Space tankers

The story of the satellite MUOS 5, which was stuck on the way to the target orbit, raised questions about the possibility of refueling satellites in orbit with other special satellites. Let's see where and how physics allows us to refuel satellites, who have been refueling for about forty years, as well as what space refueling projects (and not only) have been and are planned.

A bit of physics

In order to understand who and where you can refuel, let's first see how difficult the task of creating a tanker satellite is. It is logical to assume that such a tanker will have to move between target satellites and consistently refuel them. And then there is the problem of fuel consumption for such movements.



In low orbits, satellites fly in apparent chaos, to move from one satellite to another, you will need to change the orbit altitude and inclination. Now there are good online resources, you can see for yourself how different the orbits of different satellites are:

• The orbit of the apparatus is shown here .
• And here is the trajectory with respect to the rotating Earth.

If you change the height of the orbit relatively cheaply, for example, from 200x200 km it is possible to climb 400x400 by spending



only 115 m / s using the formula , then with the change of inclination everything is very sad. For a circular orbit, a 45 ° inclination change will cost us



11 km / s, more than putting a satellite into orbit. It follows that:

A satellite tanker capable of serving several target satellites only makes sense for constellations of satellites that are in the same plane.

Are there any such groups? Yes there is.



8 satellites in the same plane are located at GPS / GLONASS. In these planes, satellites sometimes have to maneuver to replace those that have failed, but at 20,000 km altitude there is no serious interference, and no fuel is needed to maintain orbit. In the same plane are also all the devices in the geostationary orbit. And then there is a systematically acting disturbance. Because of the impact of the moon, satellites constantly have to spend fuel to maintain the required standing point, and, given the reliability of modern electronic components, it sometimes happens that a working satellite leaves its place and stops bringing money because of the fuel that has ended.

Conclusion: The main target for tanker satellites is the geostationary orbit.

A bit of history

This is little thought, but refueling of objects in space has been successfully used for forty years. True, not satellites, but orbital stations. Starting with the Salyut-6 (launched into orbit in 1977), Soviet / Russian orbital stations are refueling from Progress cargo ships. Orbital stations regularly spend fuel on the rise of the orbit and maneuvers to avoid space debris, so refueling extends their life. But Progresses work as one-time tankers and do not fly to other targets. Similar can be realized for satellites, but here the question arises of the economic feasibility of filling only one target.

As for the refueling of satellites, this technology is at the level of separate experiments. In 2007, according to the Orbital Express program, two specially designed satellites ASTRO and NEXTSat were launched into orbit.



In orbit, ASSTRO got close and docked with NEXTSat. He then poured fuel (hydrazine) into NEXTSat and replaced the special module ORU, which symbolized the satellite's batteries. The mission was successful, it was proposed to use such technologies for military satellites, but there is no information about their use since.



In 2011, the last flight of the shuttle to the ISS brought the experimental stand of the Robotic Refueling Mission, where the technologies of servicing and refueling satellites that were not designed specifically for such refueling were to be worked out. Therefore, on the stand there were special tools for cutting the retaining wire filler neck and unscrewing the covers with seals. Here is a video with animation and ground tests:



In January 2013, the stand was successfully tested on the ISS. The standard disposable filling points, through which the satellites ran on Earth, were opened, and the filling manipulator was successfully connected to them. In August of the same year, additional equipment was delivered to the ISS - new units with satellite valves and necks, as well as a borescope to observe the “satellite repair from the inside”. But this equipment has not yet been tested.

In 2011, Canadian firm MacDonald, Dettwiler and Associates announced the creation of the Space Infrastructure Servicing satellite for the geostationary orbit, but in 2012 the project was frozen due to the lack of potential customers.

Some news

In the summer of 2016, NASA announced the creation of the Restore-L satellite, which in the mid-2020s will have to dock and refuel the Landsat-7 remote sensing satellite (launched in 1999) in polar orbit. Using this orbit means that the refueling unit will be one-time, but the documents also mention the version of Restore-G for the geostationary orbit.

At the end of June this year, the China Space Agency announced the successful refueling of a satellite in orbit. Two special satellites were launched on June 25 at the first launch of the new launch vehicle “The Great March-7”. Since then, no photos or videos have appeared, it is logical to assume that the experiment was similar to Orbital Express.



In the spring of this year, there was news about the signing of a contract between Orbital and Intelsat about the launch of the Mission Extension Vehicle satellite in 2018, which will have to extend the life of the satellite with the fuel that has ended by five years. Interestingly, from an engineering point of view, the problem here will be solved differently. Instead of bothering with the opening of filling lines on the satellite with sophisticated tools, as suggested in the Robotic Refuting Mission, the MEV satellite simply locks onto the cruising engine and the adapter ring around it at the target satellite. As a result, MEV will not be a tanker, but a tugboat that will move and rotate the target satellite with its engines. The device will most likely be disposable, but, theoretically, if there is a fuel reserve and target failure, no one will interfere with the flight to another satellite.

A bit of spying

The specifics of the geostationary orbit ballistics means that it is possible, having entered it and having slowed down a bit, to switch to such an orbit that will visit other standing points. If necessary, you can linger at the desired point, a little speeding. This feature, convenient for satellite tankers, can also be used for less altruistic interests. Just recently, two satellites built by the above-mentioned Orbital ATK went into space. But the GSSAP satellites were commissioned by the US Department of Defense and will be engaged in observing satellites in geostationary orbit from close range. This is the second pair of such satellites, the first two have been observing the geostationary orbit for two years already. Their maneuvers are not disclosed to the general public, and the satellites themselves are too small to be easily noticed by amateur astronomers. According to rumors they make wonderful photos of satellites in geostationary orbit, and a recent press release from the US Air Force said that one of the newly launched satellites would take an emergency MUOS 5 photo (this is possible when it will fly through the apocenter in the geostationary orbit region). One problem - to see these photos we will have to wait for many years until they are declassified.

Conclusion

The technology of refueling satellites has not yet decided which way to go. Perhaps, we are waiting for tankers in the style of Robotic Refuting Mission, and, perhaps, tugs a la Mission Extension Vehicle. The economic benefits are also not yet known, for example, in Orbital ATK they compare the economic effect of satellite service with experiments on the reusable Mask launch vehicles. Well, wait and see.