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#26 2018-01-01 17:04:54

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 19,222

Re: ISRU propellant production - energy requirement for the BFR?

Then send a solar powered mining operation to process all the ore you want to make for the empty reactor....problem solved....

We will be most likely going with a kilopower reactor which is a beefed up RTG design or more like a pebble reactor....as it must bring its working fluid with it as well as the radiators to make it work not just the generator system...

OR
Several smaller landed solar battery combinations to await a manned landing using a pump to store ahead of time the CO2 for use in the landers empty tanks since that seems to be the slowest process. You could use the payload shroud as the backshell all the way if you alter the top for the parachute to exit if we are not using retro-propulsion for the landing.

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#27 2018-01-01 18:50:00

louis
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From: UK
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Posts: 5,854

Re: ISRU propellant production - energy requirement for the BFR?

Where are you getting this from Spacenut?  Space X have stated explicitly Mission One will be solar powered.

As for the kilopower reactor - that's 100Kwe isn't it?  You'd need 30 of those to get up to the sort of power requirement Space X are looking at.

The Kilopower reactor have NEVER been built. But solar panels have operated on Mars and in space. There is absolutely no doubt that PV power can deliver.

SpaceNut wrote:

Then send a solar powered mining operation to process all the ore you want to make for the empty reactor....problem solved....

We will be most likely going with a kilopower reactor which is a beefed up RTG design or more like a pebble reactor....as it must bring its working fluid with it as well as the radiators to make it work not just the generator system...

OR
Several smaller landed solar battery combinations to await a manned landing using a pump to store ahead of time the CO2 for use in the landers empty tanks since that seems to be the slowest process. You could use the payload shroud as the backshell all the way if you alter the top for the parachute to exit if we are not using retro-propulsion for the landing.


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#28 2018-01-01 21:48:13

SpaceNut
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From: New Hampshire
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Posts: 19,222

Re: ISRU propellant production - energy requirement for the BFR?

That would be in the Nasa plans louis sorry for implying that nuclear had a work around for a paid payload to mars if Nasa should chose.
Prototype has been partially built we should see a full size unit at some point soon if funded.

The BFR is way to large for even after a few mission have happen so something smaller must be in the works or this plan fails as solar will not be enough for the refueling as its not able to deploy once the ship lands. Remember how high off the ground the payload area is just how do you propose that magic happens to mount them on the ground?

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#29 2018-01-02 12:19:55

louis
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Re: ISRU propellant production - energy requirement for the BFR?

I don't see why there is any longer any requirement for an intermediate ship size.

The great thing about a BFR that can deliver 150 tonens to Mars is that even if you don't send the maximum number of humans possible, you can still build up the base infrastructure.  One year, you can take out a fully-fitted out gym, or a surgical facility or several explorer rovers...


SpaceNut wrote:

That would be in the Nasa plans louis sorry for implying that nuclear had a work around for a paid payload to mars if Nasa should chose.
Prototype has been partially built we should see a full size unit at some point soon if funded.

The BFR is way to large for even after a few mission have happen so something smaller must be in the works or this plan fails as solar will not be enough for the refueling as its not able to deploy once the ship lands. Remember how high off the ground the payload area is just how do you propose that magic happens to mount them on the ground?


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#30 2018-01-02 13:47:57

kbd512
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Re: ISRU propellant production - energy requirement for the BFR?

Louis,

The sub-scale KiloPower demonstrator has already been built at NNSS.  All non-nuclear tests have already been performed, so DoE is pretty sure that the basic design works.  Nuclear tests are now underway and will end in January of this year.  The project started in October of 2015 and the team has gone from paper concept to nuclear tests in less than three years.  The nuclear tests will take the concept through TRL6.  After that, it's only a matter of flying it in space.  Lunar missions require this power source and so do Mars missions for people who do not choose to ignore current technology state of solar power for ideological reasons.

The more advanced variants of SAFE-400, all paper designs using the same core format as SAFE-400, had output levels up to 4MWt / 1MWe for roughly the same core dimensions as the original unit.  This was accomplished with higher operating temperatures, using the same ceramic fuel conventional utility electrical power reactors use, and smaller diameter fuel pins to increase the fueled volume within the core.  US DoD is now exploring a concept called MegaPower.  The total weight of this 2MWe system is between 35t and 45t.  Even if it was 50t, it's still 30t lighter than solar panels that only exist on paper.

