An airlock is a device which allows movement from one kind of environment to another, while maintaining separation between the two. Airlocks were used for hundreds of years before the development of spaceflight in undersea, mining and construction work, as well as for medical isolation. All spacecraft and many other vehicles designed to operate in hostile environments have airlocks. Most spacecraft have more than one, some large enough for vehicles and cargo to pass through. This article will concentrate on the systems typically found in personnel airlocks. Many of the systems will also be found in airlocks constructed for other purposes.
In its most elementary form an airlock consists of a room with at least two doors. Each door opens to a different environment. In the case of a spacecraft access airlock one door will open into the normal environment in the ship's interior. The other door will open to the hostile environment outside the ship. Usually there is a system to allow the normal air from inside the ship to be pumped out of the airlock and a system to allow the outside atmosphere to be pumped in. The system will also allow the reverse to be done, so that the airlock can be used as both an egress point and an entrance.
There are two very different kinds of personnel airlock doors in common usage. The manual hatch is often used on vehicle airlocks and for emergency airlocks on spacecraft. The iris valve is the most common airlock door and is almost always used for standard personnel airlocks on spacecraft.
The hatch is manually operated using a central wheel mechanism located on each side of the door. Hatches may have separate locking mechanisms, though many spacecraft and vehicles do not have locks. It is also possible to have an operating wheel on only one side of a hatch, though this is very unusual. Airlock hatches are typically mechanically interlocked to prevent both airlock hatches from being opened at the same time.
The iris valve is a powered portal that consists of a set of plates that slide into position to block an opening. Most airlock iris valves are controlled by a set of switches located both inside and outside the airlock. Iris valves can also be controlled from the bridge, often from the damage control station, on small vessels, or the security station, on large vessels. An iris valve will shut automatically when pressure drops on one side of the valve. An airlock iris valve typically has a more sophisticated control system than a standard bulkhead or deck iris valve. Iris valves are usually electrically interlocked by their control system to prevent both irises from being opened at the same time. Many ships will have control systems that will allow this interlock to be overridden under the correct circumstances, to allow a ship to be vented out on a suitably habitable world or in an enclosed hanger.
Gas Control System
The simplest airlocks designed for use in space will have a valve that allows air in the airlock to be vented out into space. Another valve will allow air from inside the spacecraft to be vented into an evacuated airlock. The valves are mechanically interlocked (they are often the same valve) to prevent venting the entire ship's atmosphere to space. This is the only system used in some emergency airlocks and many standard airlocks will have a backup system that will allow operation in this mode for use during complete power failures.
A more advanced system will allow air from inside the airlock to be reclaimed by being pumped back into the ship. This is sufficient for use on vessels which only operate in space. For ship's and vehicles that operate on worlds which possess high pressure, non-breathable atmosphere a more complex system must be used. This requires that outside air be pumped out of the airlock to lower the interior pressure enough so that air can be pumped into the airlock from the ship. Particularly insidiously corrosive atmospheres or other environmental concerns, such as dust or biological contaminants will require an even more advanced system. This will be described later in more detail.
Environment Monitoring System
All but the most basic airlocks will include an environmental monitoring system. The simplest system, and the one most often installed on emergency airlocks, is the simple telltale. The telltale either consists of a single panel, or more commonly (to better service color blind sophonts) three panels. When the atmosphere on the far side of the airlock door is at acceptable pressure and oxygen content the panel will be green (or the top green panel will be lit.) When the pressure is not acceptable and the oxygen content is too low (or non existent) then the panel will be red (or the bottom red panel will be lit.) If the device cannot determine if the atmosphere is safe, because the pressure is too low or too high or the oxygen content is not safe the panel will be yellow (or the middle yellow panel will be lit.) The user must then use a more sophisticated device to analyze the atmosphere, or play it safe and use a suit. This system is unable to detect contaminants or unsafe concentrations of dangerous gases.
A more common system is the chemsniffer. It can not only analyze atmospheric composition, but detect contaminants. The most advanced systems, like those on scouts spacecraft will also include a biosniffer, whose database can detect unsafe biological contaminants. Large ships will sometimes have a central atmospheric testing station and install repeater displays at all airlocks instead of individual testing units.
Spacecraft designed to typically operate in extremely hostile environments, such as the scout/courier or tramp trader, almost always have extensive decontamination facilities installed in their airlocks. These systems usually consist of a chemical spray down system to remove surface contaminants, a tunable EM projector system to kill UV- and IR-sensitive biological agents and a vacuum system to remove fine dust and powered spores. Most airlocks also have sufficient pump capacity to reduce air pressure into the vacuum region, even on high pressure worlds, which is usually sufficient to kill most biological agents.
A standard two door airlock will typically have three control stations. One will be inside the inner door, one in the airlock itself, and the last outside the outer door. Most airlocks can also be controlled and monitored from a remote station elsewhere on the craft, typically on the bridge.
The inner control station will usually have controls for opening both doors (usually one at a time, but sometimes at the same time, utilizing a special key or other interlock bypass safety device.) An atmospheric testing station, which measures the environment outside the outer door will also typically be located there. A telltale system for the airlock itself is often installed as a backup to this system and lets the operator know when the airlock has finished cycling during normal operation. A pump control station allows equalizing to external pressure or evacuating the airlock. Other controls, to lock the outer door, control the airlock gravity, activate any installed decontamination system, will also be at this location. A communication unit, and sometimes a computer access terminal, is also a standard option for such a station.
The airlock internal control station typically will have controls to open both the inner and outer doors, interlocked so that only one door can be opened at a time. A repeater for the atmosphere testing station will often be installed. A backup telltale system (or repeaters) for the airlock itself and for the exterior of the airlock are also usually present. A pump control station allows the airlock to be evacuated or external pressure to be equalized. A separate control station is the most common method for operating any decontamination systems installed in the airlock. It is also common to have a com station inside the airlock, to allow communication with both the inner control station and other parts of the vessel. A com relay is sometimes included (to allow portable devices to be interfaced from inside the airlock.)
