New Mars Forums

Hey Zachoi,

Welcome to newmars!

Interesting question and interesting concept.

I'm going to start by restating the question as I understand it to make sure I'm going about answering it in the right way.

You're designing a drone to fly over the surface of Mars which you'd like to power with compressed CO2.  You want to figure out the most efficient way to use this energy source to maximize the range of the drone, and you've come up with two possible alternatives: First, using the compressed CO2 to power an engine to turn a propeller or second to use jets of compressed CO2 in a cold gas thruster, with the aim of maximizing flight time and therefore range.  Both alternatives will run in tandem with a system of electric motors (and presumably batteries) familiar from drones on Earth.

I must admit that in reading your post I don't understand why you'd want to design a system using both compressed CO2 and electric propellers.  I have limited exposure to principles of aeronautical engineering, but here's how I think about the issue:  For any given drone design, you will have a certain amount of payload mass you can dedicate to energy storage/production.  In order to maximize range and flight time you will want to maximize the number of newton-seconds you can get out of that mass.  You will therefore look at your options, determine the efficiency of each, and dedicate 100% of that available mass to the most efficient one, providing for redundant systems where necessary.

I discussed the energy storage ability of compressed CO2 at length in this thread and much of what follows is based on information presented there.

One thing I want to push you on specifically is your choice to use CO2.  I can see the appeal, maybe, if you want your drone to be a sort of hopper, which flies until the battery runs down, then lands to recharge (in this case, recharging by running the engines backwards to compress atmospheric gas), then taking off again.  If this is your goal (much more complicated than a drone that flies until it runs out of power and then crashes!) then CO2 is the only gas you could potentially use (atmospheric separation would be crazy for this sort of mission imo).

Anyway, whatever your plans are I want to address the two scenarios you mentioned and then try to compare them to battery-electric and chemical-fueled engines.

The real problem with CO2 is that, compared to other gases like Nitrogen, it has an extremely low vapor pressure.  Consequently, you can't pressurize it very much before it either liquifies or solidifies.  This is made worse by the fact that Martian temperatures are much lower than Earth's, and there's no real way for an airborne drone to absorb thermal energy from the ground to vaporize CO2.  For reference, at 230 K (typical Martian temperature), CO2's vapor pressure is 10 atmospheres; Nitrogen and Helium are far above their critical points and therefore cannot liquefy at any pressure, and for reference it is not unusual for bottles of compressed N2 or He to have pressures of 150 atm or above in regular commercial use.  Cold gas thrusters using Nitrogen can see specific impulses as high as 75 s (for comparison, the SSME got 450 s).  Isp scales in proportion to the inverse square root of the molar mass, so you might get up to 200 s with compressed Helium (which is, uh, pretty good actually?).

Anyway, to get back on track CO2 has both a lower possible pressure and a higher molecular mass than either of those gases; Accounting for molar mass alone you're looking at about 60 seconds, but the low vapor pressure really limits the possible expansion ratios and I'd be surprised if you could hit 20 s.  That's 200 newton-seconds per kilogram of compressed CO2, which frankly is awful given that the gas itself is a pretty low density and requires comparatively heavy tanks to store.

When they can work, propeller-based systems are inherently more efficient ways to generate impulse than rocket-based systems because they move more mass at slower speeds.  Ideal conditions for propellers are slow travel speeds through a dense medium (Think boats).  Whatever the travel speed, Mars's atmosphere is not a dense medium and the design of a propeller that will work in it is nontrivial.  Presumably what you want is a very large one with a low speed of rotation.  Frankly I have nothing to contribute here.  What I'm trying to get at is that there's no good way to make an apples-to-apples comparison between a cold gas thruster and a propeller system.  If the propeller works it will presumably be better, but that's a tautologically true and therefore useless statement.

Hey GW,

Yes, that was very helpful, thank you.  No doubt I will have more questions when the time comes to throw everything together in one simulation. 

