Safety Hazards and Mitigation

Rockets are inherently dangerous, as they necessarily store large quantities of chemical energy within structures that are often designed with narrow margins to save weight. Liquid motors are no different than solids and hybrids in this matter. However, liquids do have different hazards that persons working on and around the system must be aware of. The most important safety measure, and usually the only one that matters for nitrous oxide (N2O) systems, is standoff distance. While there are dangers for which being far away is not possible or not relevant, standoff distance is the only effective precaution against the most serious and lethally dangerous hazards. There is no substitute for standing far away when operating the system. In other words, don’t have people near it when it’s hazardous!

Oftentimes, the risk to hardware is conflated with the risk to people. The former is at the discretion of the hardware’s owner, the latter is unacceptable. What this means is that often-quoted “safety measures” like fully pressure-rated parts, material compatibility, oxygen service cleaning, etc. are optional on the rocket. If personnel are standing near the system when it is pressurized or has oxidizer in it, when these would matter, the situation has already gone wrong! It will be stated repeatedly: The only hazard mitigation that matters is keeping all persons clear of the system when N2O is present anywhere other than the supply bottle.

There is only one exception to this rule, and that is the hose between the N2O supply bottle and the remotely actuated fill valve. Since the supply bottle must be opened by hand, there will be N2O present up to the fill valve. This hose should be kept to the absolute shortest length required to reach from the supply bottle fill valve (ideally only 12 inches or shorter), and no larger than ¼” nominal hose diameter, in order to minimize the volume pressurized with N2O when the supply bottle is opened. In some cases, the fill valve can be mounted directly onto the supply bottle using a brass CGA fitting adapter. Hoses between the supply bottle and fill valve must be made entirely from oxidizer-compatible materials (i.e., PTFE, brass, and stainless steel) and must have a safe working pressure rating of at least 1200 psi. (Manufacturer-rated working pressures include a large safety factor, typically 4X.) The remotely actuated fill valve is held to the same requirements; brass-bodied valves with a stainless steel ball and PTFE seats, such as those used in Half Cat Rocketry standard ground systems, are commonly available. Furthermore, “whip checks” - also called hose whip restraints - are strongly recommended, to limit the ability of a failed or improperly connected hose to strike and injure nearby personnel. No other components beyond the hose and remote fill valve can be allowed to contact N2O before all people have been removed from the area.

Are Liquid Rockets Extremely Dangerous?

Like all types of rockets, liquid bipropellant motors can cause harm if mistreated. However, they are not intrinsically more dangerous than other types of rockets – they are simply different. Solid rocket motors, although commonly used, are pre-mixed combustibles which can burn or explode due to stray sparks or electrical charges. Unlike solids, where an inadvertent initiation of the igniter during installation would light the propellant and cause catastrophic injuries, a standard liquid motor simply cannot be ignited by accident during transport, handling or pre-launch preparation; for one, its fuel is sealed away inside the tank, and secondly, no oxidizer is loaded.

If one was to stand near a solid rocket motor during operation, that would be extremely dangerous and a grave violation of every safety principle regarding rocketry. The same goes for hybrid and liquid rocket motors. There is no unique aspect of bipropellant systems that makes them fundamentally more dangerous. No one can ever be near a rocket motor during operation. It has been said and will be said repeatedly: Distance is the only safety precaution that matters. To approach a live rocket motor of any kind is dangerous.

The following is a discussion of potential safety hazards which may be present with liquid rockets, and mitigation strategies for each.

High Pressure

High pressure is the most straightforward hazard of a rocket. High pressure can cause hardware to break, projecting shrapnel in any direction at high velocities. Pressure can also cause ordinary injuries purely from gas momentum, where escaping fluid acts like a knife or a bullet. Although it is mandatory that all personnel be cleared when the system is pressurized, it is likely that one will encounter small amounts of trapped pressure, such as between a closed supply bottle and its remotely actuated fill valve. In these situations, fittings may be slowly opened to relieve the pressure while keeping hands away from the escaping gas. If a valve is actuated to relieve the pressure, the line it eventually exits from may whip around if not secured ("whip-checked"). High pressure hazard can be mitigated by keeping all persons clear of the system when N2O or other pressurized gas is present anywhere other than the supply bottle.

To prevent pressure from being trapped inside the oxidizer run tank, a static vent (i.e., a small hole, typically .020-.047” in diameter) must always be included at or near the top of the volume occupied by N2O. The static vent serves other purposes besides safety, but that is its most important function: Even in the event of total control failure, the rocket will always depressurize itself given enough time because the static vent is a built-in leak. There should never be a need for firearms to be a part of safety procedures, as was the case at the 2018 Spaceport America Cup when a hybrid rocket was shot with a rifle to depressurize the sealed oxidizer tank.

