Production information
Manufacturer Defiance Industries[1]
Production Year 2652[2]
Model STN-3L[1]
Class Medium
Cost 3,292,030 C-bills
Technical specifications
'Mech type Inner Sphere BattleMech
Mass 40 tons
Chassis Defiant V
Armor Valiant Lamellor
Engine Pitban 240
Communications System StarLink/Benicia Model AS829G
Targeting Tracking System Targa-7, Vid-Com-17
Heat Sinks 10 single
Speed 97 km/h
BV (1.0) 614
BV (2.0) 717[1]


The Sentinel is a medium BattleMech built by Defiance Industries as an infantry support and installation defense 'Mech, originally under exclusive contract for the Lyran Commonwealth. While initially pleased to have a then-Terran Hegemony controlled factory manufacturing the 'Mech exclusively for the LCAF, the Lyrans were less so when a Star League Council amendment in 2652 permitted the Star League Defense Force to purchase a highly advanced version of the design which Houses Steiner, Marik and Davion would not receive until years later.

The Sentinel was one of many designs to suffer from the technological decline of the Succession Wars era, with Defiance first losing the ability to manufacture its advanced weapon systems before discontinuing production entirely in 3014 due to lack of both parts and interest. Rather than disappear from the battlefield, the Sentinel now began to appear in increasing numbers thanks to ComStar and its massive stockpile of SLDF matériel, both providing the design to the Draconis Combine prior to the War of 3039 and using it to help outfit its Com Guards.[3][4][5][6][7][8]

Weapons and Equipment[edit]

The Star League Sentinel carried a somewhat unique and highly advanced armament for its time. Its primary weapon was a Kawabata Weapons Industries UAC/5 developed especially for the 3L, and is capable of firing at double the rate of a standard AC/5 at the cost of slightly increased possibility of jamming the weapon until it can be overhauled. The Sentinel also mounted an advanced Defiance Streak SRM-2, the launcher designed to only fire if it gains a lock, saving ammo until the entire salvo is virtually guaranteed to hit. Unfortunately, the weapon's benefits were hampered by the companion small laser, the swap from the fragile Defiance A-1 for the sturdier Defiance B-1A moved the launcher's ammo feed by half a meter, resulting in ammo jams during abrupt maneuvers. Though MechWarriors discovered that they could clear a jam in the field by thumping the 'Mech's chest with its right fist, opponents quickly learned to concentrate fire on any Sentinel performing this, forcing pilots to retreat from battle anyway.[3][4][5][8]

Ignoring its weapons loadout, the Sentinel was a relatively mundanely constructed 'Mech built on a standard Defiant V chassis and clad in five and half tons of Valiant Lamellor standard armor. A dependable Pitban 240 fusion engine gives the 'Mech a top running speed of 97.2 km/h, but when first manufactured the engine was also blamed as the cause for chronic overheating of the 'Mech. An investigation eventually found instead it was an issue with the heat circulation of the 'Mech's 10 single heat sinks, which required a field replacement program and redesign for future production.[3][4][5][6][7][8]

With a powerful Targa-7, Vid-Com-17 targeting system and StarLink/Benicia Model AS829G communications package, the Sentinel was ideal for long-range patrols or mobile observation. Able to operate on multiple frequencies, it could not only closely monitor and command units operating under it, but also maintain contact with rear-area artillery for fire support.[3][4]


  • STN-1S 
    The 1S Sentinel was the original production model commissioned by House Steiner in 2651. The 1S lacked the advanced technologies of the 3L, mounting a Defiance Model F AC/2 instead of the advanced ultra autocannon model, and a Coventry Quad-Rack SRM-4 instead of the Streak SRM-2. The 1S also mounted a half ton more armor, but is otherwise identical to the 3L. After years of hassles and bureaucratic maneuvering, all Lyran 1S Sentinels were eventually upgraded to 3L, only to be all but destroyed during the Succession Wars.[8] BV (2.0) = 632[9]
  • STN-3K 
    The 3K Sentinel is the downgraded Succession Wars era version of the 3L model. The UAC/5 and Streak SRM-2 have both been downgraded to a Defiance Type J AC/5 and a Holly SRM-2 rack. While its speed has stayed the same, the 'Mechs endurance in the field is actually enhanced by an extra ton of autocannon reloads and an extra half ton of armor.[1][4][5][8] BV (1.0) = 536;[10] BV (2.0) = 652[11]
  • STN-3KB 
    The 3KB Sentinel is an uncommon field refit of the 3K. This variant replaces the AC/5 with a PPC and three extra single heat sinks.[5] BV (2.0) = 823[13]
  • STN-5WB 
    This variant was produced by Defiance after it was lost to the Word of Blake during the Jihad. Built on the same chassis as the 3L, the 5WB adds one ton of armor, uses a light engine for power, and replaces the weapons with a pair of light AC/5s that are fed from a single, one-ton ammunition bin, backed up by three ER medium lasers.[1][8] BV (2.0) = 999
  • STN-6S 
    This Sentinel variant uses a Clan-spec XL engine and an XL Gyro to free weight. Its primary weapon is an UAC/5, supplemented by a Clan-grade ER medium laser and two Thunderbolt 5 missile launchers. Two tons of ammo are present for the Thunderbolts and the autocannon gets one ton of ammunition, all protected by CASE II systems. Six and a half tons of armor protects the 'Mech. BV (2.0) = 1,089[19]

Design Quirks[edit]

The Sentinel is subject the following Design Quirks:[20][21]


  • In German products, the unit's proper name was translated to Wächter. The model code was accordingly changed to WCT-3R.



  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Technical Readout: Succession Wars, pp. 62–63: "STN-3L Sentinel"
  2. MUL online date for the Sentinel
  3. 3.0 3.1 3.2 3.3 Technical Readout: 2750, p. 26: "STN-3L Sentinel"
  4. 4.0 4.1 4.2 4.3 4.4 Technical Readout: 3025 Revised, p. 40: "STN-3KSentinel"
  5. 5.0 5.1 5.2 5.3 5.4 5.5 Technical Readout: 3039, p. 254: "STN-3KSentinel"
  6. 6.0 6.1 6.2 Technical Readout: 3050, pp. 100–101: "Sentinel"
  7. 7.0 7.1 7.2 Technical Readout: 3050 Revised, pp. 90–91: "STN-3M Sentinel"
  8. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 Technical Readout: 3050 Upgrade, p. 194: "STN-3L Sentinel"
  9. Record Sheets: 3039 Unabridged, p. 411
  10. Combat Operations, p. 130
  11. Record Sheets: 3039, p. 104
  12. Record Sheets: 3039 Unabridged, p. 413
  13. Record Sheets: 3039 Unabridged, p. 414
  14. Historical: Operation Klondike, p. 159: "SLDF Royal Units - BattleMechs"
  15. Record Sheets: Operation Klondike, p. 35
  16. 16.0 16.1 Combat Operations, p. 125
  17. Combat Operations, p. 127
  18. Record Sheets: Upgrades, p. 70
  19. Recognition Guide: ilClan, vol. 14, p. 17
  20. BattleMech Manual, Fourth Printing, p. 94: "BattleMech Quirk Table"
  21. BattleMech Manual Errata (Version 6.01), p. 9