Author Topic: BFR and NASAs Kilopower Reactor  (Read 20377 times)

Offline RoboGoofers

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Re: BFR and NASAs Kilopower Reactor
« Reply #20 on: 05/14/2018 03:26 pm »
Wind turbines are usually too inefficient on earth to be commercially viable without subsidies, and on mars the thinness of the atmospehre makes them ~170 times less effective than on earth.

What? Commercial wind farms are viable. Are you talking about small scale wind?

Offline niwax

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Re: BFR and NASAs Kilopower Reactor
« Reply #21 on: 05/14/2018 04:55 pm »
On the topic of energy sources that aren't the topic of this thread, what about not going via electricity at all? Electrolysis can be supported by high temperatures all the way up to pure thermal water splitting and there is definitely need for heat to melt ice. So anywhere between simple thermal solar collectors to concentrated solar power with mirrors could be used for part of the ISRU needs. In addition, the Sabatier reaction is exothermic so a useful heat cycle could recover quite a lot of spent energy and significantly increase efficiency.
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Offline Robotbeat

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Re: BFR and NASAs Kilopower Reactor
« Reply #22 on: 05/15/2018 02:38 am »
You know, a kite-based wind power system might actually be good enough for power on Mars, potentially providing complementary power that wouldn't be dramatically reduced with a dust storm.
https://kitepower.nl/tech/
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Offline speedevil

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Re: BFR and NASAs Kilopower Reactor
« Reply #23 on: 05/15/2018 10:01 am »
On the topic of energy sources that aren't the topic of this thread, what about not going via electricity at all? Electrolysis can be supported by high temperatures all the way up to pure thermal water splitting and there is definitely need for heat to melt ice. So anywhere between simple thermal solar collectors to concentrated solar power with mirrors could be used for part of the ISRU needs. In addition, the Sabatier reaction is exothermic so a useful heat cycle could recover quite a lot of spent energy and significantly increase efficiency.
Solar with mirrors raises all of the problems with solar panels, except that instead of 'try to get them right-side up' - you need to precisely align the panels - you need an active pointing system on each mirror.
Solar collectors that collect heat at 1000+C are not very simple.

Offline dglow

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Re: BFR and NASAs Kilopower Reactor
« Reply #24 on: 05/15/2018 11:55 am »
May I ask the basis for "500kW" cited upthread – from where did this figure originate? I've scanned a few other threads (e.g. 'powering Mars via eBay') that cite less than half this number for BFS propellant production, so I'm afraid I missed the work/logic that leads to 500kW. Thanks. :)

Offline speedevil

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Re: BFR and NASAs Kilopower Reactor
« Reply #25 on: 05/15/2018 12:30 pm »
May I ask the basis for "500kW" cited upthread – from where did this figure originate? I've scanned a few other threads (e.g. 'powering Mars via eBay') that cite less than half this number for BFS propellant production, so I'm afraid I missed the work/logic that leads to 500kW. Thanks. :)
500kW is about the average power on Mars you can get out of a BFS full of commercial semi-flexible mono-crystalline panels plus Tesla batteries. $25M is approximate cost of purchase and shipping to Mars on $130/kg lift. (this number assumes BFS reuse)

The exact number is of course not going to be exactly that, given the need for structure, but that structure is very lightweight, with storm winds equivalent to perhaps 10MPH on earth, ...

But it's of the order of magnitude you need to work out required costings for nuclear.

If solar is of a comparable or lower cost to nuclear (including shipping), and has no proliferation concerns, or concerns about political winds no longer backing you, then nuclear may be a hard sell to a project.

Once you're out past Jupiter, nuclear is pretty much unquestionably a winner.

Before then, even in the worst case (moon), given cheap enough launch (meeting the $5M/launch aspiration) - the benefits of reactors are questionable, if you assume that you can deploy solar panels.

(A 50kW battery capable of 20 days uptime costs 7 BFS launches or so to the moon with a empty tanker already around the moon)



Offline dglow

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Re: BFR and NASAs Kilopower Reactor
« Reply #26 on: 05/15/2018 12:58 pm »
I see. So 500kW is not a need-defined number but rather capacity-defined, as in, "here's how much solar power a BFS can carry", correct?

Offline speedevil

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Re: BFR and NASAs Kilopower Reactor
« Reply #27 on: 05/15/2018 01:52 pm »
I see. So 500kW is not a need-defined number but rather capacity-defined, as in, "here's how much solar power a BFS can carry", correct?

Quite.
With the obvious corollary that solar reduces proportionally in costs, to a degree, but even assuming a 500kWe reactor can be had for $25M, it's pretty clear a 1kWe one will never be $50K, in the foreseeable future.

