Working full time on the Martian surface is within US Radiation Worker limits

The US Radiation Worker annual radiation limit is 5 rem, or 50mSv/year. Divided into the 2000 annual working hours, that’s 0.025mSv/hour.

The Mars Curiosity rover measured an average dosage on Mars of 0.67mSv/day at about -4km altitude. That’s 0.028mSv/hour.

If you worked somewhere lower altitude, like Hellas Basin (-7km or even possibly -8km) or a place like Valles Marineris at -5km and valley walls nearby, you should be able to get that down to the 0.025mSv/hour of US Radiation Worker limits.

Or work French hours. 😉

Or limit it to 1000 hours per year like commercial airline pilots, the rest indoors.

So really, at least during solar max (when GCR is at a nadir), the surface radiation levels of Mars don’t seem like any insurmountable barrier at all, provided you can adequately shield everywhere else and provided you’re okay with US radiation worker limits. For instance, 3m of polyethylene at -5km altitude (Valles Marineris) gives you 22mSv/year (although the model I use for that calculation is questionable to me… it seems there are too many low-energy neutrons). 1m of water/PE is 48mSv/year. Of course, you can also bury below a bunch of regolith or burrow to achieve arbitrarily low radiation levels.

Long-term, I suspect we’ll find drugs that are effective. Or we’ll find out if the Linear no-threshold (LNT) model is correct or not. In any case, when there are millions of people on Mars, it’ll be much easier to produce high-quality data about relative risks for low-dose radiation and also easier to develop enough statistical power to show whether or not drugs like Amifostine can protect against low-dose chronic radiation as well as the acute radiation we know it’s effective for.

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Homo Cosmicus: Vestibular Implants

EDIT: David Birchler mentioned in the comments that an implant is not required. The technology is called galvanic vestibular stimulation and it can stimulate the sensation of pitch, roll, and yaw. Since surgery is not required, it sounds like this really IS doable as a countermeasure for dizziness on landing (perhaps combined with training) and the sensation of coriolis in a short arm centrifuge. In fact, for the former, it looks like this is already being tested as a training tool for astronauts: http://nsbri.org/researches/galvanic-vestibular-stimulation-augmented-training-for-exploration-class-missions/
and here, it shows that GVS training can allow quicker adaption to different vestibular environments: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4458607/
Here’s a company which is developing the tech (along with Mayo Clinic) synced to Virtual Reality: VMocion

For a few years, now, I’ve been convinced that one of the best ways of making human spaceflight affordable and even competitive with robotic spaceflight in some areas is by engineering as close to the human as possible.

For instance, to solve radiation issues, you could shield the entire spacecraft. But this requires a lot of mass. Mass-wise, better is to shield just the habitable areas. Better still is to focus on areas the crew spends the most time in, like the sleeping quarters. Better still (though weird and awkward) might be a garment with radiation shielding. And yet we can do better still: radiation countermeasures in drug form, which protects at the cellular level. Drugs like Amifostine. (But we need better ones.) Or even closer, the nuclear (as in, cell nucleus) level: You could propose either genetically modifying people themselves to produce radioprotectants (or perhaps engineering our microbiome to produce radioprotectants without requiring engineering of actual humans). In each case, the closer to the human you engineer, the lower the mass and ultimately the cheaper it is (at scale).

Another example of this would be microgravity. There have been drugs that have been used to help maintain bone density, such as those used for osteoporosis, particularly Bisphosphonates, which have already been tried on ISS (although there are more powerful drugs available which haven’t been tried, such as Forteo). But exercise seems largely sufficient for the usual durations. And on a larger scale, you could try internal short-arm centrifuges. And on a larger scale, tethers for artificial gravity. But at each point, the mass overhead becomes greater. So I prefer the drug-based countermeasures if possible.

Another effect is a sense of dizziness after astronauts return from long stints in microgravity. The dizziness doesn’t last too long, but it’s feared to prevent rapid escape from the vehicle after landing (say, on Mars) if there’s a problem. I think this is a corner-case-of-a-corner-case, i.e. you have to have a survivable landing but still have to have a reason to immediately exit the vehicle AND be close enough to other help while also not being too injured too move AND you have to be so dizzy that you can’t exit the vehicle.

But let’s say that’s the only showstopper to microgravity. (and it’s not a showstopper, but let’s say it is) Another possibility is vestibular implants. Some people actually have damage to their vestibular system from disease or injury, and so they can be given an artificial vestibular implant, using external MEMS gyros attached to their head, to restore a sense of balance:

First Successful Installations of Vestibular Implants in Humans

But because the signal is now synthesized, you can now modify it. You can impose the feeling of gravity on an astronaut in microgravity, prepping the astronaut for landing. Or perhaps smoothing out the strong Coriolis effect from short-arm centrifuges. You should be able to reduce the dizziness an astronaut feels after returning to gravity.

Also, it’d allow for some crazily-immersive VR.

Now, I personally think that the brain is already sophisticated enough that we can actually train ourselves to tolerate the Coriolis forces (and this is borne out of a study from MIT), and probably also learn how to combat the dizziness that is felt upon landing (perhaps by regularly spinning around just in the air while in microgravity). So I don’t think this is purely necessary. But I do think that long-term, we need to start thinking in this direction in order to make mass human spaceflight more feasible. Human spaceflight seems intrinsically expensive because of all the overhead required for humans in space versus robots. But we can engineer ourselves, much like we developed clothing to enable living in colder climates.

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The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 4–Propellantless Horizontal Soft-Landing Methods

As mentioned previously in Part 2 of this series, one of the key elements in establishing a lunar beachhead is developing ways to safely and affordably land payloads and people on the lunar surface. In Part 3, we discussed a way of hard-landing bulk raw materials on the surface, but in this post, I’ll focus on two related methods for horizontally soft-landing equipment and people on the Moon, while significantly reducing the amount of propellant required compared to traditional rocket-based soft-landing.

