By the end of the Amaranthine military bases were either planet or moon bound or relatively slow moving orbital defence platforms and, even with liquid filled acceleration suits, ship acceleration was limited to ten gravities due purely to human frailty. With missile accelerations routinely exceeding one hundred and fifty gravities and the threat of "planet busters" able to devastate a planet all too easily conceived, military commentators had become more outspoken. In the years following the cessation if conflict it was hard to find a new channel that didn't speak routinely of the need for military ships to reach engagement with enemy forces quickly enough to prevent the use of such super weapons as well bring battles back to something the human mind could truly deal with.
Still, an alienated and homeless culture the N'Chari began experimenting with immersion techniques to create a supportive fluid that would allow their already impressive fighters to accelerate and manoeuvre more quickly; they needed a fluid that approximated the viscosity of human blood and, ideally, the carrying properties of air. The result was Immersaline, an organic fluid four times as viscous as water (slightly more viscous than human blood). Immersaline is a transparent light blue liquid that, in its "natural" state, is incapable of supporting flame or allowing explosions to proceed; although it would suffocate any person immersed in it without an air supply it is not poisonous. Under specific conditions it was found that Immersaline could be made to carry other gasses and had a high affinity for oxygen; when oxygenated, although it remained non-explosive, it could support the reactivity of other chemical mixes and several lives were lost during freak explosive accidents. The experimenters confirmed that Immersaline could carry oxygen at around twenty percent, easily enough to support mammalian respiration as well as allow the removal of waste gasses and that the liquid permeated the entire body yet was somehow held back from replacing bodily fluids. They established that around sixty percent of small to medium sized mammals survived several hours in oxygenated Immersaline and only then decided to initiate human trials.
Aware of the likely public objections to human trials the N'Chari decided to invite the UCS, one of their greatest political supporters, in as a partner and, at UCS insistence, they reluctantly agreed to limited Skean involvement. A new multi-national research team, the Joint Acceleration Research Program, was founded at Space City University on Autonomy in the Enterprise system. A spy was found in the team some three years in which was too late to prevent a huge swathe of technical information being leaked to at least one other agency; who that agency or nation was, remains a mystery since the spy had a poison-filled false tooth and used it leaving virtually no information behind to track.
As human trials progressed it was found that a subject's awareness of impending immersion massively improved their handling of the process and that, in combination with rapid extraction and fluid evacuation, improved survival rates to very nearly one hundred percent. Further pre-training reduced the risks further still but there were those who could never fully adapt to the process. A notable side effect of long exposure to Immersaline is a light blue staining of the skin that can take several days to fade; a common nickname for allied marines is "Ultras" whilst UCS naval personnel are often referred to as "Indigos" and Skean as "Royals". Five years after its inception, JARP was able to demonstrate a ship flown by fully immersed personnel that was able to accelerate at over thirty gravities and manoeuvre with stresses in the high twenties.
There are a number of potential health issues associated with long-term Immersaline exposure including poor respiratory response to negative air pressure as well as negative effects on musculature. There are also issues with respect to rest, recuperation and emergency treatment amongst others but, with highly automated ships largely managed by sophisticated computer control systems, around half a ships complement are regarded as non-essential during battles. For these reasons along with cost and strategic considerations the decision was made to only partially flood the ship and then only during high alert states; compartments that needed to be immersed included the bridge, engineering, weapons control and, on carriers and larger ships, fighter control and launch areas. It also reduced the need for large offline Immersaline tanks.
Standard operational procedure for ships is to run immersed only at Battle Alert (red) although some captains and fleet commanders have elected to do so at Ready Alert (amber). Battle alert is usually entered just prior to engaging the enemy and during that period ship's personnel, already in battle suits from Ready Alert, will lock down the ship ensuring all bulkhead doors are sealed and locked, that battle-staff are on station or on their way to their stations and non-essential personnel are in the bulk immersion tanks. As soon as battle-staff are "on station" in an isolable section the computers flood the compartment(s). Likewise, individual bulk tanks are flooded as soon as they are fully occupied. Ship computers will always warn personnel that flooding is imminent as the process is known to badly (even fatally) affect unprepared personnel.
Another benefit of flooding during high alert status only is that there are psychological and physical benefits gained from non-continuous immersion; crew allowed "normal" down time during voyages (when the ship is not at battle alert) are judged to be less stressed. Some strategists, however, have suggested there may be increased military risk because battle mode is less familiar when personnel spend so much time non-immersed; some militaries now operate longer in immersed mode which it is claimed better acclimatises personnel to the environment.
The use of immersion technologies means ships are more costly to build and run; not only with the additional, though relatively minor, costs associated with stronger bulkheads and doors but the addition of large, micro-filtered tanks that impact manoeuvrability during battle. Bulkhead doors tend to be manual rather than automatic which, aside from expense and maintenance issues, allows personnel to check compartments as they seal them and retreat to action areas or tanks. Bulkhead doors remain closed and locked during battle alert and are only unlocked once the ship reduces its alert level and the Immersaline has been recovered; advanced airlock style systems have been considered but have been deemed too costly to implement.
As mentioned above the flooding of "action areas only" leaves other areas clean and dry to serve more quickly as rest, recuperation and emergency treatment areas once the ships drop from battle alert.
Battle personnel are those considered to be essential during a naval engagement and include the ship's Captain and XO, engineering, system's support, radar detection technician, weapons specialists and ratings in essential roles. On carriers, battleships and larger cruisers pilots, air traffic control and aircrew will be included. All naval battle staff will be equipped with lightweight immersion suits which can double as space suits in emergency situations. Non-essential battle roles include all medical, scientific and logistical staff as well as chefs, trainers, financial staff and the chaplain. Marines are often equipped with full immersive suits so the choice of whether they will tank or remain in their suits becomes a command decision and most marine units will elect to strap in and prep in their own quarters.