Category Archives: The Moon

Dust off the Moon

regolith_rocket

A ready and ample supply of propellant in space sure would be useful, on the Moon or Mars, particularly. What if we could use regolith—the sand or dust present on planetary surfaces—as a propellant, directly? Would a spacecraft accelerated by dust even lift off the surface? If so, could this technique serve as a means of transport from point to point on the Moon?

Maybe you’ve already encountered the idea of a regolith rocket, which is not quite what I’m talking about. But let me describe it anyway. A classic regolith rocket involves preheating the regolith to give it some additional energy and then mixing it with a high-pressure gas. As the expanding gas flows out of the spacecraft, it sucks in and transports, or “entrains,” the regolith particles, thus producing thrust. It’s very similar to a sandblasting machine.

That’s all well and good, but you need to bring that gas with you, perhaps from Earth or from some mining operation off Earth. Once it’s used up, you’ll need to replenish the gas.

I think we can do better. Specifically, we can accelerate the regolith without any entrainment, without adding a gas. A vehicle with such a propulsion system could take off and land repeatedly without needing to recharge a tank of working fluid. It would scoop up dust to refuel. That’s all.

How would such a regolith-only propulsion system work? I propose a design that uses a mechanical pump—a fan or turbine—that expels the regolith from the back of the rocket as the propellant. Let’s consider the specific case of a lunar transport. Clearly there’s no lunar air with which to blow the regolith through the rocket. I’m thinking of a turbine that mechanically accelerates the regolith that falls into it from a hopper, or a tank, thanks to gravity. Unlike a classic regolith rocket, the regolith here provides all the mass flow, and a source of power on the spacecraft provides all the energy to spin the turbine.

Here’s a potential problem. In the process of moving the regolith through the turbine, the blades might ablate somewhat. So, one goal would be to choose a material for those blades that is tough enough to withstand all this internal sandblasting for the duration of liftoff and landing. However, it turns out that if the dust can be sifted down to particles of 20 microns or smaller, we don’t need to take any extraordinary measures. Even terrestrial turbine blades aren’t damaged by such tiny grains. They carry too little energy to impart stress in the blades beyond the yield strength of the material.

Another issue: how do we get the regolith into the turbine? You have to fill up the tank somehow—a shovel and a sieve?—but then mere gravity feeds the regolith into the fan, just like it pulls sand through an hourglass. As the spacecraft lifts off and accelerates further under the power of the engine, the mass flow rate could be allowed to increase naturally or could be throttled back with an adjustable aperture, depending on which is more efficient.

Now let’s look at a design that may support lunar businesses, i.e. commerce among Moon bases. I’ll describe the design of a printer-size transport vehicle: 10-20 kg total mass. Sure, there is probably some economy of scale that would make a larger vehicle more efficient. But let’s take this small example to be an existence proof for a larger one. Also, this little one is inexpensive enough that a person might actually build and test a prototype in the near term. (Or maybe someday Amazon will want to deliver little packages via drones on the Moon, too. They’ll see the wisdom in this idea and will come knocking at the door of my lab at Cornell looking to sponsor the research. One can dream.)

Let’s first consider a lunar transport design that offers very low specific impulse (abbreviated Isp, a measure of the efficiency of propulsion). This transport hops, if you can call kilometer-scale trajectories “hops.” The engine expels propellant at 0.14 kg/s and lifts off at an angle of 45 deg. to the vertical, which optimizes the trajectory for distance. It accelerates until it reaches maximum speed at an altitude of about 540 m. Doing so expends about 74% of the propellant, at which point the engine shuts off. The transport continues to coast ballistically, through a peak altitude of 940 m, and then drops down to 390 m altitude, at which point regolith is fed into the turbine once more. Applying that final thrust slows the descent, and the vehicle comes to a stop at the surface of the Moon. It lands 3.7 km from its starting point, if it’s carrying no load. The flight takes only a minute or two. Carrying a payload means sacrificing some of the regolith propellant, so that the total mass can still lift off. And less propellant leads to shorter trajectories, of course.

