Sustainable Access to Mars: Interplanetary Transportation Architecture

Russel · 2012-04-21 08:07:36

Ok, let me add some more random speculation here..

I've started with figures from this:

http://smartech.gatech.edu/jspui/bitstr … tories.pdf

What they say is, 4.2Km/s from LEO will get you to Mars in under 180 days.
Likewise, 3Km/s from LMO will get you to Earth in under 180 days.

Incidentally when you look at the graphs what you see is a trade-off between fuel mass and the mass of the vehicle (supplies and shielding) and it appears that the above figures are about optimal. So much for a 3 month trip but there you have it.

Now, what I'd like to suggest is the following.

First, start with a transit vehicle parked in LEO. Send a hydrogen/oxygen fueled tug to push that into a higher orbit (an extra 2.5Km/s) - in other words on par with GTO.

Second, transfer the crew, docking with the transit vehicle as it passes closest to Earth. Such a transfer requires that extra delta V but is only for the capsule.

Now, the transit vehicle is already in a higher orbit, so it needs less delta-V to achieve TMI. Now we're talking about 1.7Km/s.

Transit vehicle heads to Mars. Capsule lands. Transit vehicle aerobrakes into a high Mars orbit. This time you gain an extra 1Km/s over LMO. On a par with Deimos transfer orbit.

The capsule returns from the surface with delta-V around 5.1Km/s and again docks with the transit vehicle.

Now, the transit vehicle is already in a higher orbit, so it needs less delta-V to achieve TEI. Now we're talking about 2Km/s.

The transit vehicle returns to Earth. The capsule lands. The transit vehicle could aerobrake into the original high orbit (or it could go back to a lower orbit for maintenance).

The original hydrogen/oxygen fueled tug can gently aerobrake back into low orbit - or you could ditch it.

Now, lets do some basic figures.

Assumptions. The transit vehicle is about 20 tonnes. The propulsion parts of the vehicle another 6 tonnes. The capsule 4 tonnes. In other words about 30 tonnes all up (Yes, I did that deliberately).

To get this vehicle out of high Mars orbit and back to Earth on methane you need 2Km/s or a mass ratio of roughly 1.7.

In other words you start with a mass of 51 tonnes. 21 tonnes of propellant.

To get all of that out of high Earth orbit in the first place you need 1.7Km/s or a mass ratio of roughly 1.6 or a total mass of 81 tonnes. Total propellant (methane/oxygen) is now 51 tonnes.

And for the sake of completeness, the hydrogen/oxygen fueled tug (which strictly speaking you only have to use once, unless you have a need to service in LEO)

That requires a delta V of 2.5Km/s or a mass ratio of roughly 1.7. Now add say 4 tonnes for the vehicle itself and you need to supply it with 60 tonnes of propellant.

Now you see why its handy to to not bring the transit vehicle back to LEO.

Now I've left out some nit picky details, like how to fuel the capsule on Mars etc. But lets not worry about that too much. I'm just trying to present an example of what you can do with the benefit of aerobraking/aerocapture. And then trying to do a simple like with like comparison.

Here's the comparison.

We'll try instead a direct landing vis.

Start with a transit vehicle in high Earth orbit as before. Only this one travels to Mars, lands, and then returns directly.

Lets assume the same basic vehicle as above can land. So I'm being quite fair here. In this case the total Delta V from Mars surface is something like 4.1 + 3 = 7.1Km/s.

That's a mass ratio of roughly 6.8 on methane. So what just took off from the surface of Mars started with 204 tonnes. That's 174 tonnes of propellant..

Ouch. But hang on, you get a 16:1 mass leverage ratio if you just bring Hydrogen with you, so I'll do that.

In this case you need about 10.8 tonnes of Hydrogen to land with it. So on approach to Mars this thing weighs 40.8 tonnes.

Back tracking and you now need again a mass ratio of 1.6 so you've started in high Earth orbit with 65 tonnes. That's as opposed to 81 tonnes where you take your fuel all the way, but only to orbit.

Of course I've left out the issue of getting the capsule fueled. But then I've also left out on the flip side the issue of the extra mass the vehicle now needs to land, and by virtue of this extra structural strength to cope with actual landing.

On the basis of this crude calculation, would you save much mass? Probably. Enough to make a difference? Probably not. Its only a crude comparison and the devil is in the detail but my take on it is you can make it to Mars, safely and elegantly, with conventional propellants - and personally I'd prefer to have the safety in having something in Mars orbit at all times.

