Burn Time Calculator KSP
Estimate stage burn duration, propellant flow, total impulse, and launch TWR for Kerbal Space Program. This premium calculator uses the standard rocket propulsion relation mass flow = thrust / (Isp × g0), then derives burn time from available propellant mass and your selected engine setup.
Calculator
Choose a stock engine preset or enter custom thrust and specific impulse. Propellant mass should be the usable fuel mass for the stage, in metric tons.
Results
Your computed burn profile will appear below.
Burn Chart
This chart shows predicted propellant remaining over the full burn.
How to Use a Burn Time Calculator in KSP
A burn time calculator for KSP helps you answer one of the most practical stage design questions in the game: how long will my engines keep firing before this stage runs dry? That single number has a huge effect on ascent control, transfer window accuracy, suicide burns, orbital insertion timing, and docking maneuvers. In Kerbal Space Program, delta-v often gets most of the attention, but burn duration matters almost as much because a long burn spreads thrust over time and changes how efficiently you can execute a maneuver node.
This page calculates burn time from three core propulsion values: available propellant mass, engine thrust, and specific impulse. In simplified rocketry terms, your engine consumes mass at a rate determined by thrust and Isp. Once you know the mass flow rate, burn duration is simply usable propellant divided by that flow rate. The result is especially useful when building upper stages, landers, transfer stages, or low thrust vacuum craft where node timing can become sensitive.
If you are trying to tune a Mun transfer stage, improve a Duna injection vehicle, or decide whether to cluster multiple engines, a burn time calculator gives fast insight before launch. It also lets you compare tradeoffs. For example, adding more engines can reduce burn time dramatically, but it also increases dry mass and may reduce vacuum efficiency depending on your design.
The Core Formula Behind KSP Burn Time
The standard propulsion relationship used here is based on real rocket physics. Specific impulse measures how efficiently an engine uses propellant, and thrust tells you how strongly it pushes. The mass flow equation is:
mass flow rate = thrust / (Isp × g0)
Where:
- Thrust is in newtons. Since KSP engine thrust is commonly shown in kilonewtons, the calculator converts kN to N automatically.
- Isp is specific impulse in seconds, using either vacuum or sea level values.
- g0 is standard gravity, 9.80665 m/s².
- Burn time is available propellant mass divided by total mass flow.
That means an engine with higher thrust burns through fuel faster, reducing total burn time. An engine with higher specific impulse uses fuel more efficiently, increasing total burn time for the same propellant mass. In KSP, those two values are often in tension. Vacuum engines like the Terrier or Poodle have excellent Isp, but less thrust than big launch engines. High thrust engines like the Mainsail or Vector can finish a burn quickly, but they consume much more propellant per second.
Why Burn Time Matters More Than Many Players Expect
Many players focus only on whether a stage has enough delta-v. That is important, but not complete. A stage with enough delta-v can still be hard to fly if the burn takes too long. For instance, if your transfer stage has a very long burn around periapsis, your actual maneuver occurs over a broader arc rather than at a single ideal instant. This can reduce maneuver efficiency and produce larger targeting errors. Long burns are especially noticeable for interplanetary transfers and capture burns at moons or planets with short optimal windows.
Burn time is also central during launch. A lower stage with very short burn duration may feel powerful but can be harder to throttle smoothly and may waste efficiency due to drag or over-acceleration. A very long first stage burn, by contrast, can feel underpowered and may struggle against gravity losses. The sweet spot depends on mission type, but the key point is that burn duration affects real gameplay decisions, not just spreadsheet aesthetics.
Inputs Explained
- Engine preset or custom engine: Use a stock engine preset for fast estimates, or enter your own thrust and Isp if you are using a modded engine or a custom test case.
- Environment: KSP engines often have different Isp values at sea level versus vacuum. Choose the environment that matches where the stage operates.
- Engine count: Total thrust scales with engine count, so burn time decreases as more identical engines are added.
- Thrust per engine: Enter the thrust output for one engine in the selected environment.
- Specific impulse: Enter the selected environment Isp in seconds.
- Usable propellant mass: This should be the actual mass the stage can burn. If your tanks contain 8 t of propellant, enter 8.
- Stage starting mass: This is optional and used only to estimate starting TWR. It does not change burn time in this simplified calculator.