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#31 2018-01-02 15:41:53

louis
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From: UK
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Posts: 5,854

Re: ISRU propellant production - energy requirement for the BFR?

As pointed many times, you can't go on a human mission with just one power source whose failure will result in death of the humans...you need to have a replacement machine, so doubling your mass. Also, are you going to be doing any surface exploring at all? Because you can't lug the reactor around with you.

kbd512 wrote:

Louis,

The sub-scale KiloPower demonstrator has already been built at NNSS.  All non-nuclear tests have already been performed, so DoE is pretty sure that the basic design works.  Nuclear tests are now underway and will end in January of this year.  The project started in October of 2015 and the team has gone from paper concept to nuclear tests in less than three years.  The nuclear tests will take the concept through TRL6.  After that, it's only a matter of flying it in space.  Lunar missions require this power source and so do Mars missions for people who do not choose to ignore current technology state of solar power for ideological reasons.

The more advanced variants of SAFE-400, all paper designs using the same core format as SAFE-400, had output levels up to 4MWt / 1MWe for roughly the same core dimensions as the original unit.  This was accomplished with higher operating temperatures, using the same ceramic fuel conventional utility electrical power reactors use, and smaller diameter fuel pins to increase the fueled volume within the core.  US DoD is now exploring a concept called MegaPower.  The total weight of this 2MWe system is between 35t and 45t.  Even if it was 50t, it's still 30t lighter than solar panels that only exist on paper.


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#32 2018-01-02 16:56:07

Oldfart1939
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Registered: 2016-11-26
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Re: ISRU propellant production - energy requirement for the BFR?

louis wrote:

As pointed many times, you can't go on a human mission with just one power source whose failure will result in death of the humans...you need to have a replacement machine, so doubling your mass. Also, are you going to be doing any surface exploring at all? Because you can't lug the reactor around with you.

No rational person would suggest that we ignore a backup power system. The solar power supply seems rational for the actual Mars base and probably recharging the rover batteries, but is totally incapable of providing enough output for the fuel and oxidizer production to occur within a Marsonaut's lifespan.

That's why I've argued for NUCLEAR FIRST, and we can supplement the base with lots of solar.

As I've said innumerable times: we need BOTH!

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#33 2018-01-02 17:16:51

louis
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From: UK
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Posts: 5,854

Re: ISRU propellant production - energy requirement for the BFR?

Based on Zubrin's figure of 17 MWhs per tonne of propellant, a solar powered facility is well within capacity of a mission involving 450 tonnes to Mars.

Oldfart1939 wrote:
louis wrote:

As pointed many times, you can't go on a human mission with just one power source whose failure will result in death of the humans...you need to have a replacement machine, so doubling your mass. Also, are you going to be doing any surface exploring at all? Because you can't lug the reactor around with you.

No rational person would suggest that we ignore a backup power system. The solar power supply seems rational for the actual Mars base and probably recharging the rover batteries, but is totally incapable of providing enough output for the fuel and oxidizer production to occur within a Marsonaut's lifespan.

That's why I've argued for NUCLEAR FIRST, and we can supplement the base with lots of solar.

As I've said innumerable times: we need BOTH!


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#34 2018-01-02 18:43:02

kbd512
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Re: ISRU propellant production - energy requirement for the BFR?

Louis,

You can run a cryo plant and multi-megawatt power facility 24/7 or you can explore, but you can't do both at the same time with a skeleton crew.

I would like to use power sources that do not require lots of setup and multiple landings to carry all the equipment required.  Nuclear reactors require emplacement in small holes that cover the core and remote activation after emplacement.  KiloPower requires a pair of D cell batteries for that purpose.  I want to use a minimal power storage architecture because power production is the name of this game.  Nuclear power best services the power requirement as a function of mass and overall system complexity.

You're choosing to ignore the basic math regarding what's required to do what Mr. Musk proposes doing for personal reasons.  Mr. Musk has been quoted in a video stating that he thinks we should turn the reactors at Fukushima on again after appropriate safety precautions are in place.  He recognizes the engineering reality of different situations and applies basic math in his thought process, rather than ideology.

