Guide

Game grenade and throwable systems explained

Harbor Ruins' warehouse breach encounter gave every squad member two frag grenades and no guidance. Players tossed them at doorframes, watched them bounce into corners they could not see, and complained that enemies behind cover took full damage through concrete. Worse: the blast used a flat radial check with no line-of-sight test, so a grenade on the wrong side of a crate still one-shot anyone within five meters. Playtesters called throwables “random death orbs.” The refactor added a visible throw arc preview, audible fuse ticks, distance-based damage falloff, and cover occlusion for explosion rays. Breach completion rose from 41% to 78% and friendly-fire incidents dropped 62%. Grenade and throwable systems are the authored rules for arcing, timed, area-of-effect weapons — frags, flashes, smokes, molotovs, sticky bombs, and utility items that are thrown rather than fired as sustained gunfire. They sit above projectile simulation (trajectory, bounce, pooling) and below encounter design (when the player earns charges). This guide covers throw input and arc preview, fuse and cook timing, blast radius and falloff, throwable variants, friendly fire and team rules, multiplayer authority, the Harbor Ruins refactor, a technique decision table, pitfalls, and a production checklist alongside our weapon systems guide.

Throw input and arc preview

A throwable starts with aim mode: the player holds a button to enter a throw stance, camera may shift slightly, and movement speed often drops. Release throws; cancel returns the grenade to inventory without consuming it (unless your design charges on pull-pin).

The most important readability tool is an arc preview — a dotted parabola or ghost mesh showing landing point and first bounce. Compute it with the same integrator the live grenade will use (gravity, initial velocity from player facing and charge time, optional air drag). Sample 20–40 points along the path and raycast down at the end to snap the indicator to floor geometry.

Charge-to-distance curves

Many games map hold duration to throw speed on a curve — fast initial ramp, soft cap at max range. Without a cap, competitive players always hold full charge; with too short a cap, lobs over cover become impossible. Document min, max, and sweet-spot ranges in your tuning sheet. Twin-stick titles often use right-stick magnitude instead of hold time; see our twin-stick design guide for stick-based aiming patterns.

Validation before spawn

Reject throws when the arc preview intersects invalid volumes (out of bounds, past max range, inside spawn protection). Showing a red arc when invalid teaches constraints better than silent failure.

Fuse timing and cooking

Fuse is the delay between arming and detonation. Classic frags use 3–5 seconds from throw; some titles start the fuse on pull-pin (cook in hand) rather than on release. Each model changes skill expression:

Fuse on release — simpler; players learn bounce timing.
Fuse on arm — enables cooking for short lobs; risks self-damage if held too long.
Impact detonation — no fuse; skill is placement only (common for rockets and sticky bombs).

Communicate fuse state with escalating audio (tick rate increases), a blinking mesh emissive, or a UI ring around the grenade icon. Players should predict detonation within ~0.3 s without reading numbers. For cooked grenades, show a hand animation and optional subtle screen vignette as the fuse burns down in inventory.

Blast radius, falloff, and cover

Explosion damage usually combines three layers:

Inner kill radius — full or near-full damage.
Outer hurt radius — linear or quadratic falloff to zero.
Line-of-sight rays — sample directions from blast center to target; if a cover collider blocks the ray, apply reduced or zero damage.

Flat spheres without occlusion feel unfair in destructible or cover-heavy spaces. A practical compromise: full damage in inner radius only when LOS exists; outer radius applies chip damage through thin cover tags (wood, drywall) but not stone.

Knockback and status on top of damage

Explosions often apply knockback scaled by falloff. Pair with brief stun or deafen for flashbangs. Keep knockback direction away from blast center so victims read the causal link.

Bounce, roll, and stick behaviors

After spawn, the grenade is a physics object or a kinematic sim with authored bounces. Key parameters:

Restitution — energy retained per bounce; frags often 0.3–0.5.
Friction — slows rolling on slopes; prevents infinite hallway ping-pong.
Max bounces — cap at 3–5 then force roll or stop.
Sticky / magnet — attach to surfaces or enemies on contact; fuse continues.

Use the same swept collision approach as bullets to avoid tunneling at high speed. Pool grenade actors like other projectiles; simultaneous smokes in a 4-player breach can spike allocations if each spawn is a new prefab instance.

