An Updated Propellant Depot Taxonomy Part V: Human Spaceflight Fixed Depots (Low-Orbit)

For this next depot taxonomy post, we’ll finally be talking about what people usually think about when they hear the term orbital propellant depot — larger, cryogenic refueling facilities, focused on enabling large-scale human spaceflight missions, performed by a diverse variety of users, and going to/from a wide variety of destinations. The idea for such propellant depots for enabling interplanetary human spaceflight dates back to at least 1928 with the writings of Guido von Pirquet¹.

This blog post will be focused on what I call “Low-Orbit” human spaceflight depots. These are depots located near the lowest stable orbit around a planetary body. This is the first place at which you can realistically refuel or switch vehicles on your way from a planetary body, and is the last place at which you can refuel or switch vehicles on your way down. As a firm believer in the idea of refueling early and refueling often, low-orbit depots are an important piece of infrastructure for any planetary system humanity wants to travel to/from regularly. A later blog post in the series will talk about “High-Orbit” depots — depots operating in fixed locations² located out near the edge of a planetary system’s gravitational sphere of influence, and will include a discussion of where those types of depots might make sense.

Before we jump into the weeds about low-orbit human spaceflight depots, I did want to address a recent train of thought I’ve seen that suggests that just using tankers and directly refueling a vehicle is superior to having a depot involved. While this could easily be the topic of a series of its own, I wanted to briefly highlight a few of the biggest advantages I can see of having a depot vs just using direct refueling with tankers:

• Flexibility: A depot, properly designed, with published, standardized grappling, refueling, and power-data interfaces, can be agnostic about who it gets propellant from and who it sells propellant to. Depots can quickly take advantage of whatever the cheapest source of propellant is at a given time (RLVs, ISRU, propellantless launch, buying leftover capacity from other missions going to destinations near a depot, atmospheric gathering, etc), and can easily service both smaller missions and bigger ones. Tanker-based approaches tend to be a lot less adaptable, typically being optimized for one or two specific vehicles that needs refueling.
• Robustness: With a fixed installation, that only has to be launched once for a long mission lifetime, you can afford to throw way more resources (dry mass, volume, and power) at making rendezvous, prox-ops, docking (RPOD), and manipulation as safe and reliable as possible. This could include beacons, larger more capable (and/or redundant) relative navigation sensors and comms, longer reach capture robotics that minimize the dry mass requirements on tanker and client vehicle alike, etc.
  • For tankers, on the other hand, you want to minimize parasitic dry mass that has to be launched every time, and for the departing vehicle you want to minimize mass you have to carry through large, high delta-V in-space maneuvers. You could in theory carry a nicer RPOD kit on your departing vehicle that you jettison before leaving LEO, but now you’re amortizing that mass and cost over a much smaller number of missions, or starting to add more complexity than just doing a depot.
  • Another question to ponder is with vehicles that require large numbers of refueling events per mission, is it best to have the client vehicle handle all of those docking maneuvers with its (by definition) less-capable RPOD capabilities? With a depot, each tanker and each client vehicle only has to perform one mission-critical RPOD/refueling operation per mission, whereas with direct refueling via tankers, the client vehicle would typically have to perform a larger number of mission-critical RPOD events. A depot would also have to handle the same larger number of RPOD/refueling events, but as mentioned before, can throw more resources at making these as reliable and safe as possible.
  • Longer refueling cycles from using a direct-tanker refueling approach also increases the MMOD³ risk to the departing vehicle, which by definition can’t afford to throw as many resources at MMOD protection as a depot can.
  • A corollary to this is that depots make the most sense if you use them in a way that offloads as much of the refueling-unique hardware/software as possible from the delivery and client sides of the system to the depot. Ideally a tanker would be a minimally modified upper stage⁴, and client vehicles would also have similarly minimalistic hardware needed to be grappled and receive propellant. If you’re doing your delivery vehicle or your client vehicle in a way that makes you question the utility of a depot, that may be a hint that you’re doing something wrong.
• Non-Integerality: Yes I may have made up that word, but the point is that tankers tend to come in integer quantities. Unless you always design your departing vehicles to use only integer quantities of tankers, you’ll almost always end up having wasted propellant. This is especially true given how launch vehicles, and in-space vehicles tend to increase performance and get upgraded over time, and don’t necessarily upgrade at the same rates. If you only ever had a monopoly/monopsony situation where you only had one tanker provider, and only one vehicle needing tankers, you might be able to keep tanker size locked to an integer fraction of the amount of propellant needed, but in reality that isn’t going to happen. So a tanker-based system is always going to end up wasting propellants, and this is even more the case when you have diverse customer vehicle and delivery vehicle sizes. The more diversity you have in your space transportation ecosystem, the more a depot makes sense.

