#1
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Isotopes and Isp
Does the Isp of a propellant change if you change the isotopes of (say)
Oxygen, Carbon, or Hydrogen in the constituent molecules? Is there anything to be gained from this? |
#2
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Isotopes and Isp
Roger Stokes wrote:
Does the Isp of a propellant change if you change the isotopes of (say) Oxygen, Carbon, or Hydrogen in the constituent molecules? Is there anything to be gained from this? The Isp is inversely proportional to the square root of the average molecular weight of the gas in a thermal rocket, so changing the isotopes would change the Isp. The most common isotopes of the light elements used in propellants are the lightest stable isotopes(*), though, so there is little to be gained. Separating the isotopes would also be very expensive. Paul (*) except for lithium. |
#3
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Isotopes and Isp
Does the Isp of a propellant change if you change the isotopes of (say) Oxygen, Carbon, or Hydrogen in the constituent molecules? Is there anything to be gained from this? Yes, Isp is a function of the molecular mass, the smaller the mass the larger the Isp. Think for example about water H2O, The mass is 2 x 1 + 16 = 18. If you use deuterium, the molecular mass becomes : 2 x 2 + 16 = 20 and the Isp goes down. The same for heavy oxygen: O17 or O18. So the ordinary isotopes are the best. The same is true when burning carbon: CO2 has the smallest mass with common oxygen and carbon 12. Yvan Bozzonetti. |
#4
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Isotopes and Isp
In article ,
Roger Stokes wrote: Does the Isp of a propellant change if you change the isotopes of (say) Oxygen, Carbon, or Hydrogen in the constituent molecules? Is there anything to be gained from this? Yes, it would change a little bit -- same reactions, same energy release, almost identical gas properties, but a different mass. Unfortunately, to get a *useful* change, you want lighter isotopes... and for all the common propellant elements -- hydrogen, carbon, nitrogen, oxygen -- the dominant natural isotope is also the lightest stable one. (In fact, the lighter ones not only are unstable, but they all have very short half-lives.) So you could *reduce* performance a little, at considerable cost (isotope separation is not cheap for hydrogen and it's rather worse for heavier elements), but not increase it. -- MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer first ground-station pass 1651, all nominal! | |
#5
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Isotopes and Isp
Henry Spencer wrote:
So you could *reduce* performance a little, at considerable cost (isotope separation is not cheap for hydrogen and it's rather worse for heavier elements), but not increase it. Of course he could always add certain isotopes, say isotopes of plutonium, and get quite noticeably hotter reactions, which could certainly improve performance, atleast in theory. |
#6
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Isotopes and Isp
"Henry Spencer" wrote:
Yes, it would change a little bit -- same reactions, same energy release, almost identical gas properties, but a different mass. Unfortunately, to get a *useful* change, you want lighter isotopes... and for all the common propellant elements -- hydrogen, carbon, nitrogen, oxygen -- the dominant natural isotope is also the lightest stable one. (In fact, the lighter ones not only are unstable, but they all have very short half-lives.) So you could *reduce* performance a little, at considerable cost (isotope separation is not cheap for hydrogen and it's rather worse for heavier elements), but not increase it. Quite. Deuterated chemicals are about the cheapest to come by and they are not cheap at all, nor available in the kind of bulk needed for rocket propellant either (unless I miss my mark by several orders of magnitude). And C-13 enriched chemicals cost an arm and a leg. Though I think Deuterium itself *might* be available cheaply enough and in large enough quantities to *maybe* be workable as a propellant. Interestingly, I thought, if you run the numbers with LOX/LH2 vs LOX/LD2, the increased density of the fuel doesn't buy you a whole lot mass fraction wise. Which is actually pretty obvious now that I think about it. On the one hand you get a denser fuel (which is good for dry mass fraction), but on the other hand it's still not nearly as dense as LOX or Kerosene (by a long shot) and you're going to have to use twice as much of it (by mass), so it balances out more or less. Except you also lose Isp in the bargain, and that doesn't help. Plus, of course, buying tens of tonnes of Deuterium is likely to take a monstrous chunk out of your wallet and bring some uncomfortable scrutiny from your local "deparment of making sure you aren't building nuclear weapons, or helping others do so". More to the point, if you need to boost the average molecular weight of the exhaust of a LOX/LH2 rocket you can just adjust the fuel/oxidizer ratio. |
#7
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Isotopes and Isp
In article ,
Christopher M. Jones wrote: Quite. Deuterated chemicals are about the cheapest to come by and they are not cheap at all, nor available in the kind of bulk needed for rocket propellant either (unless I miss my mark by several orders of magnitude). Heavy water, from which you can make the others (in principle) is available by the tens of tons, if you have the budget and can deal with the bureaucracy (since it is useful for building nuclear reactors, and hence bulk purchases will get scrutinized by both security people and politicians). And both price and availability would probably improve for this application, since you'd be more tolerant of moderately impure deuterium. But it would still be hideously expensive to little purpose. -- MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer first ground-station pass 1651, all nominal! | |
#8
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Isotopes and Isp
Deuterium is far more common in the ocean than helium in the atmosphere.
Something like 0.014% of all hydrogen in deuterium. whereas helium makes up only 0.0005% of the atmosphere. Although helium is a major component in hydrogen bombs along with lithium I believe. -- "Yea, all israel have transgressed thy law, even by departing, that they might not obey thy voice; therefore the curse is poured upon us, and the oath that is written in the law of Moses the servant of God, because we have sinned against him." Daniel 9-11 |
#9
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Isotopes and Isp
In article ,
Anthony Q. Bachler wrote: Something like 0.014% of all hydrogen in deuterium. whereas helium makes up only 0.0005% of the atmosphere. Although helium is a major component in hydrogen bombs along with lithium I believe. There's no significant helium in nuclear weapons (as far as is known publicly, anyway). The fusion fuel is lithium deuteride, i.e. lithium hydride made with deuterium, possibly with isotope separation of the lithium as well. Small amounts of deuterium and tritium are reportedly used in "boosted" fission bombs -- which use a small fusion component to supply extra neutrons for the fission -- but helium is present only as a contaminant. (Tritium decays to helium-3, which is very much unwanted because it's a strong neutron absorber, so the tritium has to be purified occasionally.) -- MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer first ground-station pass 1651, all nominal! | |
#10
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Isotopes and Isp
"Henry Spencer" wrote in message
... (Tritium decays to helium-3, which is very much unwanted because it's a strong neutron absorber, so the tritium has to be purified occasionally.) In the book and audio version of The Sum of All Fears (which has almost nothing at all to do with the movie of the same name), the effects of helium in the mix is well described. It's also an example of why you don't shoot the engineer until after you use the weapon. -- If you have had problems with Illinois Student Assistance Commission (ISAC), please contact shredder at bellsouth dot net. There may be a class-action lawsuit in the works. |
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