An Updated Propellant Depot Taxonomy Part III: GEO Depots

Of the six depot types I’ll be describing in this series, GEO Depots are probably the least fully-baked of the concepts for me, mostly because I’ve only had limited involvement in the traditional GEO satellite world. But I wanted to share a few preliminary thoughts about how one would do depots in support of activities in the Geostationary belt before moving on to talk about more traditional depots. And before getting into the specific characteristics I think GEO depots will likely have, I wanted to share some background thoughts on design drivers for GEO depots, to walk you through my logic on where I think things will go.

Background on Geostationary Orbit

First off, for those of you newer to spaceflight, what is the Geostationary Belt1? Basically, when people talk about GEO they’re typically talking about a circular equatorial earth orbit whose altitude (~35,786km) is such that its orbital period is exactly one day, meaning that to an observer on earth, the satellite always stays in the exact same position in the sky. Sir Arthur Clarke came up with the idea of using such orbits for telecommunications in the 1940s, and today there are hundreds of GEO satellites, providing not just communications, but also weather observation, and other commercial, scientific, and military functions.

Animation of a geostationary satellite
Illustration of Geostationary Orbit (from the NASA Basics of Space Flight page)

Satellites in GEO experience various orbital perturbations2, that require stationkeeping maneuvers to prevent the satellite from slowly drifting out of position. These stationkeeping maneuvers amount to ~52m/s per year of delta-V requirements. Satellites are designed with enough propellant to not only get to GEO, but to perform stationkeeping for a certain amount of time, and then at end of life boost themselves up to a “graveyard orbit” that’s typically ~200km above GEO.

One nice thing about GEO compared to LEO is that pretty much everything is in the same plane, heading in the same direction, all moving at about the same velocity3. Which means that moving between different GEO satellites never requires costly inclination changes. You basically raise or lower your orbit into an orbit with a slightly different orbital period and either catch up with the new satellite, or let it catch up with you. But to give you an idea of scale4, an object in the graveyard orbit, 200km higher than GEO, will take ~10min longer to complete an orbit, which means that over the course of ~140days, the GEO satellite will “lap” a spacecraft in the graveyard orbit5. I’ll get into why this matters later.

Another characteristic of GEO is that most GEO satellites are pretty big. Some GEO satellites are bigger than a school bus, can weigh several tonnes, and cost hundreds of millions of dollars. There are some groups like our friends at Astranis that are trying to develop GEO smallsats (~350kg in their case), but the vast majority of GEO satellites, whether commercial, civil, or military, are over 1 tonne.

Because of the very expensive nature of satellites in GEO, there has been a lot of interest in servicing GEO satellites, primarily to extend their life. Most GEO birds are designed with ~15yrs of propellant on-board, and normally when they run out, they have to be boosted to a graveyard orbit to avoid becoming a “zombiesat” that endangers other GEO operators. However, in many cases by the time the propellant starts running low, the satellite may still be economically useful. Maybe it’s been transferred from a higher value orbital slot to a lower value one (one owned by a less populous or less well developed country), but in many cases, the satellite can still be producing millions or dollars per year of revenue. Because of these realities, several companies have proposed servicer vehicles that could fly up to a satellite that’s almost out of fuel, dock with it, and then either refuel it or provide “jet pack” services where the servicer takes over stationkeeping maneuvers. Our friends at Northrop Grumman Space Logistics Services finally pulled off the world’s first successful commercial satellite servicing mission just this year, with their MEV-1 vehicle. Other players in the field include our friends at Astroscale/Effective Space6, as well as our friends at Maxar7. Since GEO satellite aren’t typically designed for servicing, these missions have focused on leveraging structural features on GEO satellites (liquid apogee motor nozzles, or the separation system hardware left on the satellites after they’re released by their launch vehicles) to mechanically grapple the satellites. Initial servicing missions are focused on what I was calling “jet pack” services where after grappling with the client satellite, they take over stationkeeping and other propulsion requirements. But most of the players have plans to move to fancier services, such as using a servicer to clip on a Mission Extension Pod to provide the jetpack services without tying up the more expensive servicer, or using robotic manipulators to fix stuck deployable structures, etc.

Illustration of a MEV providing stationkeeping services to a client satellite (Credit: Northrop Grumman Space Logistics)

One final attribute of GEO is how things get there. Most GEO satellites are launched on a rocket into a Geostationary Transfer Orbit (GTO), which has a perigee down in LEO, and an apogee up at or above GEO8. Occasionally, mostly for military GEO satellites, the rocket will also then perform a second burn at apogee to circularize the orbit and drop the satellite off directly in GEO, but for most commercial satellites, the satellite itself provides the ~1.5km/s worth of maneuvers to raise perigee, lower inclination, and circularize in GEO. Some satellites do this quickly, in a small number of chemical propulsion burns9, others take months slowly spiraling out using more efficient solar electric propulsion.