On the topic of solvers and Matlab there are no doubt lots and lots of options available out there.  Typically I will build my own, which isn't that difficult and guarantees maximum understanding of the task at hand.  Normally I start with a rough and dirty method (making no attempt to economize on computational power; In the first run I'll probably set the step size to 0.1 s for maximum accuracy) and then only work in improvements to the code if I find that the runtime is excessive on the equipment at hand.  You can get substantial improvements even with simple modifications, like lowering the step size and compensating with a trapezoidal rule and introducing a variable step size.  Given the things I find myself modelling in Matlab and the power of a modern PC this is generally adequate.  If I were to attack something much more complicated (for example modelling the dynamic response of a rotating skyhook to various peturbations) I'd need to really think about whether my programming ability and PC are up to the task.

Edit: It's multidimensional optimizations that really kill you on computational power.  Taking the timing, angle, and duration of the pitchover maneuver as an example you might try to optimize by looking at 100 different possible times, 50 different angles (presumably the typical pitchover is only a few degrees and you might go in increments of 0.25 degree for precision), and 100 different durations.  This is 500,000 different scenarios, and even if your initial simulation of a single trajectory can run in 4 seconds the complete optimization will take over three weeks.  The best way to speed things up is not by improving your optimization method but by bringing new information to bear.  By researching the gravity turn trajectory and thinking about it analytically, hopefully I will be able to reduce the volume of the parameter space substantially.  After that I might look to techniques such as a "breadth-first" multistep optimization (check out a few points then hone in on promising regions for subsequent rounds to increase precision) and googling "how to do an efficient numerical optimization"

The key point is that the energy in this case is *not* stored through capacitance (IE charge storage) as we normally think of it.  It's stored in the ionization energy of the elements.  The key point here is that the second ionization energy is larger than the first, the third larger than the second, etc., and so while first and second ionization energies are on the order of a chemical reaction, the more stripped a nucleus becomes the closer the energies become to nuclear-scale energies*.

The key point here is that there exists a tremendous potential energy between a normal neutral atom and a stripped atom when they are next to each other regardless of the local macroelectric field or potential.  This means that these stripped nuclei cannot be allowed to get within arcing distance of any normal matter if they are to continue existing.  This is why your suggestion to embed them in a capacitor cannot work, because they will steal electrons from the matter from which the capacitor is made.

*Is it possible to induce a beta decay by stripping an atom of its electrons? I do not know; perhaps it is.

Hey RGClark,

I'm very interested in this project and would be glad to offer what help I can.  As kbd512 noted I have access to and experience with matlab and have done some similar-ish modelling in the past.  I will read your posts on researchgate and do some thinking about this tonight and write up a reply with my thoughts as soon as I can.

I'm with louis on this one. One kWh is 3.6 MJ, meaning you're suggesting it takes 3 GJ/kg to compress and liquefy natural gas.

Compressing gases really is a work-intensive activity, but not *that* work intensive.  Is it possible you've mixed up your units?  850 kJ/kg (0.25 kWh/kg) is a much more believable number for the work required to compress a gas to 70 bar.

A *very* rough ideal-gas approximation at room temperature (methane is not even close to an ideal gas under these conditions): 1 kg of Methane at 70 bar would take up 0.021 cubic meters.  Using W=PV (very rough approximation, *not* a good formula) that corresponds to about 150 kJ/kg of energy.  This should be correct to within +/- one order of magnitude.

This is a great distillation of a lot of discussions you see in various domains, on here of course relating to Mars (+the asteroids, the Moon, etc.), settling the Arctic, Self-Replicating Machines, self-sufficient communes/communities, seasteading, etc. (#FlashBackFriday to the Clean Slate Forums)

As far as common vs rare: I think your example of a stick brings up an important point, which is that common vs. rare is not abstract (You couldn't publish a table for it like you could publish the atomic weights of the elements) but depends mostly on local environment and infrastructure.  Sticks are common in the forest but rare in the tundra, the ocean, Mars, etc.  Iron ore is common, except for where it isn't (carbonaceous chondrites and, to my understanding, the oceanic crust are short on Iron), and good orebodies become less common as we mine them out.  Sometimes you've got to dig. 

The complexity operator seems close to a measure of the required tolerances to make something out of raw materials.  Thinking of a spear with a stone tip, the tolerance is centimeter-scale and a wide variation in rock properties will result in an acceptable final product; raw human sensoria are more than adequate to the task, although there's surely a lot of skill involved and you need the right tools.

