Roger stokes 2007-08-30 23:40:35
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?
Christopher m. 2007-08-31 18:05:24
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.
Ian woollard 2007-08-31 18:05:29
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.
Azt28@aol.com 2007-09-01 06:42:17
Yes, Isp is a function of the molecular mass, the smaller the mass the larger
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
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.
Henry@spsystem 2007-09-01 06:42:23
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
But it would still be hideously expensive to little purpose.
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