Rocket lab
Build a Moon rocket stage by stage, see whether the rocket equation says it can land, then fly the whole mission.
Start from
Fuel check: ready to land
Can land on the Moon!Enough fuel, with 2,309 m/s spare in the lander.
- S1 S-IC2,039 t3,301 m/s
- S2 S-II435 t4,450 m/s
- S3 S-IVB122 t4,516 m/s
- S4 Service stage25 t2,374 m/s
- Lander12 t2,978 m/s
- S1100%2,039 t
- S2100%435 t
- S3100%122 t
- S4100%25 t
- Lander100%12 t
- Stage 1 (Earth)33,850 kN · T/W 1.22 → 4.40
- Stage 2 (Earth)5,150 kN · T/W 0.80 → 2.34
- Stage 3 (Earth)1,030 kN · T/W 0.57 → 1.71
- Lander (Moon)45 kN · T/W 1.47 → 3.91
Black line = 1.0. Thrust stays the same but the rocket gets lighter as fuel burns, so T/W climbs during each burn. Upper stages can start below 1: they are already high up and mostly need sideways speed.
Mach 1 ≈ 343 m/s · yellow = max-Q zone (about 394–1,007 m/s) · orbit ≈ 7.8 km/s · Moon ≈ 10.9 km/s.
Every tank full on the pad.
Your rocket, top to bottom
Rocket guidebook
Every label and every drop below is worked out from your design, so it changes as you edit.
Tap or hover over a label to see what that part does. 19 parts labelled.
Worked out by flying your rocket with the autopilot. Times are mission time after lift-off.
- Stage 1 (S-IC), 123 t emptyT+2 min 35 sabout 42 km up · 1,590 m/s · +1,091 m/s gainedWhy: Its fuel is used up, so it is just dead mass: empty tanks and engines. Without it, stage 2 gives about +1,091 m/s more (rocket equation with your numbers).Then: Falls back and splashes into the sea, far downrange.Automatic while auto-stage is on
- Nose cone (fairing), 1.6 tT+7 min 10 sabout 110 km up · 4,267 m/s · +9 m/s gainedWhy: Above about 110 km the air is about 4,17,140 times thinner than at sea level, so the lander no longer needs the shell. Dropping it gives about +9 m/s for the rest of this burn.Then: Its two halves tumble back down and fall into the sea.Automatic while the launch autopilot flies
- Stage 2 (S-II), 41 t emptyT+8 min 24 sabout 149 km up · 5,819 m/s · +1,286 m/s gainedWhy: Its fuel is used up, so it is just dead mass: empty tanks and engines. Without it, stage 3 gives about +1,286 m/s more (rocket equation with your numbers).Then: Falls back and burns up in the atmosphere like a shooting star.Automatic while auto-stage is on
- Stage 3 (S-IVB), 14 t emptyT+21 min 33 sabout 200 km up · 10,657 m/s · +733 m/s gainedWhy: Its fuel is used up, so it is just dead mass: empty tanks and engines. Without it, stage 4 gives about +733 m/s more (rocket equation with your numbers).Then: Coasts on in a long, stretched orbit round the Earth (or out past the Moon): more space junk.Automatic while auto-stage is on
- Stage 4 (Service stage), 2.9 t empty (lander separation)T+3 d 0 habout 9.1 km up · 444 m/s · +615 m/s gainedWhy: After this the lander is on its own. Its fuel is used up, so it is just dead mass: empty tanks and engines. Without it, stage 5 gives about +615 m/s more (rocket equation with your numbers).Then: Falls and crashes onto the Moon's surface.Automatic while auto-stage is on
- Nothing: the lander staysT+3 d 0 hon the Moon · 3 m/sWhy: Probes under the legs touch the ground, the engine cuts and the lander drops the last 1.5 m.Then: Touched down at 2.6 m/s with 8.6 t of fuel left.You (or the landing autopilot) fly this
Mission ladder
- 1. Launch to low Earth orbit≈ 9,400 m/sDone by S1, S2, S3
- 2. Trans-lunar injection≈ 3,100 m/sDone by S3, S4
- 3. Lunar orbit insertion≈ 900 m/sDone by S4
- 4. Descent and landing≈ 1,900 m/sDone by S4, S5 · braking 1,650 + final approach 250
- Total Δv 15,300 m/s covers all 15,300 m/s of the mission ladder, with 2,309 m/s spare in the lander at the end.
