🔥 Rocket Engines: How They Work

Module 3: Space Systems & Propulsion Engineering

What You’ll Learn

Types of rocket engines
Specific impulse (Isp) - the key efficiency metric
Thrust-to-weight ratio
Trade-offs in engine design

1. How Rocket Engines Generate Thrust

All rocket engines work by: 1. Combustion - Mixing fuel + oxidizer and burning them 2. Expansion - Hot gas expands through a nozzle 3. Acceleration - Gas accelerates to very high speeds 4. Reaction - Rocket moves opposite to exhaust (Newton’s 3rd Law)

The Key Metrics

Metric	What It Measures	Units
Thrust	Force produced	Newtons (N) or kN
Specific Impulse (Isp)	Fuel efficiency	Seconds
Thrust-to-Weight	Power density	Ratio

Code

import numpy as np
import matplotlib.pyplot as plt
plt.style.use('dark_background')

G0 = 9.80665  # Standard gravity (m/s²)

# Real rocket engine data
ENGINES = {
    'Merlin 1D': {
        'thrust_kn': 845, 'isp_vac': 311, 'isp_sl': 282,
        'mass_kg': 470, 'propellant': 'RP-1/LOX', 'company': 'SpaceX'
    },
    'Raptor 2': {
        'thrust_kn': 230, 'isp_vac': 380, 'isp_sl': 327,
        'mass_kg': 1600, 'propellant': 'CH4/LOX', 'company': 'SpaceX'
    },
    'RS-25': {
        'thrust_kn': 2279, 'isp_vac': 452, 'isp_sl': 366,
        'mass_kg': 3177, 'propellant': 'LH2/LOX', 'company': 'Aerojet'
    },
    'RD-180': {
        'thrust_kn': 4152, 'isp_vac': 338, 'isp_sl': 311,
        'mass_kg': 5480, 'propellant': 'RP-1/LOX', 'company': 'NPO Energomash'
    },
    'BE-4': {
        'thrust_kn': 2400, 'isp_vac': 341, 'isp_sl': 312,
        'mass_kg': 2200, 'propellant': 'CH4/LOX', 'company': 'Blue Origin'
    },
}

print("✅ Engine database loaded!")

✅ Engine database loaded!

2. Specific Impulse (Isp): The Efficiency Metric

Specific Impulse tells you how efficiently an engine uses fuel. Higher is better!

Isp = exhaust velocity / g0

Propellant	Typical Isp	Used By
Solid	250-280s	SRBs
RP-1/LOX	280-320s	Falcon 9
CH4/LOX	320-380s	Starship
LH2/LOX	400-450s	SLS

Code

# Compare real rocket engines
print("🔥 ROCKET ENGINE COMPARISON")
print("=" * 80)
print(f"{'Engine':<12} {'Company':<15} {'Thrust':>10} {'Isp (vac)':>10} {'T/W Ratio':>10}")
print("-" * 80)

for name, data in ENGINES.items():
    thrust_n = data['thrust_kn'] * 1000
    weight_n = data['mass_kg'] * G0
    tw_ratio = thrust_n / weight_n
    
    print(f"{name:<12} {data['company']:<15} {data['thrust_kn']:>8} kN {data['isp_vac']:>8} s {tw_ratio:>10.1f}")

print("-" * 80)
print("\n💡 Key Insights:")
print("  • RS-25 has highest Isp (452s) but is expensive and not reusable")
print("  • Raptor has excellent Isp (380s) AND is designed for reuse")
print("  • RD-180 has highest thrust but Russia won't sell them anymore")

🔥 ROCKET ENGINE COMPARISON
================================================================================
Engine       Company             Thrust  Isp (vac)  T/W Ratio
--------------------------------------------------------------------------------
Merlin 1D    SpaceX               845 kN      311 s      183.3
Raptor 2     SpaceX               230 kN      380 s       14.7
RS-25        Aerojet             2279 kN      452 s       73.1
RD-180       NPO Energomash      4152 kN      338 s       77.3
BE-4         Blue Origin         2400 kN      341 s      111.2
--------------------------------------------------------------------------------

💡 Key Insights:
  • RS-25 has highest Isp (452s) but is expensive and not reusable
  • Raptor has excellent Isp (380s) AND is designed for reuse
  • RD-180 has highest thrust but Russia won't sell them anymore

🎓 Key Takeaways

Specific Impulse (Isp) = fuel efficiency. Higher is better.
Thrust-to-Weight determines if a rocket can lift off
Propellant choice involves trade-offs between Isp, density, and cost
Methane (CH4) is the future - good Isp AND reusable engines
SpaceX Raptor is revolutionary: high Isp + full reusability

🚀 Try the Simulators!

# Explore trade-offs between payload and delta-V
cd ../Projects/Starship_Trade_Simulator
python trade_simulator.py

# Design satellite constellations
cd ../Projects/Constellation_Designer  
python designer.py

“The best part is no part.” — Elon Musk (on rocket engine design)

2. Specific Impulse (Isp): The Efficiency King

Specific Impulse tells you how efficiently an engine uses fuel.

\[I_{sp} = \frac{v_e}{g_0}\]

Where: - Isp = Specific impulse (seconds) - vₑ = Exhaust velocity (m/s) - g₀ = Standard gravity (9.81 m/s²)

What Isp Really Means

Isp = 300s means 1 kg of fuel produces 1 kg of thrust for 300 seconds
Higher Isp = more efficient = less fuel needed
BUT high-Isp engines often have lower thrust

Propellant Comparison

Propellant	Typical Isp	Notes
Solid	250-280s	Simple, reliable, can’t throttle
RP-1/LOX	280-320s	Dense, good for first stages
CH4/LOX	320-380s	Clean burning, reusable engines
LH2/LOX	400-450s	Highest Isp, but low density