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#26 2019-10-04 02:23:12

Terraformer
Member
From: Lancashire
Registered: 2007-08-27
Posts: 3,271
Website

Re: Methane backup not nuclear vs solar

What, exactly, is wrong with Strontium-90 radio-isotope generators? People will freak out with launch, but then they'll do that with nuclear reactors. Allocating a few tonnes to get a baseline backup level of power would be worth it.


"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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#27 2019-10-04 05:17:29

tahanson43206
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Registered: 2018-04-27
Posts: 2,561

Re: Methane backup not nuclear vs solar

For Louis re #43

You have not read very deeply into the Kilowatt reactor design reports.  Your worry about radiation is likely coming from your newspaper level reading about earlier reactor designs.  I'd be interested in your views AFTER you've taken the time to read the specifications for the new design.

When you come back, please specify the document you are referencing, and the specific location within the document which speaks to the concern about radiation risk from this design.

(th)

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#28 2019-10-04 06:04:59

louis
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From: UK
Registered: 2008-03-24
Posts: 5,788

Re: Methane backup not nuclear vs solar

This has been discussed many times before. I'm giving my overall impression. No need for the ad hominem - I've read reasonably deeply into it on previous occasions.

The Wikipedia page indicates that the most achieved so far has been operation of a 1 Kwe test reactor at full power for 28 hours in 2018.  So we are nowhere near an operational 10Kwe reactor fully tested in Mars-like conditions. So all we've got so far is a tenth scale model that can run for just over one day.

https://en.wikipedia.org/wiki/Kilopower

This slide presentation states:

"For more than 30 years, NASA has relied on Radioisotope Thermoelectric Generators (RTGs) to produce electricity that is used to power instrumentation in spacecraft or Rover vehicles. In the 2000’s, NASA decided to explores other sources of energy to produce electricity for spacecraft that will be used for a deep space exploration mission..."

https://ncsp.llnl.gov/TPRAgendas/2017/2 … 017_R1.pdf

To me that sounds like it is designed to be used on robotic missions, not on human missions.

The following makes it sound like it could be used on the Moon and on Mars eventually, but note it is scheduled to stay in the development programme to 2020  and only then does it move into the "technology demonstration" phase. But does that mean for robotic or human missions? Note also that "additional risk reduction" activities will be tried out. What does that refer to?  Not entirely reassuring is it?

https://www.nasa.gov/press-release/demo … tion-power

All in all, I can't see this technology being used for human missions any time soon. If I had to bet, I'd say they may have something ready for real-world testing in 2025.

If you want to wait till Kilopower is completed before we go to Mars - even though we can go to Mars with solar power as Musk and his team propose - then it seems to me a weirdly skewed way to arrange priorities.

This is a v recent update:

https://www.space.com/nuclear-reactor-f … -2022.html

It's still v. v.vague. 

"No off-Earth demonstration is on the books yet, but Kilopower should be ready to go by 2022 or so" (my emphasis).

A possible lunar mission was cancelled. The talk of NASA landing people on the Moon by 2024 is obvious BS unless they use the Starship.

The developer thinks they can be ready for flight in 3 years - with what? a 1 Kwe or 10 Kwe version? But what sort of flight? Human or non-human? It's all vague, vague, vague...while they have to carry more development on "additional risk reduction".

My objection is to people claiming Kilopower is the way forward for Mars Missions when it is clearly not ready, has not been proven in a real world setting and does clearly create risks that are not present with solar.



tahanson43206 wrote:

For Louis re #43

You have not read very deeply into the Kilowatt reactor design reports.  Your worry about radiation is likely coming from your newspaper level reading about earlier reactor designs.  I'd be interested in your views AFTER you've taken the time to read the specifications for the new design.

When you come back, please specify the document you are referencing, and the specific location within the document which speaks to the concern about radiation risk from this design.

(th)


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

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#29 2019-10-04 06:09:12

Terraformer
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From: Lancashire
Registered: 2007-08-27
Posts: 3,271
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Re: Methane backup not nuclear vs solar

The Cassini RTG system achieved 5 W/kg, with Plutonium but using thermoelectrics rather than the more efficient Stirling cycle, or betavoltaics (an option with Strontium). How much power do we need to weather a dust storm? 2 tonnes would give us 10 kWe, constant. If the efficiency could be tripled with a Stirling cycle, we'd need 5 tonnes to give us 75kWe. Which should be plenty of power to keep the base running.


"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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#30 2019-10-04 08:28:33

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 354

Re: Methane backup not nuclear vs solar

This thread started out as a discussion of the wisdom or lack thereof, of Donald Trump's decision to focus NASA manned spaceflight efforts towards Mars instead of the moon.  It has degenerated into yet another pointless nuclear vs. solar discussion, in which engineers argue for a sensible solution and an idealist argues otherwise because he doesn't like the idea of it.  Sorry if I sound obtuse, but that is the truth of the matter.

I find it rather strange that people have to hammer home the need for nuclear power on a planet that receives 43% of Earthly solar flux; temperatures that can freeze human flesh as hard as stone and has dust storms that can block out the sun for weeks at a time.  For a human mission you clearly need a very reliable power supply that works all of the time, because you need to breathe and keep warm all of the time.

It is also worth noting that it will cost billions of dollars for every mission that sends men to Mars.  For every kg of equipment sent to Mars, the cost will be tens of thousands of dollars.  Each day of astronaut time on Mars will cost on the order of $10million.  If that astronaut time is used in a way that is sub-optimum due to limitations imposed by Martian weather or Martian night-time; the cost of lost productivity is huge.  If you have to include extra mass to meet the needs of energy storage, the cost is huge.  If you need to oversize equipment to maintain propellant production rates with an intermittent power supply, the cost is huge.  If you are forced to land at equatorial locations, because of limits imposed by your power supply; then you are severely limiting mission flexibility and degrading the value of investment.  An investment that runs into billions of dollars per mission.

Is it really logical to incur all of these costs so that we don't have use a technology that some people just don't like the idea of?  If we do end up using solar power as the primary energy source for these missions, it will be because nuclear power is simply unavailable.  That could happen for political reasons, which have tended to grind new nuclear projects to a halt and push costs through the roof.  But under any scenario where we actually have a choice and can develop these technologies at a sensible cost; we will be using nuclear reactors as the primary power supply for Mars missions.

Last edited by Calliban (2019-10-04 08:36:41)


Interested in space science, engineering and technology.

