An Asteroid 1.2 km across has been discovered at the Earth Sun L4 region. This is the second asteroid discovered in one of the Lagrange volumes of the Earth Sun with the other one about a third of that size. It is suggested that it is possible that there are more, quantity unknown, bodies in those regions. They are very hard to locate from Earth due to the distance and sun angle so actual number, makeup, and size in each L region is mostly speculation.
I think it might be possible that this is an ideal target for in situ resource utilization. The advances in cubesat capabilities open many new possibilities. A lander could test out a number of resource extraction techniques for immediate use. Regolith shielding? Steam rocket with whatever volatiles are located? Sintered structure in vacuum and low gravity? Low tech metal extrusion for structural components? Low tech film Solar sails?
The purpose would be to use it as a base to locate other materials in the region with a relatively small scope. And go visit them if found using local resources. Prospecting and exploring and evaluating. If there does turn out to be a large number of bodies in the L4 region, DeltaV per visit should be very low. This one is a C type, dark and carbon. There might just be enough variation in different bodies in the area to make it a resource rich destination.
How simple and cheap could a probe be and still investigate the possibilities??
As humanity’s ambitions in space increase, one of the biggest potential risks to to those ambitions is the dangers caused by the debris we’ve left on orbit over the first half century of space development. Since Sputnik’s launch in October of 1957, we’ve left over 18,000 pieces of debris on orbit that are larger than 10cm, and millions of pieces too small to track but big enough to destroy a satellite. One of the biggest threats comes from large satellites and more especially rocket bodies that were left on orbit during the early days of space development before countries properly realized the risks that leaving debris on orbit could pose. At last year’s IAC conference, a joint group of researchers from 11 countries published a list of the 50 most dangerous pieces of space debris on-orbit, and the vast majority of those 50 were rocket bodies, with most of them being ones launched by the now no-longer existent USSR.
Why Should We Care About Large Debris?
Derelict rocket bodies and large satellites have been known to breakup on-orbit, sometimes due to pressure vessels or batteries failing, and those explosions can create hundreds or thousands of new pieces of debris1. More importantly, derelict space objects can’t dodge, so while we can track them, there’s not much we can currently do if two school bus-sized derelict space objects are on a collision path at relative velocities several times faster than the fastest bullet2.
As Joe Carrol once put it, the best way to avoid creating “BBs” (untrackable but lethal space debris) and “hubcaps” (barely trackable lethal debris) is to get rid of the derelict school buses. If we want to see a world with multiple commercial LEO facilities, propellant depots, space settlements, and especially things like large fleets of Starship-sized spacecraft heading out to Mars or other destinations, we can’t allow the Low-Earth Orbit environment to become a shooting gallery of space debris.
What’s Standing in the Way of Solving this Problem?
Almost all of the most dangerous space debris up there was launched by governments as part of civil or military space missions. As such, it makes sense that governments should pay to clean up the environmental mess their activities created. While there are now early efforts in Europe and Japan to begin tackling Active Debris Removal of dangerous space debris launched by Europe and Japan, that still leaves most of the most dangerous pieces of space debris currently unaddressed.
As I understand it from conversations with my friends in the Space Law community3, a big part of the problem with addressing many of the most dangerous pieces of space debris is that as part of the Outer Space Treaty, which governs the actions of all major spacefaring nations, you’re not allowed to interfere with objects launched by other countries without their permission. Unlike on the oceans, there is currently no space equivalent of salvage law, or “flotsam and jetsam”. Once an object is launched by an actor within a state, that state retains responsible for those objects indefinitely. However, as I understand it, those states are only liable for damages if the object deorbits and damages something or hurts someone on the ground — if it damages something in space, holding them accountable would be really hard unless you could prove that they violated the accepted standards of care at the time they last had control of the object. So, as I understand it, most of the most dangerous space debris is owned by a country that can’t realistically pay to clean it up, can’t be deorbited by someone else without the original country’s express permission4, and space liability law won’t actually hold the owner responsible if that debris creates a ton of new debris, so they don’t have any incentive to clean things up, other than wanting to be a good citizen.
Additionally, the US government has been very reticent to fund developments in Active Debris Removal, because of the fear that adversaries might see these technologies as dual-use technology5, causing them to invest in similar technologies that could be used against the US.
