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The Chemistry of Rocket Science

Learn how combustion and ionisation power rockets

Chemistry is not rocket science. But rocket science sure has a lot of chemistry! From the very beginning of a launch, chemical reactions go into overdrive to lift off a spacecraft. While the latest launch of SpaceX Demo-2 featured NASA astronauts in futuristic suits, the function of the reactions is rudimentary: to generate one big fart of hot gases to push the rocket upward.

RP-1 Rocket Fuel

SpaceX Demo-2 was launched into orbit by its powerful Falcon 9 rocket. Designed by SpaceX, Falcon 9 ran on a special mixture of fuel called RP-1.

To formulate RP-1, chemists use the kerosene fraction of crude oil. They process it further to remove sulfur-containing compounds. This prevents the corrosion of fuel tank made of an aluminium-lithium alloy. The remaining hydrocarbons in the processed kerosene are mostly straight-chain alkanes (Lovestead et al, 2010).

When RP-1 is ignited with oxygen in the combustion chamber, the alkanes undergo combustion to produce carbon dioxide gas and water vapour. The reaction is highly exothermic, giving off a large amount of energy to heat up and expand the gases. As the hot gases rush out of the combustion chamber, they exact an equal but opposite force. This thrusts the rocket into orbit, escaping the clutch of Earth’s gravity.

Hydrogen Rocket Fuel

Other rockets burn liquid hydrogen as an alternative fuel. While it was proposed as early as the 1940s, it was only successfully used as a rocket fuel in the 1970s. It was difficult to store. To liquefy hydrogen gas, engineers must cool it to -253 °C. The rocket must also be thoroughly insulated to prevent liquid hydrogen from boiling off and expanding dangerously.

Liquid hydrogen reacts with liquid oxygen to produce water vapour. It works pretty much like our test for hydrogen gas in the lab, whereby we place a burning splint to ignite hydrogen with atmospheric oxygen. However, instead of a ‘pop’ sound, liquid hydrogen and liquid oxygen react with a bang, blasting the rocket off the face of the Earth.

2H2(l) + O2(l) ⟶ 2H2O(g)

Unlike the RP-1 kerosene fuel, the combustion of hydrogen fuel produces water vapour as the only product.

Powerful but Inefficient

Chemical fuels are easily scaled up to give you the power you need. Use more fuels and you go from a ‘pop’ sound in a test tube to a loud bang in the combustion chamber.

Yet, chemical fuels are inefficient. Their fuel efficiency is around 35%. This means that only a small fraction of the chemical potential energy is unlocked and transformed into kinetic energy. It spells trouble for longer missions to the moon or other planets. A mission ends whenever the fuel on board runs out.

Ions to the Rescue

To allow satellites to go further in space, ion engine is the answer.

Instead of burning fuels to produce hot gases, ion engine uses electricity to strip atoms of their electrons. This forms positively-charged ions. The ions of like charges repel away from each other, accelerating and escaping the engine at even higher speed. This exerts a greater force, conferring an efficiency of over 90%.

The atoms used are usually xenon, a noble gas. Unlike chemical fuels, inert xenon has a lower risk of exploding accidentally. However, its relative stability also means that energy is needed for ionisation.

Xe(g) ⟶ Xe+(g) + e

Sounds like science fiction? The ion engine was successfully deployed to send the SMART-1 satellite to the moon in 2003.

A Big Solution for Small Satellites

The ion engine has also been used in miniature satellites called CubeSats. Just slightly larger than a Rubik’s cube, they cannot carry a lot of fuel to go far. The ion engine provides a way out, giving them a lifeline by producing a lot of energy with a bit of xenon.

The ion-powered CubeSat provides a more affordable way for companies and research groups to launch scientific instruments into orbit. In 2019, Nanyang Technological University launched a CubeSat into orbit. It carried a special low-light camera for Instagram imaging, with a newly-developed ion engine to fine-tune its altitude while in orbit.


Data-Based Questions à la Paper 2 Section B

QUESTION 1: Elements, Mixtures & Compounds
When a batch of RP-1 was heated, the liquid boiled from 206 °C to 256 °C.

Explain why RP-1 boils over a big range of temperature. [1 mark]

As RP-1 is an impure mixture of different hydrocarbons from the kerosene fraction of crude oil, it boils over a range of temperature instead of at an exact and constant boiling point.

QUESTION 2: Air
To make RP-1, the kerosene fraction of crude oil is obtained and processed to remove sulfur-containing compounds.

Given that the combustion of sulfur-containing compounds produce sulfur dioxide, explain why their removal prevents the corrosion of the aluminium-lithium fuel tank. [2 marks]

Sulfur dioxide is an acidic oxide that dissolves in water to form sulfurous acid, H2SO3. Sulfurous acid may further oxidise to form sulfuric acid, H2SO4. The acids will react with aluminium and lithium, which are both reactive metals. This may damage the fuel tank.

QUESTION 3: Alkanes
In a study, it was found that a batch of RP-1 contained 5.32% (by mass) of n-dodecane.

Given that it is an alkane with 12 carbon atoms, state the chemical formula of dodecane. [1 mark]

C12H26

QUESTION 4: Alkanes
Write a balanced chemical equation, with state symbol, for the complete combustion of dodecane in excess oxygen gas to form carbon dioxide gas and water vapour. [1 mark]

2C12H26 + 37O2(g) ⟶ 24CO2(g) + 26H2O(g)

QUESTION 5: Energy Change
The complete combustion of hydrocarbons in RP-1 and that of hydrogen gas are both exothermic reactions.

Define the term exothermic with regard to both the energy changes and temperature changes during the reactions. [2 marks]

In an exothermic reaction, heat energy is given out from the reactants to the surroundings. This causes the temperature of the surrounding to increase.

QUESTION 6: Mole Concept & Energy Change
When 1 mol of n-dodecane is completely burned, -7091 kJ/mol of energy is produced.

Calculate the enthalpy change when 1 g of n-dodecane is completely burned. Give your answer in kJ/g, to 3 significant figures. [2 marks]

STEP 1: Convert mass data to number of moles
Molar mass of n-dodecane = 12×12 + 26×1 = 170 g/mol
No. of moles in 1 g of n-dodecane = 1/170 = 0.005882 mol

STEP 2: Calculate energy change
Energy change when 1 g of n-dodecane is reacted = 0.005882 × -7091 = -41.7 kJ/mol

QUESTION 7: Mole Concept & Energy Change
On the other hand, when 1 mol of hydrogen gas is completely burned, -286 kJ/mol of energy is produced.

Calculate the enthalpy change when 1 g of hydrogen is completely burned. Give your answer in kJ/g, to 3 significant figures. [2 marks]

STEP 1: Convert mass data to number of moles
Molar mass of n-dodecane = 2×1 = 2 g/mol
No. of moles in 1 g of n-dodecane = 1/2 = 0.5 mol

STEP 2: Calculate energy change
Energy change when 1 g of n-dodecane is reacted = 0.5 × -286 = -143 kJ/mol

QUESTION 8: Fuels & Crude Oil
Use the information given in the article, as well as, your answers to Q6 and Q7, to evaluate the use of RP-1 and hydrogen as rocket fuels. [2 marks]

Hydrogen fuel is a more efficient source of energy, as it releases a larger amount of energy per unit mass than RP-1.

The combustion of hydrogen fuel is less polluting, as its complete combustion in excess oxygen only produces water, while RP-1 produces the greenhouse gas carbon dioxide.

However, the cooling of hydrogen fuel to -253 °C is energy consuming. RP-1 is more environmentally friendly in this sense, as it can be stored as a liquid without cooling.

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