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?

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.

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.

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! |

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.

"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.

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! |

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.

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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
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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! |

"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.
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