People aren't telling you what you want to hear in this thread because they have accepted that there are some mathematical realities that won't change to favor anyone's ideology.  Whereupon both batteries and durable solar panels both become lighter and more power dense than they are today, an all-solar power plant may become feasible.  That is not current engineering reality and we have quite some ways to go if we want it to become reality.  A decade or more of additional development work is likely required.

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#35 2018-01-02 21:06:45

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 19,222

Re: ISRU propellant production - energy requirement for the BFR?

Show me the math you are saying will make 17 MWhs during a 600 sol days at only 3 hours of energy time per day with heavy 35% panels when mars recieves only 440 watt to the surface not 1100 like on earth.

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#36 2018-01-02 22:54:50

3015
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Registered: 2017-01-08
Posts: 30

Re: ISRU propellant production - energy requirement for the BFR?

SpaceNut, why assume heavy panels? NASA has a design for a 1000 m^2 array that has a panel/structure mass of 1.5 t and a PMAD mass of 1 t. They estimate that at 50 N the array could produce a mean power of 26.1 kW.

In 600 sols, the array would produce 0.0261 MW * 24.67 h/sol * 600 sols = 386 MWh. 386 MWh / 17 MWh/t = 22.7 t of fuel from 2.5 t of equipment, which is 9.08 t of propellant per t of equipment.

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#37 2018-01-03 06:22:40

louis
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From: UK
Registered: 2008-03-24
Posts: 5,854

Re: ISRU propellant production - energy requirement for the BFR?

Why would you be located 50 degrees north?  My understanding is mid to late 20s N gives the best results.

3015 wrote:

SpaceNut, why assume heavy panels? NASA has a design for a 1000 m^2 array that has a panel/structure mass of 1.5 t and a PMAD mass of 1 t. They estimate that at 50 N the array could produce a mean power of 26.1 kW.

In 600 sols, the array would produce 0.0261 MW * 24.67 h/sol * 600 sols = 386 MWh. 386 MWh / 17 MWh/t = 22.7 t of fuel from 2.5 t of equipment, which is 9.08 t of propellant per t of equipment.


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#38 2018-01-03 10:39:21

RobertDyck
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Re: ISRU propellant production - energy requirement for the BFR?

louis wrote:

Why would you be located 50 degrees north?  My understanding is mid to late 20s N gives the best results.

My requirements for a human base are:

  • close to the equator so it's warm, and consistent sunlight through the Mars year for solar arrays

  • low altitude so lots of atmosphere overhead for radiation shielding

  • flat and smooth, so safe to land. Not down a ravine.

  • something interesting for scientific study

  • plenty of water (ice)

  • other resources: iron ore (eg hematite concretions), aluminum ore (eg anorthite or bytownite), white sand for glass (eg opal sand found by Spirit), potash for potassium fertilizer in a greenhouse (not found yet, but found on Earth in dried-up ocean/sea basins)

The most promising site found so far is the frozen pack ice of Elysium Planetia. That's 8° North latitude.

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#39 2018-01-03 11:52:52

3015
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Registered: 2017-01-08
Posts: 30

Re: ISRU propellant production - energy requirement for the BFR?

louis, I selected 50 N because it is close to the latitude of a location SpaceX sees as especially promising, Arcadia Planitia. The latitude does not make a huge difference anyway though, the mean at the equator would be 29.7 kW and at 30 S it would be 28.9 kW.

RobertDyck, I really like your requirements for a human base. I think that if Elysium Planitia has significant water ice, it will be an excellent choice for a base site. But it's possible that Elysium Planitia is not actually very water rich, we will have to scout the location first.

This paper improved the resolution of data from Mars Odyssey's neutron spectrometer and has this to say on Elysium Planitia:

The locally adaptive pixon reconstruction of the region around the proposed, buried sea is shown in figure9. The location of the water ice sea, 5°N, 150°E, corresponds to one of the locations with the highest epithermal neutron flux on the entire surface of Mars. This suggests that the top few tens of centimetres of soil, in this region, are unusually dry with <1wt.% WEH. The dry feature in the reconstruction extends beyond the region identified by Murray etal. (2005) and covers much of the smooth plains that are believed to be young basaltic lavas from Cerberus fossae (outline with a black contour in Fig.9). Additionally, Boynton etal. (2007), using Mars Odyssey Gamma Ray Spectrometer data, find this region to be enhanced in Fe. Taken together, these result suggests that the plate-like features at Elysium Planitia are unlikely to be a buried water ice sea but are, instead, young, Fe-rich, volatile-poor basalts.

