For this first type of propellant depot, we have an idea that several of us seem to have independently realized. My company is developing a family of servicing tugs focused on servicing customers in Low Earth Orbit. We realized pretty early on that a lot of the economics of satellite servicing depended on how many missions a given servicer could complete over its lifetime. Much like with reusable launch vehicles, the more missions you could spread a servicer/tug’s replacement cost over, the lower the cost per mission. Which leads you really quickly to the idea of refueling. On the one hand you could try and make your servicer capable of doing a ton of missions off of a single tank, but that quickly bloats your vehicle, and you end up spending most of your propellant lugging around the propellant for future missions1. So it was obvious that to maximize the economics of LEO servicers you want to be able to refuel early and refuel often. Last year as part of an AFRL SBIR Phase I contract, we worked with SpaceWorks to create an economic model for LEO servicing with our Bulldog vehicle2, to try and figure out how small of satellites we could economically service3. One of the big takeaways from this research is that for LEO servicing, the ideal way to do propellant depots isn’t to have one big centralized depot that everything goes to, but to have a disaggregated constellation of small propellant pods that you could position at strategic places near where your servicer was operating, to minimize the amount of propellant needed to move back and forth between the depot and your clients. Our friends at OrbitFab independently came up with the same concept that I’m calling Distributed LEO Nano-Depots.
Distributed LEO Nano-Depots
Application: Refueling servicer/tugs and other satellites in LEO and eventually other areas.
Location: LEO orbits near where servicing and/or refueling needs to take place (e.g. Sun Synchronous Orbit, in planes near constellations, etc.)
- As mentioned earlier, SSO4 is a good place to start focusing on for satellite servicing because there are a small number of planes, all with approximately the same inclination5, with all of the satellites heading in the same direction at the same speed. To avoid a long discussion, this all makes it easier for servicers to maneuver around between clients without having to perform a lot of propellant-intensive inclination change maneuvers. SSO is also where a lot of satellites go because it’s useful for many kinds of earth observation applications, and because of this SSO is also where a lot of the biggest and most capable LEO satellites exist.
- Eventually you’d also want distributed nano-depots placed in planes near other large constellations, as they begin to become more interested in serviceability (especially including backup post-mission disposal services, and refueling). Right now a lot of these constellations believe that they can get their replacement cost so low that there’s no way servicing could ever be useful for them. But space is the only domain where anybody would be dumb enough to treat an object that takes several hundred thousand to several million dollars to replace as a throw-away item.
Depot Size: Cubesat scale (likely 16U) up to ESPA class (up to ~180kg)
- The size of the depot would likely be driven by both the propellant tank size of servicers and client satellites, but also by convenient sizes that can be purchased as “excess capacity”.
- [Edit 9/17: I realized that I didn’t really clearly explain the configuration I had in mind. I think that most of these Nanodepots will basically be one or more tanks packed into a Cubesat body or ESPA-class satellite body, with the minimal avionics needed to maintain attitude, perform collision avoidance/deorbit maneuvers, and maybe perform simple relocation maneuvers, and a grappling fixture and a fuel interface. To me less like a tiny space station, and more like an expendable “smart” propellant canister. I’ll get into more in later posts about what situations where a more permanent facility that people normally think of when you use the term depot makes sense.]
Propellant Types: Storable monpropellants or bipropellants (e.g. Hydrazine, MMH, UDHM, HAN, ADN, NTO, MON, HTP, non-cryogenic hydrocarbons), working fluids for electrothermal systems (water, ammonia, etc.), and electric propulsion propellants (e.g. noble gases like Xenon and Krypton, sublimable solids like Iodine or Bismuth, liquids like mercury or ionic fluids, etc.), and maybe pressurants (neutral gases like nitrogen or helium).
- Most servicers, short-range (intra-LEO) tugs, and satellites use some form of storable chemical or electric propulsion. Unfortunately while there is some commonality among larger, more traditional spacecraft6, smaller commercial missions have not really settled on one or two most common propellant types. Frankly almost all of them have at least one serious drawback (toxic, corrosive, carcinogenic, expensive, unstable, low performance, easy to freeze, hard to light reliably, etc, etc.). Picking which propellants to focus on for a nano-depot constellation is going to be challenging.
- My guess is that the way this will evolve will depend on the propellant choices of whoever starts doing LEO servicing seriously first, combined with the propellant choices of whichever constellations start buying refueling services first. Once you have a large enough customer to justify a nano-depot constellation, there are now strong incentives/network effects for future groups that want to benefit from refueling to pick what’s already being used by others.
Other Key Characteristics/Considerations for Distributed LEO Nano-Depots:
- In many cases these nano-depots may actually be single-use tanks. I haven’t run the numbers yet, but I’m skeptical it will make economic sense to collect empty nano-depots, move them back to some central refueling place, refuel them off of bigger tanks, and then move them back out again. That said, I reserve the right to change my opinion if I run the numbers and they suggest that could actually work.
- Ideally you’d want to scatter these as close to the planes and altitudes where they would be used. Though at the same time, these will likely be wanting to be launched as cheaply as possible, which means they’ll likely almost always be launched on a rideshare basis. This may mean that they’ll need either some modest on-board propulsion capabilities, or they may want to be tugged to a final location after launch, if feasible.
- Since we’re talking about a large number of objects, you’ll almost certainly want them to have at least enough maneuverability to dodge potential conjunctions with non-maneuverable pieces of debris7, and to safely dispose of a tanker once it has transferred as much propellant as possible to the servicer client.
- While in theory you could make each of these tankers capable of performing rendezvous, proximity operations, and docking (RPOD) maneuvers with services or client satellites, I’m really skeptical things will optimize in this direction. Especially if it turns out that most tanks only get used once, my opinion is that you’re almost certainly going to want to offload as much of the cost and complexity of RPOD to the reusable element in the system (the servicers), so you can minimize the complexity on the tankers and the client satellites.
- Eventually, similar concepts will likely want to be copied in other domains with similar orbital dynamics, where you have a large number of satellites and/or constellations distributed out between multiple planes (e.g. MEO, eventually Mars orbit, etc.)
My guess is this class of depot is going to be the first one that gets built, even if relatively speaking they look nothing like what most people would think of when they think of orbital propellant depots. If you’d like to learn more about the concept (and get their opinions rather than just mine), I’d suggest going over to the OrbitFab website, and poking around their resources and whitepapers.
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- And all the dry mass associated with more tankage, bigger thrusters, more solar arrays, etc. to handle that increased propellant load
- Focused on servicing in Sun Synchronous Orbits, because they’re the closest analogy to GEO, where you have a ton of satellites in a small number of planes, all in very similar inclinations, all going the same direction at the same speed. Also, the customer discussions I’ve had to-date suggested that earth observation customers, which tend to congregate in SSO, might be some of the more interested customers for satellite servicing.
- tl;dr a lot smaller than we had expected — I still want to flesh out the model further, and revisit some assumptions, but it looked like we could get prices low enough to be interesting even to high-end cubesat operators, eventually
- Slightly retrograde, near-polar LEO orbits that are designed so that their planes precess by ~1/365th of a rotation per day, so they always keep the same relative orientation to the sun, and always pass over a given point of ground at the same time of day
- Technically, the inclination needed for sun synchronous behavior varies slightly with altitude
- Which tend to use some flavor of hydrazine, sometimes in combination with some flavor of nitrogen tetroxide or MON
- One of the worst feelings in the world for a Space Situation Awareness operator has got to be watching a slow motion train wreck that you can’t do anything about, like watching a conjunction between two hazardous, dead and thus non-maneuverable satellites