Safety by Design: Safety of Rocket Nozzles

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A rocket is a transportation means which delivers people and/or cargo such as scientific equipment from Earth to outer space. A critical subsystem of the rocket to reach its destination is its nozzle. The nozzle’s main function is to direct and accelerate the flow of high temperature and pressure fluids resulting from the combustion of the rocket’s fuel (propellant) at the engine to produce thrust force that propels the rocket. Thrust force is defined as force generated due to acceleration and ejection of matter (engine exhaust fluids in this case) [1]. In this report, the rocket’s nozzle is studied from a safety perspective in terms of stakeholders, system’s safety risks, working concept and past design failures.

Stakeholders and their needs

Rockets meant to transport cargo or people to space have been traditionally operated by governmental space agencies due to their high cost, risk and their cutting-edge technology and research used in their design. Also, their potential to use for military purposes such as deploying spying satellites as well as strategic purposes. Recently, due to technological advancements lowering the costs of the rocket components and with more access to available information than before, private companies and university student teams started to also design and produce their own rockets.

Both the public and private sector organizations are important stakeholders of rocket nozzle technologies as they need to ensure that this subsystem meets safety standards before launching the rocket. Manufacturers would particularly be accountable for the nozzle’s design, assembly, and testing.  In Europe, the European Space Agency (ESA) is a key stakeholder as an international regulating body that sets safety standards for the rocket nozzles.

Furthermore, the researchers, scientists and engineers also need to use rocket nozzles for experiments to guarantee safe rocket launches based on the nozzle’s performance. Moreover, insurance companies also can cover packages for financially covering the risks of rocket launch or the nozzles failure. Therefore, they need to assess the risks associated with the technology to make convenient packages.

Finally, members of the public are stakeholders since rockets may present a serious safety risk if they fail and drop on property or on populated regions. The rocket’s launch can also impact the people’s environment through the engine’s carbon emissions in the air [2]. This may lead to environmental groups pressuring companies to stop launching rockets, which can potentially lower or halt the technology’s development pace. These stakeholders are summarized in table 1, where they are ranked by their interest and influence on the nozzle/system under construction (SUC) development on a scale consisting of 0, + and ++. Where “0” signifies low interest and influence of the stakeholder in the SUC, “+” represents moderate interest, and “++” stands for high interest and influence on the SUC. Figure 1 shows the relation between stakeholders in terms of common interests.

The space companies in the public and private sector are given the highest rank due to their active role in developing the nozzle’s safety measures, standards and efforts to abide by them in design and testing. Environmental groups and land/property owners are ranked as “+” as they have less influence on the SUC development compared to companies in the space industry. Despite their moderate interests, the general public and insurance companies are rated the lowest, as they have the least influence on SUC development.

Stakeholder Ranking Relation to SUC
European Space Agency (ESA) ++ Responsible for funding research, development, certification, and legal procedures.
Research and public organizations ++ Research and experiments
Private space companies ++ Design, research, production, testing
Residents/General public 0 People who live near a launch, landing, crash site
Land/Property owners + Own the property or land where a rocket is launched, landed, or crashed.
Insurance companies 0 Making suitable insurance policies for covering the nozzles
Environmental groups + May stall the development of rocket nozzles.
Table 1: Overview of stakeholders and their interests in the SUC

System concept and environment

As mentioned in the introduction, the rocket nozzle is a device that accelerates high temperature fluids from a rocket motor to produce thrust. Maximum thrust occurs when pressure of the exhaust fluids (exiting the nozzle) is equal to the atmospheric pressure. To achieve this, the nozzle generally consists of three parts namely, the nozzle throat, nozzle bell/contour, and nozzle exit (summarized in table 2).

The nozzle throat is a narrow cross-sectional area which receives the fluids from the combustion chamber (where the rocket propellent is burned) and compresses the fluids passing through it, and after which the fluids expand and accelerate. The nozzle bell/contour is a large cross-sectional area after the throat which allows for the expansion of the fluids and determines the efficiency of the nozzle by shaping the fluids’ flow direction. The nozzle exit is the last region of the nozzle where the fluids are expelled to the atmosphere at high speeds, generating thrust.

The rocket nozzle system operates in a harsh environment, with extreme temperatures (above 3000 K), high pressure, and high speeds [1]. The nozzle must be able to withstand these conditions without suffering damage, and it must be designed to operate efficiently at the target altitude and speed at which the rocket will be flying. Furthermore, the nozzle needs to handle the type of propellant used and must be designed to withstand the forces generated by the rocket’s launch and flight.