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#40 2018-01-03 12:54:13

louis
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From: UK
Registered: 2008-03-24
Posts: 5,854

Re: ISRU propellant production - energy requirement for the BFR?

Noted - I wasn't aware that Space X had expressed an opinion on a landing site...that's v. interesting.  Do you have a link?

Robert's list is a good one.  Personally I favour the north east corner of Chryse Planitia about 25 N and 35 W.

3015 wrote:

louis, I selected 50 N because it is close to the latitude of a location SpaceX sees as especially promising, Arcadia Planitia. The latitude does not make a huge difference anyway though, the mean at the equator would be 29.7 kW and at 30 S it would be 28.9 kW.

RobertDyck, I really like your requirements for a human base. I think that if Elysium Planitia has significant water ice, it will be an excellent choice for a base site. But it's possible that Elysium Planitia is not actually very water rich, we will have to scout the location first.

This paper improved the resolution of data from Mars Odyssey's neutron spectrometer and has this to say on Elysium Planitia:

The locally adaptive pixon reconstruction of the region around the proposed, buried sea is shown in figure9. The location of the water ice sea, 5°N, 150°E, corresponds to one of the locations with the highest epithermal neutron flux on the entire surface of Mars. This suggests that the top few tens of centimetres of soil, in this region, are unusually dry with <1wt.% WEH. The dry feature in the reconstruction extends beyond the region identified by Murray etal. (2005) and covers much of the smooth plains that are believed to be young basaltic lavas from Cerberus fossae (outline with a black contour in Fig.9). Additionally, Boynton etal. (2007), using Mars Odyssey Gamma Ray Spectrometer data, find this region to be enhanced in Fe. Taken together, these result suggests that the plate-like features at Elysium Planitia are unlikely to be a buried water ice sea but are, instead, young, Fe-rich, volatile-poor basalts.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#41 2018-01-03 13:13:20

3015
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Re: ISRU propellant production - energy requirement for the BFR?

Here's an article on landing sites considered by SpaceX. They are focusing on Arcadia Planitia, Deuteronilus Mensae, Phlegra Montes and Utopia Planitia. All of those seem to be centered somewhere around 40 N.

I don't know much about Chryse Planitia, what are its advantages?

Edit: I should note these are landing sites considered for the now cancelled Red Dragon, but since those missions were partly scouting for later SpaceX landings I think it is safe to say that they will consider similar locations for BFR.

Last edited by 3015 (2018-01-03 13:15:38)

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#42 2018-01-03 13:14:14

louis
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From: UK
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Re: ISRU propellant production - energy requirement for the BFR?

[Caculation error - will repost]

Last edited by louis (2018-01-03 15:34:06)


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#43 2018-01-03 13:30:50

3015
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Registered: 2017-01-08
Posts: 30

Re: ISRU propellant production - energy requirement for the BFR?

louis, I think that 4.5 kWh figure is per kWh of "installed capacity", not kWh per m^2 per day.

I have done some number crunching on solar irradiance on Mars, here is a post I made on Reddit giving values for various latitudes. The peak is about 130 W/m^2 on average at the equator, which suggests about 3.21 kWh per day of irradiance, only about 0.963 kWh/m^2/day even for 30% efficient panels.

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#44 2018-01-03 15:23:49

louis
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Posts: 5,854

Re: ISRU propellant production - energy requirement for the BFR?

Chryse Planitia is an area we know well, being the site of two previous major missions.

I'd say about 25 degrees north and 35 degrees west...north of the Pathfinder landing site by about - I'm guessing from the latitude difference - 300 Kms.

https://mars.jpl.nasa.gov/mgs/target/marsmap1b.jpg

Iron ore deposits are fairly high there.

http://www.psrd.hawaii.edu/WebImg/Mars-GRS-FeMap.jpg

Water resources look reasonable as well there - maybe 5% bound in the soil:

https://3c1703fe8d.site.internapcdn.net … lyseso.jpg

I like it as a central location for development of that part of the Nortern Hemisphere - you've got Valles Mariensis and Olympus Mons not too far away. There seem to be some obvious road trail routes that could be created from that site.