What I think This Means for GEO Depots

My first opinion when I started looking at GEO depots, was that because it was a lot easier to get around GEO without using lots of propellant, that maybe it would make sense to have a single aggregated depot, located in a circular orbit somewhat above GEO. But then several factors made me reconsider that as I dug deeper:

  • First, at 200km above GEO, you’re talking a very long time for the depot to make it’s lap around the GEO belt. Servicers are expensive, so you don’t want to have to wait forever to replenish/resupply. You can cut into that time by having your depot higher than GEO (where the relative orbital periods mean that it takes less time to do a lap around the belt), but the round trip delta-V requirements start growing pretty quickly on you. Including a rendezvous with the depot, you’d be at over 60m/s of round-trip dV with a depot 800km above the GEO belt, but it would still take over 1 month for it to do a lap, which means you could be waiting for weeks. If media reports that Intelsat is paying Northrop Grumman $13M/yr for MEV jetpack services are correct, that means that waiting a month could be costing you $1M worth of revenue.
  • Also, because most missions, and thus most rideshare opportunities go to GTO, not GEO, you’re going to have to use a decent amount of propulsion to get any propellant or supplies (spare extension pods, tools, etc) up to the depot10. Sending a tug down to pick things up in GTO and bring them back to GEO may be possible, but that’s a lot of dV 11, and for low-thrust, potentially several months. I haven’t done the detailed trades, so I can’t be sure, but I think this suggests that in many cases you’ll want the depot deliveries to be self-propelled to some extent.
  • Also, because no GEO satellites are currently designed for refueling, most of what the depot(s) would be storing will be propellant, extension pods, tools, and other supplies for servicers themselves. That means that almost all depot customers in GEO will be satellite servicing vehicles with full rendezvous, proximity operations, and docking (RPOD) capabilities, and in many cases with sophisticated servicing robotics.
  • Lastly, all GEO servicers I’ve seen use either storable chemical bipropellants or storable electric propulsion gases. Nobody tends to use cryogenic propellants or other things that need careful thermal design or active cooling.

To me, all of these things undermine the case for a unitary depot, and push me in the direction of a more distributed depot arrangement, like in LEO. Basically, you don’t want to have to take a long time to get to/from a depot, so having more than one of them spread throughout GEO makes sense. Since you likely will need on-board propulsion anyway to get your supplies into the depot orbit, it’s harder to have a dumb tanker that supplies a more sophisticated unitary storage facility. Since your customers all have to have RPOD capabilities, and most have robotics, there’s less need for that on the depot side, so once again less benefit to a centralized depot that can amortize its robotics/RPOD capabilities over lots of customers. Lastly, since the propellant mostly doesn’t need much propellant conditioning, there’s not a big advantage for storing it in a larger quantity like there would be if the propellant needed was cryogenic.

Anyhow, so based on all of that, here’s my best guess at what I think we’ll see for GEO depots.

GEO Depot Characteristics and Considerations

Application: Propellant for refueling GEO servicers and “extension pods”12, spare extension pods, and tools for servicers.

Location: In a circular orbit at moderately higher altitude than GEO (likely 100-400km over GEO), spread out fairly evenly to minimize the wait time before a depot has drifted close enough to boost to and rendezvous with.

  • Note that for propellants, one depot location is like another, but for specific tools or pods, it may be important to have the depot preposition itself in the right orbital position relative to the servicer, rather than just going to any old GEO depot.
  • For payloads launched via PODS or similar GEO-insertion opportunities where the material may not have its own propulsion, it may need a servicer to capture it, and drag it up to a safe operating altitude for temporary storage if it’s not needed right away.

Depot Size: Likely ESPA class (180-400kg). Most rideshare opportunities to GTO are via ESPAs or similar adapters. PODS-delivered options may be more in the 100-150kg range.

  • As described elsewhere, I see these depots as likely being one or more propellant tanks, attached to a propulsion system, with some basic spacecraft bus functionality. They likely wouldn’t have RPOD, but would likely have grapple fixtures, and possibly some servicing ports for attaching tools, pods, or transfering propellant, or attaching dumb payloads that need to be attached to something capable of stationkeeping.
  • It’s possible that the propulsion/bus system that delivers the payloads from GTO could be some sort of deployer tug like what Rocketlab is doing for Photon or what Spaceflight is doing with their SHERPA vehicle. Which means that in theory, the system might also be delivering some smaller GEO satellites along the way to getting into a depot parking orbit.

Propellant Types: Storable chemical propellants and EP propellants. Unlike LEO, there’s a lot more standardization of propellant types in GEO. Most GEO satellites, once they’re all the way in GEO use either something in the Hydrazine family as a monopropellant, or Xenon as a EP propellant.

  • Though as with all other depot types, the propellant type chosen is going to be driven by what clients (in this case servicers) are looking for. Xenon seems like a likely first bet for most customers, though Hydrazine or one of its variants (MMH or UDMH) might also be first.
  • In addition to just the propellants, the depots would also likely be storing tools, “pods”, and other hardware supplies.

