The Saturn V rocket used during the Apollo missions of the 1960s and 70s carried an incredible 810 700 liters of RP-1 fuel in its first-stage boosters. But what exactly is RP-1, and why is it so widely used in orbital rockets?
RP-1, also known as Rocket Propellent-1 or Refined Petroleum-1, is a highly refined form of kerosene primarily used to power orbital rockets. Its higher density levels make it more energetic and fuel efficient, and the removal of unwanted compounds improves engine performance and protection.
In recent times, a lot of emphasis has been placed on the use of clean-burning rocket propellants like liquid hydrogen and methane. Not only are they more environmentally friendly, but they also have a higher specific impulse than RP-1 rocket propellant.
Liquid hydrogen and liquid methane are arguably the most well-known examples of these environmentally-friendly fuels that are continuing to gain popularity, with an increasing number of rocket engines being designed and researched that utilize these propellants.
However, despite all their advantages, RP-1 rocket propellant is still used in the vast majority of orbital launch vehicles, specifically in the first stage sections of these rockets, and with good reason.
The following sections will explain why RP-1 is such a popular propellant among spacecraft manufacturers and also highlight its various advantages and drawbacks. Bur one first needs to define what exactly RP-1 is.
What is RP-1 Propellant?
As summarized in the introduction, RP-1, also known as Rocket Propellent-1 or Refined Petroleum-1, is a highly refined form of kerosene that is primarily used to power orbital launch vehicles.
(Learn more about the different types of fuel orbital rockets use and each one’s characteristics, advantages, and disadvantages in this article.)
It is a hydrocarbon, meaning it is an organic compound consisting entirely of hydrogen and carbon. It has a higher density than commonly-used propellants like regular kerosene, diesel, and even refined aviation fuel and is much more energy efficient as a result.
It is also cleaner burning due to the removal or reduction of several unwanted compounds, including sulfur content that can attract and weaken engine components. Other compounds that can result in coking and residue buildups in critical engine parts are also removed.
From the earliest launch vehicles used during the space race between the USA and former Soviet Union to some of today’s most advanced spacecraft, most orbital rockets use RP-1 in at least one or more components or stages.
Like all other fuels used in orbital rockets, RP-1 needs an oxidizer to combust. This comes in the form of liquid oxygen (LOX). The fuel and oxidizer are mixed in the combustion chamber, where they combust to form the hot gases that propel the spacecraft.
The list of rockets using RP-1 includes:
- Saturn V (First Stage)
- SpaceX Falcon 9 (First & Upper Stage)
- Atlas V (First Stage)
- Soyuz (First & Upper Stage)
- Antares (First Stage)
- Rocket Lab Electron (First & Upper Stage)
How RP-1 Propellant Is Made
Like many other highly refined chemical substances, RP-1 propellant is refined through several dedicated steps. This is done to optimize its performance by increasing the fuel density and making it more energetic while also removing unwanted compounds.
One of these steps involves the removal of corrosive compounds like sulfur to reduce their corrosive effects on the engine components.
Aromatics, alkenes, and alkyne levels are also kept to low levels during the refining period since they lead to polymerization under high temperatures and also after sustained periods of storage.
Numerous other steps are involved in the syntheses of RP-1, but making the fuel denser (and more energetic as a result) and removing unwanted compounds to optimize performance are two critical components in the refining process.
Advantages Of Using RP-1 In Orbital Rockets
Some of the main advantages of using RP-1 include:
- Relatively Cheap To Produce
- Storage At Room Temperature
- High Energy Density
- Low Toxicity
- Smaller Fuel Tanks
- Increased Ground Crew Safety Due To Low Vapor Pressure
- High Flashpoint Makes Lowers Risk Of Catching Fire At Lower Temperatures
1) Relatively Cheap To Produce
All fuels used in any form of transport need to be refined to some degree to make them usable for combustion in modern engines. Even diesel needs to go through several steps to turn crude oil into this relatively simple form of propellant.
The more steps involved in the refining process, the more expensive the end product becomes. As a result, the highly refined RP-1 propellant is a much more expensive fuel than even aviation fuel, which, in turn, is much more costly than other forms of kerosene.
