The ramjet is an incredibly simple device – no moving parts, no hassle. Air comes in, gets heated up and expelled through a nozzle. Ramjets are commonly found in high-end jet aircraft but these are all powered by liquid fuel.
In our age of electrifications, the obvious question remains: Is there a practical application for an electrical ramjet? The simple answer is yes – satellites could greatly benefit from this technology. Once the air rams into the engine, a set of ionizing grids would turn it into plasma before ejection. The problem with electrical ramjets as with any engine that operates on electricity is the power source. A sufficient electrical power source would be required to run this system (Battery, solar or nuclear).
Here on the surface of earth, there are no practical applications for such a device. This is because the power requirement to run a ramjet against high-density drag and gravity makes the power source (batteries or otherwise) prohibitively massive for flight. Furthermore, a ramjet cannot take-off on its own, it only begins working at flight speeds of roughly half the speed of sound. As a result, we would need some sort of an auxiliary system to get the whole aircraft up to speed from take-off which further adds mass and complexity to the system.
There are actually some very interesting “sci-fi” applications of the electrical ramjet – the interstellar (Bussard) ramjet. This kind of ramjet flies at very high speeds (a few percent of the speed of light) while collecting the propellant from the interstellar medium. Unfortunately, space has incredibly low density of matter – roughly 6 atoms per cubic meter! This means that our spacecraft would have to fly at a significant fraction of the speed of light just to gather enough material to run the engine. This is certainly viable for a sufficiently advanced civilization but not for humankind, at least not in the near future. The energy source for the Bussard ramjet is a fusion-reactor, another piece of technology that is simply unfeasible in our day and age.
Looking at more practical applications, there is one more place that we should explore, the interface between a planet’s atmosphere and space. This is an interesting location since gravity and drag are fairly low but at the same time the density of empty space is much higher thanks to the atmosphere beneath. Think of this as skimming over the atmosphere. As a matter of fact, all satellites and even the International Space Station are constantly skimming the atmosphere which generates drag. This means that all objects in Low Earth Orbit lose velocity over time and therefore have to implement course corrections. To do this, they burn onboard fuel, but once the fuel runs out, the orbiting object will begin slowing down, ultimately spiraling inwards and burning up in the atmosphere.
An electrical ramjet could offer a solution and ESA (European Space Agency) is actively exploring this concept. As air impinges on an orbiting satellite, it is channeled into an inner chamber where it is ionized and expelled via a magnetic nozzle. The thrust is not high, especially since the power comes from solar panels. However, the thrust is just sufficient to maintain a stable orbit for the satellite – this type of engine could be thought of as a drag compensator. Since no propellant is consumed in the process, such a system could, in theory, run indefinitely – truly a game-changer for smaller satellites in Low Earth Orbit (larger orbiting objects would require an onboard nuclear reactor to power such an engine). Furthermore, the electric ramjet is agnostic to the atmospheric composition, so it is useful for any satellite orbiting any atmospheric body be it Mars, Venus, Jupiter, or Titan. Don’t let the cool name fool you, an electric ramjet is not some type of exotic device from science fiction books, it is a real thing (with a real prototype already built in Italy) and has practical applications to satellites orbiting Earth as well as other celestial bodies.
