When discussing liquid rocket propellants, one often thinks of RP-1, liquid hydrogen, or methane. But another liquid propellant known as hypergolic fuel has been used for decades in spaceflight due to its unique properties.
Hypergolic rocket propellant is a liquid propellant in which the fuel and oxidizer spontaneously combust upon contact with each other. It is a highly toxic substance, but it is widely used in the upper stages of launch vehicles and spacecraft due to its capacity for multiple and reliable ignitions.
Whenever a discussion about the best, most efficient, and cleanest liquid rocket fuel takes place, the three usual suspects, namely liquid hydrogen, RP-1, and liquid methane, are predominantly headlining the conversation. Even solid rocket propellant is often mentioned.
However, a type of liquid propellant that is seldom mentioned and often frowned upon due to some of its characteristics dates back to World War II and forms a crucial part of many spacecraft that needs to be reignited several times and spend extended periods in Space.
These propellants are collectively known as hypergolic fuels, and as upcoming sections will illustrate, there are several reasons why they are widely used, but also several reasons why they should be avoided. But one first needs to define what a hypergolic propellant is:
What Is Hypergolic Rocket Propellant?
As mentioned, hypergolic rocket propellant is a liquid propellant in which the fuel and oxidizer spontaneously combust upon contact with each other. To best understand how it works, one first needs to define the difference between propellant and fuel.
Difference Between “Fuel” And “Propellant”
The words “propellant” and “fuel” are often interchangeably used when referring to the substances we use in vehicles to power their engines. However, technically they are not quite the same.
“Propellant” refers to the fuel as well as the oxidizer used in any particular vehicle to allow combustion to occur, which drives the engine. Typically in a rocket, the fuel will be RP-1, while the oxidizer is liquid oxygen. Together they form the vehicle’s propellant.
The word “fuel” is predominantly used when referring to the substances used in engines operating within our atmosphere. This is because fuels like petroleum, diesel, and jet fuel all use the oxygen in the air as the oxidizer. In this case, using “fuel” is technically correct.
However, orbital rockets primarily operate in the upper atmosphere and the vacuum of Space where no oxygen is present. As a result, they need to take their oxygen with them, typically in the form of liquid oxygen.
As a result, whenever one refers to the “propellant” of an orbital rocket, it refers to both the fuel component used (e.g., liquid hydrogen, RP-1, or liquid methane) as well as the oxidizer.
The most common hypergolic fuel type is hydrazine and its different variants, more specifically, monomethylhydrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH), while the most widely used oxidizer is dinitrogen tetroxide (NTO) or nitric acid.
The two primary characteristics of this type of propellant are its ability to spontaneously combust, as well as its highly toxic nature. It is these two characteristics that make the use of this propellant attractive for specific purposes, while it should be avoided for others.
When a hypergolic fuel and oxidizer, for example, unsymmetrical dimethylhydrazine and dinitrogen tetroxide, come into contact with each other, they react violently and spontaneously combust.
These propellants can be used as the launch vehicle’s primary propellants, like the Proton rocket used by Russia and various satellites and deep space spacecraft, or to reliably ignite a rocket’s primary propellants like RP-1 and liquid oxygen (collectively known as kerolox).
Several historical & current spacecraft use this form of propellant. For example, the Cassini spacecraft (that explored Saturn) & the DAWN probe (that studied the asteroids Vesa & Ceres) used hydrazine to perform the complex maneuvers required for these missions.
The International Space Station (ISS) uses two main thrusters and 16 smaller thrusters for movements that fall beyond the scope of its Control Moment Gyroscopes (CMG). The thrusters use unsymmetrical dimethylhydrazine as fuel and dinitrogen tetroxide as oxidizer.
The Apollo Lunar Module used a hypergolic fuel/oxidizer combination (Aerozine 50 fuel & nitrogen tetroxide as oxidizer) in its Reaction Control System.
This fuel combination was also used for the engine of the Service Module, which orbited the Moon with the Command Module during the Apollo Program of the 1960s and 70s.
To get a better understanding of the characteristics of hypergolic propellants and why they are used and sought after in some cases and avoided in others, one needs to take a closer look at their various advantages and drawbacks.
Advantages Of Using Hypergolic Propellants In Orbital Rockets
Apart from being a relatively unknown fuel source outside the aerospace industry and being frowned upon and criticized by many scientists due to their toxic and volatile nature, hypergolic propellants have some clear advantages, of which the most important ones are:
- No Ignition Mechanism Required
- Allows For Multiple Ignitions
- Can Be Stored At Room Temperature
- Require Less Complex Rocket Engine Designs
- Long Storage Capacity
- Higher Density Means Smaller Fuel Tanks
By taking a closer look at each advantage, one will get a better understanding of the unique benefit that it might provide over other propellant types:
1) No Ignition Mechanism Required
Although most rocket propellants are considered highly combustible, getting them to ignite inside a rocket engine is not that straightforward. The fuel and oxidizer are injected into the rocket engines at extremely high pressures and velocities.