Cheers

MatthewRRobinson · 2012-04-22 16:18:27

Russel wrote:

Ok, let me add some more random speculation here..
I've started with figures from this:
http://smartech.gatech.edu/jspui/bitstr … tories.pdf
What they say is, 4.2Km/s from LEO will get you to Mars in under 180 days.
Likewise, 3Km/s from LMO will get you to Earth in under 180 days.
[...]
Back tracking and you now need again a mass ratio of 1.6 so you've started in high Earth orbit with 65 tonnes. That's as opposed to 81 tonnes where you take your fuel all the way, but only to orbit.
Of course I've left out the issue of getting the capsule fueled. But then I've also left out on the flip side the issue of the extra mass the vehicle now needs to land, and by virtue of this extra structural strength to cope with actual landing.
On the basis of this crude calculation, would you save much mass? Probably. Enough to make a difference? Probably not. Its only a crude comparison and the devil is in the detail but my take on it is you can make it to Mars, safely and elegantly, with conventional propellants - and personally I'd prefer to have the safety in having something in Mars orbit at all times.
Cheers

Brilliant work with the elliptical orbits! That's actually a bit of a game-changer the mass savings are so drastic.

But, working off the assumption of 4.3 km/s for a free-return, and an ERV amassing 32-34 tons (with LH2 for ISRU), the ERV could be sent to Mars with 3 F9H launches, two boosters and the ERV itself, though an MTV would mean an additional launch for a total of four, or even more, depending on how the packaging can or can't be arranged.

The difference is only 75% the launch mass, and personally I don't really see the benefit of having a spacecraft in orbit, maybe a GEO satellite or even use the MRO that's already there for communications, but a manned habitable spacecraft sitting unused in orbit for about two years just seems like a bit of a waste of mass.

Essentially, an ERV is a MAV and an MTV already docked together - albiet a bit less luxurious, more along 15-25 m^3 per person instead of the ~50 of an MTV (The Shuttle had ~8.6, IIRC that it had 60 m^3). What this means, is that you have the safety of having something in orbit at all times, except instead of having to launch a MAV and go dock with it - it's already sitting right there.

Last edited by MatthewRRobinson (2012-04-22 16:22:56)

Russel · 2012-04-23 08:54:48

"But, working off the assumption of 4.3 km/s for a free-return, and an ERV amassing 32-34 tons (with LH2 for ISRU), the ERV could be sent to Mars with 3 F9H launches, two boosters and the ERV itself, though an MTV would mean an additional launch for a total of four, or even more, depending on how the packaging can or can't be arranged."

Can you expand on that? 4.3Km/s for free return? Have to know exactly what you're getting at

SpaceNut · 2012-04-23 20:51:09

Question for the theoretical numbers is for where in the train that leaves Earth with its first crew the Mars lander for the crew, a surface habitat for long duration stay ect as these cause the number of staged launches to continue to increase in numbers.....
Also has anyone run the numbers for a modified Dragon to be made into a Mars lander for the crew as well as what changes would need to be made to the basic design to make it work.....

MatthewRRobinson · 2012-04-26 00:04:29

Russel wrote:

"But, working off the assumption of 4.3 km/s for a free-return, and an ERV amassing 32-34 tons (with LH2 for ISRU), the ERV could be sent to Mars with 3 F9H launches, two boosters and the ERV itself, though an MTV would mean an additional launch for a total of four, or even more, depending on how the packaging can or can't be arranged."
Can you expand on that? 4.3Km/s for free return? Have to know exactly what you're getting at

Sorry if I glossed over things a bit too quickly - the 4.3 km/s figures just comes from earlier in the thread, I really don't know much on free return trajectory calculation, but the page you linked stated 4.2 km/s for less than 180 days to Mars, and I know the trajectory for an Aldrin Cycler takes about 4.45 km/s, so I think it's fairly safe for now to work off of the 4.3 km/s figure stated earlier in the thread, though sometime - maybe this weekend, I'd like to check it's validity and get more info on free returns.

What I meant was, in an ERV-type mission, you could send an ERV on TMI with 3 Falcon Heavy Launches. Two of the launches would carry a booster each, and the third launch would carry the ERV itself, for a total of two boosters and the ERV.

Meanwhile, the total mass required for a propellant-filled MTV, according to some estimates I ran, would require four full launches, and maybe more if the packaging can't be arranged in four launches.

