How Rockets Work: Newton's Third Law and the Physics of Thrust

A rocket engine is one of the simplest machines in engineering: burn propellant, expel the exhaust, move the other way. Newton's third law — every action has an equal and opposite reaction — is all you need to understand the core idea. But translating that principle into a vehicle capable of reaching orbit at 28,000 km/h, surviving re-entry, and landing softly on a drone ship involves some of the most demanding engineering humans have ever done.

The fundamental equation governing rocket flight is the Tsiolkovsky rocket equation, derived in 1903 by the Russian schoolteacher Konstantin Tsiolkovsky: Δv = v_e · ln(m₀/m_f). Here, Δv is the change in velocity the rocket can achieve, v_e is the exhaust velocity, m₀ is the initial (fully fuelled) mass, and m_f is the final (empty) mass. The equation reveals an uncomfortable truth: to go faster, you need more propellant, but more propellant means more mass to accelerate, which requires even more propellant. The ratio of fuel to payload grows exponentially with the desired speed.

"The rocket equation is the fundamental constraint of spaceflight. Everything else — staging, propellant choice, lightweight structures — is engineering's answer to it."

Thrust: Where the Force Comes From

Thrust is generated when propellant is accelerated out of a nozzle at high velocity. The thrust force equals the mass flow rate of the exhaust multiplied by the exhaust velocity, plus a pressure term at the nozzle exit. A rocket engine does not push against the atmosphere — it pushes against the exhaust itself. This is why rockets work in the vacuum of space, where there is nothing to push against except the gas they eject.

The efficiency of a rocket engine is measured by its specific impulse (Isp) — the thrust produced per unit of propellant consumed per second, measured in seconds. A higher specific impulse means more speed from the same amount of propellant. Chemical rockets are typically limited by the energy stored in the propellant's molecular bonds. The best chemical propellant combination — liquid hydrogen and liquid oxygen, used by the Space Shuttle Main Engine — achieves an Isp of around 450 seconds in vacuum. Kerosene-oxygen engines (like the Falcon 9's Merlin) reach around 311 seconds at sea level, 348 in vacuum.

The Nozzle: Converting Heat Into Speed

The combustion chamber is where propellants meet and burn, reaching temperatures above 3,000°C and pressures exceeding 200 atmospheres. The de Laval nozzle — a convergent-divergent duct — converts this hot, high-pressure gas into a directed supersonic stream. Gas accelerates as it passes through the narrow throat, reaching the speed of sound there, then continues accelerating in the divergent (expanding) section to supersonic velocities.

The shape of the nozzle is critical. At sea level, the optimal expansion ratio is different from the one in vacuum. An over-expanded nozzle (designed for vacuum but operating at sea level) creates internal shock waves that reduce thrust. An under-expanded nozzle (sea-level optimised, operating in vacuum) loses potential performance. This is why some upper-stage engines have large, bell-shaped nozzles — they are optimised for the vacuum environment where they do most of their work.

Propellant Types

Rocket propellants fall into three broad categories, each with tradeoffs in performance, handling, and cost:

Liquid propellants: Stored separately as fuel and oxidiser and mixed in the combustion chamber. Liquid oxygen (LOX) paired with kerosene (RP-1) or liquid hydrogen (LH2) offers the best performance among chemical options. LOX/LH2 delivers the highest Isp but requires cryogenic storage at −183°C and −253°C respectively. LOX/RP-1 is denser and easier to handle, making it the choice for most first stages (Falcon 9, Soyuz, Atlas V).

Solid propellants: Fuel and oxidiser are pre-mixed and cast into a solid grain. Once ignited, they cannot be throttled or shut down. Solid rockets offer simplicity and long shelf life, making them common in military missiles and as strap-on boosters (Space Shuttle SRBs, Ariane 5, the SLS boosters). Specific impulse is typically lower than liquid engines.

Hybrid propellants: One propellant is solid (usually the fuel), the other liquid (usually the oxidiser). They can be throttled and shut down, and are safer to handle than solid motors. Performance falls between solids and liquids. Virgin Galactic's SpaceShipTwo uses a hybrid motor burning nitrous oxide and a polyamide plastic.

Staging: The Engineering Answer to the Rocket Equation

The rocket equation demands a very high mass ratio to reach orbit. For a single-stage vehicle using LOX/kerosene, reaching orbital velocity of 9.4 km/s (accounting for gravity and drag losses) would require the propellant to be roughly 93% of the launch mass — leaving only 7% for structure and payload. In practice, structural mass alone typically consumes 5–10% of a stage's mass, leaving almost nothing for payload.

The solution, dating to Tsiolkovsky himself, is staging: jettison empty structure as you go. A two-stage rocket drops the first stage (and its engines, tanks, and plumbing) once the propellant is gone, allowing the upper stage to start fresh with a much better mass ratio. The Falcon 9 is a two-stage rocket; the Saturn V that sent Apollo missions to the Moon was three stages. Each staging event improves the overall mass fraction at the cost of hardware lost — or, in the case of SpaceX's reusable first stages, hardware recovered and reused.

Cooling: The Thermal Engineering Challenge

Combustion temperatures in a rocket engine far exceed the melting point of any known structural material. Rocket engines survive this through regenerative cooling: propellant (usually the fuel) is circulated through channels in the engine wall before being injected into the combustion chamber. The fuel absorbs heat from the wall, keeping it below melting temperature, and arrives in the chamber already warm — slightly improving combustion efficiency. The SpaceX Raptor engine (used on Starship) operates at the highest chamber pressure of any production engine — over 300 bar — and relies on methane regenerative cooling to survive the thermal environment.

Some engines supplement this with ablative cooling (a sacrificial liner that chars and insulates) or film cooling (a thin layer of propellant injected along the chamber wall to protect it). Upper-stage engines and nozzle extensions often use ablative coatings, trading a modest performance penalty for simplicity.