Liquid air carrying Skylon spaceplane

The special feature of the spaceplane’s SABRE engine is the heat exchanger, which cools the intake air by using the low temperature of the stored liquid hydrogen. This could be a real turning point in space exploration, not because it makes launching satellites into orbit cheaper, but because the working principle of the SABRE engine creates a new opportunity.

Deep space exploration actually depends on water. If the water supply is unlimited, everything from the oxygen supply of astronauts through crop production to radiation protection can be assured, not to mention that fuel is also available for the rocket or ion engines. However, since humanity has neither asteroid mines nor colonies on other planets and moons, therefore, water supply can currently only be provided from Earth.

To do this, a Skylon spaceplane would be needed which is much smaller in size and can’t carry a satellite or any similar payload mass at all, that is, this version of the Skylon spaceplane would not have a payload bay. Instead, it would have a secondary liquid hydrogen tank, and along with it the spaceplane could carry more liquid hydrogen than it needs to reach the orbit around the Earth.

The secondary liquid hydrogen tank would be small in size, and to take off and reach a height of a few kilometers, the spaceplane would first consume the liquid hydrogen from this smaller tank, completely emptying it.

Then, passing through the atmosphere, the spaceplane would fill the empty secondary liquid hydrogen tank with continuously produced liquid air. Since, for example, at -140 degrees Celsius only 32 atmospheric pressure is enough to liquefy the intake air, the liquid air can easily be produced during in-flight.

Of course, leaving the atmosphere, the spaceplane would reach the orbit around the Earth by using the liquid oxygen from its liquid oxygen tank.

Thereby, after reaching the orbit around the Earth, some liquid hydrogen will still remain in the spaceplane's primary liquid hydrogen tank, and it would have liquid air in the secondary liquid hydrogen tank. That is, practically the payload mass that this spaceplane carries will be the unused liquid hydrogen and the created liquid air, which can then be used by space operation as desired, as supply of oxygen, water or even fuel.

The nitrogen content of the liquid air can also be useful, because, for example, it is ideal as a fuel for robots like CIMON. Releasing high-pressure nitrogen gas through their maneuvering nozzles, such robots could be ideal for external repairs at space stations. 

Shell modules:

If it is possible to transport liquid air into space on a regular basis, for example for a space station such as the International Space Station (ISS), it would be worthwhile not to store the nitrogen in a liquid state in a cryogenic storage tank, but rather in other way that exploits the possibilities of the huge amounts of available nitrogen.

The Bigelow Aerospace has developed an expandable space station module, which allows modules with much larger living space or cargo space to be used in space compared to traditional modules. Using the materials of this expandable module for development, to almost every traditional space station module an extra shell module filled with nitrogen gas could be attached.

The diameter of the shell modules can be multiple compared to the traditional module, and each shell module would be sized to accommodate a selected traditional module. The middle of the shell modules, where the traditional modules are placed, would be empty, that is, the shell modules would be sized for fit like a suit.

The inner structure of the cylindrical shell modules would be divided into cells with an edge length of half meter, and each cell would be connected with valves that can be remote controlled separately.

When installing a shell module, first the cells along the outer wall of the shell module would be blown with nitrogen gas. The traditional module would then be inserted into the center of the cylindrical shell module and secured with straps. After that, the cells of the shell module would be blown in a row, moving inwards, until finally, by blowing the innermost cells, the shell module would fully adhere to the outer surface of the traditional module. The pressure of the nitrogen gas in the cells of the shell modules would be about three times more than the atmospheric pressure.

The shell modules would not interfere with the interconnection of the traditional modules, however, of course an observatory module would not get a shell module.

Benefits of the shell modules:

When a space debris impacts into a space station module, no matter how small it is, it is sure it will punch through the module because of the extreme speed difference.

The shell module will still not absorb this impact, but since the nitrogen gas pressure is higher than the air pressure in the traditional module, first the nitrogen gas will be ejected through the resulting holes, through the inner holes inwards into the traditional module, and through the outer holes outwards into the space.

Depending on the diameter of the holes and how many nitrogen gas cells are damaged by the impact, a few extra seconds or even a few extra minutes may be available for the astronauts to take action before the air of the traditional module begins to eject into space.

Since the nitrogen gas does not feed the fires, and with the oxygen it can be inhaled, therefore, the nitrogen gas is perhaps the least dangerous for the astronauts, but nonetheless the rate of the nitrogen gas in the air should not be too high because it causes suffocation then death, therefore, in any case, the first thing for astronauts would be putting on an emergency oxygen mask.

Developing an exterior repair robot similar to CIMON would be worthwhile, which is much larger in size and developed specifically for the space environment. Such repair robots can be on standby at all times, reach the point of an impact in a minute, easily find the holes in the outer surface of the shell module by looking at the ejecting gases, and then, depending on the size of the holes, patch them immediately, even completely prevent the escape of the air from the traditional module.

The expandable space module developed by Bigelow Aerospace has a multi-layer wall specifically designed for the harsh space conditions, which allows, among other things, the use of the nitrogen gas stored in the shell module as an additional thermal insulation layer. The nitrogen gas can be continuously maintained at a given temperature of about twenty degrees Celsius, either by radiators built into the inner surface of the shell module wall, or by a central device and the continuous flow of the nitrogen gas between the cells.

Thus, no matter how small the holes in the wall caused by the impact of a space debris, due to the difference of almost three hundred degrees Celsius between the escaping nitrogen gas and space, it will look like a volcanic eruption in an infrared camera.

(The first version of this concept was written in May 2020.)

Co-author: Joshua Swindell