The vehicle is designed to carry 13 kg of propellant, which is enough to accomplish these hops. The rocket equation tells us that this 0.14 kg/s mass-flow rate, with an exit velocity of 240 m/s, leads to an Isp of 24 seconds. That Isp would be shamefully low for a chemical propulsion system, where Isp is typically in the hundreds of seconds. But remember—the propellant is everywhere you look. A scoop and a sieve constitute the gas pump here. This convenience comes at the cost of efficiency, but I think it’s a good trade for this particular concept.

Limitations on the speed of the turbine ultimately drive the design. The speed at which a turbine rotates is limited by the tensile strength of the material that it’s made of, as well as the bearings that support the rotating portion. The AMT Mercury small turbine for unmanned aircraft, for example, is designed to rotate at 151,000 RPM. Let’s base our Moon transport design on this turbine, but I’ll assume that we can boost its rotation by about 20%, for 181,200 RPM. It will need considerable redesign so that the flow is straighter, since the goal here is not to compress gas but simply to accelerate the particles. But I hope that this example suffices as an estimate of the weight. They say it’s about 2.2 kg for this great little turbine.

We’ll model the turbine blades so that the twist along the blade length reaches 45 deg. halfway along each blade. Using two turbine stages might be important so that the expelled propellant doesn’t carry significant angular momentum, which would be in the same direction as the rotation of the turbine that imparts its axial velocity. That momentum would cause the vehicle to spin up in the opposite direction, and that spin would be difficult to dissipate before the vehicle lands. This simple model has the propellant exiting the turbine stage(s) at an axial velocity equal to that of the blade midpoint in the plane of its rotation: i.e. the dust bounces off the 45 deg. angled blade and acquires an axial speed equal to the blade’s lateral speed when they were in contact.

As the regolith passes through the blades, it tends to slow down the turbine. The power required to keep this turbine spinning is roughly equal to the change in energy of that accelerated dust. With a mass-flow rate of 0.14 kg/s, accelerated to 240 m/s, the change in kinetic energy (or power) is 3.98 kW. Let’s use a 4 kW electric motor to provide this power. A nice brushless DC one is available off-the-shelf and weighs only 3.3 kg. We’ll also need a transmission of some kind that gears up the 8000 RPM peak rate of the motor to the 181,200 RPM that the turbine demands. Prof. Brandon Hencey at Cornell tells me that some aircraft-engine turbines, e.g. from Pratt and Whitney, include a transmission that reduces speed, sometimes for noise purposes. Our vehicle would use a similarly designed transmission but run backwards so that the gear ratio allows the motor to drive the high-speed turbine.

Batteries. Ah, batteries. It is my belief that the highest-priority technology advance for space technology is increasing energy density (J/kg) and power density (W/kg). So many extraordinary systems would be enabled if arbitrarily large energy storage were possible and if it could be expended as power arbitrarily fast. That technology development is not necessarily NASA’s responsibility, though.  Using commercially developed technologies might be the fastest way to see improvements. Today the best lithium-ion batteries provide about 360,000 J/kg. They’re very slow to discharge, i.e. to expend their energy. That discharge rate is limited to 10-15 A, which means that the energy drools out over the course of half an hour or longer. The 13 kg of propellant in this design has to be expelled over the course of only 93 seconds. So, ultracapacitors are better choices than batteries here. They’re less mass-efficient in storing energy (only 230,400 J/kg) than lithium-ion batteries, but they can discharge quickly enough to impart the power that the motor demands. These ultracaps total about 1.6 kg. Another alternative would be flywheel energy storage. Interesting as it may be, this technology is not much more appealing than high-performance ultracaps when you take into account the hardware necessary to make the system work.

The motor, batteries, turbine, transmission, electrical harness, mechanical components, and propellant of our lunar transport total 18 kg. The thrust available from the engine is 34 N, which means that the 20 kg vehicle taking off in lunar gravity has another 2 kg available for everything else: tank, structure, guidance sensors, solar cells, flight computer. That’s not much, perhaps, but remember that the “tank” is just an unpressurized bucket, and other than the batteries discharging, there is no thermal activity on this vehicle to require special materials, insulation, et al. This tank needs to hold 13 kg of regolith powder, which (at 1800 kg/m3) could fit in a cube 20 cm on a side. And the electronic guts of a CubeSat (no more than 0.5 kg worth of boards and solar cells) should be sufficient for the rest of the electronics.