- Lander thrust-to-weight on the Moon is 1.69 at the final approach, safely above 1.5. A test run of the Autopilot touches down at 2.5 m/s with 8.6 t to spare.
| Stage | Full / dry | Exhaust speed | Δv |
|---|---|---|---|
| S1 S-IC | 2,162 t / 123 t | 2,579 m/s (sea level) | 3,301 m/s |
| S2 S-II | 477 t / 41 t | 4,129 m/s (vacuum) | 4,450 m/s |
| S3 S-IVB | 136 t / 14 t | 4,129 m/s (vacuum) | 4,516 m/s |
| S4 Service stage | 28 t / 2.9 t | 3,079 m/s (vacuum) | 2,374 m/s |
| S5 Descent stage | 14 t / 2.4 t | 3,050 m/s (vacuum) | 2,978 m/s |
| Total (if every stage burns completely) | 17,620 m/s | ||
Ladder values are approximate: launch to low Earth orbit about 9,400 m/s (including gravity and air-drag losses), trans-lunar injection about 3,100 m/s, lunar orbit insertion about 900 m/s, descent and landing about 1,900 m/s (1,650 m/s braking from orbit down to 2 km, then about 250 m/s for the final approach you fly yourself). Stages burn in order; a lower stage can help with braking but must separate before the final approach, and any fuel it still holds is wasted.
Why rockets drop parts
An empty stage is just tanks and engines: dead mass. If the next stage had to carry it, the same fuel would give less speed, because the rocket equation depends on the ratio of starting mass to final mass. Here is what dropping each empty part is worth for your rocket:
| Drop | Dead mass | Next part if carried | If dropped |
|---|---|---|---|
| Empty stage 1 | 123 t | 3,340 m/s | 4,430 m/s (+1,091) |
| Empty stage 2 | 41 t | 3,231 m/s | 4,516 m/s (+1,286) |
| Empty stage 3 | 14 t | 1,642 m/s | 2,374 m/s (+733) |
| Empty stage 4 | 2.9 t | 2,363 m/s | 2,978 m/s (+615) |
Δv = vₑ × ln(m_start ÷ m_end) for the next part, with and without the empty part still attached. The fairing is dropped at about 110 km, once the air is too thin to heat the lander.
Incidents: when real missions went wrong
Every step of the mission ladder has failed for real at least once. Engineers learn from each one. You can fly some of these as failure drills on the flight page.
SLV-3 E1 · 10 August 1979 · India (ISRO)
India's first attempt to launch a satellite on its own rocket. During the second stage's flight a valve in its control system reportedly failed, the stage lost the fluid it used for steering, and the rocket fell into the Bay of Bengal.
A. P. J. Abdul Kalam's team found and fixed the fault, and SLV-3 put the Rohini satellite into orbit in July 1980. Small parts in steering systems now get very careful checks.
GSLV-D3 · 15 April 2010 · India (ISRO)
The first flight of India's home-built cryogenic (liquid hydrogen) upper stage. Soon after the stage lit, the turbopump feeding it hydrogen stopped, the stage lost thrust, and the rocket and its GSAT-4 satellite fell into the sea.
After years more testing, the same kind of stage flew successfully on GSLV-D5 in January 2014. Cryogenic engines are hard: parts go from colder than −250 °C to burning hot in seconds.
PSLV-C39 · 31 August 2017 · India (ISRO)
The heat shield (fairing) did not separate. The IRNSS-1H navigation satellite reached orbit but stayed trapped inside the shell, so it could never work.
