Aerospace companies have always aimed for low cost and high reliability in commercial launches. Whether it’s the choice of engines or the use of ultra-light aluminum-lithium alloys to reduce structural weight and maximize payload capacity, this pursuit is evident. Notably, the much-hyped “controlled recovery and reuse of the core stage,” hailed as a pioneering milestone in human spaceflight, also reflects an element of technological commercialization. However, one particular technology—helium pressurization via COPVs (Composite Overwrapped Pressure Vessels) immersed in liquid oxygen—has puzzled the author. This method, which drew global attention after a catastrophic in-flight breakup, is widely regarded in the industry as being “unrelated to low cost, or even in stark contrast to it.”
I. The Disclosed Cause of the Failure
On June 28, 2015, a Falcon 9 rocket tasked with resupplying the International Space Station disintegrated 139 seconds after launch, releasing a massive white plume before losing control. Telemetry and observed phenomena indicated that the second-stage liquid oxygen tank had experienced an abnormal overpressure and ruptured. On July 21, Elon Musk confirmed that the failure was due to the fracture of a COPV support strut. Under a 3.2g load, the strut holding the high-pressure helium bottle broke, leading to a helium leak that triggered the explosion. According to the description, the support strut securing the helium bottle inside the LOX tank snapped, allowing the buoyant bottle to rise and collide with the upper tank structure. This resulted in a rapid pressure increase as high-pressure helium entered the LOX tank, causing an explosion. Visual schematics available online vividly illustrate this sequence.
Further details from the aerospace company clarified: “The strut used in the Falcon 9 rocket was approximately 60 cm long and 2.5 cm thick, supplied by an external vendor. It was designed to withstand 10,000 lbf (about 44.5 kN), roughly three times the expected load. However, at the time of failure, the strut bore only 2,000 lbf (about 8.9 kN), one-fifth of its rated strength. This type of strut had been used in hundreds of Falcon series rockets without issue, including ground testing.”
From this, we can derive the following key points:
- The high-pressure bottle was immersed in liquid oxygen (LOX);
- The container was a composite overwrapped pressure vessel (COPV);
- The bottle was filled with helium;
- The volume was substantial, likely ≥ 230 L.
II. Why Immerse the Bottle in −183°C Liquid Oxygen?
Liquid oxygen has a temperature of just −183 °C. Immersing a high-pressure bottle in it while maintaining internal pressure is inherently challenging and risky. So why did the aerospace company choose this path?
In the April 2012 issue of Rocket Propulsion, Chief Designer Fan Ruixiang of the Long March 7 program detailed high-pressure helium pressurization at ambient temperature. In contrast, the aerospace company’s intent behind immersing the COPV in LOX was to pressurize the engine.
Liquid-fueled rocket engines typically use turbopumps. Due to the large flow of propellants, rapid pressure drops at the pump inlet can cause cavitation—similar to boiling water—leading to vibrations or damage to the impeller. To prevent cavitation, inlet pressure must be increased, usually by pressurizing the tank.
Three common pressurization methods are used today:
| Pressurization Method | Principle | Pros & Cons |
|---|---|---|
| Autogenous Pressurization | Vaporizing the propellant itself | Simple and low cost, but risk of impurities condensing |
| Gas-Cooled Pressurization | Injecting cooled combustion gases | Complex design, requires cooling gas management |
| Inert Gas Pressurization | Using helium or other inert gases | Flexible and reliable, but gas bottles are bulky and heavy |
The second stage of Falcon 9 uses a Merlin engine burning LOX and kerosene. Its exhaust gas contains substantial water vapor; using autogenous pressurization could introduce water or CO₂, which may condense and clog the system. Helium, by contrast, is a non-condensable inert gas with a low molecular weight and remains gaseous even when cooled. Immersing the bottle in LOX significantly increases helium density, reducing the number of required tanks and cutting down weight. By lowering temperature and increasing pressure, immersing COPVs in LOX becomes the most weight-efficient and performance-optimized solution.
III. Global Comparison: Not a New Invention
This technique is not unique to the aerospace company. As early as the 1960s, the United States employed submerged gas bottles in LOX or liquid hydrogen on Saturn V stages S‑IC and S‑IVB. The Ares I J2‑X and the Centaur upper stage also used this approach. Other major launch systems—Soyuz, Angara, CZ-3A, H-IIA/B, Ariane V—have adopted cold helium bottle immersion in LOX. The key difference is the aerospace company’s use of COPVs instead of traditional metal tanks, achieving significant weight savings. So, while the concept itself is not novel, the integration of COPVs with LOX immersion is a breakthrough in cost-performance optimization.
IV. Cost-Driven or Performance-Oriented?
Some argue that COPV pressurization involves high costs and structural complexity, clashing with the low-cost philosophy. In reality, it’s a comprehensive trade-off between weight and system reliability:
- Autogenous pressurization introduces H₂O and CO₂ impurities, which may condense and block the system at cryogenic temperatures, risking engine failure;
- High-pressure turbopump inlets—like those on RD-170-class engines—can reach 40–50 MPa, posing pressure challenges for autogenous systems;
- Helium’s low molecular weight means that for the same pressurization, helium requires only about 1/7 the amount of oxygen, reducing mass;
- COPVs enable higher density storage and lighter system weight compared to metal tanks.
Thus, in terms of performance, redundancy, and risk control, cold helium pressurization surpasses the cheaper but unstable autogenous method. The fact that the aerospace company continued using COPV cold helium systems after the CRS‑7 failure indicates that its commitment is rooted in reliability first.
V. Conclusion: The Best Technology is the One that Fits
Reading through all this, you might be astonished by the “helium + LOX + COPV” combination. Is it truly at odds with low-cost goals, or is it a comprehensive optimization of cost, performance, and reliability? Although internal evaluations from the company are unavailable, the above technical analysis suggests a rational choice grounded in a trade-off between weight savings and system safety.
No matter how advanced or novel the technology is, fitness-for-purpose remains the ultimate principle of design. Falcon 9’s pressurization system proves once again: the key is not achieving the lowest cost, but achieving the best system suitability.
