Exotic Propellants

Rockets are toil and trouble to begin with.  Why add certain death into the mix?

Exotic Propellants

Rockets harness the same chemical energy released in bomb explosions in a controlled fashion.  By definition, this means the chemicals used as rocket fuels are themselves explosives which poses a great deal of risk to their operation.  Some of the chemicals used in rocketry aren’t just explosive but also bad for the environment and others will outright murder you on contact from their corrosive and poisonous nature.  So why do engineers design rockets that are even more hazardous to life and property than they inherently need to be?

Story Time

22 years ago, the predecessor of today’s Great Wall Industry Corporation launched a Long March 3B carrier rocket out of the Xichang Satellite Launch Center.  The only details I have been able to find on the cause of failure for this mission is that a weld broke somewhere in the vehicle immediately after lift-off.  From the grainy video taken that day, it appears that thrust vectoring system broke and caused an engine to fire at an angle relative to the flight path.  This was perhaps due to leaking of the working fluid that powered the thrust vectoring system through the broken weld joint.

In any case the rocket veered off course immediately.

The Chinese seemed to  have borrowed from the Russians the practice of not using a self-destruct system aboard their rockets.  The end result of that decision was that the rocket ended up cratering a small village 1.2 km downrange from the launch pad.  The Russians don’t have the problem of nearby settlements at Baikonaur so their use of rockets that can’t be self-destructed carries little risk.  This was not the case back in 1996 in Xichang.

The rocket exploded and dumped thousands of gallons of dinitrogen tetroxide and monomethyl hydrazine onto the village.  The official death total acknowledged by the Chinese is six dead and 57 injured.  This may be a dramatic understatement but there is no real reason to believe that the Chinese wouldn’t have evacuated the residents of the village for their own protection before the launch.  The village (the name of which the western media is uncertain) was subsequently abandoned at the order of the Chinese government.

While working on the initial draft of this article, I had intended to argue that the village would have been rendered permanently uninhabitable by the propellants dumped on it but a bit of research disproved my hypothesis.  It turns out that given how reactive the compounds used in that rocket were, they tend to readily decompose in the atmosphere and would quickly break down into far less harmful components.   It still would have been a hellish nightmare of death and destruction to anyone engulfed in either the fireball or the propellants.

The hazardous rocket propellants used that day were dinitrogen tetroxide (NTO) and unsymmetrical dimethylhydrazine (UDMH).  These are common enough rocket propellants and their safety issues have been well-documented over the decades of their use:

It can’t be that bad, even NASA uses this stuff.

NOTE:
NTO is potent enough that by the time you can smell it, you’ve already inhaled a dangerous dose that is forming nitric acid inside your lungs.

So why do engineer choose to use such dangerous propellants?

Despite their dangers, these propellants are downright useful.  You may have noticed in the NASA video all of the Apollo-era hardware being moved around and tested.  That’s because that Apollo hardware needed these toxic fuels to accomplish the missions.

NTO and UDMH (or one of the other nitrogen and hydrazine compounds) are liquids at room temperature and react spontaneously with each other without an ignition source. fire-triangle This is the opposite of many rocket propellants which boil off at room temperature and are hard to ignite properly without causing damage to the engine.  There is a very fine line between stable combustion inside a rocket chamber and an explosion.  When engineers must add special equipment to a combustion chamber to help the process along (special fuel/oxidizer injectors, ignition systems, plumbing and injector systems for ignition systems, etc) it adds risk to the design and decreases safety margin.

The spontaneous reactions that hypergolic propellants go through allows engineers to omit many of the above features from their rocket engines.  The fact that these hypergolic propellants will sit in a tank and remain liquid is also a tremendous benefit.  For missions like the Apollos, there are long coasts between rocket firings. A propellant combination such as hydrogen and oxygen would have begun to boil off and evaporate into space during such long coasts.  In this alt-reality scenario, the Apollo astronauts would have arrived at the moon only to turn into the newest crater on it when they attempted to light the engines with empty gas tanks.

If you are Captain Tom Hanks, this is not so good for your chances of returning to Earth.

iss-31_spacex_dragon_spacecraft_is_grappled_by_canadarm2
This moment brought to you by deadly hypergolic propellants.

Hypergolic propellants are one of the most practical solutions for these types of problems.  Missions to deep space are  reliant on dependable, restart-able rockets systems as they can coast for years in space between engine firings. Currently there are no good systems capable of storing LH2 and LOX indefinitely. Because of these considerations, nearly every rocket and spacecraft manufacturer use hypergolic propellants to this day, primarily in spacecraft and satellites such as the Dragon produced by SpaceX or the SSL 1300 satellite platform from SSL (formerly Space Systems/Loral)

These characteristics also made hypergolic propellant choices desirable for the second  generation of ballistic missiles.  The Titan II ICBM was capable of sitting in a silo for weeks at a time fully fueled and ready for launch.  First generation ballistic missiles like the Soviet R-7 and American Atlas had to be loaded with fuel and liquid oxygen before a launch and could not maintain ready-to-launch status more than a few hours.  You can see how the extra loading time and limited ‘ready’ status would hinder the mission of these weapons of mass destruction.  Largely due to the use of hypergolic propellants, the Titan II was able to remain on active deployment for nearly two decades after the Atlas was retired. The Russians use hypergolic propellants in their ICBM’s to this day in the form of the R-36 ‘Satan’ which in turn is planned to be replaced by the hypergol-propelled RS-28.

GODWIN’S LAW NOTICE: Nazi Rockets Ahead

Going back even further, military engineers for the Nazis created rockets that depended on other storable propellants such as the various nitric acid compounds and concentrated hydrogen peroxide.  These compounds are even less stable and more corrosive than NTO/UDMH and tend to eat their way out of their containers over a short period and then explode.  Various nitric acid-based compounds were used for a few decades but by now have largely fallen out of use.  It’s hard to justify using something that itself is more hazardous than other propellants which will “only” melt your lungs.

Even crazier propellant combinations utilize liquid fluorine or fluorinated compounds as an oxidizer.  Despite the name, substances can act as oxidizers without containing any oxygen.  Flourine and fluorinated compounds can perform this role quite well as they react with just about everything vigorously.  For example, liquid fluorine (LF2) is a more energetic oxidizer of liquid hydrogen than is liquid oxygen.

Theoretically, a rocket so fueled could have a specific impulse around 480- 490s, which is quite excellent and beats LH2/LOX by 10-20s.  The downsides of this comparison are rather extreme, however. The use of liquid fluorine would cause tons of Hydrofluoric acid to be released as a primary combustion product.  I don’t feel like I need to explain why this is bad on multiple levels for living things and inanimate objects alike.

nto-leak-1-meatball
Oh hey, NASA used to just dump a potent toxin into the open air.  I guess it’s no big deal as long as they only did it in Texas, right?

There is one semi-mythical rocket propellant that gets brought up from time to time that is affectionately known as FOOF.  Dioxygen difluoride is hypergolic with just about everything it comes into contact with.  One way of producing FOOF is by bombarding oxygen and flourine with bremsstrahlung radiation at sub-zero temperatures. That sounds like a plot point in a superhero movie.

If the fabrication process of your rocket fuel requires radiation and sounds like a Lex Luther subplot, you probably shouldn’t use it.  Just sayin’.

Hypergolic fuels have their uses and limitations.  NASA, the US Air Force and the ESA are all funding research into replacements for these deadly propellants right now.  These potential replacements offer higher efficiency with far less toxicity.  When these new propellants are available on the market it will mark a major win for the aerospace industry, worker safety and the environment.

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