A fairing must split and fall away cleanly. Separation systems use backup (redundant) firing circuits and are tested on the ground.
SSLV-D1 · 7 August 2022 · India (ISRO)
On SSLV's first flight, vibration while the second stage separated briefly upset its accelerometers. The software decided the sensors had failed and switched to a back-up mode that left the two satellites in an orbit far too low (about 76 km at its lowest), so they soon fell back.
ISRO changed the logic so a short sensor glitch is not treated as a total failure. SSLV-D2 reached the right orbit in February 2023.
Luna-25 · 19 August 2023 · Russia (Roscosmos)
During a burn to lower its orbit before landing, the engine ran for about 127 seconds instead of the planned 84, so Luna-25 crashed into the Moon.
Engines must cut off exactly on time. Burns are timed and checked by more than one system.
Chandrayaan-2's Vikram · 7 September 2019 · India (ISRO)
Vikram was lost a couple of kilometres above the Moon. ISRO's review reported that its engines gave more thrust than expected during braking, errors built up, and limits in the guidance software meant it could not correct fast enough. It hit the surface hard.
Chandrayaan-3's lander got stronger legs, more fuel, extra sensors, a much bigger landing zone and software designed to cope when things go wrong.
Beresheet · 11 April 2019 · Israel (SpaceIL)
During the descent a sensor glitched, and the commands sent to recover from it led to the main engine switching off. It could not restart in time and the lander crashed.
Every command sent during a landing is a risk; teams now rehearse fault recovery again and again before the real thing.
Hakuto-R Mission 1 · 25 April 2023 · Japan (ispace)
As it passed over the rim of a crater, its altitude reading jumped suddenly. The software decided the reading must be faulty and ignored it, so the lander believed it was at the surface while still about 5 km up. It crept down until the fuel ran out, then fell.
Landing software is now tested against realistic Moon terrain, and different sensors are cross-checked against each other.
Apollo 11 · 20 July 1969 · USA (NASA)
During the descent the guidance computer flashed '1202' and '1201' alarms: it was overloaded with extra work, so it restarted and kept only the most important jobs running. Neil Armstrong then steered by hand past a field of boulders and landed with less than a minute of fuel left before the 'land or abort' limit. Buzz Aldrin called out 'contact light' when probes under the legs touched.
Software that knows its priorities, and a team that decided within seconds it was safe to carry on, saved the landing.
Chandrayaan-3 · 23 August 2023 · India (ISRO)
Vikram flew rough braking, attitude hold, fine braking and a hover at about 150 m to check for hazards, then touched down near the Moon's south pole region. India became the fourth country to soft-land on the Moon and the first near the south pole.
It was designed to survive failures, not just to work when everything goes right: wider landing area, more fuel, more sensors and legs built for about 3 m/s.
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How the maths works
Rocket equation
Δv = ve × ln(mstart ÷ mend)
ve = Isp × 9.81. Each stage carries everything above it, so the payload and upper stages count as dead weight for the stages below. Dropping empty stages is why staging works.
Thrust-to-weight
TWR = T ÷ (m × g)
On the pad g = 9.81 m/s²: TWR must be above 1 to rise, and about 1.2 or more to climb briskly. The lander needs TWR above 1 on the Moon (g = 1.62 m/s²) to hover, and about 1.5 to brake firmly.
Boosters (simplified)
Boosters and stage 1 light together. Until the first of them runs dry they burn as one rocket whose exhaust speed is total thrust ÷ total fuel flow. Then the empty boosters drop off and stage 1 carries on alone. Stage 1 and boosters use sea-level Isp; upper stages use vacuum Isp.
Tanks are cylinders; 60% of a liquid tank's volume holds propellant (85% for solid motors). Structure mass grows with tank area and propellant mass, so big tanks have a smaller dry fraction. Engine and propellant figures are rounded “about” values inspired by real hardware. Everything ignores steering, the Earth's spin and orbital timing.