Chemical Reaction Energy Release and Propellants

From: Robert J. Bradbury (
Date: Sat Feb 05 2000 - 12:04:00 MST

> Sarcasm mode on... >

Jeepers, I can't believe this group, how come you don't all have
Physical Chemistry books on your desk? I'm surprised you can even
survive in this world... After all its the basis for life itself...

... I wounder if I should move over to sci.chemistry, where the
*real men* hang out...

> Sarcasm mode lowered somewhat... >

And then what is it with these lazy asses, god it seems like they
are multiplying. Aren't they supposed to be sterile? Even if not,
shouldn't there be some selection affect against their long term

> Sarcasm mode off... >

Reaction heats of fusion:
    Reaction DeltaHf (kJ/mole)
  H2 + F2 --> 2HF -271 (*2)
  2H2 + O2 -> 2H2O -286 (*2)
  C + O2 --> CO2 -394
  4Fe + 3O2 --> 2Fe2O3 -824 (*2)
  3Fe + 2O2 --> Fe3O4 -1184
  2Ca + 2C + 3O2 --> CaCO3 -1207 (*2)
  2Al + 3O2 --> Al2O3 -1675
  12C + 11H2O --> sucrose -2222
  4P + 5O2 --> P4O10 -2984

Obviously its difficult to make some of these reactions happen
exactly as outlined, but I think they make clear a point.
The most energy gets generated when you collapse the greatest
number of molecules into most highly constrained compound.
I.e. most of the energy isn't in the bonds but in the motion
of individual molecules (i.e. the entropy). Ideally what you
want is something that would take two gases and generate
a solid (since the energy in the molecular motion of the
gases has to be released). Lacking that, the Fe/Al reactions
seem nice since one of the reactants is a gas and the result
is a solid.

Of course if you want a rocket fuel, you might want something
else unless you can construct an engine that expels the energy
only as photons or accelerates sapphire pellets. Wouldn't want
to stand in the exhaust though... :-)

Now, getting back to the atomic hydrogen question, there is
a ref on the net:
But unfortunately it doesn't provide much detail.

Going through my files I find:
  "Electric Propulsion: A Far Reaching Technology", Graeme Aston,
  JBIS 39:503-507 (1986)

  Table 1. Candidate Space Propulsion Energy Sources
  Type Energy Release (J/kg)
 Chemical (O2/H2) 1.5 x 10^7
 Free Radical (H+H->H2) 2.2 x 10^8 { == "atomic fuel" }
 Nuclear Fision 8.2 x 10^13
 Nuclear Fusion 3.9 x 10^14
 Matter-Antimatter Annihilation 6.3 x 10^16

So the Atmomic Hydrogen in He (current NASA work) does provide
significant benefits but manufacuturing this propellant is likely
to be quite expensive I would presume. The value outlined in the
table is presumably the theoretical limit and you have to discount
it if you are "freezing" atomic H in He by the H/He ratio. If you've
got energy to burn (or atomic assembly), it might be a very nice fuel,
particularly for things like "deceleration" in remote star systems.


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