Adding to what ka9q said about hypergols:
After WW2 it was believed that ballistic missiles would be a big part of future warfare, so research was conducted to identify the most likely propellant combinations.
For long range missile everybody agreed that LOX was the optimum oxidizer. For the fuel, liquid hydrogen was obviously the best performing, but it was hard to come by, hard to handle, and had an extremely low density. Alcohol, gasoline and kerosene all worked pretty well and were easier to deal with. Kerosene would win out, but new clean-burning formulations had to be developed. And, in time, the US perfected the use of liquid hydrogen.
For uses such as JATO and short-range tactic missiles, storable propellants were desired. Available oxidizers were nitric acid, hydrogen peroxide, and nitrogen tetroxide. Hydrogen peroxide and nitrogen tetroxide both froze at temperatures likely to be experienced at high altitude or in Siberia, thus nitric acid was the leading candidate. Which fuel to use was much less clean-cut. There were many candidates that all had similar performance. Only hydrazine stood out - it was hypergolic with the prospective oxidizers, it had a high density, and its performance was better. But its freezing point (1.5o C) was higher than water.
Prior US experience was with aniline and RFNA (red fuming nitric acid). This stuff was extremely nasty and dangerous. RFNA was so corrosive that is had to be loaded just before firing. It produces dangerous and painful burns, and simply pouring it produces dense clouds of highly poisonous nitrogen dioxide. Aniline was almost as bad. Everyone agreed a better alternative was needed.
Considerable research and experimentation ensued. The first breakthrough came in 1951 when contracts were granted to Metallectro and Aerojet to synthesize certain derivatives of hydrazine and to determine their suitability as rocket propellants. The three derivatives were monomethylhydrazine, symmetrical dimethyl hydrazine, and unsymmetrical dimethyl hydrazine. The hope was that a very slight alteration to the structure might give it a reasonable freezing point without changing the energetics enough to matter.
Symmetrical dimethyl hydrazine turned out to have too high a freezing point, but the freezing points of monomethylhydrazine (MMH) and unsymmetrical dimethyl hydrazine (UDMH) were -52.4o C and -57.2o C. After determining their thermodynamic properties, MMH and UDMH were both found to be very good fuels. MMH was a little denser than UDMH and had slightly higher performance. But UDMH was less liable to catalytic decomposition and had such good thermal stability that it could be used for regenerative cooling. UDMH was also more soluble and would tolerate a large percentage of water in the fuel. Both were hypergolic with nitric acid. The final decision to concentrate on UDMH was made on economic grounds. Military specifications for UDMH were first published in 1955.
RFNA was largely hated by all who used it. Not only for the reasons previously given, but also in cold weather it would react with its aluminum storage drum to produce a slimy white precipitate that would settle to the bottom of the drum. If this sludge got into a rocket engine it would plug up the injectors. If stored in a stainless steel drum the results were even worse - the corrosion was faster and the acid's performance was seriously degraded.
Mixed acids and WFNA (white fuming nitric acid) were tried, but they weren't much better than RFNA, though at least they didn't produce the deadly clouds of nitrogen dioxide.
The fourth possibility was nitrogen tetroxide. Although it is poisonous, if you can avoid handling it in the field, this doesn't matter much. As long as nitrogen tetroxide is kept out of water it is practically noncorrosive to most metals. In fact, ordinary mild steel would do. Thus the tanks of a missile could be filled at the factory and the operators would never have to see, smell, or breathe the nitrogen tetroxide. And it was perfectly stable in storage and didn't build up any pressure. The only problem was that it froze at -9.3o C, which the military would not accept.
A major breakthrough came in 1951 when a chemist named Eric Rau thought that a coating of fluoride on stainless steel might protect it from the acid. It was found that adding 0.5% hydrofluoric acid to WFNA reduced the corrosion rate of steel by a factor of 10 or more. As it turned out, HF was even more effective at inhibiting the corrosion of aluminum, and it was just as good with RFNA as with WFNA.
With the corrosion problem solved, the military now had an oxidizer that could be loaded at the factory and had a low enough freezing point to meet their requirements. Military specifications were written up in 1954. The terms RFNA and WFNA were thrown out and the designation "Nitric Acid, Type I, II, III, and IV" were adopted. These contained 0%, 7%, 14% and 21% nitrogen tetroxide respectively. HF inhibited acid was designated Type I-A, II-A, III-A, and IV-A, and contained 0.6% HF.
It was type III-A that gradually edged out all the others and became the nitric acid oxidizer. Engineers came to call it IRFNA, or Inhibited Red Fuming Nitric Acid.
IRFNA was widely used in tactical missile, where freezing point of the propellant matters. For strategic missiles, which are fired from hardened and heated sites, nitrogen tetroxide, with its somewhat greater performance, is the oxidizer used. Nitrogen tetroxide is typically mixed with nitrogen oxide to depress the freezing point. The resulting oxidizer in called MON (mixed oxides of nitrogen) and is given a number (e.g. MON-10, MON-25) designating the percentage of NO in the mix. When "nitrogen tetroxide" is listed as the oxidizer it is likely actually some form of MON.
For the Titan missile, Aerojet developed Aerozine 50, a 50-50 hydrazine-UDMH mixture. Aerozine 50 has better performance than straight UDMH but is still stable enough to be used for regenerative cooling. Aerozine 50 was also the fuel of choice for Apollo.
For anyone wanting to read more, I recommend the book Ignition, An Informal History of Liquid Rocket Propellants by John D. Clark.