The other way nuclear works is if you assume the solar panels cost is $500/W.
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Offline dglow

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Re: BFR and NASAs Kilopower Reactor
« Reply #28 on: 05/15/2018 02:58 pm »
Okay, a few points, questions, and thoughts:


1. This is a Kilopower thread.
While comparisons with alternatives are inevitable, a pure solar vs nuclear debate may belong elsewhere... though IANAM(oderator).


2. I've read comments, elsewhere, to the effect "a Kilopower will never produce X" or "you'd need too many Kilopowers" to produce X... but what is X? We don't necessarily need 500kWe, do we?

What do we need for ISRU propellant production for a single BFS? Assume that task, alone. Is the 200kWe cited in the 'eBay' thread sound?


3. This seems specious to me...

500kW is about the average power on Mars you can get out of a BFS full of commercial semi-flexible mono-crystalline panels plus Tesla batteries. $25M is approximate cost of purchase and shipping to Mars on $130/kg lift. (this number assumes BFS reuse)

The exact number is of course not going to be exactly that, given the need for structure, but that structure is very lightweight, with storm winds equivalent to perhaps 10MPH on earth, ...

when upthread we have this:

The atmosphere is 1% the pressure but 2% the density. And wind speeds are significantly greater (average 10m/s), which is relevant because wind /power/ scales as the wind speed CUBED, so if the wind speed is 3 or 4 times that on Earth, the wind power is actually the same. That all said, not a near term source of power.

The solar I see on this planet requires quite a bit of 'structure' relative to the panels themselves. Will Mars really be that different?


4. Regarding cost:

With the obvious corollary that solar reduces proportionally in costs, to a degree, but even assuming a 500kWe reactor can be had for $25M, it's pretty clear a 1kWe one will never be $50K, in the foreseeable future.

The other way nuclear works is if you assume the solar panels cost is $500/W.
A cosy relationship with a particular program might get you the first reactor free.
It may not get you the second.

Dollar cost occupies but one axis in a complex set of tradeoffs to be made.

Your argument that the cost of goods for solar will be less expensive seems overwhelming and carries the day. However, the same does not hold for cost of transport – at best the two, Kilopower and solar, appear to be a wash.

But dealing with 'cost of transport' in a pure $/kg manner is misleading. Everything occupying the payload bays of the first BFSs to Mars will accrue a much greater value in opportunity costs alone. Which is why 500kW of solar "because we can carry it" is not a sound way to approach this, IMO.
« Last Edit: 05/15/2018 03:08 pm by dglow »

Offline IRobot

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Re: BFR and NASAs Kilopower Reactor
« Reply #29 on: 05/15/2018 03:11 pm »
So NASA just released some press about the success of a new Kilopower fission reactor. Sounds like great news for space travel. Is this the sort of thing that SpaceX could use within the BFR? I realize they have these animations of fancy solar ray deployments, but the sun is not always available (on the surface of the moon for instance) and it would reduce complexity it seems too. For a ship like BFR, it seems like having a constant power supply for 10 years would be a perfect fit.

The figures given above are from memory about 1500kg for a 10kW electrical solution.

This is 7 watts a kilo or so, which is  very bad from a solar panel perspective in the inner solar system.
Juno, for examples panels get about 50W/kg around earth, and around half of that at Mars.
10kW means you need about (of poor solar cells) 40m^2 near earth, or 100m^2 near Mars.

Note that 10kW is only twice of the power of the Dragon solar arrays. You'd want four of these near Mars of course.
You are forgetting that the BFS needs all the extra weight and complicated moving parts to deploy and retract the solar panels.
An internal power source reduces this risk. Also power does not scale linearly with weight for the Kilopower.

But I agree that this goes against the concept of BFS, as it is supposed to land on Earth again. Too much risk at the moment to have a nuclear power source going up and down the atmosphere over and over.

Offline dglow

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Re: BFR and NASAs Kilopower Reactor
« Reply #30 on: 05/15/2018 03:21 pm »
But I agree that this goes against the concept of BFS, as it is supposed to land on Earth again. Too much risk at the moment to have a nuclear power source going up and down the atmosphere over and over.

Why would BFS return to Earth with its Kilopowers? They're needed on Mars. Leave them there.

Offline johnfwhitesell

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Re: BFR and NASAs Kilopower Reactor
« Reply #31 on: 05/15/2018 03:49 pm »
Burning fuel to generate electricity to generate that fuel.. just wasting energy for the inefficiencies of the process, also makes no sense.

