DOES MASS HAVE ANY ROLE IN ROCKET PROPULSION?

The Mass of a Rocket is important for two reasons :

• An object with less mass accelerates more quickly.
• 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 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?

• 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.

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 Rutherford Engine and Electron Rocket:

Rutherford engine

RUTHERFORD ENGINE

• The Rutherford engine is an oxygen/kerosene pump-fed engine specifically designed for the Electron rocket.

• The engine is named after the New Zealand scientist Ernest Rutherford.

• The Electron is Rocket Lab’s small, all carbon-composite rocket.

• The Electron’s main propulsion system is the Rutherford engine, with the rocket’s first stage composed of nine of these engines, which are powered by liquid oxygen and kerosene.

• With both the Rutherford engine and Electron rocket, Rocket Lab has used innovative technologies to eliminate mass. Learn about the work of Rocket Lab and Kiwi innovator Peter Beck

ROCKET LAB’s SUCCESSFUL ELECTRON ROCKET

PROPELLANTS OF ROCKETS:

• Rocket propellant is the reaction mass of a rocket.

• This reaction mass is ejected at the highest achievable velocity from a rocket engine to produce thrust.

• The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines.

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