Prowler

"If you saw a prowler, it probably wasn't a prowler."

- Anonymous Prowler Corps officer

A prowler is a line of warship, represented by the hull classification symbol PRO. Originally, the term properly referred to the slipspace-capable reconnaissance ships used by the Office of Naval Intelligence. However, it has been adopted by many human organisations as a moniker for any ships that are similar in size to a corvette which are designed for covert operations.

Technology
Up until the late 25th century, the notion of a truly invisible spacecraft was considered a pipe dream. Although visual and radar stealth were viable, any ship would be blatantly visible on infrared sensors a system away the moment it fired its thrusters. Although trickery involving decoys and electronic countermesures could confuse or disable enemy sensors over short ranges, no ship (or at least any ship with human crew expected to move about a star system over reasonable spans of time) could be made truly undetectable to opponents with access to appreciable telescope equipment. High-resolution sensors are near-ubiquitous across the Human Sphere, as they are relatively cheap and widely commercially available due to their importance to space traffic monitoring; this also means all but the most poorly-armed insurgent groups have access to such systems. Given that sensor satellites are much easier to camouflage than ships, most systems with insurrectionist activity had numerous hidden sensor platforms (coupled with occasional insurrectionist collusion with local space traffic control), which made it impossible for the UNSC to deploy into such systems undetected. Instead, early generations of reconnaissance spacecraft relied more on deception than true stealth, often being designed to mimic civilian craft in drive signature and physical makeup along with hiding among high volumes of civilian traffic.

The heightened tensions of the UNSC-CMA Cold War, coupled with advances in spacecraft thermal management, fusioneering and slipspace theory, saw the UNSC reopening research on thermal emissions masking. The solution, as it turned out, arose from a novel application of slipspace. As early experiments with slipspace-based communication had shown, moving energy into slipspace is not particularly difficult; the trouble is keeping that energy coherent and controlling its trajectory past entry. Any radiation transmitted into slipspace (including radio waves or tightbeam) rapidly decoheres, then radiates back into normal space predominantly as random patterns of microwave radiation scattered across wide swathes of space in what is known as the Muratovski effect.

Prowlers rely on the Muratovski effect for concealing their engine burns, using emissions baffles coupled into their fusion engines to shunt drive emissions into non-spatial dimensions, which then scatter it widely enough to fully or partially obfuscate the ship's drive flame. When operating properly, the radiation backscatter is virtually indistinguishable from cosmic background radiation. Though this does not make the ship completely invisible — an observer can still detect the minute variance in black-body radiation if they look hard enough, and persistent active scans will reveal the presence of an object — it does make prowlers very difficult to detect, especially over long distances and to an unsuspecting enemy.

The baffling arrays have to be coupled with a highly specialized "dumb" AI, which calculates the precise output of the engines at any given time and synchronizes it with the baffle field generator attached to the drive. This is coupled with a system of hull cooling, insulation and internal heat sinks for other onboard heat sources. For this reason, prowlers and their engines have to be constructed to far stricter tolerances than normal ships, which increases their costs and makes manufacturing a specialized and expensive affair. By the Human-Covenant War, even the prowler's slipspace exit can be convincingly masked or obfuscated, though this means fissile materials cannot be carried onboard as their Cherenkov radiation aura interacts with slipspace in ways too unpredictable to be accounted for by the baffles.

The first generation of baffling devices were finicky and had to be constantly tuned, with even a minor miscalibration or failure to account for a drive flare exposing the ship's presence. In the early years of the Insurrection, prowlers mostly relied on appearing as something else than warships by using baffles to alter their drive signature; the near-complete emissions masking that would characterize later prowlers is a later innovation. Since letting some emissions escape also improves drive efficiency, many prowlers still rely on disguising themselves as civilian or logistics craft. Such deception is complemented by building the prowlers into hulls that mimic the designs of civilian yachts, for example. This tactic was effective against most Insurrectionist groups, particularly those working with limited technological resources.

Most Covenant War-era prowler types forgo such deception and instead optimize their hull configuration for more complete stealth via the use of angled surfaces and radar-resistant materials. Ideally, the goal is for the prowler not to be detected in the first place; while a "soft" detection will not directly reveal the size or type of ship to long-range sensors, it will alert the enemy to the presence of a ship, leading to more detailed scans that will expose the prowler. UNSC stealth proved surprisingly effective against the Covenant; despite their superior sensor technology, the Covenant's lack of AI-based detection means they rely mostly on organics to monitor their sensor feeds, meaning Covenant sensors are only as good as their operators.

Even with advances in the technology since its inception, engine baffles are not completely efficient and cannot be used indefinitely. By the end of the Covenant War, baffled engines could only be run at up to 30% power before a geometric increase in the likelihood of detection. As a rule, any burn of the main engines is regarded as a costly maneuver, as this contributes to the heat buildup that will eventually compromise stealth. Although modern baffling systems combined with internal heat sinks are very efficient at removing heat, they can only be used for a limited time before the prowler must radiate out its stored heat. Prowlers use cold-gas thrusters and gravity-assist maneuvers over sustained fusion engine burns as much as they can; this also means that the ideal prowler mission is one where the prowler can afford to take its time. This is not often the case, however, and prowler crews must balance stealth with the schedule they are working on in a given operation.

These challenges also multiply as the prowler's size increases, meaning that prowlers should ideally be as small as possible. First conceptualized in the later years of the Insurrection, the Human-Covenant War saw the introduction of destroyer-sized prowlers termed stealth cruisers, albeit in extremely limited numbers due to their great expense and diminishing returns in most fields of performance.

Although secondary to their emissions masking and only relevant on short ranges, modern prowlers also make use of visual camouflage. This provided by a relatively simple chameleoflage stealth coating, though advances during the Human-Covenant War have seen prowlers shift increasingly toward photoreactive metamaterials.