Thanks for asking, it's much better than posting a reply without fully understanding, lol

GW Johnson · 2012-04-28 16:06:08

Would it be worthwhile to point out that the first mission or two to Mars need not be "sustainable" in the sense that y'all are discussing here? The ones following, yes, because those will be more-or-less permanent bases of some sort. And we will know a whole lot more about ISRU after that first mission or two. If (AND ONLY IF) we are very smart about what we do on those first one or two missions!

No more Apollo-style "flag-and-footprints" nonsense, please!

Don't forget about light gas gun technology for launching water (as ice) into orbit for processing into propellants. It's just about ready for that job on Earth right now. It's more than powerful enough for the moon and Mars. Right now! And processing water into hydrogen and oxygen can be done (slowly, yes, at low power levels) by solar PV. Right now!!!

Just an odd thought or two to consider, from an old guy.

GW

RobS · 2012-04-28 22:38:41

Is there a link you can send us about the idea? It'll probably be cheaper to develop and deploy a system to make hydrogen, oxygen, and methane on Phobos than to develop a gas gun and launch it in pieces to Mars. Besides, anything you launch is on a trajectory that still has a periapsis below the top of the atmosphere, right? How do you raise the periapsis so it doesn't fall back and burn up?

louis · 2012-04-29 03:45:52

GW Johnson wrote:

Would it be worthwhile to point out that the first mission or two to Mars need not be "sustainable" in the sense that y'all are discussing here? The ones following, yes, because those will be more-or-less permanent bases of some sort. And we will know a whole lot more about ISRU after that first mission or two. If (AND ONLY IF) we are very smart about what we do on those first one or two missions!
No more Apollo-style "flag-and-footprints" nonsense, please!
Don't forget about light gas gun technology for launching water (as ice) into orbit for processing into propellants. It's just about ready for that job on Earth right now. It's more than powerful enough for the moon and Mars. Right now! And processing water into hydrogen and oxygen can be done (slowly, yes, at low power levels) by solar PV. Right now!!!
Just an odd thought or two to consider, from an old guy.
GW

Yes, I think you are probably right GW about it not needing to be sustainable - but on the other hand, I think we effectively have the tonnage available to begin ISRU. Clearly we will be doing that with energy - taking PV panels (to some degree I expect, even if you favour a nuclear reactor). We need to gain experience of what works with agriculture so at the least I think we would want from Mission 1 onwards a modest salad farm facility growing lettuces and tomatoes and so on (of the type that exist already in the Antarctic).

The light gas gun sounds good.

GW Johnson · 2012-04-29 14:47:48

There's already a light gas gun launching small (around 5 pound) payloads at M17 for USAF for hire. It's a private venture. I saw their paper at the 14th annual Mars Society convention in Dallas last summer, same advanced technology session as my Mars mission design paper.

Bigger diameter, higher launch angle (more than anything else), a bit more fuel-oxidizer combustion power, and you reach orbital altitudes at almost-orbital speed. From there it's a very small rocket burn to circularize in LEO. Almost nothing but an old standard Mark 25 solid JATO motor, for a pretty substantial payload (hundreds to thousands of pounds). I don't remember the specific numbers, but it looked pretty good to me. Perfect for refueling an already-existing vehicle, especially if what you shoot up there is water, as tougher-than-an-old-boot ice. The real problem is launch gees. Thousands of them. Just like an artillery shell. Something we already know know to handle, just not with fragile stuff. Gun launch cost looked like about $100-200/pound, compared to Falcon-Heavy at $800-1000/pound.

As for ISRU on the first mission or two to Mars, by all means send such gear for trials, but absolutely don't count on it working right! Chances are, it will not work right, perhaps not at all, especially on the first mission. Probably not even the second.

Accordingly, it would be entirely stupid to count on ISRU for crew survival and return on the first mission. Nothing is more expensive than a dead crew. Ask NASA. They've seen it 3 times now (Apollo-1, Challenger, and Columbia). Nearly saw it on Gemini-8, Gemini-7, and Apollo-13.

It takes on the average 1.5 to 2 full scale, all-up trials of new equipment before it comes close to working "right", and that's with some very talented, artful people working the problem. That's nearly 20 years' aerospace engineering experience talking. Rocket science ain't science, it's about 50% art never written down. It's about 40% science actually written down somewhere. And, it's about 10% blind dumb luck, and you have to plan for that.