Other Characteristics/Considerations:

  • If PODS or other direct GEO insertion options result in significant numbers of deliveries of dumb cargo pallets or tanks, it could be beneficial to haul them up and attach them to one of the self-propelled depots, because in that case the PODS themselves would lack the stationkeeping and other functionality you’d get from a self-propelled tanker.
  • But given the tradeoff between lapping time and round-trip dV to depots, I think you’ll still want to keep things fairly small overall, since you’ll want to have a large number (maybe eventually more than a dozen) smaller depots.
  • Because GEO is so hard to reach from any place you could likely get propellant from, I don’t see the likelihood of reusable tankers being used to fill up a permanent depot tank, I really think you’ll be talking about expendable tankage for the most part.
  • However, the tankage is a lot more likely to last for more than one refueling — since there’s a lot of variation in sizes of servicers, so most tanks will likely be involved in multiple refueling. And if a tank starts getting relatively low, it might be worth transferring a little from one tank to another tanker/depot before retiring the empty tank.
  • Because these depots would already be operating in a safe GEO graveyard orbit, they won’t need further disposal delta-V. Though since the use of graveyard orbits may not always and forever be best practices for GEO disposal, having grapple fixture on-board could be a good idea to enable future disposal missions — but you’d probably want those anyway just for making refueling operations easier.

As I said up-front, of all the depot concepts, this is the one I feel least definite about. I’d love to hear other people’s thoughts, but I tried to do my best to put together some logic and rationale for how I think things would turn out, and why.

Next-Up An Updated Propellant Depot Taxonomy Part IV: Smallsat Launcher Refueling Depots

Posted in Orbital Dynamics, Propellant Depots, Satellite Servicing, Space Transportation | Tagged , , , , , | 6 Comments

An Updated Propellant Depot Taxonomy Part II: Distributed LEO Nano-Depots

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.

Conceptual illustration of OrbitFab’s Distributed LEO Nano-Depot architecture

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.

Next Up: An Updated Propellant Depot Taxonomy Part III: GEO Depots

Posted in Altius Space Machines, Commercial Space, Propellant Depots, Satellite Servicing | Tagged , , , | 5 Comments

An Updated Propellant Depot Taxonomy

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)
  • Human Spaceflight Fixed Depots (High-Orbit)
  • Human Spaceflight Tankers/Roving Depots

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.

Next Up: An Updated Propellant Depot Taxonomy Part II: Distributed LEO Nano-Depots

Posted in Commercial Space, ISRU, Lunar Commerce, Lunar Exploration and Development, Mars, NASA, Orbital Dynamics, Propellant Depots, Space Development, Space Exploration, Space Transportation, Venus | Leave a comment

Blog Maintenance

Hey everyone, I just wanted to share a quick note that over the next day or so, we’ll be migrating our domain name registrar and hosting service. In theory, you shouldn’t actually notice anything on this or my other blog 1. The urls aren’t changing, we’ll still be a WordPress site, we’re just changing who we’re paying for hosting things.

I just wanted to publicly thank Michael Mealling, one of my fellow cofounders at Masten Space Systems, who has been handling, and paying for, the hosting for these two blogs since the beginning. I figure now that I was able to cash out a little of my Altius stock when Voyager majority-acquired us last year, that I can afford to start paying my own way around here.

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How much mass can we put in orbit before running into atmospheric constraints?

In January, Elon Musk mused that the design goal for Starship was 3 flights per day, for about 1000 flights per year per Starship (assuming for the sake of simplicity he’s talking about a whole stack) with a payload of about 100 tons to LEO. That’s 10 Starships flying a total of 10,000 flights per year to reach 1 megaton in orbit. He points to 100 megatons annually to orbit (100 Starships per year for 10 years… 1 million Starship flights per year) as the goal.

Building 100 Starships/year gets to 1000 in 10 years or 100 megatons/year or maybe around 100k people per Earth-Mars orbital sync

Just how big is that? Is it realistic?

Note: I decided to make a couple images to illustrate the difference between 1 Megaton IMLEO/year (corresponding to roughly 1000 larger Starships heading off to Mars each synod) and 100 Megaton IMLEO/year (roughly 100,000 larger Starships heading off to Mars each synod). There are no stars in these images, just Starships:
1000 Starships doing trans-Mars-insertion simultaneously:

1000 Starships doing TMI
(alternately, for 100,000 Starships, they might do this once per orbit for a week straight)


Going by rough figures, there’s about 3 billion tons of methane (in the form of 4 trillion cubic meters of natural gas) produced every year. Each Starship takes about 1000 tons of methane (very rough numbers…). So you’d need 1 billion tons of methane every year to reach Elon’s 100 Megaton-to-orbit goal.

1 billion tons of methane is approximately 0.75 billion tons of carbon. About 5 billion tons of carbon is absorbed total by land and sea every year (Candela and Carlson 2017), so if all the world was doing as far as carbon emissions was launching SpaceX’s rockets, this wouldn’t be a problem. 1 gigaton of payload to LEO (so about 7.5 billion tons of carbon), however, WOULD be beyond the current ability of the land and sea to absorb carbon, slightly less than the current global carbon emissions from human civilization.