Fire Hazard

Fire hazards are an intrinsic part of rocketry due to the use of flammable substances as fuel and igniters. Fires can occur when free fuel is ignited and spread rapidly if there is a puddle or leak present. Firefighting equipment, such as a Class B handheld extinguisher or a large bucket of water or sand, should always be kept on-hand in case a fire threatens personnel safety. Solvent fuel fires may damage hardware but are mostly benign - the biggest danger is the potential to start secondary fires of nearby equipment, structures, or vegetation. Fire hazards can be mitigated by keeping open flame, sparks, and other ignition sources away when handling fuel especially if there is a known spill or suspected leak. Whenever possible, fires should be allowed to burn themselves out with personnel at a safe distance unless the fire is very small and able to be safely extinguished.

Oxidizer Hazard

Oxidizer hazard is present due to N2O, which splits into nitrogen and oxygen when it decomposes. This decomposition is thermally triggered, meaning that it will not decompose ordinarily unless exposed to a heat source. The main danger to be aware of is that N2O will accelerate fires, so it should not be vented near hot parts, embers, sparks, or open flame. Although it may appear to act as an extinguisher due to how cold it can get, N2O should NEVER be used to fight fires. Oxidizer hazard can be mitigated by keeping all persons clear of the system when N2O is present anywhere other than the supply bottle.

N2O Decomposition

Decomposition is the most critical and unique hazard of N2O. When it reaches its decomposition temperature, N2O splits into nitrogen and oxygen while releasing significant heat energy. This makes it an exothermic process. In addition to causing a rapid pressure spike if contained, the free hot oxygen will combust with most anything flammable - simultaneously creating an oxidizer hazard. The decomposition temperature is normally around 1000°F, but a catalyst such as fine metal particles and other contaminants can lower that threshold. Without a significant heat source like an open flame there is little risk of decomposition, but the potential for contact with catalysts and heat sources during normal operation of the rocket is precisely the reason that personnel must be kept away from the system when N2O is outside the supply bottle.

A less intuitive source of heat is adiabatic compression, where N2O is either compressed or slams into a stoppage with sufficient momentum to instantaneously raise the local temperature above the decomposition threshold. The most common place for this to happen is in a solenoid or other fast-acting poppet valve. All N2O valves other than the supply bottle valve must be actuated remotely when there is N2O present. Cavitating N2O can also provide the requisite compression heating as bubbles of N2O vapor collapse - N2O pumps should never be used around personnel. Decomposition hazard can be mitigated by keeping all persons clear of the system when N2O is present anywhere other than the supply bottle.

In addition to regular fire hazards, there is a secondary risk that a fire occurring around a vessel containing N2O gas, even at ambient pressure, may cause a decomposition event. For this reason, no persons may approach a rocket, test setup, or any other N2O tank for a minimum of two minutes following the last indication of fire or smoke unless it is thoroughly purged with an inert gas such as CO2. The heating of residual N2O vapor from a lingering fire inside the combustion chamber or anywhere in proximity to the rocket can cause a tank to explode long after the initiating event. It is especially important to recognize that unlike a normal high-power rocket being on fire, approaching an inflamed N2O rocket is more hazardous than leaving it alone to burn itself out. If this presents a risk of igniting vegetation and starting a brush fire, emergency fire response should be available at the site.

BLEVE

Boiling Liquid Expanding Vapor Explosion, BLEVE, refers to the situation in which a saturated liquid under pressure loses containment and rapidly flashes into gas, resulting in a vapor explosion. Although unlikely, a BLEVE can occur if the N2O tank experiences a sudden large leak. A key point to note is that a BLEVE will almost certainly not be the primary hazardous event if one occurs, but it will increase damage to hardware and the projected distance of any thrown debris. For example, some mechanism that causes a bulkhead to tear out of an N2O tank will have already critically damaged the system but will also result in a BLEVE as the N2O rushes out. The main danger to be aware of is the potential for an explosive event which is beyond the energy release of an equivalently sized pressurized gas container. BLEVE hazard can be mitigated by keeping all persons clear of the system when N2O is present anywhere other than the supply bottle.

Fuel-Air Explosion

A fuel-air explosion occurs when enough fuel vaporizes into a volume of air (containing oxygen) to reach the explosive threshold and ignites, resulting in a detonation. Unlike propane, which readily becomes a gas, alcohol and other room temperature liquid solvent fuels evaporate quite slowly and are rapidly dispersed by ambient air movement. This makes solvent fuels generally immune to fuel-air explosive mixtures under normal circumstances unless they are purposely kept in a stagnant volume with the right oxygen concentration. As is good practice for solvent fuels, they should be handled in a ventilated area. The only real risk of a fuel-air explosion is when a gross leak of both fuel and N2O gas occurs in a confined space near an ignition source, where the N2O acts as the air. Fuel-air explosion hazard can be mitigated by handling fuel only in well ventilated areas, such as near an open roll-up door or outside, and by keeping all persons clear of the system when N2O is present anywhere other than the supply bottle.

