After a brief hiatus, I’d like to continue this series with the type of depot I’ve spent the most time thinking about over the last several years, and the first type of depot that really fits the mold of what people typically think of when they hear the word propellant depot–depots focused on refueling smallsat launcher upper stages. Before I get into the details of expected characteristics and considerations of this type of depot, I’d like to give some backstory on what led me to thinking of this type of depot1.
For those of you who haven’t been neck deep in the politics around propellant depots over the past decade and a half, suffice it to say, large-scale depots focused on human spaceflight missions have excited more than their fair share of political opposition. Because of that, I started looking at other situations where cryogenic propellant depots could make economic sense independent of any change in the political dynamics around depots for human spaceflight2. Of several potential economically-viable depot markets, the one that I became most interested was in using depots to support sending small satellites to destinations beyond LEO.
Over the past two decades, there has been a steady and now rapid increase in the utilization of satellites smaller than 500kg (“smallsats”) in LEO, driven in part by the continuing miniaturization of electronics and sensors, new manufacturing techniques, and increasing rideshare and dedicated launch capabilities. We’re now also starting to see interest in sending smallsats further afield, with some companies doing GEO smallsats for serving smaller GEO telecom markets and Bringing into Use applications3, and even several groups working on interplanetary smallsats missions to the Moon, Mars, Venus, and beyond. One of the biggest challenges with beyond LEO smallsat missions is that almost all of the options for getting smallsats to destinations beyond LEO suck.
Specifically, here are some of the current options for launching smallsats beyond LEO and some of their limitations:
- Rideshare: If you’re going to a popular enough orbit, sometimes you can hitch a ride as part of a bigger mission. Unfortunately, as a secondary payload, you have little control over the timing, you can only go to places where others are going (or get dropped off along the way)4, and there are often lots of restrictions and added scrutiny of secondary payload propulsion systems. If you’re going someplace unpopular, rideshare may at best modestly reduce the amount of delta-V your spacecraft has to produce.
- Buy a Bigger Flight: You could also just secure a larger flight than you need, like say on a Falcon 9, and try to sell the rest of the space. But then you’re stuck herding cats, and if one of them isn’t ready on time, you may have to foot more of the bill if you can’t afford to delay the mission5. And selling the rest of the space only works if you can find an orbit you can drop people off in along the way that they actually want to go to, which can add additional mission constraints to an already complicated mission.
- Make a High Delta-V Smallsat: You could also just try to make a really high performance smallsat, maybe with staging, drop tanks, and/or an electric propulsion system. But in most of these cases, your propulsion system now dominates your satellite, the amount of net usable payload may be very sensitive to even modest mass growth in your propulsion system, and in the case of EP systems, you may dramatically add to the amount of time it takes to get to your destination.
- Fly on a Dedicated Smallsat Launcher with a Third Stage: If your payload is small enough, many of the dedicated smallsat launchers are now either offering or contemplating the use of a small chemical or EP third stage. RocketLab for instance can send 15-40kg net to destinations beyond LEO. But the cost in $/kg delivered to the destination can be >$400k/kg.
From talking with at least a few developers of beyond LEO smallsats, what would be nice would be a way to launch your satellite on a dedicated mission that was reasonably right-sized for your spacecraft, with a propulsion system that could get you to your desired orbit as quickly as possible, while minimizing the propulsion requirements on your satellite. And that’s where Smallsat launcher refueling depots come in.
Smallsat Launcher Refueling Depots
Application: Refueling the upper stages, kick stages, and payloads launched by smallsat launchers, for sending dedicated, on-demand missions beyond LEO to MEO, GEO, Cislunar space, or interplanetary destinations.
Location: LEO, likely a singular station (or a small number of stations), ideally at a low altitude (<500km), mid-inclination, and near where other missions are going to. My current favorite location is in an ISS-trailing ~400km x 51.6 degree orbit, trailing as closely as NASA will allow (hopefully <200km behind).
- The moderate inclination is to maximize the range of interplanetary departure declinations that can be hit6, without being too high of inclination for practically getting to GEO or equatorial MEO destinations.
- For deep space departures, you’d prefer to have your perigee at departure be as low as possible, to maximize the benefit of the oberth effect. Also for most smallsat launchers the LEO payload falls off somewhat rapidly. But on the other hand you don’t want to be so low that you’re having to waste a lot of propellant on stationkeeping for your depot. 400km like ISS is a reasonable tradeoff.
