New Mars Forums

Good to see you again Spacenut

GW, Tahanson, good to see you guys again.

Was talking about the Northrop Moon Train study with some friends on Twitter, had a look at the forums to show them how long I've been banging on about trains in space for. I'm doing engineering for Rocketlab these days on the space systems side.

I tried to copy the post text into the above post and it seems like it's something about the post itself that the forum doesn't like. Pretty strange since it was all ASCII

Thermal expansion/contraction problems are way worse on the Moon than anywhere on Earth. It's a fun design question though. I'd do something like this (probably occurring over a much longer distance than shown here). Rails in the top of the picture allowed to slide against the rail on the bottom but shall always overlap.  Ties, probably stiffer at the joint, keep everything aligned left to right. Rails are probably made stiffer also at the joint to partially compensate for tapered shape.

The ability to use regular steel or maybe aluminum (price difference much smaller if both are smelted electrically: See this post) is going to be pretty critical for rail to make sense. If you're using fancy temperature insensitive alloys you might as well just use trucks.

As much as I love trains, it's not clear to me that any kind of lunar train network will pencil for the foreseeable future. What kinds of goods will even be shipped, and in what quantities, and over what distance? It's a bit hard to say.

But for starters, there will be spaceports at least some distance from each base, iron mines, aluminum mines, silicon mines, water mines, carbon mines, nitrogen mines (though water, carbon, and nitrogen will all likely come from permanently shadowed craters they may not come from the same permanently shadowed craters).

On Earth the case for rail over road is that the rails give you better fuel economy, longer-lived equipment, and the ability to use less labor to ship more stuff. But building a rail line costs money—more than a road, despite having a narrower use case—so it doesn't pencil unless there's high volume (coal and oil are often shipped by train, along with other stuff) or someone's already built the rail line.

On the moon I think the driving factor towards something like a railroad is different: Electric power.

I don't want to get into a big argument over power sources, but in my view solar photovoltaics are the path of least resistance for the Moon if you can make them work.  The biggest challenge there is the two-week long lunar night, which would mean very large storage requirements.

But there is another way: Since bases will be clustered around the South Pole, you can build a grid that circumnavigates the pole with solar sites spaced at, say, 120 degrees. At near-polar latitude the panels will cast long shadows, meaning your array will use a lot of land, but the moon has as much land as all of South America and I don't think this is a big deal. You can also build your power stations significantly north and run lines down to the base locations. China has power lines that go 3200km (diameter of the Moon is 3400km) and they're not at the physical limit, meanwhile the low gravity, stable environment, and vacuum make it even easier to build power lines on the Moon.

There is a point to this digression, which is that we should expect the Moon to have a pretty well-built out power network early on. This ends up being pretty important for the idea of long range transport, because burning hydrocarbons is a waste of hydrogen and carbon unless you recapture, which comes with difficulties, nuclear is poorly suited to the size of transport, batteries are inconveniently heavy—especially if you want really good ones brought from Earth, etc.  The most logical way to build out a transportation network is to piggyback on your electricity transmission network.

So what you end up with is roads—paved in the busiest corridors (with what I'm not sure, possibly just sintered) or cleared and leveled elsewhere, under an electrical catenary which provides power to the vehicles running underneath it.

I asked Bing's AI image generator to make this, just to see what I could get. Didn't capture everything I'd want (had a tough time getting it to put Earth in the sky) but it's a cool look anyway:

On Mars though? Plain old combustion engines in trucks, in all likelihood. Possibly bigger trucks. Oxidizer is a tougher question than fuel. Pressurized oxygen is probably the easiest thing. Electrified routes probably come later.

Hey kbd512,

Happy New Year!

Glad to see I was on the track of something real and potentially useful.

Using one of these guys as a temporary fix for climate change seems like a definite possibility.  From there I would say there are obvious applications also to Mars and Venus for their respective terraforming projects, and at that point possibly any outer-system body looking to heat itself up (in combination with other terraforming methods like greenhouse gases).

Looking even further into the future, plasma mirrors would definitely be useful for interstellar lightsail missions.  By locating one close-in to the Sun you might direct concentrated light at a far-away lightsail hosting a similar plasma mirror array.  You might achieve a multiplier of the intensity by bouncing the light back and forth between the craft and the concentrator array.

Plasma mirror telescopes?  It's hard to imagine that this technology could achieve the necessary precision to get good imaging, but it's not impossible, and if planet-sized plasma mirrors are already in use I have to think astronomers will do anything to get that much signal.  At that point (and this is unfounded speculation) I have to think you'd be able to look at extrasolar planets light-years away in substantial detail.  As a point of comparison, James Webb Space Telescope is supposed to have a mirror 6.5m across; The one I'm talking about is literally a million times bigger, and thus would have a million million times more area/signal.

Where this ends, I suppose, is with a dyson sphere, which becomes at least conceivable when you do not need to build it from solid matter.