That puts current Starship launch efficiency to about 580MJ/kg, compared to the absolute minimum of ~32MJ/kg, or about 5% efficient. This is remarkably efficient if you think about it, but it’s nowhere near the efficiency that’s *possible*. Stretch estimates for what SpaceX hopes to eventually achieve with Starship might be 2320 tons of methane for 380 tons of payload (propellant in this case), given the 2016 ITS tanker figures (from “making_life_multiplanetary_2016.pdf“). They had a higher O:F mixture ratio (3.8), higher Isp, and lower dry masses. That’s just under 10% efficiency. Significantly better. That would mean half as much methane would be needed, perhaps lowering the carbon emissions to 4 or so gigatons of carbon for 1 gigaton to orbit, below the 5 gigatons the land and sea can absorb… Back to the 100 megaton goal, that’s 340MJ/kg times 100 billion kilograms… divided by about 31 billion seconds in a year, and you’re talking about a terawatt of methane per year.

Of course, you could switch from fossil methane to CO2-direct-air-captured methane. That’s about 50% efficient, so it’d take about 2 terawatts average per to produce. Or, given about a 20% capacity factor, about 10 terawatts of solar nameplate capacity.

But I think they’re potentially still leaving energy on their payload. (See previous posts) If they operate at yet higher O:F, even deeply oxygen-rich, for the first stage, they can get closer to an optimum Isp for early in flight. They can switch to a near-stoich hydrolox upper stage. Maybe we continue making advances in structural materials. Maybe there’s a small launch assist in the beginning (at least getting the vehicle to an altitude where vacuum-optimized first stage engines are feasible). Or they use gigantic expansion nozzles on the upper stage; higher chamber pressures; adjustable Isp. I can imagine achieving 20% or even 30% efficiency. Perhaps 100MJ/kg could be achieved without miracles. That lowers to about 300Gigawatts of chemical energy per year. Hydrogen may be more efficient to make (75%?), so maybe 400Gigawatts of average electricity per year… The US electric grid produces about 475Gigawatts average, so for the first time we’re below the US’s electrical output to power Elon’s 100 megaton/year dream.

However… Pumping all that water vapor up in the atmosphere could cause problems, too. But let’s say we avoid that somehow. There are other problems:

NOx emissions (nitrous oxide and similar) in the high atmosphere cause several problems. One is breaking down the ozone layer. Another is acid rain (although this is also part of the normal nitrate cycle on Earth, where lightning fixes nitrogen into the soil). Another is greenhouse effects… NOx emissions are approximately 250 times worse than CO2, pound-for-pound. It’s estimated that the Space Shuttle produced about 5% (but perhaps up to like 15%) of its reentering mass in NOx emissions (see: Global atmospheric response to emissions from a proposed reusable space launch system). Air-breathing rockets would make this worse by also producing NOx on the way up (see Skylon). We currently emit about 13 megatons of NOx every year from burning fossil fuels (compared to another 8 megatons annually from lightning). Considering the current fairly high dry mass of Starship, there’s basically about a 1-to-1 ratio of Starship mass reentered to payload delivered. So 1 megaton of payload would produce about 50,000kg of NOx. Not nothing, but not a showstopper. 100megatons, however, would produce about 5 megatons of NOx emissions… almost half of what we already make, but could be even higher, if the higher estimates of reentry NOx production are accurate (I don’t think they are).

However, I think we can do much better. The 2016 ITS tanker had a propellant payload to reentry mass ratio of about 4, reducing the amount of NOx production for 100 Megatons by a factor of 4 again. And we can maybe do better by changing the staging situation… Because NOx production and reentry temperature are really non-linear, there would be very little NOx production from a reentering 1st or 2nd stage in a 3-stage-to-LEO rocket. Falcon Heavy, in expendable mode, has an upper stage dry mass of around 4.5 tons (guesstimate from spacelaunchreport.com), and a payload of about 63.8 tons. That puts the ratio at about 14! Over an order of magnitude better than first-generation-Starship. Maybe knock that back to 10:1 for a reusable upper stage (but still using really advanced structures) for a launch vehicle optimized for this constraint, and we could be talking only 500,000tons of NOx per year. MUCH more manageable. To equal the current 20 megatons tons (combined human and lightning) NOx per year, we can reenter about 400 megatons of material, or launch (with an upper stage empty mass to payload ratio of 10) about 4 gigatons.

It may also be possible to scrub NOx from the atmosphere. This concept (backup link: https://doi.org/10.1007/s11356-016-6103-9 by Ming et al) suggests using a solar tower to help scrub NOx from the atmosphere and generate solar electricity at the same time. At really high launch rates, that might be necessary. In fact, any plan to use space resources to “deindustrialize” Earth (like Bezos and O’Neillians like to mention) would have to deal with the problem of (re)entry of massive amounts of material to Earth and the NOx emissions that causes.

It is also possible to pump Ozone into the stratosphere or maybe even suppress lightning to compensate.

