Using Amercium RTGs for a reusable Manned Lunar Lander-Could it work?

[Sanic]

Yay! MOAR Pu-238!

Praise the kraken!

[SargeRho]

7 minutes ago, Albert VDS said:

From the  article:

 

Which is exactly what I've said.

[fredinno]

12 minutes ago, Albert VDS said:

From the  article:

 

That's still painstakingly slow, and not enough for manned landers. Also, the DOE is requesting NASA pay for every gram they make right now, along with all the costs associated with it, raising the cost enormously, compared to Americium, which is commercially produced.

Edited January 16, 2016 by fredinno

[Albert VDS]

4 minutes ago, SargeRho said:

Which is exactly what I've said.

From what you posted it read as if they need to order them to make it for a specific mission.
But the article actually state they are getting a set  amount each year, with an increased amount at one point, and they can use it on any space mission they see fit.
 

[Albert VDS]

Apparently NASA tested Am-241 and it's great for mission with a low energy requirement:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwj4pbyGiq_KAhWBeA4KHXUIAG0QFggdMAA&url=http%3A%2F%2Fntrs.nasa.gov%2Fsearch.jsp%3FR%3D20140008746&usg=AFQjCNHAr1lLfw_M0mAX_khLCeXGOpzSGg&sig2=K9786Sd8CuE8-Tdny8kzFA

[fredinno]

4 hours ago, Albert VDS said:

Apparently NASA tested Am-241 and it's great for mission with a low energy requirement:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwj4pbyGiq_KAhWBeA4KHXUIAG0QFggdMAA&url=http%3A%2F%2Fntrs.nasa.gov%2Fsearch.jsp%3FR%3D20140008746&usg=AFQjCNHAr1lLfw_M0mAX_khLCeXGOpzSGg&sig2=K9786Sd8CuE8-Tdny8kzFA

ESA is actively developing it.

[Mad Rocket Scientist]

Just buy a couple thousand smoke detectors and you can get started building your own!

[fredinno]

10 minutes ago, Mad Rocket Scientist said:

Just buy a couple thousand smoke detectors and you can get started building your own!

More like a couple billion.

[Rune]

Awful idea. Just how many kW-hours can be stored in 2mT of fuel? I mean, Apollo did the whole mission on fuel cells alone, and that's a week of power for three guys. Don't even bother with batteries, go full one-shot. It's not as if you are reusing the lander in any case...

The cost for a 2mT RTG would be horrendous too, dwarfing launch costs. You are talking about something 25 times bigger than NH's powersource, which in itself has taken a tenth or so of current US Pu₂₃₈ reseves.

 

Rune. I am a horrible rounder, I know. Still, order of magnitude.

[fredinno]

28 minutes ago, Rune said:

Awful idea. Just how many kW-hours can be stored in 2mT of fuel? I mean, Apollo did the whole mission on fuel cells alone, and that's a week of power for three guys. Don't even bother with batteries, go full one-shot. It's not as if you are reusing the lander in any case...

The cost for a 2mT RTG would be horrendous too, dwarfing launch costs. You are talking about something 25 times bigger than NH's powersource, which in itself has taken a tenth or so of current US Pu₂₃₈ reseves.

 

Rune. I am a horrible rounder, I know. Still, order of magnitude.

It's using Americum 241. Also, the lander is supposed to be a reuable SSTO- which is something you would have known if you read the first post, Rune. 

[SomeGuy123]

37 minutes ago, Rune said:

Awful idea. Just how many kW-hours can be stored in 2mT of fuel? I mean, Apollo did the whole mission on fuel cells alone, and that's a week of power for three guys. Don't even bother with batteries, go full one-shot. It's not as if you are reusing the lander in any case...

 

Depends on fuel type, of course.  Using liquid methane, which I particularly like because it has a better density, lower cryogenic temperature, and also can be burned in fuel cells like H2 (Apollo era fuel cells may not have been able to burn methane, which would explain why they used hydrogen), it's 55 megajoules per kilogram.  With 50% efficient fuel cells (space ones might do a little better), that's 27.5 megajoules per kilogram of fuel.

It's CH4 + 2 02 -> CO2 + H20.  So that's 10 grams/mole methane : 32 grams/mole oxygen.  So for every kilogram of methane you bring, you need 3.2 kilograms oxygen.  So to get 27.5 megajoules, you need 4.2 kilograms of fuel.  What about the tank it's in?  Let's be really conservative and assume the mass ratio is only 80%.  So to carry 4.2 kilograms of fuel, if you include the tankage, it's a total of 5.2 kilograms.  You'll also need a fuel cell, but high end ones are 1 kilowatt per kilogram, so it's probably negligible.  Also, I don't think the numbers for the americium are including the conversion apparatus.

