SpaceX is not afraid to experiment and challenge the fundamental classics of rocketry. The transformation of understanding what a modern rocket should be: from the time-tested Falcon series with its “modest” 500 cubic meters of tanks to the colossal 5000 cubic meters of the Starship system. Why switch to liquid methane? How did it happen that the same company can launch 16 tons into orbit… or 150 tons at once? This article will analyze the difference between the “kerosene classic” Falcon and the “methane giant” Starship regarding numbers, tanks, and capabilities, and why it is the door to the Moon and Mars.
Fuel combinations
Falcon 9 rockets combine kerosene (RP-1)* and liquid oxygen (LOX)**. The kerosene is stored at lower than standard temperatures to increase its density and fit more fuel into the same dimensions. Liquid oxygen is also further cooled to increase its density and improve combustion efficiency.
*RP-1 is a specially refined and improved type of kerosene used as fuel in rocket engines. It is cleaner than conventional aviation fuel and has more stable combustion characteristics, making it effective for space launches.
** (LOX) is oxygen cooled to an ultra-low temperature to be in a liquid state. It is used as an oxidizer in rocket engines because it provides the necessary oxygen for fuel combustion in space where there is no air.
Starship (along with Super Heavy) is switching to a combination of liquid methane (CH₄) and liquid oxygen (LOX). Compared to kerosene, methane “soots” the engines less, making them easier to maintain and repair. In addition, methane can theoretically be synthesized on Mars using the Sabatier reaction*, which opens up prospects for long-distance space missions.
*The Sabatier reaction is a chemical process in which carbon dioxide (CO₂) and hydrogen (H₂) are combined to form methane (CH₄) and water (H₂O).
The transition to methane fuel vapor allows SpaceX engineers to more efficiently ensure the reusability of engines and also lays the groundwork for future interplanetary missions where fuel can be produced on-site. This approach allows us to dream of longer and larger flights, bringing the space program one step closer to colonizing other planets.
Tank pressurization: helium versus autogenous pressurization
In rocketry, maintaining a stable pressure in fuel tanks is critical. If the tanks “crumple” or their walls deform, this can lead to loss of control and sometimes to a complete accident. SpaceX uses two different pressurization systems depending on the type of rocket: helium and autogenous.
Falcon rockets use helium stored in special high-pressure COPV* (Composite Overwrapped Pressure Vessels) to pressurize liquid oxygen tanks. During fuel consumption, helium is fed into the tank to maintain the required pressure and prevent the deformation of the shell. Helium is also needed during rapid engine start-up or at critical moments of flight (for example, when the engines are turned off and on again).
*COPV – small but extremely durable composite reservoirs are installed inside the tanks, which can withstand pressures up to 350 bar. They supply helium for pressurizing liquid oxygen. This pressure can be developed by hydraulic pressing units using special cylinders. Details of the structure and life of such systems can be found in the NASA report.
However, these tanks are associated with an incident during the Falcon 9 tests of the Amos-6 mission (2016), when an explosion occurred due to complex chemical interactions between helium, carbon fiber, and liquid oxygen. Since then, SpaceX has improved the COPV design and manufacturing processes to reduce risks and increase system reliability.
The new Starship system (including the Super Heavy booster) uses autogenous supercharging: part of the methane and oxygen is vaporized and returned to the top of the tanks. This gas raises the pressure and replaces the need for large volumes of helium. This scheme reduces the weight of the rocket (fewer additional tanks) and simplifies the design, as there is no need to keep large helium reserves on board. At the same time, small gas tanks (COPVs or steel tanks) are still needed for auxiliary operations (orientation control, emergency modes, engine restart), but they perform secondary functions and are not the main pressurization system.
Why give up helium?
- Simplicity and weight reduction: the absence of large high-pressure tanks simplifies the design and saves weight.
- Mitigating risks: using helium in composite tanks at ultra-low temperatures can cause undesirable effects (a lesson learned after Amos-6).
- Mitigating risks: using helium in composite tanks at ultra-low temperatures can cause undesirable effects (a lesson learned after Amos-6).
This approach from SpaceX illustrates the desire for maximum efficiency and safety. It seems that autogenous pressurization will be a key element of future rockets designed not only to deliver cargo to orbit but also for large-scale interplanetary missions.