For something like an airplane turbine, though, you've got a much different set of requirements.  Alloy compositions need to be correct to within 0.05% or so for consistency, which requires an ability to measure mass accurately.  Dimensional tolerances are presumably small, although I don't know how small (A few hundred micron?  Naturally the answer is "it depends" but probably something around there).  Some alloys get annealed, meaning you need to be able to produce and measure temperature, plus time.

Louis mentioned something that I'd like to elaborate on, which is that there's a difference between having the ability to do something and the ability to do that thing efficiently.  Let's say, for argument's sake, that you need to produce an aluminium cylinder with a diameter of 0.1000 meters diameter with a tolerance of ±0.0001 m (i.e. acceptable diameters range from  0.9999 m to 0.1001 m).  The best tool for the job is a lathe.  Let's say the lathe you have on hand has a precision around 0.001 m.  You can still make this component, as long as you have the measurement capability to check that it's within spec.  It'll be pretty wasteful, though: You'll be throwing out "bad ones" (out of spec but within machining precision) until you get a good one.  Huge waste of time and material, but it can be done.  Naturally it's much more efficient when your machining precision is as good or better than your specified tolerances, but precision begets precision: It's hard, though not impossible, to design a machine more precise than the parts it's made from.

I think there's a sort of set of underlying ideas here that could be put together into a comprehensive physical economic analysis of any given community, which I can comment on in another post (or another thread if you'd rather keep this one more narrowly on topic)

Maybe I'm wrong about this but I bet it says right on the form that lying is a federal crime, punishable by (whatever the penalties are) under U.S.C. (legal designation whatever).

Then I'm sure there's also classes of people who fail their background check because they have outstanding bench warrants against them or are otherwise on the run from the law--people who shouldn't be able to purchase guns and people who ought to comply with their warrants.  Sure, it's extremely dumb to submit personally identifying forms to the FBI if you're a fugitive from the law, but in a nation of 325 million there's plenty of dumb criminals out there.

If it were the case that it was a federal crime for someone to try to buy a gun who doesn't pass a background check, I would be against that since there's no way to find out if you can legally try to buy the gun without trying to buy the gun.  I don't think that's how this law works, though.

I myself don't own a gun and have never really wanted to.  I think it would probably be better if there were fewer guns in the country.  The murder and suicide rates would probably be lower if it were harder to get machines designed to be effective at killing.  Most murders are crimes of passion, and most people who attempt suicide and fail don't try again; there's not "good guys" and "bad guys" out there, really.  I guess there are bad guys, but the good guys sometimes get angry or drunk too.  No personal guns would be fine with me: I have no issue with the idea of repealing the second amendment, and if solid majorities of the country were on board I'd say let's go for it.

We live in a democracy, and people have and like their guns for lots of reasons, many of which are valid. I'm fine with letting people keep their guns while curbing some of the excesses of the current gun regime: Universal background checks would be a good start, and banning the 2 or 3 specific kinds of guns that can be used to kill a bunch of people at once pretty easily would be another good thing to do. It might be a good idea to enforce titles for guns (or certain kinds of guns) the way we do with cars, where you need the piece-of-paper legal document, and you need to get a new one when it changes hands.

Strictly speaking we don't actually know RobertDyck (or at least I don't).  It might be better in some senses to either have entirely different characters (short stories in a common world with a common theme) or fabricate characters by agreeing on general personalities and then drafting off each other's descriptions as we write more.

Another thing we could do would be to each write sections in the first person based on ourselves, in which case RobertDyck is more than welcome to be a founder as long as I get a ride on the ship.

Hey kbd512,

I've seen on the forums that you're a strong proponent of carbon fiber, CNT, and graphene-based materials (including composites) for their strength and high temperature tolerance.  I will readily admit that I do not know a lot about these materials and haven't kept up with new developments in the last few years.  If you could recommend good sources (orNnewmars threads?) for me to learn about these materials, including current best-practices for their production, molding/forming/use in manufacturing, and bulk properties I would be grateful.

Having said that, I'd like to mount a broad defense of the continued importance of traditional materials, in most applications chiefly steel and to a lesser degree aluminium.  It is of course the case that each particular application has different constraints that call for different materials choices, and while we are very often talking about aerospace on these forums it represents a small minority of our actual material use.  This will remain the case for the foreseeable future, even if we mount a full-scale settlement effort of the solar system.

