Nitrous Oxide Safety
SPG has developed industry-leading expertise in the theoretical and practical aspects of nitrous oxide. We have developed precise analytical models of nitrous oxide ignition and flame propagation as well as vented explosion models all of which are anchored by experimental data. For a more detailed overview of nitrous oxide decomposition, the interested reader is pointed to our paper on the decomposition of nitrous oxide
Nitrous oxide and liquid oxygen are the most commonly used oxidizers in hybrid rocket systems and are commonly used in liquid rocket systems. This is primarily due to their cost, safety, availability and handling advantages compared to the other liquid oxidizers that can be used in propulsion applications. Despite its moderate Isp performance and poor impulse density at room temperature, N2O has been the choice for small motors for which the systems and operational simplicity are the dominant driving forces. This fact explains the extensive use of N2O in amateur rocketry and in many sounding rocket programs. The two good examples of the N2O based hybrid sounding rockets are the Hypertec system which has been designed and flown by a private company eAc under the HPDP program and the currently ongoing NASA/Stanford Peregrine effort.
The primary hazard associated with N2O is related to its exothermic decomposition reaction which can liberate substantial energy. Although this exothermic behavior presents benefits in terms of the theoretical Isp performance and motor stability/efficiency characteristics, it also introduces a chemical explosion hazard in the various components of the rocket system including the oxidizer tank and the feed lines.
Arguably, the most impressive demonstration of a N2O hybrid to date is the Ansari X-prize-winning SpaceShipOne system built by Scaled Composites. The larger follow on SpaceShipTwo vehicle, which is designed to carry tourists into space on a sub-orbital flight, is also baselined for a N2O hybrid propulsion system. If successful, this will be the first operational large scale hybrid rocket in commercial or military use. However the decomposition process is not yet well understood in practical systems. This introduces unknown risk in the development of large-scale nitrous propulsion systems.
Chemical Decomposition of Nitrous Oxide
It is well established that nitrous oxide exothermically decomposes into the reference species, O2 and N2, following the global reaction:
The most relevant decomposition scenario for rocket propulsion system is that of local forced ignition in a tank filled with mostly vapor. This scenario is illustrated in the graphic to the left. If a powerful enough ignition source exists locally in a vessel full of nitrous oxide vapor, it will initiate a deflagration wave (flame) that propagates through the vapor, releasing energy and increasing the specific volume of the vapor. This can lead to extremely large pressure buildups in excess of 15x initial pressure, which are capable of bursting most conceivable tanks. A scenario of this type is easy to imagine in a close-coupled hybrid system when the liquid in the tank is exhausted. Pressure fluctuations in the chamber can flow hot combustion gases into the tank, initiating a decomposition flame that results in a catastrophic failure.
SPG's Capability and Expertise in Nitrous Oxide Systems
SPG has developed ignition, flame propagation and vented explosion models for nitrous oxide. In addition, we have conducted decomposition testing in our high pressure closed volume, bomb-type reaction vessel and at larger-scale in the 10-inch and 36-inch diameter vented explosion vessels. Based on our models, test results and first-hand experience with nitrous oxide, we have created a summary of decomposition behavior of nitrous oxide below:
- Nitrous oxide is a widely used material with very interesting physical, chemical and biological properties. The number of serious chemical accidents reported for N2O is quite limited considering the wide use of this material.
- The use of N2O in propulsion applications presents unique hazards. This is mainly due to 1) the high temperature storage of N2O in very large quantities (in run tanks and flight tanks), 2) large flow volume and high flow velocities and 3) the abundance of ignition sources in chemical propulsion systems. Thus, the relative safety enjoyed in other applications should not be used as a guideline for safe operation.
- The relative safety of nitrous oxide compared to the other energetic propellants such as H2O2 is due to its slow kinetics which is partly caused by the abnormal nature of the decomposition step of the reaction which is governed by a spin-forbidden process.
- Models for homogenous and local thermal ignition have been developed. Local thermal ignition has been identified as the more feasible ignition mode for propulsion systems. It has been shown that diluents such as Helium or Nitrogen substantially increase the minimum ignition energy enhancing the safety of the system.
- Local thermal ignition of the vapor in the tank ullage is arguably the greatest hazard that exists in a N2O based propulsion system. The deflagration wave induced overpressurization in the tank has been demonstrated using an imaginary example case. The propulsion systems that have closely coupled oxidizer tanks and combustion chambers are believed to be particularly vulnerable to this mode of catastrophic failure.
General safety recommendations for nitrous oxide systems
SPG has compiled a list of practical safety recommendations for design and operation of nitrous oxide systems (Ref1).
- Nitrous oxide is an energetic material and it must be respected! When N2O is used in any risky testing or other operations, all personnel should be at a safe distance and/or in a protected area. A comprehensive hazard analysis is recommended, especially for large scale operations and testing.
- In manned systems, a properly designed pressure relief system must be installed on the flight tank. This system should be tested at full scale.
- The dilution of N2O in the tank ullage by supercharging is highly recommended. Blow down N2O systems, especially the ones that are allowed to burn in the vapor phase are inherently hazardous. Some of the possible diluents that can be used with N2O systems are helium, molecular nitrogen and molecular oxygen.
- For small scale motor testing, the oxidizer tanks should be run in the vertical configuration such that a liquid layer of N2O always separates the vapor in the tank ullage from the combustion chamber.
- In order to prevent accumulation of N2O in the combustion chamber, the N2O flow should always lag the igniter action.
- One should follow strict oxygen cleaning procedures for N2O. Note that very small concentrations of fuels in N2O might change the entire decomposition dynamics, making most of the findings with uncontaminated N2O irrelevant. Also note that the lean flammability limit for N2O is zero, resulting in high sensitivity at very low concentrations of fuel.
- Nitrous oxide is a good solvent for a number of hydrocarbons which includes a lot of the common polymers. Any polymeric or nonpolymeric materials that will be used in the N2O system should be tested for compatibility. This includes the valve seals, o-rings or gaskets.
- One should avoid using catalytic materials in N2O systems.
- All past experience indicates that it is impossible to ignite and sustain a decomposition wave (detonation or deflagration) in liquid N2O. Note that fuel contamination in the liquid changes this situation, potentially resulting in very dangerous conditions.
- One must understand that it is almost impossible to eliminate all of the ignition sources in a practical system. A system should be designed to mitigate a potential decomposition event. The famous quote by Trevor Kletz is worth remembering in the design and operation of N2O based propulsion systems: "Ignition Source is always free."