the other. This is called space docking.
The rail coaches are assembled in the rail yard, and the train is ready. However, the crewed spaceship may need to dock with another craft or space station. This can happen only while it is sailing in space. This is space docking, comparable to when a ship arrives at a port and berths.
Unlike a vestibule between two train carriages, the space docking mechanism should be sealed to prevent air from escaping and endure the space vacuum.
Why is it formidable?
Imagine that your friend is ahead on a highway, and you want to catch up and travel beside. You press the accelerator, speed up, and, on arrival, slow down to the same speed as your friend. Now, both of you will travel side by side.
Like highway lanes, orbits are circular or elliptical paths from the earth's surface at a given height. An orbit at 500 km is distinct from an orbit at 500.5 km. A satellite's orbital period is 94.47 minutes at an altitude of 500 km and 94.48 minutes at an altitude of 500.5 km. But the crafts will speed at around 7500 meters per second, and even a fraction of a second will take them far apart.
Suppose the target spacecraft is ahead of you in the same orbit, the same height as your craft, and you fire the rocket to speed up your craft.
Zoom, your craft's speed will indeed increase, but thanks to Kepler's laws, you will no longer be in the same orbit. Your altitude will increase. If you slow down, your orbit will decrease. Therefore, reaching the target craft is not like catching up with a vehicle ahead of you on a highway. It is complicated.
A spacecraft performs a series of precise manoeuvres known as a "rendezvous" to dock with another spacecraft.
First, the chaser approaches the target craft by carefully matching its orbital velocity and position. Then, in complicated steps, the chaser craft uses the small thrusters to align itself to the target craft's docking mechanism and gently slams onto it to create a secure connection.
Chaser craft can be seen with a cylindrical antenna on front-middle of top deck and a mounted sensor on front-middle of bottom deck.
Target can be seen with a mounted sensor on front-middle of top deck and empty space on front-middle of bottom deck.
What ISRO is planning to do?
Like dance moves, the experiment involves executing a series of precise moves.
First step: ISRO will launch two identical microsatellites, chaser (SDX01) and target (SDX02), each weighing 200 kg mass, aboard the PSLV launch vehicle.
Second step: Once the PSLV reaches an altitude of 470 km, the two crafts will be deployed in space one by one. Both crafts will orbit the earth in a circular orbit at 55° inclination.
Step three: Due to the slight difference in the time of separation, one would be moving at a relatively higher velocity than the other. Due to this incremental difference in the velocity, the distance between the target and the chaser crafts will swell to about 10-20 km within a day. Now, the stage is set for the rendezvous.
Step four: Using its propulsion system, the target spacecraft will operate to reduce the relative velocity to zero. Now, both crafts will be orbiting at the same speed, and in the same orbit, but with a gap between them. This is the first step in a series of complex rendezvous manoeuvres. At this time, the inter-craft distance will be approximately 20 km.
Now, the chaser's thrusters will be activated, and a small relative velocity will be introduced. The chaser will approach the target, and the inter-satellite distance will be reduced to 5 km. Once again, the target craft will fire its thrusters to compensate for the difference in velocity of both craft. Now, the relative velocity will be zero.
Courtship dance
Steps five to nine are virtually smiler courting dance moves. In this set of manoeuvres, the inter-craft distance will be gradually lowered from 5 km to 1.5 km, 500 m, 225 m, 15 m, and 3 m. The chaser will use its thrusters to approach the target throughout these activities. The target vessel will engage its propulsion system to adjust for the relative velocity.
When the crafts are at grabbing distance, the chaser will use its thrusters to precisely align itself with the docking port on the target craft, adjusting its position on all three axes. Before fully docking, a gentle contact is made to absorb impact and ensure a secure bond.
In step ten, the docking mechanism will take over when the crafts are just 3 meters apart. A Mechanism Entry Sensor (MES) will be used from 8 cm to 4 cm to detect chaser entry into the target spacecraft during docking. Once the entry is made gently, the crafts will touch each other, and the clamps on each craft will catch and latch each other. The ring-like structure on each craft's docking mechanism must touch each other like a jar and its cap. A set of clamps latch from one craft latch onto the ring of the other to secure the crafts.