Causing this large amount of vaporized propellants to ignite reliably and uniformly in this high-pressure environment is no easy task. Typically, a pyrotechnic detonator, high-voltage electric currents (using a powerful spark plug), or hypergolic propellants are used.
(Learn more about the different methods used to ignite an orbital rocket engine and how each one works in this in-depth article.)
The first two methods require specialized ignition mechanisms that need to be integrated with the rest of the rocket engine, adding complexity to the system, which may also potentially lead to reliability issues.
In contrast, using hypergolic propellants as an ignition mechanism is relatively simple. Keeping them in separate tanks and allowing a small amount to enter and ignite inside the combustion chamber will ensure the reliable combustion of the main propellant.
What makes this process even simpler is that some hypergolic fuels do not even need a hypergolic oxidizer to combust. Triethylborane/triethylaluminium (TEA-TEB) is a hypergolic fuel and pyrophoric liquid that reacts with any form of oxygen, including liquid oxygen.
Since most liquid fuels use liquid oxygen as their oxidizer, simply injecting TEA-TEB into the combustion chamber will cause it to react with the liquid oxygen present, resulting in an instantaneous and powerful combustion.
2) Allows For Multiple Ignitions
Not all launch vehicles and spacecraft are used only once. Spacecraft used for deep space exploration or those in orbit that need to change their position or attitude on a regular basis need a form of propulsion that can be repeatedly used over extended periods.
In orbit, spacecraft like the now-retired Space Shuttle, the International Space Station (ISS), the Hubble Space Telescope, and a large number of satellites typically use what is called a Reaction Control System (RCS) to change their attitude and position.
Although Reaction Control Systems often utilize reaction wheels and gyroscopes to change a spacecraft’s orientation and position, smaller rocket engines are often needed for larger or faster maneuvers that fall beyond the capacity of reaction wheels or gyroscopes.
(Learn more about Reaction Control Systems, how they work, and the different mechanisms involved in the following in-depth article.)
This is where the advantages of hypergolic propellants really come into play. Since the fuel and oxidizer spontaneously combust upon contact, it is as “simple” as opening a valve to allow the propellants to combine & combust to allow a craft to perform a specific maneuver.
Depending on the amount of hypergolic propellant a spacecraft carries, the engine can theoretically reignite as many times as there is propellant available for combustion.
Together with the propellant’s ability to be stored for extended periods makes it ideal for use in a spacecraft’s Reaction Control System (RCS), orbital maneuvering system (OMS), and larger rocket engines that need to ignite repeatedly during interplanetary missions.
In the vacuum of Space, hypergolic propellants also have little to no contact with humans and are far away from any environment where their highly toxic nature can negatively impact them. This makes them ideal for long-term and repeated use in spacecraft.
3) Can Be Stored At Room Temperature
One of the big challenges of many liquid rocket propellants is storage. Hydrogen, methane, and oxygen are all cryogenic substances, which means they have to be stored at temperatures below -153° Celsius (-243° Fahrenheit) to remain in liquid form.
For example, oxygen needs to be cooled to -183° Celsius (-297°Fahrenheit) or below to remain a liquid, methane to -162° Celsius (-260° Fahrenheit), while hydrogen needs to be kept at an extremely low temperature of -253° Celsius (-423° Fahrenheit) to remain a liquid.
(Learn more about the different fuel types orbital rockets use, their characteristics, as well as the advantages and drawbacks of each fuel in this article.)
However, like RP-1 fuel, hypergolic propellants can be stored at room temperature, making it easier and less expensive to store for extended periods without the additional expensive and complex mechanisms & structures needed to keep cryogenic propellants in liquid form.
The ability of different variants of specific hypergolic propellants to freeze at different temperatures also allows them to be matched to specific engines and spacecraft for different purposes.
For example, the freezing point of hydrazine is 2° Celsius (36° Fahrenheit), monomethylhydrazine -52° Celsius (-62° Fahrenheit), while unsymmetrical dimethylhydrazine freezes at -58° Celsius (-72° Fahrenheit).
This allows fuel like hydrazine to be used as a monopropellant for use in spacecraft like those used in the auxiliary power unit of the Space Shuttle, while dimethylhydrazine is used in combination with dinitrogen tetroxide in regeneratively cooled engines like the Proton rocket.
(Learn more about the different cooling methods used to prevent rocket engines from melting due to the extreme heat generated in this in-depth article.)
4) Require Less Complex Rocket Engine Designs
As previously stated, allowing rocket propellants to combust in the high-pressure environment of the combustion chamber of a rocket engine is no simple matter.
The RS-25 rocket engines, which were used in the Space Shuttle and are currently used in NASA’s SLS rocket, use a torch igniter system. The process starts when gaseous hydrogen & oxygen are directed through a small area around a spark plug, which causes the gas to ignite.
