A crawler or climber is a reaction in which two objects (or groups of) collide, leaving one of the objects moved backwards or forwards to a degree, and the other forward. Crawler reactions are commonly used to create engineered spaceships, also known as caterpillars.
List of crawlers
17c/45 pi heptomino crawler
Perhaps the first and most well known climber is the infamous 17c/45 reaction, consisting of pi heptomino running over a blinker, moving it back 6 cells and rephasing it, whilst the pi heptomino moves forward 17 cells. This takes place in 45 generations.
The reaction was known for a long time, however the idea of a spaceship powered by this reaction was mentioned much later in 2002 by David Bell who had discovered reactions between multiple pies, which emitted gliders. After much work, Gabriel Nivasch finished construction of caterpillar, the first 17c/45 orthogonal spaceship, on 31 December 2004.
Research has been conducted into building much smaller 17c/45 spaceships with pi crawlers.
31c/240 Herschel crawler
Another orthogonal crawler, the 31c/240 reaction, consisting of a Herschel crawling though blocks.
Four 31c/240 spaceships are known, three of which use the crawler; shield bug, centipede, and silverfish. The fourth, centipede caterloopillar, relies on universal constructor technology instead of conventional crawlers.
(23,5)c/79 Herschel climber
The (23,5)c/79 Herschel climber involves a Herschel reacting with a stream of gliders, emitting more gliders behind it in the same way, displacing the Herschel by 23 cells vertically and 5 horizontally in the space of 79 generations. It can also crawl on certain still lifes as well as gliders.
(13,1)c/31 Pseudo-B climber
An interesting climber, the (13,1)c/31 Pseudo-B climber involves a climbing reaction that never isolates itself into a Herschel or B-heptomino. It was first mentioned by Gabriel Nivasch as an afterthought on his caterpillar article.
No spaceships have been built with this reaction as of yet.
(27,1)c/72 Herschel climber
Much research has been conducted into a spaceship based on the (27,1)c/72 Herschel climber.
(34,7)c/156 Herschel climber
The (34,7)c/156 Herschel climber is a little-known, mainly unspoken climber.
There is a well-known reaction in which a glider crashes into the side of a half-bakery, shifting the loaves and releasing a glider. This reaction has been used to create Half-baked knightship and parallel HBK, and research has been conducted into a potential caterpillar-like knightship using the reaction.