Compared to the high costs of producing liquid hydrogen or methane, though, RP-1 is a much cheaper and cost-effective choice as rocket fuel. This makes it the first choice for many rocket manufacturers to use in the first stages of their launch vehicles.
2) Storage At Room Temperature
Fuel cost is not the only consideration when it comes to choosing the fuel for an orbital launch vehicle. The logistics of transporting, handling, and storage of fuel also play a crucial role, considering the large volume of propellants that are needed at the launch site.
Hydrogen needs to be stored at temperatures below -253° Celsius (-423° Fahrenheit) to remain a liquid, which requires special equipment and care to keep it cool and protect ground crews from the dangerous cryogenic temperatures.
Like liquid hydrogen, liquid methane is a cryogenic fuel that must be cooled to a temperature of -162° Celsius (-260° Fahrenheit) to turn into a liquid.
(Learn more about liquid methane, what exactly it is, how it is made, and its various advantages and disadvantages in this article.)
RP-1 propellant, on the other hand, can be transported and stored at room temperature, making it a lot easier to handle, transport, and store for sustained periods.
3) High Energy Density
In recent years, much of the focus in the space industry has been on a rocket fuel’s Specific Impulse. Specific Impulse refers to how energy-efficient a rocket engine is and is typically measured in seconds.
(Learn more about Specific Impulse, what it is, and why it so important in this article about possible nuclear propulsion for spacecraft.)
Although it doesn’t have the high specific impulse of fuels like liquid hydrogen, it is much much denser, which means it requires much smaller fuel tanks to carry the same amount of fuel. The resulting higher energy density also allows it to provide more thrust during launch.
The higher thrust (as a result of the fuel’s larger mass per volume and resulting increased density) is crucial in a launch vehicle’s first-stage boosters to allow the rocket to push through Earth’s thick atmosphere and escape the planet’s gravitational forces to reach space.
4) Low Toxicity
Some of the fuel types used in orbital rockets include hyperbolic fuels. Hypergolic fuels are propellants that spontaneously combust upon contact with each other without the need for an ignition mechanism. Their ignition can also easily be stopped and restarted.
However, hypergolic fuels are highly toxic and should be handled with extreme care. It makes them unsuitable for most launch vehicles operating in the atmosphere since a rocket failure on the launchpad or close to the ground will have a major environmental impact.
(The fuel component of hyperbolic propellants is usually a form of hydrazine like monomethylhydrazine or unsymmetrical dimethylhydrazine. The oxidizer component is typically nitrogen tetroxide or nitric acid.)
Although RP-1 is a carbon-based fuel (which means one of the byproducts of its combustion is the creation of carbon dioxide in the atmosphere), it is a lot less toxic than hypergolic and many other fuel types, making it safe to handle and use in a relatively safe environment.
The removal or reduction of sulfuric compounds and other impurities during the refining process also contributes to this propellant being less toxic than other alternatives.
5) Smaller Fuel Tanks
The high density of RP-1 propellant mentioned earlier in this section also means that it requires smaller fuel tanks than a lighter fuel like liquid hydrogen, which lower density needs much larger tanks for the same amount of fuel.
In orbital rockets, this is a crucial factor since any savings in the vehicle’s mass is invaluable and means less energy, and as a result, less fuel will be required for the spacecraft to reach space and establish an orbit around the Earth.
6) Increased Ground Crew Safety Due To Low Vapor Pressure
Fuels like liquid hydrogen and methane are environmentally friendly, as they produce little to no byproducts that can contribute to air pollution. Hydrogen also has a much higher Specific Impulse than RP-1, making it a popular choice, specifically for rocket upper stages.
(Learn more about liquid hydrogen, what it is, as well as the different advantages and drawbacks of this fuel in this article.)
However, both liquid hydrogen and methane are cryogenic fuels, and as soon as they are pumped into a launch vehicle’s fuel tanks on a launchpad, they warm up and start to evaporate, causing a buildup of pressure inside the tanks.
As a result, these fuels need to be vented to prevent the tanks from fracturing under increased pressure from the continuous evaporation. This causes a substantial amount of potentially dangerous gases to build up in and around the rocket and launchpad.
The buildup of gases at the launch site can become toxic and also increase the likelihood of an unintentional ignition. Both scenarios create a dangerous situation for ground crews that need to work in the area.
