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#1 2017-12-30 17:02:32

louis
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From: UK
Registered: 2008-03-24
Posts: 5,875

ISRU propellant production - energy requirement for the BFR?

Think I and others have been too negative about Space X's ability to provide enough energy for ISRU propellant production. This forum discussion:

https://forum.nasaspaceflight.com/index … c=36504.20

suggests Zubrin's original sums gave an energy input of 17 MwHs per tonne of propellant.

If that is the case, for a returning BFR that requires 1500 tonnes of the propellant, that would imply an energy input of 25500 MwHs or, over 660 sols,  an input of 38.64 MwHs [per sol] or about 1.6 Mw constant.

For a solar power system this would probably imply something like 80-100 tonnes mass.

So with an overall budget of 300 tonnes, that would be doable.

As regards my own miscaculation, that probably has a lot to do with the fact that previous discussions of solar power were looking to delivering a constant minimum, which involves a lot of losses.  Of course, propellant production takes place when the energy is available, so far more efficient.

Last edited by louis (2018-01-03 11:00:27)


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#2 2017-12-30 17:45:53

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

The issue is also what is the insitu sources for what we will make since that changes the energy requirement, it is not that simple....

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#3 2017-12-30 18:57:35

louis
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From: UK
Registered: 2008-03-24
Posts: 5,875

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

True, but those are far more marginal issues. Even if they swing 40% either way on energy demand, they wouldn't make the Space X plan impossible.

Unless I see convincing figures the other way, I think 1500 tonnes of propellant production is well within the scope of a 300 tonne cargo delivery, probably taking up no more than 33% of the cargo figure. 






SpaceNut wrote:

The issue is also what is the insitu sources for what we will make since that changes the energy requirement, it is not that simple....


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#4 2017-12-30 20:21:27

SpaceNut
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From: New Hampshire
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Posts: 19,908

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

Big difference between water from  the regolith rather than being from an ice field..... one needs extra equipment to gather and more energy to process...

To which CO2 from the atmosphere versus a frozen sheet of co2 under the regolith....is the same problem.

We need to send better than what we have so far to the surface to quantitatively pin down the measurements for a given location...

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#5 2017-12-30 22:29:01

Oldfart1939
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Registered: 2016-11-26
Posts: 1,912

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

SpaceNut wrote:

Big difference between water from  the regolith rather than being from an ice field..... one needs extra equipment to gather and more energy to process...

To which CO2 from the atmosphere versus a frozen sheet of co2 under the regolith....is the same problem.

We need to send better than what we have so far to the surface to quantitatively pin down the measurements for a given location...

Agreed! The optimistic 300 Tonnes to the Mars surface may be a high estimate.

I'm with GW on the issue of spacecraft stability upon landing as another significant problem. Energy estimates always tend to assume nothing goes wrong and no safety factor built in. In this case, the margin should be nearly 100 %. WHAT IF the Moxie unit doesn't work as well as designed? Not enough feedstock (CO2) coming in? Too many imponderables here. What if, what if, what if....

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#6 2017-12-31 13:44:45

louis
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From: UK
Registered: 2008-03-24
Posts: 5,875

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

Space X will have plenty of opportunity to test landing stability on a variety of rock fields.  I don't think much will be left to chance. There may well be some observation of the landing site prior to landing.


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#7 2017-12-31 13:52:03

GW Johnson
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From: McGregor, Texas USA
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Posts: 4,103
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Re: ISRU propellant production - energy requirement for the BFR?

What falls over easier,  the tall skinny box,  or the short,  fat box?  There's a reason the diagonal span from landing pad to landing pad of the Apollo lunar lander exceeded the height of the vehicle. 

You can only build so much stroke into the the individual legs to accommodate setting down on a rock,  and if it slips off the rock,  there's a limit to how fast the leg can re-stroke out to the new required length.  That time may exceed the time it takes to topple over.  That's why Armstrong delayed touchdown until he cleared the boulder field,  risking running out of fuel.  He only had 20-30 seconds left at shutdown.

