In 1993, I published a Ph.D. dissertation at Caltech titled, “An Experimental and Theoretical Study of the ECR Plasma Engine”. At the time of publishing I only had a decade of professional experience as a NASA-JPL technologist and technology consultant and a passion to push the envelope of the current space population systems. This was my first leading edge research paper exploring computational modeling, plasma physics and experimental testing of a prospective new technology. In the following decades, I have applied my skills to many different technical areas, but have often looked back on my motivations for that original work with an eye toward making it happen in space.
What were those early motivations? I was driven by a desire to solve a problematic feature common to all high performance electric propulsion systems: electrodes.
What were those early motivations? Electrodes.
Electrodes are problematic because they wear out and die limiting the life of electric propulsion systems. Additionally, they only work with inert or metallic propellants like argon, xenon, or mercury. While those limitations are accepted for lavishly funded NASA science missions or huge communications satellites that fill a niche without much real competition in the communications industry, the effects of electrodes are a serious problem for anyone trying to do truly competitive work in space. Today’s NewSpace industry and the bigger space industries that Elon and Jeff plan to enable simply can not afford expensive or toxic propellants; complex systems integration driven by high pressure tanks or dangerous propellants; or the life and performance limitations that arise from electrodes in electric propulsion.
The holy grail of a great NewSpace propulsion system will combine high specific impulse, (two or three times that of chemical propulsion); long life; and low integration costs afforded by low cost, non-toxic propellant that can be stored at room temperature pressure.
The perfect propellant? Water!
Of course there are water based propulsion systems on the market today. They tend to be simple resistively heated steam jets that use the same kind of nichrome heaters used in home toasters. They heat water into vapor and squirt it out of a nozzle to produce thrust. These are great, low cost devices for low performance CubeSats, but their performance is limited and even less effective than chemical rockets, and they will not scale well for use in main propulsion even on a 100 kg SmallSat.
For high performance water based propulsion you need electroless microwave rockets and so the motivation of my thesis work at Caltech has finally come full circle. The systems we have developed here at Momentus are going to blow the lid off of NewSpace in-space transportation in the near term and I am delighted that the knowledge and expertise I gained from my work at Caltech, and since, is going to have application here and now.
Sometimes good things come from work even decades hence.
I forgot to mention the other important thing about water based propulsion! In the next wave of space industrialization we will be harnessing resources from the Moon and the asteroids, and they are full of water.
By doing the right thing now from a business perspective and building a water based transportation economy in low Earth orbit and cislunar space, we are building a market for asteroid miners. Momentus propulsion systems will be on the first asteroid mining missions and will be key to using the mined water to return resources, especially water, from asteroids to Earth orbit.
Joel C. Sercel, PhD
Chief Technical Advisor