LFB KR-1x2 "Twin-Boar" Liquid Fuel Engine

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LFB KR-1x2 "Twin-Boar" Liquid Fuel Engine
Part image
Liquid fuel engine by

Radial size Large, Radial mounted
Cost (total) 17 000.00 Funds
(dry) 14 062.40 Funds
Mass (total) 42.50 t
(dry) 10.50 t
Drag 0.2
Max. Temp. 2000 K
Volume  ?
Impact Tolerance 20 m/s
Research Heavier rocketry.png Heavier Rocketry
Unlock cost 65 000 Funds
Since version 0.23.5
Part configuration part.cfg
Liquid fuel engine
Maximum thrust (1 atm) 1 866.67 kN
(vacuum) 2 000.00 kN
Isp (1 atm) 280 s
(vacuum) 300 s
Fuel consumption 135.96 Units of fuel/s

Thrust vectoring 1.5 °
Liquid fuel tank
Liquid fuel 2 880 Units of fuel
Oxidizer 3 520 Units of fuel

Testing Environments
On the surface Yes
In the ocean Yes
On the launchpad Yes
In the atmosphere Yes
Sub orbital Yes
In an orbit Yes
On an escape Yes
Docked No
Test by staging Yes
Manually testable Yes

The LFB KR-1x2 "Twin-Boar" Liquid Fuel Engine is a liquid fuel engine with a built-in liquid fuel tank.

Like all liquid fuel engines, the Twin-Boar is throttleable, both by the main throttle and a thrust limiter tweakable. The minimum thrust level required for this booster to lift its own laden weight is 17.5%. In addition, the liquid fuel booster can be run longer by adding additional rocket fuel tanks, which the Twin-Boar will draw from.

This booster contains two engines at the bottom which are added radially, so the base of the booster is wider than a usual large tank. It also does not allow any parts to be attached below it. It contains the same amount of fuel as the Rockomax Jumbo-64 Fuel Tank (2880). The thrust is higher than the Rockomax "Mainsail" Liquid Engine and is thus the most powerful large-sized engine. However, its Isp is slightly (5-10 s) lower than that of the Mainsail at all altitudes.

The Twin-Boar is the only part that is simultaneously a liquid fuel tank and engine.

Product description

This liquid fuel booster system is a very purposeful piece of engineering. Instead of relying on single engines or solid rocket boosters, the KR-1x2 employs two modified KR-1 engines, fed by a built-in fuel system, as the source of its immense lifting power. All that self-contained goodness comes at the cost of not having a mounting point for its underside, and not being quite as efficient as dedicated engines. It also comes at quite a literal cost as well.



The Twin-Boar is an unusually good engine. Once the weight and cost of the fuel + tankage is subtracted out, it weighs almost the same as a Mainsail, but has 33% more thrust while actually being 13% cheaper. It's similar to a pair of lower-efficiency Vectors. The Twin-Boar also has highest thrust to weight ratio of any liquid fuel engine in the game and the second highest thrust to cost ratio, second only to the LV-T30 "Reliant" Liquid Fuel Engine. This makes it an excellent and cost-effective mid-game launcher for larger payloads.

It is also the best engine in the game for first stage recovery. It has a high impact tolerance of 20m/s, and being very tall makes it extremely buoyant in case of a splashdown. Even in the case of a bad landing which results in partial disintegration of the rest of the stage, the integrated fuel tank is always recovered, so the recovery percentage in case of a crash landing is higher than for other engines.


  • The name LFB KR-1x2 might be a reference to the Advanced Boosters, a future Space Launch System Block 2 component.
  • The names "KR-1x2", "Twin-Boar", and the part description all imply the existence of an LFB KR-1 "Boar" Liquid Fuel Engine. However, no such part exists.


  • Changed cost (5900 -> 16400), reduced Isp from 320-360 s to 290-340 s, changed description.
  • Initial release