All of THAT taken together is why I say it pays to have either (1) a short,  fat vehicle with extra fuel,  or (2) a well prepared (very smooth,  very flat) landing field of size big enough to overcome landing accuracy errors,  plus some extra fuel. 

The common item is extra fuel (and a way to pilot with it,  when something goes wrong,  which it will,  sooner or later).

It's just prudence and common sense,  plus a tiny bit of basic physics. 

GW

Last edited by GW Johnson (2017-12-31 13:55:09)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#8 2017-12-31 14:41:28

3015
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Registered: 2017-01-08
Posts: 30

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

I wish I had access to the paper where Zubrin achieves a value of 17 MWh per tonne of propellant, that value seems very high to me. Most of the power in a system like that should be for electrolysis, cryocompression of CO2, and RWGS, which even at low efficiencies should add up to far less than 17 MWh/t. Also, Zubrin's system is much different from what SpaceX is planning, if you don't bring the H2 you have to mine water and you have to electrolyze twice as much water.

I have created a spreadsheet here to estimate the power and equipment requirements for fueling a BFR. The parameters I have used come mostly from this PhD thesis. The spreadsheet is read-only, but you can save a copy and enter your own parameters. The parameters I have used suggest about 100 t of solar panels and other equipment is required to fuel one BFR spacecraft on 600 days, though that value is very sensitive to panel mass which I am very uncertain about. If that value is correct, then 2 BFRs can bring enough equipment to fuel 3 ships per transfer window. This allows one ship to return as long as production is at least 1/3 of the expected value, resulting in a very safe margin, which as Oldfart1939 has stated is very important.

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#9 2017-12-31 14:55:51

Oldfart1939
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Registered: 2016-11-26
Posts: 1,912

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

All we are doing here is what we used to refer to at the scientific conferences I attended as "Hand waving." That's what speakers would do when there was no concrete data to back up their assertions. Zubrin was very thorough in his calculations of energy required, but didn't really have a BFR to deal with in his projections.

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#10 2017-12-31 16:24:34

louis
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From: UK
Registered: 2008-03-24
Posts: 5,875

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

That's pretty much the range I was coming up with (80-100 tonnes) based on Zubrin's figure.

Have you including for cabling and ancillary equipment? I tend to add a third in terms of mass on to the mass of the panels. I couldn't see any specific reference to that...obviously the further away you go from your propellant facility, the more cabling you need...

Your work seems to confirm at least that this is certainly within the scope of Space X's announced mission.

Of course, effectively,  propellant manufacture can continue after humans arrive on the first mission...so there is a degree of "failsafeness" built in there as well.

3015 wrote:

I wish I had access to the paper where Zubrin achieves a value of 17 MWh per tonne of propellant, that value seems very high to me. Most of the power in a system like that should be for electrolysis, cryocompression of CO2, and RWGS, which even at low efficiencies should add up to far less than 17 MWh/t. Also, Zubrin's system is much different from what SpaceX is planning, if you don't bring the H2 you have to mine water and you have to electrolyze twice as much water.

I have created a spreadsheet here to estimate the power and equipment requirements for fueling a BFR. The parameters I have used come mostly from this PhD thesis. The spreadsheet is read-only, but you can save a copy and enter your own parameters. The parameters I have used suggest about 100 t of solar panels and other equipment is required to fuel one BFR spacecraft on 600 days, though that value is very sensitive to panel mass which I am very uncertain about. If that value is correct, then 2 BFRs can bring enough equipment to fuel 3 ships per transfer window. This allows one ship to return as long as production is at least 1/3 of the expected value, resulting in a very safe margin, which as Oldfart1939 has stated is very important.


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

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#11 2017-12-31 16:29:23

louis
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From: UK
Registered: 2008-03-24
Posts: 5,875

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

I think that's unfair to Zubrin and 3015. We have a reasonably good idea of the energy parameters. I think we can now see that Space X's plans are realistic even if you apply plus or minus 40% to the calculation.

