The recent launch of Starship attracted worldwide attention, but after the stages separated, a leak occurred in the fuel line, leading to an explosion a few minutes later. This is not the first time fuel supply problems have become critical in complex aerospace systems. However, each such incident provides engineers with valuable lessons, helping to improve the design and make future versions of the rocket safer and more reliable.
In this article, we will examine the fuel lines of Starship and more classical rockets. We will compare their designs, materials, sealing systems, and ways to prevent leaks and find out what measures engineers are taking to fix such problems and prevent similar accidents in the future.
Methods of eliminating leaks and emergency measures
If a leak is detected before launch, an emergency refueling shutdown is immediately performed: the supply of fuel and oxidizer is stopped, and the fuel in the lines is discharged through drainage systems. For example, when liquid hydrogen leaked on Artemis I, the launch was postponed several times while engineers replaced the seal. After replacing the problematic seal, a test refueling (test cryogenization) is performed to eliminate the leak. If a leak occurs at the launch complex and leads to a fire, fire extinguishing systems are activated (some installations have water, rain or foam). Personnel at the launch site are trained to keep a safe distance during refueling; in an emergency (fire or risk of explosion), a warning system is activated and fire brigades from the safe zone extinguish the flames after the tanks are discharged.
However, during flight, a fuel leak would almost certainly result in the loss of the rocket, as the components are very aggressive. Nevertheless, some rockets can survive a single engine failure (for example, Saturn V could shut down one of its five engines and still reach orbit). In the event of a serious accident (line rupture, pressure drop), an emergency flight termination system is triggered: for example, if the trajectory is uncontrollable, powder charges are triggered and destroy the rocket in the air to avoid an uncontrolled fall. The incident with the Titan II leak in the mine (1980) showed a clear protocol: after a tool fell in the mine and punctured the fuel tank, the crew was evacuated and emergency crews were sent to the site. Unfortunately, one technician was killed before the explosion, but a nuclear catastrophe was averted because the warhead did not detonate. After this incident, a rule was introduced to secure the tool (to prevent it from falling) and improved procedures for ventilating shafts in case of leaks. Modern missiles repeatedly train for scenarios such as “fuel leakage at launch” – up to the point of taking the missile back to the hangar and replacing the main line segments.
After a series of Starship explosions, SpaceX improved the system: water nozzles appeared during landing to extinguish possible fires and shield each engine to localize the fire.
For example, the explosion of Starship SN11 was caused by a “relatively small” methane leak that caused a fire in the engine area and system damage – SpaceX management noted that this problem is “being fixed in every way possible”. Thus, each accident leads to engineering changes: better sealing materials, new refueling procedures (e.g., slower cooling of connections), and additional sensors.
To understand how to prevent similar accidents in the future, it is important to understand how spacecraft fuel lines are designed. We will look at their design, materials, sealing methods, and safety measures. Using the example of Starship and other technologies, we will see how real projects are adapting designs to improve their reliability and efficiency. We will also analyze how engineers improve these systems after identifying problems, and how one of the secrets of the SpaceX team, which we analyzed earlier, “Fail fast learn faster” works in action.
Materials and design features
The main structure of the Starship is made of stainless steel, which also applies to the fuel tanks and lines. The use of steel is due to its strength at cryogenic temperatures and its ability to withstand high temperatures during reentry. The fuel lines in Starship are mostly welded into a single system with the steel tanks to minimize flange connections and potential leakage points. The design provides for separate “header tanks” – small landing tanks for fuel and oxidizer located inside the main tanks to ensure a stable supply during landing. These auxiliary tanks are additionally insulated and their pipelines are equipped with thermal insulation.
In classical launch vehicles, the fuel lines are made primarily of lightweight metals or alloys such as aluminum or titanium. For example, the heavy Saturn V rocket was constructed primarily of aluminum; large cryogenic tanks (LOX, LH2) were connected by a common composite bulkhead to reduce weight. In areas of high thermal load or when working with aggressive components (e.g., hydrogen peroxide or diazote), stainless steel, titanium, or special alloys resistant to corrosion are used. Structurally, the rocket lines can be located inside the tanks (to save weight and protect them, as in Titan II) or outside the body (for example, external LOX pipelines in Saturn V). Pipe segments are connected using flanges with seals or welding; at the joints, fasteners are installed to compensate for thermal expansion (wavy compensators) during the transition from cryogenic fuel temperatures to warmer sections.
Starship uses liquid methane (CH4) as a fuel and liquid oxygen (LOX) as an oxidizer. Both components are cryogenic, stored at temperatures of approximately -150…-180 °C. Rocket systems in general can use cryogenic components or traditional mixtures, such as kerosene (RP-1) in Falcon 9. Self-igniting vapors (e.g., UDMH and N2O4) avoid complex ignition systems but require additional safety measures due to their high toxicity and corrosiveness. Read more about fuel and fuel tanks in our article “Starship vs. Falcon: Why did SpaceX increase the volume of tanks 10 times?”. Therefore, Starship has implemented special measures for their storage. The tanks are made of stainless steel, which becomes even stronger at very low temperatures. There is no external insulation (foam insulation) on the Starship prototypes, as frost appears on the outer surface during refueling. Instead, to minimize evaporation, the fuel lines are vacuum-insulated (made as a pipe within a pipe with a vacuum between the walls). Early prototypes used conventional insulation only on the lines to the small landing tanks, but the latest versions have vacuum insulation on all the main lines.
