In our previous article, we covered the basics of rocket engines, but we only scratched the surface when it came to fuel-oxidizer combinations. While hydrogen-oxygen and kerosene-oxygen dominate the world of launch vehicles, numerous other propellant combinations are in common use with equally numerous applications. Here, we will explore some popular propellant combinations.
A brief history of fuels
Like nuclear energy, the course of modern rocket engine development was set in the 1950s and 1960s by the Cold War – specifically by the development of ballistic missiles designed to carry nuclear weapons. Consequently, propellant choices were determined by both engine performance requirements and by the operational demands of the battlefield. For example, ICBMs would need to remain dormant and fully-fueled for years before use, precluding fuels such as liquid hydrogen that are difficult to store. Later rockets used for peaceful applications were descended from these, but used similar fuels and techniques. History is just as important as engineering in understanding the selection of propellants in use today.
The number of propellant types and combinations are as varied as their applications. Hydrogen and oxygen are used in the “Ferraris” of the rocket world, where performance is absolutely paramount. However, hydrogen is a finicky fuel that is difficult to store. Kerosene and oxygen are the workhorses for most launch vehicles, being reliable and with decent performance. Monopropellants and plain pressurized gases dominate reaction control systems, and hypergolics rule where reliability and durability are key. Emerging from SpaceX and Blue Origin are methane and oxygen, which are convenient to produce on Mars, have better performance than kerosene, and are easier to store than hydrogen.
Of course, we are most interested in learning how to get a spaceship to Mars. These involve high-delta-V, high-thrust maneuvers like launching to orbit and trans-Mars injections. For these applications, no mature technology has superseded bipropellant engines that burn a fuel and oxidizer… yet.
Launch vehicles generally use bipropellants, meaning that a fuel and oxidizer are mixed and burned to produce thrust. To maximize their density (thereby minimizing the bulk of the tanks), these are stored as liquids. The most common fuels are liquid hydrogen, or LH2, and rocket-grade kerosene, or RP-1. These are usually burned with liquid oxygen, or LOX. Usage varies with application: LH2/LOX has a high specific impulse, while RP-1/LOX has a higher density and is less difficult to store.
In other words, while LH2/LOX has high performance, it is inconvenient for numerous reasons, such as needing to be stored at ultracold temperatures and having a very low density, requiring heavy tanks. On the other hand, RP-1/LOX, while having a lower performance, is more convenient; kerosene can be safely stored at room temperature and has a high density, reducing the weight of the tanks. We will explore the tradeoffs and handling considerations of the fuels later in the article.
A special class of bipropellants is the hypergolic propellant combinations, which ignite spontaneously on mixing. These chemicals are more exotic, with tongue-twisting names like unsymmetrical dimethylhydrazine and dinitrogen tetroxide. Hypergolic propellants will be covered in detail in the next section.
Here is a small sampling of the numerous fuel-oxidizer combinations used in bipropellant spacecraft engines. Note that specific impulse varies by both engine and propellant:
|Vacuum specific impulse
|Space Shuttle, SLS*
|BE-4 (Blue Origin)*
|Rutherford (Rocket Lab)
|Dinitrogen tetroxide (NTO)
|Space Shuttle orbital maneuvering system (Aerojet)
|Unsymmetrical dimethylhydrazine (UDMH)
|Long March 5 (Yuanzheng third stage)
* Still under development.
** The BE-4 uses liquefied natural gas (which is mostly methane, but with other trace gases and impurities) and will probably have a specific impulse in the mid-300 s range.
Why are there so many combinations and applications? An engine’s choice of propellant is usually the result of a complex interplay between engineering requirements, local weather, applications, organizational experience, history, politics, the local industrial base, and even the nation’s physical geography.
The highest-performing fuel in common use is liquid hydrogen, which can reach specific impulses of 453 seconds on RS-25 engines. However, it also comes with a plethora of engineering headaches, such as:
- Cryogenic storage: Liquid hydrogen is cryogenic, meaning that it must be kept extremely cold or it will boil: -253 °C, to be precise. This means that the fuel tanks need heavy insulation and that handling becomes more difficult and dangerous. Additionally, most materials become stiffer and more brittle at cryogenic temperatures. The Titanic’s steel hull sailed through water at merely -2 °C, and it became so brittle that survivors reported that the hull sounded like shattering china when it fractured. However, the SpaceX Starship actually takes advantage of this effect to improve the strength of its hull when filled with cryogens by using a type of stainless steel that retains ductility at low temperatures.