Anyhow, at a World Economic Forum meeting on the topic of space debris and space traffic management, someone floated an idea for a potential way to solve these problems. I can’t remember who suggested the idea, to give proper credit, but I wanted to run with it, and try to bake it a little more6. The idea is what if we propose to clean up the most dangerous pieces of large debris via a joint international cleanup effort?
Concept for a Joint International Debris Remediation Effort
While we’re on the cusp of having the technology to solve the problem of large space object active debris removal (ADR), the policy and international relations issues are the bigger unsolved pieces of the puzzle. So the idea is, what if we find a way to jointly tackle the problem in a way that incentivizes all of the key players to act, and removes some of their bigger concerns about acting?
What I’m Proposing: A joint effort, involving at least the US and Russia, but possibly also China, Japan, and Europe, to capture and recycle7, and/or controllably dispose of the most dangerous 100-200 pieces of space debris.
Key Elements of the Concept:
Most of the funding for the effort would need to come from the wealthier spacefaring nations. We have the wealth to do this, and stand the most to lose from inaction.
To make it worth Russia’s while to participate, there should be a way for them to economically benefit from this effort — my suggestion would be to have them involved at least with the on-orbit recycling part of the effort. Helping them build up new and useful space economic capabilities and industries is a good incentive for cooperation. Though in theory, the funding states could maybe insist on these being joint ventures between companies in their countries and Russian companies, as a way to share in the upside.
If China also wants to participate in this a mutually-acceptable portion of the project could be done via Chinese companies, or joint ventures with Chinese companies.
It’s a good idea to have dissimilarly redundant RPO sensors, especially for something as dangerous as capturing and detumbling a spinning rocket stage. So, it might be possible to have both commercial-grade US and Russian (and Chinese?) RPO sensors on the vehicle, with the feeds from both of them being live streamed during RPO events for enhanced transparency8
Having the most sensitive operations, like the RPO and detumbling/grappling efforts, lead by commercial companies in more neutral countries might also help. Companies from the US, Russia, or others could still develop technologies for use on these missions, but have them operated by someone that is as innocuous as possible to both US and Russia. Conveniently, two of the main companies working on active debris remediation are a Swiss company, Clearspace9, and the Singaporean/Japanese/US/UK/Israeli company Astroscale.
There are probably plenty of other elements here, but I wanted to get the idea out there. A joint US/Russia or US/Russia/China mission like this could be a way to break the logjam on cleaning up the space environment before the situation degrades further, while also helping greatly accelerate development in advanced on-orbit operations that could help enable not only in-space recycling, but also in-space construction and eventually asteroid mining.
Watched a bit of the two suborbital flights, not that much though, certainly not with the interest I would have had roughly a decade and a half ago. I had thought back then that the technical route to routine space travel would be through incremental advancements from suborbital to orbital to cislunar to other planets and asteroids. Looks like I was wrong again.
One thing that stood out to me was the people trying to play in microgravity with limited room in which to do it. Much more limited in volume and more clutter than on the ZeroGee flights. Looked to me like time was limited enough that there was just time to glance out the windows and play half a round of ZeeGee catch.
A balloot in the spirit of the inflatable space structures might help here if some problems of fast deployment, rapid access, and safety could be addressed. A 5-10 meter diameter inflatable without seats and other clutter would give considerably more play room than either SS2 or NS. Fast deployment might be a bit of a challenge as there wouldn’t be time for a leisurely five minute deployment and check out.
I don’t see how the idea could be adapted to SS2, but it does seem that a VTVL ship could have a fast clamshell hatch in the nose that could allow people in a fairly tight cabin access to a large play area within seconds of reaching vacuum and engine cut off. So a ship that would have had six people might fly with a dozen, each of which would have a lot more room to play and experiment.
With a large balloot starting deceleration at much higher altitude, it seems possible that a ship could fly much higher without subjecting the participants to excessive gee loads. Every extra Mach number gives on the order of another minute of play time in microgravity. On the way back, the participants get back to their seats while the hatch closes. The balloot remains deployed until landing as a drag device incidentally reducing terminal velocity considerably, possibly enough to make a failure of engine relight survivable, if painful.
This seems to me an idea that, even if feasible, is very late to the party. It looks like orbital tourism is likely to become fully operational in the same timeframe as the two suborbital contenders. The contrast may well cut into the desirability of the short pop ups. One interesting factoid though is that the people that were insisting that orbital was 64 times as hard as suborbital have to be wondering what the two companies could have accomplished with 64 times the investment.
