Hypergolic reactions

[KuzkinaMat]

A hypergolic reaction is a highly exothermic redox reaction between two substances that occurs spontaneously at room temperature. They cause combustion, deflagration, or even an explosion if left to progress inside a sealed container. Old-school Soviet rockets exploited hypergolic reactions to provide lift, combining liquified dinitrogen tetroxide (N2O4) with hydrazine (N2H4), which spontaneously combust on contact. Another well-known example of a hypergolic reaction is the addition of potassium permanganate (KMnO4) with glycerol (C3H8O3), which catches fire either immediately (if excess permanganate is used), or after a delay, the duration of which depends on the relative quantities of the two reactants and how they are mixed. This is the true danger of hypergolic reaction, in that it can appear that nothing is happening, only for the mixture to ignite while left unattended. A similar "ghetto" hypergolic reaction occurs when calcium hypochlorite 'pool shock' tablets are added to automobile break fluid, which causes the break fluid to catch fire (permanganate can also react with break fluid, and does so more vigorously).

 

Obviously, not all fuel-oxidizer mixes cause this effect, or our hobby would be impossible. The permanganate ion is avoided in pyrotechnics on general principle, but my question is this: what kinds of oxidizers cause these reactions? Are there others besides MnO4 and OCl (and OB and OI) that should be avoided? Does a mixture of, for example, calcium hypochlorite, and powdered sugar (or benzoic acid, or microcrystalline cellulose, etc.) catch fire at room temperature? Which oxidizers should be avoided when acetone or NC lacquer is being used? Which kinds of petroleum products and which kinds of oxidizers can be mixed without bursting into flames if left unattended?

[BurritoBandito]

Calcium hypochlorite reacts with glycerine the same as breakfluid.

[SekndAmendment]

Not just the old-school Soviet stuff used hypergols as propellants. Plenty of modern rockets do as well. They're in common use today. The shuttle used them in the OMS system. Now, they aren't typically used as the main propellant during launch, but the technology still gets used for stabilizers or supplemental power. We still use several types of hypergol fuels on some Raytheon missiles. They're great for DACS (direction attitude control system) of some of the fancier missile defense stuff. I've never worked on a program that uses one so I'm not super familiar with it.

[brimstoned]

Undiluted antifreeze (ethylene glycol) will react with KMnO4 as well, also sucrose if a few drops of water are added to the mixture.

[Jonathan]

Watch out too for mixing urea peroxide with KMnO4 or MnO2. The peroxide ion decomposes with very, very little frictional input as well as with the addition of any water. No flame, but a lot of fussing and fuming.

[experienced]

If you value your hair and eyelashes, do not mix ether with any thing including chromium trioxide.

[hindsight]

Of course, one of the common examples of spontaneous combustion using a material found in our hobby is the boiled linseed oil (oily rags) concern. Edited October 5, 2013 by hindsight

[taiwanluthiers]

I'm not sure if that qualifies as hypergolic as the reason it combusts is because linseed oil reacts with air to polymerize. However the process is exothermic so if too much linseed oil soaked rags were exposed to air it would get hot enough to ignite the oil and start a fire. Without air the reaction would not happen. If the linseed oil soaked rags were spread out in the open it will not ignite, heat can't build up fast enough to reach the oil's flashpoint.

[hindsight]

I'm not sure if that qualifies as hypergolic as the reason it combusts is because linseed oil reacts with air to polymerize. However the process is exothermic so if too much linseed oil soaked rags were exposed to air it would get hot enough to ignite the oil and start a fire. Without air the reaction would not happen. If the linseed oil soaked rags were spread out in the open it will not ignite, heat can't build up fast enough to reach the oil's flashpoint.

Agreed. Just a potential hazard for spontaneous combustion, since the main topic is Safety.

Edited October 5, 2013 by hindsight

[taiwanluthiers]

I think the safe way to dispose of oil soaked rag is to either put it under water in a sealed can, or just seal it in a can... I usually just leave them on the workbench until it hardens before throwing it in the trash.

[Jonathan]

Another one: calcium hypochlorite (73%) + urea hydrogen peroxide

[Jonathan]

Just discovered one to avoid: sodium chlorite (80%) and thiourea. Friction sensitive. Big flash.