It's no different in any of the other disciplines, either. That's why the non-flight engineering disciplines use such whopping huge safety factors. Those of us designing things that fly could not afford that luxury. Fundamentally, that's why flying things are more expensive.

GW

louis · 2012-04-29 15:01:49

GW Johnson wrote:

There's already a light gas gun launching small (around 5 pound) payloads at M17 for USAF for hire. It's a private venture. I saw their paper at the 14th annual Mars Society convention in Dallas last summer, same advanced technology session as my Mars mission design paper.
Bigger diameter, higher launch angle (more than anything else), a bit more fuel-oxidizer combustion power, and you reach orbital altitudes at almost-orbital speed. From there it's a very small rocket burn to circularize in LEO. Almost nothing but an old standard Mark 25 solid JATO motor, for a pretty substantial payload (hundreds to thousands of pounds). I don't remember the specific numbers, but it looked pretty good to me. Perfect for refueling an already-existing vehicle, especially if what you shoot up there is water, as tougher-than-an-old-boot ice. The real problem is launch gees. Thousands of them. Just like an artillery shell. Something we already know know to handle, just not with fragile stuff. Gun launch cost looked like about $100-200/pound, compared to Falcon-Heavy at $800-1000/pound.
As for ISRU on the first mission or two to Mars, by all means send such gear for trials, but absolutely don't count on it working right! Chances are, it will not work right, perhaps not at all, especially on the first mission. Probably not even the second.
Accordingly, it would be entirely stupid to count on ISRU for crew survival and return on the first mission. Nothing is more expensive than a dead crew. Ask NASA. They've seen it 3 times now (Apollo-1, Challenger, and Columbia). Nearly saw it on Gemini-8, Gemini-7, and Apollo-13.
It takes on the average 1.5 to 2 full scale, all-up trials of new equipment before it comes close to working "right", and that's with some very talented, artful people working the problem. That's nearly 20 years' aerospace engineering experience talking. Rocket science ain't science, it's about 50% art never written down. It's about 40% science actually written down somewhere. And, it's about 10% blind dumb luck, and you have to plan for that.
It's no different in any of the other disciplines, either. That's why the non-flight engineering disciplines use such whopping huge safety factors. Those of us designing things that fly could not afford that luxury. Fundamentally, that's why flying things are more expensive.
GW

I agree we need testing of ISRU - but a lot of that testing could be done on Earth or the Moon. I certainly favour a Mars Simulation Chamber - a full mock up of the base inside a giant warehouse with low atmospheric pressure and Mars gases/Mars simulation regolith. It might cost a couple of hundred million dollars but I think in terms of the overall success of the mission it would be vital.

But we don't disagree that Mission 1 ISRU has got to be failsafe. That's one reason why I favour pre-landed supplies, because you can establish they are present and in good order before the mission lands. I suppose you could argue that the mission might land thousands of Kms off course! Well I think that's a bit like arguing the rocket might explode between Earth and Mars - it might, but there's not a lot you can do about that.

GW Johnson · 2012-04-29 15:09:01

Well, you try to rig the rocket so that if one engine explodes, the rest don't. That's the kind of suspenders-and-belt (and armored codpiece!!) design that is needed to take on the challenge of a Mars mission.

GW

SpaceNut · 2012-04-29 19:57:09

GW Johnson wrote:

Would it be worthwhile to point out that the first mission or two to Mars need not be "sustainable" in the sense that y'all are discussing here? The ones following, yes, because those will be more-or-less permanent bases of some sort. And we will know a whole lot more about ISRU after that first mission or two. If (AND ONLY IF) we are very smart about what we do on those first one or two missions!
No more Apollo-style "flag-and-footprints" nonsense, please!
Don't forget about light gas gun technology for launching water (as ice) into orbit for processing into propellants. It's just about ready for that job on Earth right now. It's more than powerful enough for the moon and Mars. Right now! And processing water into hydrogen and oxygen can be done (slowly, yes, at low power levels) by solar PV. Right now!!!
Just an odd thought or two to consider, from an old guy.
GW

Apollo was that as it was pure exploration and science.....