A lunar lander concept I haven’t seen elsewhere is a layover concept. A tall ship lands on its’ tail the way the several suborbital hoppers and the Falcon9 has been doing for several years now. After landing and checking the area to one side, a small thruster tips it over on two legs. Before reaching the surface, some landing rockets in the nose fire to bring velocity to zero as the nose lands on a couple more much smaller legs.
If the lander is 100 meters tall, the terminal velocity that must be braked before the nose landing is under 20 m/s. Stopping 100 tons just at the surface would require between 400 and 700 kg of propellant depending on the Isp in use. If feasible, seems like a fair trade off as opposed to raising and lowering people, supplies, and equipment 100 meters for the duration of the mission.
The lower legs on the pivot side would need to be able to handle loads from two directions, though the legs at the nose could be quite light. One alternative leg option is to have the lower pivoting legs articulate up under the body of the vehicle to split the loading better and to reduce braking thrust requirements a bit more.
The tip over landing should be quite simple as there would be no question of where the vehicle is in relation to the ground at any given time. Of course smaller vehicles would have even less trouble and use less propellant. A 20 meter high vehicle with a 10 ton payload would require only 20-30 kg of braking propellant.
[Author’s Note: Back in summer of 2019, shortly after Mike Pence announced the goal of having NASA return astronauts to the lunar surface for the first time in over half a century, I had the idea of doing a blog post about the benefits of fully-reusable lunar landers, and then going over a few of my favorite unorthodox reusable lunar lander configurations. I got side-tracked at the time by my entrepreneurial day job1, and by the time I freed up from that, Altius was busy supporting one of the three Artemis HLS teams, so I felt a blog post like this might be impolitic. Now that we’re no longer actively supporting that team, and given that I no longer have running a startup as an excuse for not blogging, I wanted to finish fleshing out these ideas and at least get them out there for discussion.]
While traditionally most lunar landers and lander conceptual designs have been fully-expendable, many people, including NASA have begun to see some of the benefits of reusable lunar lander systems. Some of those benefits as I see them include:
Lowers the cost of sending hardware to the Moon: This is the obvious one that people get about reusability, is that so long as it’s done right, it can significantly lower the per mission cost, especially if the lander can be reused many times over its lifetime.
Makes the program easier to throttle up or down: One of the big challenges the Augustine Commission noticed for NASA Human Spaceflight missions was that they could rarely afford to both develop new capabilities while operating previous ones. With a fully-reusable lander system, especially one designed to not require a huge standing arm to support it, much of the cost of a given mission could be the marginal cost of launching new crew/cargo/propellant, which means it’s easier to throttle down temporarily without losing the capability.
More amenable to non-critical-path international participation: With reusable landers, once they’re launched, incremental missions mostly require refueling, reprovisioning, and a crew swapout. Government space agencies typically don’t want to spend money outside of their country–they typically try to find a way where they can handle things through barter and no-exchange-of-funds agreements. For instance, in exchange for getting some of the ISS crew slots, Japan and ESA both developed cargo vehicles to bring some of the cargo that ISS needed. So they could spend their space agency money locally, and use that contribution to get ISS astronaut slots without having to develop their own crew launch capabilities2. Reusable lunar landers provide an easy barter option for international participation in NASA lunar missions — launching propellant and/or cargo. The nice thing is that if done wisely (say using a Low-Orbit HSF Depot in LEO), this might require minimal development cost for foreign countries, while allowing them to usefully contribute, but in a way where they’re not on the critical path, and they can throttle up or down their involvement as desired.
Creates near-term demand for lunar ISRU: Once you have reusable lunar landers, the vast majority of the mass needed per mission is propellant. Being able to source that locally could significantly reduce the cost of missions3, and could increase the capability of landers by enabling them to be refueled both in orbit and on the surface4. With reusable landers, you have established demand at an established price point, which makes closing the business case for lunar ISRU easier, so long as you can truly extract it cheaper than shipping it from Earth5.
Enables a much more ambitious exploration program: This should be obvious, but once you have reusable landers, you have tons more flexibility for doing things beyond simple flags-and-footprints missions. Things like lunar search and rescue, doing suborbital sorties from a bigger outpost to explore areas of economic or scientific interest, etc. become more feasible.
Anyhow, if you’re reading this blog, I’m probably preaching to the choir here, but I wanted to lay out some of my thoughts on why reusable lunar landers matter.