There are probably other good arguments I’m glossing over, but long-story short, unless you’re interested in a boring monoculture world where only one type of in-space transportation system exists, depots make a lot of sense. So, without further ado, let’s jump into some of the taxonomical considerations of human spaceflight low-orbit depots.

Human Spaceflight Low Orbit Depots

Application: Refueling large transfer stages or in-space transports for ferrying people and cargo between LEO, the Moon, Mars, Venus, and other destinations of interest.

Location: As discussed earlier, this blog post is focused on depots located near the lowest practical stable orbit around a planetary body. Details vary for different planetary bodies, as described below⁵:

• For Earth, the low-orbit depot location is in LEO, ideally in the lowest inclination that still lets you hit required departure asymptotes and maximize the number of economically useful launch sites to send propellant, people, cargo, and materials to/from the depot⁶. You probably only need one or two such low-orbit depots, though if you get to high-enough earth departure throughput there may eventually make sense to spread more depots out at different RAANs⁷. Likely for a first human spaceflight depot, as with the previously discussed smallsat launcher depots, you’ll want to locate it in LEO near other human-occupied facilities like ISS — far enough away to be safe, but close enough to conveniently move between each other, ideally within one work shift⁸.

• For a low-orbit depot around the Moon, this would likely be a polar or near-polar LLO⁹, though due to the very slow rotation and practically zero J2 perturbation¹⁰, if you have a lot of non-polar surface sites, you eventually may want multiple smaller depots in equally RAAN-spaced near-polar LLO planes, and maybe one in an equatorial orbit. If you’re trying to do lunar surface missions, having your depot in LLO makes way more sense than in a higher orbit like NRHO, for reasons I should probably go into in another blog post.

• For Mars this would also likely be a LMO orbit, with an inclination high enough to be able to access any points of interest on the surface, while still being low enough to minimize delta-V penalties¹¹, and keep the nodal precession rate fast enough to minimize phasing orbit time for three-burn departures. You’ll likely also have to put some thought into perturbations from Phobos and Deimos¹².

• For Venus, the extremely low rotation speed and therefor very low J2 pertubation may require you to do multiple smaller depots in similar inclinations but equally RAAN-spaced planes, as you won’t pass over a given point on the surface very frequently, and the very slow nodal precession rate could potentially require very very long phasing orbits for a 3-burn departure. Venus has a deep enough gravity well that you do want to refuel in LVO coming to/from, but it’s not trivial from an orbital dynamics standpoint¹³.

Size: As big as you can practically get away with — ideally you’d want this depot to be at least 2x the propellant capacity as whatever the largest vehicle you’re refueling. So somewhere in the 100-2000mT range, or even bigger¹⁴. Early versions will want to be single-launch if possible, in many cases repurposing at least one of the main propellant tanks from the stage that delivered them to their destination as one of the depot tanks¹⁵. Eventually, it may be possible to do multi-tank depots, but if you can do a single launch depot big enough for refueling two missions, you may be better off making more than one depot instead of trying to make the depot super big.

Propellant Types: For low depots, you’re primarily going to be dealing with large transfer stages (Centaur V, Starship, New Glenn Upper stage), which typically use LOX, and either Methane or LH2 for the fuel. Most of these use autogenous pressurization, and use the main propellants for RCS. So most of the depot will be for LOX, LH2, and/or Methane.

• For Mars or Venus you may eventually also want to store liquid CO for some applications, since it’s an easier ISRU propellant, but that remains TBD.
• A lunar low-orbit depot may also want to stock storable propellants, depending on what lander propellants end up being most popular¹⁶.
• You may eventually also want to store some secondary fluids (Helium or Neon for active cooling loops, life support consumables like water, air, etc), but you may not explicitly need a depot for that function.

