How to Build a Slime Block Flying Machine

Six blocks fly: two sticky pistons, two slime blocks, two observers. Build the engine on top of a tower, give it one pulse, and it walks itself across an ocean at a steady crawl while you stand on it doing nothing. No rockets, no elytra durability, no fuel. Everything depends on six block faces pointing the right way, and on one rule that stops the whole thing welding itself into a useless brick: the two slime blocks must never touch each other. There is no honey anywhere in the engine. Honey only earns its place later, when you bolt a deck onto both halves.

How a flying machine actually moves

Nothing here is flight. A flying machine is a two-part contraption that pushes half of itself forward, anchors that half, then pulls the other half up to meet it. One block per cycle, forever. Four mechanics carry the entire design:

• A piston pushes the blocks in front of it. It never moves itself.
• A sticky piston also pulls the block touching its head when it retracts.
• Slime and honey blocks stick to the movable blocks they touch, on all six faces, so moving one slime block moves everything hanging off it. The two exceptions are each other and glazed terracotta, which neither of them ever drags along.
• An extended piston, and the piston head in front of it, cannot be moved by anything.

That last rule is the one nobody expects, and it is the trick the whole machine turns on. Half A pushes half B forward. Half B's piston then extends, which anchors it in place. When half A's sticky piston retracts and tries to drag half B straight back where it came from, that pull silently fails against the extended piston. Half B keeps the ground it gained. Then half B retracts and drags half A up to it. If you have built a piston door you already know pistons push and pull; the flying machine is just those two actions taking turns with nothing underneath them.

Slime, honey, and why you want both

A slime block is nine slimeballs; a honey block is four honey bottles. They behave almost identically as building material, and four things decide how you use them:

• They do not stick to each other. Slime sticks to slime, honey sticks to honey, and both stick to pistons, observers, stone and almost everything else movable. But a slime block sitting flush against a honey block stays completely independent, and neither of them will ever drag glazed terracotta along. That is how you bolt cargo onto both halves of a machine without gluing the halves together.
• Slime carries a redstone signal; honey does not. A slime block is redstone conductive, so the engine below feeds each piston through its slime block. A honey block is not, so rebuild the engine in honey and the pistons never fire.
• Slime throws entities, honey carries them. A slime deck that moves under you flings you off it in the direction of travel. Stand on honey and you get carried along with the machine instead.
• Sourcing differs. Slimeballs come from slime chunks deep underground or swamp spawns at night, which is worth reading up on in slime chunks explained. Honey bottles come from a full hive or nest, easiest from a proper bee and honey farm.

The 12-block push limit

A piston moves at most 12 blocks in a single push or pull. That is 12 blocks of everything glued together by slime and honey, not 12 blocks of slime. The bare engine is three blocks per half, so each half has room for nine more blocks of deck, walls and cargo. Go over the budget and the piston simply refuses to extend.

Some blocks cannot be moved at all, and a single one of them anywhere in a stuck structure kills the entire push:

• Obsidian, crying obsidian, bedrock and reinforced deepslate.
• In Java Edition, anything with a block entity: chests, barrels, furnaces, hoppers, shulker boxes, beacons. Bedrock pushes most of these happily, which is why Bedrock tutorials will lie to you here.

The two-observer engine, block by block

You need two sticky pistons, two slime blocks and two observers. An observer is six cobblestone, two redstone dust and one nether quartz; a sticky piston is a piston plus a slimeball.

Two facing rules before you start. A piston's pushing face is the one with the wooden plate. An observer's output is the side with the two small red dots, and the opposite side is the eye that watches.

Stand facing the direction you want to travel, ideally on a tower with clear air ahead. You are building a flat rectangle, three blocks long and two blocks wide, one block tall. Call the rows left and right, and the slots back, middle and front.

1. 1.Right back: sticky piston, pushing face pointing forward.
2. 2.Left back: slime block, directly beside that piston.
3. 3.Left middle: observer, eye looking forward, red dots touching the slime block you just placed.
4. 4.Left front: sticky piston, pushing face pointing backwards, staring straight into that observer's eye.
5. 5.Right front: slime block, beside the front piston.
6. 6.Right middle: observer, eye looking backwards at the rear piston, red dots touching the front-right slime block.