Unorthodox Reusable Lander Concepts
Given the desirability of fully-reusable lunar landers, it’s sad that most of the best-known reusable lander concepts have used a very similar landing configuration — tall skinny landers with the crew and cargo mounted on top.
In this series I wanted to highlight a few other potential fully-reusable lander configurations worth considering, some thoughts on variations on the themes, and their pros and cons. Only maybe one of these configurations is one I could claim to have invented, but I thought it would be worth highlighting some other good ideas that may or may not be as well known, especially to younger space engineers or enthusiasts.
In the following parts I’d like to discuss the following reusable lunar lander configurations:
I may think of and add additional configurations later if time permits. But next up: Bottom-loader SSTO landers.
Okay, so for the first leg of my sabbatical, we went on a trip to Utah to visit my family. It had been two years since the last time we had seen any of my family in person. Our last trip had been to go be there for my “little” brother’s wedding in the Salt Lake temple. A lot has happened since then, including Jeremy’s wife Emily graduating from BYU, my older brother Adam graduating from Western Governor’s, and Jeremy and Emily having their first baby.
So as early as last December, when it was clear that effective COVID vaccines were soon to be available, we had been notionally planning on going out sometime in May or June. Once Tiff and I had our shots scheduled, we decided to plan something for end of May. Serendipitously, by the time we went to leave, Tiff, Jonny, and I were fully vaccinated, and Jamie and Peter had at least been able to get their first shots. At the time we planned this trip, we had no idea that I was going to be taking the full-time position at Voyager and transitioning out of my Altius role. Once we decided to do that, we set my last day at Altius as the day before we wanted to leave for Utah, and then worked extra hard to make that happen.
I ended up working late my last day at Altius, so we ended up taking an extra day (the 21st) to finish packing and take care of passport things. It turns out that in our part of Colorado, all of the places you can apply for a passport require an advanced appointment to be scheduled, and all of them were booked over a month in advance. With expedited passports taking 4-6wks, and us wanting to leave on our Canada trip by the second week of July, we couldn’t wait that long. Fortunately, Utah didn’t have as much of a backlog, so I scheduled a passport meeting at the civic center for my parent’s town for our third day there. And then of course made it look like on Twitter that the whole Utah trip was some last-second idea we decided to do on a lark as a way to get our passport applications submitted, because silliness.
We had a great time out there, participating in the baby blessing for Jeremy and Emily’s little girl, Victoria, visiting with my family and all of my siblings (and Tiff’s little brother), getting to see Jenny and Alan’s new place they’re remodeling, visiting and getting longsword lessons from my long-time friend Danny Farnsworth (and then later catching dinner with him and his girls), catching lunch and playing pool with Tiff’s “fake adopted sister” and her family, visiting BYU and getting a tour of the new engineering building1, getting to “hike the Y” with Jonny, hiking Bridal Veil falls with the family, a visit to the cemetery on our way out of town, and a side trip to Arches National Park on our way home.
We also got to stay in scenic Parachute, CO because the Eastbound I-70 was closed due to a huge accident in Glenwood Canyon. All the hotels in the town where the road closure started (Rifle) were completely full by the time we got there around midnight, but fortunately we only had to backtrack to Parachute. And we were able to make it home Saturday afternoon.
Anyhow, here are a few pictures from favorite parts of the trip:
Anyhow, that was the first leg of our adventure. We’re now back in Colorado, starting to pack up so we can move out of the house we’ve been renting for the last 8yrs. Tomorrow morning the dumpsters arrive, and Friday we get several of U-haul’s equivalence of the PODS mobile storage units. Wish us luck!
I imagine most of you who read Selenian Boondocks probably also follow me on Facebook and/or Twitter. But for those of you who don’t, as of May 21st, I decided to move up to a full-time position at my parent company, Voyager Space Holdings, and transitioned out of my role as President and CEO of Altius Space Machines1.
This will be an interesting change, basically moving into more of an intrapreneurial role rather than the entrepreneurial role I’ve been in for most of my career. As the VP of On-Orbit Servicing at Voyager, I’ll be providing leadership for their whole On-Orbit Servicing portfolio, including helping guide Voyager investments/acquisitions in the OOS space, developing and implementing a go-to-market and technology development strategy for Voyager OOS capabilities, and generally leading all things OOS at Voyager.
As part of this transition, I’m taking a 3-month sabbatical to recharge before jumping into my new role2. As part of my “unplugging” efforts, I’ve signed off of social media, unplugged from things work related, and am planning on using this blog as my primary means of communicating with people about my family’s misadventures this summer.