Nuclear conversation in four steps:
1) Nuke guy: Energy smoothing is indispensable so we need nuclear
2) Solar guy: Well the fuel systems can provide that energy smoothing easily if you just burn the fuel
3) Some other guy: Why on mars would you burn the fuel?
4) Solar guy: Hey, does anyone know how to repair a computer keyboard after I smashed it with my face?

when upthread we have this:

Wind pressure (i.e. stress on a strut) follows a square law.  Wind power (i.e. turbine output) follows a cube law.
« Last Edit: 05/15/2018 03:53 pm by johnfwhitesell »

Offline RoboGoofers

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Re: BFR and NASAs Kilopower Reactor
« Reply #32 on: 05/15/2018 09:48 pm »
The solar I see on this planet requires quite a bit of 'structure' relative to the panels themselves. Will Mars really be that different?

it doesn't need much, certainly not as much as on earth. most of the support structure on earth is to keep the panels from blowing away, as well as adjust the panels to an adequate angle. you don't have to worry about them blowing away on Mars, so they only need something as simple as a kickstand. if there's any worry about them moving, staking them down would be sufficient.


Offline guckyfan

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Re: BFR and NASAs Kilopower Reactor
« Reply #33 on: 05/16/2018 02:23 am »
The solar I see on this planet requires quite a bit of 'structure' relative to the panels themselves. Will Mars really be that different?

it doesn't need much, certainly not as much as on earth. most of the support structure on earth is to keep the panels from blowing away, as well as adjust the panels to an adequate angle. you don't have to worry about them blowing away on Mars, so they only need something as simple as a kickstand. if there's any worry about them moving, staking them down would be sufficient.

There's also no rain or hail or snow to worry about. Panels can be much more lightweight, so a lot less structure.

Offline Robotbeat

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Re: BFR and NASAs Kilopower Reactor
« Reply #34 on: 05/16/2018 03:12 am »
Kilopower still needs a deployable radiator to dump heat.
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Offline johnfwhitesell

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Re: BFR and NASAs Kilopower Reactor
« Reply #35 on: 05/16/2018 04:19 am »
Kilopower still needs a deployable radiator to dump heat.

I believe the thinking was that on Mars your problem is getting too cold not too hot.

Offline speedevil

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Re: BFR and NASAs Kilopower Reactor
« Reply #36 on: 05/16/2018 10:12 am »
Kilopower still needs a deployable radiator to dump heat.

I believe the thinking was that on Mars your problem is getting too cold not too hot.
Even with only earthly insulation, and not bothering with vacuum insulation, cold isn't really a problem - with BFR.

50kg of commercial cheap building insulation insulates 2.4m*2.4m of ceiling and floor to 10W/m^2, to the point that the waste heat from one human will warm it to comfortable temperatures assuming -60C, and only a hundred watts of heating takes you to the point that you're OK with the coldest recorded temperature on Mars.


Offline Nomadd

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Re: BFR and NASAs Kilopower Reactor
« Reply #37 on: 08/24/2018 06:50 pm »
 Just sat through a kilopower presentation by Dave Poston and it was pretty impressive. The fission is a low enough level that it could still be producing power in 200 years. The main thing limiting lifespan should be the Stirlings and they're making those easily replaceable. Testing is pretty well along and the main thing to do is wade throught the paperwork/permit maze.
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Offline john smith 19

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Re: BFR and NASAs Kilopower Reactor
« Reply #38 on: 08/24/2018 10:19 pm »
Just sat through a kilopower presentation by Dave Poston and it was pretty impressive. The fission is a low enough level that it could still be producing power in 200 years. The main thing limiting lifespan should be the Stirlings and they're making those easily replaceable. Testing is pretty well along and the main thing to do is wade throught the paperwork/permit maze.
Don't underestimate the size of that maze.  :(

Of the many achievements of the Kilopwer programme one of the most amazing is they have managed to negotiate the hugely complex modern H&S environment on (by nuclear standards) a pittance of funding.

This was vital as AFAIK the old SNAP 10 design the US last flew in 1965 would simply be viewed as a massive H&S fail before it was even built (by modern standards).
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Offline john smith 19

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Re: BFR and NASAs Kilopower Reactor
« Reply #39 on: 08/24/2018 10:26 pm »
2. I've read comments, elsewhere, to the effect "a Kilopower will never produce X" or "you'd need too many Kilopowers" to produce X... but what is X? We don't necessarily need 500kWe, do we?

What do we need for ISRU propellant production for a single BFS? Assume that task, alone. Is the 200kWe cited in the 'eBay' thread sound?
The big energy consumer is ISRU mfg of BFS propellant for a single BFS.
Estimates on another thread have this as 16GWhrs. Roughly 860Kw 24 hrs a day over 26x  30 day months

No a single Kilopower won't cut it. 

But the Kilpower architecture is a) Designed to be clustered and b) Individual units are designed to scale from 1Kw to at least 100Kw and (IIRC) 1MW.

So 10 100Kw units could be widely spaced to eliminate point failure (Mars is hit by c395 objects a year that add craters that are detectable from obit (IE 5m in dia IIRC).
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