One important distinction to make is the difference in properties between individual carbon fibers, nanotubes, and graphene macromolecules and the bulk properties of materials formed from these components.  To the best of my knowledge in most practical applications these components are formed into useful engineering materials by forming them in an epoxy adhesive matrix.  This has two major offshoots:

1. Your ability to produce the epoxy resin (or whatever you're using for your matrix) needs to be taken into account when talking about the bulk cost of the material and synthesis methods

2. The material properties of the composite are strongly affected by the properties of the matrix and the interaction between the matrix and the fiber

The first is obvious but important, while the second has all sorts of effects.  For starters, while Carbon itself is a very high temperature material, most polymers are not.  Because both materials need to remain within their operating temperature range for a component to function as intended, the actual operating temperature of a carbon composite will in generally be lower than Steel or even aluminium.

The interaction between the matrix material and the fiber has other effects, too.  Composites have failure modes that simply don't exist in metals, things like delamination (layers of fiber reinforcement come apart from each other and from the matrix), fiber pull-out (individual fibers lose their connection to the matrix and pull out from the material), and debonding (the fiber de-bonds from the matrix).  One potentially severe failure mode is thermal strain caused by a difference in thermal expansion between fiber and matrix leading to debonding.  Composite materials also tend to fail in a brittle manner, cracking and losing strength entirely, rather than in a ductile manner, stretching and bending in a visible, gradual way.  Composite materials are also often stronger in one direction than the others.

Finally, composite materials in general don't have a fatigue limit.  Here's what that means: All materials get weaker as they age and go through repeated thermal/mechanical stress cycles.  Some materials, chiefly steel, have a limit to how much weaker they get (Steel will lost half its strength after enough stress cycles but will lose no more after that; other materials get weaker and weaker and weaker until they fail).

As far as the difficulty of manufacture goes, metals are typically easier and therefore cheaper than fibers or composites.  Before tariffs, Steel goes for around $0.25/kg and Aluminium for around $1.80/kg*.  Because composites don't exist as abstract materials in the same way metal alloys do (indeed, the composite doesn't exist at all until its components are molded into a part), it's not really possible to establish a bulk price.  Because polymers and fibers are generally not traded as commodities, it's difficult to pin down a price for them from my computer, but it seems that in general epoxy can't be had for less than a few dollars per kilogram (and presumably you want good stuff for your composite, not the bottom-of-the-barrel cheap stuff) and high-strength carbon fibers a good deal more than that.

For native martian, lunar, etc. materials like we talk about in this thread relative prices will almost certainly be different due to the different structure of the Martian economy.  Just as one example, Mars is not gifted with petroleum oils and fossil fuels like Earth is.  Oil currently trades around $70/barrel ($0.50/kg) and is an important feedstock (it or fossil fuels of other kinds) for most polymers.  One of the main reasons Iron is cheaper than Aluminium is that Iron is produced in a smelting process using coal as an energy source while Aluminium is produced electrolytically.  Carbon Nanotubes specifically are usually created in small amounts electric arcs and then harvested based on their molecular weight, if I understand correctly.

As a final note, I want to point out that traditional forming and machining methods like cutting, CNC, etc. are generally not available for composite materials.  Instead my understanding is that they are molded as a component (in a process I would compare to casting) and sanded down at the end.

By comparison, metals and alloys are strong, common, fairly easy to produce, have a wide variety of forming methods, broadly useful across applications, often fail gently, and can achieve fairly high strength-to-weight ratios.

Having said all that, I want to make clear that I definitely think carbon fiber composites have their uses.  Here's some design paradigms under which it definitely makes sense to look at carbon fibers:

• High-cost applications where performance is critical

• Applications requiring not just high tensile strength but high tensile strength per weight

• Applications with a lower number of cycles ("Low" meaning less than 10,000 or so full loading-unloading cycles)

• Applications requiring a high-strength, nonconductive material

While the use of carbon fiber composites is growing as costs fall and experience with them increases, for reasons of cost, simplicity of use, temperature tolerance, failure tolerance, and failure resistance, we should look to metal alloys first. 

*Note that because of the difference in densities, the cost per cubic meter is closer, $1950/m^3 for Steel and $4860/m^3 per Aluminium; Tensile strengths for plain carbon steel and aluminium are similar.

