🎈 Superpressure Balloon Calculator

Follow the physics step-by-step | Based on UKHAS SPLAT

📚 About This Calculator

What this calculator does: Computes the equilibrium state of a superpressure balloon - where it will float and the stress on the envelope. This is a static equilibrium calculator that solves for the final steady-state conditions.

New feature: Optional pressure relief valve simulation shows how venting gas affects float altitude and safety margins.

For full trajectory prediction: Use predictor.stratosonde.org for multi-day flight paths with real weather data.

📊 Results Summary (Updates automatically)

🎯 Float Altitude
-- m
-- ft
📍 Onset Altitude
-- m
-- ft
⬆️ Initial Ascent
-- m/s
-- ft/min
💨 Fill Volume
-- L
@ --% full
⚠️ Pressure Safety
-- psi
--
1

Define Your Balloon System

Enter balloon, payload, and launch conditions.

💡 Intuition: Think of this like packing for a trip - you need to know what you're carrying. Every gram matters because it affects how high the balloon can float!
Gas used to fill balloon
m³ - When fully inflated
grams - Envelope only
grams - All non-balloon
grams - Extra lift
meters - Above sea level
°C - At ground
⚙️ Pressure Relief Valve (Optional Safety Feature)
Limits max pressure differential
psi - Max differential pressure
📐 Physics: The system's total mass (envelope + payload + gas) determines how much air must be displaced to achieve neutral buoyancy. More mass = deeper into atmosphere = higher float altitude.
2

Launch Conditions & Fill Volume

Calculate atmospheric conditions at launch and required gas fill.

💡 Intuition: Like inflating a balloon until a scale shows the right weight. The "neck lift" is what you'd measure with a spring scale at the balloon's neck before release.
Gas mass
Gas Mass = Volume × Density × 1000
Total mass
Total = Balloon + Payload + Gas
System density
Density = Mass / Volume
Gas Mass
0g
Total Mass
0g
System Density
0kg/m³
📐 Physics: The ISA (International Standard Atmosphere) model calculates pressure and temperature at any altitude. We use actual launch temperature (not ISA) for accuracy. The fill volume creates enough buoyant force to lift the total system mass plus provide excess "free lift" for ascent.
3

Forces & Initial Ascent Rate

Analyze forces and calculate how fast the balloon rises at launch.

💡 Intuition: Why does the balloon go up? Because the upward buoyant force (weight of displaced air) is greater than the downward weight force. The difference drives acceleration until air resistance (drag) balances it out.
Buoyancy force (upward)
F_buoyancy = ρ_air × V × g
Weight force (downward)
F_weight = m_total × g
Net upward force
F_net = F_buoyancy - F_weight
Terminal ascent velocity (with drag)
v = √(2×F_net / (C_d×ρ_air×A))
📐 Physics: The balloon doesn't accelerate forever - drag force increases with velocity until it balances the net force. This is "terminal velocity."
Initial Ascent Rate
0m/s
Initial Ascent Rate
0ft/min
Net Force
0N
4

Superpressure Onset Altitude

Where the balloon reaches maximum volume and stops expanding.

💡 Intuition: As the balloon ascends, decreasing atmospheric pressure allows the gas to expand (like a sealed bag of chips on an airplane). The balloon reaches max size when this expanded gas fills the entire envelope.
Finding altitude
V_expand = V_fill × (P_launch/P_h) × (T_h/T_launch)
📐 Physics: Below onset altitude, the balloon is in "free expansion" - it grows like a latex balloon as you blow it up. Above onset, it's in "superpressure" - the envelope is fully taut and internal pressure exceeds external pressure.
Onset Altitude
0m
Onset Altitude
0ft
5

Float Altitude (Equilibrium)

Final altitude where buoyancy equals weight - neutral equilibrium.

💡 Intuition: The balloon rises until it reaches an altitude where the air is so thin that the weight of displaced air exactly equals the weight of the balloon system. It's like a submarine adjusting ballast to hover at a specific depth.
Float Altitude
0m
Float Altitude
0ft
Altitude Gain
0m
Temp at Float
0°C
📐 Physics: At float altitude, net force = 0, so ascent rate = 0 (ignoring atmospheric vertical motions). The balloon has reached mechanical equilibrium. In reality, thermal effects cause ±500-2000 m altitude oscillations around this mean value.
6

Pressure Analysis & Safety

Calculate envelope stress and check against burst limits.

💡 Intuition: Don't confuse two different things! At float altitude: (1) NET FORCE goes to ZERO (buoyancy = weight), but (2) PRESSURE DIFFERENTIAL is at its MAXIMUM! The balloon is floating, but the envelope is under maximum stress.
Internal pressure
P_int = (P_l × V_f / T_l) × (T_float / V_max)
Differential
ΔP = P_internal - P_external
⚠️ Safety: Qualatex 36" burst at 4.7-5.4 kPa. Keep ΔP below this!
Internal Pressure
0kPa
External Pressure
0kPa
Differential (kPa)
0kPa
Differential (psi)
0psi
✅ Calculating...

📊 Pressure vs Altitude

Chart: Blue = external pressure (decreases with altitude). Red = internal pressure (equal to external until onset, then increases). Gap = differential pressure (stress on envelope).