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#31 2019-10-04 09:31:09

Oldfart1939
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Registered: 2016-11-26
Posts: 1,835

Re: Methane backup not nuclear vs solar

Oldfart1939 wrote:

Louis-
I really didn't want this discussion to devolve into another solar power versus nuclear discussion, but I need to state now that your proposal for methane-oxygen as a substitute for nuclear is absurd. We don't need another experimental power supply to contend with, and any methane produced will be required facilitate the trips back to Earth. Re-read what GW has written in the section you quoted. You are violating one of Musk's basic rules by making things overly complicated. A small portable nuclear reactor as suggested by Dr. Zubrin is EXACTLY  what is required for Mars Base Alpha. We're talking about human survival--the crew needs energy for all life support functions, and if a massive storm swamps the solar farm--everybody dies. Not if we have that Kilopower unit up and running.

I was trying to point out to Louis that we didn't need this ridiculous and extreme, grasping at straws, discussion of his pet topic of solar energy and refusing to accept the EFFICIENCIES  and RELIABILITY  that we need for Mars.

I am possibly the ONLY person on this thread who has ever had the opportunity to operate a Thorium based nuclear reactor--in Graduate School. I'm still alive, have reproduced successfully, and the building housing the reactor still doesn't glow in the dark. But Louis contends that Nuclear Reactors are unsafe. So are rockets. Mars is a dangerous place for those improperly equipped. I wouldn't want the first crews to die 25,000,000 miles from home, cold and in the dark.

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#32 2019-10-04 11:55:17

GW Johnson
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From: McGregor, Texas USA
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Posts: 3,903
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Re: Methane backup not nuclear vs solar

The methane-fired generators Louis points at are IC engines that can run on LNG instead of liquid fuels,  which means they could be run on methane gas here on Earth.  That is while breathing Earthly air,  which is 15 times the mass of the fuel.  That methane capability is because natural gas mostly is methane,  but not entirely.   

However,  a generator like that CANNOT run on Mars,  because (1) there is no 21% oxygen air at (2) near 1 full atmosphere of pressure available at the air intake inlet.  Note that there are TWO requirements,  not just one!  You will NOT succeed unless you satisfy BOTH requirements.

You can't just feed it gaseous oxygen instead,  because in-cylinder flame temperatures are about 2000 F (1000 C) higher.  That was tried multiple times looking for high speed submerged submarine propulsion before the advent of nuclear power instead.  It doesn't work,  no hardware ever proved capable of withstanding the hotter gas temperatures. True for piston and turbine.

You could dilute the oxygen with Martian CO2 down to 21% oxygen,  but then you are faced with real-time compression from 6 mbar to 1013 mbar.  That's a gigantic,  power-hungry,  heavy machine,  which then eats up your generator's electrical output,  and more.  It's not your typical garage air compressor at all!  It CANNOT be,  it must look more like an extreme-hard vacuum pump.  Grams/sec compressed gas output from tons of machine pulling 10's of KW to run.  The 6 mbar ambient pressure at the compressor inlet is what causes that. 

Methane plus oxygen combustion at useful pressures is a near-6000 F (near-3000 C) process.  That's why rocket engines are regeneratively cooled and have finite lives.  They are NOT industrial equipment,  not by a long shot.  There is NO shaft output power from a rocket engine. 

Meanwhile,  there are NO generators available from anyone anywhere in the world that would run on methane and oxygen!  Not one!  Not from ANY supplier ANYWHERE!  That's not to say it could not be done,  because it can.  But it is not easy.  It is not cheap.  And it will NOT be available in less than about a dozen years,  if work on it began today.  Which it has not.

Meanwhile,  the NASA Kilopower thing is nearing readiness.  It could be more-or-less ready in a couple of years or so.  I'd like to see it deployed and used successfully,  before risking lives on it on Mars,  but that's just me and my suspenders-and-belt (AND armored codpiece) approach to things that risk lives.  They are relatively safe and not much of a hazard,  before you start the reactor. 

You would not do that reactor startup until you set up your power supply on Mars.  You set this thing up,  hook it up,  check everything out,  bulldoze a berm around it,  start the reactor,  and never go inside that berm again!  Simple.  Effective.  Safe.  End-of-life,  you bulldoze over it to cover it up,  and fence off the area.  10 of these coupled together into a common bus bar layout would be 100 KWe. 

That's not bad:  night time light and warmth for a small crew in a hab,  plus a bit more to charge batteries in rovers and machines.  Same would be true in a blackout dust storm months long.  Let the solar do the propellant manufacture. 

But I'd personally recommend just enough nuke not to have to shut propellant manufacture completely off,  even in the worst case dust storm.  That's the "armored codpiece". 

GW

Last edited by GW Johnson (2019-10-04 11:59:19)


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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#33 2019-10-04 12:11:15

louis
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From: UK
Registered: 2008-03-24
Posts: 5,788

Re: Methane backup not nuclear vs solar

An idealist?  Me?  or Elon Musk (currently way out ahead in the race - if race it can be called - to Mars).

It's not idealism, it's practicality.

You are just retelling the nonsense you've picked up elsewhere. Nowhere does a dust storm on Mars "block out the sun".  Insolation is probably around 35% of norm at its lowest (and that sort of low level only lasts for at max a week or so) and even with dust accumulation you can pick up 20%. People get carried away with those pics from the Rover site which NASA admitted were artistic rendering more than actual photos.

Furthermore for the Space X mission there will be four Starships that have been sitting there for two years recharging their fin and other batteries, so there will be no shortage of power on arrival in any case. My proposal for meth-ox generation of electricity is simply a neat way of ensuring a strong degree of failsafeness for the Mission. It's v. unlikely to be needed. 

I've no problem with nuclear power on Mars because on Mars, in contrast to on Mars, it is not going to be destroying huge swathes of productive land and endanger tens of millions of people's lives if that's what people want to do but it's not my choice. And I don't think it's Space X's choice either.

One important point is that it will not take forward the development of the Mars economy: you'll be dependent on importing reactors from Earth for a long time. But Mars could very quickly have a PV manufacturing facility producing its own PV panels from Mars silicon and other raw materials in a scaled down factory.

You need to answer some basic questions rather than peddling fantasy:

1. When will Kilopower really be available for human missions? 2025? 2030? 2035? There isn't even a full size test model yet. Are you prepared to put a Mars Mission on hold until Kilopower is available.

2. What are the risks (given NASA says they are working on "additional risk reduction" - a rather opaque phrase). Pretending there are risks when NASA says there are risks isn't responsible.

3. Will Kilopower be used on the outward journey? If not, what energy source are you relying on and what happens if you are relying on PV power but find yourself in the middle of a dust storm on landing, unable to open the cargo door for some reason. What's your alternative onboard energy source?

4. Is Kilopower going to be located on the surface, if so where and how far from the habs? Will it be buried under regolith? If so is that safe and practical?

5. Will Kilopower be required on the return journey? - what are the practicalities of getting a radioactive generator back on board?