Each 27.5 megajoules = 7.64 kilowatt hours.  So, with 2000 kilograms (2 metric tons), (2000/5.2) * 7.64 = 2938 kilowatt hours.  If we need 4000 watts all the time, then we get 734 hours or 30 days of runtime.  It is possible to capture the products (dry ice and water) and recycle them back to fuel, forming an inefficient (in power efficiency) battery.

If we also have solar panels, that actually means we can stay 56 days.  Odds are we're gonna run out of Scooby Snacks before then.  And we can leave a tiny RTG to run any scientific instruments we leave behind.

To me, this makes sense.  An organization like SpaceX can just order fuel cells, some vacuum insulated tanks, and put together something like this.  At their current capabilities they could probably just get it ready to launch with a few years lead time and a measly few billion dollars.

Getting more Plutonium, especially 2000 kilograms of it, sounds like a nightmare of red tape and waiting.

Edited January 17, 2016 by SomeGuy123

[SomeGuy123]

Proposal : let's put it all together.  Bring 1000 kilograms of methane and LOX.  Bring at least 25 kilowatts of solar panels.  High end panels are only a couple kilograms/kilowatt, so 50 kilograms.

During the day, 4 kW of the 25 kW goes to running the Altair.  20 kW goes to recycling the dry ice and water back to methane and oxygen (reaction is H20 -> H2 + 02, then H2 + C02-> CH4, uses catalyst and heat in a tiny chemical reactor and an electrolyzer).  Assume generously that the equipment that does this weighs 500 kilograms.  (probably much much lighter).

So total : 1000 + 500 + 25 = 1525 kilograms of apparatus, no need for nuclear fuel, and we can run indefinitely, at least until the pumps break.  (this system has several mechanical pumps involved in compressing gas to drive it.  You can use parallel pumps for redundancy but eventually they will wear out.  RTGs can obviously run for decades maintenance free)

We can also get lots more peak power.  Want 20 kilowatts for a brief period?  100 kilowatts?  Just bring a slightly heavier fuel cell stack.

Also, the methane and LOX used for this can be the same methane used in our ascent rocket!  We can use the same tank.  Just recycle so we don't actually consume any, net!

NOW we're talking.  I just realized this.  This cuts our weight costs down drastically.  In essence, some of the reserve fuel for the ascent rocket is being used during nights to power the equipment then being reformed back to methane and LOX during the days on solar power.  It depends on how you do the weight accounting but basically the only extra weight is the Sabatier/electrolyzer/compressor assembly and the extra solar panels for the power needed to do the reforming.  In the event we have a failure of that equipment, we just don't bring back any Moon rocks so we have the same effective fuel safety margin on the return trip.

 

TLDR : There's a far better solution than RTGs that is much, much lighter, cheaper, and much faster to deploy.  It uses technology that is mostly off the shelf today.

Edited January 17, 2016 by SomeGuy123

[Rune]

14 minutes ago, fredinno said:

It's using Americum 241. Also, the lander is supposed to be a reuable SSTO- which is something you would have known if you read the first post, Rune. 

Yeah, because using another rare nuclear material will help things a lot. And I checked again, yes, you mention the word "reusable" once (no mention of SSTOing, though). So I guess my bad, even though it doesn't change things much: a fuel cell or battery is just as reusable as a RTG.

12 minutes ago, SomeGuy123 said:

Depends on fuel type, of course.  Using liquid methane, which I particularly like because it has a better density, lower cryogenic temperature, and also can be burned in fuel cells like H2 (Apollo era fuel cells may not have been able to burn methane, which would explain why they used hydrogen), it's 55 megajoules per kilogram.  With 50% efficient fuel cells (space ones might do a little better), that's 27.5 megajoules per kilogram of fuel.

It's CH4 + 2 02 -> CO2 + H20.  So that's 10 grams/mole methane : 32 grams/mole oxygen.  So for every kilogram of methane you bring, you need 3.2 kilograms oxygen.  So to get 27.5 megajoules, you need 4.2 kilograms of fuel.  What about the tank it's in?  Let's be really conservative and assume the mass ratio is only 80%.  So to carry 4.2 kilograms of fuel, if you include the tankage, it's a total of 5.2 kilograms.  You'll also need a fuel cell, but high end ones are 1 kilowatt per kilogram, so it's probably negligible.  Also, I don't think the numbers for the americium are including the conversion apparatus.