Thinking long-term, what is the ultimate limit to ability to launch stuff with rockets, of any type? Current anthropogenic global warming from the greenhouse effect from fossil fuel emissions is much larger than, say, fundamental waste heat from any energy usage whatsover. Waste heat is on the order of 18 TW (same as primary energy usage), with global warming effect from fossil gas emissions (and land use changes) about 100 times that, so about 1-2 Petawatts. If we take current global warming level to be the ultimate limit that we could safely pursue long-term, then human society could grow to use approximately 100 times as much energy as it does right now relying on fossil fuels, or about 1-2 Petawatts. If all of that was used for chemical rockets with each kg of payload into LEO requiring 100MJ/kg, then we could get about 300 gigatons of payload into LEO per year before producing too much waste heat. Maybe with perfect launch systems, about 1 trillion tons per year.

So there are a lot of constraints. We, long-term, probably want to off-load much of that into space. That means maybe using solar-electric propulsion eventually. Before we get much beyond 1 megaton per year, I hope we’re looking seriously at scaling up solar electric propulsion, asteroid mining for propellant, and tethers. Using rotovators (discussed elsewhere on this blog), we could drastically reduce the amount of energy needed to be expended on Earth to launch payloads. And maybe just important (at that scale), we don’t need to use the atmosphere to slow down payloads to the surface of the Earth, either. Tethers combined with megastructures ~100km tall would allow payloads to be launched at higher efficiency and returned to Earth without massive aerobraking… in fact, even reducing reentry from 7.8km/s to 5.5km/s using a modest rotovator would halve the orbital energy input into the atmosphere and probably would non-linearly reduce NOx emissions as well.

Humans move on the order of 50-100 gigatons of material per year (with trucks, bulldozers, etc). See: https://www.sciencedaily.com/releases/2004/07/040709083319.htm#:~:text=In%201994%2C%20Hooke%20published%20the,%2C%20glaciers%2C%20oceans%20or%20wind.
That’s more than all the sediment moved by all the rivers of the world every year.
To move that much of material *to space* every year would require some clever thinking, but wouldn’t be impossible. It’d just take on the order of 100 Terawatts. Current solar cell prices are just 5.5 cents per watt… if they were illuminated constantly, that’s less than $10 trillion dollars worth of solar cells to move more material into space than all the material that all of humanity moves anywhere every year. And we might not even have to cook ourselves to do it.

However, Musk’s 100 Megatons to LEO every year would use up about a third of the world’s annual natural gas production. Might want to move beyond fossil fuels (and maybe optimize launch vehicle efficiency) if we’re going to really launch that much stuff…

And to continue the crude visual thought experiment from earlier, this is what it might look like if those 100 Megatons IMLEO were used to send 100,000 Starships to Mars at once each synod, with a total power output of approximately 2 Petawatts for 8 minutes, maybe even regionally outshining the Sun for a few minutes:

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Adding an Earth-sized magnetic field to Mars


Mars has only the ancient remnants of a magnetic field. What little chunks of field it does have (imprinted into magnetic rocks) are regional in scale and do nothing at all for radiation shielding (I once calculated this). Additionally, over a long enough timescale (tens of millions of years), the solar wind will erode the atmosphere of a terraformed Mars. So, let’s just get on with replacing the lost magnetic field.

To do this, we aren’t going to do something silly like restart the core. We’re going to rely on a tried and true existing technology: cryogenic superconductors. Just put a superconducting ring around the equator of Mars. Turns out, this wouldn’t even cost that much to build. I will be using Magnesium Diboride (MgB2) because it’s cheap, has pretty good performance (critical temperature of 39K, critical current of like 105 A/cm2 at 5K), and both Boron and Magnesium are known to exist fairly commonly on Mars. (Magnesium is super common, and Boron has been found in clays at >150ppm concentrations–according to this paper–and probably exists in far higher concentrations somewhere since Mars once had a quite active water cycle. I’ll assume that by the time the residents of Mars want to do this sort of thing, the costs of extracting these minerals will be similar to that of Earth (maybe a poor assumption, but why would you do this unless there are like millions of people on Mars, at a minimum?). We’ll also assume that because of the ridiculous scale being operated at, and because MgB2 is a pretty easy superconductor to make (just need to heat the mixture of Magnesium and Boron powders), the cost of actually building the ring will be some single-digit multiple of the raw costs of the material.

Okay. So just how big is the Earth’s magnetic field? We’ll use its total energy (when approximated as current on a sphere) to estimate what we’ll need as far as current in the ring. According to this, the Earth’s magnetic field stores about 1026 erg, or 1019J (roughly half the energy the world uses in a year). The energy stored in an inductor is just:
E\,=\,\frac{1}{2}\,L\,I^{2}
(according to Wikipedia)
Where L is inductance and I is the current in the ring.

