Talk:Rocket fuel
Chemistry discussion
[edit]I'm using this page to prototype a chemistry section before I insert it into the article. Feel free to edit. Iain McClatchie 23:45, 27 Jan 2005 (UTC)
Before discussing the sometimes complicated chemistry of rocket propellants, it's worthwhile discussing the simpler chemistry of rocket exhaust.
An ideal chemical rocket exhaust is composed of low mass, tightly bound simple molecules. Tightly bound, so that a great deal of energy is liberated in their formation. Low mass, so that the energy/mass ratio is high, so that the particle velocity and eventually exhaust velocity is high. Simple (ideally diatomic), so that energy stored in their vibrational and rotational states can be quickly converted to velocity in the nozzle (see mixture ratio, below).
The lightest chemically active elements by approximate atomic mass are Hydrogen(1.01), Lithium(6.94), Beryllium(9.01), Boron(10.81), Carbon(12.01), Nitrogen(14.01), Oxygen(16.00), and Fluorine(19.00). After that come Sodium(22.99), Magnesium(24.31), and Aluminum(26.98). We are searching, then, for tightly bound pairs of these elements.
There are not very many candidates.
- H2 is one of the best molecules from a mass perspective, but propellants (e.g. hydrazines) tend not to energetically liberate H2. It is usually included in exhaust because it helps quickly convert combustion chamber heat into velocity through the nozzle.
- CO and N2 are the most common diatomic molecules in rocket exhaust.
- HF and LiF liberate appealing amounts of energy, but HF is a toxic acid, flourinated propellants are very dangerous, and lithium is rare.
- BeF doesn't liberate enough energy to compete with hydrocarbons, and Beryllium is very toxic.
Some larger molecules allow the release of larger amounts of energy. Most rocket engines liberate their energy from the formation of H2O and CO2. Solid and hybrid rocket motors can use metals as fuel: Al2O3 is a terrible exhaust component (it comes out as an abrasive solid), but liberates so much heat that it is common in solid rocket exhaust streams.
| exhaust species | enthalpy of formation (0 K) in KJ/mol | molar mass in g |
| H2 | 0 | 2.016 |
| CO | -113.8 | 28.01 |
| CO2 | -393.1 | 44.011 |
| H2O | -238.9 | 18.02 |
| N2 | 0 | 28.013 |
| HF | -273.3 | 20.01 |
| CF4 | -927.2 | 88.01 |
| Al2O3 | -1675.7 | 101.96 |
| OH | 39.1 | 17.0 |
| H | 216.0 | 1.0 |
| C | 711.2 | 12.011 |
Exotics:
| exhaust species | enthalpy of formation (0 K) in KJ/mol | molar mass in g |
| LiF | -340.6 | 25.94 |
| AlF | -265.6 | 45.98 |
| AlF3 | -1205.6 | 83.98 |
| BeF | -172.2 | 28.01 |
| B2F4 | -1435.6 | |
| BF3 | -1133.2 | 67.81 |
Propellants
[edit]Storage temp, density, stable?, corrosive?, toxic?, hypergolic?
| propellant species | enthalpy of formation (0 K) in KJ/mol | molar mass in g |
| O2 | 0 | 32.00 |
| F2 | 0 | 38 |
| H2 | 0 | 2.016 |
| H2O2 | -129.9 | 34.01 |
| CH4 | -66.6 | 16.04 |
| C3H8 | -82.4 | 44.11 |
| N2O4 | 20.4 | 92.013 |
| N2H4 | 109.3 | 32.045 |
| Al2 | 500.2 | 53.96 |
Exotics:
| propellant species | enthalpy of formation (0 K) in KJ/mol | molar mass in g |
| O3 | 144.5 | 47.99 |
| LiH | 140.8 | 7.95 |
| Li2 | 215.5 | 13.88 |
| AlH3 | 135.7 | 30.00 |
- Check Al2O3 enthalpy -- may be high
2AlH3 + 2O2 => 2Al2O3 + 3H2O + 4339.5 (156) = 16.8 KJ/g
BH3 + 3F2 => BF3 + 3HF + 2063.6 (127.84) = 16.1 KJ/g
AlH3 + 3F2 => AlF3 + 3HF + 2161.2 (141) = 15.3 KJ/g
H2 + F2 => 2HF + 546.6 = 14.38 KJ/g
LiH + F2 => LiF + HF + 613.9 (44.95) = 13.6 KJ/g
2H2 + O2 => 2 H2O + 477.8 (36.02) = 13.26 KJ/g
Li + 0.5F2 => LiF + 340.6 (25.94) = 13.13 KJ/g
Plagarism
[edit]Is there no Wikipedia policy about flat-out plagiarism -- please at least gesture at giving people credit? The last paragraph ("The highest specific impulse... effect on the environment") is taken almost verbatim from Henry Spencer's post at [1]. -- 69.0.51.208
I'm the one that put Henry's post up there. I would love to give credit, but I don't see how to do that with the current Wiki style. And since it's not a direct quote (I've added a bit), I can't format it as a quote. Finally, a quote would be wrong here. Why don't you have a shot at rewriting the paragraph to avoid plagarism but capture the message? Iain McClatchie 23:23, 8 Feb 2005 (UTC)
Nevermind, I did it myself. Iain McClatchie 00:55, 9 Feb 2005 (UTC)
Add for Nuclear propulsion, ion propulsion, etc.
- You could easily ask Henry; I seriously doubt he'd mind. His email address is: henry@zoo.utoronto.ca WolfKeeper 00:40, 6 February 2006 (UTC)
Solid propellants
[edit]Is there any evidence of white powder being used as a propellant? That recipe is also known as flash powder, and has a *very* high potential for detonation when enclosed. AFAIK, older solid propellants consist mostly of gunpowder, eventually replaced by double-base propellants and composite propellants (the first being GALCIT-60, IIRC). Amatuer rocketeers also used Zn-S powders, but I don't think flash powders see use except in fireworks for strobe effects. Evand 17:34, 24 January 2006 (UTC)
Equation
[edit]Umm, the equation given for the theoretical exhaust velocity seems very optimistic. I'm assuming that Tc is the chamber temperature and it gives a exhaust velocity three hundred times less than I found using Ve = SQRT[ (2 x k / (k - 1)) x (R' x Tc / M) x (1 - (Pe / Pc)(k-1)/k) ] 24.137.78.34 23:43, 21 February 2006 (UTC)