Each 27.5 megajoules = 7.64 kilowatt hours.  So, with 2000 kilograms (2 metric tons), (2000/5.2) * 7.64 = 2938 kilowatt hours.  If we need 4000 watts all the time, then we get 734 hours or 30 days of runtime.  It is possible to capture the products (dry ice and water) and recycle them back to fuel, forming an inefficient (in power efficiency) battery.

If we also have solar panels, that actually means we can stay 56 days.  Odds are we're gonna run out of Scooby Snacks before then.  And we can leave a tiny RTG to run any scientific instruments we leave behind.

To me, this makes sense.  An organization like SpaceX can just order fuel cells, some vacuum insulated tanks, and put together something like this.  At their current capabilities they could probably just get it ready to launch with a few years lead time and a measly few billion dollars.

Getting more Plutonium, especially 2000 kilograms of it, sounds like a nightmare of red tape and waiting.

Thanks for doing the math to prove my point, I know it takes time. I'm surprised that we have such a low margin, actually, but it is more than enough to prove the point: RTGs are the wrong size for this, and the mission duration screams fuel cells.

 

Rune. Which, in a reusable lander, would get refueled just like the main engines.

[SomeGuy123]

7 minutes ago, Rune said:

Thanks for doing the math to prove my point, I know it takes time. I'm surprised that we have such a low margin, actually, but it is more than enough to prove the point: RTGs are the wrong size for this, and the mission duration screams fuel cells.

Rune. Which, in a reusable lander, would get refueled just like the main engines.

Oh, it's better than that.  I just figured out before you finished typing your reply that you can use the methane and LOX stored in the same tanks on the lander itself to run your fuel cells.  Then, during the day, you use solar power to reform it back to methane/LOX in a reactor.  This gets you amazingly better margins, it makes the mass penalty of your power system for the nights tiny.  Also, that methane reforming apparatus is the same equipment you would use for ISRU on Mars (well, if the ISRU refinery is modular, it's only about half the components).

The other interesting thing I figured out is that fuel cells are incredibly better.  The fact they are energy inefficient - batteries are about 90% efficient, from charge to discharge, while fuel cells are only about 20% (you lose about half the energy doing electrolysis, more energy doing Sabatier, and you lose half the energy in the resulting fuel in the fuel cell conversion) - becomes line noise if you get to have solar cells that are around 2 kilograms/kilowatt, which we already have today.  Solar power is so powerful and amazing, at least close to the Earth, that fuel cells >>>> batteries.  

Reason current spacecraft don't use em is they are complex and there's more to go wrong.

Edited January 17, 2016 by SomeGuy123

[fredinno]

How about RTGs for a Pressurized rover?

Also, RTGS can heat up the spacecraft, it might be a good idea to carry at least some to make sure nothing ever freezes up.

Edited January 17, 2016 by fredinno

[njmksr]

2 minutes ago, fredinno said:

How about RTGs for a Pressurized rover?

Also, RTGS can heat up the spacecraft, it might be a good idea to carry at least some to make sure nothing ever freezes up.

"As with most of life's problems, this one can be solved by a box of pure radiation."

--Mark Watney

 

Though seriously, IDK. Would the risk of cancer be worth it?

 

Maybe have the RTG superheat water which would heat air? So the radiation isn't as bad?

[fredinno]

6 minutes ago, njmksr said:

"As with most of life's problems, this one can be solved by a box of pure radiation."

--Mark Watney

 

Though seriously, IDK. Would the risk of cancer be worth it?

 

Maybe have the RTG superheat water which would heat air? So the radiation isn't as bad?

It can be surrounded by say, backup H2O tanks. The Apollo LEMs used RTGs for heating too, so...

[SomeGuy123]

13 minutes ago, fredinno said:

How about RTGs for a Pressurized rover?

You're up against similar weight penalties as with the extra landing weight.  The rechargeable fuel cell solution discussed above would be even more dominant for rovers.

[fredinno]

1 minute ago, SomeGuy123 said:

You're up against similar weight penalties as with the extra landing weight.  The rechargeable fuel cell solution discussed above would be even more dominant for rovers.

But rovers do not produce fuel for fuel cells...

[SomeGuy123]

18 minutes ago, njmksr said:

Maybe have the RTG superheat water which would heat air? So the radiation isn't as bad?

