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 stealth 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, no ship (or at least any ship with human crew expected to move about a star system) could be made truly undetectable to opponents with access to appreciable telescope equipment.

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 concealment, 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 the cosmic microwave background. 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.

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, 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.

In the early years of the Insurrection, prowlers mostly relied on appearing as something else than warships by altering 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. However, most Covenant War-era prowler types forgo this and instead optimize their hull configuration for stealth via the use of angled surfaces and radar-resistant materials.

The early 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. Even with advances in the technology, baffles are not completely efficient and cannot be used indefinitely. As a rule, any burn of the main engines is generally regarded as a costly maneuver with an ever-present risk of stray particles escaping. While this 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, typically leading to more detailed scans that will expose the prowler. Prowlers use cold-gas thrusters over their fusion engines as much as they can, but the inefficiency of said thrusters (even compared to the resistojets mostly used on non-stealth craft) means they can only be used for attitude control and local maneuvers. Heat buildup is another issue. Although modern baffling systems combined with internal heat sinks are efficient in removing heat, they can only be used for a limited time before the prowler must radiate out its stored heat.

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.