I can see where my own calculations went astray (I was confusing the basic energy requirement of propellant manufacture with my previous calculations relating to providing a minimum constant electricity supply via solar - a very wasteful use of solar energy).  Basically on Mars the propellant manufacture can take place when the solar energy is there, making full use of the energy flow.

I am sure Zubrin would have been only too happy to have a BFR to hand! smile 

Oldfart1939 wrote:

All we are doing here is what we used to refer to at the scientific conferences I attended as "Hand waving." That's what speakers would do when there was no concrete data to back up their assertions. Zubrin was very thorough in his calculations of energy required, but didn't really have a BFR to deal with in his projections.


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

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#12 2017-12-31 16:38:36

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,875

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

Well, speaking as one who can never believe those returning Space X rockets stay upright having landed on a floating platform that's basically moving all the time, I see what you are saying!  But Musk is surely fully aware of the problem. 

With the cargo landings you can accept an element of risk. If they fail, all that has been wasted is money, not lives.

So it is perfectly possible the plan might be to land robot rovers as part of the cargo delivery which will then clear a landing area for the humans' BFR.

Regarding the landing legs, I feel there must be scope for technical development. Six legs better than four? Legs with their own stabilisers?


GW Johnson wrote:

What falls over easier,  the tall skinny box,  or the short,  fat box?  There's a reason the diagonal span from landing pad to landing pad of the Apollo lunar lander exceeded the height of the vehicle. 

You can only build so much stroke into the the individual legs to accommodate setting down on a rock,  and if it slips off the rock,  there's a limit to how fast the leg can re-stroke out to the new required length.  That time may exceed the time it takes to topple over.  That's why Armstrong delayed touchdown until he cleared the boulder field,  risking running out of fuel.  He only had 20-30 seconds left at shutdown.

All of THAT taken together is why I say it pays to have either (1) a short,  fat vehicle with extra fuel,  or (2) a well prepared (very smooth,  very flat) landing field of size big enough to overcome landing accuracy errors,  plus some extra fuel. 

The common item is extra fuel (and a way to pilot with it,  when something goes wrong,  which it will,  sooner or later).

It's just prudence and common sense,  plus a tiny bit of basic physics. 

GW


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

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#13 2017-12-31 18:51:32

3015
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Registered: 2017-01-08
Posts: 30

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

Hi louis, I'm actually kind of fudging the mass of cabling, voltage conversion, power protection, etc right now by using the panel mass parameter to represent the mass of 1 m^2 of panels plus the associated power management and distribution equipment. I've added PMAD to  the equipment mass section, but I haven't filled in numbers for it yet since I can't find any modern references for PMAD mass.

I think that the current SpaceX plan does not involve propellant manufacture starting before humans arrive to finish setting up the propellant manufacturing plant. I was surprised when Elon Musk mentioned this (either during the 2016 or 2017 BFR presentation I think) since it introduces a significant risk relative to a plan like Mars Direct.

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#14 2017-12-31 20:06:55

Oldfart1939
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Registered: 2016-11-26
Posts: 1,912

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

3015-

This is probably why Zubrin definitely espouses the use of nuclear power early on. Yes, solar can pick up a significant portion of the net power requirement, but NOT immediately. This is why, were I still young enough to become a Marsonaut, I would say that any mission reliant 100% on solar power for production of return flight propellant--I AIN'T GOING! We really need all potential sources of power simultaneously--in parallel.

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#15 2017-12-31 20:11:06

louis
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From: UK
Registered: 2008-03-24
Posts: 5,875

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

Hi 3015.

I don't understand your last para. Are you claiming that Space X are not looking to setting up propellant production from 2022?  Because as far as I can see that is what they propose: land cargo craft in 2022 and begin propellant production shortly after that.