- Picking an orbit where many other missions are going to, like an ISS-like orbit, potentially enables buying excess propellant from those other missions. As it is, most missions to the ISS massively underutilize the mass capacity of the launch vehicles, meaning there is potential for buying leftover propellant from commercial crew/cargo launches. Because the primary customer has paid for the whole mission, selling this leftover propellant would be pure profit for the launch operator, potentially enabling pretty interesting price points.
Size: At least 5-10mT capacity, maybe up to 20-40mT on the high end.
- You want something small enough to launch on a single launch, either as a secondary payload on one of the larger launch vehicles, or as a dedicated launch on one of the larger smallsat launchers (Relativity, Firefly, ABL, etc.)
- You’d like enough propellant capacity to handle at least 2-3 missions with your largest customer, because mission demand may not be well synchronized with when you can get propellant especially if you’re buying excess propellant from ISS missions.
- Ideally you want to be bigger than the excess propellant capacity of say a Falcon 9 Dragon mission to ISS, or an Atlas or Vulcan mission with other crew/cargo vehicles, so you can buy as much propellant as possible when its available.
Propellant Types: LOX plus Kerosene and maybe Methane for the upper stages, some form of storable bipropellant for the kick stages and payloads, and helium for pressurization.
- Most of the existing smallsat launchers use LOX plus a hydrocarbon propellant (mostly Kerosene, but with a few looking at Methane) for their main stages, and some form of storable bipropellant combo (many using HTP plus some sort of hydrocarbon) for the kick stages. Exactly which storable propellant combos get settled on will likely be driven by which companies first start taking this type of depot most seriously.
- Even though LH2 is almost certainly not a propellant smallsat launcher customers are likely to buy anytime soon, buying some leftover LH2 from a Vulcan or other LOX/LH2 upper stage might still be useful as an expendable coolant to supercool the other cryogens, potentially eliminating cryogenic boiloff potentially without requiring active cooling if you can get LH2 frequently enough.
- For the helium, it might be worth trying to recover the helium from the upper stages being refueled, removing any oxidizer/fuel trace impurities, and recompressing it. In that way you wouldn’t actually need much helium other than to make up for losses and cold-gas usage on the upper stages. The helium you’ll likely want to try to keep as cold as possible, to make storage easier, so you likely want to have it in close thermal contact with the LOX or methane tanks. though for safety reasons you might not want the helium tanks inside the LOX tanks.
Other Considerations for Smallsat Launcher Refueling Depots
- You’ll almost certainly want to design the depot to last as long as reasonably possible. This will likely drive you to make the depot robotically serviceable with deliberate modularity for likely wear components.
- This class of depots will almost certainly be designed for purely robotic operation, without any human habitation capabilities. Though you might want to make the serviceability designed for both robotic and manual servicing. Maybe.
- You’ll almost certainly want to have your depot repurpose at least one of the main propellant tanks of the upper stage that launches the depot, once the depot has been delivered to orbit. As I’ve shown in previous papers, this is a great way to get free depot tankage capacity. If the depot was launched as a secondary payload on Vulcan, you could get up into the ~40mT capacity for LOX and Kerosene if you wanted to.
- You’ll almost certainly want to have a fairly capable capture and manipulation robotics capability, with RPO sensors on-board. The goal is to offload as much requirements-wise from the customer upper stages/kick stages to the depot as possible. My personal preference has been to see if the customer smallsat launcher upper stages/kick stages can maneuver their stack adequately to do a drift-by near-rendezvous7 close enough that one or more deployable capture arms can magnetically grapple and retract the stacks — ie avoiding trying to make the smallsat launcher stacks capable of full RPO/docking maneuvers, while also trying to see if you can avoid the depot having to do rendezvous maneuvers in nominal cases. The depot is probably the much heavier of the two vehicles/stacks, so it’s more efficient to have the smaller vehicle move, but you want to do it with the least mods to a stock upper stage–ideally just grapple and refueling ports, and an upper stage capable of being remotely guided by the depot8.
- Most upper stages and kick stages have RCS thrusters that enable at least 3-axis attitude control. Most upper stage engines also likely have a cold gas purge function to blow excess propellant out of the engine prior to a relight (to avoid the risk of a hard start). It may be possible to use such a cold gas purge with the main engine to provide enough axial thrust, and with a sufficiently tight minimum impulse bit, to enable the close flyby rendezvous maneuvers. This is something I haven’t had the budget to simulate in detail, but it’s my preferred approach.