One more thing. This spinning turbine will take time to reach full speed. There is likely a lot of kinetic energy stored in the turbine itself. I propose that solar power be used to charge up the ultracaps and spin up the turbine while it’s on the ground, without expending energy from the batteries that would be indispensable in flight. Once it’s spinning, a single-stage turbine would helpfully store angular momentum that gyroscopically stiffens the attitude in response to disturbances, such as misalignments of the rocket engine or liftoff and landing perturbations.

This low-Isp design is based on a 2.5 cm radius turbine, in which the regolith impacts the blades at, on average, about halfway along their length. Now, let’s consider a higher-Isp design. A 4 cm-radius turbine gives the regolith higher exit velocity: 380 m/s instead of 240 m/s. However, for the same 4 kW electric motor, the system is a lot lighter: only 11 kg, of which 4.6 kg is propellant. The higher exit velocity means that more power must be applied to a given amount of regolith, and that requirement limits the total performance. The mass flow rate must drop to 0.055 kg/sec, and the total engine on-time to 83 sec. The upshot is that the trajectory of the higher-Isp design is only about half as high (460 m) and half as far (1.9 km) as the low-Isp vehicle. What we see here is a classic trend in propulsion design: that you pay a power penalty for higher efficiency. And that’s why in-space propulsion is often electric, and launch propulsion chemical. No current electric-propulsion technology can even lift its own weight on Earth. This regolith transport concept lives in a sandy border town between the two.

So, dust off your plans for lunar colonization. Now we have a way to deliver water, rare elements, spools of 3D printer material, and really just about anything to neighboring lunar outposts, in small batches at least. The demonstration-scale vehicle described here may be a precursor to a much more capable, larger transport vehicle. And thanks to bypassing chemical propulsion systems and avoiding the use of refined materials and expendable matter from Earth, we may have found a pretty cheap way to fulfill lunar e-commerce orders.

Lunar Xistera

xisterawithcrater

What if you could land on the Moon without a rocket motor—in fact, just by landing on a runway and rolling to a stop with more-or-less familiar mechanical brakes? And what if you could take off again without a rocket, simply by using an electric motor?

Yes, I’m aware that there is no air on the Moon, and only a few centimeters of what one might call atmosphere (and is in fact electrostatically levitated dust particles). I’m not proposing that an aircraft would lift off the surface with aerodynamic effects. I’m considering the possibility of a spacecraft that lands at orbital velocity and slows down from there. To take off again, the spacecraft must reach orbital velocity as it travels along this runway.

Why bother? Quite simply, a rocket takes a lot of fuel. For reference, each Apollo mission that landed on the Moon comprised both a lunar descent stage (10,149 kg) and an ascent stage (4,547 kg), for a total of 14,696 kg. About half of that total was propellant. In our example, let’s assume that we want to land 20,000 kg on the surface of the Moon. And why that number? It’s the low end of the total mass NASA calculates a human Mars mission needs to land on the surface of Mars. So, that total might make sense for a lunar landing as well—for instance, the case of a mission with a longer duration than any of the Apollo landings. For a spacecraft to be in orbit just above the surface of the Moon—assuming it’s at the average lunar radius of 1,740 km—it has to be traveling faster than 1,679 m/s. So, for it to stop by the end of the runway, it has to get rid of 1,679 m/s worth of energy. And for it to take off again, it has to attain that speed (or more). Unlike Apollo, our spacecraft may be able to use the same hardware—the wheels or something else mechanical—to land and take off. It does not necessarily expend propellant in the process. So, right there, I anticipate some significant mass savings. And this same vehicle can land and take off repeatedly, without refueling. Nice.