Some Other Considerations for Human Spaceflight Low-Orbit Depots

• As mentioned before, human spaceflight depots want to be designed in a way to enable offloading as much of the RPO and docking/or berthing from the vehicles they’re servicing. The less parasitic dry mass that tankers and clients have to lug around on every mission, the better.
• Storing cryogens in low orbits tends to be hard — you have a warm planet blocking half of the sky. So launching the propellant in a subcooled state or even partially frozen (i.e. slushy propellants) can help a lot. Also a lot should be done to minimize heat leaks between the cold part of the depot and any hot sections (habitation, power, etc). If you can’t get to zero boiloff, LH2 is a great thermal sponge, and can be used to chill other propellants, and intercept heat from heat sources before being vented. You may have to vent some hydrogen boiloff, but if you’re smart, you can use that hydrogen boiloff on the way out to eliminate boiloff issues for everything else.
• These depots are also big debris targets¹⁷. Deployable MMOD/MLI¹⁸ solutions could be very helpful to avoid a puncture, which would probably be very hard to patch. Since these depots are fixed, and are typically only performing stationkeeping maneuvers, it may be possible to augment their MLI/MMOD protection over time using in-space assembly/manufacturing techniques¹⁹.
• Especially for low-orbit depots around the Moon/Mars/Venus, there may be a benefit to having some temporary habitation/shelter collocated with the depot, especially if you’re supporting multiple sites, as a search and rescue option during exploration phases, and as a stop-over point to act as a buffer between different sizes of transportation between planets and between the depot and the surface.
• Over time you may want to add in other facilities such as dry docks for assembling, and repairing/maintaining large in-space vehicles/structures, habitation facilities, etc. But they should probably be coorbital, near the depot, not attached (as that will make cryo thermal management all the harder, and the depot is a big hazardous work location, where you should probably minimize the amount of time people spend in close proximity to it). This could be done in two ways — coorbital facilities, spaced where the safe time to travel from one to the other is as short as practical (definitely less than an 8hr work shift if at all possible, and much closer if possible), or by having the two facilities connected by a connecting tether or other structure that includes elevator facilities.
  • If your depot facility starts wanting to have permanent collocated habitation (say for in-space assembly/repair/maintenance of in-space stages), and having the two be coorbital doesn’t work, you’re likely going to want to keep the people separated as far away from propellant tanks as possible, both to minimize heat-leak into the tanks, but also to minimize hazard to the people²⁰.
  • For larger depots, and ones where people will be there more, putting some thought into spatially separating the fuels and oxidizers more could be a good idea. In rockets you often can’t do much to keep the two separated, and many use a common bulkhead, but in a fixed facility it’s more of a possibility. Having fuel and oxidizer that close together for long periods of time is somewhat tempting fate — you kind of have to do it for high efficiency rockets, but there’s something to be said for having your fuel and oxidizer many meters apart for a long-duration facility.
• One area of disagreement I have with other depot advocates is whether propellants should be shipped to a depot as cryogenic propellants (LOX/LH2/LCH4), or if you should ship them as something more storable like water and CO2, and have the depot itself have large-scale electrolyzing, separation, and propellant refrigeration systems. My concern is that while in theory very large solar arrays could be done in space, combining large flexible structures like multi-MW solar arrays and radiators with a facility that sees a lot of docking, propellant slosh, etc seems like a bad idea from a structural dynamics standpoint. Also depots with very large power generation and heat rejection capabilities are likely to come later in the process, since they’ll almost certainly require multiple launches and in-space construction.

Anyhow, I probably could go on, but as with the previous parts of this series, I am only trying to scratch the surface with considerations and operating details, as I introduce each new type of depot. This definitely isn’t the last you’ll hear from me on the topic.

Next-Up An Updated Propellant Depot Taxonomy Part VI: Roving Depots

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Jonathan Goff

President/CEO at Altius Space Machines

Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.

