Launching MISAT into orbit
MISAT would either have a custom launch vehicle developed for itself, or would launch atop a fully reusable rocket such as SpaceX's Starship rocket. After all MISATs have been launched and are in a proper orbit, not many more launches to place new MISATs in orbit would occur, as MISATs are able to refill their propellant tanks in orbit by docking with a nearby MISAT refilling station. Launches would need to occur regularly to refill these stations, but other than that, no new MISATs need to be placed into orbit, unless there is some massive upgrade that requires all current MISATs to de-orbit and new ones to be placed in orbit to replace them.
The term “refilling” is used rather than “refueling” because MISAT refills on not just fuel, but also oxidizer, which is necessary for stable combustion to take place in the near-100% vacuum of outer space. MISAT can remain in orbit, operational, for years without having to be taken back down to Earth, as it can refill its propellant tanks in orbit, and receives electricity from the sun via its two large solar panels. Orbital refueling (with hypergolics) has already been demonstrated on various ISS (International Space Station) missions, so it should not be extremely difficult for MISAT to do the same.
MISAT utilizes hypergolic propellants for its main thrusters. Hypergolic propellants consist of a fuel and an oxidizer, which, when they come into contact, immediately combust. The specific hypergolic propellant that MISAT uses is Monomethyl Hydrazine (as fuel) and Dinitrogen Tetroxide (as oxidizer). These are stored in two large tanks on the walls of MISAT, and are used for the 24 main thrusters on MISAT, which are located at the bottom/rear of the vehicle. These main thrusters are primarily used to adjust the orbital velocity of MISAT to place it on a trajectory where it will intercept a non-functional satellite, when scheduled to.
MISAT Thrusters & Tank Pressurants
The main thrusters/engines are currently pressure-fed. They use standard bell nozzles, and unfortunately not aerospikes (if you know, you know). For propellant atomization (to maximize propellant mixture, therefore maximizing combustion efficiency) in the combustion chamber, each engine uses a like-doublet propellant injector to mix the fuel and oxidizer in the combustion chamber for a stable and efficient combustion to take place. For those unaware, the like-doublet propellant injector is a commonly-used rocket propellant injector design, where you essentially have pairs of jets which, with extreme precision, shoot out liquid fuel and liquid oxidizer, at a calculated angle so that the liquids being shot out of the jets will collide with each other, producing a spray containing a combustible mixture (hypergolic propellant) that fans out across a wide angle in the combustion chamber, delivering good atomization and spreading out the propellant. The most remarkable rocket that used this propellant injector design was the Saturn V - the rocket that landed humans on the Moon in the year 1969. As the MISAT design evolves, the engines may use coaxial swirl injectors instead of like-doublet injectors. In this case, for the main thrusters, combustible hypergolic propellants are used rather than pure nitrogen gas for adjusting orbital velocity because MISAT need something more powerful to efficiently intercept and capture a dead satellite. 8 out of 10 of the Composite Overwrapped Pressure Vessels (COPVs) inside MISAT contain high-pressure nitrogen gas inside, used for accelerating carefully and precisely when capturing a satellite. These Nitrogen COPVs, though containing extremely high-pressure Nitrogen gas, could also be refilled with Nitrogen by a MISAT refueling station - more on that later. The remaining two COPVs store high-pressure helium used for pressurizing the main propellant tanks. Helium is preferred for this as it is less reactive.
MISAT saves much propellant, pressurant, and nitrogen ACS gas by adjusting its attitude/orientation and rotation via internal reaction wheels powered by electricity via the solar panels rather than hypergolic/nitrogen ACS (attitude control system) thrusters. MISAT has separate thrusters for actually moving/propelling itself on the X, Y, and Z axes.
The computer on board this spacecraft, which controls all systems, would actively take note of any problems/difficulties it encounters while attempting to capture a satellite, so that it could send information regarding the issue to humans where they will come up with a solution, send it back to the MISAT satellite, where it will share the solution with all other MISATs in orbit. If this MISAT becomes too far away from Earth (somewhere beyond Earth or Lunar orbit) to reliably broadcast a direct signal to people on Earth or other MISATs, it will send the information through the Gbit Space Network.
MISAT Refilling Stations
Multiple MISAT stations carrying extreme amounts of hypergolic propellant, tank pressurant, and nitrogen gas for MISAT’s attitude control system, would be placed in orbit so that when MISATs are beginning to run low on propellant, they can maneuver and dock to a MISAT station nearby to refill on hypergolic propellant for the main thrusters as well as nitrogen gas for the Nitrogen ACS and ullage thrusters, and Helium for the main propellant tank pressurant. These large stations would be refilled by a reusable launch vehicle carrying a tanker. The tanker will dock to the refilling station, provide it with more propellant, and then return to Earth (or which ever planet it's orbiting) where it will be prepared for its next mission.
In-depth technical details regarding MISAT and its internal systems will come in the future. Lastly, we would like to make it clear that we do not put this project under our "ambitions" list, because we simply don't see this as a super ambitious project/idea. A system like MISAT is something that should be normalized in space and part of everyday space activity.