How long this runway would have to be involves some guesswork. Here are some of the numbers involved in my guesses. The FAA says that 1,829 m is sufficient for most aircraft under 90,718 kg. Longer runways exist, e.g. O’Hare’s runway 28, at 3,963 m. However, different issues are in play here, notably that gravity is lower on the Moon and likely doesn’t let the brakes decelerate the vehicle as quickly as they would on Earth without skidding. And the absence of atmosphere gives greater determinacy to the spacecraft’s touchdown, i.e. landing precisely at the beginning of the runway. So, I take it that the overrun area and other forms of margin are less necessary. Of course, most aircraft land at speeds far below that of our orbiting spacecraft. In fact, they land at speeds lower than that of the Space Shuttle, which used to land on Earth at 366 km/hr (104 m/s). Good automotive brakes can decelerate at about 1 g, and let’s assume that that principle holds for lunar gravity, where g=1.622 m/s2.

Decelerating at that rate from orbital velocity therefore requires at least an 869 km-long runway. That seems very long. Inconveniently, expensively long. Let’s try scaling up from those FAA guidelines, using the ratio of kinetic energy as a scale factor. For the same mass, that ratio is (1679/104)2=260.6. So, that basic runway distance would scale up to 477 km. Still quite long!

My dad, a former Marine Corps pilot, has a better answer, or I think he would if I were to ask him. Naval aviators extend a so-called tailhook from the back of their aircraft to snag a cable (one of several available attached to hydraulic cylinders) that slows down the aircraft over a relatively short distance: from 240.8 m/s to zero in about 104 m. Those are high gs, and I doubt it feels very nice. Nevertheless, at no more than 23,000 kg, those aircraft are about the mass of our spacecraft, albeit with about 1/50 of the spacecraft’s kinetic energy. So, how about a series of arresting gear, like those cables, over a distance of about 25 km? The spacecraft would decelerate at a withstandable 56.4 m/s2 (5.7 g, or 5.7 times Earth gravity), and it would take about 7.7 seconds. Quite a ride, but nothing the Navy hasn’t seen, and actually quite a bit more gentle than landing an aircraft at sea. But let’s add 20% margin to that length and call it 30 km.

Making a lunar runway requires something like concrete. We have an estimate of its length, but the volume of concrete depends on its thickness and width, too. Now, width seems not to be very important, judging by the lessons learned from aircraft carriers. Let’s say 10 m, which is more generous than what naval pilots require. As for thickness, we need enough concrete to prevent the landing from crushing the runway and to resist deformations of the Moon’s surface (yes, there are moonquakes). Our spacecraft is traveling a lot faster than an airliner, but its weight is far less—both because it has less mass and because the Moon’s gravity is only a sixth of Earth’s. While airport runways range between 25 cm and 1 m in thickness, I don’t see why ours needs to be any thicker than the most extreme terrestrial runway, at 1 m. Even that is probably excessive, but the point here is to come up with an upper limit of how much material is needed.

It certainly would take a lot of effort to bring concrete from Earth. And this runway requires 300,000 m3 of material. Naturally, we would build this runway from local materials—the lunar regolith. There are myriad recipes for lunar concrete, although the only solutions I’ve encountered require bringing quite a bit of material from Earth. So, here’s my recipe, which doesn’t:

Let’s begin by considering small rovers that can create regular, comparatively smooth bricks by sintering lunar material, as has been demonstrated at the Johnson Space Center (JSC) and elsewhere, or perhaps some other process. For sintering, the rovers will need solar panels and time, and an insulated (ceramic) mold that compacts the regolith (like WALL-E compacts trash), but not much else. And they can be at it for years as part of a robotic precursor mission before anyone is ready to land on the runway. Say each brick consists of a liter of material (10 cm on a side), with about 3.1 kg of mass each. There are 1,000 in a cubic meter and therefore 300 million bricks in this runway.

There’s a classic solution: melt regolith into bricks, whether with a 3D printer or some sort of oven and a mold. A 400 W rover could create a brick every eight hours—one hour during which it collects the material and later places it, and seven for heating up the sample (as in the JSC tests) from 350°K to 1,100°K. The bricks need to reach 1000°C for the silica to fuse, and although the JSC tests held it there for three hours, this slower heating would require less hold time. How much? I don’t know. This is just a ballpark figure. The specific heat of the regolith varies with temperature and was reported by some folks at Harvard to be about 1 J/g/degK at room temperature—i.e. 1,000 J/kg/degK in useful units. I modeled the specific-heat variation as linear, extrapolating from that Harvard data. This estimate is based on 1 m2 solar panels at 30% efficiency, for about 400 W of power during the 14 1/2 Earth-day-long lunar day. And I assume that about 20% of this available power is lost in electronics, imperfect insulation in the mold, et al. So, about three bricks per Earth day per rover—at least, while the sun is shining. Half the time it doesn’t shine, even on the Moon, simply because the Moon rotates as it orbits the Earth. (Incidentally, this sintering could all be done a lot faster with nuclear power, but I’d rather focus on a readily built system.) So, this approach would take 54,800 rovers 10 years to complete.