Since RP-1 one remains a liquid at room temperature, no pressure buildup occurs, and the resulting necessary venting is not required. This makes working in and around an RP-1 fueled rocket much safer for ground crews in and around the launch site.
7) High Flashpoint Lowers Risk Of Fire
Since more than 85% of an average orbital rocket’s mass consists of fuel, one of the biggest concerns before and during launch is the dangers of an unintended fire and resulting explosion. Many spectacular rocket failures on the launchpad illustrated this point.
RP-1 one has a high flashpoint, meaning it needs to reach much higher temperatures than many other fuels before its mixture with oxygen can result in ignition. This contributes to the relative safety of using this fuel type in such large quantities.
Disadvantages Of Using RP-1 In Orbital Rockets
Despite the many advantages of RP-1 propellant, it also has several drawbacks, some of which were already mentioned. The most notable disadvantages of using RP-1 include:
- Lower Specific Impulse
- Residue Buildup In Engines
- Increased Air Pollution
- Dead Space Needed Between Fuel And Oxidizer Tanks
1) Lower Specific Impulse
In the section covering the advantages of RP-1 propellant, the benefits of having a high energy density were highlighted. Unfortunately, the greater molecular mass of the fuel that contributes to its high energy density also has a drawback.
Although it provides more thrust during launch, the heavier particles in RP-1 make it more difficult to be accelerated through the rocket’s nozzle at the same speed as the much lighter hydrogen particles.
As a result, the fuel is not able to match the impressive Specific Impulse achieved by liquid hydrogen, which makes the latter a much more engine-efficient propellant with a Specific Impulse of approximately 381 seconds versus the 281 seconds achieved by RP-1.
(The above values were calculated at sea level.)
2) Residue Buildup In Engines
As stated earlier in the article, RP-1 is a type of hydrocarbon, which means it produces carbon dioxide and other byproducts that can result in coking and other residue buildups within the rocket engine and fuel lines.
Although some coking can actually be advantageous and cause a thin layer of film that helps to protect the engine walls from the hot gases, excessive residue buildup can lead to the clogging of critical engine components, heat buildup, and reduced overall performance.
Heavy amounts of residue buildup also make it harder and much more expensive to clean and refurbish rocket engine boosters for reuse at subsequent launches.
3) Increased Air Pollution
With a global move towards reducing global emissions and being more environmentally friendly, clean burning fuels like liquid hydrogen and methane are becoming increasingly popular as the fuel of choice for many launch vehicle manufacturers.
Although it remains relatively clean compared to many other fuel types used in other forms of transport, RP-1 propellant still produces a fair amount of air pollution compared to its two cryogenic counterparts.
As already mentioned, RP-1 is a type of hydrocarbon, which means part of the byproducts of combustion is carbon dioxide, which contributes to air pollution. It is also a hothouse gas, meaning it is also a contributor to Global Warming.
Other substances that are also released into the atmosphere during RP-1 combustion include nitrogen oxides, carbon soot, and some sulfuric compounds, all of which contribute to the amount of air pollution present in the air.
4) Dead Space Needed Between Fuel And Oxidizer Tanks
One of the main advantages of RP-1 propellant is that it remains a liquid at room temperature, which allows for easy and safe transportation and storage. To burn, however, it needs an oxidizer in the form of liquid oxygen (LOX), which present some challenges.
Liquid oxygen is also a cryogenic substance that needs to be stored at temperatures of approximately -183° Celsius (-297° Fahrenheit) to remain a liquid. This is not a problem when used with other cryogenic fuels like liquid hydrogen or methane.
However, the large temperature difference between RP-1 and liquid oxygen means a fair amount of dead space is needed between the two propellant tanks to keep the fuels insulated from each other and maintain their respective temperatures.
This adds size and mass to the launch vehicle, which is exactly the opposite of what most launch vehicle manufacturers strive for when designing an orbital rocket.
As this article clearly illustrated, RP-1 propellant, which has been used for more than half a century in orbital launch vehicles, has both its advantages and drawbacks.
However, for the time being, the fuel’s advantages far outweigh its drawbacks, and one can expect to see it used in the majority of first-stage boosters of most orbital rockets for the foreseeable future.
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