3015 wrote:

Hi louis, I'm actually kind of fudging the mass of cabling, voltage conversion, power protection, etc right now by using the panel mass parameter to represent the mass of 1 m^2 of panels plus the associated power management and distribution equipment. I've added PMAD to  the equipment mass section, but I haven't filled in numbers for it yet since I can't find any modern references for PMAD mass.

I think that the current SpaceX plan does not involve propellant manufacture starting before humans arrive to finish setting up the propellant manufacturing plant. I was surprised when Elon Musk mentioned this (either during the 2016 or 2017 BFR presentation I think) since it introduces a significant risk relative to a plan like Mars Direct.


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

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#16 2017-12-31 20:26:29

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 19,908

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

The standard solar panel has an input rate of around 1000 Watts per square meter, however on the solar panels available at present you will only gain roughly 15-20% efficiency at best. Therefore if your solar panel was 1 square meter in size, then it would likely only produce around 150-200W in good sunlight.

At optimal orientation, the solar irradiance for several locations is: (in kilowatt-hours of solar energy per sq. meter and per year) / 365 for the day less the panels efficiency

* Northern Germany: 1220 kWh/m2/a (@36 degrees inclination) / 365 = 3.3 kwhr
* Northern Italy: 1720 kWh/m2/a (@36deg inclination) / 365 = 4.7 kwhr
* Southern Spain: 2100 kWh/m2/a (@32deg inclination) / 365 = 5.75 kwhr
* Northern Africa: up to 2700 kWh/m2/a (@18deg inclination) / 365 = 7.4 kwhr

As you can see the number for hours in a solar day go down as you get farther from the equator...

http://store.sundancesolar.com/how-much … n-a-day-1/

how do you set up a field of arrays when its like this image:

learn-about-solar-panels-the-martian7.png

learn-about-solar-panels-the-martian.png

a near flat alignment will look something like this:

spirit_panels.jpg

https://www.nasa.gov/pdf/163008main_SES … rt_dc3.pdf

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#17 2017-12-31 21:33:01

3015
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Registered: 2017-01-08
Posts: 30

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

Oldfart1939, I strongly agree about nuclear power. Easier to set up, runs all the time, and is more reliable. I've done the mass estimates with solar because that's what Musk intends to use.

louis, in slide 31 in this presentation, it says that the propellant plant will be set up in 2024, at the time of the first manned landing. I think it is because Musk thinks it would be too complicated to set up a solar array and water processing without boots on the ground.

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#18 2017-12-31 22:55:44

Oldfart1939
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Registered: 2016-11-26
Posts: 1,912

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

SpaceNut, Louis, and 3015;

The numbers given in the table above are optimal, and would assume there is NO major dust storm activity reducing the solar irradiance. Then there is the issue of no power generation during Martian nights, cutting down the output by 50%, and seasonal variability of irradiance angle changes. My position has been that solar power will always be supplementary to nuclear or possibly geothermal sources (should they be discovered).

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#19 2018-01-01 09:10:14

louis
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From: UK
Registered: 2008-03-24
Posts: 5,875

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

Thanks I hadn't spotted that before. I guess Space X must feel that robotic propellant production set up is a bridge too far...although it does sound like they are going to be going for robotic power set up and water sourcing... all a little confusing.

Nuclear sounds fine in theory until you look at the practicalities of safe set up and operation in a human settlement setting on Mars. There is also the issue of putting tens of tonnes of nuclear power infrastructure into a rocket. It's all very well saying "nothing" will happen but you could irradiate the whole of Cape Canaveral and maybe beyond if something did go wrong if we are talking about launching a 3MW nuclear reactor.

The speculation is redundant in any case as Space X have made clear this will be a solar powered mission.

3015 wrote:

Oldfart1939, I strongly agree about nuclear power. Easier to set up, runs all the time, and is more reliable. I've done the mass estimates with solar because that's what Musk intends to use.

louis, in slide 31 in this presentation, it says that the propellant plant will be set up in 2024, at the time of the first manned landing. I think it is because Musk thinks it would be too complicated to set up a solar array and water processing without boots on the ground.


