Liquid fluorine has great potential as a rocket fuel; per the Encyclopedia Astronautica, LF2/LH2 has a specific impulse of 470 seconds. http://www.astronautix.com/l/lf2lh2.html Lithium/LF2/LH2 can get you over 500 seconds, but requires you to have molten lithium at over 450K stored near cryogenic liquid hydrogen.
Krypton difluoride (KrF2) is a compound with some interesting properties, aside from being a noble gas compound. Annoyingly, it breaks apart at temperatures above about 195K. More importantly for the purpose of using it as a rocket propellant, it is an incredibly strong oxidizing agent. In fact, it is a more powerful oxidizer than fluorine gas, a consequence of the extremely dissociation energy (delta Hf) of the krypton-fluorine bond (54 kJ/mol, vs 157 kJ/mol for F2, via https://labs.chem.ucsb.edu/zakarian/armen/11---bonddissociationenergy.pdf )
Hydrogen fluoride (HF), the product of burning hydrogen with fluoride compounds, has a bond dissociation energy of 568 kJ/mol. Modelling the combustion of KrF2 and H2, we get the following equation;
KrF2 + H2 = Kr + 2HF
-(2*55 + 436) = -2*569 + E
for a net energy production of 592 kJ/mol. Compared to 545 kJ/mol for hydrogen/fluorine combustion, this is slightly better. Also, it has the advantage that your fuel is at a higher temperature (195K for KrF2 vs. 85K for LF2), and KrF2 is more dense than liquid fluorine. Unfortunately, KrF2 is a solid with no known liquid phase, and as mentioned, is unstable above 195K.
A better option might be xenon fluorides. The xenon-fluorine bond has a bond energy of a mere 13 kJ/mol, and xenon compounds are much more understood than krypton compounds. Likely the best option is Xenon Hexafluoride (XeF6). Xenon hexafluoride melts at 322K, and boils at 348K. A fairly narrow temperature range, and unfortunately one that would require heating, but likely not insurmountable. (Sadly, I cannot find information on the liquid density of XeF6).
XeF6 would react with H2 according to the following formula.
XeF6 + 3H2 = Xe + 6HF
-(6*13+3*436) = -6*569 + E
for a net energy production of 2028 kJ/mol
Granted, this combination would almost certainly have lower specific impulse than LH2/LF2 due to the large size of the xenon atom (average mass of exhaust products is 35.8 vs. 19, so exhaust velocity would be roughly 37% less). Assume roughly 300 isp assuming flame temp is equal to LH2/LF2, possibly more if the flame temp for xenon hexafluoride is higher. However, xenon hexafluoride fuel would give a higher thrust, in addition to being more dense and making your first stage smaller. Although at that point, why not use kerolox and ditch liquid hydrogen altogether. Still, an interesting theoretical exercise, and I'd be keen to see data on xenon hexafluoride flame temps if anyone has it.
I've posted this on SomethingAwful before, but here's a repost for those of you who haven't given Lowtax tenbux. Warning: it's very long.
The Swedish original is called JA 37: pilot och system and is a transcript of a witness seminar held at the Stockholm Museum of Technology in 2007, with a lot of old geezers who had worked on the development of both the Viggen and other Saab aircraft.
A very brief glossary:
- FMV is the Defense Materiel Administration, which was (and is) the government authority responsible for buying and developing all kinds of equipment for the Swedish armed forces. At the time when the Viggen was being developed, FMV still had a lot of engineers in house, which was considered important since it was thought at that time that in order to buy or develop a good system you had to know how it worked.
- SRA is Svenska Radioaktiebolaget (Swedish Radio Company). Acquired by Ericsson in 1983.
- LM Ericsson is just Ericsson. The name comes from the full formal name of the company, "Telefonaktiebolaget L M Ericsson", which in turn is named after its founder Lars Magnus Ericsson.
- JA 37 is the fighter version of the Viggen.
- AJ 37 is the strike version, which predates the fighter version by about ten years (entered service around 1970; the JA 37 around 1980).
- 35, or aircraft 35 is the Draken.
- 37, or aircraft 37 is the Viggen airframe in general.
- 39, or aircraft 39, or JAS 39 is the Gripen.
My comments are in parentheses.
Please ask away if there's anything that seems unclear.
Plutonium, in addition to being radioactive and fissile, has some rather exotic physical properties that make it... shall we say, exciting, to work with.
One of these is that, like uranium, plutonium is pyrophoric. That is, it burns spontaneously on contact with air. The greater the surface area of the plutonium, the faster it burns. This makes the management of the metal shaving from any machining operations critical.
In addition, plutonium has six solid allotropes, and they vary wildly in density:
Finally, and unusually, plutonium contracts when it freezes.
It's rather odd stuff.
Well, scientists just announced that they now understand a bit more about why it does this bizarre shit.
Here's the paper, I don't claim to have understood anything past the abstract.