6. How will you power your Rovers?

7. How will you power exploration camps? Will you be moving Kilopower reactors to such camps?






Calliban wrote:

This thread started out as a discussion of the wisdom or lack thereof, of Donald Trump's decision to focus NASA manned spaceflight efforts towards Mars instead of the moon.  It has degenerated into yet another pointless nuclear vs. solar discussion, in which engineers argue for a sensible solution and an idealist argues otherwise because he doesn't like the idea of it.  Sorry if I sound obtuse, but that is the truth of the matter.

I find it rather strange that people have to hammer home the need for nuclear power on a planet that receives 43% of Earthly solar flux; temperatures that can freeze human flesh as hard as stone and has dust storms that can block out the sun for weeks at a time.  For a human mission you clearly need a very reliable power supply that works all of the time, because you need to breathe and keep warm all of the time.

It is also worth noting that it will cost billions of dollars for every mission that sends men to Mars.  For every kg of equipment sent to Mars, the cost will be tens of thousands of dollars.  Each day of astronaut time on Mars will cost on the order of $10million.  If that astronaut time is used in a way that is sub-optimum due to limitations imposed by Martian weather or Martian night-time; the cost of lost productivity is huge.  If you have to include extra mass to meet the needs of energy storage, the cost is huge.  If you need to oversize equipment to maintain propellant production rates with an intermittent power supply, the cost is huge.  If you are forced to land at equatorial locations, because of limits imposed by your power supply; then you are severely limiting mission flexibility and degrading the value of investment.  An investment that runs into billions of dollars per mission.

Is it really logical to incur all of these costs so that we don't have use a technology that some people just don't like the idea of?  If we do end up using solar power as the primary energy source for these missions, it will be because nuclear power is simply unavailable.  That could happen for political reasons, which have tended to grind new nuclear projects to a halt and push costs through the roof.  But under any scenario where we actually have a choice and can develop these technologies at a sensible cost; we will be using nuclear reactors as the primary power supply for Mars missions.


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

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#34 2019-10-04 12:28:41

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,788

Re: Methane backup not nuclear vs solar

GW - If you read my posts you'll see I've referred to the oxygen supply requirement and included it in my mass estimates.

Are you really sure it's impossible to burn methane with pure oxygen?  Exytron don't seem to agree:

"When electricity or heat is required, Exytron’s machines then burns the methane in an oxygen-only atmosphere. A turbine makes electricity from this combustion."

https://www.carboncommentary.com/blog/2 … -and-reuse

Or this: "Oxyfuel combustion uses pure oxygen instead of air to burn carbonaceous materials, resulting in a CO2 separation efficiency theoretically close to 100 % should the fuel and oxygen be free of contaminants. This chapter examines several oxyfuel systems, considering two categories of power cycle – those based on steam cycles and those based on gas cycles – both of which generate oxygen using a cryogenic air separation unit."

https://www.sciencedirect.com/topics/en … ure-oxygen

I love the stuff about Kilopower: "nearing readiness", "more or less ready", "relatively safe and not much of a hazard"...

And if you only activate the reactor on the surface, where is your energy supply coming from if for some reason there is an emergency that traps you in the Starship?

GW Johnson wrote:

The methane-fired generators Louis points at are IC engines that can run on LNG instead of liquid fuels,  which means they could be run on methane gas here on Earth.  That is while breathing Earthly air,  which is 15 times the mass of the fuel.  That methane capability is because natural gas mostly is methane,  but not entirely.   

However,  a generator like that CANNOT run on Mars,  because (1) there is no 21% oxygen air at (2) near 1 full atmosphere of pressure available at the air intake inlet.  Note that there are TWO requirements,  not just one!  You will NOT succeed unless you satisfy BOTH requirements.

You can't just feed it gaseous oxygen instead,  because in-cylinder flame temperatures are about 2000 F (1000 C) higher.  That was tried multiple times looking for high speed submerged submarine propulsion before the advent of nuclear power instead.  It doesn't work,  no hardware ever proved capable of withstanding the hotter gas temperatures. True for piston and turbine.

You could dilute the oxygen with Martian CO2 down to 21% oxygen,  but then you are faced with real-time compression from 6 mbar to 1013 mbar.  That's a gigantic,  power-hungry,  heavy machine,  which then eats up your generator's electrical output,  and more.  It's not your typical garage air compressor at all!  It CANNOT be,  it must look more like an extreme-hard vacuum pump.  Grams/sec compressed gas output from tons of machine pulling 10's of KW to run.  The 6 mbar ambient pressure at the compressor inlet is what causes that. 

Methane plus oxygen combustion at useful pressures is a near-6000 F (near-3000 C) process.  That's why rocket engines are regeneratively cooled and have finite lives.  They are NOT industrial equipment,  not by a long shot.  There is NO shaft output power from a rocket engine. 

Meanwhile,  there are NO generators available from anyone anywhere in the world that would run on methane and oxygen!  Not one!  Not from ANY supplier ANYWHERE!  That's not to say it could not be done,  because it can.  But it is not easy.  It is not cheap.  And it will NOT be available in less than about a dozen years,  if work on it began today.  Which it has not.

Meanwhile,  the NASA Kilopower thing is nearing readiness.  It could be more-or-less ready in a couple of years or so.  I'd like to see it deployed and used successfully,  before risking lives on it on Mars,  but that's just me and my suspenders-and-belt (AND armored codpiece) approach to things that risk lives.  They are relatively safe and not much of a hazard,  before you start the reactor. 

You would not do that reactor startup until you set up your power supply on Mars.  You set this thing up,  hook it up,  check everything out,  bulldoze a berm around it,  start the reactor,  and never go inside that berm again!  Simple.  Effective.  Safe.  End-of-life,  you bulldoze over it to cover it up,  and fence off the area.  10 of these coupled together into a common bus bar layout would be 100 KWe. 

That's not bad:  night time light and warmth for a small crew in a hab,  plus a bit more to charge batteries in rovers and machines.  Same would be true in a blackout dust storm months long.  Let the solar do the propellant manufacture. 

But I'd personally recommend just enough nuke not to have to shut propellant manufacture completely off,  even in the worst case dust storm.  That's the "armored codpiece". 

GW


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

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#35 2019-10-04 13:03:00

Oldfart1939
Member
Registered: 2016-11-26
Posts: 1,835

Re: Methane backup not nuclear vs solar

To all posters here on this thread-

We have gone past the point of a "discussion," and ventured into dealing with an emotional argument. One side has attempted rationality and  evidence based on engineering and physical science. The other side in this now argument stubbornly clings to a position which only has some passing banter statements to support. Since I started this thread, we need to stop wasting time and electrons; otherwise I'll ask the moderators to lock down any further posting.

