Safety
A system is safe when no personnel are ever in close proximity to any oxidizer, pressurized fluid, or energetic material capable of causing serious injury.
This page contains information about how we stay safe and avoid injuries when working with rockets. Be aware at all times that stored energy sources, especially fuels, oxidizers, and pressurized gases, are extremely dangerous and must be treated with caution.
Nitrous Oxide
Nitrous oxide (N2O) must be treated with respect. There are three main safety concerns associated with N2O:
Compressed gas
Stored energy
Oxidizer
Examining each of these in closer detail:
Any compressed fluid has the possibility of rapid expansion which can cause serious injury. Besides the force of the gas itself (which is often stored at 1000+ psi), if a gas bottle fails it usually does so catastrophically, releasing all of its contents instantaneously and fragmenting the container. The Department of Transportation requires that all commercial compressed gas cylinders be rated to very high safety factors and include a relief valve in case of overpressure. While risk can never be 100% eliminated, the DOT safety rating is treated as the highest standard.
As mentioned, nitrous oxide decomposes exothermically. This means that once a decomposition event begins, it rapidly spreads while adding energy. Inside of a contained pressure vessel this will become a detonation faster than human reflexes, so it is important to keep heat sources away while personnel are present. It should be noted that even in the presence of catalysts (which can lower the decomposition temperature), N2O does not randomly explode; N2O is commonly used in motorsports where it is subjected to much more extreme conditions without incident.
As an oxidizer, N2O will accelerate fires and inflame substances which are not normally thought of as combustible. Care must be taken in considering what N2O will come into contact with – the process of removing unwanted fuel residuals (where fuel is anything that easily burns) from equipment and hardware is referred to as oxygen cleaning, and it is done so that stray heat (especially as a result of adiabatic heating in valves and tube bends) does not ignite a fire where it is not desired.
Many rocketeers avoid nitrous oxide under the fairly legitimate impression that it can be scary (usually citing the 2007 Scaled Composites accident which killed three employees and injured three others) given the above concerns. However, we have never feared using N2O because we respect its destructive capabilities and abide by one crucially important tenet above all else:
A system is safe when no personnel are ever in close proximity to any oxidizer, pressurized fluid, or energetic material capable of causing serious injury.
Since the liquid fuel we use is mostly benign, and there are only small amounts of other stable energetic materials present (blackpowder, solid rocket propellants, etc.), the main safety hazard belongs to nitrous oxide. The principle spelled out above is applied by always evacuating the test or launch area to an appropriate safety distance before N2O is ever allowed outside of its DOT-rated container, and keeping the area clear until all pressure has been relieved and no flames or embers are present. Only once there is no N2O remaining outside of the DOT-rated vessel (save for residual wisps in plumbing at ambient pressure) and no energy source to start a decomposition event do we approach the system.
A common fallacy is believing that the same safety principles apply to hardware. Hardware, unlike human life, has no intrinsic value. Once the personnel safety tenet has been fulfilled, the risk to hardware is entirely at the discretion of its owner and his or her willingness to accept the destruction of the rocket, GSE, launchpad, or test stand. Put another way, oxygen cleaning and oxygen safety has no relevance to motor design and operation if people are never near the system when an oxidizer is present. It is only relevant to the parts in contact with nitrous oxide that a person will be around, and these parts – which must be properly rated well in excess of their maximum expected operating pressure – should be kept to an absolute bare minimum.
Following this guideline, along with general good practice and common sense, ensures that a rocketeer will never be in any greater danger than they are with standard solid motors. To this day, many hobbyists safely use nitrous oxide in hybrid rocket motors, and liquid engines can achieve an equivalent level of safety by following the same practices.
Storage and Handling
No unusual hazard exists from the proper storage of N2O or alcohol. N2O is stored and transported in a DOT-rated automotive bottle and kept away from heat sources; it is also good practice to keep nitrous oxide bottles out of the sun until necessary to prevent the pressure from rising higher than desired. Any solid propellant or blackpowder charges present must be kept away from ignition sources and protected from accidental ignition, which includes shunting any E-Matches or initiators that are installed during motor assembly.
It was the case with Half Cat, and likely will continue to be so for all future engines, that the fuel was loaded during motor assembly one or more days before the planned firing. Since the room temperature vapor pressure of alcohol is less than 1 psi, there is no pressurization hazard as a result of this practice. The fuel is kept completely sealed and leak-tight until the moment of propellant valve opening; Half Cat has been pre-fueled and transported horizontally several dozen times without issue.
Tripoli Research Safety Code
The Tripoli Research Safety Code (TRSC) governs all experimental motors at officially sanctioned Tripoli research launches. The most important aspects are its limitations on construction materials (frangible metals are prohibited) and the safe standoff distance table. Although not directly applicable - the TRSC prohibits liquid motors (5.2.1) - it is a very useful guideline for the safe construction of an engine.
Aside from exclusion 5.2.1, our motors comply with all other aspects of the TRSC. There are occasional minor exceptions, such as the pyrotechnic valve pellets and combustion chamber igniter of Half Cat which were made from KNSU (sucrose-based sugar propellant); not technically an allowed formulation, but not fundamentally more dangerous than the other sugar propellants.
Our motors are constructed primarily from 6061-T6 aluminum, and the nozzle of an ablative engine is typically made of copper. Both of these are very ductile materials and allowed under the TRSC, as is the brass that the fluid fittings are made from. There are some steel components (bolts, washers, snap rings, etc.) but these are exempt under 7.2.2.2. We have used graphite nozzles on two occasions, and the damage to them was consistent with what is seen in solid and hybrid motors during anomalies. The CHAMBERSAFE material, being epoxy-based, is akin to a hard plastic. In the only explosive failure it has been subjected to, it cracked into several chunks that were found no more than a few feet from the combustion chamber. Although brittle, its debris is low-energy like graphite.
During every static firing to date, and for every future firing and launch, we adhere to the safe standoff distance table. In fact, we have always exceeded the required distance out of an abundance of caution.