- Low density: Hydrogen’s high performance is due to its low molecular weight – unfortunately, this also means that it has a low density. Accordingly, larger, heavier tanks are needed. The size of liquid hydrogen tanks compounds the insulation problem. Larger tanks are also more difficult to transport to the assembly site, especially if they have to be transported by roads or trains. This is why liquid hydrogen may be a bad choice for nations with tough terrain.
- High ignition sensitivity: Hydrogen-oxygen mixtures ignite easily. This means that starting the engine is easy. Unfortunately, this also means that a leak is more likely to cause an explosion.
- Invisible flames: In the event that a leak ignites and produces a flame, hydrogen-oxygen flames are nearly invisible. This means that fires are much more difficult to detect.
Those are some of the tricky problems that apply to the design of launch vehicles, but it doesn’t stop there. When trying to store liquid hydrogen for an extended period of time, such as during a deep-space mission, we encounter even more difficulties. These include:+
- Boil-off: Liquid hydrogen is extremely cold. A deep-space vehicle will be heated by the Sun and by its onboard systems, which warm the hydrogen and cause it to boil. The resulting gas must either be vented into space (and be lost) or be captured and re-liquified (requiring heavy refrigeration systems that consume energy.) For expendable launch vehicles, hydrogen boil-off can just be vented and replenished while awaiting liftoff on the pad. A deep-space mission using liquid hydrogen, like a mission to Mars, would need to be equipped with cryogenic refrigeration systems.
- Ortho-para transition: Hydrogen in its natural state is diatomic, meaning that each molecule is two hydrogen atoms bonded together. The nuclei of these atoms are single protons, each of which has a quantum mechanical property called spin (they aren’t literally spinning, it’s just a name for the property – no, I don’t understand what it means either.) These spins may be parallel (orthohydrogen) or antiparallel (parahydrogen.) Hydrogen gas at room temperature is about 75% orthohydrogen and 25% parahydrogen. However, when liquefied, the orthohydrogen begins turning into parahydrogen, releasing heat. This heat is enough to vaporize the hydrogen. So after liquefaction, hydrogen has to be actively chilled for several days to allow all the orthohydrogen to transition into parahydrogen, or it will begin rapidly vaporizing inside the rocket.
- Leakage: Hydrogen is an extremely tiny molecule, meaning that it leaks readily through the smallest seams and gaps.
- Embrittlement: Alternatively, hydrogen molecules may get stuck between the tank wall’s atoms, jamming them apart. So over time, the tank will become more and more brittle. This is combined with the fact that subjecting most materials to cryogenic temperatures will already make them brittle.
With these numerous problems, hydrogen as a fuel is mainly confined to applications where extreme performance is required and storage times are short. In modern use, hydrogen shines as a fuel for high-performance upper stages, such as with the Centaur. It doesn’t usually find use as a lower-stage fuel (with the notable exception of the Space Shuttle) because enormous tanks would be required.
Oxidizers – the other component required for combustion – may also have many of the same issues, and then some. For launch vehicles, liquid oxygen (or LOX) is a commonly-used oxidizer. It must be stored below -183 °C, meaning it is still a cryogen, but it is still much less problematic to store than liquid hydrogen. Insulation and boil-off are still concerns; launch vehicles do have to continuously vent and top-up on liquid oxygen as it boils in their tanks while awaiting launch, or the pressure would burst the tanks. A serious concern is that liquid oxygen is also a ferociously powerful oxidizer, easily creating flammable mixtures with many materials (such as steel, Teflon, engineers, and cloth.)
One of the upsides of liquid oxygen is that it has 16 times the density of liquid hydrogen and 1.4 times the density of kerosene, meaning that it does not require large tanks. This is why the Space Shuttle’s external fuel tank required much more volume to store its liquid hydrogen than to store its liquid oxygen, despite the fact that it required a much greater mass of oxygen.
This laundry list of issues highlights some of the engineering and logistical considerations that go into choosing a fuel-oxidizer combination.
One of the most common fuels used in launch vehicles today is RP-1, a high-purity kerosene usually burned with oxygen. Here, we see how history affects modern-day rocketry. RP-1 is based on the formulation of kerosene for jet fuels, but with higher purity and thermal stability. It was initially developed for applications in nuclear ICBMs such as the Soviet R-7 and American Atlas.
Despite having a poorer performance than hydrogen, with a specific impulse of only 330 seconds even with the best engines, RP-1 has far better density and storability. Ballistic missiles had to be on standby for years on end and be small enough to fit into silos, favoring RP-1. Its heritage in missiles, and later in orbital launch vehicles, has made RP-1/LOX the propellant combination of choice for rockets like the Saturn V, Falcon 9, and Electron.
