The Momentus Ardor thruster. It will be used for the Ardoride service. Credit: Momentus.

Water is the first resource many in the space resources community are targeting for exploration and utilization missions. Not only is water abundant throughout the Solar System, but it is one of the most useful resources. Its uses include the creation of propellant and oxidizer, use in chemical processes, and as drinking water and breathing oxygen for astronauts. Water is the oil of space. Similar to how oil drives terrestrial economies, water in space will propel the space economy, literally and figuratively.

Currently all resources utilized in space are launched via rockets from Earth. The cost of these trips are high, adding up to thousands of dollars per kilogram. If we wanted to launch water from Earth, it would mean spending thousands of dollars per kilogram to launch a commodity that costs around $0.001 per kilogram.

It is clear that there is an opportunity to save money by using water from space instead of launching water from Earth. This is achievable, as water can be extracted from space sources for significantly less than the cost of launching it from Earth.

There are two core steps required on the demand and supply side to enable the extraction and utilization of water in space. The first is testing and deploying systems that can extract and deliver water from locations where it's found in space to destinations where it's needed. The second is building up space systems that are capable of receiving and utilizing water while in space. This second step includes the development of systems that use water as propellant. These propellant systems could utilize water from Earth now and switch to space-sourced water when it’s available.

Great strides have been made on water based propulsion systems in recent years. Some of the systems include:

A thruster called Comet, from Deep Space Industries who was recently acquired by Bradford Space.
A thruster called HYDROS, from Tethers Unlimited.
Multiple thrusters, such as Vigor, Ardor, and Fervor, being developed by Momentus.

These thrusters each provide different and unique capabilities to different sizes of spacecraft. All of these thrusters are using H²O for propellant.

The Bradford Space Comet thruster. It was originally developed by Deep Space Industries before they were acquired. Credit: Bradford Space/Deep Space Industries.

Bradford Space has developed the Comet thruster. It has a 174 second specific impulse and 17 mN of thrust. With a scalable tank, it is targeted at CubeSats and small microsatellites. The Comet thruster is planned for use on 60 spacecraft that will launch in the next two years, and is currently flying on four.

Four examples of the Tethers Unlimited HYDROS-M thruster. Credit: Tethers Unlimited.

Tethers Unlimited is developing the HYDROS thruster. It is offered in two configurations: the HYDROS-M and HYDROS-C. Both versions are capable of over 310 second specific impulse and over 1.2 N thrust. The M version is targeted at microsatellites, and the C version is intended for CubeSats and NanoSats. NASA selected the HYDROS-C for a demonstration on the first Pathfinder Technology Demonstration (PTD-1) CubeSat mission in late 2019.

The Momentus Vigoride system, using the Vigor thruster. This service has the ability to move customer masses up to 250 kg. Credit: Momentus.

Momentus is building a collection of water based thrusters to cover many spacecraft weight classes. Their goal is to use these thrusters for moving spacecraft from orbit to orbit.

The Vigor thruster is being used for Momentus' transport services for moving masses up to 250 kg from the ISS to orbits around 2000 km. The Vigoride Extended service planned for 2020 is expected to move 300kg from LEO to GTO, or 100 kg from LEO to GEO, Lunar orbit, or beyond.

Momentus has indicated plans to increase the size of their water based engines for larger services past 2020. The current thruster development timeline includes the Ardoride in 2021, the Fervoride in 2022, and the Valoride beyond that.

These larger Momentus thrusters will offer large thrust capabilities. The expected figures start at moving 4 tons from GTO to GEO, and extend to transferring over 100 tons of water from near-Earth asteroids to cis-lunar space. In the coming months Momentus plans to launch propulsion flight hardware for Vigor that will demonstrate their core propulsion technology.

Building a sustainable space economy is all about balancing supply and demand. A key driver for an early space economy will be based on water. Using water for propulsion will place a large demand for cheap and plentiful water in Earth orbit. These pure water based thrusters are a prime driver for starting this demand curve, while also offering effective means for transportation.

RESOURCES

https://twitter.com/amaragraps/status/1092305170728394752
https://twitter.com/jetcitystar/status/990003904891961344
http://www.tethers.com/HYDROS.html
http://www.parabolicarc.com/2018/06/30/nasa-selects-tuis-hydrosc-thruster-ptd-cubesat-mission
https://www.nasa.gov/content/upcoming-elana-cubesat-launches
http://bradford-space.com/products-comet-smallsat-propulsion.php
https://spacenews.com/momentus-developing-water-engines
https://www.aerospace-technology.com/news/bradford-buys-deep-space