Comparison Table, Popular KSP Stock Engines
The following table summarizes several stock engine values commonly used in KSP career and sandbox builds. These figures are representative stock values used by many players when making stage comparisons.
| Engine | Sea Level Thrust (kN) | Vacuum Thrust (kN) | Sea Level Isp (s) | Vacuum Isp (s) | Typical Role |
|---|---|---|---|---|---|
| LV-T45 Swivel | 167.97 | 215 | 250 | 320 | Steerable early launcher |
| LV-T30 Reliant | 205.16 | 240 | 265 | 310 | Cheap booster core |
| LV-909 Terrier | 14.78 | 60 | 85 | 345 | Vacuum upper stage and lander |
| RE-I5 Poodle | 64.29 | 250 | 90 | 350 | Heavy vacuum transfer stage |
| RE-M3 Mainsail | 1379.03 | 1500 | 285 | 310 | Heavy lifter first stage |
| S3 KS-25 Vector | 936.51 | 1000 | 295 | 315 | High control launch core |
Example Burn Time Logic
Suppose your KSP vacuum stage uses one Poodle with 250 kN of thrust and 350 s vacuum Isp, carrying 8 t of usable propellant. Convert thrust to newtons, then compute mass flow:
250,000 / (350 × 9.80665) ≈ 72.82 kg/s
Eight tons of propellant is 8,000 kg. Burn time is:
8,000 / 72.82 ≈ 109.9 seconds
That tells you your transfer burn lasts just under two minutes. If you are executing a maneuver node, you would typically split the burn around the node, so you start roughly 55 seconds before the planned center point. This simple estimate is incredibly helpful in mission planning.
What the Results Mean
- Total thrust: Combined output from all selected engines.
- Mass flow rate: How many kilograms of propellant the stage consumes every second.
- Burn time: Total duration from full propellant to empty stage under constant full thrust.
- Total impulse: A useful performance figure equal to thrust multiplied by burn duration, shown in kN·s.
- Starting TWR: The stage thrust to weight ratio at the entered starting mass. This is useful for launch and landing planning.
Comparison Table, Burn Time Sensitivity by Engine Setup
The table below demonstrates how different engine choices alter burn duration for the same 8 t propellant mass. These figures use representative stock vacuum values and show why engine selection changes handling characteristics so much.
| Setup | Total Thrust (kN) | Isp (vac, s) | Estimated Mass Flow (kg/s) | Burn Time with 8 t Propellant |
|---|---|---|---|---|
| 1× Terrier | 60 | 345 | 17.73 | 451.2 s |
| 1× Poodle | 250 | 350 | 72.82 | 109.9 s |
| 2× Swivel | 430 | 320 | 137.03 | 58.4 s |
| 1× Vector | 1000 | 315 | 323.70 | 24.7 s |
Interpreting These Numbers for Actual Missions
A long Terrier burn can be ideal for highly efficient vacuum transfers on light probes or landers, but it may be frustrating if you need precise burns at periapsis. A Poodle shortens that same burn drastically while keeping strong vacuum efficiency, making it one of the best all-around interplanetary upper-stage engines in stock KSP. On the other end, a Vector can empty the same propellant load extremely fast, which may be perfect for launch stages or aggressive landings, but usually overkill for a delicate transfer stage.
That is the real value of a burn time calculator. It does not just provide a number. It helps you decide whether your stage behavior fits the mission profile.
Best Practices for Better KSP Burn Planning
- Use vacuum Isp for orbital transfer stages, landers, and interplanetary propulsion.
- Use sea level Isp for launch engines operating near Kerbin’s surface.
- Remember that engine count scales fuel use quickly. Doubling engines often halves burn time for the same propellant load.
- Check TWR alongside burn time. A short burn is not useful if your vehicle is too heavy to lift efficiently.
- For maneuver nodes, begin your burn about half the burn duration before node time when using a simple centered approximation.
- For landings, shorter burns provide stronger response but can reduce precision if overpowered.
Real Rocket Physics References
The formulas used in this KSP calculator are grounded in real aerospace principles. If you want to understand the engineering concepts more deeply, these resources are excellent references:
- NASA Glenn Research Center, Specific Impulse
- NASA educational page on rocket thrust and propellant relations
- MIT propulsion notes on rocket performance
Common Mistakes When Estimating Burn Time in KSP
- Using tank volume instead of propellant mass. Burn time depends on mass flow, not tank size alone.
- Using the wrong Isp environment. A Terrier at sea level behaves very differently from a Terrier in vacuum.
- Ignoring engine clustering. Four engines can drain a stage far faster than expected.
- Confusing thrust with efficiency. More thrust means faster acceleration, but not necessarily more efficient operation.
- Assuming maneuver node timing is instantaneous. Long burns should be centered around the node and monitored closely.
Final Takeaway
If you want cleaner ascents, more accurate transfer burns, and fewer failed insertion attempts, understanding burn duration is essential. A burn time calculator for KSP is one of the fastest ways to improve stage design because it turns engine specs into actionable mission timing. Combine this with your delta-v planning, TWR checks, and staging strategy, and your craft will become much easier to fly.
Use the calculator above whenever you change engine type, stage fuel mass, or engine count. Even small configuration changes can alter burn duration enough to affect gameplay. In KSP, successful missions often come down to timing, and timing begins with knowing exactly how long your engines will burn.