To calculate the inductance L of a ring of radius R with wire radius a and number of turns N, we use the approximation:
L \approx N^2 R \mu_0 \mu_r \left ( \textup{ln}\left ( \frac{8 R}{a} \right ) - 2\right )

(from here)

Since N=1 and we’ll conservatively (very conservatively) say the relative permeability  \mu_r = 1, and since the current I is related to the critical current density Jc such that: I=J_c \pi a^2, we can write the equation as:

E\,=\,\frac{1}{2}\,R \mu_0 \left ( \textup{ln}\left ( \frac{8 R}{a} \right ) - 2\right )\cdot\left (J_c \pi a^2\right )^{2}
If we let a=.42m, R=rMars, and Jc = 105 A/cm2:
https://www.google.com/webhp?#q=.5*r_Mars*(mu_0)*(ln(8*r_Mars/(.42m))-2)*(10^5A/cm^2*pi*(.42m)^2)^2 = 1.047*10^19 Joules, when we only needed 10^19 J to equal the same energy as Earth’s magnetic field.

Given the density of MgB2 is 2.57g/cc (source), the mass of the superconductor is:
https://www.google.com/search?q=2*pi*r_Mars*pi*(42cm)^2*2.57g/cc or about 3*1010kg, 30 million tons, almost have of which is boron. The Earth mines about 4 million tons of Boron a year, so the Earth produces enough boron to build that thing in about 4 years (we’ll mine this on Mars, of course). Pretty reasonable, considering we’re doing some pretty hardcore terraforming, here.

Given a price of about 10USD/kg for Boron (just spitballing here, since Ferroboron is half boron by molarity and 15-20% boron by mass and is 1-2USD/kg… of course, boron ore is much cheaper) and like 2USD/kg for magnesium metal (just look up the spot prices for Mg and Ferroboron), so about USD6/kg of bulk MgB2.

This whole thing would cost about 180B USD in raw materials but would store about 3 trillion kWh for a ridiculously low price per kWh of storage (like 6 cents/kWh! For storage that can be reused!). Of course, there is also insulation and cooling, plus some method to inject power into the ring.

 

NOTE: Some Japanese researchers recently (last 2-3 years I think?) published a paper about the thing I am proposing here. I can’t find the paper now, but I assume they did I better job than I did. Also, I don’t really buy into Jim Greene’s L1 magnetosphere, since the solar wind does actually shoot straight out from the sun but actually follows the spirals of the Interplanetary Magnetic Field, so I’m pretty sure solar wind would hit Mars because the shadow of a big magnetosphere at L1 would miss Mars.

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A human tribe is a Von Neumann probe

After thinking a while about why self-replicating robots do not exist (thanks, Casey: https://caseyhandmer.wordpress.com/2019/09/02/self-replicating-robots-do-not-exist/ ), we’re reminded that living things do this regularly. I’m tempted to write “a human is a von Neumann probe,” but that wouldn’t be accurate. A single human cannot reproduce, and even a pair would quickly run into survival problems unless they got lucky (and there’s some amount of genetic variety needed, with the minimum number of individuals being somewhere between 50 and 5,000 to ensure enough genetic diversity). The human is by default in society, usually a band or tribe in the past and now in cities. Our survival depends on it. This has enabled us to span from pole to pole with a vast array of lifestyles.

Even a band (10-50 people) enables specialization and folklore. A full tribe (made of several bands, enough for genetic diversity) is a self-contained unit of humanity. Enough to replicate and perpetuate neolithic technology, which includes domesticated animals, plants, spoken language, and perhaps even written language. An individual or family would have difficulty maintaining this, but a tribe should be capable of it. And with the tools of written technology, could maintain knowledge and learn between generations.

What’s interesting about neolithic technology versus later developments is that this is before humanity became dependent on vast trade routes and city-state social structures (although those did develop in that time). Still small enough to be self-contained within a tribe and replicated most places on Earth (with adaption). Other than biological materials (seeds, animals… which are reproducible and not a fundamentally limited geological resource), it is still really easy to bootstrap neolithic technology and could be done by a small group of people. The Bronze Age requires tin and bronze, which are very limited in geologic availability (requiring vast trade networks) and require more sophisticated processing.

My favorite exploration of Neolithic technology is the Primitive Technology Youtube channel. He bootstraps from nothing (no knives, etc, just himself in shorts) and has gotten extremely far in technology development. https://www.youtube.com/watch?v=P73REgj-3UE

No doubt his efforts rely on a lot of free time enabled by modern food distribution systems, but they all could be replicated by a tribe. He has made some interesting advances, such as a sort of centrifugal blower and the beginnings of a bootstrapping of iron technology using iron bacteria (found all over) instead of geographically limited iron ore. He also makes use of domesticated yams.

Now I’ve been thinking a lot about potatoes. They’re remarkably easy to grow and extremely efficient in area, store reasonably well, easy to propagate, etc. It really is a good choice if you had to pick one staple to survive on Mars with (hello, Matt Damon). But they were not introduced into the New World until the 1500s. Same with corn (maize), and a bunch of other things. Corn is a particularly efficient way of growing calories. A similar thing is true of other domesticated crops. …and animals, such as donkeys, horse, oxen, etc. Any neolithic tribe could’ve utilized these resources, but these resources weren’t all available until the modern era (i.e. starting in the 1500s). There’s some evidence that domestication of, say, potatoes, helped speed the industrial revolution due to their efficiency. You could say that it was these domestic plants and animals that enabled the industrial revolution as much as any other particular scientific, economic, etc advance.