Bipropellants usually require igniters to provide the small kick needed to get the reaction going. Igniters can be broadly classified into spark and chemical types.
Spark igniters are exactly what they sound like – larger, beefier versions of the sparkplugs found in common automotive internal combustion engines. By causing an electrical arc to jump across the terminals, they ignite a small region of mixed propellant vapors, and the flame spreads out from there. The SpaceX Raptor, fueled by methane and oxygen, uses a spark igniter. This allows it to restart many times, since the igniter can fire as long as electrical power is available.
However, for larger engines and those that use fuels that are more difficult to ignite (like kerosene), spark igniters do not deliver enough energy evenly enough. Unburned propellant may pool in one region of the engine while the flame is spreading, then detonate all at once – this can cause harmful resonances that destroy the engine. For these situations, we turn to more powerful chemical igniters.
You may hear about TEA-TEB ignition fluid. TEA-TEB stands for triethylaluminum–triethylborane (rolls right off the tongue), which is a mixture of two chemicals. Both of these chemicals are pyrophoric, meaning they ignite spontaneously on contact with air. Both the Saturn V and Falcon 9 light their engines by injecting flaming TEA-TEB into their combustion chambers, effectively igniting the fuel with a flamethrower. While chemical igniters are much more effective at uniformly igniting stubborn propellant combinations, they also limit the number of restarts possible. Once a rocket runs out of ignition fluid, restarts are no longer possible.
Developing in parallel with RP-1/LOX and LH2/LOX were the hypergolic fuels, which need no igniters. These combinations too, have roots in both history and engineering.
Through the mid-1930s and 1940s in Nazi Germany, development was progressing on rocket propellant combinations that self-ignited. These fuels later became known as hypergols, with the suffix ‘-gol’ being a portmanteau of Greek ‘ergon’ (work) and Latin ‘oleum’ (oil.) The moment a hypergolic fuel and oxidizer come into contact with each other, they ignite spontaneously.
An engine using hypergolic fuels can be started and restarted simply by opening the propellant valves, dramatically improving its simplicity and reliability. With the performance of many hypergols coming close to that available from RP-1/LOX while being much easier to store, they found their way onto many ballistic missiles and into a number of older orbital launch vehicle designs like the Russian Proton and Chinese Long March.
They also have numerous applications for orbital and interplanetary vehicles, where long-term storability and reliability are critical. For example, the Dragon capsule’s Draco and SuperDraco engines, the Apollo lunar lander and service module, and the Space Shuttle’s orbital maneuvering engines all used hypergolic fuels.
However, these advantages come at the cost of extreme toxicity. Fuelling a spacecraft with hypergolic fuels requires substantial protective equipment for the operators. In July 1975, the last Apollo spacecraft was returning to Earth from the Apollo-Soyuz mission, when an accident occurred that underscored this danger. The maneuvering thrusters, which used hydrazine and nitrogen tetroxide, had been accidentally left on throughout the descent. When the ventilation valve opened to bring fresh outside air into the cabin, the astronauts were exposed to the toxic fumes, causing one to lose consciousness and requiring all three to be hospitalized for two weeks.
Note the self-contained breathing apparatus used by NASA technicians in this video needed to handle hypergolic fuels:
The most common hypergolic fuels are based on hydrazine, a chemical consisting of two nitrogen atoms linked together, surrounded by four hydrogen atoms:
They are frequently burned with dinitrogen tetroxide or NTO:
Hydrazine-based fuels are especially easy to store because they remain stable over long periods of time, have high densities, and can be liquefied at room temperature by simply pressurizing them, unlike cryogenic propellants. This makes them especially useful for long-duration space missions – and ballistic missiles. This means that cryogenic refrigeration equipment is not necessary, reducing power requirements for space travel – and allowing nuclear ICBMs to be launched at shorter notice.
One of the most common hypergolic fuels is MMH, or monomethylhydrazine. It is hydrazine with one of its hydrogen atoms replaced with a methyl group:
Another common hypergolic fuel is UDMH, or unsymmetrical dimethylhydrazine. It is hydrazine with two of its hydrogen atoms replaced with methyl groups on one side:
The choice between UDMH, MMH, or a blend of the two is based on application and engine design. While MMH is slightly denser and has a slightly higher performance, UDMH has better thermal stability – good enough that it can be pumped through a rocket engine’s walls for cooling (also known as regenerative cooling.) Both have the low freezing points needed for ballistic missile applications; however, blending UDMH and MMH together results in a lower freezing point than either would have alone (also known as an eutectic mixture.)