Forget that! Way too many rovers, obviously. I say fabricate the bricks out of frozen mud: regolith+water, which is far more brittle but could be repaired every time with far less effort than building a sintered-brick runway. With a supply of water, and in the cold, each brick would take maybe 5 minutes. And at that speed, we’d need only 570 rovers. That still seems like a lot of rovers, but at 50 kg each (the scale of the Violet spacecraft), that’s about 28,550 kg—one landing’s worth of rovers. Maybe plan for three landings, since we’ll need all that fuel etc. before the runway is built. Still, three landings and 10 years gets us permanent infrastructure for lunar transportation.

In preparation, a strip of the lunar surface is cleared of boulders. Also not hard for some small-scale rovers to achieve.

An adhesive mortars the bricks together. Again, proposed solutions for such mortar are legion. A particularly appealing one is the use of sulfur as a binder and as mortar, and that’s not hard to find on the Moon. That solution would also require no materials to be brought from Earth. However, if the mud bricks are solid enough to resemble terrestrial bricks, I would propose that we use water again as mortar—simply freeze the bricks in place, which is a solution that both uses in-situ material and also lends itself to straightforward repairs, as long as the ice is not exposed to the sunlight. So, protecting the surface of the mortar would be necessary, again using local material. Regolith itself—a thin layer of dust—might be enough. And this principle raises an important issue: sunlight would soften these bricks, turning them into mud. The simplest solution would be to land at night. Another, less simple, solution would be to build this runway in a permanently shadowed crater, of which there are several. The constant shadow ensures constant sub-freezing temperatures, which would keep the runway solid. In fact, these locations are also where water is found on the Moon’s surface. However, the location would be near a lunar pole, which may be limiting (although there are many reasons why a polar outpost could be a good idea, such as the availability of permanent sunlight and shadow).

After each landing, rovers inspect the runway. They seal cracked bricks and dribble water into the interstices as needed. Or they remove bricks entirely and replace them. Now for an interesting adaptation. What if the runway were not straight but, in fact, curved and banked—for example, along the edge of a crater. Some of the larger craters, like Tycho, are wider than 50 km. The tighter the radius and greater the bank angle of this runway, the higher the centripetal acceleration that would keep the spacecraft from skidding, thanks to increased friction at the wheels. So, how about a runway with an initial, flat landing region that curves into a lower, circular track? With no atmosphere, the Moon doesn’t require that the runway be entirely horizontal. The spacecraft’s initial approach simply allows the vehicle to begin tracking the runway’s kinematics, its path, which I suggest should be tilted so that the vehicle’s path is parallel to the lunar surface, but curved, like a jai alai xistera.

extrathickxistera

Let’s say the spacecraft decelerates at 5 g (Earth gravity, again), i.e. 49.05 m/s2. The force it feels would be inward, i.e. toward the runway surface, to keep the wheels in contact, as in the case of terrestrial runways. At this deceleration, the runway spirals inward over its roughly 28 km length, and there’s no need for the arresting device. This banked runway’s shape is a little harder to build than that of a flat runway, but it’s likely easier to operate, is a lot shorter than the alternative, and requires no hardware sent from Earth other than the rovers that build it. The image above is an exaggerated view of the runway: too thick and wide (I am showing it that way for clarity), but the curvature and other parameters are exactly what would accomplish this goal.

The runway merely needs to withstand 49.05 x 20,000 = 981,000 N inward, toward its surface as the spacecraft travels along the curve. It also has to withstand that same amount as a shear, along its surface, as the vehicle brakes. Even doubled (2,774,000 N), that force is far lower than commercial airliners apply to terrestrial runways.

