Production information
Manufacturer Defiance Industries
Production Year 2652[1]
Model STN-3L
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
Speed 97.2 km/h
BV (1.0) 614
BV (2.0) 717


The Sentinel is a medium BattleMech built by Defiance Industries, originally under exclusive contract for the Lyran Commonwealth, as an infantry support and installation defense 'Mech. 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 material, both providing the design to the Draconis Combine prior to the War of 3039 and using it to help outfit its Com Guards.[2][3][4][5][6][7]

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 Ultra Autocannon/5. Developed especially for the 3L, this advanced weapon was 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. However the weapon's benefits were hampered by the companion Small laser, the swap from the fragile A-1 for more sturdy 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.[2][3][4][7]

Ignoring its weapons load-out, the Sentinel was a relatively mundanely constructed 'Mech, built on a standard chassis and clad in five and half tons of armor. A dependable Pitban 240 rated Fusion Engine gives the 'Mech a top running speed of 97.1kph, 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 ten standard heat sinks, which required a field replacement program and redesign for future production.[2][3][4][5][6][7]

With a powerful targeting and 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.[2][3]


  • 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 an Defiance Model F AC/2 instead of the advanced Ultra model and a Conventry Quad-Rack SRM-4 instead of the Streak Missile Launcher. 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.[7] BV (2.0) = 632 [8]
  • STN-3K 
    The 3K Sentinel is the downgraded Succession Wars era version of the 3L model. The Ultra Autocannon and Streak SRM have both been downgraded to a Defiance Type J Autocannon 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. [3] [4] [7] BV (1.0) = 536[9], BV (2.0) = 652[10]
  • STN-3KA 
    The 3KA Sentinel is an uncommon field refit of the 3K. This variant replaces the autocannon with a large laser, adds more armor and three extra heat sinks.[4] BV (2.0) = 847 [11]
  • STN-3KB 
    The 3KB Sentinel is an uncommon field refit of the 3K. This variant replaces the autocannon with a PPC and three extra heat sinks.[4] BV (2.0) = 823 [12]
  • STN-3Lb 
    The Royal version of the Sentinel featured a significantly upgraded weapon load-out, mounting a Gauss Rifle in place of the autocannon and a Medium Laser in place of the Streak rack, retaining the small laser. To handle the weight of the Gauss Rifle, it was almost entirely rebuilt using a XL engine and Endo-Steel structure, with the one and a half tons of Ferro-Fibrous plate enhancing its protection. [13] BV (2.0) = 1260[14]
  • STN-3M 
    The downgraded STN-3M model provided by ComStar to the Draconis Combine as part of Operation ROSEBUD carried attributes of both the 3L and 3K Sentinels, retaining the advanced 'Mech's Ultra autocannon, but replacing the Streak rack with a standard Markin Mini SRM-2 launcher. While House Kurita was angered as much by the loss of the LosTech system as the use of a "foreign" Magistracy of Canopus manufactured launcher, the swap allowed the small laser to be upgraded to Magna Mk II Medium Laser. [5] [6] [7] BV (1.0) = 655[15], BV (2.0) = 758
  • STN-4D 
    The 4D adds a Davion touch to the Sentinel. The SRM launcher has been completely done away with on this model and the Ultra Autocannon has been replaced with a rotary model. The Medium Laser from the 3M version has been upgraded to an ER Medium Laser, and its five and a half tons of standard armor have been replaced with seven tons of Ferro-Fibrous plate. [7] BV (1.0) = 1,064[16][17], BV (2.0) = 1,179
  • STN-5WB 
    This variant was produced by Defiance after it was lost to Word of Blake during the Jihad. Built on the same chassis as the 3L, the 5WB removes the weapons and adds a ton of armor. It uses a Light Engine for power, allowing it to mount a pair of Light Autocannon/5s that are fed from a single, one-ton ammunition bin. Three ER Medium Lasers back up the autocannon. [7] BV (2.0) = ???
  • STN-6S 
    This Sentinel variant uses a Clan-spec XL engine and an XL Gyro to free weight. Its primary weapon is an Ultra AC/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 a ton of ammunition. These ammunition bays are protected by CASE II systems. Six and a half tons of armor protects the 'Mech. It can reach a top speed of 96km/h. BV (2.0) = 1,089[18]
  • STN-C 
    The STN-C is a Combine upgrade of the 3M model, with the medium laser removed in order to carry a C3 Slave Unit. [7] BV (1.0) = 586 (C3: 47)[15], BV (2.0) = 689

Design Quirks[edit]

The Sentinel has the following Design Quirks:[19]


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



  1. MUL online date for the Sentinel
  2. 2.0 2.1 2.2 2.3 Technical Readout: 2750, p. 26 "STN-3L Sentinel"
  3. 3.0 3.1 3.2 3.3 3.4 Technical Readout: 3025 Revised, p. 40 "STN-3KSentinel"
  4. 4.0 4.1 4.2 4.3 4.4 4.5 Technical Readout: 3039, p. 254 "STN-3KSentinel"
  5. 5.0 5.1 5.2 Technical Readout: 3050, pp. 100-101 "Sentinel"
  6. 6.0 6.1 6.2 Technical Readout: 3050 Revised, pp. 90-91 "STN-3M Sentinel"
  7. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 Technical Readout: 3050 Upgrade, p. 194 "STN-3L 'Sentinel"
  8. Record Sheets: 3039 Unabridged p.411
  9. Combat Operations, p. 130
  10. Record Sheets: 3039, p. 104
  11. Record Sheets: 3039 Unabridged, p. 413
  12. Record Sheets: 3039 Unabridged, p. 414
  13. Historical: Operation Klondike, p. 159 "SLDF Royal Units - BattleMechs"
  14. Record Sheets: Operation Klondike, p. 35
  15. 15.0 15.1 Combat Operations, p. 125
  16. Combat Operations, p. 127
  17. Record Sheets: Upgrades, p. 70
  18. Recognition Guide: ilClan, vol. 14, p. 17
  19. BattleMech Manual, p. 94 Design Quirk Table - Sentinel Entry.