Physics

2.3 - Space Travel - Propulsion & Re-Entry

4 min

note 1 of 4

Ion drives

An ion drive is a highly efficient engine that uses far less fuel than a conventional chemical thruster.

How it works

It ionises a propellant by adding or removing electrons to produce ions.
It then accelerates those ions to produce thrust.

Small force, big change in speed

The thrust is tiny, producing only a small unbalanced force.
Because it can fire continuously for months or years, that small force produces a large change in speed.
Needing far less fuel makes the spacecraft lighter and cheaper to launch.

A left-to-right sequence: blue circles labelled propellant, an atom losing an electron labelled electron knocked off and propellant ionised, a positive ion, then pink negative plates accelerating it into an ion stream labelled thrust.

note 2 of 4

Gravitational catapults (slingshots)

A gravitational catapult (or slingshot) uses the gravity of a moving planet, moon or asteroid to change a spacecraft's speed or direction without using fuel.

How it works

As the spacecraft enters the body's gravitational field, it's pulled towards the body and accelerates.
It leaves the field moving faster than it arrived, because some of the body's kinetic energy is transferred to the spacecraft.
The body is so massive that its own loss of speed is negligible.

A rocket at bottom left, labelled pulled by planet's gravity, curves up past a planet marked fast moving planet to a second rocket labelled moves faster, planet moves slower.

note 3 of 4

Precise manoeuvres with no friction

Spacecraft often have to make precise manoeuvres, such as entering Earth's atmosphere at an exact angle or docking with a space station.
This is difficult because space is a zero-friction environment - there's nothing to slow the spacecraft down or hold it in place.
Small thrusters fire tiny, carefully timed bursts of fuel to nudge the spacecraft into the correct position and orientation.

note 4 of 4

Re-entry heating and heat shields

During re-entry, traveling at high speed through the atmosphere produces a very large frictional force on the spacecraft's surface.
This friction does work on the spacecraft, transferring kinetic energy to heat and reaching temperatures of around 1,600 °C.

Ablation heat shields

An ablation heat shield is a protective layer designed to burn away on re-entry, carrying heat away from the spacecraft.
Heat is absorbed in two ways:
1. As the shield material rises in temperature
2. As some of the material turns into a gas

key terms

TermDefinition

Ion driveA highly efficient spacecraft engine that turns a propellant into ions (charged particles) and accelerates them to produce thrust.

Gravitational catapultA manoeuvre that uses the gravity of a moving body to change a spacecraft's speed or direction without using fuel.

Ablation heat shieldA protective layer designed to burn away and carry heat away from a spacecraft during re-entry.

end of notes

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2.4 - Space Travel - MechanicsNext up

Ion drives

An ion drive is a highly efficient engine that uses far less fuel than a conventional chemical thruster.

How it works

It ionises a propellant by adding or removing electrons to produce ions.

It then accelerates those ions to produce thrust.

Small force, big change in speed

The thrust is tiny, producing only a small unbalanced force.

Because it can fire continuously for months or years, that small force produces a large change in speed.

Needing far less fuel makes the spacecraft lighter and cheaper to launch.

Gravitational catapults (slingshots)

A gravitational catapult (or slingshot) uses the gravity of a moving planet, moon or asteroid to change a spacecraft's speed or direction without using fuel.

How it works

As the spacecraft enters the body's gravitational field, it's pulled towards the body and accelerates.

It leaves the field moving faster than it arrived, because some of the body's kinetic energy is transferred to the spacecraft.

The body is so massive that its own loss of speed is negligible.

Precise manoeuvres with no friction

Spacecraft often have to make precise manoeuvres, such as entering Earth's atmosphere at an exact angle or docking with a space station.

This is difficult because space is a zero-friction environment - there's nothing to slow the spacecraft down or hold it in place.

Small thrusters fire tiny, carefully timed bursts of fuel to nudge the spacecraft into the correct position and orientation.

Re-entry heating and heat shields

During re-entry, traveling at high speed through the atmosphere produces a very large frictional force on the spacecraft's surface.

This friction does work on the spacecraft, transferring kinetic energy to heat and reaching temperatures of around 1,600 °C.

Ablation heat shields

An ablation heat shield is a protective layer designed to burn away on re-entry, carrying heat away from the spacecraft.

Heat is absorbed in two ways:

As the shield material rises in temperature
As some of the material turns into a gas