Kerosene as rocket fuel

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Kerosene as Rocket Fuel, RP-1

Kerosene, specifically highly refined kerosene like RP-1, is a common rocket fuel, called RP-1 (Rocket Propellant-1). RP-1 is a mixture of long chain hydrocarbons that has been purified to remove unstable or contaminant components. 

RP-1 fuel. Source: Wiki
RP-1 is a refined petroleum distillate similar to aviation kerosene but processed to strict specifications. Compared to ordinary kerosene it contains fewer unsaturated compounds and has tighter limits on impurities and sulfur. 

Specific properties of RP-1:
  • Density: At ambient temperatures (around 15°C to 20°C), RP-1's density is about 810 kg/m³ (0.81 g/cm³). Density is a key factor in rocket design for determining the volume of fuel needed. 
  • Freezing Point: RP-1's freezing point is well below normal operating temperatures, -60°C or lower. This is a desirable property for a storable propellant, as it doesn't require cryogenic temperatures like liquid hydrogen. 
  • Boiling Range: As a complex mixture, RP-1 doesn't have a single boiling point but rather a boiling range. A typical range is from about 150°C to 275°C, with a normal boiling point around 231°C. 
  • Specific Energy: RP-1 provides an energy density of approximately 47 MJ/kg. While lower than liquid hydrogen, this is sufficient for many applications. 
  • Flash Point: The flash point for RP-1 is a minimum of 60°C, ensuring it's relatively safe to handle and less likely to form explosive mixtures with air compared to lower-flash-point fuels. Much safer than for example gasoline.
Compared to other rocket fuels, RP-1 provides several advantages with a few tradeoffs. Compared to liquid hydrogen (see Hydrogen as Rocket Fuel), it offers a lower specific impulse, but can be stored at ambient temperatures, has a lower explosion risk, and although its specific energy is lower, its higher density results in greater energy density. 

Hydrogen is very light (0.07 g/cm³), requiring huge insulted tanks. RP-1 has density about 0.81 g/cm³, which allows more compact tanks and critical point for the first stages where rocket's diameter and aerodynamics play crucial role. 

Example: 
In case of 1000 kg of propellant, what volume is required?
  • RP-1: about 1.23 m³
  • LH2: about 14.3 m³
What does it tell us? Hydrogen needs about 10 times bigger tanks than RP-1.

Kerosene is burned with an oxidizer, most commonly LOX, inside liquid rocket engines to produce high temperature, high pressure gases expelled through nozzles of the rocket. 

Merlin engines of a Falcon 9 rocket use RP-1.

Let's talk about Engine performance with RP-1/LOX combination. RP-1 is almost always paired with LOX as oxidizer. The actual performance depends on engine design but typical values are for Mixture Ratio 2.2-2.7 (O/F = oxidizer mass to fuel mass), Chamber Pressure about 6-18MPa (that varies by engine), Specific Impulse (Isp) about 265 - 300 s at sea level and about 310 - 350 s in vacuum. 

Despite having a poorer performance than hydrogen, RP-1 has better density and storability. Ballistic missiles had to be on standby for many years and to be small enough to fit into silos, which plays in favor for RP-1. Its heritage in missiles, and later in orbital launch vehicles, RP-1/LOX is the propellant combination used for rockets like the Saturn V, Falcon 9, and Electron, etc. 
RP-1 is a fuel in the first-stage boosters of the Electron, Soyuz, Zenit, Delta I-III, Atlas, Falcon, Antares, and Tronador II rockets. It also powered the first stages of the Energia, Titan I, Saturn I and IB, and Saturn V.

List of LOX/RP-1 examples of use

There are many advantages, but as well as other disadvantages, we didn't talk about yet.

RP-1 used in rocket engines, undergoes thermal decomposition in the high-temperature environment of regenerative cooling channels, forming carbon-rich deposits (coke) on the channel walls. This coking process degrades heat transfer, increasing wall temperatures and potentially leading to reduced engine performance, pressure loss, and even nozzle blockage in severe cases. Coking is most prominent at wall temperatures between approximately 600 K and 800 K (327°C to 527°C), with peak formation around 700 K (427°C). 

RP-1 generates as well significant amounts of black carbon (soot) and other pollutants, like carbon dioxide and nitrogen oxides, from its combustion during rocket launches. This soot contributes to atmospheric pollution, stratospheric temperature increase, and ozone layer thinning, with potential impacts on the environment. Researchers are developing cleaner alternative fuels, such as liquid methane.

Advantages vs. Disadvantages

RP-1 remains popular choice for first stage boosters, where rockets require high thrust, dense propellant, such as Falcon 9, Soyuz. However, as we already mentioned earlier, and in earlier articles, new engines are using different fuel, such as methane in Raptor, BE-4 etc (see Methane as Rocket Fuel). 

Summary in numbers


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