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#20 2018-01-01 10:48:15

Terraformer
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From: Lancashire
Registered: 2007-08-27
Posts: 3,312
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Re: ISRU propellant production - energy requirement for the BFR?

you could irradiate the whole of Cape Canaveral

I've not seen anyone suggest launching a *live* nuclear reactor. Uranium toxicity would be a far bigger worry, but that would only happen if the containment vessel broke apart.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#21 2018-01-01 12:43:09

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 19,908

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

The thing about my numbers for the kwhrs for a day is that the number also represents the number of hours the panels are producing power. This does not change for mars but even is less due to the distance from the sun just to achieve that same performance requires even better efficiency panel cells for the arrays.

So what reactor that is ready to go now do we have in this wattage?
How much is the radiators mass?
How much is the shielding mass?

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#22 2018-01-01 15:54:43

Oldfart1939
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Registered: 2016-11-26
Posts: 1,912

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

Louis-

There are at least 3 nuclear powered submarines with *live* reactors somewhere in Earth's oceans that have sunk and are *lost.* Launching a reactor prior to it's *activation* or being started is not particularly hazardous. Yes, Uranium is toxic; yes Uranium is pyrophoric; the battlefields of Iraq are covered with the residue of depleted Uranium used in antitank ammunition. Uranium residues have been tossed around like candy during the Middle East wars. Your fears are founded in misinformation about reactors. I would count on Dr. Zubrin's proposals before anyone else's since he's a PhD. Nuclear Engineer.

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#23 2018-01-01 16:00:06

louis
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From: UK
Registered: 2008-03-24
Posts: 5,875

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

I am simply pointing out the PR obstacles to a nuclear power mission. SpaceX won't have to explain to the public why no one is at risk from a 15 tonne nuclear reactor in the event of the rocket breaking up. Some researchers do think all that depleted uranium has harmed humans in the Middle East.


Oldfart1939 wrote:

Louis-

There are at least 3 nuclear powered submarines with *live* reactors somewhere in Earth's oceans that have sunk and are *lost.* Launching a reactor prior to it's *activation* or being started is not particularly hazardous. Yes, Uranium is toxic; yes Uranium is pyrophoric; the battlefields of Iraq are covered with the residue of depleted Uranium used in antitank ammunition. Uranium residues have been tossed around like candy during the Middle East wars. Your fears are founded in misinformation about reactors. I would count on Dr. Zubrin's proposals before anyone else's since he's a PhD. Nuclear Engineer.

Last edited by louis (2018-01-01 16:46:50)


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#24 2018-01-01 16:04:15

Oldfart1939
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Registered: 2016-11-26
Posts: 1,912

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

As a follow up comment to my previous post: I know Elon Musk has stated he wants to use solar power, but I also believe that he is a pragmatic individual who will reconsider the facts once they have been presented to him. He isn't really a "doctrinaire" type person, and is a smart guy. I seriously don't expect him to risk his billions on something that has even a remote possibility of failure, and "going solar" raises that specter.

Last edited by Oldfart1939 (2018-01-01 16:04:48)

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#25 2018-01-01 16:57:33

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,875

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

I really can't see what the issue is.  The Cargo BFRs land two years ahead of the human missions.  I suspect that there is going to be some serious battery capacity on board. The propellant/fuel is going to be methane, which itself can be used to generate electricity.  I'd be surprised if there wasn't a  methane-fuel electricity generator on board.

I think when humans land they will probably be able to survive several months on minimal power purely from battery power.

Dust storms don't stop solar power generation. 

What exactly do you think the risk is?



Oldfart1939 wrote:

As a follow up comment to my previous post: I know Elon Musk has stated he wants to use solar power, but I also believe that he is a pragmatic individual who will reconsider the facts once they have been presented to him. He isn't really a "doctrinaire" type person, and is a smart guy. I seriously don't expect him to risk his billions on something that has even a remote possibility of failure, and "going solar" raises that specter.


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

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