Cryogenic components (LOX, LH2, methane, etc.) require careful thermal control. Rockets with liquid hydrogen have the most massive preservation systems: the LH2 tank must be insulated with foam or vacuum panels, as hydrogen boils at -253 °C. In Saturn V, the second stage S-II had a common partition between the LOX and LH2 tanks with fiberglass insulation so that the difference of ~70 °C between the components did not lead to excessive hydrogen heating. The shuttle’s external tank (ET) was covered with polyurethane foam insulation to reduce evaporation – its orange layer is well known (unfortunately, a piece of such foam damaged Columbia in 2003).
The supply lines from the ground systems to the rocket are also vacuum-insulated (especially for hydrogen). Ventilation systems are provided on launchers: during refueling and waiting for launch, the cryogen in the tanks gradually boils, and the gas is vented through special valves (for example, a “vent mast” for the upper stage). This gas is often vented away or burned (for methane and hydrogen, to prevent an explosive plume from accumulating). Some modern rockets (Vulcan Centaur, Ariane 6) plan to actively cool the fuel before launch (subcooling) to increase its density – this requires highly efficient insulation of tanks and pipelines to keep the fuel “supercool” without excessive heating. In the case of a long stay in orbit with cryogenic propellant (as in the upper stages for launching into geotransition orbit), cooling and ventilation systems are used: for example, periodic discharge of a small amount of gas for cooling, protective shielding and vacuum insulation on the tanks, etc. For short-term use (launching within a few hours after refueling), a certain amount of boiling and gas discharge is often allowed as a design value.
Methods of sealing and preventing leaks
Due to the all-steel construction of the Starship, most of the connections are welded, which ensures a high level of tightness. In places where a detachable connection is required (for example, the point of connection to the ground refueling hose, the so-called quick disconnect, QD), reusable seals are used. SpaceX designs these assemblies with cryogenic conditions in mind, using fluoropolymer O-rings* suitable for -180 °C and heating the interface before disconnection to avoid brittleness. After leakage incidents during Starship testing, improvements were made: SpaceX engineers added fire shields and engine shielding that isolate each Raptor engine in case of leakage or fire. Ventilation systems in the tail section have also been improved. To detect leaks, the Starship is equipped with pressure sensors and, presumably, methane sensors – the slightest drop in pressure in the line or the presence of gas in an unplanned place will automatically interrupt the launch. Excess fuel is held in ground lines and safety compartments until the last minute during refueling, and valves shut off the supply immediately when the QD is disconnected. Sealing methods include the use of a minimum number of threaded connections – most of the valves and pipes in Starship are directly integrated. There are no helium cylinders in the boost system (which have been a source of accidents in the past on other rockets), eliminating another potential leakage point.
*Today, fluoropolymer O-rings are widely used in aviation, rocketry, petrochemicals and medicine, replacing traditional rubber seals in extreme temperatures and aggressive environments. The emergence of advanced fluoropolymer seals, such as FEP (fluorinated ethylene propylene) and PFA (perfluoroalkoxy alkane), which combine flexibility and chemical resistance, has become an important development in this narrow field and opened the door to new designs based on this invention. This proves once again that such a small detail can affect the lives and safety of the entire team. Read our article to find out how small details affect the lives of astronauts.
Historically, a combination of welds, flange connections with seals and controlled valves have been used to ensure leakage. In inter-tank pipelines, spherical or conical seals with O-rings are often used. After the Challenger shuttle accident (1986) – caused by a gas leak through a cold-hardened rubber O-ring at a solid-fuel accelerator joint – NASA introduced a new joint design with three O-rings and heating to prevent similar leaks. In liquid rockets, quick-release connections between the rocket and the refueling station are critical. They usually have a double sealing system: two rings and an intermediate “leakage chamber”. For example, in the SLS (Artemis I) system, the hydrogen leak occurred in the intermediate cavity of the 8-inch QD between the “unearthly” and “rocket” plates – this cavity is equipped with a sensor that detected hydrogen and signaled the problem. To prevent leaks, hydrogen systems use Teflon or other fluoroelastomer sealing materials that do not “tan” in the cold, and the flanges are tightened to a precise torque. (Incorrect tightening torque can cause leaks – this is what happened in 1990 with the shuttle Atlantis: insufficiently tightened bolts on the flange of a 17-inch fuel line led to hydrogen leakage. Tightness is monitored by repeated pressure tests before refueling: for example, slowly “leaking” a small amount of liquid hydrogen through the connection (the so-called kick-start bleed, which was tested before Artemis I) allows you to see if the concentration of H2 in the compartment is increasing. If even a small leak is detected, the operation is interrupted. In some designs, metal seals (e.g., soft metal gaskets) and duplicate valves are used to avoid leaks of highly toxic components. In general, rocket systems have reusable electromechanical valves to shut off the flow in abnormal situations and drainage lines to safely drain the fuel after aborting the launch.
So SpaceX engineers implemented several measures to solve the problem:
- Improved seals: sealing elements at the joints of the fuel lines were replaced or upgraded to ensure better sealing.
- Optimization of the pipeline design: the design of fuel pipelines was revised to minimize possible leakage points and reduce stress on the connections.
- Improving the monitoring system: additional sensors were installed to detect possible leaks or anomalies in the fuel system at an early stage.
- Updating of testing procedures: more rigorous testing of fuel lines under various conditions, including simulation of extreme temperatures and pressures, has been introduced.
When we analyze the fuel lines in Starship and other rockets, we see that their design depends on the operating conditions, working fluids, and safety requirements. Rocket systems face unique challenges, such as cryogenic storage, high pressures, and the need to minimize joints that can become sources of leaks.
The recent Starship explosion due to a fuel leak is another lesson for SpaceX engineers. As in previous cases, the company has implemented improvements: better insulation, more reliable seals, and additional leakage controls. The history of technology development shows that every failure is a step towards improvement, and it is through continuous error analysis and engineering solutions that future spacecraft will become safer and more efficient.
