Momentus of water plasma propulsion

In space propulsion arena, there are many competing engine designs and technologies beyond the chemical rockets. Nuclear thermal. Nuclear electric. Solar electric. And then the types of engines. And propellants. Xenon. Argon. Krypton. Iodine. Not to mention the Impossible Drive.

But with all this high-tech gear, why not having something simpler? Nuclear means expensive. Kilo power reactors are first reactors that might be designed in 40 years in the US. How long until it really flies? In this risk averse era, it might take decades.

So that leaves solar electric propulsion. While it is nice to achieve 5000s Isp, it requires a  lot of solar panels, as required power increases as square of exhaust speed. And having highly pressurized tanks that have to withstand years of operation...does increase the cost.

There are various satellites busses and space tugs (like Sherpa family) that can provide various propulsion, navigation and electric power services to its payload. But they rely either on hydrazine as fuel or high pressure tanks containing noble gas. In other words, they are dangerous. And use specialized fuel. Which means they are and will remain expensive to design, test, manufacture and (re)fuel. On the ground and in space. In other words, they just don't scale well.

It would be really great to have something that does not require high pressures, is completely environment friendly, and is liquid to enable easy filling and refilling? Something that could be scaled from low thrust to high thrust configurations? Something humans could handle without all protective equipment and all the precautions? And with Isp better than the best of chemical propellants? Something that would not require huge "wings" or rolls of solar cells? Imagine an astronaut filling in a cubesat inside of the space-station and tossing it out through the airlock?

There comes Momentus, self-proclaimed space infrastructure company. They view SpaceX as shipping/trucking company, delivering goods to the nearest logistics warehouse. And they position themselves as doing door-to-door delivery. In space, of course.

But that vision depends upon easy, reliable and cheap propulsion method. Their use of electric propulsion does not pursue Isp as efficiency measure. Thrust is more important measure, as the current electric propulsion devices takes months or years to complete their mission. With choice of water (plasma) does provide quite low Isp in the range of 600-800s. But that is compensated by much higher thrust, leading to 4-9 times shorter mission times. Instead of waiting six months for GTO-GEO transfer, their propulsion method can do it in a month. And it does not need Xenon. And no high pressure tank. 

That places them in the "sweet spot" between chemical and typical electric propulsion systems. Jack of all trades, but master of none. Their Vigoride Extended, Ardoride and Fervoride vehicles offer up to 6km/s of velocity change. For example, such vehicles could provide regular monthly shuttle service between Lunar Gateway and ISS. Fervoride could push lighter GEO satellites from LEO to their proper orbit and get back to LEO. Deorbit old satellites and spent rocket stages. 

The beauty of Momentus engines is that is safe for humans unlike any other propulsion method. It makes it ideal for co-manifested launches, where payload can be fueled even before any integration work. Plus it fills the gap between LEO optimized Starship capacity and realistic satellite weights in use today, which scale up to less than 10.000kg. Since every satellite today uses solar panels, its moderate power requirements fits perfectly inside any space mission that is not weight constrained. And the propellant will always be available in any space station or human inhabited vehicle.