Matija Milenovic, Co-Founder & CEO of porkchop
As you may know, porkchop is developing a reusable orbital transfer vehicle for last-mile satellite delivery in Low Earth Orbit. It’s reusable not because we bring it back to the ground and launch it again, but because it’s able to perform its mission multiple times over in orbit. This is made possible by being able to rendezvous with other satellites, dock with them, and move them to other orbits.
Rendezvous, proximity operations and docking (RPOD) is nothing new. The first missions involving RPOD were performed in the early days of the space race, and we humans very quickly mastered it. RPOD was used twice on the Apollo moon missions (apart from Apollo 13…) — the first time to re-configure the command and service modules, and the second time to allow the lander crew to rejoin their third crew member in the command module.
The point to be made is that RPOD is nothing new. It’s been used on a regular basis to supply the International Space Station with people, new modules and cargo.
What’s changed, however, is that RPOD is now being used beyond primarily human spaceflight. From in-orbit servicing to reusability of space hardware, RPOD is seeing a rapid resurgence in interest by the space industry.
Docking two satellites is like chess — it’s easy to learn the rules, but difficult to master the game.
Step 1: Put the two satellites into almost the same orbit.
If you have two satellites in space that you want to dock, there’s a high likelihood that they’re not in the same orbit to begin with. They might have different altitudes; one may be in a circular orbit while the other is in an elliptical one (different eccentricities); one may be orbiting around the equator whereas the other is going from the north pole to the south pole (inclination); one satellite may be trailing the other by 180 degrees in the orbit (phase angle); there are some other differences in their orbits which may exist.
As the mission planner, your job is to figure out which manoeuvres you need to execute on one of the satellites in order to bring it closer to the other. I recommend playing Spaceflight Simulator, Kerbal Space Program, or Simple Rockets 2 to quickly and easily get an intuition of how these manoeuvres can be done.
This process is known as orbital phasing and far-range rendezvous. The goal is to get the satellites “close enough” such that one satellite can get a lock on the other in terms of navigation and comms. Once this step is complete, you’re ready to start the close-range rendezvous!
Pro tip: Make sure both satellites orbit the earth in the same direction. Each satellite moves at around 8 km/s. If the satellites are moving in the same direction, this high speed almost cancels out — if the satellites are moving in opposite directions, you’ll have a collision at 16 km/s and have successfully built an anti-satellite weapon. Please don’t do this.
Step 2: Bunny hop
Now that your satellites are close enough, you can start doing smaller, more controlled manoeuvres to get them closer to one another. Without getting too into relative orbital mechanics, as satellites approach one another, the way they move relative to one another becomes somewhat counterintuitive. Braking makes you go faster, whereas moving “up” or “down” relative to your target allows you to hop relative to it.
Also, at this point, the satellites are sufficiently close such that relative navigation can be used to dock them. Relative navigation is where you primarily care about the position, velocity, orientation and rotation speed of one satellite relative to the other, rather than relative to the Earth or some other reference. Relative navigation tends to be more accurate, but needs to have the satellites sufficiently close to each other (see Step 1) in order to be activated.
Relative navigation also tends to be part of a closed loop control system. This is where the satellite is able to automatically perform relative navigation, perform a certain manoeuvre, do mid-course correction manoeuvres, and repeat until a certain goal is reached. Relative navigation is typically done with some combination of RGPS/CDGPS, lasers, LIDAR, radio, and/or computer vision.
Step 3: Dock!
You’ve come this close, but you’re not done until the two satellites are firmly connected.
Sometimes satellites use robotic arms to connect to one another (berthing), but in our case, we want to use docking ports on both satellites.
To dock, the relative navigation systems need to bring the satellites within a range of several centimetres of one another. They continue bringing the satellites closer and closer together, until the two docking ports are able to connect. There are plenty of different types of docking systems, from the gendered ones used in Soyuz (probe & drogue) to the genderless ones used in US vehicles such as Dragon (androgynous).
Following docking, after verifying that the connection has been properly established, the two spacecraft can begin exchanging data, power, fuel, astronauts, or become part of a larger structure.
References worth checking out if you want more technical details:
- Automated Rendezvous and Docking by W. Fehse. I consider this the bible of RPOD, as it starts from basic principles/concepts and explains the whole procedure very well. It’s a bit out-dated and focuses on ISS docking missions in LEO, but the fundamentals haven’t changed.
- I’m reiterating the games I mentioned above: Spaceflight Simulator, Kerbal Space Program, and Simple Rockets 2 are all fantastic as they let you build an intuition for orbital mechanics and manoeurvres. Another fun browser game is on SpaceX’s own website: ISS Docking Simulator.
- In terms of papers, the best ones I’ve seen are ones which go through entire mission concepts. Camille Pirat’s EPFL PhD thesis is great (along with many of his papers).
- The papers about the CPOD mission and AAReST mission are great starting points too.
