rocketry/docs/ROCKET_DESIGN.md

Rocket Design Guide

Overview

The Rocket Design tool calculates vehicle mass budget, tank geometry, structural requirements, and generates 3D models. It integrates engine data from the engine designer and computes the complete rocket's performance characteristics.

Main Sections

Vehicle Geometry

Define tank configuration, dimensions, and structural layout.

Tank Structure

Calculate wall thicknesses, masses, and pressurant requirements.

Nose Cone Design

Select and visualize nose cone shapes (conical, ogive, Von Kármán).

Mass Budget

Integrate all components (engines, tanks, payload, structure) to compute total mass and performance metrics.

3D Visualization

Interactive three.js model showing tanks, nose cone, and flight orientation.

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Vehicle Geometry

Tank Configuration Options

Tandem (Sequential):

• Two cylindrical tanks stacked vertically
• Fuel (lower), Oxidizer (upper)
• Domes: hemispherical on both ends
• Advantage: Compact, simpler plumbing
• Disadvantage: Longer vehicle

Coaxial (Concentric):

• Outer tank (fuel) surrounds inner tank (oxidizer)
• Reduces diameter
• Inner tank: flat bulkheads (simpler)
• Advantage: Compact, low diameter
• Disadvantage: More complex, thermal management

Single Tank:

• One tank with both propellants mixed
• Rarely used for high-performance (lower Isp)

Tank Dimensions

Inputs:

• outerRadius (mm) — outer tank radius
• propellantVolume (L) — total propellant quantity
• tankConfiguration — tandem, coaxial, single
• ullagePercent (%) — pressurant volume as % of propellant

Key Relationship:

Ullage volume = propellant volume × (ullage% / 100)
Effective volume = propellant volume × (1 + ullage%)

Ullage accommodates:

• Thermal expansion of propellant
• Gas bubble for pump inlet
• Margin for fill level uncertainty

Tank Geometry (Tandem Case)

Dome Volume (hemispherical):

V_dome = (2/3) × π × R³  (one hemisphere)
V_domes = (4/3) × π × R³  (both domes)

Cylindrical Section:

L_cyl = (V_eff - V_domes) / (π × R²)

Total Tank Length:

L_total = L_cyl + 2R  (two dome heights)

Example:

• R = 1.5 m, V_prop = 5000 L, ullage = 10%
• V_eff = 5000 × 1.1 = 5500 L
• V_domes = 4/3 × π × 1.5³ ≈ 14.1 m³ = 14100 L (larger than prop!)
• Need smaller R or accept small L_cyl

Note: For smaller rockets, domes dominate volume. Design must account for this.

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Tank Structure

Pressure Sources

Pressure-Fed System:

P_tank = 1.2 × P0_chamber  (pressure margin)

Typical: P0 = 200 bar → P_tank = 240 bar

Pump-Fed System:

P_tank = 2 MPa (minimum for turbopump inlet)

Much lower pressure (pump provides most acceleration)

Selection affects:

• Wall thicknesses (direct proportionality)
• Mass (linear with pressure and wall thickness)
• Propellant pump requirements

Structural Material Properties

Available materials from knowledgebase:

• Aluminum 6061-T6: density 2700 kg/m³, σ_y = 275 MPa
• Stainless Steel 304: density 8000 kg/m³, σ_y = 215 MPa
• Inconel 718: density 8190 kg/m³, σ_y = 1240 MPa
• Titanium 6-4: density 4430 kg/m³, σ_y = 880 MPa
• CFRP (Carbon Fiber): density 1600 kg/m³, σ_y = 600 MPa

Selection factors:

• Cost (Al < SS < Ti < CFRP)
• Weight (CFRP < Ti < Al < SS)
• Corrosion (SS, Ti, CFRP better; Al needs coating)
• Temperature (Inconel for hot engines; others at T < 150°C)

Wall Thickness (Hoop Stress)

Hoop stress formula:

σ_hoop = (P × R) / t

With safety factor:

t = (P × R) / (σ_yield / SF)

Cylindrical section mass:

m_cyl = 2π × R × t × L_cyl × ρ_material

Dome mass (two hemispheres):

m_domes = 4π × R² × t × ρ_material

Total tank mass:

m_tank = m_cyl + m_domes

Example Calculation

Tandem LOX/RP1 Tank:

• Radius: 1.0 m
• Pressure: 240 bar = 24 MPa
• Material: Aluminum 6061-T6
  • σ_y = 275 MPa
  • ρ = 2700 kg/m³
• Safety Factor: 2.5

Wall Thickness:

t = (24 MPa × 1.0 m) / (275 MPa / 2.5)
  = 24 / 110
  ≈ 0.218 m = 2.18 mm

Cylindrical Section (assume L_cyl = 3 m):

m_cyl = 2π × 1.0 × 0.00218 × 3.0 × 2700
      ≈ 110 kg

Domes:

m_domes = 4π × 1.0² × 0.00218 × 2700
        ≈ 74 kg

Total tank mass: ~184 kg

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Pressurant System (Pressure-Fed Only)

Helium Requirements

For pressure-fed engines, pressurant gas maintains tank pressure throughout burn.

Ideal gas law:

P × V = n × R × T

Helium mass:

m_He = (P_tank × V_prop) / (R_He × T)

Where:

• P_tank — tank pressure (Pa)
• V_prop — propellant volume (m³)
• R_He — helium specific gas constant = 2077 J/kg/K
• T — temperature (K), typically 293 K (20°C)

Example:

• P_tank = 24 MPa = 24×10⁶ Pa
• V_prop = 5 m³
• T = 293 K

m_He = (24×10⁶ × 5) / (2077 × 293)
     ≈ 20 kg

Helium Bottle Mass

Helium stored in high-pressure bottle. Typical storage pressure: 250 bar.

Bottle mass (conservative):

m_bottle = 4 × m_He

Rule of thumb: bottle is 4 times the helium mass. (More sophisticated: use MEOP analysis.)

Total pressurant system:

m_pressurant = m_He + m_bottle

Example: 20 kg He → 80 kg bottle → 100 kg pressurant system

Note: This is a major mass penalty for pressure-fed systems. Pump-fed systems avoid this.

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Nose Cone Design

Available Profiles

Three aerodynamic nose cone shapes, all with L = 2 × R (length = 2 × base radius).

Conical:

• Simple linear profile
• r(x) = R × (x / L)
• Sharpest tip, moderate drag

Tangent Ogive:

• Smooth circular arc meeting base tangentially
• ρ = (R² + L²) / (2R) (radius of curvature)
• r(x) = √(ρ² - (L - x)²) - (ρ - R)
• Smooth, less drag than conical
• Common in rocketry

Von Kármán:

• Power law profile minimizing drag
• θ = acos(1 - 2x/L)
• r(x) = (R / √π) × √(θ - sin(2θ)/2)
• Theoretically optimal for transonic flight
• Used on high-performance rockets

Selection Criteria:

• Conical: Simplest to manufacture, sharp tip
• Tangent Ogive: Better aerodynamics, smoother base transition
• Von Kármán: Best drag coefficient (Cd ≈ 0.15 vs 0.25 conical)

3D Visualization

Nose cone is rendered using THREE.js LatheGeometry:

• Profile points sampled from mathematical formula
• Rotated around vertical axis to create 3D shape
• Interactive rotation/zoom in 3D viewer

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Mass Budget

Components

Structure:

• Tank walls and domes: m_tanks
• Other structure (nosecone, bay, interstage): m_other
• Engine dry mass (from engine designer): m_engine

Propellant:

• Fuel + oxidizer: m_propellant

Pressurant (pressure-fed only):

• Helium + bottle: m_pressurant

Payload:

• Avionics, recovery system, instruments: m_payload

Calculation

Dry mass (no propellant):

m_dry = m_tanks + m_other + m_engine + m_pressurant + m_payload

Wet mass (with propellant):

m_wet = m_dry + m_propellant

Payload fraction:

fraction_payload = m_payload / m_wet

Structure fraction:

fraction_structure = m_tanks / m_wet

Thrust-to-weight ratio:

TWR = F_thrust / (m_wet × g)

Example Mass Budget

Small LOX/RP1 rocket:

Component	Mass (kg)	%
Tanks (structure)	200	14%
Engine	50	3.5%
Pressurant (He + bottle)	100	7%
Avionics + recovery	30	2%
Dry mass	380	26.5%
Propellant (LOX + RP1)	1050	73.5%
Wet mass	1430	100%

Performance:

• TWR = 150 kN / (1430 kg × 9.81 m/s²) = 10.7
• Delta-v (no gravity loss): 310 s × ln(1430/380) ≈ 1300 m/s