Rodger

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#36 2019-10-04 13:43:22

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,788

Re: Methane backup not nuclear vs solar

Emotional argument? A poster states that it is impossible to have pure oxygen combustion of methane for electricity generation. I find two references to it being perfectly possible. I presented facts, as I have done throughout this discussion. The questions I have asked of Kilopower advocates are all lucid and valid, given the unproven nature of the technology and the vagueness of proposals for its deployment.

I see no reason to curtail debate, but would accept that this discussion should perhaps be in another thread.

Oldfart1939 wrote:

To all posters here on this thread-

We have gone past the point of a "discussion," and ventured into dealing with an emotional argument. One side has attempted rationality and  evidence based on engineering and physical science. The other side in this now argument stubbornly clings to a position which only has some passing banter statements to support. Since I started this thread, we need to stop wasting time and electrons; otherwise I'll ask the moderators to lock down any further posting.

Rodger


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

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#37 2019-10-04 16:22:34

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 18,555

Re: Methane backup not nuclear vs solar

The argument of one versus the other is old in light that we do not have a BFR or Starship with no cargo manafests as you can only shove so much stuff inside a box of a given measurement. The taking of excess fuels in the cargo area as the main tanks will not have it means adding fire barriers and well insulated tanks to prevent boiloff going to mars. While we know that its limited one can only hope that we would never need to use methane as a backup as it does not last due to generator efficiency even if its 30 plus. The methane sabetier reactor is another energy intense device needed to make the methane and the addition energy needed for the oxidizer from water electrolysis means that the backup to the backup could mean no return home if it last to long or comes to early in the production cycle that we can not recover from reduced power levels in time. All devices have a unknown shelf life to failure with some being a bit more dependable from the get go as they are heavily tested for mars conditions.

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#38 2019-10-04 18:37:36

SpaceNut
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From: New Hampshire
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#39 2019-10-05 01:49:09

Calliban
Member
From: Northern England, UK
Registered: 2019-08-18
Posts: 354

Re: Methane backup not nuclear vs solar

Perhaps imported solar is a better fix for underground agriculture systems when we establish a Martian base.  In that case, no backup power is needed as the crops are illuminated by LEDs when the sun is up and illumination levels just follow the day-night cycle.  Other equipment can be directly coupled to solar panels as well.  It all depends on what the equipment is and if you are only going to use it intermittently anyway.  Ideally, it is always more cost effective to operate expensive equipment on a continuous basis.  As I have said before, if you build a solar powered propellant plant, it must be 3 times the size of a nuclear powered equivalent to produce the same output per day.  Some things really just work better with a continuous power supply.  On the other hand, if I need to use a lathe for an hour every day, then I can simply schedule that activity for daylight hours.

Magnox reactors?  Those things had enormous carbon moderator blocks and terrible low power density.  The UK more or less dropped Magnox as soon as enriched uranium became relatively cheap.

Last edited by Calliban (2019-10-05 01:57:25)


Interested in space science, engineering and technology.

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#40 2019-10-05 02:02:38

Calliban
Member
From: Northern England, UK
Registered: 2019-08-18
Posts: 354

Re: Methane backup not nuclear vs solar

Two articles from Kris DeDecker's low tech magazine that discuss how a society might live on intermittent energy.

https://www.lowtechmagazine.com/2017/09 … -powe.html

https://www.lowtechmagazine.com/2017/09 … ather.html


Interested in space science, engineering and technology.

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#41 2019-10-05 08:39:50

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,788

Re: Methane backup not nuclear vs solar

No way would the propellant plant have to be three times the size. That's where the use of chemical batteries and the meth-ox generators would help level production. 

I don't think anyone knows for sure how much electricity will be required, but, to put this in context, if 80Kwe is the average input into the propellant plant, that would be 1960 KwHes per sol. The amount of battery storage available from the fin batteries in the 6 Starships would be 2400 KwHes. So that's freely available chemical battery storage. To that you could add up to 24,500 KwHes of power available from meth-ox storage allowance (based on my proposal).

So, there will be no need for major fluctuations in the power input to the propellant plant.

The energy demand for agriculture is actually huge. Once the population is in the tens of thousands you probably need to think in terms of natural light farming. What you could do on Mars is use reflectors to boost light levels in domed structures. That way you could get up to maybe 70% of surface insolation on Earth. Mars has the advantage of virtually no cloud cover so I think that 70% figure would probably enable you to grow all the crops we grow in places like the USA and UK. The only problem is that in severe and prolonged dust storms crops would die unless there were supplementary lighting.


Calliban wrote:

Perhaps imported solar is a better fix for underground agriculture systems when we establish a Martian base.  In that case, no backup power is needed as the crops are illuminated by LEDs when the sun is up and illumination levels just follow the day-night cycle.  Other equipment can be directly coupled to solar panels as well.  It all depends on what the equipment is and if you are only going to use it intermittently anyway.  Ideally, it is always more cost effective to operate expensive equipment on a continuous basis.  As I have said before, if you build a solar powered propellant plant, it must be 3 times the size of a nuclear powered equivalent to produce the same output per day.  Some things really just work better with a continuous power supply.  On the other hand, if I need to use a lathe for an hour every day, then I can simply schedule that activity for daylight hours.

Magnox reactors?  Those things had enormous carbon moderator blocks and terrible low power density.  The UK more or less dropped Magnox as soon as enriched uranium became relatively cheap.


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

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#42 2019-10-05 08:53:54

kbd512
Administrator
Registered: 2015-01-02
Posts: 3,452

Re: Methane backup not nuclear vs solar

Louis,

(a) KiloPower won't be operated aboard Starship.  It will be emplaced a couple of kilometers away using a rover- the same rover required to transport other cargo around on the surface of Mars.  NASA specifically designed their rover to be able to carry the weight of a 10kWe KiloPower reactor.

(b) The KiloPower units are not aircraft / spacecraft gas turbine APU's, which seems to be what you're talking about.

(c) I agree with this point.  There's no point comparing the total output of any other power generation technology with a fission reactor because there's no contest between the output-to-input ratio of a fission reactor when compared to any other power generation technologies that humans actually know how to make.  The crudest nuclear reactors ever made were still powered by a material that's more than a million times as energy dense as any hydrocarbon fuel.  As a result, nuclear fission wins the total power output argument every time.  Nothing can ever change that as long as the physical laws that dictate how this universe works remain as they are.  It's an empirical fact not subject to alteration or interpretation to appease anyone's personal beliefs or desires.

(d) Nobody is talking about using a fission reactor aboard Starship except you.  This is another straw man argument against using nuclear power.  To what end, I don't really comprehend.

