The History of Rocket Propellants: Hypergolic Fuels
One of the main components of rockets is its propellant, without fuel rockets would just sometimes be an over engineered metal tube. The engines would not be able to propel the rocket into space without propellant. In this series of articles about the history of rocket propellant, the chemistry, usage, and types of fuels and oxidizers will be explained.
In the first part of the series, I will cover one of the most interesting types of liquid rocket fuels. With this being hypergolic fuels, these types of rockets fuels are corrosive and highly toxic. Hypergolic fuels are also very reactive meaning even just adding the oxidizer with the fuel is enough to ignite it. Due to its toxic nature, when introduced in the environment, it can cause massive damage to the surrounding area. Improper handling of this fuel can also cause harm to the people using it, even death is possible when exposed to the fuel without proper equipment
Introduction to Hypergolic Fuels
Hypergolic fuels seem unconventional and would not be a safe option to use due to its toxic nature. But in reality not much harm could be done by this fuel if handled safely and properly. These fuels are also advantageous in the climates that rockets experience. Being used as early as 1931 by Russian engineers as an igniter for the main engines. Hypergolic fuels are great for engines that need quick ignition times, such as reaction control systems or smaller engines like SpaceX’s Draco thrusters on their Dragon capsule.
One of the main reasons of use for hypergolic fuels is its stable nature with it being a liquid at normal temperatures. This gives it the advantage of being able to be stored for a long period of time unlike other fuels like liquid hydrogen. With liquid hydrogen having to be cryogenic in order to be liquified, requiring very cold temperatures and is not ideal for long missions.
The most common use of hypergolic fuels are upper stage engines that are much smaller or reaction control systems (RCS) that steer the spacecraft. It’s very practical to use them for RCS thrusters because of the spontaneity of hypergols (another term for hypergolic fuel). The fast ignition of the fuel is practical for the quick bursts of propellant RCS thrusters use to control a spacecraft. With its drawbacks being only its corrosiveness and toxicity which can cause a disaster if improperly handled
Types of Hypergolic Fuels
The most common types of hypergols used are hydrazine and its derivatives as well as nitrogen tetroxide. Most of the time these two hypergols are used in combination as the propellant of a spacecraft or its control systems.
Some of the most used combinations of hypergols, mostly because of their relative safety compared to other combinations. One such combination is monomethylhydrazine (MMH) added with nitrogen tetroxide (NTO). This is used by the Dragon spacecraft which already successfully completed its Demo 2 launch and soon the Crew 1 launch, with the Dragon spacecraft named Resilience. Another combination used primarily by historic American spacecraft is Aerozine 50 and NTO, with Aerozine being a hybrid mix of just hydrazine and unsymmetrical dimethylhydrazine (UDMH).
On the other side, the Russians also have their combination of hypergols. With the first use of these fuels as a chemical ignitor for their rocket engines. This was a mix of phosphorus and carbon disulfide which ignited engines with kerosene and nitric acid as propellant. More modern mixes of hypergolic propellant that the Russians use is UDMH and NTO. The Russians also use a fuel called Tonka created by the Germans. Tonka is a mix of half triethylamine and half xylidine, an oxidizer of NTO or nitric acid would be used.
One hypergol that was used in the past was aniline. Aniline as a fuel was used early in the development and testing of different rocket fuels. However, aniline was never used after, due to its low freezing point of -6.3°C (20.66°F). Anailine can be paired with nitric acid or hydrogen peroxide, with nitric acid being used in the first sounding rocket developed in the United States (sounding rockets are the name for rockets that carry instruments to take measurements).
Spacecraft With Hypergols
As stated earlier spacecraft mostly use hypergols as upper stage or RCS propellant. It is rare to see spacecraft that has a main stage that uses hypergolic propellant.
Proton
One such spacecraft that differs from most is the Russian Proton rocket that is fully fueled with hypergols. The Proton has been in service since 1965 for the former Soviet Union and now the Russian Federation. This family of rockets is modular and uses NTO added with UDMH on all of its stages. With the rocket having a 90% success rate, 425 total flights and 47 total failures. Today this rocket is still in service and launches from the Baikonur Cosmodrome.
It’s quite unfortunate to know that the Proton has had failures. These failures were catastrophic causing much damage to the ecosystems around the cosmodrome. It also caused damage to the nearby residents, breaking windows and causing a large explosion. One failure of the Proton caused the rocket to pitch down towards the earth and eliminated by the range safety officer before it hit the ground. The cause of this was a failure of proper installment of the angular velocity sensors. Some of the sensors were installed facing downward instead of the proper upward position. This gave the rocket different readings on its trajectory and caused it to account for it. This led the rocket to change its direction, which eventually led to its failure.
Apollo & Titan II
The Saturn V which was the rocket used for the Apollo missions that NASA conducted to get to the moon used hypergolic propellant. The command and service module (CSM) had its AJ-10 propulsion system and RCS fueled by hypergols. The AJ-10 was developed by Rocketdyne and used the Aerozine 50 with NTO combination. The RCS however used MMH and NTO for its propellant. Another part of the Saturn V that uses hypergols is the Lunar Excursion Module (LEM). The LEM also uses the Aerozine 50 and NTO combination of hypergols.
The Titan II one of the first rockets to bring Americans into space, was used as the primary rocket to launch the Gemini capsule for NASA. Created by the Glenn L. Martin Company the Titan II was originally an intercontinental ballistic missile (ICBM). This rocket also uses the Aerozine 50 with NTO mix of hypergols because it was able to be stored for long periods of time, perfect for ICBMs.
The Future of Hypergols
With the development of newer fuels, hypergols might become obsolete. For now without those more advanced fuels and the tech to use them, hypergols will stay around in the near future. At the moment, some promising types of fuels that can replace hypergols are xenon or krypton which are used for ion thrusters. These are different from hypergols but have the potential to replace them if the power output of ion thrusters increase over time. Ion thrusters for now have very low thrust and use fuels that are rare like xenon. Ion thrusters already are in use for smaller satellites and deep space probes because of its practicality in being efficient. These satellites and probes also need much less thrust which makes ion thrusters very practical, this will be covered in future parts of the series.