SpaceNut · 2012-04-29 19:59:43

louis wrote:

I agree we need testing of ISRU - but a lot of that testing could be done on Earth or the Moon. I certainly favour a Mars Simulation Chamber - a full mock up of the base inside a giant warehouse with low atmospheric pressure and Mars gases/Mars simulation regolith. It might cost a couple of hundred million dollars but I think in terms of the overall success of the mission it would be vital.
But we don't disagree that Mission 1 ISRU has got to be failsafe. That's one reason why I favour pre-landed supplies, because you can establish they are present and in good order before the mission lands. I suppose you could argue that the mission might land thousands of Kms off course! Well I think that's a bit like arguing the rocket might explode between Earth and Mars - it might, but there's not a lot you can do about that.

This is something that has been proposed on MarsDrive but the cost is not in reach for the group at this time....

Mark Friedenbach · 2012-04-29 20:55:32

GW, you've been consistently against ISRU on early missions. But in proposals which make use of ISRU on the first mission, like Mars Direct, the resource used is the atmosphere (of known and consistent composition no matter where you land), and the fuel is extracted and stored prior to the human crew ever leaving Earth. How is that objectionable?

louis · 2012-04-30 06:36:46

SpaceNut wrote:

louis wrote:
I agree we need testing of ISRU - but a lot of that testing could be done on Earth or the Moon. I certainly favour a Mars Simulation Chamber - a full mock up of the base inside a giant warehouse with low atmospheric pressure and Mars gases/Mars simulation regolith. It might cost a couple of hundred million dollars but I think in terms of the overall success of the mission it would be vital.
But we don't disagree that Mission 1 ISRU has got to be failsafe. That's one reason why I favour pre-landed supplies, because you can establish they are present and in good order before the mission lands. I suppose you could argue that the mission might land thousands of Kms off course! Well I think that's a bit like arguing the rocket might explode between Earth and Mars - it might, but there's not a lot you can do about that.
This is something that has been proposed on MarsDrive but the cost is not in reach for the group at this time....

And of course another advantage is you can simulate the Mars night and day sequence.

YOu can even simulate journeys to remoter places using rover simulators.

GW Johnson · 2012-04-30 12:55:46

Please don't misunderstand ... I'm not against ISRU on the first mission. I'm against betting lives on unproven equipment needlessly.

Nothing never-before-done-in-situ can be considered proven. We can't really do "real ISRU" till we land on Mars. Simulations can be quite good sometimes, but it just ain't the real thing. Because our estimates of site conditions are only estimates.

It is essential to thoroughly try out everything we can dream up for ISRU from mission-1 on. I just don't think it'll work as good as folks wish. More than 50% of our early rocket shots in the 50's were failures. This is no different: it takes real trials to find and fix all the "gotchas", and believe me, there will be "gotchas".

I do have some qualms about using a nearly-pure CO2 atmosphere to make stuff like fuel. The density is so low. And how do you compress in any practical equipment from 7 mbar to 10,000 mbar or more? We have never built compressors like that before, recip or turbine, other than near-zero throughput lab devices. Not very many of them, either.

One "out" for compression might be to refrigerate a large volume of atmospheric CO2 to solidify it, pack it into a much smaller volume sealed container with no free volume, then re-heat it. Re-gasification confined like that is automatic compression of the product gas. Energetically, that's not a very efficient process because of the refrigeration, but it might be more practical to do. I just don't know.

We might be better off just mining solid dry ice near the poles, and doing confined-heating compression that way to get CO2 gas bottled at pressures we can really use. But that's not viable ISRU unless you land near one of the poles. See what I mean?

GW

louis · 2012-04-30 14:12:34

GW Johnson wrote:

Please don't misunderstand ... I'm not against ISRU on the first mission. I'm against betting lives on unproven equipment needlessly.
Nothing never-before-done-in-situ can be considered proven. We can't really do "real ISRU" till we land on Mars. Simulations can be quite good sometimes, but it just ain't the real thing. Because our estimates of site conditions are only estimates.
It is essential to thoroughly try out everything we can dream up for ISRU from mission-1 on. I just don't think it'll work as good as folks wish. More than 50% of our early rocket shots in the 50's were failures. This is no different: it takes real trials to find and fix all the "gotchas", and believe me, there will be "gotchas".
I do have some qualms about using a nearly-pure CO2 atmosphere to make stuff like fuel. The density is so low. And how do you compress in any practical equipment from 7 mbar to 10,000 mbar or more? We have never built compressors like that before, recip or turbine, other than near-zero throughput lab devices. Not very many of them, either.
One "out" for compression might be to refrigerate a large volume of atmospheric CO2 to solidify it, pack it into a much smaller volume sealed container with no free volume, then re-heat it. Re-gasification confined like that is automatic compression of the product gas. Energetically, that's not a very efficient process because of the refrigeration, but it might be more practical to do. I just don't know.
We might be better off just mining solid dry ice near the poles, and doing confined-heating compression that way to get CO2 gas bottled at pressures we can really use. But that's not viable ISRU unless you land near one of the poles. See what I mean?
GW

GW - My position is I agree with your failsafe approach - you can't bet lives on novel ISRU.