Summarizing threads for the blog is a great idea regardless, and if we're going to collaborate on a work of fiction that's definitely the first place we should post the results

Two key design principles that I want to mention are failure resistance and failure tolerance.  Failure resistance means reducing the probability of failure.  Above, I used a safety factor of 5.0 in calculating the thickness of the dome.  This means that my thickness was five times higher than the theoretical physical limit needed to contain the pressure.  This ratio is standard for pressure vessels.  Increasing the amount of material means that stress concentration, fatigue, mechanical shock, etc. are much less likely to exceed the dome strength and cause a failure.  Failure tolerance, on the other hand, means that when failure comes (and no matter how well-maintained, how often replaced, how carefully inspected, all systems fail eventually) it is not catastrophic.  If failure means a sudden explosive decompression, that would really catastrophic.  If failure means that a single strut yields and nearby struts bear the load while it's replaced, that's not so bad.  Likewise, if failure means that there's a gas leak which can be located and sealed, that's not so bad.

One open question common to every approach is the need for strong, clear materials to roof in these enclosures.  The strength of this material determines how far the spacing can be between beams of structural material/cable.  Glass is easy to make but has extremely poor mechanical properties, plus its brittle behavior gives it both poor failure resistance and poor failure tolerance (if anything happens it will crack and fracture).  Plastics are better but still weak and much harder to make than glass on a planet with no known oil reserves: HDPE is roughly 20 times weaker than plain carbon steel.  I don't have any novel ideas on this front and I'd be interested in hearing what you all think.

For the purposes of this post I have ignored the question of radiation shielding.  There is a need for it, but with clear-dome designs it's more or less impossible to build in a thick shield.  It's a question that's worth circling back to, sooner or later.

Conceptually, the simplest habitat style that conforms visually to the "domed crater" idea is the partly-buried sphere.  The idea is pretty simple, really: Dig a big hole in the ground and build a spherical pressure vessel within.  Put a clear roof on the aboveground portion. You can understand it as being similar to a large, half-buried ISS module with a glass roof.  The upwards pressure on the domed part is cancelled out by the downwards pressure on the belowground portion, transmitted through the structural members of the habitat.  There's no problem with this approach per se, but there's also no reason to prefer a crater to the flat plain.  Indeed, crater floors seem likely to be substantially harder than the surrounding regolith, and therefore worse locations to dig on; although on the other hand perhaps that makes it a good foundation, and you can pile regolith up around the habitat instead of building down into the ground.

You could build it as an inflatable.  In that case, you'd probably start with a compressive structure (much like a tent, in fact) and wrap your elastic "tent fabric" around it, finally connecting up all the cables with a bit of tension so that the tent fabric expands into it once the structure is pressurized.  I don't know what sort of thickness you'd need for the tent fabric.  A good design might be a multi-layered, clear, elastic plastic.

The second kind is a dome with a counterweight.  In this design the upward pressure force is counterbalanced by weight from below, probably with tethers  coming straight down from bracings on the roof.  In this design, the regolith below needs to be sealed against gas exchange, but the dome can have a foundation like a normal (extremely heavy) building.  To give an idea, a pressure of 50 kPa (0.5 atm) corresponds to about 13,500 kg/m^2 under Mars gravity.  At a bulk density of 1500 kg/m^3, that's regolith to a depth of 9 meters.

You might build it by pouring a concrete foundation on the floor of the crater, flooring it over and building the dome structure, loading the floor up with regolith, putting up the panes of glass or tenting, and pressurizing the dome.  This one is sort of like the structure I described in the other thread for an enclosed building, but upside-down.  I guess this isn't too far from the popular conception of a domed crater, and even has many of the benefits (the crater apron for some craters may be smooth enough to use as a ramp and many large craters are deeper than 9 m), but it still requires substantial modification of the crater floor.

The third method is not something that I have fleshed out very well, but what I'm thinking is that much in the way skyscrapers dig their foundations to rest down on the bedrock, you might do something similar, but instead of gravitational forces pushing down you would anchor your dome to the bedrock pulling upwards.  It might work, and if so could be the easiest way to anchor large domes (there being more than enough weight pushing down on the bedrock to make it work, and the dome pressure adding even more), but it depends on the strength of the bedrock as an anchoring point.