(e) Please provide an example of that hypothetical emergency scenario you made reference to, wherein nobody can get off the ship for a few hours to go attach a power cable to a fission reactor, but somehow having 12t of excess rocket fuel onboard would save the day.  If such an emergency scenario is even possible, then surely a 12t RTG that isn't powered by an explosive cryogenic gas would be a better option.  A hypothetical 12t MMRTG, in response to your hypothetical emergency scenario, would provide a little over 30kWe, every hour of every day for at least a decade.  That's 265MWh of power over the course of a year- assuming we actually wanted to use nuclear power in an even more absurdly inefficient manner than what KiloPower is capable of providing.  If your O2/CH4 engine was 100% efficient, it could produce 166MWh to 183MWh with 12t of CH4 (50MJ/kg to 55MJ/kg, though this also ignores the mass of the O2 required for combustion).

(f) If the efficiency of a hypothetical O2/CH4 combustion engine was 100%, then fission reactors would still produce substantially more power for the same mass.

(g) Musk proposes lots of things, but that doesn't mean all of his proposals are practical.  They're mostly just ideas in his head.  Math determines what will work in the objective physical world.  No amount of wishes, beliefs, desires, etc have one little thing to do with what will or won't work in a practical manner using the technology that we have, rather than what we wished we had.

This CH4 backup power scheme doesn't appear to best nuclear power at any level, even when compared to something with a power-to-weight ratio as low as NASA's MMRTG.  Pound for pound, KiloPower can roughly double what a MMRTG is capable of providing when the reactor is run at 75% of rated output.  Anything slightly more sophisticated than a Uranium metal slug fission reactor would vastly out-produce anything that a very finite supply of CH4 could produce.

The math doesn't work out in favor of hydrocarbon fuels or batteries.  That's all there is to it.  It only gets worse as continuous energy requirements increase.

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#43 2019-10-05 09:06:27

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 3,903
Website

Re: Methane backup not nuclear vs solar

Louis is a Mars dust storm denier,  because that is a killer for solar-only on Mars. 

The kind of a planet-girdling dust storm that Mariner 9 saw in 1969 happened again about 6 times in the decades since,  including the one that killed the Opportunity rover. 

Insolation levels at the surface during these events is a lot lower than the "35%" figure Louis quotes.  Closer to the 5-10% range,  which is not enough for silicon solar cells to function at all,  especially at half the exoatmospheric solar constant we have here at Earth.

Opportunity used electric power to keep its electronics warm enough to survive Martian winter nights.  That was batteries at night,  charged by solar during the day.  When the dust storm cut off solar charing,  the batteries ran dry,  the spacecraft froze too cold,  and the electronics died of that cold.  Which is why the JPL guys could not get a peep out of it after the dust storm passed.

It died in the cold for lack of solar electricity due to a big dust storm.  Literally. 

THAT is what you MUST plan on happening roughly once a decade,  since that has been quite literally the track record since Mariner 9.  You cannot fight the facts of history!

As for Spacex's "Starship",  we'll see what the design really looks like after the prototypes (really just experiments) have been tested.  In the concepts (and that's all they were) presented in 2017 and 2018,  "Starship" had big solar panel wings that folded out for power during the transits.  Most of the Mars mission transit designs I have seen have had solar electricity.

Out in space like that,  solar electricity is essentially "24/7 no matter what" because there is no night.  No obscuring clouds or dust.  Nothing.  You only need enough battery to supply your needs during departure,  until you can orient the ship and its solar wings correctly. 

It's different,  even in Earth orbit.  There is "night" when you are behind the Earth in its shadow.  You need enough battery to get through the shadow at full power,  and it needs to be rechargeable rather fast.  In low orbit,  that's about an hour's day and an hour's night to deal with.

It's quite different on the moon.  Daylight there is some 14 Earth days long,  and night there is also some 14 Earth days long.  That's one whale of a battery to get a habitation of some kind through the lunar night at full power. 

Mars in many ways is like the Earth for solar power,  due to its just-about-24-hour day/night cycle.  The exceptions are half the solar constant,  and those occasional giant,  sun-obscuring dust storms.  To do only solar on Mars doesn't look like Earthly solar/battery sizing,  it's far worse than the moon because of those once-in-a-decade dust storms.  Things can go dark for months.

Is it any wonder why NASA has been exploring nuclear power designs?  The government is the only outfit in America that can freely and easily experiment with nuclear:  nukes are a government monopoly in America by law.  Contractors are licensed to do these things for the government.

Louis is quite right to point out that Kilopower is not yet ready to use.  It might be in 2-4 years,  unless NASA flubs that up the way they have mismanaged SLS/Orion.  Let's just say my confidence is not high.

I would point out to Louis that "Starship" is not ready to use,  either,  and given my real-world experiences developing new flight vehicles from scratch,  I would have to say flying manned to orbit in 2 years is UNLIKELY IN THE EXTREME. 

My point is that by the time "Starship" is really ready to go to Mars,  Kilopower is also likely to be ready as well.  You don't need the nukes for the transit,  you need it at your settlement on Mars.  Like many others on the forums,  I advocate taking both forms of power.  Some time a year or a decade after setting up on Mars,  that giant dust storm will hit,  and you will die without the nukes.

It's really that simple,  and that stark.

As for methane-oxygen generators as backup power,  while there might be some machines out there that I was unaware of,  these are Earthly machines.  They must be adapted to work at lunar or Martian conditions.  They are not yet Mars-ready either.  It takes time to do that. 

But why draw backup power from the propellant supply that you need to get home?  What if something unexpected and unforseen happens,  requiring the settlement be evacuated?  If you have insufficient propellant,  you cannot do that.  Unexpected and unforseen things happen all the time,  here at home.  Out there will be no different.  Why take that risk?

A better option might be hydrogen-oxygen fuel cells,  using local water on Mars as the source.  But until you have built up multiple-months' supply,  you are NOT prepared for those erratically-occurring giant dust dustorms.  It all depends upon how much risk you are willing to bet lives on. 

If you take enough nuke power with you,  nearly all that risk goes away. The logic is inescapable.

Musk is going to need a Mars-ready propellant manufacturing plant of near a ton/day scale from "somebody".  He's going to need a nuclear electric generator from "somebody".  He's going to need electric bulldozers and similar equipment from "somebody".  He's going to need Mars-ready construction techniques,  tools,  and materials from "somebody".   

All that stuff,  and more,  is needed for any sort of base or settlement on Mars.  Musk supposedly supplies the rocket to get there and back.  Those various "somebodies",  whoever they are,  had better get busy.  4-5 years,  even a decade or so,  is not very long at all,  to get all that stuff Mars-ready.

As far as I can tell,  there is nobody "minding the store" to see that all of this comes together at the time the rocket is finally ready to go there.  That would be a perfect fit as NASA's job,  but that is NOT what they are doing.

I really do hope that Spacex people,  NASA people,  Bigelow people,  and many others (including the likes of Boeing and Lockheed-Martin) actually look at these forums from time to time.  They're all going to have to work together to bring all of this to a timely fruition. 

If not,  then Musk's vision will remain a fantasy.  He and his Spacex cannot do all of this alone. 