High level plans for the sabbatical include:
A trip to Utah to visit family for the first time in two years (last week).
Packing up and moving out of our current rental (most of June).
Some camping stuff (late June/early July).
A roadtrip to Canada3, the US Pacific Northwest, and possibly Alaska4 (mid July/early August).
Looking for a new house to buy or rent5 in the Denver North area, and getting resettled (August).
My first week back at Voyager should be just in time for Space Symposium in Colorado Springs this August. Between now and then, I’ll mostly be blogging about our travels, but I may do a space blog post or two (not work related) that had been sitting on the backburner due to running a bootstrapped aerospace startup.
Oh, and if you want to comment on the blog posts, please comment here instead of on Twitter. I’m trying to avoid getting sucked into Twitter comments while I’m gone, and will only be on long enough to post a link6.
I noticed the other day that it had been over a year since I posted, and at least double that since posting on a space topic. I’ve had a bit going on for distraction, and haven’t had anything new to add in the way of space related concepts.
A few years ago with the recession going away, we started trying to expand the construction company back out to where it was pre-recession. Went through a couple of dozen people trying to expand with no success. Most couldn’t pass a drug test, or didn’t have transportation, or were unreliable, etc etc. Focusing on the company tends to restrict hobbies like blogging about space. I have my core crew and myself with no new people.
Hired an intern (paid) from Florida Polytechnic two years ago for the summer. His job was to work on remote controls for various small equipment we use. If I could not find the people to do the hard work, possibly make the work itself easier. He accomplished nothing useful in three months. Literally nothing. I have been slowly learning to get this done myself. It would be far more efficient if I could hire someone that understands the field, but have failed to find them.
We had a slowdown during the Covid lockdowns last year followed by a slammed schedule from last May through April this year. Slowing down some now with material shortages and difficulties with building departments permitting and inspections.
I got married in February this year and have had other things on my mind. We are buying a lot and will build over the next year mostly with cash. Granddaughter graduated University of Georgia this year with a degree in Biochemical Engineering. (don’t know what that means but she gets a good job right out of school) Other things going on locally.
Noticed a comment on another site that suggested that many of the concepts discussed here have been made obsolete by the success of SpaceX. While I will acknowledge that some of the concepts that made sense in the first place (many didn’t) may be less important, I still believe there is room for improvement all the time. SpaceX may be on the way to becoming the Walmart of space launch, but I believe there are many companies that are well poised to become the Dollar Stores and Discount Auto Parts of space. Those companies may benefit from anything that gives a competitive advantage. One of them may also pass on the outside when we are looking at the infield. As an aside, I am impressed with the progress SpaceX has made, while being very uncomfortable with the “hand it to Elon” meme going around.
I am inclined to restrict my limited blogging to things that are either new, of where I have some experimental evidence on a previous idea. I did an experiment a few years back that seems to support a method of doing the Thrust Augmented Nozzles that Jon wrote up only quite low tech. I have an idea of an inexpensive way to focus sunlight better over long (GEO to surface) distances and will run an experiment or two before discussing. And of course my various pump concepts still fascinate me, but would bore anyone else unless I produce some experimental proofs. Same with compensating nozzles.
Consider a vehicle that undertakes a first , then picks up a payload and undertakes a second . It carries the propellant for both maneuvers, but only on the second maneuver does it have the added mass of the payload. This situation might be representative of:
1. a one-way lunar vehicle, arriving in low Earth orbit fully fueled but with no payload. It executes a trans-lunar injection burn and then a lunar orbit insertion burn, all with no payload. It then picks up a payload in lunar orbit and descends to the surface with the payload and lands with essentially all its propellant expended.
2. a reusable lunar lander, based on the surface of the Moon, fully fueled by lunar propellant but lacking any payload, which ascends to a lunar orbit and recovers a payload in that orbit, then descends to the surface, landing with nearly all its propellant expended.
3. a space tug that departs for geosynchronous orbit and recovers a satellite, then returns with it to a low-Earth orbit for repair.