While the idea of horizontally landing on a planet with negligible atmosphere may sound odd at first, there is a method to my madness.

Horizontal Soft Lithobraking
I’ve mentioned this concept in the introduction a previous blog post1, but to recap for those who haven’t read the previous post, back around the time of the Apollo Program, Krafft Ehricke2 invented a concept for horizontally landing payloads and people on the lunar surface.

Lithobraking Slide Landers

Lithobraking Slide Landers

Ehricke’s concept involved having a lander with ski-like skids land on a very long (10s of km long) bulldozed regolith track. The lander skids would drag along the surface exchanging momentum with the lunar regolith. In this way the rocket delta-V associated with landing could be significantly diminished. Ehricke even invented a new field of engineering called “harenodynamics” to study the fluid-dynamics-like properties of regolith particles in that situation.

While probably doable, and while the concept would likely significantly increase the payload deliverable from Earth, it’s not without its share of drawbacks and challenges. Obviously, prepping such a landing strip would be no small feat. You would have to clear all boulders, level the ground as smoothly as possible, probably brake up and rake the regolith, and then you’d probably have to redo the strip after each landing, as the landers spray regolith in every direction. You’d probably also want a really, really accurate gravimetric map of the approach trajectory, and some good navigation aids, to enable the vehicle to hit the track, and hit it with the right velocity vector.  This concept would likely result in a lot of wear and tear to both the track and the lander. Not to mention getting dust on pretty much everything, and being on the “that scares me, and I’m fearless” side of sketchy. But there’s also been a decent amount of research put into it, so it might be feasible.

That’s all I’ll say about his concept, but you can read more about it here on page 27. For the rest of the post, I’d like to focus on a newer variation on the theme that I think has real potential.

MFA™ Hover-Brake Horizontal Landing
This second concept is a combination of a concept I heard from a friend at XCOR about a decade ago3 combined with some newer technology from a cool company in the Bay Area called Arx Pax. Arx Pax is probably most famous for their “Hendo Hoverboard”, which can levitate while carrying a full-weight human about an inch over a copper sheet, without using superconductors.

The Hendo Hoverboard

The Hendo Hoverboard

Their Magnetic Field Architecture (MFA™) technology works by using four “Starms” or “Hover Engines” which are rotating discs with magnets arranged on them in a way that projects most of the magnetic field down from the disc into the conductive medium you’re trying to hover over.

Arx Pax Hover Engine 3.0

Arx Pax Hover Engine 3.0

As the Hover Engines rotors rotate, they induce a circular eddy current in the conductive medium, which creates a magnetic field that pushes back on the Hover Engines. Using two pairs of counter-rotating Hover Engines at the four corners of their hoverboard, they can push against the conductive medium with enough force to lift both the hoverboard and a human rider. They can also induce lateral forces by rotating the Hover Engines relative to the surface of the conductive medium. They think there may also be ways to modify their Hover Engines to induce push, shear, and pull forces on a conductive target.4 For those of you interested in learning more, Arx Pax just announced this last week that they’re now selling both a pair of the Hover Engine 3.0 modules shown above (capable of levitating 60kg at a height of 6mm over a 12.5mm thick sheet of 6101-T6 aluminum, with a system weight of <7kg each, as per this spec sheet)5 for $9,999 for the pair, as well as a MFA™ bundle kit with smaller hover engines and all the pieces you need to make and control your own hovering device for $1,589, both of which you can now order online here.

For this lunar landing application, you would need three elements: a long conductive track, multiple Hover Engines on the horizontal lander in order to levitate the lander over the track, and a magnet array on the bottom of the vehicle to decelerate the vehicle using eddy current braking with the long conductive track. The Hover Engines can help both keep the vehicle from touching the track, or from bouncing away from it, while also keeping it centered on the track, absorbing a lot of the shocks associated with a lunar landing, and providing additional deceleration force as the vehicle slows6. As the Hover Engines approach the conductive track, the repulsive force should increase, and as the Hover Engines get too far above the track, the repulsive force should drop off, thus providing at least some natural feedback The amount of power needed to keep the Hover Engines rotating is probably modest enough to be powered by an APU like ULA is proposing for their Integrated Vehicle Fluids system7. While there still are rather demanding requirements for hitting the landing track within the right cone of velocity vectors, the non-contact nature of this landing makes it somewhat less scary. And depending on the design of the eddy current braking magnets, the braking drag may be able to be applied gradually after the vehicle has established a clean hover on the track, if so desired.

One important question is still how much up-front infrastructure this may require. You’ll still need the bulldozer to remove boulders, and level the road. But you also no need a conductive layer of some thickness. The thickness is going to be driven by the float height, gravity, number and size of Hover Engines, and the conductivity of the conductive track. For earth hovering, they used a copper track of decent thickness8. If we needed that thick of a track, that could add up relatively quickly, even if we go with lightweight aluminum instead of copper9, to get the thickness down to much more reasonable levels.

Residual Resistance Ratio
Most people are familiar with superconductors, but did you know that if you chill high purity metal conductors to cryogenic temperatures, that their resistivity also drops off dramatically? In most cases not all the way to zero, but enough to make a real difference. You need a very pure alloy (typically 99.99% or more of the base metal), fully annealed, with minimal impurities. But if you can get the metal in that condition, the conductivity can go up by 50-100x or more compared to their room temperature values. For instance, the figure below (from page 1135 of this paper) shows the conductivity vs temperature curve for high-purity aluminum. As you can see, the resistivity at room temperature is ~2.7×10^-8 Ohm*meters, but at LN2 temperatures it’s 10x lower, and LH2 temps it is ~3400x lower.