Electrical Hazard

Danger exists with both high and low voltage electrical sources. The direct hazard of low voltages is typically a large battery, such as a 12V car battery, unintentionally shorting to itself and starting a fire. High voltages, such as those found in sparker systems, can cause electrical shocks, damage equipment, and start fires. Keep high voltage devices powered down until needed, and always insulate exposed wires to prevent shorts. Keep E-matches and other electric initiators shunted (leads twisted together) until ready to make a connection to prevent accidental initiation by induced current or stray charges. Always spark-check electrical leads (by briefly touching the supply wires together) before connecting initiators to ensure that current is not flowing, which could cause an unintended pyrotechnic event.

Burns

Components such as the thrust chamber assembly may be very hot after firing and can cause burns if touched before being allowed to cool down. If it is necessary to make contact, such as when recovering a rocket in the field, place the back of your hand about half an inch away from the surface to feel for radiant heat. If no radiant heat is felt, quick-touch feel parts (by touching for as little time as possible) to judge the temperature. In most cases, hot metal parts can be quickly cooled to safe temperatures by pouring a small amount of water over the surface.

Pyrotechnic ignition materials, including E matches and solid propellant grains, also pose a risk of burns if mishandled. When connecting igniter electrical leads to the ground system wires, keep hands and other body parts clear of the nozzle, to avoid exposure to flame or hot gas if the pyrotechnic igniter is accidentally set off. As mentioned in the previous section, electrical leads should be spark-checked before connecting to prevent this from occurring.

Frostbite

On the flipside, parts that have recently been exposed to liquid N2O may be very cold and can cause frostbite. Quick-touch feeling parts may also be used to test for cold. It is possible for liquid N2O to leak out of fittings in a place where it could make contact with skin, particularly the fittings connecting N2O supply bottle to hose and hose to fill valve. As previously mentioned, it is sometimes preferred to vent this hose by loosening a fitting. The rapid boiling of escaping liquid will absorb heat from anything it touches, which can cause frostbite. This hazard can be mitigated by keeping hands and exposed skin away from the fitting being loosened to avoid contact with liquid N2O.

Fume Inhalation

Most liquid fuels are inherently volatile to some degree, meaning they constantly evaporate into air. The health effects of breathing a large volume of fumes varies by chemical, but typically results in headache, nausea, or respiratory irritation. In order for this to occur, the fumes must build up in a closed room – therefore, this hazard can be mitigated by only handling fuels in well-ventilated areas, such as near an open roll-up door or outside. As a general rule, personnel should avoid breathing in any airborne chemicals, which also include cleaning agents and rocket motor exhaust.

Asphyxiation

Commonly used gases like N2O and CO2 can rapidly displace air in a closed volume, like the inside of a room. Venting a large volume of non-air gas in a closed room may result in drowsiness, or at an extreme, asphyxiation due to lack of oxygen. This hazard can be mitigated by only working with compressed gases in well-ventilated area, such as near an open roll-up door or outside. If a gas bottle is actively leaking indoors, immediately remove personnel from the room and open a door or window if possible. Close the bottle or take it outside, and turn on any available fans to clear the contaminated air from the room.

A rare but occasional scenario can occur in which a gas bottle may inadvertently begin leaking: If a bottle has been in a relatively warm environment, and is then moved to a much colder environment, the thermal contraction of the hand valve may be sufficient to open a leak path. Most notably, this can occur when a bottle has been sitting outside on a hot day and then put into an air-conditioned car or room. Always ensure that bottle hand valves are fully tightened, and if leakage does occur then the same procedure as above should be followed to clear the air in the room.

Standoff Distances

The following table provides recommended minimum standoff distances by impulse class. Note that this distance extends in all directions from the rocket. These distances are to be treated as a minimum recommendation, and also apply to occupied buildings and vehicles. Limited exceptions may be made on a case-by-case basis only when persons are behind adequate protection, such as the bunkers used at FAR and RRS.

Pressure relief devices, oxidizer service cleaning, “fail-safe” software, etc. are not an acceptable replacement for standoff distance. In fact, the presence of such “safety mitigations” can instill a false sense of security and invite complacency towards the still-present dangers of being near a pressurized tank containing oxidizer, fuel, and/or inert gas.

Note: The above distances are based on the total stored energy in the propellant of a typical motor in each size class. These distance guidelines are based on those originally set forth by the Tripoli Rocketry Association (TRA) and were intended primarily to apply solid and hybrid motors, although the stored energy in liquid propellants is similar. Some edge cases unique to liquid propulsion may require modification of these guidelines (always in the direction of increased conservatism), such as extremely short-duration tests of very high-thrust liquid motors. If a test is planned to only partially deplete the propellant tanks, the standoff distance should be based on the maximum potential total impulse if all loaded propellant were to be consumed, regardless of the intended duration.