Any of these could’ve been introduced to neolithic tribes 10,000 years ago or even earlier. Maybe 100,000 years ago. One could have taught them writing. A single tribe had enough resources to bootstrap these “technologies,” and their productivity would’ve been vastly improved. Unlike our heavy industry today, they do not require a vast, globe-spanning economy to replicate. They can be planted and replicated by a single, neolithic tribe to their great benefit. Self-contained, self-replicating… (or requiring just some assistance from people to replicate).

Domestic plants and animals are remarkable technologies. Seeds in particular… little, unassuming von Neumann machines. Iron bacteria could’ve bootstrapped the Iron Age 100,000 years earlier. Give a tractor to a neolithic tribe, and it would stop as soon as it ran out of gas. A steam tractor maybe could’ve lasted longer and maybe animal grease could serve as oil, but the industrial toolchain in order to maintain any such engine would be beyond a single tribe’s ability. But oxen or other beasts of burden? Easy to maintain and replicate in comparison. Even with some amount of semi-autonomous intelligence. There are your self-replicating robots!

This is the potential of biology in simplifying the technology bootstrap process. It’s unfortunate that biological processes tend to be so inefficient. Their relevance to bootstrapping a Mars civilization may be difficult to gauge relative to the more energy-efficient heavy machinery approach… Also, not only is it inefficient, it’s also only viable in a relatively narrow temperature and pressure range which mostly makes it irrelevant to space settlement…

…but perhaps this is worth another look. I think the fact that biology can play a part in bootstrapping is one of the most important arguments for (at least partial) terrraforming… if you can make Mars Earth-like enough for at least SOMETHING to grow, maybe we can use biology to help bootstrap human civilization there. If Mars is terraformed, then the basic human unit, the tribe, would be sufficient to replicate civilization given a continuity of knowledge of written language.

…but maybe something smaller than full terraforming is sufficient? Humans, even with just neolithic technology, are remarkably adaptable (if we can find an energy source). We can live indefinitely in the Arctic by harvesting animals (including fish, etc) using neolithic technology. Some humans live nearly their whole lives on the water using pre-modern tech. Perhaps with some clever new inventions that could be bootstrapped with neolithic-level-tech, there may be some future domesticated plants or animals (or other?) that enable humans to live on a partially terraformed Mars with something as small as a tribe. After all, we used animal intestines to produce the impermeable gas bags of the mighty zeppelins. What new domesticated life form might enable us to live on Mars with a much simpler bootstrap chain?

Can we harden living things for the Martian environment? I’m reminded that the Armstrong Limit of pressure is dictated by the boiling point of water at the human body temperature. Other living things, such as lizards (not to mention hardy plants), can still live and move with body temperatures low enough that Mars’ pressure at Hellas Basin would be high that water would not boil. There’s also the possibility of some sort of toughened skin designed to maintain internal pressure and temperature. Lichen or similar lifeforms may even be able to photosynthesize under Martian conditions: https://pubmed.ncbi.nlm.nih.gov/20402583/

…so what is the REAL Martian potato? Could some domesticated lichen produce food and important chemicals for a growing Martian civilization? Maybe some sort of domesticated lichen that produces hydrogen peroxide? Could some sort of genetically modified reptile beasts of burden serve as our self-replicating robots? Could we grow tough membranes (with built-in molecular pressure pumps powered by photosynthesis) to make our pressurized cities? How can we simplify the industrial tool-chain so self-sufficiency becomes tractable?

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COVID 19 and Global Warming

It seems to me that we have an accelerated time experiment on the realities and effects of government reactions to a major problem. Governments’ around the world handling of COVID 19 is a microcosm of the handling of Global warming.

Both are considered to be disasters of epic proportions by some and a tempest in a teapot by others. Some consider massive government intervention to be absolutely critical to controlling and solving the problem. While others see no reason for government interference at all.

COVID 19 is an issue that is working on the time scale of days and weeks while global warming is working on the scale of years and decades. My thought is that watching how governments and populations interpret and handle COVID 19 across the next year is a fair indication of how global warming will be handled across the next century.

So I suggest that people of many viewpoints should track the reactions, truths, and lies of the current epidemic with an eye to how global warming will be played out. the relevant timescale is about 100 to 1. Are the leaders of the various countries operating in the best interests of their people, or just using a crisis to gain more power and wealth? Are they creating a crisis for their own manipulative needs. Or are they doing everything right. Let’s all keep an eye on this with the long view.

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Modeling COVID-19: When will the peak occur in the US?

CDC Statement on first possible community transmission case in the US

CDC Statement on first possible community transmission case of COVID-19 in the US

So, given that the CDC recently confirmed the first possible instance of community transmission of COVID-19 (Novel Coronavirus) in the US, I thought I’d guesstimate roughly when the peak of the epidemic would occur in the US with some (extremely) rough modeling.