High Purity Aluminum Resistivity vs. Temperature

High Purity Aluminum Resistivity vs. Temperature

If you could chill pure Aluminum cold enough to reach ~125x the room temperature conductivity of 6101-T6 sheet (ie resistivity = .025 Ohm*m), you would need ~100µm of aluminum to conduct electricity as well as the 12.5mm thick sheet of aluminum at room temperature used for the Hover Engine 3.0 modules. From the chart that’s somewhere around 45-50K. 100µm is a lot more workable than 12.5mm. The 100µm thick layer could be thermally deposited onto a microwave fused regolith track to provide a strong mechanical backing, while keeping the initially imported aluminum mass quite low10.

Running Some Initial Numbers
If you say had a 7.5m wide track with 100µm thick aluminum, the aluminum would mass approximately 2.03mT/km of track11. So if you could hard-land a 70mT sample of such high-purity aluminum 12, and could recover approximately 2/3 of the aluminum, that would let you lay about 23km of conductive track from a single hard-landing flight. If you were using that 23km track to decelerate something from lunar orbital velocity (~1800m/s), and assumed that you were landing on ~80% of the track length13, you could get the landing Gs down to ~9Gs (over approximately 20s). That’s only a little bit more intense than a Gemini launch on a Titan II missile and a bit gentler than a ballistic reentry in a Soyuz capsule, but probably within what a healthy human could handle safely14. With two hard landings worth of aluminum under the above assumptions, you’d be able to get the G rate down to a totally survivable 4.5G deceleration (with a 46km long track).

But how much would all the Hover Engines mass to do this? Not as much as you’d think. So, if you assume that you’ve chilled the aluminum enough that 100µm of pure aluminum can conduct as much as 12.5mm of 6101-T6 aluminum can at room temperature (40-50K as mentioned above), so it behaves similarly to the nominal track thickness, that means that for landing a 20mT payload attached to a 6mT ACES/Xeus/HoverEngine lander, you’d need ~64 of the Hover Engine 3.0 modules (say 8 pods of 8 Hover Engines each), which without further weight optimization would mass around ~450kg. You could probably cut that down substantially with clever design15, but that’s already in a similar order of magnitude to the mass of the landing kit for a rocket-powered Xeus lander.

As an aside, can you see why I was so interested in having a way to land large amounts of bulk raw materials on the lunar surface? Even though there’s plenty of aluminum in the regolith, and you could eventually setup facilities that could produce many tonnes of aluminum per year, being able to land that early-on dramatically lowers the cost of landing those facilities in the first place.

How to Chill
Getting back to the topic, can we really get aluminum cold enough to enable that level of conductivity? I’m honestly not sure. It may be possible to have the bulldozer that makes the path intentionally sink the track into the ground with berms around it in a way to keep the aluminum in shadow as much of the time as possible. If you combine that with some sort of “cryogenic selective thermal coating”16 that could help keep the temperature of the track passively cool for most of the day. Will it be enough to get the track down to the desired ~40-50K? I’m not sure. According to this chart from a site with data from the LRO Diviner mission, polar sites get down to around ~50K at night, so I don’t think it’s entirely crazy at least for polar locations. At more equatorial positions, even with a properly dug trench, you’ll likely need a thicker aluminum track to make things work.

Lunar Surface Temperatures over Time at Various Latitudes (from LRO Diviner Instrument)

Lunar Surface Temperatures over Time at Various Latitudes (from LRO Diviner Instrument)

One additional knob to turn is that apparently if you have a two layer track, with a highly conductive non-ferromagnetic upper layer, and a lower ferromagnetic layer, that the ferromagnetic sheet modifies the induced magnetic field in a way that actually increases the force pushing back on the Hover Engine. So theoretically, if you did a three layer design (first the microwave sintered regolith underneath, then a layer of melted NiFe material magnetically extracted from the lunar dust, and finally a layer of thermally deposited aluminum), you might be able to get a bit more bang for the imported aluminum buck.

Multi-Use Track
One other nice thing about a track like this is that it can provide two other uses other than just propellantless landing of people and payloads. First, the track is actually a pretty impressive conductor. A 100µm thick, 7.5m wide conductor has about the cross sectional area of a 1.25in (30mm) diameter rod. That’s a lot of conductor. And if chilled to 125x the room temperature conductivity of aluminum, you’re talking about a conductor equivalent to a 14in (350mm) diameter bar of aluminum at room temperature. A conductor that big could likely conduct megawatts of power over non-trivial distances. This means that you could have solar farms and installations located along the track, which all use the track as the high-power backbone to connect themselves to the main beachhead facility. If you had a polar facility with four tracks spaced 90 degrees from each other (say earthward, anti-earthward, and along and against the moon’s orbital velocity vector), you could locate solar farms along each track in a way that half of them are always in sunlight, so you wouldn’t need a lot of power storage capacity.

Second, the tracks also would serve as ideal highways/railroads for moving stuff between the main facility and other facilities down the line. Without air, and without rolling friction, these tracks could basically function as maglev railways linking installations up and down the track. The same Hover Engine kits used for landing could form the backbone of a hover vehicle that could rapidly move very heavy payloads up and down the track at very little energy cost.

And so long as you locate a track along a “great circle” route (ie a surface track that is in a plane that intersects with the center of the Moon), the tracks could be used for all three applications (landing, transportation, and power). While the first one or two could be built using Earth resources to accelerate their availability, once you have access to lunar aluminum, you can start putting tracks like this down wherever they are convenient, and as long and wide and thick as is convenient. I even have an idea 17 for how you could use tracks like this for propellantless launching of payloads, but that’s a post for another day.