However, I think you are being unduly pessimistic about ISRU. ISRU is not really "untried" in the way early rockets were. PV panels are known to work for years on Mars without malfunctioning.
We will have to bet lives on those I think.

We know from the Antarctic bases that you can grow a range of foods under artificial lights.

We know already you can miniaturise steam and stirling engines, lathes and so on. There may be some issues about how these things operate in low gravity - but we can experiment on that to a certain extent on the lunar surface.

Also, with robot pre-landings you can try automated ISRU e.g. for rocket fuel and know before you launch whether it has worked.

SpaceNut · 2012-04-30 18:39:48

There are several processes that I can think of that can be employed in the collection and compression of the mars atmosphere.
Sorbent Bed Acquisition and Compression of Carbon Dioxide from the Mars Atmosphere which was protyed for the inclusion on the Mars 2001 Surveyor Lander that failed.
Development of a Microchannel In Situ Propellant Production System

Then once you have it what do you do with it also varies...SEPARATION OF CARBON MONOXIDE AND CARBON DIOXIDE FOR MARS ISRU
http://ntrs.nasa.gov/archive/nasa/casi. … 030161.pdf

Now for the inventiveness, how about a combination solar chimney and solar panel unit that couples the undraft and powered collection into a compression method.

I know that submarines take 1 atmosphere 14.7psi up to well over 5000psi for use and while its not as thin as mars the principle would not differ...

RobS · 2012-04-30 21:05:35

No one is proposing to do ISRU with the first crew anyway. Mars Direct sends the ERV 2 years before the crew goes, so when they are launched, the ERV is already fueled and ready to go. The ERV for the second manned mission goes out the same time the first crew goes out and would be fully fueled by the time they are ready to leave, thereby providing a backup.

If we can compress terrestrial gasses to thousands of pounds per square inch, why can't we compress Martian gasses to 100 Martian atmospheres? Surely an electric motor and a cylinder would do it, rather like a bicycle pump! You may not need to compress it beyond a tenth of a terrestrial atmosphere, either. Zubrin also talks about a zeolite adsorption bed that would adsorb atmospheric CO2, then release it under some pressure when the bed is heated.

But if one isn't sure, Mars semi-Direct solves the problem.

GW Johnson · 2012-05-01 08:56:15

On the compression thing: I cannot claim to be familiar with everything that has been proposed. I am familiar with recip and turbine. And basic thermodynamics. Yes, we have compressors on submarines that charge air banks from 1 atm to 500 atm. They are gigantic, heavy things, carried by a sub weighing 4-7000 tons. That kind of thing does not miniaturize well. And, we're talking about a compression ratio final/input of 500.

Most of the aircraft compression machinery we have is under-30 for compression ratio, most of them under 15 or so. What goes on in a shop air compressor and what goes on inside a piston engine are comparable at compression ratios in the 6-11 range. All of these devices are light enough to fly, and all of them miniaturize well. It is the lower compression ratio that allows them to be miniaturizable and be flightweight.

Mars's atmosphere is 7 mbar, except up on the highlands and mountains where it's nearer 2 mbar. But let's go with 7 mbar. Picking a challenging miniaturizable/flightweight compression ratio like 30, that's a bottled gas pressure of 210 mbar, or about 20% of 1 atm. Most of the chemistry processes I know of take place at 1-30 atm. Well, it seems like that might be a serious problem. Although, exploring low-pressure chemistry is something we can do, right here. But dollars to doughnuts, I'll bet you it ain't ready yet to go to Mars.

On the other hand, lets look at the submarine high-pressure air bank compression ratio of 500, and use that at 7 mbar. 3500 mbar, about 3.5 atm, that's usable with chemistry processes we already use industrially. The only problem is the huge tonnage of machinery for a throughput that scales down directly with inlet density (about 0.6% of that here). Heavy, inefficient. I see not much promise down that path.