Latest posts by Jonathan Goff (see all)

• NASA’s Selection of the Blue Moon Lander for Artemis V - May 25, 2023
• Fill ‘er Up: New AIAA Aerospace America Article on Propellant Depots - September 2, 2022
• Independent Perspectives on Cislunar Depotization - August 26, 2022

1. Sykora, Fritz, “Guido von Pirquet-Austrian Pioneer of Astronautics,” History of Rocketry and Astronautics, R. Cargill Hall, ed., AAS Publications, San Diego, 1986, p. 151. And yes, I’d love to see someone actually fly a depot before we hit the 100th anniversary of von Pirquet first discussing the idea
2. Or at least orbits/halo orbits where the only maneuvers being performed are for stationkeeping purposes
3. MicroMeteorite and Orbital Debris
4. With some lightweight grapple fixtures, refueling modified T-0 umblicals, and maybe an upgraded comms/controls system
5. Most of these points could easily justify their own blog post, and knowing me, I’ll probably eventually do said blog posts, but I’ll try to keep things in this post brief.
6. I’m currently noodling doing a paper looking specifically at this optimization — higher inclinations for the depot give you the ability to hit higher departure declinations, and are accessible by more launch sites, but at the cost of slower nodal precession driving longer hang-times for a three-burn departure, and you also have decreased payload mass to higher inclinations. My gut says ISS-like orbit is probably not far from the optimal point, but it would be fun to run the numbers
7. As a reminder, RAAN stands for Right Ascension of the Ascending Node, which is a measure of where a given orbit plane crosses the equator heading northward. Here’s a decent wikipedia explainer. When you see a multi-plane constellation, like say OneWeb or Starlink, in a lot of cases the satellites will be put into multiple planes with each plane having the same inclination and altitude, but a different RAAN. The different RAANs help with more frequent passes over the same point in the ground, and for depots by increasing the frequency with which a depot passes through the plane of a specific departure asymptote. If that makes any sense.
8. I could easily see a scenario where the first human spaceflight depot evolves from smallsat launcher depots.
9. Though how low of a LLO is open to debate — lower orbits have higher stationkeeping requirements, and are harder to do sunshields with because the Moon takes up more of the sky surrounding the depot. I haven’t done an optimization analysis myself, but 250-500km above the Moon might reduce both of these effects noticeably compared to the 100-150km parking orbits used for previous lunar missions
10. The J2 parameter is a measure of how oblate or “round about the middle” a planet is. This is typically tied to the rotation speed of the planet. The J2 perturbation is what causes orbital planes to precess slowly. If a planet had no J2 or very low J2, the planes would stay in a fixed orientation relative to the stars, which is problematic if you’re trying to get the plane to line up with a departure asymptote for an interplanetary departure… Earth, Mars, and most of the gas giants have high J2s, while the Moon, Venus, and Mercury all have very low J2s.
11. The equatorial velocity of Mars is ~half that of earth, so it’s not as big of a deal to launch from or into a higher inclination, but it still is some losses
12. I’m not much of a Mars guy, so am handwaving that part a bit
13. If someone is looking for a PhD dissertation topic, this feels like an area that could use a lot more skullsweat investigating.
14. Depending pretty strongly on how successful Starship ends up being, and how open Elon is to using other people’s depots. In a world where Starship either doesn’t pan out, or Elon insists on doing his own thing, the required size for refueling other missions can be a lot more modest initially. Also a world with depots in low-orbits around destination planets, and roving depots and/or high-orbit fixed depots may not need LEO depots to be quite so big.
15. As shown in Part IV of this series.
16. For long duration lunar landers there are definitely differences of opinion about whether storables or cryogens make more sense.
17. Big LEO facilities that don’t want to dodge things all the time is one of those reasons why moving to a leave-no-trace approach to satellite operations is going to be an important part of growing up as a spacefaring civilization.
18. MLI, or multi-layer insulation is a type of very effective insulation for use in vacuum environments. It is typically made of many layers of thin metalized plastic films separated by nets or spacers so that most of the heat has to transfer via radiation instead of conduction or convection. Having more spacing between layers can help. And in some cases, MLI can be combined with MMOD (which also wants to be multiple thin layers with spacing between them), like what our friends at Quest Thermal have worked on.
19. Most hypervelocity impact shielding benefits a lot from extra spacing between bumper layers, so an in-space assembled/manufactured MMOD solution could be particularly useful
20. Though as my wife pointed out, if your depot facility starts wanting anything, it’s probably a sign of an impending robot apocalypse, so the point might be moot.