SpaceX’s newest rocket, Starship, uses Methane(CH4) and Liquid Oxygen (LOX) as fuel. However, the Falcon 9 and Falcon Heavy uses rocket-grade Kerosene (RP-1) and LOX as a propellant.
So, why does SpaceX use Kerosene?
SpaceX uses Kerosene in Falcon Rockets because up until 2007; there were no Methane engines. And Kerosene is cheaper, denser, stable at room temperature. It’s easy to handle, is not toxic like hypergolic propellent, easily transportable, and has no leakage issue similar to Liquid Hydrogen (LH2). It also has a higher energy density and presents a lower explosion hazard than LH2, and Kerosene rocket engines produce more thrust per dollar.
Since childhood, Elon Musk has been fascinated by space and rockets. When Elon Musk made some money during the PayPal sale to eBay, he wanted to buy a rocket to send a greenhouse to Mars.
However, soon he found that all the commercial rockets were too expensive because none were reusable.
So, he invested $100 million and formed SpaceX.
SpaceX’s primary goal is to make human life multi-planetary. To make this successful, they started working on a reusable rocket.
Types of available rocket propellant:
As we mentioned earlier, Elon Musk started his SpaceX in 2002. At that time, these were common types of rocket fuels:
- Solid propellants.
- Highly refined Kerosene (RP-1) and Liquid Oxygen (LOX).
- Liquid Hydrogen (LH2) and Liquid Oxygen (LOX).
- Hypergolic propellants.
Nowadays, we have seen that many rocket engines use Methane (CH4) and Liquid Oxygen. However, up until 2007, there were no Methane rocket engines. 
Why SpaceX did not choose Solid Propellent:
The primary goal of SpaceX rocket development was to design and manufacture a reusable rocket.
Solid propellant rockets are very easy to design and manufacture. This type of rocket generally has a steel tube filled with solid rocket fuel and oxidizer. When ignited, this fuel burns rapidly and expels hot gasses through a nozzle, and produces thrust.
However, this solid propellant rocket has a problem. Unlike liquid propellant engines, we can’t shut down “solid propellant” rocket motors once we ignite them. They will burn until all the propellant is finished.
Therefore, SpaceX can’t use solid rocket propellent.
Hypergolic Propellant is toxic:
Hypergolic propellants are well understood, storable, and give the best rocket performance. The Proton rocket uses Hypergols.
Hydrazine, unsymmetrical dimethyl-hydrazine (UDMH), and monomethyl-hydrazine (MMH) are common types of hypergolic fuels.
However, there are severe issues with this rocket fuel.
Hypergolic propellants are incredibly toxic, corrosive, explode on contact, give off corrosive and poisonous vapors, have a high freezing point, and are too unstable for use as a coolant.
One has to put on a HAZMAT suit to handle this propellant.
Therefore, SpaceX decided not to use this propellant for their Falcon rocket.
So, Why didn’t they choose Liquid Hydrogen?
For a long time, we have been using Hydrogen as rocket fuel. As a rocket fuel, Hydrogen has numerous benefits.
Benefits of Liquid Hydrogen:
Hydrogen is a lightweight and extremely powerful rocket propellant. In combination with liquid oxygen, Liquid Hydrogen generates the highest specific impulse or efficiency. Moreover, liquid Hydrogen has three times more energy density than Kerosene in terms of mass.
In short, Hydrogen-LOX rocket engines are very efficient.
However, the critical benefit of Liquid Hydrogen ends here.
Drawbacks of Liquid Hydrogen:
Liquid Hydrogen(LH2) poses an immense technical challenge. Hydrogen boils at -423°F (-253°C); therefore, we must store it at -423°F, and to store and transport, it needs a highly specialized storage container.
Hydrogen density is 0.071 g/ml, whereas the density of Kerosene (RP-1) is 0.820 g/ml. In short, LH2 is 12 times less dense than Kerosene. Therefore, an LH2 tank would be bigger in volume for the necessary propellant than an RP-1 rocket tank. As a result, tank weight would be more and less aerodynamic.
Moreover, to prevent liquid Hydrogen from boiling off, the rockets need insulation from all heat sources, such as rocket engine exhaust, air friction during launch, and radiant heat from the sun.