Take some dry ice frost, pack it tightly into a steel can with a valve and an outlet pipe, and seal it up. Heat it with low-grade energy (solar thermal would work, albeit slowly). The CO2 vaporizes, but cannot expand, so the pressure rises by the specific volume ratio at a constant process temperature. That's a factor in the hundreds to around a thousand (don't have a thermodynamic properties table for CO2 handy).

There's a compression ratio comparable to the sub's high-pressure air bank, done with steel cans, high-pressure tubing, ordinary gas bottle plumbing, and a solar-thermal panel. Admittedly, it's a batch process, not so amenable to robotic operation. But it could be done. All you need is a source of dry ice.

That's the numbers for the kind of problem we are talking about here. If you want to make methane and oxygen out of CO2 and H20, on Mars, the poles are the place to do it. Unfortunately, that's not where the first landing(s) will take place.

That's also a part of my point: every site on Mars is different. There is no one "average Mars" we can put in a simulation chamber here on Earth.

GW

RobS · 2012-05-01 11:19:22

This is very helpful, GW, to understand the situation. But surely one of the reasons a thousand to one compression ratio requires heavy equipment is because the output gas requires heavy containment. Why couldn't you:

Take Martian air and run it through a 10 to 1 compression ratio and store it in a tank at 70 millibars

Take the 70 millibar air and run it through a compressor (a different one, or even the same one) and store it in another tank at 700 mb.

Repeat a third time if need be?

ISRU needs maybe 100 tonnes of CO2 over 18 months. That's less than 6 tonnes per month, 1.5 tonnes per week, about 200 kg per day, about 8 kg per hour.

And this can be tested on Earth, then tested on Mars; you would certainly test it on Mars 2 years before people arrived, maybe 4 years.

The process would generate waste heat. Possibly it could be used to heat a zeolite bed to force it to degas CO2, or to keep parts of the system at certain operating temperature.

GW Johnson · 2012-05-01 15:30:53

RobS:

Unfortunately, the problem really isn't containment. That's just about an inch of steel in a gas bottle's wall at well above 2000 psig, and there's not very many of those bottles in real system, not compared to the mass of the compressor itself, even here.

The problem is the size and efficiency (more than one sense here, see also next paragraph) of the compressor. That machinery here is 30 to 70% energetically efficient, meaning throughput massflow x enthalpy rise (more or less proportional to compression ratio) compared to shaft power input. On Mars there is the same basic machinery friction to fight, but only about 0.6% of the throughput. So, the energy efficiency is way-to-hell-and-gone far lower there, unless you reduce compression ratio very, very, very drastically in proportion. Which trends toward very little product. Which trends toward not being usable. Not a picture I like.

There is also the problem of throughput x time-to-accumulate-a-given-mass compared to the mass of the compression machinery. This is proportional to inlet density ratio, no matter what else. On Mars, that's 0.6% of here. No compressor capable of filling a welding gas bottle at a useful pressure will ever be small on Mars. Laws of physics preclude it. If it ain't small, who's going to pay to ship it there? Another picture I don't like.

Sorry. Too much knowledge is a dangerous thing, just like too little.

But I am still very intrigued by thermal self-compression of CO2 and H2O in confined spaces. Low grade heat is cheap and lightweight, even on Mars.

GW

SpaceNut · 2012-05-01 16:45:17

I think the problem of the CO2 collection and pressure is partially answered at lower levels when using the Reverse Water Gas Shift which takes CO2 + H2 = CO + H2O I think we can operate down on the curve....

Then wait and save up the CO until we have the volume and pressure for the Sabatier reaction to produce methane...
http://w2energy.com/technology/ft-reactor/

something extra to think about
http://www.ecn.nl/units/bkm/co2-capture … -reactors/

GW Johnson · 2012-05-01 17:13:53

Spacenut:

That conversion ratio vs reactor pressure curve you posted shows exactly what I am talking about. Yield is better the higher your reaction pressure, in a nonlinear fashion. Everything "falls off a cliff" between 0.1 and 1.0 atm. Mars is 0.007 atm in its open "air". Compression above 30:1 is a real problem, as I have already described.

All that says is "do it a different way".

I've never said not to do ISRU. But most of the pre-conceived notions of exactly how to do ISRU look like crap to me.

GW

SpaceNut · 2012-05-01 18:55:53

I struggle with units when we talk several units of measurement in a topic but 1 Bar is 14.7 Psi and Mars is at 7 millibar or .101 Psi thats a problem agreed. Saw in my searching that the 12.4 millibars, occurs at the bottom of the Hellas Basin.

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