Why do you think the radiation is bad?  We're talking about RTGs, not nuclear reactors.  Plutonium-238 emits 100% alpha particles, which are blocked cold by a sheet of paper, aluminum foil, a plastic bag, the outer layer of human skin...

Literally harmless as long as you don't lick it or inhale the metal dust.

The drawbacks of it have to do with it costing an incredible amount of money to make even a small amount of it, it will kill lots and lots of people if the rocket explodes and the people inhale the dust, and it's actually fairly heavy and low power density.

6 minutes ago, fredinno said:

But rovers do not produce fuel for fuel cells...

You're right, they don't.  But, if you store your waste - dry ice and water, you could store it in empty fuel tank compartments - you could recycle it if you make it back to base.  

It would take a very long mission before the mass of these fuel tanks and plumbing is less than the mass of bringing an RTG along to give you the same amount of power.  And if you have astronauts along, they'll probably run out of snacks before you run out of fuel.

Reliability could be an advantage.  A sealed Pu-238 RTG attached to thermocouples is probably almost bulletproof.  You could drop it and the whole rover, probably shoot it, crash it into things, and it more likely than not will still produce power.  Even if the thermoelectric power components are damaged, they are arranged in an array and you could probably wire around the damaged ones with a quick bit a jury rigging and get some power off it.  

With fuel cells that depend on methane and oxygen gas, obviously a hole in the plumbing can leak out all your fuel and then you're in trouble.

Edited January 17, 2016 by SomeGuy123

[fredinno]

7 minutes ago, SomeGuy123 said:

Why do you think the radiation is bad?  We're talking about RTGs, not nuclear reactors.  Plutonium-238 emits 100% alpha particles, which are blocked cold by a sheet of paper, aluminum foil, a plastic bag, the outer layer of human skin...

Literally harmless as long as you don't lick it or inhale the metal dust.

The drawbacks of it have to do with it costing an incredible amount of money to make even a small amount of it, it will kill lots and lots of people if the rocket explodes and the people inhale the dust, and it's actually fairly heavy and low power density.

You're right, they don't.  But, if you store your waste - dry ice and water, you could store it in empty fuel tank compartments - you could recycle it if you make it back to base.  

It would take a very long mission before the mass of these fuel tanks and plumbing is less than the mass of bringing an RTG along to give you the same amount of power.  And if you have astronauts along, they'll probably run out of snacks before you run out of fuel.

RTGs are actually pretty safe even in a launch failure due to their casing.

 

And CO2 and water would almost certainly be in the form of urine and spent scrubbers when it is thrown out. So no.

[SomeGuy123]

Just now, fredinno said:

RTGs are actually pretty safe even in a launch failure due to their casing.

 

And CO2 and water would almost certainly be in the form of urine and spent scrubbers when it is thrown out. So no.

If the casing doesn't break.  It's a liability and legal issue.  So, SpaceX could slap together a fuel cell system and basically launch it tomorrow with minimal approval.  Or go through years of reviews for an RTG casing.  Even NASA has to waste a lot of time in reviews and they are an arm of the government itself.

I'm talking about the exhaust of the fuel cells.  That's pure CO2/water. 

Although...recycling water and co2 from your crew to get more rocket fuel does have a certain elegance, does it not?

 

[Rune]

1 hour ago, SomeGuy123 said:

Oh, it's better than that.  I just figured out before you finished typing your reply that you can use the methane and LOX stored in the same tanks on the lander itself to run your fuel cells.  Then, during the day, you use solar power to reform it back to methane/LOX in a reactor.  This gets you amazingly better margins, it makes the mass penalty of your power system for the nights tiny.  Also, that methane reforming apparatus is the same equipment you would use for ISRU on Mars (well, if the ISRU refinery is modular, it's only about half the components).

The other interesting thing I figured out is that fuel cells are incredibly better.  The fact they are energy inefficient - batteries are about 90% efficient, from charge to discharge, while fuel cells are only about 20% (you lose about half the energy doing electrolysis, more energy doing Sabatier, and you lose half the energy in the resulting fuel in the fuel cell conversion) - becomes line noise if you get to have solar cells that are around 2 kilograms/kilowatt, which we already have today.  Solar power is so powerful and amazing, at least close to the Earth, that fuel cells >>>> batteries.  

Reason current spacecraft don't use em is they are complex and there's more to go wrong.

Of course, using the main tanks and tank residuals to power the fuel cells is a very nice bonus: little additional plumbing needed, and maximum efficiency, like ULA's IFV tech. Pretty nifty, actually, if you add stuff like self pressurization and the like, you might be talking about a reusable vehicle that can run on water, basically. Methane is a bit hard to come by on cismunar space...

