Author Topic: Weight Savings Using Light Metal Isotopes  (Read 2455 times)

Offline RobLynn

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Weight Savings Using Light Metal Isotopes
« on: 05/16/2012 11:07 am »
Weight saving is worth about $500/kg for airliners, on the order of $5000/kg for fighters and rockets and up to $50000/kg for some space payloads.  Eg see page 5:http://eagar.mit.edu/EagarPapers/Eagar192.pdf

There are a number of important aerospace materials/elements in which the average Isotopic mass is significantly higher than the lowest stable isotopic mass:
   average mass   lightest stable   weight saving   abundance of lightest
Lithium   6.941   6.015   13.3%   7.5%
Magnesium   24.305   23.985   1.3%   79.0%
Titanium   47.867   45.952   4.0%   8.0%
Chromium   51.996   49.946   3.9%   4.3%
Iron   55.845   53.939   3.4%   5.8%
Nickel   58.693   57.935   1.3%   68.1%
Copper   63.546   62.929   1.0%   69.2%
Zirconium   91.224   89.904   1.4%   51.5%
Molybdenum   95.96   91.906   4.2%   14.8%
Tungsten   183.84   181.948   1.0%   26.5%

Isotopic Separation via gas centrifuge can be a relatively cheap process, (costing $10's-100's/kg in large scale), particularly when there are large mass ratio differences and the metal can be turned into a gas easily at low temperature (eg Nickel or Iron Carbonyl, Uranium Hexafluoride, Titanium Tetrachloride).  The newly developed Silex laser enrichment process is apparently as little as 10-30% the cost of gas centrifuges.

Unlike Uranium processing the rejected heavy isotopes will be worth just the same in the open market for weight insensitive applications.

As an example enriching Uranium to 4% U235:U238 from 0.7% natural costs as little as $200/kghttp://web.mit.edu/stgs/pdfs/RothwellBraunEnrichment.pdf, (though $700/kg is typical spot market price set by older less efficient machines in what is a fairly uncompetitive market where enriched fuel costs are dominated by other factors), even though the mass difference for U235:U238 is a relatively small 0.85% in hexafluoride form.  Their big cost comes from the discarded depleted Uranium tailings that are about 10x the mass of the enriched product.

For each application depending on value of weight saving there will be an economic optimum degree of enrichment for the low mass isotope, but some degree of enrichment is likely to be worthwhile for a large number of aerospace applications.

For Rotomachinery blades and disks the savings are greatly enhanced by the further mass savings possible in disks, shafts, bearings and containment structures.  Material weight savings in other areas can also lead to slight additional structural weight savings due to lower inertial loadings.

Titanium enrichment in particular is almost certainly worthwhile for at least fighter aircraft and spacecraft.  In the F35 there is about 3300kg of titanium, reducing Ti mass by 4% using the lightweight Ti46 could save 130kg, and probably more with added weight savings in the engine.  Key superalloy constituents Iron54, Nickel58, Molybenum92 and Chromium50 could also save about 1-3% of their component mass - perhaps another 50-100kg of savings in an F35.

On the A350 and 787 both with about 16,000kg Titanium going to Ti46 could save 650kg, though economics may not favour as much enrichment

Hi End motorsports will probably also be interested for titanium conrods as well as Fe, Ni, Cr and Mo crankshafts, wrist pins, valves, valve springs and other driveline components.

Lithium6 in AlLi alloys would be another easy gain, and may be available already from the nuclear industry.  For example on a Shuttle SLWT (assuming 20,000kg of AlLi at 2% Li) it would save on the order of 50kg from the 400kg of Li.  It would save a similar amount in an A350 which uses AlLi extensively.
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Offline DMeader

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Re: Weight Savings Using Light Metal Isotopes
« Reply #1 on: 05/16/2012 11:19 am »
Not worth the trouble. Easier to save weight by optimizing systems and structure and simply using lighter materials where possible.

I suspect you seriously underestimate the cost of this.

Offline go4mars

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Re: Weight Savings Using Light Metal Isotopes
« Reply #2 on: 05/16/2012 11:43 am »
Isotopic Separation via gas centrifuge can be a relatively cheap process, (costing $10's-100's/kg in large scale), particularly when there are large mass ratio differences and the metal can be turned into a gas easily at low temperature (eg Nickel or Iron Carbonyl, Uranium Hexafluoride, Titanium Tetrachloride).  The newly developed Silex laser enrichment process is apparently as little as 10-30% the cost of gas centrifuges.
Truly an interesting thought RobLynn.  Do you have any links that describe separation processes and/or the companies that offer this?
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Offline RobLynn

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Re: Weight Savings Using Light Metal Isotopes
« Reply #3 on: 05/16/2012 01:15 pm »
Not worth the trouble. Easier to save weight by optimizing systems and structure and simply using lighter materials where possible.

I suspect you seriously underestimate the cost of this.

