"This is emphatically not fun."  
"RFNA (red fuming nitric acid) attacks skin and flesh with the avidity of a school of piranhas. And when it is poured, it gives off dense clouds of nitrogen dioxide, which is a remarkably toxic gas"

- John D. Clark, "Ignition: An Informal History of Liquid Rocket Propellants"

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Below is a picture of four different rocket engines: the Titan II's LR-87, the Apollo service propulsion system and lunar module engine, the Russian Proton's RD-275, and the SpaceX Dragon's SuperDraco thrusters. Completely independent designs spanning several decades, two superpowers, and one private company: what do they all have in common? The answer, as this article's title suggests, is that they all use hypergolic fuel!

Stay aware from the launchpad after one of these bad boys fires - hypergolic fuel is some nasty stuff!

Unlike conventional liquid rocket fuels like RP-1 kerosene + liquid oxygen or liquid hydrogen + liquid oxygen, which need an ignition source to combust (see my article on that here!), hypergolic fuels ignite on contact, no spark needed. They're the literal definition of spontaneous combustion! This begs the question, with so many classes and combinations of available rocket fuel, which one is the best to use? The answer, as is often the case in engineering design, is it depends on the purpose of the vehicle you're building, since rocket fuel selection is always a balancing act between many important attributes: performance, reliability, storability, and toxicity, just to name a few!

The spontaneous combustion of one droplet of hypergolic fuel | Credit: YouTube

To appreciate hypergolic fuel, it's best to understand the demands that led to its creation, which takes us to the year 1945 as the Allied Powers were closing in on Germany. Once the Americans and the Soviets had scooped up as many Nazi rocket scientists as they could, it became clear that guided and ballistic missiles would be the high-tech artillery of the Cold War future. The Germans had been building the V-2 rocket towards the end of the war, but its limitations became apparent since the liquid oxygen it burned as oxidizer needed to be cryogenically stored, making it impractical for scenarios that required launch at a moment's notice. The race was on to find a fuel-oxidizer combination that ignited on contact, was liquid at room temperature, and wasn't too corrosive, toxic, or otherwise unwieldy to store in missiles deployed on the battlefield

The Germans successfully developed a mobile launcher for the V-2 (the Meillerwagen), but it still took about 2 hours to fuel and fire the rocket

The chemists tried every compound imaginable, from alcohols and amines to some downright wacky stuff. It quickly became apparent that hydrazine ($N_2H_4$) was the frontrunner to be the hypergolic fuel - it had good performance, high density, and combusted with a number of oxidizers. The only problem - its freezing point was 1.5$^\circ$C, way too high for the military's specifications. The Air Force and the Navy needed a fuel that would stay liquid down to -54$^\circ$C, necessary for tactical missiles aimed at Siberia. The engineers tried all sorts of additives to bring the freezing point down, ranging from the simple (water - no good because it ruined performance) to the exotic (sodium perchlorate monohydrate - utterly unstable and prone to detonation). When it became clear that additives weren't the answer, they discovered that close derivatives of hydrazine, bearing methyl ($CH_3$) groups, produced effective results. There were two winners - monomethylhydrazine ($MMH$) and unsymmetrical dimethylhydrazine ($UDMH$); researchers found that a proper mixture of the two could go down to -80$^\circ$C before freezing

If you recall from chemistry, asymmetric compounds tend to not freeze as easily.  Same reason why butter (saturated fat) is solid but olive oil (unsaturated fat) is liquid at room temperature

So we've got the fuel, what about the oxidizer? For a while engineers had been using $RFNA$ (red fuming nitric acid), a mixture of ~84% nitric acid ($HNO_3$), ~13% dinitrogen tetroxide ($N_2O_4$) and ~3% water aptly named for the noxious clouds of poisonous nitrogen dioxide ($NO_2$) emitted as the $N_2O_4$ gradually breaks down. Though its performance was good, its spectacular corrosiveness ate away at any metal container it was stored in, posing enormous problems for missile deployment. Using pure $N_2O_4$ was a viable alternative with slightly better performance, but similar to hydrazine, its freezing point of -11$^\circ$C was too high to make the cut. Fortunately, the chemists discovered that adding just a little hydrogen fluoride to the $RFNA$ completely eliminated the corrosion problem. Thus, rocket scientists can take their pick of hypergolic oxidizer: for low freezing points needed in military ICBMs, $RFNA$ is the answer, but for slightly better performance in temperature controlled environments (like with space vehicles), $N_2O_4$ is the way to go

Dinitrogen tetroxide - breathe in this stuff, and you're about to have a very, very, VERY bad day