The mass of a rocket is important for two reasons – an object with less mass accelerates more quickly, and an object with more mass has more gravitational force acting on it.
To understand these ideas, it is useful to think of objects moving in a horizontal direction before thinking about the vertical motion of a rocket.
Horizontal rocket motion
If you push two people with different masses on different swings, the person with less mass (measured in kilograms) is easier to get moving. This is because a person with less mass speeds up more quickly than a heavy person pushed with the same amount of force.
Newton’s second law of motion sums up this idea. This is often stated as force = mass x acceleration. If the same force is applied to two objects, the object with less mass will have more acceleration.
For a balloon-powered toy car (which is a simple rocket in action), a lighter car (less mass) will speed up more quickly than a car with more mass.
Vertical rocket motion
A rocket launched vertically has the same effect. A rocket with more mass will speed up more slowly, just as in the horizontal example, but there is another effect. The force of gravity is now acting in the opposite direction to the thrust, so the resultant force pushing the rocket upwards is also less.
Making a rocket as light as possible will affect how quickly the rocket will speed up and the height that it will be able to reach.
What affects the overall size of a rocket?
Some rockets are much bigger than others. If rockets with less mass speed up more quickly, then why are some rockets so huge?
The three contributors to the overall size of the rocket are:
- payload – the total amount of measuring devices, satellites, spacecraft or astronauts that needs to be carried into space, the Moon or towards other planets
- propellant load – the fuel plus oxidiser that is needed to get a rocket into space or its desired location, and any extra fuel needed also needs fuel to provide the thrust to lift it and the extra tanks that are also required to carry all of this fuel
- rocket engines, fuel tanks and so on.
For an ideal rocket getting into orbit, the payload should make up 6% of the total mass. The rocket engines, fuel tanks and so on should be 3%. The propellants should be 91%.
Any saving of unnecessary mass can markedly reduce the amount of propellant needed.
Keeping acceleration to safe levels
As the fuel reacts and its combustion products are ejected from the rocket, the mass of the rocket decreases. This lower mass means that the rocket starts to accelerate more quickly. Thrust is carefully reduced during launches for rockets carrying astronauts to protect them from this increasing acceleration.
Getting rid of excess mass
Multistage rockets use the idea of keeping mass as low as possible. As soon as the fuel from one tank is used up, the fuel tanks and the rocket engine(s) for that tank are released. The remaining stage of the rocket now has less mass so less thrust is needed to accelerate it.
The Saturn V rocket used for the Apollo Moon missions had three stages. The first stage (tanks plus propellant plus five F-1 engines) was 10 m wide and 42 m tall. The tanks were filled with refined kerosene and liquid oxygen. Fully fuelled, this stage had a mass of 2,300,000 kg. Once all propellant had been used, the mass remaining was only 131,000 kg. At an altitude of 61 km and a speed of 8300 km/h, the empty tanks and engines were discarded.
The second and third stages carried the Apollo spacecraft through the upper atmosphere towards the Moon.