Nevertheless, the rocket nozzle system design needs to minimize potential negative impact on the environment and to comply with all relevant national and international regulations, including safety standards set by regulating organizations such as the European Space Agency (ESA) in Europe.


Table 2: SUC main sub-sub-systems

System (Safety) Objectives

Safety is a key consideration in the design and operation of rocket nozzle systems. The main safety objectives for a rocket nozzle system include structural integrity, thermal protection, human safety (for maintenance and testing) and avionics (electronics) safety.

First, with structural integrity, the rocket nozzle must handle the forces due to launch and flight, including high temperatures, vibrations, and aerodynamic loads. Second, the rocket nozzle needs to resist the high temperatures generated during launch and landing without significant damage.

Safety risk assessment

Potential hazards from the rocket nozzle can be divided into several categories. In this section, main hazards that are preventable and realistic are mentioned. To begin with, design/construction hazards occur due to structural failure and thermal protection failure for instance. These hazards are considered high-severity risks and have a relatively low frequency of occurrence if the nozzle system is designed, constructed, and tested properly.

Operational hazards are another category that consists of problems like engine failure, control system failure, and human error. They are considered high-severity risks and can have a moderate frequency of occurrence, depending on the complexity and reliability of the rocket nozzle system.

Additionally, environmental hazards emerging from weather conditions, debris impact, or bird strikes on the nozzle’s body. These hazards are considered moderate-severity risks and have a relatively low likelihood of occurrence, due to the possibility to minimize or avoid them with careful planning of launches time and procedures before they happen.

Furthermore, propellant leakage hazard can cause uncontrolled/undesirable explosions. These hazards are considered high-severity risks and have a relatively low likelihood of occurrence, but it is crucial to plan for them in advance and counteract by performing proper inspections before launching the rocket.

However, for a visual presentation, the above safety risks are illustrated in table 3. Some hazards are listed at multiple places, meaning that the risks they encompass may vary in the severity and frequency.


 Table 3: Severity against frequency of foreseeable risks

Past design failures

There have been several notable past events of rocket nozzle design failures such as the Space Shuttle Challenger disaster in 1986, in which the Space Shuttle Challenger rocket broke apart 73 seconds after launch. This resulted in the deaths of all seven crew members. The reason behind this catastrophe was a failure of an O-ring seal on the rocket nozzle, which allowed high temperature fluids to escape and weaken the structure of the nozzle.

Another incident is the Ariane 5 Flight 501 failure in 1996. The rocket failed during its flight, resulting in losing the rocket and its payload (cargo). The failure was due to a software error in the rocket’s guidance system, which caused the rocket to pitch up steeply, leading to an extreme positioning of the rocket’s nozzle [3]. These failures show how crucial sufficient testing and quality control are in the design and construction of rocket nozzles, as well as the need for continuous development of the technology, and the importance of risk assessment and mitigation to ensure safety.


For safe and reliable operation of a rocket nozzle system, setting maintenance procedures is necessary. Common maintenance practices include inspections, cleaning, calibrations, parts replacement, and maintenance record keeping. The maintenance team receives training on how to safely maintain the rocket’s nozzle.

Inspections of the nozzle system are of regular visual and non-destructive nature. They are focused on identifying and addressing issues like surface cracks, corrosion, or damage. Moreover, the nozzle system should be kept clean to prevent debris and contaminants from building up, which can affect the performance and safety of the nozzle. Not to mention, the nozzle system should be periodically calibrated to check that it is operating within the correct parameters, providing the correct thrust and guidance. Lastly, records of the maintenance activities like the date, type of maintenance and the team member who performed it as well as any issues or repairs done need to be kept to have an overview of potential problems and schedule future maintenance times.

Human factors

Human factors and culture play a critical role in the safety of rocket nozzles. Clear communication between maintenance team members for instance is indispensable to check that no damages or required repairs are overlooked which can prevent accidents. Not all nozzle designs are the same. Therefore, staff directly involved in handling the nozzle need to receive training on operating and maintaining that specific nozzle design.

Moreover, engineers and scientists involved in designing the nozzle need to account for the limitations and capabilities of human operators to make it easy for them to operate and maintain it.


[1] European Space Agency, ESTEC, Space Engineering Propulsion General Requirements (2009). Accessed via

[2] L. David, How Much Air Pollution Is Produced by Rockets (2017)

[3] European Space Agency, Ariane 501 – Presentation of Inquiry Board report (1996)