3015 wrote:

Here's an article on landing sites considered by SpaceX. They are focusing on Arcadia Planitia, Deuteronilus Mensae, Phlegra Montes and Utopia Planitia. All of those seem to be centered somewhere around 40 N.

I don't know much about Chryse Planitia, what are its advantages?

Edit: I should note these are landing sites considered for the now cancelled Red Dragon, but since those missions were partly scouting for later SpaceX landings I think it is safe to say that they will consider similar locations for BFR.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#45 2018-01-03 15:32:37

louis
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From: UK
Registered: 2008-03-24
Posts: 5,854

Re: ISRU propellant production - energy requirement for the BFR?

My apologies for the schoolboy error. I will repost.

3015 wrote:

louis, I think that 4.5 kWh figure is per kWh of "installed capacity", not kWh per m^2 per day.

I have done some number crunching on solar irradiance on Mars, here is a post I made on Reddit giving values for various latitudes. The peak is about 130 W/m^2 on average at the equator, which suggests about 3.21 kWh per day of irradiance, only about 0.963 kWh/m^2/day even for 30% efficient panels.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#46 2018-01-03 15:49:32

louis
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From: UK
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Re: ISRU propellant production - energy requirement for the BFR?

So for propellant production of 1500 tonnes, according to the Zubrin figure, we require 38,600 Kwhs of power per day.

According to the link below one of the better sites in the USA (California) gives 4.5 Kwhs per Kw installed capactiy of PV Panel per day (equates to 7 sq metres at 15% efficiency).

https://www.solar-estimate.org/solar-pa … ls-produce

Taking that figure and applying a figure of 35% per sol for an equivalent location on Mars, gives a figure of 0.22 Kwhs per sol per square metre. (I think 35% is quite a conservative figure - since we could be looking at much higher than average power production per sq. metre on an advanced mission.)

Applying that figure to the 38,600 Kwhs power requirement suggests a square metreage of 172,095 sq. metres.

Depending on mass per sq. metre this provides the following indicative tonnages:

0.5 kg per sq. metre =  86.03 tonnes

1 kg per sq. metre = 172.06 tonnes

1.5 kgs per sq. metre =  258.09 tonnes

2 kgs per sq. metre = 343.42 tonnes

2.5 kgs per sq. metre =  429.45 tonnes

Even if we add to that a 30% allowance for cabling and ancillary equipment,  that provides a range of 111.83 tonnes to 558.27 tonnes.

However, this is at standard 15% efficiency. If we can double efficiency to 30% the tonnage range becomes 55.92 tonnes to 279.13 tonnes.  On that basis I think Space X's cargo delivery could handle any mass up to 1.5 kgs per square metre (which would require 167.4 tonnes at 30% efficiency).


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#47 2018-01-03 16:45:01

Oldfart1939
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Registered: 2016-11-26
Posts: 1,872

Re: ISRU propellant production - energy requirement for the BFR?

Louis-

You know, we're not simply going to Mars in order to set up a monster solar array, one which consumes an inordinate amount of payload capacity (both mass and volume). There needs be a LOT of other equipment brought along for infrastructure construction. Your pet project aside, we can get a lot of power from nuclear that does NOT take the tremendous setup time and associated labor. I for one, as a Marsonaut, would NOT want to spend time erecting/laying out this power grid system you keep proposing. I'm a professional research scientist--not a coolie putting up solar panels. Neither is Robert Zubrin, who was first to propose the nuclear option. We're initially going there to look for evidence of life both present and past. Not as a demonstration that solar power can/cannot handle the fuel production!

Last edited by Oldfart1939 (2018-01-03 16:47:09)

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#48 2018-01-03 17:00:15

louis
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From: UK
Registered: 2008-03-24
Posts: 5,854

Re: ISRU propellant production - energy requirement for the BFR?

Well this isn't my proposal, this is trying to work out how the Musk/Space X proposal could work, based on solar power, which is all Musk/Space X have referenced so far.

Personally I would just build up Mars's industrial infrastructure and use a small scale Apollo-style ascent craft to get people off the surface, rather than making 1500 tonnes of propellant at an early stage.