Conclusions
It’s probably more complicated than this. These ideas are very conceptual, and could use a significant amount of further baking. In fact, if I get a chance to flesh this out further, I’ll probably do follow on posts as Part 4.1, 4.2, etc. when I get the time. But they at least suggest that there may be ways to eventually cut the cost of delivering payloads to the Moon in-half, by getting rid of most of the propulsive landing delta-V. And for setting up heavy infrastructure on the Moon, the sooner you can get that big of a landing-cost savings, the more your overall system cost goes down.

Next Up: Slings

[Update: I still need to dig into this more, but it may be necessary to put more thought/analysis into the eddy current damping part of this idea. All of the kinetic energy effectively has to be dissipated as thermal energy (by resistance losses by the eddy currents flowing through the metal). The problem is that as the metal heats up, it becomes less conductive. So there’s probably a mass limit that a given track thickness/length can realistically handle. The math is currently above my hand-calc levels, but I’ll do a follow-on Part 4.x post if I get a chance to dig into this more.

One other interesting idea I’d like to look at is using pure Beryllium as the conductor instead of pure aluminum. Beryllium has 2x the heat capacity of aluminum (~1800J/kg*K vs ~900J/kg*K), and has a much higher melting point (~1550K vs ~933K), so it is much better as a heat sink. It isn’t as good of an electrical conductor though, with 3/2 the room temperature resistivity of aluminum, but it is only 2/3 the density, so for a given mass/length, you get approximately the same resistance, just with 3/2 the cross-sectional area for the Beryllium. It’s RRR (ratio of resistivity near absolute zero compared to room temperature resistivity) of ~2000, which while not as high as the highest grade coppers or aluminums, may still be good enough to be quite interesting. Needs more analysis though.]

Posted in Lunar Commerce, Lunar Exploration and Development, Slings and Arrows of Outrageous Lunar Transportation Schemes, Space Development, Space Settlement, Space Transportation, Technology | 26 Comments

Perfectly energy-efficient rocket vehicle

So I was watching a video of a talk by Geoffrey Landis (my old mentor when I was an intern at Glenn), and he made a very interesting point.

If you’re trying to maximize energy efficiency for a rocket and you have the ability to change Isp, you should set the exhaust velocity equal to the rocket’s current velocity. Think about it for a little bit, and you’ll see that it’s true.

We make some simplifying assumptions here, such as assuming arbitrarily good mass fraction, and thrust occurring outside any gravity wells (or, equivalently, thrust that is arbitrarily higher than acceleration due to gravity at that point). But with these assumptions, this finding is true.

One thing that’s a little surprising is that you’d have PERFECT efficiency under this scenario. That means you’re wasting exactly no energy by accelerating all that propellant, which goes a little counter to the intuition. A perfectly efficient rail gun (with an arbitrarily low-mass acceleration cart) would not be any more energy efficient.

…the downside, here is that your initial exhaust velocity is zero, which implies an infinite amount of fuel. This isn’t so bad because we can just truncate the initial portion of the flight by picking a certain minimum Isp to keep mass fraction to a reasonable number. We lose some efficiency this way, but it’d small. Alternatively, you could use rail-launch for the low-speed portion of the flight and maintain arbitrarily-good efficiency. The other side of this is that it implies 700-900s Isp for the last portion of the flight if going all the way to Earth orbit. That implies use of non-chemical thermal rockets (such as NTR), but you’d be no better off energy-wise since the only practical propellant for those (while still reaching those Isps) is hydrogen, and producing hydrogen requires a lot of energy (energy which is not at all utilized in the rocket! Hydrogen is considered basically inert… you could also use helium). Besides, terrible mass fraction.

Anyway, there is also an energy-ideal exhaust velocity if you can’t adjust the Isp. I believe it’s about 5/8th the final delta-v, if I remember geoffrey’s talk. I may try to recreate this calculation.

Another point: It makes things like Aerojet’s thrust-augmented rocket look pretty interesting from an energy standpoint.

I have a bunch of thoughts related to this that I may blog later on. Here’s Landis’s talk. (He argues for NTR here. I disagree with him, but very interesting talk, and he definitely makes a convincing case that NTR is not some unobtainable technology.)

Overall, I appreciate the change in focus to energy. I think we have an (understandable) obsession with mass in the rocketry world, but that doesn’t tell the whole picture and can somewhat bias our intuition.

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The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 0–An Elevator Pitch for the Moon

[Note: After getting a few posts into this series on the “how” of alternative lunar transportation concepts, I realized that some commenters were asking about why we should even be going to the Moon, or what we’d be doing there. So even though this series focused more on the “how” of lunar transportation development, I figured a short up-front post about why we should care might be useful.

Also, I should add an up-front caveat that I’m not doing this post or series in an effort to bad-mouth other destinations such as NEOs, Mars, Venus, etc, or to propose that NASA should focus all of its money an effort on the Moon, or anything like that. I think all of those destinations are potentially interesting, and that humanity will be better-off if it can figure out how to pull all of them into its economic sphere. The point of this post and series is to explain the ways the Moon could be interesting, and to explore what technologies or approaches might enable that.]

