Build a rocket part by part, fuel it, and fly it to the Moon

Is there a game where you build rockets?

Yes, and this one lands where you live. Openworld is a voxel sandbox on a whole, round planet — walk straight and you come back to where you started — under a real sky with Moon phases and eclipses, all the way to the Sun, by way of the Moon and the planets. On Mac (native Apple Silicon), Windows and Linux. The rocket is how you leave: you assemble it on the ground next to your base, fill it, climb in, and the same world you built in turns into a planet below you.

The physics underneath is the real thing — gravity that weakens with distance, engines with real thrust and fuel flow, air that thins out up to 80 km, re-entry heating, a Moon with its own sphere of influence. But you don't need an engineering degree: the screen does the math, simulates the flight for you, and the autopilot flies the whole mission. Scroll to watch one go up.

Step 1
The engine firstRight-click flat ground with it.

Step 2
Stack it upTanks, cargo module, cockpit, fairing: each part right-clicked onto the one below.

Step 3
Fill it upDrag dewars of LOX and fuel onto its screen. The tanks frost over.

Step 4
Board, press SpaceThe autopilot switches on as you climb in.

Lift-off
Next stop: orbitThe legs fold away above 150 m.

Which parts does a rocket need?

A rocket is a stack, always in the same order, bottom to top. The game refuses a part in the wrong place (“Stack a tank on the engine first”), so you can't get it wrong.

• Rocket engine — 3.4 m, 1.6 t. The first part, set on flat ground: a titanium bell and chamber, pumps of cogwheels and four landing legs. It burns 340 kg of propellant per second.
• Rocket tank — 6 m, 1 t empty, 26 m³. On the engine or on another tank, up to four. Each one holds oxygen and fuel, split by a common bulkhead.
• Cargo module — 4 m, 1.5 t. Optional, up to two, on the tanks: a hold of 36 slots that flies.
• Cockpit — 3 m, 2.4 t. The crew capsule and its instruments, four seats, on the last tank or module. Right-click it to board; leashed animals board too.
• Fairing — 2.6 m, 0.4 t. It closes the rocket and takes the heat of a nose-first re-entry.

To build it: right-click the ground with the engine, then right-click the rocket with each part in hand — it goes on top. Hold left click on the rocket to take the top part back. Everything is made of 25 cm voxels, like the rest of the world.

• 5kinds of parts
• 31meters: the /rocketkit rocket
• 4tanks at most
• 72cargo slots with two modules

/rocketkit
/rocketkit kerosene
/rocketkit hydrogen

Fuel: liquid oxygen plus one of three

Openworld uses the propellants of the real space industry. Every rocket burns liquid oxygen (LOX) with one fuel of your choice:

• Liquid methane (methalox, like SpaceX's Raptor): efficiency (Isp) of 327 s at sea level, 350 s in vacuum. The balanced choice, and the fuel of reusable rockets.
• RP-1 kerosene (kerolox, like the Merlin or Soyuz): 282 / 311 s. The least efficient, but the densest: a tank holds the most mass of it.
• Liquid hydrogen (hydrolox, like the RS-25 or Vulcain): 366 / 452 s. The most efficient, but so light that a tank holds 2.3 times less of it.

Propellant comes in dewars: one dewar is 1 m³ of cryogenic liquid. Drag one onto the rocket's screen and it fills all the tanks at the right mixture ratio. The first dewar chooses the fuel; to change, Vent the tanks (two clicks). A fuelled tank wears a coat of frost.

Making propellant: from oil, water and air to dewars

In Survival, nobody hands you propellant: you make it, in a chain of machines. By hand, you carry buckets and cylinders from one machine to the next; in a factory, pumps, pipes, conveyors and power plants do it. The chain:

• Natural gas and kerosene: oil pumped from the ground goes into the refinery, which splits each cubic metre into kerosene, petrol, diesel, natural gas and tar.
• Empty dewars: steel flasks padded and sealed in epoxy, four at a time — every propellant is one dewar, 1 m³.
• The gas plant (a small power cable, 6 to 9 kW): it splits water into hydrogen and oxygen, then fills empty dewars with liquid oxygen (from its holder, or straight from the air where there is air), liquid methane (from natural gas) or liquid hydrogen. On the Moon, Moon ice gives LOX and hydrogen at once.
• RP-1: the refinery fills an empty dewar with 64 voxels of kerosene.

A methalox tank takes about 15 dewars of LOX and 11 of methane: one refinery and one gas plant need a good while for it, so flying often to the station or the Moon means building more of them. The dewars go round and round: pouring one into a rocket gives you the empty dewar back (venting the tanks loses the propellant). On the pad, a fuelling tower does the pouring: 9 slots of full dewars that conveyors fill, 9 of empty dewars that leave, and one dewar every half second into the nearest rocket within 12 m.

How much fuel to reach the Moon?