First, define the mass conditions at the beginning and end of :
Alternatively, and just as importantly, the conditions bracketing can be described in terms of an initial mass:
This expression can be conveniently rearranged to yield the propellant mass consumed by the vehicle in :
In a similar manner, we define the mass conditions at the beginning and end of :
We can also express the conditions bracketing in another way, in terms of initial mass:
Yet another approach will later yield a useful relationship:
The propellant mass consumed by the vehicle in can also be expressed in a manner analogous to prop1:
Now we are positioned to calculate the total propellant load:
substituting the definitions for prop1 and prop2
collecting terms and simplifying
now let us define the vehicle’s “dry” mass entirely in terms of initial-mass-sensitive (), propellant-mass-sensitive (), and payload-mass-sensitive () mass terms. This is a substantial simplification, but it should do for now.
substituting the definition of the vehicle’s mass in
we collect terms related to the initial mass on the left hand side
Take equation(3) and multiply it through by :
then substitute the RHS for equation(5) in equation(4) and collect terms
With all terms relating only to initial mass and payload mass, a general expression for payload fraction can at last be defined:
In the last post in this series, I discussed the idea of large-scale depots for human spaceflight applications, which operate in fixed, low-orbits. While the final post in this series will investigate human spaceflight depots that operate in fixed higher orbits1, in this post I want to talk about situations where you want to refuel with large amounts of cryogenic propellants in an orbit where a permanent depot doesn’t make sense — in temporary high-elliptical orbits on your way into/out of a deep gravity well like Earth’s. The background on these depots involves getting into the weeds on some orbital dynamics, but I’ll try to keep it as understandable as possible for the layperson2.
Orbital Dynamics Background for Roving Depots
For an outbound interplanetary mission, there are two obvious places to do refueling — one at the first safe stopping point after leaving the planetary body3, and other is in an orbit that’s just shy of leaving the planetary body’s gravity well4. For a high-thrust/low-Isp departure, like you’d have with a chemical or nuclear thermal rocket, this would be a highly elliptical orbit, with the periapsis as low as possible5, and a apoapsis as high as practical6. There’s just one problem — in order to leave on an interplanetary trajectory from a highly elliptical orbit, that elliptical orbit’s periapsis has to be in the right place7, going in the right direction8, at the right time9. As you can probably guess, the odds of a specific highly elliptical orbit for one interplanetary departure trajectory lining up with another specific interplanetary departure trajectory at a specific point in the future is really, really low.
While I was working with Mike Loucks and John Carrico on our three-burn departure orbital dynamics papers (reviewed previously on Selenian Boondocks here, here, and here), I realized that there might be a way around this problem of elliptical orbits not lining up for future missions, especially if you had a reusable, mobile depot capable of both entering into and then returning from that highly elliptical orbit.
One or more such “roving depots” could work with reusable tankers10 and a low-orbit depot to preposition propellants into a highly-elliptical orbits for specific departure missions, with the mission stack for a given mission only having to travel out and rendezvous with the depot on the last time the departure highly-elliptical orbit plane lines up with the low-orbit depot’s plane. Once the mission stack has refueled, and left the roving depot (on its way to doing its final plane change and departure burn), the roving depot can return to the low-orbit depot the next time its plane lines up with the low-orbit depot, enabling it to be refueled and prepared for its next mission.
Application: Significantly enhancing the performance benefits of using a low-orbit depot, by providing one last chance to top off before heading out into interplanetary space. This can range from topping up a smallsat interplanetary stage to assembling and fueling a large interplanetary human mission or multi-ship convoy.
Location: As described earlier, roving depots are mobile depots that start a given mission at a low-orbit depot, maneuver into a highly elliptical orbit that is targeted for a specific interplanetary departure, and then returns to the low-orbit depot after the refueling operation is over.
Size: Depends strongly on what is being refueled. These could be as small as 100’s of kg for refueling a smallsat launcher interplanetary stage, up to 10’s to 100’s of mT for refueling larger human spaceflight missions.
Propellant Types: The propellant type for a roving depot will be driven by the propellant type used for the mission stack stage that is performing the interplanetary injection. For smallsat interplanetary stages, this is almost certainly storable, while for human spaceflight missions this would likely be LOX/LH2 or LOX/CH4.
For roving depots and tankers operating around planets with an atmosphere, you’ll almost always want to use some form of aerobraking or aerocapture system11. Because you want your highly-elliptical orbit to have a relatively low periapsis for departure performance reasons, it only takes a little burn at apoapsis to lower the periapsis far enough for practical aerobraking/aerocapture.