It is relayed through a channel and enters through a hole in the center of the main injector of the combustion chamber, creating a large enough heat source that allows the propellant to combust evenly. (Several launch vehicles use a similar type of ignition device.)
The Soyuz rocket, used by Russia’s Roscosmos agency, uses 32 pyrotechnic detonators placed on birch sticks underneath each of its 32 nozzles before launch, which are monitored and simultaneously detonated by a central control system.
Hypergolic propellants housed in separate smaller tanks and released into the combustion chamber by simply opening a valve at the appropriate time require a much less complex design compared to these intricate ignition devices and the resulting possible failures.
5) Long Storage Capacity
The ability of hypergolic propellants to be stored at room temperature not only makes them much safer and less complex to store than their cryogenic counterparts, but it also has an added benefit.
If the substances are correctly and properly contained, they can be stored for extended periods of time. This means that these propellants can be manufactured and safely stored for long periods before it is used.
This feature is also especially useful for spacecraft spending months to years in Space and only need to fire their thrusters periodically.
6) Higher Density Means Smaller Fuel Tanks
Hypergolic propellants are much less dense than their cryogenic counterparts. For example, liquid oxygen has a density of 1.14 g/ml, while dinitrogen tetroxide has a density of 1.45 g/ml. Certain forms of hydrazine can be up to ten times denser than liquid hydrogen.
As a result, hypergolic propellants require smaller fuel tanks than cryogenic propellants, which is especially useful in space probes that need to fit inside larger launch vehicles to be carried into Space before deployment.
Disadvantages Of Using Hypergolic Propellants In Orbital Rockets
With the numerous advantages of hypergolic propellants mentioned in the previous sections, one might find it surprising that it is not used more frequently and replace other types of propellants. However, they have a few disadvantages that make them very dangerous.
The primary drawbacks of hypergolic propellants are:
- Extreme Toxicity
- Highly Corrosive
By taking a closer look at each disadvantage, one will get a better understanding of why this propellant type is only used in specific and carefully controlled circumstances:
1) Extreme Toxicity
By far, the most significant disadvantage of hypergolic propellants is their extreme toxicity. Hydrazine has a median lethal dose of 60 mg/kg, which means that just 4.2 grams of this substance are enough to kill an adult human.
(The Centers for Disease Control And Prevention or CDC classify it as “Immediately Dangerous To Life And Health” at gaseous concentrations higher than 15 parts per million.)
As a result, extreme care needs to be taken when handling these substances, and scientists & technicians need to be fully covered in hazmat suits (hazardous material suits) when coming into direct contact with them.
It is also this toxic nature of hyperbolic fuels that make them unsuitable as the main propellant for launch vehicles. Russia’s Proton rocket is one of the only remaining rockets still using it as its primary propellant for its first-stage boosters.
The number of toxic fumes that will be released in case of an accidental spill or unintended ignition & resulting explosion at a launch site will have a massive environmental impact on all life in the surrounding area. It will also require extensive & hazardous cleanup operations.
This also explains why, apart from their other benefits, these propellants are so widely used in the upper stages and Reaction Control Systems of spacecraft, where they are predominantly only exposed to Space and removed from any human contact.
2) Highly Corrosive
Hypergolic propellants, like monomethylhydrazine (fuel), dinitrogen tetroxide (oxidizer), and fluorine gas (oxidizer), are also very corrosive. This means it cannot be used with some types of materials in a rocket engine or in specific types of rocket engines.
It can erode away crucial components within a rocket engine, causing it to malfunction and, in some cases, lead to catastrophic failure of the entire engine.
The highly reactive nature of hypergolic propellants that makes them so attractive to use to ensure reliable and instantaneous combustion also makes them very dangerous.
Typically, traditional fuel/oxidizer combinations like RP-1 and liquid oxygen (collectively known as kerolox) will not spontaneously combust upon contact and need some kind of ignition mechanism.
This is not the case with hypergolic propellants with their unstable characteristics since the smallest amount of exposure to possible compounds with which they can react may result in a spontaneous and violent reaction with catastrophic results.
Therefore, extreme precautions should be implemented when handling these propellants, and extra care must be taken to ensure no accidental contact, leaks, or spillage occurs within a rocket’s hypergolic propellant tanks and engine.
(Unintended propellant combustions have led to many orbital rocket failures in the past. Learn more about why rockets explode before and during launch in the following article.)
As this article clearly illustrated, hypergolic propellants have numerous advantages that make them very desirable for use in specific applications in spaceflight. However, their toxic, corrosive and unstable nature also makes them dangerous and not suitable for others.
What is clear, though, is that these propellants have obvious advantages that, if the necessary care is taken, make them almost indispensable for the reliable operations of a rocket engine in certain circumstances.
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