Why did the Axiom-4 mission want 28 hours to achieve the ISS?

At 12.01 pm IST on June 25, a crew capsule containing 4 astronauts, together with India’s Group Capt. Shubhanshu Shukla on his first spaceflight, lifted off atop a Falcon 9 rocket from NASA’s spaceport in Florida.

The lift-off marked the beginning of the long-awaited Axiom-4 mission.

The launch was clean. By the point the rocket’s two phases had accomplished their work, the crew capsule — referred to as Dragon — was travelling at a number of thousand kilometres per hour.

The capsule’s vacation spot was the Worldwide Area Station (ISS), which orbits the earth at roughly 400 km above sea degree. On the time of lift-off, Axiom Area, the corporate orchestrating this mission, mentioned Dragon would dock with the ISS in 28 hours.

Why does a spacecraft travelling so quick want 28 hours to achieve a spot that’s little greater than the gap between Chennai and Bengaluru away?

It helps to image the capsule and the ISS as two race-cars on separate lanes of the identical observe reasonably than as a automobile attempting to drive straight as much as a spot 400 km forward. The whole lot occurs sideways across the earth, and the choreography is dictated by orbital mechanics and strict security guidelines.

To share the ISS’s lane, the capsule wanted to match each its altitude and its velocity vector. This was achieved utilizing uncooked pace in addition to timing.

Going straight up 400 km would depart Dragon with nearly zero sideways pace, inflicting it to fall again virtually instantly — like a ball that has been thrown up.

The Falcon 9 rocket vaulted Dragon right into a low, barely elliptical parking orbit about 200 km excessive. Its pace there was round 27,000 km/hr, which the capsule maintained simply to keep away from spiralling again down in direction of the bottom.

The ISS can be larger than this parking orbit, at round 400 km, and subsequently circles the earth somewhat extra slowly to keep away from spiralling down. Whereas the ISS takes round 92 minutes to go across the earth as soon as, Dragon began by taking round 88 minutes. Thus, by being decrease than the ISS, Dragon slowly fell behind in its orbit till it had caught up with the ISS.

After its system check-outs, the Dragon capsule carried out a sequence of small thruster burns, a.ok.a. phasing burns, to first increase its apogee (the purpose of its elliptical orbit farthest from the earth) after which its entire orbit.

Every of those burns was timed such that after finishing a number of orbits, Dragon ended up being precisely the place the ISS can be. This was considerably like merging right into a freeway on the proper exit reasonably than steering straight in direction of one other automobile.

For the Axiom-4 mission, the mission planners designed a pre-docking profile that lasted about 28 hours, or about 18 orbits. On the finish of this profile, the Dragon crew capsule was on the proper aspect of the ISS, the place a free docking port was positioned, and at a time when the ISS crew was awake.

As soon as Dragon got here inside 30 km or so of the ISS, it moved itself right into a ‘hall’ aligned with the ISS. From right here, the foundations required the capsule to maneuver at no various metres per second. As soon as it was inside 20 m of the ISS, Dragon was to gradual to a couple centimetres per second.

There are holding factors at 400 m, 220 m, 20 m, and 1 m from the ISS, the place the Dragon crew, together with Group Capt. Shukla — who’s the designated mission pilot — would have needed to carry out go/no-go polls and LIDAR checks. Even when there had been one incorrect sensor studying on this course of, Dragon would have been required to retreat from the ISS alongside a pre-programmed path.

This deliberate pacing alone added a number of hours to the crew capsule’s method in direction of the ISS.

The truth that Axiom Area and NASA used SpaceX’s Dragon crew capsule for this mission can be related. It allowed the mission operators to go for a extra conservative, fuel-rich profile that additionally allowed the crew to complete check-outs, eat, and sleep earlier than beginning the extreme docking train.