While trying to exactly predict future markets and economic trends is often an exercise in futility, I think we can still make a broad brush-strokes “elevator pitch” for why the Moon is potentially interesting as a destination for space development. As the only pre-existing destination in space that can be accessed both quickly and frequently1, the moon is interesting for at least three or four application areas: resources, adventure, science, and possibly settlement:

  • Resources: There are many resource-intensive things that can be done in space once the cost of resources in the required orbits come down far enough. These include orbital space settlements in LEO or the Lagrange points, massive communications or power-beaming platforms in high-LEO, MEO, or GEO, space tourism destinations, and all sorts of travel to destinations beyond Low Earth Orbit. If the cost of lunar-derived materials and/or manufactured goods delivered to these destinations can be competitive with Earth-launched or NEO-derived materials, then lunar mining and resource processing could be a very important reason to go to the moon.
  • Adventure: In addition to raw materials, the Moon is a potentially very interesting destination in itself. Both for scientific exploration, but also for plain adventure. People visit all sorts of dangerous places on earth just for the shear thrill and beauty of it. If the cost of travel to/from the Moon can come down, the Moon is a potentially very interesting destination for adventure tourism and public or private exploration.
  • Science: We’ve barely scratched the surface of understanding our nearest celestial neighbor, and on its far side there is a radio-pristine area for locating various types of observatories. Once again, If the cost of transportation to/from the Moon came down rapidly there are many international agencies and private societies who might be interested in studying the Moon and using it as a platform to explore the heavens.
  • Settlement: This one has more “if’s” involved, but If it turns out that 1/6g is sufficient for long-term human health2, and If the cost of travel to/from the lunar surface can come down significantly, it very well could be a good destination for Earth’s first off-world settlements. It’s probably not as ideal as Mars or Venus, but if the other three activities are going on, and humans can adapt to the lunar environment, you will see settlement of the Moon on at least some scale.

I could probably go on, but to me those provide ample justification for being interested in going to the Moon, but as you can see, the success of all those applications depend strongly on driving down the cost of going to and returning from the Moon, both in absolute terms, and also relative to other competing alternatives such as Earth launch using RLVs and NEOs.

How to actually do that is the focus of the rest of this series.

Posted in ISRU, Lunar Commerce, Lunar Exploration and Development, Slings and Arrows of Outrageous Lunar Transportation Schemes, Space Development, Space Exploration, Space Settlement | 15 Comments

The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 3–Intentional Hard Landings

The first advanced lunar transportation concept is the idea of intentional hard lithobraking landings of bulk raw materials. The idea was recently suggested by frequent Selenian Boondocks commenter Paul Dietz in the comments to this previous post. Basically you would launch a rocket with one or more large chunks of raw material on a trajectory that grazes the surface of the Moon, using the lunar regolith to decelerate the raw material to the point where it can be subsequently collected. Even if the payload is coming in on a hyperbolic trajectory from Earth without stopping in low lunar orbit, it’s theoretically possible for a very shallow impact to not vaporize the payload, leaving it largely intact.

Artificial Shallow-Impact Meteorites
In Dennis Wingo’s excellent book MoonRush, he makes the argument that due to the Moon’s shallow gravity well, it’s possible for a subset of lunar nickel/iron meteorite impactors to arrive at low enough relative velocity and a shallow enough angle for the majority of the material to survive intact in a large chunk.

This concept though would be basically trying to create that phenomenon artificially, with an earth-launched chunk of raw material, such as a big ball of some alloy that may be hard to produce on the Moon (copper conductor alloys? high strength aluminum alloys?). Impact conditions could be carefully controlled using the upper stage as a shepherding vehicle, similar to what was done with the LCROSS mission1, but potentially with the upper stage performing a last minute maneuver to avoid intercepting the lunar surface, allowing it to return to earth for recovery/reuse.

Why would you want to do something like this?
Primarily because it would dramatically reduce the cost of delivering some bulk raw material to a lunar settelement site. The delta-V from LEO to a lunar soft-landing is approximately 6km/s. With high performance LOX/LH2 propulsion, that means that only 1/4 of your LEO mass will be dry mass on the lunar surface. For instance, an ACES stage with 70mT of prop loaded in LEO could deliver approximately 24mT to the lunar surface, with the stage and Xeus landing hardware taking up at least 6mT of that (leaving a net ~18mT of payload). On the other hand, the same stage could hard land over 65mT of bulk material while recovering the stage back to LEO. In the case of a big chunk of raw material, unless it is something really exotic, most of the cost is going to be in the launch of the material and the propellant to get it to the moon. If you can increase the payload to the moon for the same propellant by a factor of 3.6x, that’s likely going to decrease the cost on the lunar surface by a somewhat similar margin.

What would you use this for?
As mentioned above, the most likely use of this would be to hard land bulk raw materials. Specifically you’d want something somewhat tough so it could survive the impact intact, in spite of impact forces probably in the 1000s of Gs, most likely a large chunk or ball of metal. Here are some specific applications I can think of:

  • Copper conductor materials — copper is relatively rare on the Moon, but is used a lot as a conductor for power transmission, and could be a key element in manufacturing electromagnets or conductors for other propellantless launch/landing systems. Landing equipment that could metal and process high-purity copper or copper alloys into useable conductors may be a lot easier than landing big spools of conductors, if this approach allows you to cut the cost of the copper by almost a factor of 3.6x.
  • High-Strength Aluminum Alloys — while aluminum itself is pretty common on the Moon, most high-strength alloys, particularly ones you’d want to use for structural components or pressure vessels, are ones that require alloying elements that may be hard to source locally, and which frankly will likely take a long time to build up the smelting capabilities on the Moon even if the alloying elements are available. These alloys could be alloys that can be cast using locally produced sintered regolith molds. Or they could be alloys designed to be converted into 3D printing powder. Or it could be wrought alloys that are designed to be melted and converted into sheet or bar forms. Once again, processing equipment to convert the bulk raw material into usable forms is likely going to be massive enough that you’ll be somewhat limited, but with proper thought the 3.6x cheaper aluminum alloy raw material may still be useful in some applications.
  • Toughness-Enhancing Alloys for Ni/Fe Asteroidal Material — in the ongoing discussion on the previously mentioned ESIL-8 post, it was mentioned that many examples of meteoric steel, while beautiful and stainless steel-like are actually rather brittle. It might be possible to send along the right mixture of alloying materials to mix with magnetically recovered Ni/Fe asteroid fragments on the Moon to turn it into a much better and more useful grade of stainless steel, for 3d printing, and/or production of pressure vessels for habitation or material storage.