GW

Last edited by GW Johnson (2019-10-05 09:35:55)


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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#44 2019-10-05 11:05:49

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,788

Re: Methane backup not nuclear vs solar

I'm not a dust storm denier.

Opportunity died of old age not dust storms. It lived 55 times over its designed life time - surviving for over 14 years (living through about 6 major dust storms) without direct human intervention. That's the definition of the triumph of PV power.

Closer to the 5% to 10%?  Where's a citation to back your claim? This academic paper gives a figure of 37% for a severe dust storm:

https://rmets.onlinelibrary.wiley.com/d … 6/qj.04.09

"In the severe dust-storm condition ofτ=5 and sun in the zenith, the transmissivity drops to 37%..."

Is it really plausible that Musk has been pressing ahead with rocket development while forgetting to address life support, habs, rovers, the energy system and all the rest? I don't think so, but who knows...?

I envisage any meth-ox generator would be housed in a pressurised hab. Why would you have it out in the open?


GW Johnson wrote:

Louis is a Mars dust storm denier,  because that is a killer for solar-only on Mars. 

The kind of a planet-girdling dust storm that Mariner 9 saw in 1969 happened again about 6 times in the decades since,  including the one that killed the Opportunity rover. 

Insolation levels at the surface during these events is a lot lower than the "35%" figure Louis quotes.  Closer to the 5-10% range,  which is not enough for silicon solar cells to function at all,  especially at half the exoatmospheric solar constant we have here at Earth.

Opportunity used electric power to keep its electronics warm enough to survive Martian winter nights.  That was batteries at night,  charged by solar during the day.  When the dust storm cut off solar charing,  the batteries ran dry,  the spacecraft froze too cold,  and the electronics died of that cold.  Which is why the JPL guys could not get a peep out of it after the dust storm passed.

It died in the cold for lack of solar electricity due to a big dust storm.  Literally. 

THAT is what you MUST plan on happening roughly once a decade,  since that has been quite literally the track record since Mariner 9.  You cannot fight the facts of history!

As for Spacex's "Starship",  we'll see what the design really looks like after the prototypes (really just experiments) have been tested.  In the concepts (and that's all they were) presented in 2017 and 2018,  "Starship" had big solar panel wings that folded out for power during the transits.  Most of the Mars mission transit designs I have seen have had solar electricity.

Out in space like that,  solar electricity is essentially "24/7 no matter what" because there is no night.  No obscuring clouds or dust.  Nothing.  You only need enough battery to supply your needs during departure,  until you can orient the ship and its solar wings correctly. 

It's different,  even in Earth orbit.  There is "night" when you are behind the Earth in its shadow.  You need enough battery to get through the shadow at full power,  and it needs to be rechargeable rather fast.  In low orbit,  that's about an hour's day and an hour's night to deal with.

It's quite different on the moon.  Daylight there is some 14 Earth days long,  and night there is also some 14 Earth days long.  That's one whale of a battery to get a habitation of some kind through the lunar night at full power. 

Mars in many ways is like the Earth for solar power,  due to its just-about-24-hour day/night cycle.  The exceptions are half the solar constant,  and those occasional giant,  sun-obscuring dust storms.  To do only solar on Mars doesn't look like Earthly solar/battery sizing,  it's far worse than the moon because of those once-in-a-decade dust storms.  Things can go dark for months.

Is it any wonder why NASA has been exploring nuclear power designs?  The government is the only outfit in America that can freely and easily experiment with nuclear:  nukes are a government monopoly in America by law.  Contractors are licensed to do these things for the government.

Louis is quite right to point out that Kilopower is not yet ready to use.  It might be in 2-4 years,  unless NASA flubs that up the way they have mismanaged SLS/Orion.  Let's just say my confidence is not high.

I would point out to Louis that "Starship" is not ready to use,  either,  and given my real-world experiences developing new flight vehicles from scratch,  I would have to say flying manned to orbit in 2 years is UNLIKELY IN THE EXTREME. 

My point is that by the time "Starship" is really ready to go to Mars,  Kilopower is also likely to be ready as well.  You don't need the nukes for the transit,  you need it at your settlement on Mars.  Like many others on the forums,  I advocate taking both forms of power.  Some time a year or a decade after setting up on Mars,  that giant dust storm will hit,  and you will die without the nukes.

It's really that simple,  and that stark.

As for methane-oxygen generators as backup power,  while there might be some machines out there that I was unaware of,  these are Earthly machines.  They must be adapted to work at lunar or Martian conditions.  They are not yet Mars-ready either.  It takes time to do that. 

But why draw backup power from the propellant supply that you need to get home?  What if something unexpected and unforseen happens,  requiring the settlement be evacuated?  If you have insufficient propellant,  you cannot do that.  Unexpected and unforseen things happen all the time,  here at home.  Out there will be no different.  Why take that risk?

A better option might be hydrogen-oxygen fuel cells,  using local water on Mars as the source.  But until you have built up multiple-months' supply,  you are NOT prepared for those erratically-occurring giant dust dustorms.  It all depends upon how much risk you are willing to bet lives on. 

If you take enough nuke power with you,  nearly all that risk goes away. The logic is inescapable.

Musk is going to need a Mars-ready propellant manufacturing plant of near a ton/day scale from "somebody".  He's going to need a nuclear electric generator from "somebody".  He's going to need electric bulldozers and similar equipment from "somebody".  He's going to need Mars-ready construction techniques,  tools,  and materials from "somebody".   

All that stuff,  and more,  is needed for any sort of base or settlement on Mars.  Musk supposedly supplies the rocket to get there and back.  Those various "somebodies",  whoever they are,  had better get busy.  4-5 years,  even a decade or so,  is not very long at all,  to get all that stuff Mars-ready.

As far as I can tell,  there is nobody "minding the store" to see that all of this comes together at the time the rocket is finally ready to go there.  That would be a perfect fit as NASA's job,  but that is NOT what they are doing.

I really do hope that Spacex people,  NASA people,  Bigelow people,  and many others (including the likes of Boeing and Lockheed-Martin) actually look at these forums from time to time.  They're all going to have to work together to bring all of this to a timely fruition. 

If not,  then Musk's vision will remain a fantasy.  He and his Spacex cannot do all of this alone. 

GW


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

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#45 2019-10-05 11:37:34

kbd512
Administrator
Registered: 2015-01-02
Posts: 3,452

Re: Methane backup not nuclear vs solar

Louis,

In a word, YES, it's entirely plausible that Elon Musk doesn't have any of the myriad of the practical problems with his ideas worked out.  Idealism alone doesn't confer any special powers to Elon Musk or anyone else for that matter.  People who are highly creative typically haven't thought through every conceivable problem with their new idea.  That's why engineers are required to make those new ideas leap into existence.  In the same way that you've never addressed the basic math of what you proposed doing, either because you can't or don't like what the math says about how well your ideas would work in practice, it's entirely plausible, if not probable, that another wishful thinker has done the exact same thing.

