In July 1969 Neil Armstrong and Buzz Aldrin stepped onto the surface of the Moon. It was the climax of the American Apollo programme. A further ten astronauts (all men unfortunately) walked on the Moon. The success of the Apollo moon landings was the result of the bravery of the astronauts, the excellence of the engineering and the application of Newton’s Laws of Motion. It also owed a lot to chemistry.
Chemistry was involved in the propulsion systems used to get the astronauts to the Moon and back, in the materials used to construct the spacecraft and in the life support systems.
Propulsion
The Apollo spacecraft that travelled to the Moon consisted of three parts, the Lunar Module, that carried two astronauts to the surface of the Moon and back; the Command Module which was home to the three astronauts throughout their voyage, and the Service Module which carried most of the fuel and supplies that was needed for the mission. Apollo was launched using the Saturn V rocket, the largest and most powerful rocket ever built, even today. The Saturn V had three stages. All six parts of the craft had rockets for propulsion and manoeuvring.
A rocket propellant expands rapidly out of the nozzles and, by the principle of Newton’s 3rd Law of Motion, pushes the rocket in the opposite direction. The propellant is a very hot gas usually produced by the chemical reaction between a fuel and an oxidising agent i.e. a RedOx reaction.
The earliest rockets invented by the Chinese, used gunpowder, a mixture of sulfur, carbon and potassium nitrate to produce a hot mixture of gases mainly carbon dioxide, sulfur dioxide, and nitrogen oxides. Gunpowder however is not a powerful enough rocket propellant to get to the Moon. Four men were pioneers of rocket flight in the early twentieth century: Konstantin Tsiolkovsky (1857-1935), Robert Esnault Pelterie (1881-1957), Robert Goddard (1882-1945) and Hermann Oberth (1894-1989). Their principles have guided rocket designers to the present day including the choice of fuels and oxidisers.
The first stage of the Saturn V, combined kerosene with pure oxygen. Kerosene is a hydrocarbon fraction of petroleum that is also used as aircraft fuel. Its molecules have between 10 and 16 carbon atoms with boiling points between 150 and 275 oC. Liquid kerosene and liquid oxygen react in the rocket motors to produce carbon dioxide gas and steam, e.g.
2C12H26(l) + 37O2(l) = 24COs(g) + 26H2O(g)
The Saturn V stage 1 carried 770,000litres of kerosene and 1,204,000litres of liquid oxygen. Every drop of that fuel was used up in under 3 minutes as the rocket reached a height of about 67 km (41 miles).
The second and third stages burned liquid hydrogen in liquid oxygen forming steam.
2H2(l) + O2(l) = 2H2O(g)
The 2nd stage carried 984,000 litres of hydrogen and 303,000litres of oxygen while the 3rd stage had 252,000 litres of hydrogen and 75,280litres of oxygen. As you can see the hydrogen/oxygen mixture requires less oxygen than the kerosene/oxygen rocket but for the same mass, hydrogen takes up a lot more space than kerosene even when cooled to a liquid. A Saturn V 1st stage fuelled by hydrogen would have had to be at least twice as big. Hydrogen/oxygen was however chosen as the main fuel for the Space Shuttle.
The Saturn V 2nd stage produced thrust for just over six minutes and carried the craft to the top of the atmosphere. The 3rd stage initially burned for 2½ minutes to take the craft into orbit and then was burned for a further six minutes to set the Apollo craft on route to the Moon.
The Service, Command and Lunar modules used a different reaction. The fuel was mixtures of hydrazine, methylhydrazine and/or dimethylhydrazine. The oxidizer was dinitrogen tetroxide. These substances are toxic so could not be used in the atmosphere where they could contaminate air and water, but they are easier to store than liquid hydrogen and oxygen.
Hydrazine was first prepared in 1895 by the Dutch chemist, Lobry de Bruyn. It was used to fuel rocket planes manufactured by Germany towards the end of the 2nd World War. It can be used on its own as a low-powered rocket fuel because it decomposes in the presence of a catalyst, indium.
3N2H4(l) = 4NH3(g) + N2(g)
More power is produced in the redox reaction with dinitrogen tetroxide: Dinitrogen tetroxide is one of the nitrogen oxides that we meet most as air pollutants released by petrol and diesel burning vehicle engines.
2N2H4(l) + N2O4(l) = 3N2(g) + 4H2O(g)
It was this reaction along with the similar reactions with the methyl and dimethyl hydrazine that were used in the Apollo craft, to manoeuvre into orbit around the Moon, land, and bring the astronauts back to Earth.
Apollo also needed a source of energy to power the instruments, communications and life support systems. The Service module carried fuel cells powered by liquid hydrogen and oxygen. A fuel cell is an electric cell where reactions take place on the electrodes producing an electric current. In a hydrogen/oxygen fuel cell the electrodes are made of platinum.
Hydrogen is bubbled through an electrolyte at the anode (+) 2H2 –> 4H+ + 4e–
The H+ ions pass through a membrane to join oxygen gas bubbled to the cathode (-)
O2 + 4H+ + 4e– = 2H2O
In Apollo the water that was formed was used for drinking. Fuel cells were invented almost simultaneously in the late 1830s by Welsh scientist, William Grove, and German, Christian Friedrich Schönbein but it wasn’t till the mid-twentieth century that they were developed for use.
The Command and Lunar Modules had batteries to supply electricity either as backup to the fuel cells or for when disconnected from the Service module. These were zinc-silver batteries, using the same electrode reactions as the very first battery invented by Alessandro Volta in 1799.