Implementation Details
Some potential implementation issues that may need to be addressed for this concept include:

  1. What materials can actually survive an impact at the velocities associated with a hyperbolic trajectory from Earth?
  2. What is the optimal impact trajectory for controlled landing in a recoverable manner? Does too shallow result in large dispersions? Does too steep result in vaporization or the material being buried below the surface? How much of this can be analyzed up-front versus needing to be experimented with?
  3. How big of an impact crater are you going to form? How far will regolith fly?
  4. What is the optimal shape for the material? A solid sphere? A hollow sphere?
  5. From an earth TLI, where on the Moon can you realistically target an impact point for that has the right impact flight path angle?
  6. Where would you want material to be targeted relative to a settlement, to minimize risk to the settlement while simultaneously maximizing the odds of being able to recover the material easily?
  7. Can you design a trajectory that allows the shepherding upper stage to avoid lunar collision and return to Earth for recovery and refueling?
  8. Do you need to “prepare the ground” any by having a robot remove large boulders from the target area, and/or “rake” the regolith?

There are probably other questions, but the potential of being able to cut the cost of raw materials delivered to a lunar site by 3.6x makes this intriguing, even if a little on the messy side.

Next Up: Propellantless Soft-Landing Options

Posted in ISRU, Lunar Commerce, Lunar Exploration and Development, Slings and Arrows of Outrageous Lunar Transportation Schemes, Space Development, Space Settlement | 42 Comments

The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 2–The Beachhead Analogy

This observation may already be bleedingly obvious to everyone else, but I feel it is worth explicitly stating:

One of the top priorities of early missions to any destination in the solar system should be to drive down the cost of future missions and increase their safety and reliability as quickly as possible.

While traditionally NASA missions have been of the one-off sortie/expedition variety, if we hope to thoroughly explore our solar neighborhood, and more especially if we want to  settle it and bring increasing portions of our solar system into humanity’s economic sphere, following this priority would be a good way of showing that we’re actually serious about it.

I’m no military historian or strategist, but I think using the analogy of a “beachhead” may be appropriate and illustrative. When conducting airborne or amphibious invasions, one of the important goals of the attacking force is to quickly secure the area in a way that enables lower-cost, more reliable transportation of people and material to reinforce the initial beachhead. While the initial forces can use inefficient transport such as paradrops or amphibious landers, the sooner they can secure a port or lagoon or airstrip from enemy operations, the faster they can bring in reinforcements using more efficient means of transportation such as passenger/cargo planes or vessels. In military campaigns, that ability to rapidly reinforce the beachhead can often be a matter of life-and-death.

While space operations may not be quite so dire, there are strong incentives for also trying to quickly transition to more affordable, safe, and efficient means of getting goods and people to and from the destination. The sooner you lower your cost of say delivering objects to the destination, the more exploration/settlement/resource-extraction you can perform with a given amount of money. The sooner you lower your cost of returning material from that destination, the lower the cost of reaching that destination (if you’re producing propellants), and the more competitive you become with shipping material from other locations. Because so much of the cost of human space exploration or settlement is in the transportation of people and goods, the sooner you can lower the transportation cost, the bigger the impact. This is why driving down transportation costs should be such a high priority for early missions to a given destination.

I originally made this first point in the context of Mars missions1, where instead of focusing on sorties, I feel that we can get a much better return on investment by having early Mars missions focus on lowering the costs of future missions by: a) establishing a landing site with good landing pads and navigation aids at a location with good ISRU potential, b) getting as quickly as possible to the ability to produce steady quantities of high-performance fuel and oxidizer (LOX and either Methane or LH2), c) getting a depot/staging base setup in low Mars orbit, and d) getting reusable tankers regularly traveling from the landing site to the depot, stocking it with propellants for future landers and return vehicles.

In this series though, I’d like to focus on some interesting options for pursuing this Beachhead strategy for lunar missions. While I agree with many lunar resource advocates that the Moon can potentially play a significant role in the exploration, settlement, and commerce of the inner solar system, the gear ratio math I discussed in this series’ first blog post suggests that while having highly reusable rocket-powered landers may be a necessary starting point for lunar development, we may need to move beyond the rocket equation to truly unlock the Moon’s potential. Fortunately, unlike most other places of interest in the inner solar system, there are several potentially realistic ways of propellantlessly landing and launching materials and equipment, and eventually people from the Lunar surface. As I mentioned in the first post, most of this series will focus on introducing some of these concepts, and explaining how they might fit into lunar development plans.

Next Up: Intentional Hard Landings

Posted in ISRU, Lunar Commerce, Lunar Exploration and Development, Slings and Arrows of Outrageous Lunar Transportation Schemes, Space Settlement, Space Transportation | 30 Comments

Random Thoughts: ACES/EUS Public Private Partnership Idea

A few years ago, I did a “Random Thoughts” blog post about synergies between the proposed ACES stage and the proposed SLS upper stage1. Now, I’m still not the world’s biggest SLS fan, and I’m still not a fan of sole-sourcing EUS to Boeing, but I was realizing today that the potential for synergies may be even higher now, and I wanted to throw out an idea for a potential public-private partnership that would benefit both NASA and ULA, and save the taxpayer some money now and in the future.