I’ll be modeling it using the usual logistical model (which I think turns out to be the wrong model for a virus… but let’s just run with it for now) you all learned in your first Differential Equations course. I’ve recently been brushing up on my differential equations (have been getting rusty) in Khan Academy: https://www.khanacademy.org/math/differential-equations/first-order-differential-equations/logistic-differential-equation/v/modeling-population-with-differential-equations

The rate of change of the population N (in this case, Coronavirus cases) with respect to time can be given as:

dN/dt = r*N*(1-N/k)

Where r is the exponential constant (related to doubling-time) and k is the “carrying capacity”, i.e. max number of cases (not actually a good definition for a virus… but again, let’s run with it).

This is solved as: 

N(t) = N0*k/((k-N0)*e^(-r*t) + N0)

Where N0 is the population at time = 0.

At the beginning, the number of cases rises exponentially. Early research says the doubling-time of COVID-19 was 7.4 days in the early days of the outbreak in China, according to: Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus–Infected Pneumonia

This is related to the exponential constant by:
r = ln(2)/(7.4 days) = 0.09366853791 (1/days)

According to Wikipedia, 28% of the US population became infected with the Spanish Flu (carrying capacity?): https://en.wikipedia.org/wiki/Spanish_flu

And US population is currently about 327 million people, so we’ll use 91.56 million as our “carrying capacity”:

k= 9.156*10^7 or 9.156e7 (in more compact notation)

And since community transmission just started, we can set N0 = 1. Therefore our equation becomes:
N(t) = 9.156e7/((9.156e7-1)*e^(-0.09366853791 (1/days)*t) + 1)

If we plot t in days: 

modeled infected population vs time

modeled infected population vs time

https://www.wolframalpha.com/input/?i=9.156e7%2F%28%289.156e7-1%29*e%5E%28-0.09366853791*t%29+%2B+1%29+for+1%3Ct%3C365

So sometime before 200 days from now, COVID-19 should have peaked in the US. Taking the derivative with respect to time, we see there will be a period of about two months when the number of new infections per day will be super high:

US infected per day vs time

US infected per day vs time (simple COVID-19 model) link

This compares fairly well with the peak of deaths for Spanish Flu in the US:

(Thanks again Wikipedia: https://en.wikipedia.org/wiki/Spanish_flu)

We can try overlaying these, and we see that the width of the peak of infections is fairly similar to the width of the peak of deaths from Spanish Flu in the US.

About two months of chaos, potentially. And we have about 5-6 months until this peak.

My model is pretty terrible. A virus doesn’t really have a carrying capacity in the same way… But it does seem to have pretty similar characteristics. I know almost nothing about virus modeling, this is COMPLETELY an amateur, toy model. A guess. There are professionals (like the CDC and the WHO) who do this for a living and you should listen to them, not me. Also, obligatory relevant XKCD webcomic: https://xkcd.com/793/


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SpaceX is great. But Mars needs more than SpaceX.

With the death of Mars One, there are no high-profile companies looking to settle Mars besides SpaceX.

Mars settlement future is currently single-string. Reliant on a single company. What can we do to change that?

Am I missing someone?

On a second note, I’m really missing XCOR. They had something special. SpaceX is doing very well. Has the right architecture for launch (reusability) and wants humans space settlement. And they have a drive to get it done quickly and execute. Blue Origin has only the first two (right architecture and wanting settlement), although hopefully the others will follow… someday?
XCOR had that (although their execution was well beyond concept phase, it did end up lacking a bit). They unfortunately did not have the financial backing of the other two. But they had something more, that’s kind of special: a sort of egalitarian sensibility. They were often fairly right-libertarian in spoken philosophy (as was common among early New Space companies), but they weren’t founded by charismatic billionaires or near-billionaires and so were more worker-focused. They offered free space rides (as part of the shakedown process for Lynx, as I understand it) to all their employees who wished it. That is something special.

So even beyond Mars, we basically have just two companies with the right mix of backing, vision, execution, and technology, and only one is really executing right now (Virgin Galactic seems too small right now… and with the split, seems less likely to be pursuing orbital anytime soon). It’d really be nice if we had a third or a fourth, particularly if they had an XCOR-like egalitarian esprit de corps. Perhaps Musk could eventually be persuaded to transition SpaceX in that direction? Or Bezos/Blue? (I wouldn’t hold my breath for either, but it is a possibility… You can’t have Musk’s democracy on Mars without, you know, democracy… but power is seductive.)

Additionally, I have some ideas for space commerce that require the ability to launch one or two people to Earth orbit (with recovery) for less than a million dollars (ideally <$100k). But on a dedicated launch. XCOR had that, see here: https://spacenews.com/34930xcor-aerospace-makes-plans-for-reusable-orbital-vehicle/

https://spacenews.com/34930xcor-aerospace-makes-plans-for-reusable-orbital-vehicle/

 

 

…but neither SpaceX nor Blue appear to be offering anything close to that now or in the future.

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