A Liquid Rocket Booster (LRB) is similar to a solid rocket booster (SRB) attached to the side of a rocket to give it extra lift at takeoff. A Liquid Rocket Booster has fuel and oxidiser in liquid form, as opposed to a solid rocket or hybrid rocket.
Like solid boosters, liquid boosters can considerably increase the total payload to orbit. Unlike solid boosters, LRBs can be throttled down and are also capable of being shut down safely in an emergency, providing additional escape options to manned spacecraft.
For the R-7 missile, which later evolved into the Soyuz rocket, this concept was chosen because it allows all of its many rocket engines to be ignited and checked for function with the rocket still on the launch pad.
The Soviet Energia rocket of the 1980s used four Zenit liquid fueled boosters to loft both the Shuttle Buran and the experimental Polyus space battlestation in two separate launches.
Two versions of the Japanese H-IIA space rocket uses (or will use) one or two LRBs to be able to carry extra cargo to higher geostationary orbits.
The Ariane 4 space launch vehicle also optionally could use two or four LRBs (the 42L, 44L, and 44LP configurations). As an example of the payload increase that boosters provide, the basic Ariane 40 model with no boosters could launch around 2,175 kilograms into Geostationary transfer orbit [1]. The 44L configuration could launch 4,790 kg to the same orbit with four liquid boosters added [2].
Various LRBs were considered early in the Space shuttle development program. More recently, after the Challenger accident, LRBs were considered to replace the existing SRBs, and four companies proposed booster designs to NASA. While very attractive from the performance and safety perspective, the cost of developing the systems resulted in the decision to stick with (and improve as much as possible) the existing solid boosters.
The Evolved Expendable Launch Vehicle program produced new liquid fueled primary stages for the Atlas V and the Delta IV rockets called Common Core Booster (CCB) or Common Booster Core (CBC). These can be used alone (with possible strap-on solid rocket boosters) or in a configuration of three CCBs tied together