But, if Space X can bring in the mass requirement for propellant manufacture at something like 170 tonnes, then it is definitely doable within the overall cargo tonnage of 450 tonnes.  Also, I don't believe roll out would take much longer a few days...could probably be done  in under a week.


Oldfart1939 wrote:

Louis-

You know, we're not simply going to Mars in order to set up a monster solar array, one which consumes an inordinate amount of payload capacity (both mass and volume). There needs be a LOT of other equipment brought along for infrastructure construction. Your pet project aside, we can get a lot of power from nuclear that does NOT take the tremendous setup time and associated labor. I for one, as a Marsonaut, would NOT want to spend time erecting/laying out this power grid system you keep proposing. I'm a professional research scientist--not a coolie putting up solar panels. Neither is Robert Zubrin, who was first to propose the nuclear option. We're initially going there to look for evidence of life both present and past. Not as a demonstration that solar power can/cannot handle the fuel production!


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#49 2018-01-03 17:22:30

3015
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Registered: 2017-01-08
Posts: 30

Re: ISRU propellant production - energy requirement for the BFR?

I managed to get my hands on a copy of the Zubrin paper that suggest 17 MWh per tonne. Here is the breakdown of power usage in that paper, values are in watts for a system that makes 1 kg of propellant per sol:

Cryocooler 165
Sensors and flow controllers  5
Reactor heater 40
Absorption column heaters 10
Electrolyzer 100
Absorption column three-way valves 2
Mars tank solenoid 0
Gas/liquid separator solenoid 2
CO2 acquisition Stage 1 144
CO2 acquisition Stage 2 74
Recycle pump 136
Total 678

The system described in the paper is for a sample return mission, it is safe to say that a larger system would experience very significant economies of scale. For example, the CO2 acquisition step in the paper suggests a power need of 5.38 kWh/kg of CO2. But this NASA paper suggests CO2 can be cryocooled for just 1.23 kWh/kg. The cryocooler power need is also much higher than would be needed for larger scale production, in Zubrin's system 4.07 kWh are required to liquefy 1 kg of propellant. The recycle pump should use much less relative power as well on a larger scale.

I want to be clear that I am in no way criticizing Zubrin's paper, it is excellent work that needs to be done to get us closer to going to Mars.  It's just that the scope of his paper does not allow for the economies of scale a Mars Direct/BFR mission would experience since that paper is focused on a Mars sample return system.

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#50 2018-01-03 17:44:25

louis
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From: UK
Registered: 2008-03-24
Posts: 5,854

Re: ISRU propellant production - energy requirement for the BFR?

Thanks 3015.  V. interesting observation...essentially you appear to be saying the 17 MWh figure is a very conservative one i.e. we can probably get that amount of propellant for far less.  Well, again, I think this would confirm that Space X's goal of a solar powered propellant production is feasible.  What would be your guesstimate for a lowest possible energy input per tonne, taking full benefit of economies of scale on a Space X style mission? Would we be saving 50% overall?


3015 wrote:

I managed to get my hands on a copy of the Zubrin paper that suggest 17 MWh per tonne. Here is the breakdown of power usage in that paper, values are in watts for a system that makes 1 kg of propellant per sol:

Cryocooler 165
Sensors and flow controllers  5
Reactor heater 40
Absorption column heaters 10
Electrolyzer 100
Absorption column three-way valves 2
Mars tank solenoid 0
Gas/liquid separator solenoid 2
CO2 acquisition Stage 1 144
CO2 acquisition Stage 2 74
Recycle pump 136
Total 678

The system described in the paper is for a sample return mission, it is safe to say that a larger system would experience very significant economies of scale. For example, the CO2 acquisition step in the paper suggests a power need of 5.38 kWh/kg of CO2. But this NASA paper suggests CO2 can be cryocooled for just 1.23 kWh/kg. The cryocooler power need is also much higher than would be needed for larger scale production, in Zubrin's system 4.07 kWh are required to liquefy 1 kg of propellant. The recycle pump should use much less relative power as well on a larger scale.

I want to be clear that I am in no way criticizing Zubrin's paper, it is excellent work that needs to be done to get us closer to going to Mars.  It's just that the scope of his paper does not allow for the economies of scale a Mars Direct/BFR mission would experience since that paper is focused on a Mars sample return system.


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