While older designs used mixtures such as aniline and red fuming nitric acid as hypergolic propellants, these rapidly fell out of favor with the emergence of MMH/NTO, UDMH/NTO, and blends of the two. Many orbital launch vehicles, satellites, interplanetary probes, and smaller spacecraft still use these propellants, although their toxicity is spurring research into safer, greener alternatives.
While hypergolic engines are simple because the fuel and oxidizer will ignite on contact, some engine designs are even simpler by using only a single propellant. These are known as monopropellants. Monopropellants are chemicals that decompose energetically when passed over a catalyst bed, producing thrust. Rocket engines can’t get much simpler than this: a pressurized tank of propellant, a valve, a combustion chamber with a catalyst bed on the inside, and a nozzle.
The most common monopropellant is hydrazine. When it comes into contact with an iridium catalyst, it violently decomposes into nitrogen and hydrogen gas at over 1,000 °C, producing thrust. Generally, monopropellants have poorer performance than bipropellants. However, hydrazine monopropellant can still achieve respectable specific impulses of up to about 250 seconds.
Many spacecraft are equipped with small maneuvering thrusters to make minor adjustments. These may need to fire in bursts dozens or even hundreds of times during a mission, demanding high reliability and rapid response. Monopropellant thrusters excel in this application, as valves can be opened and closed very quickly, and they are reliable due to their simplicity. Since the adjustments are small, the lower specific impulse matters less. However, hydrazine also finds use for larger maneuvers: The UAE’s Hope probe used hydrazine monopropellant thrusters to brake into Mars orbit, and the Perseverance rover also used hydrazine monopropellant in its skycrane.
As was the case with hypergolic fuels, hydrazine’s toxicity has driven a search for alternatives. A possible contender for small spacecraft, like CubeSats, is nitrous oxide (laughing gas), with the formula N2O. When passed over a catalyst like iridium, it decomposes into nitrogen and oxygen gas at about 580 °C. While it can only achieve a meager specific impulse of about 180 seconds, this can be sufficient for low delta-V maneuvering, such as station-keeping or deorbiting an old satellite. Nitrous oxide also has the advantage of having similar storability to hydrazine while being much less toxic. Development of nitrous oxide monopropellant thrusters for CubeSats is ongoing at the University of Toronto Institute for Aerospace Studies Space Flight Laboratory (UTIAS SFL).
In the discussion on bipropellants, noticeably missing were methane and natural gas, used by the SpaceX Raptor and Blue Origin BE-4, respectively. Methane/LOX (or methalox) is gaining popularity as a potential propellant mix for future rockets, and will likely power the Starship on Mars missions. Methalox can reach specific impulses of approximately 380 seconds, which is better than RP-1/LOX with fewer of the storage headaches of LH2/LOX.
(Liquefied natural gas [LNG] is almost entirely methane with some impurities, so we’ll lump LNG/LOX together with methane/LOX.)
While methane is still cryogenic with a boiling point of -162 °C, that is still 91 degrees higher than the boiling point of liquid hydrogen. It also doesn’t exhibit the pesky issues that hydrogen has with embrittlement and para-ortho transitions.
This does beg a question: The first methalox engine was fired by German rocket engineer Johannes Winkler in 1931. Why did it only begin to see real, large-scale usage almost ninety years later? The answer, again, lies in history. When the main applications of rocketry were in developing ballistic missiles, methane’s performance advantage over RP-1 was not considered significant enough to be worth the problems of its low boiling point and slightly lower density.
Times have changed.
Now, with the need for peaceful orbital launch vehicles skyrocketing (pun intended), engineering requirements have changed. In this context, methane being a cryogen is less problematic. Methane is also better for reusability: Having no heavy hydrocarbons, methane burns much cleaner than RP-1 and is less likely to clog the engine with soot. Methane also ignites more easily, meaning that spark igniters (that can be used an unlimited number of times) can be used instead of chemical ones.
Above all these considerations, one of the most important benefits of methane is that it can be manufactured from the Martian atmosphere by the Sabatier process. The details were covered in one of our first articles, ISRU Part I: How to Make Fuel, Oxygen, and Water on Mars. It’s no surprise that the SpaceX Starship has been designed to use methalox – the Starships will be returning to Earth with fuel made on Mars.
The choice of a rocket’s fuel depends on its application. The rockets of the past, ICBMs, carried payloads that could destroy civilization. The rockets of the future may save it.
Footnotes and further reading
For a hilariously-written yet informative and accessible account of the (frankly bonkers) history of rocket propellants, read Ignition!: An Informal History of Liquid Rocket Propellants by propellant chemist John Drury Clark. It is out of print, but has been preserved online here.
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