The outer, or external airlock control station can range from extremely basic to very sophisticated. It will usually include a control for the outer airlock door. This may be a simple opening stud or a keypad lock. Some airlocks can only be opened externally using a remote radio cipher device. The external control can automatically equalize the airlock interior prior to opening the external door or there may be a separate pump control switch. The external station sometimes includes a com device, which can act as a "doorbell", to contact the inside of the ship. Even an outer airlock door with no lock can be prevented from being opened by keeping the inner door opened. Some external airlock doors will also have a manual override, that will allow the door to be opened in the event of a power failure. Such overrides usually require that the inner door be closed and will evacuate (or pressure equalize the airlock) to the outside prior to opening the door. Most such devices will not operate if the ship has power to prevent an external intruder from exploiting them.
In the case of an airlock with manual doors, bypassing the interlock will usually require removal or disconnection of some part of the interlock mechanism, to allow both doors to be opened.
Many airlocks will have a number of remote indicators, to allow bridge or security personnel to monitor the status of the airlock doors and environment. This makes it fairly difficult to sneak into a ship's airlock.
Like other ship compartments, airlocks will often have fire protection and security devices that are centrally monitored from the bridge or another security station. These sometimes include include visual and audio pickup devices.
Because the interior of the airlock is often subjected to conditions of vacuum, varying pressures and chemically active, perhaps corrosive atmospheres, both control panels and storage lockers must be specially sealed to prevent damage to their interiors. Any items which are sensitive to vacuum or corrosive atmospheres or varying pressures must be stored in such lockers or in individual storage containers. This includes items such as vacc suits, which are likely to have internal components that are not designed to be exposed to vacuum. Such items are often stored in lockers just outside the airlock inner door. Tools and devices typically used in hazardous environments are sometimes stored in open racks in the airlock compartments, because these devices are designed to withstand the rigors of space.
Boarding Passages & Couplings
A joint TAS/Lloyds safety standard has been established for airlock fittings. This standard has been accepted by the Office of Calendar Compliance and is used by the Imperial Interstellar Scout Service and Imperial Navy, as well as the Imperial Starport Authority. Ships of the Solomani Confederation have also accepted this standard, making their airlock equipment interchangeable with Imperial standard. It is not unusual to find ships from other areas, which engage in cross border trade with the Imperium or Confederation, equipped with at least one airlock which conforms to the TAS/Lloyds standard.
Standard airlock doors are designed so that a ship can mate airlock doors directly with a station or another ship. Auxiliary ships held in couplings or clamps are designed so that airlock doors associated with the coupling can mate. Unless ships are specifically designed to dock together, as are some IISS scout ships and military cutters, they will have to use a passage tube to couple airlocks.
A passage tube is an armored flexible tube that hooks on the standard fittings around the exterior airlock doors. The standard passenger tube is 100' long by 8' in diameter. It must be manually deployed, requiring approximately 30 minutes to rig.
A powered version of the passage tube is available which will allow the tube to be deployed mechanically in less than 5 minutes. This device is used most typically by stations.
A more sophisticated version of the powered passage tube is used by military, patrol and rescue craft. The far end of the tube has a flexible seal allowing it to be used with nonstandard airlocks. It can also be used for salvage and boarding purposes by being deployed against a ship's hull, allowing cutting tools to be used in a shirt sleeve environment.
Interior airlocks, airlocks designed for passage from one area of a vessel to another, are most commonly used for access to hangers or vehicle bays, or as emergency entrances in the case of compartment integrity failure.
Most modern vessels use iris valves in vehicle bay or hanger access airlocks. These airlocks will have many of the same systems as a standard personnel airlock, including decontamination systems. They typically lack security measures as this function is provided by the exterior hanger or bay door. Sometimes no separate airlock is provided for these spaces, as the hanger/bay itself act as an airlock. In this case only the most rudimentary controls are located at the iris valve, as control of the hanger systems is normally controlled from another remote station.
The most common emergency interior airlock system is the trunk. A trunk is a shaft segmented by each deck of a vessel. A manual hatch is set in the deck and overhead of each segment as well as a hatch on one or more bulkheads of the trunk. A simple pressure telltale is installed at each door to report on the atmospheric pressure on the far side. A ladder provides the method to climb from the deck to the overhead hatch, which is typically 9 feet above, but may be much farther on large ships. The total compartmentalization of the shaft allows it's integrity to be maintained, even if the area outside the shaft has been exposed to vacuum. Should the integrity of the shaft by Compromised, the hatches at each deck will ensure that the whole shaft is not evacuated. Each level can be utilized as an airlock. Some shaft systems include emergency bottles of air, to allow the trunk level to be repressurized if it has been opened to vacuum. Trunks require the commitment of a significant amount of volume and discipline in the design of a vessel, since the trunk must traverse every deck of the ship to be effective. They are fairly useless on vessels of three or fewer decks. Trunks are standard on most military vessels and large commercial liners, but somewhat rare on small tramp vessels and scout craft.
A not entirely common airlock system, the integrated lift airlock, is used almost entirely on luxury vessels. Some such vessels will have a standard airlock which leads to a platform, which operates entirely in the external environment. This is not strictly speaking, a lift airlock system. A true lift airlock will consist of a cage or platform located in the airlock which will descend through an iris valve in the deck. Such a system will allow the occupants of a ship, which is a significant distance above the ground when standing on its landing pylons, to reach ground level without having to scramble down a ladder. Such a system requires less space than a ramp, but is too costly for general use.