Hydrogen rockets need advanced metallurgy to prevent hydrogen embrittlement. Metals become brittle when exposed to the extreme cold of liquid Hydrogen.
It also leaks very easily. Liquid Hydrogen can leak through tiny pores in welded joints.
Liquid Hydrogen is costly too.
An enormous amount of technical expertise is necessary to solve all these problems.
SpaceX was starting a new space program with a clean-sheet design. Because of all these problems related to LH2, SpaceX did not choose Hydrogen for their Falcon rockets.
SpaceX decides to use Kerosene (RP-1):
SpaceX Falcon rocket uses rocket-grade Kerosene (RP-1) and Liquid oxygen (LOX). But it is not a new thing. For more than 50 years, various rockets have been using this rocket propellent.
Though Kerosene (RP-1) has a lower specific impulse than liquid Hydrogen; it’s cheaper, denser, stable at atmospheric temperature, has higher energy density, and presents a lower explosion hazard than liquid Hydrogen.
Moreover, compared to hypergolic propellent, liquid Kerosene is not toxic and carcinogenic.
As a result, SpaceX can load more fuel into the rocket with less weight and allows them to carry larger payloads into orbit.
We can also transport rocket-grade kerosine (RP-1) in a regular 18 wheeler gasoline truck without any modification.
Note: SpaceX uses very cold Kerosene and liquid oxygen. That’s why they load this propellant right before the rocket launch.
RP-1 can also be used as a coolant as well as a propellant.
In summary, Kerosene is denser than Hydrogen; thus, we need a smaller rocket. RP-1 is liquid at room temperature, easy to handle, is not toxic, easily transportable, has no leakage issue similar to Liquid Hydrogen, and Kerosene rocket engines produce much more thrust per dollar. Because of all these reasons, SpaceX chose rocket grade Kerosene for their Falcon rockets.
Drawbacks of Kerosene (RP-1) Propellant:
Kerosene produces soot, coking, and polymerization inside the Merlin engine. As a result, after each rocket launch, SpaceX needs to clean its rocket engine thoroughly. This process is costly and prevents SpaceX from reusing their rocket in rapid succession.
Though Falcon 9 and Falcon Heavy’s Merlin engine uses liquid Kerosene, the Starship’s Raptor engine uses liquid Methane (CH4) as fuel.
Methane burns cleanly and far superior fuel for reusable rockets.
Nowadays, liquid Methane (CH4) is a commonly used rocket propellant. However, it was not as prevalent before 2007.
SpaceX is building the Starship to send payload and humans to Mars. Though there is no Methane on Mars, we can manufacture Methane via the Sabatier process. In this process, we need to mix Carbon Dioxide (CO2) with Hydrogen (H) and then heat the mixture to produce Methane (CH4) and Water (H2O.)
The Martian atmosphere is an abundant source of carbon dioxide (CO2) and has water in the ground. SpaceX can produce Hydrogen from Martian water.
On other planets in our solar system, Jupiter, Saturn, Uranus, and Neptune all have Methane in their atmosphere. Pluto has frozen Methane ice on its surface, and Saturn’s moon Titan has Methane Lake.
Methane is also safe for humans. That’s why it is called a green propellant.
Can SpaceX or Elon Musk claim Mars?
In 2020, SpaceX’s Starlink started their service. Like every other product, a new Starlink customer has to sign a legal document. In that legal document, there is a term which states that:
“For Services provided on Mars, or in transit to Mars via Starship or other colonization spacecraft, the parties recognize Mars as a free planet and that no Earth-based government has authority or sovereignty over Martian activities. Accordingly, Disputes will be settled through self-governing principles, established in good faith, at the time of Martian settlement.”
Therefore as you can see, if you want to get the Starlink internet, you have to declare, Mars is a free planet.
So, can SpaceX or Elon Musk claim Mars? Yes, SpaceX can claim Mars because according to a 2015 federal law 114-90, the US government allows a private entity to mine, own, possess, and sell a celestial body, even though the US does not claim it. Therefore, according to that federal law, Elon Musk and SpaceX can claim and mine Mars.
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When SpaceX started developing their Falcon rocket, rocket grade kerosene (RP-1) was the best choice for rocket fuel. However, later they made the Raptor engine, which uses Methane (CH4). When Starship becomes fully operational and retires the Falcon rocket, SpaceX will no longer use the Kerosene again.