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3D Model Components

Tank Rendering

Cylindrical body:

<TankSection
  radius={1.0}
  length={3.0}
  position={[0, 0, 0]}
/>

Renders:

• Cylinder: CylinderGeometry
• Top dome: SphereGeometry (hemisphere)
• Bottom dome: SphereGeometry (hemisphere)

Material: MeshStandardMaterial with color (red = fuel, blue = oxidizer)

Nose Cone Rendering

LatheGeometry:

• Takes profile curve (array of points)
• Rotates around Y-axis to create 3D surface
• Smooth, aerodynamic appearance

Example:

const profile = noseConeProfile('tangent-ogive', 0.5, 1.0, 24)
const geometry = new THREE.LatheGeometry(profile, 24)

Scene Setup

Lighting:

• Ambient light (0.6 intensity)
• Directional light from top-right
• Shadows enabled for depth

Camera:

• Orbit controls (rotate, zoom, pan)
• Auto-fit to model bounds
• Perspective view (60° FOV)

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Workflow: Design Complete Vehicle

Step 1: Import Engine

1. Go to Design > Engine
2. Configure combustion (LOX/RP1, 200 bar, etc.)
3. Size structure (Inconel, 3.0 SF)
4. Export as JSON
5. Return to Design > Rocket
6. Upload engine JSON → Result: Engine dry mass auto-populated

Step 2: Configure Tanks

1. Set tank configuration: Tandem
2. Set outer radius: 1.0 m
3. Set propellant volume: 5000 L
4. Set ullage: 10% → Result: Calculated tank length, dome geometry

Step 3: Design Tank Structure

1. Select material: Aluminum 6061-T6
2. Set safety factor: 2.5
3. Select feed system: Pressure-Fed → Result: Wall thickness, tank mass, pressurant requirements

Step 4: Set Payload

1. Enter payload mass: 50 kg (avionics, recovery) → Result: Dry mass updated

Step 5: View Mass Budget

→ Result:

• Wet mass: 1550 kg
• Dry mass: 450 kg
• TWR: 10.2
• Delta-v (vacuum): 1320 m/s

Step 6: Adjust Nose Cone

1. Select shape: Von Kármán → Result: 3D model updates with optimal nose cone

Step 7: Export & Simulate

1. Export vehicle JSON
2. Go to Design > Trajectory
3. Import vehicle & engine data
4. Run flight simulation → Result: Altitude, downrange, apogee, landing site

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Advanced Topics

Multi-Stage Rockets

Not yet implemented, but conceptually:

• Stage 1: Large engines, heavy structure
• Stage 2: Smaller engines, lighter structure
• Interstage adapter: mass penalty
• Staging sequence: defined by velocity requirements

Implementation would require:

• Multiple vehicle definitions (or list of stages)
• Trajectory system that handles stage separation
• Cumulative mass and thrust calculations

Composite Materials

CFRP offers best strength-to-weight but requires special analysis:

• Fiber direction — properties vary with orientation
• Matrix — epoxy, vinyl ester
• Layup schedule — [0°/90°/45°] typical
• Microbuckling — axial compression limit
• Matrix cracking — transverse tensile limit

Advanced model would include:

• Classical laminate theory (CLT)
• Ply-by-ply failure criteria (Tsai-Wu, Hashin)
• Buckling analysis (Timoshenko, finite element)

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Troubleshooting

Tank too long?

• Reduce ullage percentage (lower to 5%)
• Increase radius (larger tank, shorter cylinder)
• Split into multiple smaller tanks
• Switch to coaxial configuration

Vehicle too heavy?

• Switch to lighter material (CFRP)
• Reduce safety factor (if acceptable)
• Decrease payload mass
• Use pump-fed system (avoids pressurant)

Nose cone looks wrong?

• Verify radius and length (L = 2R)
• Try different shape (Von Kármán usually best)
• Check 3D viewer isn't zoomed in too far

3D model not rendering?

• WebGL must be enabled
• Check browser console for Three.js errors
• Ensure geometry dimensions are reasonable (not NaN)

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References

• NASA SP-8007: Structural Design and Test Factors of Safety for Spaceflight Hardware
• Huzel, D. K., & Huang, D. H. (1992). Modern engineering for design of liquid-propellant rocket engines.
• Rocket Propulsion Elements (8th ed.). by Sutton & Biblarz.

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Last Updated: 2025-02 | Status: Current (v3 — Domed Tanks)