Hey OldFart,

Here's what I've been able to find:

1) Compressive strengths are not often provided for polymers but the tensile strength for Polystyrene is cited in various places as 35-55 MPa.  You can correct me if you believe I'm wrong but it seems to me that the compressive strength should be in a similar range, thus roughly 50 MPa±25 MPa.  I was surprised to find that this is roughly also the compressive strength of both portland cement and typical concrete. One thing I am truly uncertain about is whether sand/aggregate will bond to a styrene matrix in the same way as it bonds to a cement matrix.  Both cement and sand/aggregate are sort of rocky materials with somewhat similar structures and similar mechanical properties (densities, hardnesses, etc).  Styrene has very different properties--would a styrene-sand composite be as well-matched as cement-aggregate?  Would it last as long? Would the components bond to each other? Or would their different thermal expansions and hardnesses cause it to shred itself from within over time?

2) As far as pricing goes, it looks like polystyrene goes for about $10/kg, which is about $9500/cubic meter.  By contrast, concrete costs around $113/cubic yard, which is about $150/cubic meter or about $0.11/kilogram--much less.  It makes sense that this would be the case, because concrete is made from lightly processed bulk materials whereas styrene monomers have to be produced via a somewhat complex process of chemical purification and synthesis.  On the one hand, the availability of CaO or CaCO3 deposits on Mars is something of a question mark.  On the other hand, Styrene monomers are ultimately produced from crude oil which is almost certainly not present on Mars. (Crude oil comes from fossilized life-forms; CaCO3 most often exists as limestone accreted from living organisms but can also form naturally in water depending on pH and Calcium availability).

3) I'll talk about 3D Printing in my next post.

Hey GW,

Yup, we were just talking about mild pressurization for the concrete to set. Understood on all sides here that pressure containment in the actual building is much harder.

Gotta divide by ~2.5 for lower Martian gravity, but making things heavier than they need to be presents no difficulty at all.

The idea of 3D Printing with concrete is a little weird because it's actually slower and harder than just pouring concrete down into a mold.

In my opinion the most interesting thing would be to start 3D Printing the molds (rather than framing them out with wood per current practice) and then pouring the structural concrete in to save labor.

Concrete is also extremely poorly suited for being a structural wall material for Mars because it has virtually no tensile strength.

I think a good design for insulation would be modular "insulation panels" that can be bolted onto the structure and onto each other.  Probably best for them to remain unpressurized: Insulation filled with gas at Martian ambient pressure is an even better insulator than insulation filled with gas at Earth ambient pressure.

I haven't done the numbers, but it would come as no surprise at all to me if air at 1 atmosphere and -20 C carried away more heat through convection than air at 0.007 atmospheres and -70 C.

Mars is also a lot drier than Canada (it basically never rains or snows) and never thaws.  This has downsides from a settlement perspective but also upsides in that the freeze-thaw cycle can be extremely damaging.

Hey Tahanson,

Absolutely feel free to share that post wherever you like.

As far as using stripped ions for digging:

You asked me to disregard practicability, so I will: Yes, it could be done.

I maintain that it is not practical.  These things are hard to create and even harder to store.  It's basically impossible for that to be the cheapest or easiest way to deliver energy into rock.

kbd512, GW, Bob Clark, et al:

I've been thinking about this some more, and here are some thoughts I've had about how I'm going to simulate the trajectory, in no particular order.

1. The point about doing the pitchover basically at the time of launch makes sense to me; a rocket launch is a fundamentally horizontal affair and time spent travelling vertically is basically wasted impulse.

2. I had initially thought to start the simulation with a zero-time impulse (meaning to start the rocket off with some horizontal velocity relative to the ground at t=0 as a simplifying assumption) and fly a non-lifting trajectory from there, then go back and calculate the correct angle/duration (given a fixed total horizontal impulse, pitchover angle and duration are directly related to each other).  Taken together this would turn a 3-dimensional optimization into a 1-dimensional one, and while a finite duration pitchover will result in a slightly lower orbit than a zero-duration pitchover, the difference is small and easily corrected for.  This will not work, because if you start with horizontal velocity and no vertical velocity (the zero-time impulse pitchover) your flight angle will be zero.

3. Therefore I need to think more carefully about how to simulate the timing, angle, and duration of the pitchover.  Suggestions are appreciated.

4. Strictly (theoretically) speaking, a pitchover maneuver is not required to reach orbit.  An object sitting on the equator that is motionless relative to Earth has a velocity of 465 m/s.  This corresponds to a two-body newtonian orbital radius around 2 million km (in practice this is interplanetary space, but you'll be orbiting something regardless).

5.  A zero lift/zero AOA trajectory is not quite the same thing as firing in the direction of motion in inertial space, because the earth and the atmosphere both rotate, and therefore a craft that is stationary with regards to the atmosphere still has rotation and velocity in inertial space.  Presumably the global effect of the difference is small for reasonable trajectories but at the beginning of the trajectory the difference in implied angle is a full 90 degrees.