Materials
Steel is the material of choice for many applications. It is relatively cheap, strong and has a high melting point. There is one problem, however. If the Saturn V rocket had been built of steel, it wouldn’t have got off the ground. Steel has too high a density. The solution was to build most of the rocket from a less dense metal. Titanium would have been ideal except that in the 1960s it was very expensive and difficult to make in large quantities. The answer was aluminium, or to be correct, alloys of aluminium. While sheets of aluminium must be thicker than steel to give the same strength, they are very much lighter.
Aluminium was discovered in the 1820s, but it wasn’t till the 1880s that a process was developed for producing it industrially. Charles Hall in the USA and Paul Heroult in France developed almost identical processes almost at the same time. It wasn’t till the 1940s when cheaper electricity from hydropower became available that the price of aluminium fell enough to make it useful for engineering purposes. Pure aluminium is quite soft but adding small amounts of other metals, such as magnesium or scandium, make it harder and tougher. Panels in the Saturn V rocket and Apollo modules consisted of sheets of aluminium alloy sandwiching a honeycomb of aluminium. These panels had great rigidity and strength but were very light.
The liquid oxygen and liquid hydrogen used in the rockets had to be kept very cold. The honeycomb aluminium panels were good insulators but a bulkhead between the two tanks which were at different temperatures was made of aluminium alloy sheet sandwiching a plastic material. This material is made from phenol (C6H5OH) and formaldehyde (methanal, HCHO). The first artificial polymer, Bakelite, was a form of this resin and was invented in 1907 by Leo Baekeland. It is a very good insulator but hard and tough with a low density. Phenol formaldehyde resin was also used in the heat shield of the Command Module. The material covered the bottom of the capsule with a maximum thickness of about 5 cm. During re-entry to the Earth’s atmosphere the heat shield slowly burned away while protecting the crew. The Command Module was the one part of the craft to have an outer skin of steel as the temperature of re-entry would have melted aluminium.
Many other materials were used in the structure of the spacecraft including metals such as titanium, nickel and chromium and polymers such as Teflon (PTFE) and mylar. These were used in specific parts such as thermal shields in the Lunar Module to keep the blast from the descent rocket away from sensitive parts of the descent and ascent stages. Even the windows were of a special design. As well as being double glazed using special glass to prevent penetration by micro-meteorites, there were also layers of coatings of metal oxides and polymers to reflect infra-red and ultra-violet light and to stop reflections and misting.
Life Support
The Apollo craft had to maintain the lives of the three astronauts for a week. All the things we take for granted in our everyday lives had to be catered for – air to breathe, food and water, waste management. Except before and during the launch, the atmosphere in the craft was pure oxygen at a pressure of about one-third of normal atmospheric pressure. This is about the same pressure as the top of Mount Everest but, because it was pure oxygen, the astronauts were able to breathe comfortably. However, breathing out releases carbon dioxide. If the concentration of carbon dioxide reaches 7% or more the astronauts would be suffocated despite having plenty of oxygen. The Command and Lunar Modules had scrubbers to remove carbon dioxide. These contained an alkali metal hydroxide to combine with the carbon dioxide. Lithium hydroxide was used as it has the lowest density.
2LiOH(s) + CO2(g) = Li2CO3(s) + H2O(l)
People also tend to release smelly compounds when they spend a long time in a confined space. The craft was equipped with odour removers made of charcoal.
Water was supplied by the fuel cells. It was kept free of bacteria (they even get carried into space) by chlorine added to the water. Most of the food the astronauts ate was freeze dried before packing. It had to have water added to make it edible and the astronauts sucked the re-hydrated foods from the packet.
Urine and faeces produced by the astronauts had to be collected and stored. A disinfectant was added to the faeces to kill microbes and prevent fermentation – the production of gases would have been an explosion hazard! The disinfectant was sodium orthophenyl phenol. It has an E-number E232 and is normally used to spray fruits such as oranges, apples and pears.
These are just some of the materials used in the Apollo programme. Thousands of chemists, physicists and engineers worked on the programme to get the astronauts to the Moon and safely home.
Activities
- Several scientists are named in the article. Find out more about their lives and work.
- A rocket reaction produces a hot gas from cold reactants. What can you say about the energy changes in the reactions?
- Give two reasons why kerosene was chosen as the fuel in the first stage of the Saturn V rocket.
- There are many fuels that will react violently with an oxidising agent such as potassium nitrate, including paper and wood. Ask your chemistry teacher to show you what happens when a wooden splint is pushed into a test tube in which potassium nitrate is being heated. CARE! Don’t try this yourself. Describe what happens.
- Why weren’t lithium batteries (used in computers and electric cars) not used in the Apollo craft?
- Explain why aluminium alloys were used for all the casing of the Saturn V rocket and Apollo craft except for the Command module which was cased in steel.
- Write down the electrode reactions that take place in the Hall-Heroult Cell used to make aluminium from alumina (aluminium oxide).
- (a) All the alkali metal (group 1) hydroxides combine with carbon dioxide. Why was lithium hydroxide used?
(b) An astronaut exhales about 2.2 g of carbon dioxide every hour. Calculate the mass of lithium hydroxide needed to absorb this much carbon dioxide. Calculate the mass of lithium hydroxide required to absorb the carbon dioxide of three astronauts for a week.
(RAM: H = 1, Li = 7, C = 12, O =16)
- Why was it necessary to add disinfectant to the astronauts’ waste products?
- There are plans to take humans to the Moon by the mid-2020s. In what ways do you think the chemistry of the mission will be different to the Apollo programme of fifty years ago?
Bibliography
Details of the Apollo Lunar Module: https://www.hq.nasa.gov/alsj/LM04_Lunar_Module_ppLV1-17.pdf
The Saturn V explained: https://www.space.com/18422-apollo-saturn-v-moon-rocket-nasa-infographic.html
By Peter Ellis