If nothing else, Musk is a bit of a showman.  If he had solutions for Mars habitation / ground transportation / life support / propellant production / etc, then he would tweet them out immediately- in much the same way he does whenever someone so much as turns a wrench or puts another part on Starship.

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#46 2019-10-05 11:41:14

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 18,555

Re: Methane backup not nuclear vs solar

sparks plus leak (boil off from fuel /lox) of a flamible with oxygen makes for an explosion....
Tank not inside will make for no danger...

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#47 2019-10-05 15:34:00

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,788

Re: Methane backup not nuclear vs solar

I've been an advocate for the Mars Mission using retro-rocket landing, planetary passes to slow down from hypersonic speed and an energy system based on solar power. All three approaches have been rubbished here by various posters at different times but Space X are running with all three. We'll see if I am proved right about use of meth-ox for complementary electricity generation.

Musk didn't tweet in advance about the switch to stainless steel, so he might just be waiting till Space X has gone orbital before he begins the slow reveal on the base itself. We'll see, I guess.


kbd512 wrote:

Louis,

In a word, YES, it's entirely plausible that Elon Musk doesn't have any of the myriad of the practical problems with his ideas worked out.  Idealism alone doesn't confer any special powers to Elon Musk or anyone else for that matter.  People who are highly creative typically haven't thought through every conceivable problem with their new idea.  That's why engineers are required to make those new ideas leap into existence.  In the same way that you've never addressed the basic math of what you proposed doing, either because you can't or don't like what the math says about how well your ideas would work in practice, it's entirely plausible, if not probable, that another wishful thinker has done the exact same thing.

If nothing else, Musk is a bit of a showman.  If he had solutions for Mars habitation / ground transportation / life support / propellant production / etc, then he would tweet them out immediately- in much the same way he does whenever someone so much as turns a wrench or puts another part on Starship.


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

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#48 2019-10-05 17:15:58

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 3,903
Website

Re: Methane backup not nuclear vs solar

Louis:

Quoting your post in part:  "I'm not a dust storm denier.

Opportunity died of old age not dust storms. It lived 55 times over its designed life time - surviving for over 14 years (living through about 6 major dust storms) without direct human intervention. That's the definition of the triumph of PV power.

Closer to the 5% to 10%?  Where's a citation to back your claim? This academic paper gives a figure of 37% for a severe dust storm:"

Louis,  that posting proves in fact that you ARE a severe dust storm denier!  I need no citation beyond the publicly-released images when Mariner 9 reached Mars in 1969 to prove my point. 

The Tharsis Mons volcanoes,  including Olympus Mons,  were invisible for nearly half a year.  The general plain was not visible for some 9 months.  The relief from the plain to the tip of Olympus Mons is about 50,000 feet (some 15 km).  The depth of dust-filled air was clearly deeper than that. 

It almost doesn't matter what the actual dust or air density is with dust-filled depths like that.  So says the physics of particles suspended in gases relative to visible light transmission.  The transmissibility of sunlight through dust-filled air over distances that long is essentially zero.  Not 37%.  Not 35%.  0%!!!  Period!!!  It's an exponential relation you use to figure it,  with that distance in the exponent,  for crying out loud!

THAT Mariner 9 data is a "worst case dust storm" on Mars!  We've already seen it.  Multiple times.  Which makes your citation inapplicable to the worst case that we already know WILL occur!  Roughly once a decade,  but irregularly,  with lesser storms more like your citation in between.

Sure,  Opportunity lasted way far longer than its intended life.  But that says ABSOLUTELY NOTHING about what sun-obscuring dust storms are like,  except for the really bad (months long) one that killed it. It was still working before the storm,  and never worked again after the storm.  QED:  the dust storm killed it.

I have never said that retropropulsive landing doesn't work.  In point of fact,  I championed that concept before you,  or anyone else on these forums,  advocated for it. 

I have NOT championed multiple passes for aerodynamic braking at Mars,  because Mars's upper atmosphere densities vary by factors of 2+,  and that's way too much variability to cope with.  Spacex does NOT propose that for "Starship" anyway.  What they propose is to "aphelion-out" on the transfer trajectory and let Mars run over "Starship" from behind,  for a (single-pass) direct entry.  No different than any lander probe all the way back to Viking.

That sets your speed at entry interface to something near 7.5 km/s,  when Mars escape speed is 5 km/s.  You only have the actual encounter geometry as your one and only significant degree of freedom.  You must control that geometry to a grazing entry between 1 and 2 degrees below local horizontal.  Too steep,  and it's a race whether you crush or incinerate first.  Too shallow,  and you bounce off into deep space forever.  Tricky,  that!

And,  I HAVE NEVER SAID NOT TO USE SOLAR ON MARS!  I ONLY said that solar should NOT be the ONLY power source you take!  The other practical source in that cold,  near-vacuum environment is NUCLEAR!  No way around that. 

GW

Last edited by GW Johnson (2019-10-05 17:39:58)


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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#49 2019-10-05 18:16:20

kbd512
Administrator
Registered: 2015-01-02
Posts: 3,452

Re: Methane backup not nuclear vs solar

Louis,

There are engineering reasons why certain ideas have been shot down.  It's not personal.  People who understand and accept some basic engineering principles tend to get short with people who clearly don't.  Should we be more tactful?  Yes.  Will we?  Probably not.  I can only speak for myself, but after I read through enough variations on the same basic proposal that I already know can't work well enough for the intended use case, from a basic math perspective, it starts to irritate me.

I imagine that GW felt the same way after reading through my parachute landing proposals.  There's just not enough atmosphere for a parachute of any size to work and that's all there is to it.

So, about the ideas...

Reentry at interplanetary velocity

Tends to vaporize every acceptably lightweight material we know how to make.  Can interplanetary reentry still work?  Sure, but at what cost?  If it's so expensive or so destructive to the reentry vehicle that it makes interplanetary travel as unattainable as it's ever been, then does it matter if it can be made to work at all?  I'm sure it'd be acceptable for exploration, but we'd still be no further along the interplanetary colonization route than we are now.

Solar power as a panacea solution to all power provisioning problems

Since this doesn't even work on Earth where solar insolation levels are double what they are on Mars and we don't have giant dust storms that blot out the Sun across the entire planet for months at a time, how well could this possibly work on Mars?  Mission planners can't design systems that consume power within specified power availability parameters when the amount of power available varies by a factor of 10 or more.