The infographic below summarizes the key characteristics of the two launch vehicles. It demonstrates the differences in fuel types and quantities, tank volumes, refueling times, and potential payloads. For a better understanding of the scale, we have provided refueling equivalents (the number of cars and Boeing 737 aircraft). It’s hard to imagine how much fuel is needed for just one launch – thousands of cars and dozens of airplanes. So, let’s learn about the capabilities of each missile and compare its effectiveness and technical potential.
Filling and tank design
Falcon’s first stage has two large tanks: one for RP-1 fuel and the other for liquid oxygen (LOX). The second stage is built on a similar principle. The process of refueling with fuel and oxidizer is fast – usually during the last half hour or hour before launch. The fuel reserve in the first stage is designed to ensure not only the main “upward” flight but also the reverse maneuvers: reentry burn (braking impulse in the atmosphere) and landing burn (final impulse for landing).
The Starship has two main tanks for methane (CH₄) and liquid oxygen (LOX), with additional “header tanks”* inside that are used during landing. The Super Heavy booster (lower stage) also contains two large tanks (CH₄ and LOX), but there are usually no separate “headers” (although special tanks for additional maneuvers are possible). When the Starship spacecraft returns for a “side flip” landing, the fuel in the large tanks can move unpredictably, so engineers have developed compact internal tanks. They guarantee a stable supply of fuel and oxidizer to the engines, regardless of dynamic loads and the position of the ship.
*Header tanks – tanks for landing. Starship has additional “header tanks” built inside the main fuel tanks. They are needed for landing: when the main tanks are almost empty or at an awkward angle, the fuel supply can become unstable. The compact tanks, located closer to the engines, hold a supply of methane and oxygen specifically for the final braking impulse during landing. These tanks are filled in advance, and at the moment of landing, the engines receive fuel from them, guaranteeing a stable supply even in case of a sharp change in the ship’s orientation.
Thus, Falcon, with its “classic” tank layout and quick refueling, paved the way for reusability. Starship takes this concept to new heights by using not only a different fuel pair but also additional “header tanks” for the most efficient landing.
Instant start systems and a “limited window” for refueling
SpaceX’s Falcon 9 and Falcon Heavy rockets use rapid fuel and oxidizer refueling technology. The process takes place in the last minutes before launch (instead of several hours). The main reason for this is the low temperature of liquid oxygen (LOX) and kerosene (RP-1), which makes it necessary to reduce the time for fuel “heating” in the tanks. As a result, the starting window is strictly limited: it is necessary to meet certain minutes while maintaining the required temperature and density of the fuel.
Advantages of a separate fuel supply for landing
In Falcon 9, it is usually enough to “precipitate” the fuel at the bottom of the tank using gas pulse thrusters (RCS) and correctly calculate the remaining amount.
In the Starship, especially during the horizontal “drop” and subsequent “turn” maneuver, fuel can splash, creating the risk of an “empty fence” and loss of thrust. This problem is solved by additional “header tanks”.
Pumping and redistribution of fuel
For re-launch, Starship has an “in-orbit refueling” capability, which is extremely important to be able to pump fuel between tanks or even between two spacecraft directly in orbit. SpaceX plans to implement an in-orbit refueling system: one Starship will serve as a “refueler” and the other as a “receiver”. They will be connected by noses, and methane and oxygen will be pumped through tubes from the “refueler” to the “receiver”. This innovation opens the way for interplanetary missions, as it allows Starship to be refueled in space without having to lift the entire volume of fuel from the Earth at once.
Thus, the transition from Falcon 9 to Starship is a qualitative leap in fuel volume and weight, refueling methods, and permissible payload. If Falcon 9 and Falcon Heavy have already proven their reliability and efficiency in the launch market, Starship is designed to expand these capabilities and open the way to manned missions beyond Earth orbit. The key is in-orbit refueling, which will allow Starship’s gigantic fuel supply to be used not only for launching from Earth but also for sending large cargoes to the Moon, Mars, and beyond.
SpaceX is still optimizing the preparation, refueling, and reuse of the Starship, but it is already clear that the scale and tasks that this system can solve go far beyond the capabilities of the current Falcon line.