The two crafts will perform what would be a celestial hug!
Who will control the crafts?
Both the crafts are uncrewed, as this is a test flight. During the docking manoeuvrer, the ground control will remotely drive the crafts when the craft distance is between 50-5 km. When the relative separation is 5 km to 0.25km, the crafts will use docking sensors such as Laser Range Finder to estimate the distance and trim its delicate movements. Between 300 meters to one-meter relative distance, the Docking Camera will be kicked into action to guide the rendezvous. Additionally, the Visual Camera for real-time imaging will guide during the relative separation of l meter to docking.
Novel technology
The first unmanned space docking was performed by the Soviet Union on October 30, 1967, when the Kosmos 186 and Kosmos 188 spacecraft automatically docked in orbit. This was followed by the first docking of two crewed spacecraft when the Soviet Union's Soyuz 4 and Soyuz 5 spacecraft joined on January 16, 1969. Space docking enabled the Soviet Union to build the world's first space station, Salyut. It was placed in orbit on April 19, 1971, and a three-member crew spent a record-setting 24-day space mission. Soon, the United States and, later, in 2011, China accomplished successful crewed space docking, enabling them to routinely send human space missions and establish space stations. India's ambitions for human space flight, sample return mission to the Moon (Chandrayaan-4) and establishment of Bharatiya Antariksh Station (BAS) hinge on the success of this in-space docking experiment.
The SpaceX experiment is crucial in another aspect. ISRO developed a space docking mechanism called "Dual-Lever Rigidisation Mechanism With A Self-Hold Down Feature For Autonomous Docking Of Spacecraft." This experiment tests not only ISRO's precision manoeuvre capability but also this advanced docking mechanism.
What are the advantages of ISRO’s docking technology
The existing rigid latch system developed by other space agencies uses 24 motor drives and 12 pyrotechnics for individual latches, which adds to the additional mass and reduces the reliability of the overall system due to additional moving components and electrical complexity to power above actuators. The ISRO version uses an ingenious single mechanism, actuated using a single motorised actuator, thereby saving mass, enhancing reliability, amplifying the output torque and reducing launch cost. The novelty of this system is the dual lever rigidification mechanism, which helps the two spacecraft to hold on to each other post docking using a single motorised actuator. Moreover, the mechanism does not have any consumable parts and hence can be used multiple times to dock and undock.
One of the key advantages of this system is that it is scalable to larger and has the minimum number of components, thereby enhancing the mechanism's reliability. When the crafts are about to dock, the contact switch-based telemetry system indicates the position, enabling precision alignment of the crafts. Further, the chaser and target have similar ports, meaning they can both berth for other crafts to dock and a docking mechanism for the craft to berth in different acrafts. The overall structure can be easily tuned to the International Docking System Standard (IDSS) interface, enabling the craft to berth with the International Space Station (ISS) in future.
The capture mechanism of ISRO's docking port
In addition, ISRO has developed "Rendezvous and Docking Algorithms," an AI suit that enables autonomous cruise of the crafts when the inter-craft distance is less than 5 km. This algorithm will also be tested for its performance during this experiment.
Clamps on the docking port holds the other craft tightly.
What next?
The docking will not only enable a portal for humans to move from one craft to another but should also enable the transfer of electrical power from one craft to another. If a craft is standard for want of power, then this will help. Alternatively, one craft may host the solar power plant. The power generated must be distributed to other docked crafts to recharge the batteries. ISRO will perform these tests.
After the docking experiment is successfully completed, the crafts will undock. Like docking, undocking is also an acrobatic exercise. With precise, gentle movements, the crafts will separate and move from each other.
All eyes on the D-day
The year 2024 commenced with ISRO launching the XPOSAT Satellite aboard its PSLV-C58 launch on January 01, 2024 09:10 Hrs IST. On the eve of year end, ISRO is all set to create a milestone, now in the arena of human space flight. If all goes well, the first-ever test of the ISRO's Space Docking mechanism will be put to the test on December 30, 2024. This experiment is imperative for future human space flights if one wants to dock with the International Space Station, construct India's own space station or even for rescue missions in case Indian astronauts are stranded in space due to craft failure.