If you told an aerospace engineer that he or she had to design a plane that normally requires 100 gallons of gas but might have to carry 1,000 gallons of gas, but irrespective of whether or not it needed 100 or 1,000 gallons of gas it had to fly just as economically, then that engineer would tell you to forget about your idea because it's simply not possible.  And you know what?  That engineer is still right, no matter any ideas to the contrary.  It's not possible to design a plane that consumes the same amount of fuel or costs the same as one that's carrying 600 vs 6,000 pounds of gas.  An aircraft carrying 6,000 pounds of gas will be the size of a small business jet, it'll consume fuel like a small business jet to stay airborne as a function of its weight, and it'll have power requirements commensurate with a small business jet as a function of its substantially greater weight when compared to an aircraft carrying ten times less fuel.

That'd be why we don't see any business jets giving aerial sight-seeing tours.  They're A to B machines, as fast as available technology dictates is feasibly affordable, such that as many flights as possible can be made by a single aircrew in a single day.  The plane carrying 100 gallons of gas won't be as fast as the one carrying 1,000 gallons of gas, but it will certainly be more economical to operate, such that giving sight seeing tours to local tourists falls within the realm of affordability for them.

How does that tie back to Mars missions?

A mission planner can't plan mass requirements when available power fluctuates that wildly.  We don't even build power grids here on Earth, where any prospective solution is incomparably less mass-constrained, with load carrying capability that fluctuates that much.

So what to do about that?

Start taking a hard look at power sources that don't have output levels with such massive swings.

Why nuclear power?

Happens to work.  Happens to not be unattainably expensive because we have nearly inexhaustible stockpiles of the nuclear materials required and the designs now under development are ridiculously simple compared to previous pie-in-the-sky efforts and tested in ways that no other types of power provisioning solutions ever have been- which typically means we know precisely how they'll handle whatever specific scenario is thrown at the power plant.  Happens to have energy density so far in excess of any other power generation solution that we can still afford horribly inefficient solutions like KiloPower that still output vastly more power than any other alternatives for a given amount of weight.  There are no free lunches, though, and nuclear power does have downsides.  That said, the "downsides" are manageable for this particular design.  It might be worth your time to actually read about how the technology works, whereupon you'd have a better basis for understanding why NASA is pushing so hard for this technology.  At the very least, you could formulate valid arguments about why we shouldn't use nuclear power if your opinion about using it doesn't change.

Unlike whatever designs have come from the engineers who work for Elon Musk, these small nuclear reactors are tested and reviewed by multiple groups of engineers, inside and outside of NASA, and test data is shared with the Europeans and the Russians so that if they spot a problem that we don't spot, the opportunity exists to correct it before we find out about what we don't know on Mars.  Believe it or not, during the height of the Cold War, nuclear engineers from NASA and ROSCOSMOS flew back and forth between labs in America and Russia to jointly develop nuclear reactors that were considered safe and reliable enough to have them whizzing around our heads in orbit, where many of those reactors remain to this day.

As far as complexity goes, there is no comparison at all between the complexity of a commercial nuclear power plant and one of these portable reactors.  KiloPower is a chunk of Uranium metal about the size of a coffee can and it has a single Boron Carbide control rod inserted into it and bolted into place so it can't accidentally "fall out".  If the Uranium is blown to bits by a launch accident, then there's no possibility of a criticality.  The geometry of the Uranium slug in KiloPower is what allows it to go critical.  If it's no longer in that geometry because it's been blown to pieces, then it won't go critical and remains only slightly more radioactive than background radiation levels.  At launch, a cold fission reactor is vastly less radioactive than any type of radioisotope thermoelectric generator.

O2/CH4 and Batteries

Cryogenic liquids require power to keep them cold.  That power has to come from somewhere.  If it doesn't come from the giant fusion reactor or a much smaller and less impressive fission reactor, then it'll have to come from the fuel taken to Mars.  If it doesn't, then the cryogenic liquids vaporize or rupture their storage vessel.  If a combustion engine or fuel cell was 100% efficient, which is thermodynamically impossible, all chemical fuels fall woefully short of the most inefficient nuclear power generation systems we use.

On first principles, how much sense does it make to consume the very fuel you've elected to take to another planet, that you also need much more of to return home, that you have to expend vastly more energy to make from scratch, merely to stay alive because your power provisioning scheme of choice isn't reliable enough to produce the power required to keep the fuel cold?

Batteries have energy densities so low that we only use them at significant scale for portable computers and power tools here on Earth.  All electric vehicles that use batteries are substantially heavier than equivalent gas powered vehicles and require so much more energy input to produce that they only represent a net emissions reduction after most owners have already sold off their vehicle to buy a newer model.  In a world where people who claim to care about Earth's environment understood basic math, they'd keep any vehicle they owned for as long as possible and they'd merely upgrade or refresh the vehicle's power plant to more efficient models.  Aerodynamics and electronic gadgets don't amount to a hill of beans in the prototypical use case.  As always, the key to dramatically improved efficiency is all about weight reduction- making stronger parts that weigh less than past models.  That axiom holds true for every type of transport vehicle we actually make, from bicycles to interplanetary transport systems.  Tesla touts the aerodynamics of their vehicles because the power density of their batteries is so poor and their weight is so great that they had to resort to that just to produce a vehicle with less range than a gas powered car.

None of that is even remotely likely to change substantially in the near future.  We already know so much about combustion engines and electric motors that what we can achieve is mostly a matter of engineering a particular design for a particular use case.  What we don't know about batteries is how to make an affordable and reliable battery with greater energy density than current Lithium-ion technology.

A new "revolutionary breakthrough" will be touted at least once per week.  And yet...  We'll most likely be using Lithium-ions for decades because no other battery technology we know of doesn't come with significant downsides.  The Aluminum-air batteries already achieve 1,300Wh/kg- half way to liquid hydrocarbon parity, but the "downside" is that after about a month of use, you have to send the battery back to the factory to re-separate the Aluminum in the electrolyte from the Oxygen, with substantial energy input and unproductive time costs.  Could this work at small scale on Mars?  Probably.  It's certainly worth considering.  The recycling plant would certainly be much closer to the point of use.  Aluminum-air has the bonus feature of not being prone to fires and explosions as many of the bleeding edge Lithium-ion batteries are.  Anything with a large removable battery could benefit from this technology.  IIRC, it also has a wider operating temperature range.  It's essentially an "electrolyte rechargeable / recyclable" primary battery technology with approximately 5 times the energy density of Lithium-ion.

BTW, how come you never talk about Aluminum-air in your battery arguments?  Seems more feasible than existing Lithium-ion battery technology in this particular use case.

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#50 2019-10-05 18:38:32

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 18,555

Re: Methane backup not nuclear vs solar

I think the issue for methane is also the term liquid as for earth its under a great pressure in the fuel tanks on earth but for space it needs to be super cold to keep the tank mass down in addition to the pressure its stored at.

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