PolarStar Kythera® SA HPA Engine — Mechanical High-Performance Power for A&K SR-25 (Model: SA)

The PolarStar Kythera® SA is a ground-up mechanical HPA engine built for shooters who want uncompromising consistency, a realistic trigger feel, and rock-solid reliability — all without batteries, electronics, or solenoids. The Kythera SA (A&K SR-25 model) brings PolarStar’s precision engineering to a fully mechanical platform, delivering a responsive single-action trigger, wide operating range, and surgical repeatability that make it ideal for DMR, competition, and multi-role AEG builds.

Real mechanical design — no electronics required
Unlike solenoid-based or FCU-dependent HPA systems, the Kythera SA is a purely mechanical engine. That means no batteries to charge, no firmware to manage, and virtually no electronics to fail. The system is waterproof by design and uses a patent-pending valve mechanism engineered for maximum efficiency: 100% of the air routed into the engine is available for the shot cycle, producing consistent velocity and excellent gas economy.

Authentic, tunable trigger feel
A defining strength of the Kythera SA is its realistic single-action trigger. Hardened alloy sears engage and release a stainless-steel spool valve just like a firearm’s sear and hammer — giving a genuine break and reset. The trigger is fully adjustable (weight, length of pull, and over-travel), so you can fine-tune the feel to match competitive disciplines or personal preference. The internal firing valve is spring-balanced, so trigger feel remains consistent regardless of input pressure or external conditions.

Complete shot cycle on trigger pull — rapid, reliable follow-ups
The Kythera completes a full shot cycle on the trigger pull, providing closed-bolt reliability akin to PolarStar’s Fusion and F2 platforms. Because the system chambers and recharges automatically without requiring trigger release, follow-up shots are rapid and consistent with a reduced risk of double feeds. No pilot valves are used, and the valve releases at the same point and force every shot — a design that yields both efficiency and predictable performance.

Adjustable dwell, nozzle speed and wide pressure range
Tuning is straightforward and flexible. The Kythera allows adjustable nozzle dwell/return speed to control timing, and it accepts input pressures from 60 to 180 psi. In testing (with a 6.05×380 mm barrel and 0.20 g BBs), this pressure window produced velocities from ~270 fps up to ~500 fps, making one Kythera capable of roles from CQB to DMR simply by changing regulator pressure — perfect for players who want a single engine to cover multiple playstyles.

Durable, serviceable, and field-ready
The Kythera’s balanced spool valve and mechanical cocking mean the system is extremely robust. A universal charging cord is provided for manual cocking if the system ever runs fully out of air — a rare situation — and can be routed through the grip or secured near the charging handle for convenient use. Installation is designed to be simple: the Kythera installs into a standard gearbox as a single assembly with minimal retained gearbox parts (trigger, spring guide, selector and safety). No external electronics or complex wiring harnesses are required.

Why builders choose Kythera SA

• Mechanical simplicity — no batteries, FCU, or solenoids to fail.

• Realistic trigger — adjustable weight, travel, and over-travel for a firearm-like pull.

• Full shot cycle on trigger pull — faster, more consistent follow-ups and closed-bolt reliability.

• High efficiency — patent-pending valve uses air direct to the nozzle for excellent economy.

• Wide tuning range — 60–180 psi supports CQB to sniper roles.

• Waterproof operation and field serviceability with the universal charging cord.

Model & Compatibility

• Model: Kythera® SA — A&K SR-25 specific configuration

• Manufacturer: PolarStar Airsoft

For players and technicians who demand a responsive, dependable HPA solution without the hassle of electronics, the PolarStar Kythera SA is a revolutionary mechanical engine that blends analog reliability with modern tuning flexibility. Convert your A&K SR-25 into a high-performance, battery-free platform and experience the refined trigger feel and consistent shot-to-shot performance that only Kythera can deliver.