- I should probably do another blog post at some point about the work Altius has done on cryogenic and storable refueling for rocket upper stages. Long-story short, our approach is focused on minimal modifications to the T-0 umbilical ports that the stages already typically have to have to enable reconnecting umbilicals on orbit, combined with poseable individual-fluid transfer lines, because you’re going to be servicing a wide range of customers where you might be able to get standardized T-0 quick disconnect interfaces, but you’re almost certainly not going to be able to standardize things at the umbilical plate level (since not all stage use the same propellant combinations). More on that some other day.
- Alternately, if such a close rendezvous approach isn’t feasible, another option would be to use a servicing tug to grapple the smallsat launcher stack and tow it to the depot. Depending on where the OOS ecosystem is by the time such a depot exists, this may be a relatively simple operation that can be “bought by the drink” by a servicer that also services other clients.
- Michael Loucks, John Carrico, and I wrote an AAS paper that I discussed in a pair of previous blog posts, on the orbital dynamics of using such a depot. Some key takeaways from that study that are worth repeating in this blog post include:
- For most beyond LEO missions, you’re going to want to refuel both the launcher 2nd stage and a storable propellant kick stage. The 2nd stage does your boost to a highly elliptical orbit, and the kick stage does the rest (plane changes at apogee, circularization burns if going to MEO/GEO, injection burns if going to lunar or deep space destinations).
- For payloads that themselves don’t need much refueling, such a system was shown to enable you to send >80% of your LEO payload for the smallsat launcher onto a TLI, TMI, or TVI trajectory, and you don’t even need to top the second stage up all the way typically. So for RocketLab, if you could refuel and reuse their 2nd stage and their Photon stage9, you could send on the order of 250kg into an interplanetary trajectory this way, instead of the ~15-25kg they can send currently. With Virgin Orbit you’d be talking up to ~400kg, and for Firefly you’d be talking about nearly a ton.
- One last consideration is the economics of such missions. Since your depot is buying its propellant wholesale, in bulk from larger, more efficient rockets, the depot enables much lower $/kg costs for interplanetary missions than you would think. I need to redo the analysis with updated details, and a good set of eyes looking at assumptions, but when I ran the numbers previously, you could run a pretty profitable depot where the depot mission was 1.5-2x the cost of a normal LEO launch by the smallsat launcher. But since using a depot increases the amount of beyond LEO payload you can launch with a dedicated smallsat launcher by typically a factor of 10x over what you could do with just putting a kick stage on without LEO refueling, my previous calculations were suggesting interplanetary missions at a price point around $50k/kg for many of the smallsat launchers was feasible, and with total mission prices much less than half the cost of a reusable Falcon 9. As I said earlier, I need to rerun the numbers, but I think those would be in an interesting price range for customers.
Anyhow, while we could definitely go on and dig way, way deeper into the technical weeds on how to do the depot, how to make an upper stage compatible with such a depot, how you’d do the rendezvous/prox ops for getting to the depot, and the economics of such a depot, hopefully you can see why this concept is potentially very powerful for enabling affordable, dedicated beyond LEO smallsat missions.
[Edit 11/16/2020: I completely buried the lede, and totally forgot to mention the fact that my company is working with Eta Space, a Florida-based cryogenic propellant management startup on their LOXSAT-1 flight demo under NASA’s Tipping Point Technologies program. This 9-month flight demonstration, which is set to launch on a Rocket Lab Electron vehicle in 2023, will demonstrate a suite of cryogenic fluid storage, management, and transfer technologies, including using a version of Altius’s cryogenic refueling coupler to transfer LOX between two tanks on orbit. Eta Space’s planned follow-on depot, LOXSAT-2, would be a LOX-Kerosene depot, potentially of the very kind described in this article. They’re targeting a 2025 timeframe for fielding this depot, and I’ll be supporting the Eta Space team on conversations with interested partners/customers, and development of design and mission concepts for LOXSAT-2. After years of talking about propellant depots, I’m personally really excited to be supporting Eta Space and NASA on this project.]
Next-Up An Updated Propellant Depot Taxonomy Part V: Human Spaceflight Fixed Depots (Low-Orbit)