Current EUS Concept (Credit NASASpaceflight.com)

Current EUS Concept (Credit NASASpaceflight.com)

Here were my thoughts/observations that led to my latest concept (in no particular order):

  1. The most complex part of an upper stage is typically the bottom of the stage where the propulsion systems are located. The tanks themselves are relatively simple comparatively. Tank stretches have always been considered much, much easier and lower risk than changing the diameter of a stage, because now you usually have to redesign all the structures and plumbing on the back end.
  2. Both EUS and ACES are looking at using four RL-10 class engines on their stage. The EUS wants to use a different RL-10 variant with a longer extendable nozzle, but not a wildly different engine2.
  3. The EUS LOX tank diameter has in the past usually been 5.4m diameter, while the ACES stage is now baselined at 5.4m diameter3.
  4. There’s already some interest on the EUS side in leveraging some of the IVF systems as a way of providing auxiliary power.
  5. On the EUS, the LOX tank is likely suspended, with the interstage reacting loads into the bottom of the LH2 tank. This means the LOX tank doesn’t have to take compressive loads on the pad unpressurized.

So here’s my crazy thought: What if NASA had Boeing and ULA develop the EUS as a public-private partnership, with the LOX tank and propulsion section for EUS and ACES sharing a high-commonality design?

  • Have EUS go with the 5.4m diameter resistance-welded CRES tankage from ACES, with a ~4% longer barrel section to compensate for the 10cm (~4in) smaller diameter.
  • Have ACES design its propulsion thrust structure to accommodate both versions of the 4x RL-10 class engines.
  • Have EUS keep the current design for the aluminum 8.4m diameter tank and intertank structure, but have ACES stay with the common bulkhead design and 5.4m diameter CRES LH2 tank.
  • Have the EUS LOX tank sidewalls and top-dome slighly modified compared to ACES to react the loads to/from the intertank structure, and to eliminate the unneeded common-bulkhead, but keep the bottom dome and engine/equipment rack designs the same between the two.
  • If EUS needs more IVF modules, either have ACES leave space and minimal scarring of its structure to allow mounting two more modules 4, or have the EUS LH2 tank designed with IVF mounts (either at the bottom or top, depending on what gives the most bang for the buck).
  • Have Boeing focus on overall stage integration, the LH2 tank, the intertank structure, and the interstage structure, and any EUS-specific long-duration hardware (sunshields, cryo-coolers, radiators, solar panels, deep-space comms, etc).
  • Have ULA focus on the LOX tank and propulsion system, and have them produced on the same line that would make ACES.

There are some risks–this would work best if Boeing was willing and able to keep interfaces between the two halves simple, and wherever possible let ULA drive the LOX tank and propulsion element design without too much micromanaging. SLS, having much higher launch capacity can probably can afford to have EUS be a tiny bit less optimized if it allows for high-commonality and minimum impact to ACES, as opposed to forcing EUS to be hyperoptimized for SLS at the expense of being suboptimal for ULA.

The benefits I could see to NASA is that this would:

  1. If done right, potentially save significant development costs by leveraging both outside investment by ULA, and by having at a more commercially-driven design for at least part of the EUS. This might also accelerate when EUS was available.
  2. If done right, EUS would now share at least some of its fixed and marginal costs with the ACES assembly line, and would benefit from higher production rates on many of the subsystems.
  3. The core subsystems on EUS would see far more flights this way than they would on SLS alone, and the manufacturing team would stay fresh even if the SLS flight rate is modest.
  4. The EUS stage would probably end up with at least slightly better dry mass numbers, and would likely have longer duration built-in.
  5. EUS would be able to leverage at least some of the ongoing enhancements ULA is trying to develop for ACES (refueling/distributed launch, longer duration missions, etc).

ULA would obviously benefit for a few reasons too:

  1. Most of the complexity of ACES is in the LOX tank and propulsion section. The only other complex ACES part that wouldn’t be needed for EUS is the common bulkhead. The LH2 tank is pretty simple. So, if done right, this could help accelerate the development of most of ACES.
  2. If done right, this would both lower the cost of fielding ACES (since NASA would be footing part of the cost of the common elements), and would likely accelerate when they could have ACES flying by 1-2 years or more.
  3. They would effectively have an additional customer stream for ACES hardware.
  4. This would probably better align the interests of at least one of their parent companies with their interests.

If done right, taxpayers would benefit from decreased development and fixed operating costs compared to the current approach.

Now, I have no idea if Boeing or ULA would even consider this. You’ll notice I used the phrase “if done right” a ton of times, and things being done right are rarely a given when talking about government contracting. But it seems like an intriguing approach in a world where SLS is unlikely to get canceled anytime soon.

Thoughts?

[Disclaimer: As the founder of a company that has done some work with ULA on their IVF system, I could potentially stand to financially benefit if NASA took this approach. I can’t claim to be an unbiased, unselfish player in this case. But I still feel it’s worth throwing the idea out there, as I think if done right it would make EUS a better stage, save NASA money, get EUS and ACES flying sooner, and generally make both systems better. I still am skeptical that SLS is worth saving per se, but assuming it isn’t going to go away, this seems like a way to get at least some benefit to the commercial space industry out of it.]

Posted in Launch Vehicles, NASA, Random Thoughts, ULA | 9 Comments

Reusable Falcon Heavy payload (upper stage staging velocity)

Jon and I were discussing the recent Falcon Heavy payload numbers.

Expendable performance to GTO is supposed to be 22 tons (metric, same for the rest of this post). Given how aggressive that is, and given the history of Falcon 9’s performance, I would expect that to be to 1800m/s-to-go (i.e. you need 1800m/s more delta-v to get to actual geosynchronous orbit). The delta-v between that and LEO is approximately 2.5km/s, though that depends on the details of exactly which LEO orbit (but I think this is a good number; provide a better number if you know of one).