Throwable variants and roles

Type	Primary effect	Design notes
Fragmentation	Radial damage + knockback	Clear arc preview; strict friendly-fire policy in co-op
Flashbang	Blind/deafen through LOS	Intensity by angle to burst and distance; PvP needs immunity caps
Smoke	Vision block + optional tag reveal	Particle budget; AI needs separate sense channel than human players
Molotov / fire	Damage over time zone	Navmesh no-go for AI; stack rules with other DOT sources
EMP / utility	Disable shields, cameras, traps	Telegraph loudly; short PvE silence is frustrating without feedback

Give each variant a distinct silhouette and sound on equip so teammates predict outcomes. Economy matters: refill at checkpoints, kills, or inventory slots — scarcity defines whether grenades are panic buttons or encounter solutions.

Friendly fire, teams, and PvP fairness

Co-op titles often disable or reduce friendly damage; PvP titles keep it for skill. Middle paths:

50% friendly damage — punishes recklessness without instant team kills.
Hostile-only damage — frags hurt enemies; flash/smoke affect all.
Per-mode toggles — hardcore vs casual playlists.

Log friendly kills for analytics. If frags team-wipe more than 5% of runs, tighten arc preview or add a “allies in blast zone” warning on the preview mesh.

Multiplayer authority

Server (or host) should own throw validation, fuse timers, and damage application. Clients predict arc preview locally; server confirms spawn position, velocity, and detonation time. On lag, avoid rewinding explosions that already played VFX — prefer slight damage delay over rubber-banding fireballs. Replicate minimal state: position, velocity, fuse end timestamp, grenade type id. See our netcode guide for snapshot patterns.

Harbor Ruins warehouse breach refactor

The encounter required clearing two dug-in gunners behind shelving units. Before the refactor, grenades used instant sphere overlap on detonation with no preview. Players either overshot into the street (wasting charges) or blew up teammates rushing the door.

Changes shipped in one milestone:

Green/red arc preview with max-range ring on the landing dot.
Fuse audio synced to server time; 4.0 s from release with visible LED on the mesh.
Damage falloff: 100% inside 2 m, linear to 20% at 6 m, 0% beyond 8 m.
LOS ray from blast center to each target's capsule; full cover blocks outer radius only.
Co-op friendly damage set to 25%; flashbangs affect all actors equally.

Average grenades used per successful breach dropped from 3.1 to 1.4 because players stopped guessing. The same systems later powered smoke deploys for the bridge boss add phase.

Technique decision table

Your situation	Prefer	Why
Cover-heavy PvE encounters	Arc preview + LOS falloff	Players learn flush angles; reduces unfair wall damage
Fast arena PvP	Short fuse, low bounce, impact option	Keeps pace; minimizes inventory management
Stealth / tactical sim	Cook-on-arm, quiet pin pull, smoke priority	Rewards timing; frags are loud telegraphs
Twin-stick horde	Stick-charged arc, generous self-damage immunity	Thumb aiming needs preview more than mouse
Large open maps	Long fuse, high throw speed cap	Without range, grenades are dead inventory slots
Ability-based hero shooter	Cooldown grenade ability, no cooking	Consistent kit reads; avoids hoarding

Common pitfalls

No arc preview — players treat throwables as slot-machine damage.
Preview mismatch — ghost arc uses different gravity than live sim; trust evaporates.
Silent fuse — victims cannot distinguish grenade death from gunfire.
Infinite bounce noise — hallway pinball annoys audio mix and AI hearing.
Full friendly damage in co-op — random teammates become the biggest threat.
Smoke without AI fallback — enemies either ignore smoke or cheat through it.
Client-only detonation — desync kills and ghost explosions in multiplayer.
Stacking unlimited charges — encounters designed around scarcity break when players hoard 20 frags.

Production checklist

Match arc preview integrator to live throw physics; test on slopes and stairs.
Define min/max throw speed, fuse length, and inner/outer damage radii in a data table.
Implement LOS or cover tags for explosion damage; playtest behind crates and walls.
Add fuse audio with accelerating ticks; sync to authoritative server time online.
Pool grenade actors; cap simultaneous active throwables per player and globally.
Document friendly-fire rules per mode; warn when allies sit inside preview blast zone.
Give each throwable type unique equip audio and mesh silhouette.
Validate throws server-side: inventory charge, cooldown, spawn position, velocity.
Profile particle and decal cost for smokes and fires in worst-case 4-player scenes.
Log wasted throws (detonate out of bounds, zero-damage blasts) to tune encounter layouts.

Key takeaways

Throwables are tactical tools when players can read arc, fuse, and blast zone before committing.
Damage falloff plus line-of-sight prevents grenades from ignoring cover geometry.
Fuse feedback (audio, light, UI) is as important as damage numbers for fair counterplay.
Harbor Ruins cut wasted grenades per breach by more than half after adding preview and occlusion.
Server-owned detonation and pooled physics keep multiplayer explosions consistent and performant.