And kudos for getting the radiation part right. People read "nuclear powersource" and they think "green glowing pills of radiation death", like in the Simpsons, when in reality it's completely different. In this case, dull red (due to the heat) pills of toxic, not radioactive, death.

49 minutes ago, fredinno said:

How about RTGs for a Pressurized rover?

Also, RTGS can heat up the spacecraft, it might be a good idea to carry at least some to make sure nothing ever freezes up.

RHUs (Radioisotope Heating Units) are a different thing altogether, and they can greatly reduce power consumption for electronics for a very reasonable weight. Humans are such power-hungry things to keep alive in space, tough, that you are talking about using radiators instead (with proper insulation), just on account of all the waste heat we produce keeping ourselves running. But yeah, RHU's make more sense than RTG's for manned missions, not everything can be kept at operating temperature inside the air-conditioned cabin. Then again, nuclear material is so darned expensive, you might instead oversize the main electrical power source a bit and use electric heaters instead.

 

Rune. There's already enough need for RTGs for the outer system.

Edited January 18, 2016 by Rune

[fredinno]

3 hours ago, SomeGuy123 said:

If the casing doesn't break.  It's a liability and legal issue.  So, SpaceX could slap together a fuel cell system and basically launch it tomorrow with minimal approval.  Or go through years of reviews for an RTG casing.  Even NASA has to waste a lot of time in reviews and they are an arm of the government itself.

I'm talking about the exhaust of the fuel cells.  That's pure CO2/water. 

Although...recycling water and co2 from your crew to get more rocket fuel does have a certain elegance, does it not?

 

Oh... I didn't know that that was what you meant, sorry. Are there any space-rated CH4 Lox fuel cells?

3 hours ago, Rune said:

Of course, using the main tanks and tank residuals to power the fuel cells is a very nice bonus: little additional plumbing needed, and maximum efficiency, like ULA's IFV tech. Pretty nifty, actually, if you add stuff like self pressurization and the like, you might be talking about a reusable vehicle that can run on water, basically. Methane is a bit hard to come by on cismunar space...

And kudos for getting the radiation part right. People read "nuclear powersource" and they think "green glowing pills of radiation death", like in the Simpsons, when in reality it's completely different. In this case, dull red (due to the heat) pills of toxic, not radioactive, death.

RHUs (Radioisotope Heating Units) are a different thing altogether, and they can greatly reduce power consumption for electronics for a very reasonable weight. Humans are such power-hungry things to keep alive in space, tough, that you are talking about using radiators instead (with proper insulation), just on account of all the waste heat we produce keeping ourselves running. But yeah, RHU's make more sense than RTG's for manned missions, not everything can be kept at operating temperature inside the air-conditioned cabin. Then again, nuclear material is so darned expensive, you might instead oversize the main electrical power source a bit and use electric heaters instead.

 

Rune. There's already enough need for RTGs for the outer system.

"Methane is a bit hard to come by on cismunar space..."

And CO2 for methane can be made from CO2 from the astronauts, though no idea how you would get it out of the scrubbers...

We know H20 is on the Moon, though, so that's probably a better thing to make a reusable lander off of.

 

Isn't Am-241 ordered of magnitude cheaper than Pu-238?

Edited January 18, 2016 by fredinno

[SomeGuy123]

1 hour ago, fredinno said:

Oh... I didn't know that that was what you meant, sorry. Are there any space-rated CH4 Lox fuel cells?

"Methane is a bit hard to come by on cismunar space..."

And CO2 for methane can be made from CO2 from the astronauts, though no idea how you would get it out of the scrubbers...

We know H20 is on the Moon, though, so that's probably a better thing to make a reusable lander off of.

 

Isn't Am-241 ordered of magnitude cheaper than Pu-238?

I don't know if there are any "space rated" CH4 cells, but that's barely relevant.  The Dragon wasn't "space rated" or any of the pumps or tanks or engines or computers or any of that equipment until it was (almost all of SpaceX stuff is newly designed).  Mr. Musk's team of engineers could space rate one in a matter of months, it's conventional technology.  

In the far future when you can actually mine for water, sure, great.  The reason to use methane until then is this chart.  Also, methane is liquid at 111 kelvin, versus 20 kelvin for hydrogen, and it's also far, far, far easier to make it stay in it's tank.  

As for Am-241 being cheaper : there's also orders of magnitude difference in energy output per gram.  So...

Edited January 18, 2016 by SomeGuy123