Assume the propellant in the upper stage is about 110t, and the dry mass is 5t (this is in range of other people’s estimates and figures from SpaceX, though I’ve seen down to 4t dry mass). With a 22t payload, that gives a full mass of 137t, empty of 27t. Given Merlin Vac’s Isp of 348 (which is staged-combustion territory, although it is gas generator), you have a bit better than 3.4km/s exhaust velocity.

Delta-v of the upper stage is thus slightly more than:
3.4km/s*ln(137t/27t) = 5.5km/s.

Given the 2.5km/s required to reach GTO from LEO, that means that the upper stage has already provided 3km/s of delta-v already by the time the stack reaches LEO.

What is the LEO velocity? Given the standard gravitational parameter of the Earth mu = 3.986E14m^3/s^2 and we’ll pick an altitude of 150km, LEO is:
sqrt(3.986E14m^3/(s^2*(r_Earth+150km))) = 7814m/s.

But we’re concerned with the speed with respect to the ground, so we have to minus the contribution from the Earth’s rotation. Given 28 degrees lattitude (I think it’s actually launching from Boca Chica, but hey), that’s:
cos(28 degrees)*2*pi*r_Earth/day = a bit more than 400m/s.

So the relative speed is around 7.4km/s, let’s round to 7.5km/s.

So by the time the upper stage has been burning through 3km/s, the stack is at 7.5km/s velocity with respect to the ground. 3km/s less than that is 4.5km/s (minus a small amount of gravity loss, which is small by the time you’re at the upper stage).

So the core stage for the expendable Falcon Heavy is going at 4.5km/s relative to the atmosphere at stage sep from the upper stage.

With the reusable variants, the upper stage will be pushing a lighter load, so the upper stage has a higher delta-v and will be doing more of the work, and so the staging velocity (relative to the atmosphere) will be even less. So we’re talking about 4.5km/s, worst case. For a 15 ton payload to 1800m/s to go, you’re talking about ~3.6km/s staging velocity. That’s a much easier reentry problem than the 6km/s I’ve seen bandied about, and it could even be handled largely by propulsion.

Posted in Uncategorized | 3 Comments

Fundamental cost of putting stuff in orbit: theoretical minimum vs RLV

What is the minimum energy of orbit, and how does that compare to the energy in a chemical rocket’s propellant?

Accessing a 150km LEO orbit requires first the energy to get to 150km. That’s roughly (in Energy/mass, or J/kg, aka m^2/s^2, the unit I’ll mostly use here): 150km*9.8m/s^2.

Orbital velocity at 150 km altitude is just v=sqrt(mu/a), where the distance from the center of the Earth a = r_Earth + 150km. Mu is the “standard gravitational parameter” of Earth, or ~3.986*10^14 m^3/s^2.

(BTW, I’ll write numbers like 3.986*10^14 in a more compact notation: 3.986E14.)

So v= sqrt(3.986E14m^3/s^2/(r_Earth+150km)) = 7814m/s ( here is the google calculation: https://www.google.com/webhp?q=sqrt(3.986E14m^3/s^2/(r_Earth%2B150km)) ).

But we can minus the speed from the rotation of the Earth: v= sqrt(3.986E14m^3/s^2/(r_Earth+150km)) – 2*pi*r_Earth/day

Now we need to make this in terms of energy in order to add that potential energy from being 150km high:
E_specific (energy/mass) = .5*(sqrt(3.986E14m^3/s^2/(r_Earth+150km)) – 2*pi*r_Earth/day) + 150km*9.8m/s^2

Which is roughly: 28,480,000 m^2/s^2 or 28.5MJ/kg. That’s 7.9kWh/kg or just under $1 per kg to LEO at typical 10-12 cents per kWh.
And in terms of delta-v, it’s: v = sqrt(2*E) = 7550m/s or so.

That’s zero aero or gravity drag, launching due East on the equator. Imagine a 150km tall tower with a 100% efficient electromagnetic launch mechanism on the top, including the energy required to lift stuff up that tower and assuming no energy loss from the sled, no mass for the encapsulating of the payload, and 100% efficiency for electromagnetic launch. None of these are realistic assumptions.

Let’s compare with chemical launch. Assume a hypothetical stoichiometric methane/oxygen rocket engine operating at 3.7km/s exhaust velocity. This is very aggressive (especially at sea level), would probably melt the engine due to operating stoichiometrically, but it may actually be possible.

A stoich methane/oxygen mix, with methane having 55.5MJ/kg specific energy and the mix having 11.1MJ/kg, would have a theoretical exhaust velocity, if you totally convert chemical energy to jet energy, of 4.712km/s, so 3.7km/s isn’t physically impossible in the least (would be feasible in vacuum, but would require incredibly high pressures at sea level).
Anyway, let’s assume a mass ratio of, say, 25 for each stage. Let’s assume a 100 ton payload. The first stage weighs 120 tons dry (25 times that wet), and the next stage 10 tons dry (etc). That gets us 9km/s delta-v, which we’ll say is good enough, launching on the equator due East to 150km altitude.

Work: 3.7*ln((25*120+(25*10+100))/((25*10+100)+120)+3.7*ln((25*10+100)/(100+10)

We assume the dry mass magically can be recovered at no mass penalty (I will address this in another post…).

Mass of the propellant is: 120*24 + 10*24 = 3120 tons. Or 31.2 kg of propellant per kg to orbit. At 11.1MJ/kg, that’s 346MJ/kg of chemical energy in the form of methane. Natural gas is about $0.30 per therm in bulk. A therm is about 105MJ. So the cost of chemical energy to put stuff in orbit via chemical rocket like I described is actually ALSO $1/kg, and with arguably more realistic (though also aggressive) assumptions.

Moral of the story: It’s not, and never ever has been, about the cost of energy to get to orbit. Such arguments are flawed.

Posted in Uncategorized | 6 Comments