Incidentally, for cargo only, a much higher g-load would be possible. Say 15 Gs. In that event, the spiral runway could be less than 10 km, as long as the runway could withstand the force.

Now, let’s free up our thinking even more. Do we really need those bricks? If landing in soft regolith—powdery sand—is possible, all that may be necessary would be for microrovers to clear the large rocks and boulders. The motivation for this banked spiral is to avoid a large number of bricks. So, one might return to the 800 km or longer runway, if it’s possible to find such a stretch of open, flat area on the Moon. A quick look at a lunar map suggests that it may be. Such a runway may be even shorter, given the drag of the regolith on the landing gear. The downside is that the drag of the regolith may be hard to predict and may overturn the landing vehicle.

So, in summary, we have several concepts here. There’s a long, flat 800 km runway that may have a sintered-brick surface or may simply be a soft, rock-free area. There’s a thoughtfully curved, banked runway that is much shorter—10-30 km. And there’s a 30 km runway with arresting gear, like on an aircraft carrier.

The spacecraft that lands on this runway needs wheels, or maybe skis. But if the goal is to take off again, a set of wheels makes more sense to me. Could they withstand the landing? A key issue is that the wheels must come in contact with the ground without too much relative velocity. Aircraft are able to land with wheels that spin up as they contact the runway, but the orbital speed is much higher. A 1 m radius wheel is not far from what large commercial aircraft use. Such a wheel, rotating at just over 16,000 RPM, would contact the runway without skidding. 16,000 RPM is fast, though. The tensile strength of the material must be quite high for the wheel not to tear itself apart at that speed, let alone the other forces associated with landing. A carbon-fiber composite wheel is necessary here.

Spinning up these wheels is not trivial. The International Space Station uses control-moment gyroscopes (CMGs) for attitude control, and their rotors require a long time to spin up—many hours. That’s because the spin motors typically are used only to keep the rotors spinning. On the rare occasion that a spin-up is necessary, they go as fast as they can. But that’s not very fast. So, I would anticipate that the wheels for this spacecraft need to begin spinning up many hours before landing. The power would come from solar energy, though, not propellant. A really useful feature of establishing that much angular momentum in the three-or-more wheels is that the spacecraft would have a high momentum bias, stiffening its attitude dynamics and allowing for a lower-risk approach to the runway, with little or no pitch or yaw motion. The landing gear need some sort of shock absorbers, like the oleo stroke gear on other aircraft. That’s a largely off-the-shelf component.

At this point I need to acknowledge that I’m not the first to consider all of these ideas. Some, perhaps, but not all. After reading the first draft of this post, a key member of my vast editorial staff introduced me to the work of Krafft Ehricke. Ehricke was a futurist and visionary technologist. He came up with the notion of a lunar runway long before I did, and lots of other great ideas besides. He had in mind several permutations, roughly along the lines of two of my three architectures: a long, dusty runway cleared of boulders and a paved surface. He put quite a bit of effort into the former, looking into the behavior of regolith for a vehicle that might land in it. But I have to say that his thinking, like mine, was driven by the spirit of his age. For him, a nuclear powered system to gather regolith and produce concrete was not much of a stretch. But for me, having seen how money is spent and how work is prioritized in Washington, I am focusing on a much leaner design that involves readily launched technologies with comparatively low cost. And, perhaps most important, I have benefited from recent discoveries, from Clementine to LCROSS, that confirm that the Moon is simply loaded with water. Ehricke had no idea. In fact, most of us assumed that the Moon was simply bone dry. Until about a decade ago, our exploration-mission architectures had humans bringing all the water they would ever need. That fundamental principle even shapes today’s architectures. It’s time for a re-think.

I claim that this banked and pitched, mud-brick runway built by robotic rovers in a permanently shadowed lunar crater is a new idea. It allows a spacecraft to land in a short distance, seems feasible to build on a useful time scale, and requires no fantastical technological breakthroughs.

Taking off again requires some more attention. I’ve given some hints at how it might be accomplished already. However, since this post is already quite long, and I’ve already offered about four new ideas here, I’ll save an analysis of this maneuver for a future post.