What counts is delta-v: how much speed your rocket can gain before its tanks run dry. More tanks, more delta-v — but more mass to push. The engine's thrust is capped at 4 g, for the pilot and the structure. With methane, full:

• 5.0km/s: orbit and back (1 tank: about 5.5)
• 7.0km/s: a Moon landing (2 tanks: just)
• 8.3km/s: the Moon and back (4 tanks)
• 3.4km/s: from the Moon back to Earth

A single-tank rocket weighs 27 t at lift-off (5.4 t empty) and has about 5.5 km/s: it reaches orbit and comes back to land. Two tanks make about 7 km/s, a Moon landing with little margin. Three or four (7.8 and 8.3 km/s) get you there and back. Low orbit alone takes about 4.2 km/s on this 1,000 km-radius Earth. Never guess: the screen draws the bar of your delta-v with each trip's need ticked on it, and the in-game guide computes it for every number of tanks.

The rocket's screen

Right-click the rocket with empty hands and its screen opens: the rocket drawn in blocks (the health of each part, oxygen and fuel in each tank), the two propellant cards to drop dewars on (click: one, right-click: half), the delta-v bar, the destinations, and the buttons — Board, Hold, Vent the tanks, Top part back, Guide.

The Guide is a small illustrated manual with nine animated tabs: Build, Propellants, Cargo, How far, Plan a flight, Flying, To orbit, The Moon, Landing. Read it once and you know as much as this page.

Carrying cargo

The cargo module turns a rocket into a truck: build a factory in one place, ship what it makes somewhere else. Each module is a hold of 36 slots, two at most (72 slots), opened with the Hold button like a chest. It flies everywhere — the Moon too — and stays where the rocket lands.

• A full stack weighs 50 kg, whatever it holds; a partial stack, its share. A full hold is 1.8 t, plus 1.5 t for the module.
• The screen says what the cargo costs (“Cargo 1.2 t: –0.4 km/s”) and the most this trip can carry.
• Too heavy, and the rocket shakes on the pad without lifting off. Destroyed in flight, it loses its cargo.

Planning a flight and choosing a landing point

The Landing point card opens the map (M) in pick mode: click the ground, in the flat view or the space view (yes, you can click the Moon from the Earth), a checkpoint, or the rocket's own launch pad, then Land here. The planner picks the route:

Then the screen simulates the whole flight, in the background, with the same physics and the same autopilot as the real one: the route, the distance, the delta-v needed against what you have, and a verdict — lands on the point with this much propellant left, or runs dry on the way. Click the plan to open the Flight plan card: the simulated flight replayed step by step, with its curves and its phases. In tests, hops of 300 m to 1,000 km land within one or two meters of the point.

One honest limit of the alpha: a point on another world (the Moon from the Earth, the Earth from the Moon) is approached as close as the planner can, not yet to the meter. Too far at the end of the braking (3 km on Earth, 6 on the Moon), the rocket lands as close as it can and tells you.

Flying: the autopilot, or by hand

Board, press Space: on the pad, that's the lift-off. The autopilot (P, on as soon as you board) flies the whole mission to the destination: climb, orbit, the burn for the Moon, corrections, braking, landing. A steering or throttle key takes the controls back, P gives them back.

• W A S D: tilt the rocket the way the key points on your screen. Seen from behind, A leans it left.
• Shift / Ctrl: more or less throttle. N: change the destination in flight.
• V: the view — the cockpit (look around with the mouse), from behind, or the orbital map with your trajectory.
• G: the landing legs (they also deploy on their own under 700 m). F: step out, on the ground.
• Time warp is automatic: up to ×10,000 when coasting, ×6 at most under thrust.

You're never stuck: your inventory (E) and the chat (T) work in the cockpit, and dying at the controls puts you back home. Pressing F twice in flight bails you out — the rocket is lost. Over the Earth, you'll want a paraglider packed in your equipment slot: the wingsuit and parachute guide shows how its canopy opens.

Landing — and when it goes wrong

• Landed: under 3 m/s down and 2.5 m/s sideways, upright on its legs.
• Damaged: from 3 to 9 m/s, the legs and the engine take the blow.
• Boom: over 9 m/s, tilted past 15° or on its side, the rocket explodes — debris and a fireball. In Survival the pilot dies with it; in Creative, you're set down on the ground.

Each part also wears, like a tool: the fairing heats up during re-entry (always nose first), the legs and the engine suffer on hard landings, the whole stack strains if it flies sideways through thick air. Repairs will come later. Coming back from the Moon, the autopilot never dives straight in: it brakes into low Earth orbit first, then comes down, because a direct entry at 4.4 km/s would burn the fairing. The whole trip is in the guide to going to the Moon.

Rockets inside a survival sandbox

Openworld's rockets are not a full spaceflight simulator with hundreds of parts and the freedom to design anything. They have five kinds of parts and a guided assembly, and they live inside a survival sandbox: you craft the parts and make the propellant in your own factories, your rocket stands next to your house, takes off from your world and lands on a Moon you can walk on, build on and mine.

Underneath, the physics is honest: real gravity, real fuel flow and efficiency, real orbits that you only reach by going sideways fast enough, re-entry heating, and a missed orbit that falls back on a true parabola. Perfect for an evening of launches with friends, and for the Moon base that comes after.

Flying with friends

On a server, the rockets you leave on the pad are saved, and every player sees them. One pilot per cockpit (“Someone already sits in this cockpit”), but everyone watches the flights live, and if your connection drops in flight, the rocket waits for you in the sky. Going to space is a server rule: open by default, an admin can close it. New to the game? Start with your first nights: the rocket is the long goal.