Six blocks, nothing left over. Read the shape and it is two identical rows of slime, observer, piston, laid side by side pointing opposite ways.

The halves are not the rows. They are:

• Half A: the rear piston, the back-left slime block it touches, and the left middle observer stuck to that slime.
• Half B: the front piston, the front-right slime block it touches, and the right middle observer stuck to that slime.

The two slime blocks sit at opposite corners of the rectangle and never touch, which is exactly why the halves stay independent. Each observer stares at the other half's piston and feeds its own half's piston through the slime block behind it. If the pulse behaviour is new to you, observers explained covers the two-game-tick output, and redstone basics covers why a powered slime block fires the piston next to it.

Starting it, and choosing a direction

Both observers face inward at pistons, so you cannot start this machine by placing and breaking a block in front of an eye. Pulse a piston directly instead:

• Place a temporary block behind the rear piston, touching its back face, put a lever on that block, and flick it on then off. The machine leaves the lever behind.
• Or hold a block of redstone, place it against the rear piston, and mine it immediately.

Whichever piston you kick first becomes the pusher, and the machine travels that way. Kick the rear piston and it goes forward. Kick the front piston and the same six blocks go backwards. That is why both pistons are sticky: either one has to be able to do either job.

What happens in one cycle

1. 1.The rear piston extends and shoves half B one block forward: three blocks moved, well inside the limit.
2. 2.Half B's observer was staring at that piston. Retracted to extended is a block state change, so it fires strong power into its slime block for two game ticks, one redstone tick, and the front piston extends backwards into the gap half B just vacated.
3. 3.The rear piston's pulse ends and it retracts. Being sticky, it tries to drag half B straight back. It fails, because the front piston is now extended and extended pistons cannot be moved. The gain is locked in.
4. 4.The front piston's pulse ends and retracts. Its head pulls the block touching it, which is half A's observer, one block forward, and the slime block beside it drags the rear piston along too.
5. 5.Half A's observer is now looking at the front piston again and has just watched it retract. It fires. Back to step one.

The machine has advanced exactly one block and all six blocks are in the identical arrangement they started in. There is no clock anywhere in the build; each piston's state change is the other observer's input, which is why it runs until something physically stops it.

How to stop it

• Break an observer. This is the handbrake you will actually use. Mine either observer mid-flight and the loop dies instantly. Replace it and pulse a piston to set off again, so carry a spare in case the broken one drops into the sea.
• Dock it against obsidian. Put obsidian in both rows at the far end of the run. The leading half gets pushed into a block that cannot move, the push fails, no observer sees a state change, and the machine parks itself one block short. Plain stone will not do it: stone just gets pushed along until the total exceeds 12 blocks, which does eventually stall the machine but leaves a scar of pushed blocks behind.
• Reverse it. Parked at a dock, kick the opposite piston and the same machine flies home.

Five ways people break this build

1. 1.Slime touching slime across the halves. Add a deck of slime to both halves and they glue into one rigid lump that cannot move. Use slime on one half and honey on the other wherever the two decks meet.
2. 2.A chest on the deck. Block entities cannot be pushed in Java. One chest, hopper or shulker box anywhere in a stuck structure freezes that half permanently.
3. 3.Over the budget. Nine extra blocks per half, maximum. Count walls and floor, not just the visible platform.
4. 4.An observer flipped. If the eye points at the slime block instead of the piston, nothing fires and nothing moves. Red dots always touch the slime.
5. 5.Terrain in the path. Trees, hills and floating islands all get shoved until the push limit blows. Build high, aim at open air, and check the horizon before you set off.

Adding a deck and passengers

Attach up to nine blocks to each half. Put a honey block where you intend to stand so the machine carries you along instead of throwing you off, and fence it in if you plan to sort your inventory mid-flight. Slime on the deck works fine as structure, it just launches anything standing on it when the machine steps forward.

This is the cheapest long-distance transport in the game after a nether tunnel: nothing to burn, nothing to repair, and it works over lava lakes as happily as over ocean. It is slower than elytra and fireworks, but it costs nothing per trip and it hauls a mining platform, a TNT load or a whole farm module along with you.