Here’s why SpaceX doesn’t use Liquid Hydrogen.

SpaceX’s new Starship rocket uses the Raptor engine. Liquid Methane and Oxygen is the fuel of this engine. The Falcon 9 and Falcon heavy use kerosene (RP-1) as fuel. Moreover, for a long time, Hydrogen has been used in various rockets. But, SpaceX chose Methane for their Raptor. No other rockets have ever used Methane as rocket fuel.

So, why doesn’t SpaceX use liquid Hydrogen?

It’s because liquid Hydrogen needs extreme temperature control, leaks easily, is less dense, thus requires a bigger fuel tank, expensive compared to Methane, not suitable for a reusable rocket and Mars mission, needs advanced metallurgy; hence, it increases rocket complexity and cost. 

Let’s elaborate.

SpaceX has developed the Raptor. It’s a Methane powered full-flow staged combustion cycle engine. The Raptor engine is the first-ever in history, a full-flow staged combustion cycle engine ever flown on a rocket. No rocket engine that uses Methane has ever reached orbit except SpaceX rockets.

SpaceX’s Starship is a rapidly reusable Mars rocket. The Falcon rockets are not fully reusable. While the first stage of this rocket is reusable, the upper stage is not.

Another problem with Falcon 9 and Falcon Heavy rocket is it’s not rapidly reusable. After every flight, this rocket needs extensive refurbishment before SpaceX can use it again. The primary reason is the fuel of the Falcon rocket is Kerosene (RP-1). The burnt fuel leaves soot in the Merlin engine compartment, making it harder to reuse it without extensive cleaning.

SpaceX’s Mars Plan:

As we mentioned earlier, Elon Musk wants to go to Mars. A spaceship will take more than 6 months to reach Mars from Earth. The round trip is 12 months. 

We know that the Mars atmosphere has CO2 and has frozen water in the ground. On Mars, we can create Methane (CH4) using these resources.

First, through electrolysis, we can split water (H2O) into Hydrogen (H2) and Oxygen (O2). 

2H2O = O2 + 2H2

Oxygen and Hydrogen will be collected in separate containers. Humans will use the Oxygen produced in this step.

On the next step, through a chemical process known as the Sabatier process, Carbon dioxide (CO2) and Hydrogen (H2) would create Methane (CH4).

CO2 + 4H2 = CH4 + 2H2O

The Methane (CH4) and water will be collected in separate containers. Starship will use Methane (CH4) and Oxygen (O2) as fuel. The Martians can drink water (H2O) produced in this step or use it to make Oxygen and Hydrogen.

Astronauts already use these steps to produce Oxygen from water and eliminate Carbon Dioxide at the International Space Station (ISS). As this process has been tested; therefore, we can also use it on Mars. The only difference is, ISS releases Methane (CH4) into outer space, while on Mars, we will collect it into containers as rocket fuel.

In the above-described process, we can make both Methane (CH4) and Hydrogen (H2). 

Kerosene (RP-1)MethaneHydrogen
Density813 g/L422 g/L70 g/L
Oxidizer vs. Fuel Ratio2.7 : 13.7 : 16 : 1
Efficiency370s459s532s
Combustion Temp3670 K3550 K3070 K
Boiling Point490 K111 K 20 K
Combustion ByproductsCO2, H2O, and black soot.CO2 and H2OH2O
Manufacturable on MarsNoYesYes

In rocketry, the efficiency of an engine is measured in specific impulse (ISP). 

A simplified comparison would be the MPG of our gasoline-powered vehicle. The higher the MPG of a car, the more mileage it can go on 1 gallon of gas, and the more efficient it is.

Now, let’s assume that we have 1lb of rocket fuel. It could be Kerosene (RP-1), Methane (CH4), or Hydrogen (H2).

ISP is measured in seconds. It’s the measurement for how many seconds the engine can push with 9.8 newtons of force with a fixed amount of fuel. The higher it can push with 9.8 newtons with the same amount of fuel, the higher the efficiency or ISP.

If we have one rocket engine with 100s ISP and another one with 150s ISP, then the second engine with 150s ISP is more efficient because it can do more work with the same amount of fuel.

From the above table, we can see that Hydrogen is efficient compared to Methane. However, Hydrogen has several issues.

SpaceX wants their rocket to be simple, cheap, and reliable. According to Elon Musk, “the best part is no part; the best process is no process.”

Though Hydrogen is efficient compared to other rocket propellants, it adds complexity to rocket engines and rocket design.

Hydrogen is a cryogenic fluid. It’s melting point is -259° C or -435°F, and its boiling point is -252°C or -423°F. Hydrogen needs to be stored in extremely cold temperatures to keep it in liquid form. Because of this, a hydrogen rocket needs insulation around tanks, thus increasing rocket weight, production complexity, and cost.

Furthermore, other problems arise because of the Hydrogen’s low boiling point. 

On a long trip to the Moon and Mars, the Hydrogen will boil off and evaporate. Moreover, during the earth’s re-entry of the Starship, the generated heat will cause a significant technical challenge to keep Hydrogen liquified in the fuel tank.

However, Methane does not have these problems. 

Hydrogen embrittlement is a serious issue. When metal comes into contact with cryogenic Hydrogen, the metal becomes brittle. Therefore, a reusable rocket design that uses Hydrogen is very complex and challenging. Hydrogen engines need advanced metallurgy to prevent this embrittlement.

Methane (CH4) does not create this embrittlement issue.

The density of Hydrogen is 70 g/L. In contrast, the density of Methane is 422 g/L. As a result, a hydrogen rocket’s fuel tank needs to be significantly bigger than a Methane powered rocket. A big tank means a heavier rocket.

Therefore, a Methane powered rocket would be lighter compared to a Hydrogen rocket.

Hydrogen is the smallest molecule on earth. It leaks easily, primarily through the welded joints of the fuel tanks. Therefore, it needs extraordinary precision and cares to make the fuel tank leak proof. 

Methane does not have this leak issue.

Hydrogen is also expensive compared to Methane (CH4). 

As we can see, though Hydrogen is more efficient, it has many drawbacks.

Therefore, SpaceX needs a rocket engine that does not have these issues. 

When they first started designing an engine, SpaceX did not want to take any risk of creating a completely new type of engine that no one developed. Therefore, they decided to use Kerosene (RP-1) for their Falcon family rockets. When they were successful with their Marlin engine, they started their R&D for the Methane powered Raptor.

In short, Liquid Methane (CH4) has several benefits over liquid Hydrogen. It’s easier to store. A passive cooling system is enough to keep the Methane at cryogenic temperature, significantly denser than Hydrogen. A Methane powered rocket fuel tank is smaller and less bulky. It’s why SpaceX uses Methane and doesn’t use Hydrogen.

Interesting Findings:

How much does Elon Musk own of SpaceX?

SpaceX is a privately held company, and Elon Musk is the principal shareholder of SpaceX. During a recent FCC filing, SpaceX revealed that Elon Musk owns 54% of the company with 78% voting control in the SpaceX board.[1] Moreover, Elon Musk is the only person who owns more than 10% share of this company.

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.

To learn more, please click here.

Bottom Line:

SpaceX made the right decision by abandoning Kerosene and Hydrogen and embracing Methane for their Starship.

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