You obviously optimise systems, structure and materials anyway.  But as with everything there comes a point where you reach diminishing returns - costs outweigh savings and it becomes uneconomic to go any further.  At that point if lightweight isotopes are cheap enough to fit in with your weight saving economic cost target (eg $500/kg for commercial jets) then you are sensible to do that too.
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Offline Jim

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Re: Weight Savings Using Light Metal Isotopes
« Reply #4 on: 05/16/2012 01:19 pm »
At that point if lightweight isotopes are cheap enough to fit in with your weight saving economic cost target (eg $500/kg for commercial jets) then you are sensible to do that too.

Never.
composites take the place of metals.

Offline kevin-rf

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Re: Weight Savings Using Light Metal Isotopes
« Reply #5 on: 05/16/2012 01:40 pm »
Well then, isotopic separation of Carbon 12 from Carbon 13 it is ;)

(for the humor impaired, ~99 of the carbon is Carbon 12, so you are only saving about 10% of 1%, eer 0.1% on the weight)
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Offline RobLynn

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Re: Weight Savings Using Light Metal Isotopes
« Reply #6 on: 05/16/2012 02:07 pm »
  Do you have any links that describe separation processes and/or the companies that offer this?

The only large scale isotopic separation that I am aware of is Uranium and Deuterium for nuclear applications.  There are relatively tiny amounts of many others done too, but this is an industry where economy of scales make a huge difference so I am going off Uranium processing numbers.

I've just done some calculations on separating Titanium.  Part of the normal Kroll Titanium purification process involves creation of TiCl4, which (usefully) boils at 136°C and so is probably a good choice for gas centrifuging.  It also means that the centrifuging doesn't add any other chemical processing costs to the production of the Titanium.

You can find some of the math needed at:
http://en.wikipedia.org/wiki/Enriched_uranium
But be aware an SWU for U235:U238 separation is different from an SWU for Ti46 separation, basically need to multiply the Ti SWU by about 0.71 to get equivalent U SWU.

I am assuming just two isotopes Ti46 and a Ti47.9, instead of whole range, it's not totally accurate, but a reasonable simplification to give a quick estimate.

In the jargon of uranium separation enriching the Ti46 from 8% (natural) to 95% takes about 15.5 Separative work units or SWU, (11 if converted to uranium SWU).  Enriching to just 50% Ti46 takes 3.7 Uranium equivalent SWU and to 25% just 1 Uranium equivalent SWU.  For comparison enriching 0.7% U235 to 5% U235 takes about 9 SWUs.

Now the lowest cost Uranium processing centrifuges cost about $30/SWU so for 11 SWU it should cost about $330/kg extra to create 95% Ti46.  For 50% Ti46 about $100/kg extra, and for 25% Ti46 about $30/kg extra.  Perhaps a lot cheaper with laser enrichment.

So as a rough estimate you can drop the density of Ti by 1% for $30/kg, 2% for $100/kg and almost 4% for $330/kg once you are working in large volume.

That is cheap enough to make it useful for commercial aviation (at some point along the enrichment - cost curve) and definitely cheap enough for high enrichment to be a no-brainer for many military and space applications.

Molybdenum enrichment is likely to be cheaper than Ti, Iron is likely to be more a bit more expensive, and other elements mentioned in earlier post mostly more expensive as well.
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Offline DMeader

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Re: Weight Savings Using Light Metal Isotopes
« Reply #7 on: 05/16/2012 02:22 pm »
One problem I see is that you are going to great trouble and expense to save small amounts of weight in materials that you use in relatively small amounts. Is not most aerospace materials usage aluminum and (more and more) composites? On top of that, to reach your weight targets per element I assume you are figuring on 100% separation. Probably the greater the percentage of enrichment the cost goes up exponentially.

The way I see it, in a situation where you need perhaps tens or hundreds of kilograms of material and there simply is no other way to reach your goal with anything else (nuclear materials), it makes sense. To make an aircraft or spacecraft a few per cent lighter, it doesn't.

Offline RobLynn

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Re: Weight Savings Using Light Metal Isotopes
« Reply #8 on: 05/16/2012 02:57 pm »
Just realised that I have been looking at the cost saving numbers wrong.     

25% Ti46 at $30/kg saves 1% weight for $3000/kg saved
50% Ti46 at $100/kg saves 2% weight for $5000/kg saved.
95% Ti46 at $330/kg saves 4% weight for $7500/kg saved.

So probably useful for fighters and spacecraft but not for commercial aircraft excepting possibly the engines.
« Last Edit: 05/16/2012 02:58 pm by RobLynn »
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Offline Jim

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Re: Weight Savings Using Light Metal Isotopes
« Reply #9 on: 05/16/2012 03:04 pm »
Just realised that I have been looking at the cost saving numbers wrong.     

25% Ti46 at $30/kg saves 1% weight for $3000/kg saved
50% Ti46 at $100/kg saves 2% weight for $5000/kg saved.
95% Ti46 at $330/kg saves 4% weight for $7500/kg saved.

So probably useful for fighters and spacecraft but not for commercial aircraft excepting possibly the engines.

not fighters or spacecraft either.  They use composites or aluminum too.

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