After way too many years in the wilderness, the concept of orbital propellant depots and in-space refueling in general, are finally beginning to be taken seriously again in public circles. A couple of examples include: Elon’s announcement that he is building his Mars architecture around RLVs and propellant depots, NASA’s baselining of in-space refueling as part of their Artemis lunar return program, even the most recent NASA Tipping Points solicitation1 had Cryogenic Fluid Management Technology Demonstration (active cooling, transfer, and pressure management) as one of its three topics. There are also now several startups out there explicitly focused on orbital propellant depots including my friends at OrbitFab, my friend Dallas Bienhoff’s company Cislunar Space Development Company, and also my startup, Altius Space Machines2.
Those of you who’ve been following this blog over the years have probably seen a lot of my previous thoughts on the topic. But I’ve been realizing that there are now a lot of new people becoming interested in the topic, and during some recent conversations on Twitter, I realized that it might be helpful to share some of my thoughts on the different types of propellant depots, and key considerations for each type of depot (things like where you’d likely put them, what sort of propellants they’d likely contain, how big they’d likely be, what you’d use them for, etc).
Instead of doing what I often do, and trying to cram six blog posts into one, I’m going to release a series of blog posts over the next week or two about the six main types of orbital propellant depots I’ve been able to think of so far3:
- Distributed LEO Nano-Depots
- GEO Depots
- Smallsat Launcher Refueling Depots
- Human Spaceflight Fixed Depots (Low-Orbit)
- Roving Depots
- Human Spaceflight Fixed Depots (High-Orbit)
I’m sure there are probably more categories than that, but I figured that it was worth at least sharing some of my thoughts about these different types of depots, and their similarities and differences.
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- STMD’s Tipping Points solicitations are focused on using NASA/industry public/private partnerships to get key technologies across the “tipping point”, via ground or flight demonstration, where they are now commercially viable or ready to infuse into NASA missions without needing additional NASA technology investment
- We tell people that while our first horizon goal is ubiquitous LEO satellite servicing, that our second horizon goal is propellant depots
- After I’ve written them, I’ll add links to them from this opening blog post