Something you may have noticed is that the distinction between a roving depot and a tanker is sort of blurry. As I see it, they exist in sort of a continuum with expendable tankers on one end and reusable roving depots on the other extreme. The key differences are that roving depots would be more likely to have more significant propellant cooling (active and passive) capabilities, likely be designed to handle a wider range of client vehicles, and likely carry a lot more rendezvous, prox-ops, and grappling hardware.
I think the tankers that top off a roving depot in its mission orbit would likely be just minimalist upper stages with an aerocapture system, with as much of the smarts and complexity offloaded to the roving depot as possible. Minimizing the dry mass per unit propellant hauled from the low-orbit depot to the roving depot.
I think the roving depots, since they move less would strike a balance of complexity/robustness between the tanker and a fixed depot. You don’t want to go too crazy by throwing dry mass at problems like RPOD12 reliability, and mission robotic flexibility, but you want to be able to make the tankers as dumb and lean as possible, by offloading capabilities to the roving depot as much as possible.
This idea of roving depots can be combined with in-space assembly/manufacturing in the highly-elliptical orbit, sending up parts/materials/propellant for the mission every few months when the low-orbit depot lines up with the departure plane, assembling the overall stack, and then only sending the mission crew up on the last time the low-orbit depot’s plane aligns with the departure plane.
For chemical propulsion, travel to interplanetary destinations like Mars and Venus typically is only feasible once every 1.5-2yrs (depending on the synodic period). The use of roving depots can allow you to spread out the propellant launches for missions to a planet like that over a longer period of time. Instead of having to launch all the propellant and hardware for a given “launch season” all in the few months leading up to that season, you may be able to set up multiple roving depots, aligned for that departure opportunity, and then top them up over the course of a year or more. This allows a much smaller fixed low-orbit depot to support a lot more mission capacity than you would otherwise think, because the low-orbit depot wouldn’t need as much surge capacity, since you could likely plan things in advance.
This also suggests to me that for busy planetary systems like Earth, if roving depots take off as a concept, they’d likely significantly outnumber the low-orbit fixed depots.
One drawback to using roving depots and these highly elliptical parking orbits is that you end up putting a lot more van allen belt passes on your hardware than you would otherwise. While you typically won’t have your crew onboard until late in the process, your electronics, especially on your roving depot, will take a lot more radiation than it might otherwise.
On the plus side, you spend a lot less time in LEO where MMOD13 issues are worse, so your mission hardware doesn’t need as much MMOD shielding for how long it is in orbit. Additionally, since you spend most of your orbit far away from the planet, these highly-elliptical phasing orbits tend to be much easier for long-term cryogenic thermal storage.
One other consideration is that the longer you spend in the phasing orbit, the more orbit adjustment maneuvers you’ll need to perform. This may put some limitations on how long you can practically build things up in a phasing orbit for a given mission. If you’re just pre-staging propellant long in advance for a large convoy mission, you may be able to let your orbit drift a bit, and only trim things up shortly before departure, but we’d need to run the numbers on how practical that is. This is more of the case for orbits with very high apogees/long periods, especially in multi-body systems like the Earth/Moon system where, lunar perturbations become more of a problem with high apogee phasing orbits. In theory, it may be possible to craft an orbit that starts in a lower, shorter-period phasing orbit initially, but that then boosts up to a higher phasing orbit shortly before the final refueling of the mission stack. Long story short, there are lots of knobs to twist on optimization14.
One other problem I’m handwaving away as probably solvable is the complexity of rendezvousing with a roving depot in a highly-elliptical orbit. This will definitely be a lot different than the relative dynamics of rendezvous with objects in LEO. Though fortunately this should be pretty similar with trying to rendezvous with a facility in NRHO, so it’s something that probably has had a lot of recent thought put into it.
Roving depots around planets with lots of moons will definitely be more challenging from an orbital dynamics standpoint, but could be really enabling for missions to the gas giant planets. Especially if you want to do return trips, and don’t have something as cool as an Epstein Drive available yet. In some ways this reminds me of the idea of using base camps and/or storage depots when planning expeditions up mountains, or the early Antarctic expeditions. Once you have the right building blocks in place, there’s a lot you can do.
When you think about what you can do with the combination of low-orbit depots and roving depots on both ends of the mission, especially supported with ISRU and reusable launch, you can actually do very large and capable missions without needing super-heavy lift vehicles. It’s kind of amazing what you can do with a refuel early, refuel often, reusable space transportation architecture.
Next Up An Updated Propellant Depot Taxonomy Part VII: Human Spaceflight Fixed Depots (High-Orbit)