You are here


Rocket Chemistry Molecular Modelling (liquid oxygen/liquid hydrogen)

Model the chemical reaction in LOX/LH2-powered rockets.
Science content (2016 curriculum): 
Chemistry: Atoms, Molecules (3-7)
Chemistry: Chemical Changes (2, 7)
Earth/Space: Extreme Environments, Space Exploration (6)
Science topic (2005 curriculum connection): 
Earth and Space Science: Stars and Planets (grade 3)
  • molecule pieces for each student/student pair: 4 white hydrogen atoms, 2 red oxygen atoms, 4 bonds

Show a dramatic video of a real rocket taking off e.g. a Delta IV Heavy rocket launching the Orion spacecraft (to be used in deep space exploration) Dec 2015:

Discuss how real rockets work:
The "rocket fuel" is mixed with an "oxidizer" in the combustion chamber. They chemically react and make new molecules, including a gas. The huge amount of gas produced (called "exhaust") can only escape out of a nozzle built into the back of the rocket. The action of the exhaust shooting out exerts an equal and opposite force on the rocket, which propels the rocket upwards. (Newton's Third Law of Motion: Action and Reaction).
(If the baking soda/vinegar rocket activity or Alka seltzer activity has been done, remind students that it is the same mechanism.)

Model the chemical reaction in real rockets:
Give the students their molecule pieces.
Write the oxidizer and fuel molecules on the board, ask students to build them:
O2 (liquid oxygen or “LOX”) + 2 H2 (liquid hydrogen or ”LH2”).
These are usually gas molecules (as some students may know), but in a rocket engine, they are stored at very low temperatures, so are liquid.
Tell them that in the fuel and oxidant are injected into the hot combustion chamber, where the oxygen and hydrogen chemically react to make a new molecule. Ask them to take apart their oxygen and hydrogens and rebuild two identical molecules.
They should make two water molecules, 2 H2O. They might recognize this molecule when it is called by it's chemical formula.
In the rocket combustion chamber the temperatures high enough that this water is a gas, which escapes through the nozzle and gives the rocket thrust.

Other facts about rocket fuels:

The fuel is a significant portion of the rocket mass, so as the fuel is burned up, the rocket gets significantly lighter and accelerates upwards.
The boosters (on the side) run out of fuel first, then the main engine.

The propellants of a rocket are a "fuel" and a source of oxygen "an oxidizer".
The most efficient combination is liquid oxygen (the oxidizer, also called LOX) and liquid hydrogen (the fuel, also called LH2), as we modelled here.
Liquid oxygen ("LOX") is the oxidizer commonly used in rockets.
Other fuels used besides liquid hydrogen are kerosene or methane.
Chemical reaction for kerosene: 2 C12H26(l) + 37 O2(g) → 24 CO2(g) + 26 H2O(g)

A rocket where the fuel or oxidizer (or both) are gases liquefied and stored at very low temperatures (below −150 °C) is called a cryogenic rocket engine. If the propellants had been stored as pressurized gases, the size and mass of fuel tanks themselves would severely decrease rocket efficiency.

Rockets have to carry their own oxygen into space, where there is no air.
This is in contrast to aeroplanes, which use atmospheric oxygen to oxidize their fuel.

First liquid-fuelled rocket in was launched in 1926. It flew 12metres. First satellite, Sputnik, launched in 1957 by the Soviet Union. 1961 was the first person in space.

Grades tested: 
Gr 6
Gr 7
Phyllis Daly
Reid McInnes
Teaching site: 
Strathcona Elementary
Activity originally developed and delivered: 

Strathcona Elementary with the VSB's Scientist in Residence Program