Post #12 - Hypergolic Fuels

 Hypergolic Fuels

By In Thrust We Trust

Hey there! Thanks for spending some of your well-earned free time here! Anyway, this post will talk about Hypergolics fuels, going over the basics of them, and talking about different types, and finally, the usage of hypergolic fuels.

Shuttle OMS pod undergoing refurbishment

The Basics

A hypergolic-powered
Agena target rocket

The word hypergolic comes from the Greek roots of hyper and erg, meaning overly and work, respectively. The reason it is named this way is the nature of hypergolic fuel. Hypergolic fuel reacts instantly upon contact, contrary to some other fuels, which needs an ignition source, which is usually a third chemical or a spark creator that sustains a "contained" explosion inside the combustion chamber. Hypergolic don't need any of that fancy stuff, they just need 2 ports that feed fuel into a centralized space where the fuel and oxidizer will come into contact, then....boom

Hypergolic Fuel Advantages and Disadvantages

A certain hyperbolic lander...(hehe)

Hypergolics are almost always very reliable. Although larger hypergolic engines are driven by turbopumps, the more common smaller hyperbolic engines (the reason for them being smaller, we will talk about that later) are driven by pressure, usually, an inert gas, such as helium is fed into the propellant tanks at high pressure, which pushes propellant into the combustion chamber. Another advantage of hyperbolic engines is that they can be re-ignited as long is there is fuel, which makes it extremely useful for upper stage engines and lander engines. This differs from conventional engines which sometimes use an ignitor fluid that is in limited quantities, and once it runs out, the engine cannot be re-ignited after shutdown. All these combined make hypergolics extremely reliable and useful to use. So why aren't all engines hypergolics?

A worker dressed in
a full hazmat suit near
hyperbolic fuel

Mainly for 2 reasons. Safety, and energy-efficiency. When I say safety, you might be going "pfffft its rocket science nothing is safe" and thinking that hypergolics aren't really dangerous, probably an overreaction. Well, short-term exposure to hydrazine (a common hyperbolic fuel) can cause damage to the liver, kidneys and sometimes damage the central nervous system, resulting in paralysis or death. So not really an overreaction...Anyways, energy efficiency. By mass, cryogenic fuels are less energy-dense (same thing as energy-efficiency), meaning that cryogenics are more worth it per mass, which is why on a large scale the extra weight from the complex ignition and feeding systems are outweighed by the extra performance by cryogenic fuels.

Hypergolic Fuels

The CST-100 Starliner leaking red smoke from
its hypergolic nitrogen tetroxide hypergolic fuel

The most common type of hyperbolic compounds involves some form or relative of hydrazine. The reason is that hydrazine (not the pure stuff cause the freezing point is 2C) is a perfect hyperbolic fuel, reacts readily with other common hypergolics like nitrogen tetroxide, but also because it can be stored long term at room temperature and fired quickly and reliably. Some relatives of hydrazine include some big names, quite literally: UDMH and MMH (Unsymmetrical dimethylhydrazine and Monomethylhydrazine, respectively). Aerozine 50 is also a type of hypergolic fuel, involving a 50:50 mix by weight of hydrazine and UDMH. The most common type of hypergolic oxidizer is nitrogen tetroxide. 

Historical and Current uses of Hypergolics Fuel

A photo by Roscosmos showing
the Proton-M rocket, a rocket
entirely made of hypergolic fuel

Throughout history, and still today, hypergolics are used widely and almost all aerospace companies have used them at least once. The first rockets used hypergolics as the main stage, such as the Agena target vehicle of the Gemini Program. Notably, the Apollo LEM (Lunar Excursion Module's) used hypergolics for the descent and ascent stages. The Space Shuttle also used hypergolics in the OMS (Orbital Maneuvering Pods). Hypergolics were also used in several Roscosmos rockets (the Russian space agency), most notably the Proton rocket. The ESA also used hypergolics for the Ariane 1 rocket, and the Ariane 5 rocket has a version of the 3rd stage that uses hypergolic fuels. Other companies like SpaceX and ULA are using hypergolics as reaction control systems and abort systems. However, due to their unsafe nature and tendency to...explode, they are slowly being phased out by Roscosmos and ESA.

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Well, that's all, folks! I hoped you enjoyed this edition of Aerospace Weekly! If you enjoyed it, follow to receive email notifications of any future posts! Remember to comment, and share the post! This is In Thrust We Trust, signing off!