Why don’t we use giant batteries to power rockets instead of fuel?

Why don't we use giant batteries to power rockets instead of fuel? by Robert Frost

Answer by Robert Frost:

Be careful about the word power. Batteries do power rockets, but they don’t propel rockets. I think your question is about propulsion. You might be thinking about how we have electric cars and we have petroleum fueled cars on Earth and wondering why the same thing can’t happen in space.

Whenever discussing the motion of an object, we should start by refreshing ourselves with Newton’s three laws of motion:

First Law: “Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.”

Second Law: “Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration.”

Third Law: “For every action, there is an equal and opposite reaction.”

The first law tells us that a spacecraft will continue along its path until forces act upon it instituting a change in the motion. This is the part that a lot of people are confused about when it comes to understanding that spacecraft are not constantly firing their engines. They only fire their engines when they need to make a change or have to counteract an external force.

The third law explains how a car moves down the road, on Earth. The rotating tires experience friction against the ground and push backwards against the ground. For that action, there is an equal and opposite reaction – the car moves forward. Electrical motors can make wheels rotate just as well as a chemical combustion engine can.

In space, there is no ground to grip. There is nothing to push against – wheels are useless, so how do we change the motion of our spacecraft? Let’s look at the second law.

We need a force to accelerate our spacecraft. The second law tells us that a force is equal to a change in momentum per change in time. Messing with the momentum can give us the force we need to accelerate the spacecraft. But there’s another rule of physics we have to consider – conservation of momentum. The total momentum of a system remains the same. So, if we throw some burned fuel out of the back of the spacecraft, the momentum of that gas has to be balanced by an equal and opposite change in momentum of the spacecraft. This is essentially the example you probably saw in sixth or seventh grade of a person in ice skates, on a frozen lake, throwing a ball. When she threw the ball forward, she moved backwards. The speed at which she moved backwards was proportional to the speed the ball moved forward, considering the masses of both the ball and girl.

We throw a small amount of fuel ([math]m_{e}[/math]) out of the back of the spacecraft at high velocity ([math]v_{e}[/math]) and to conserve momentum, our much larger spacecraft ([math]m_{s}[/math]) will travel forwards at a smaller velocity ([math]v_{s}[/math]) such that the magnitude of added [math]mv[/math] for the spacecraft equals the magnitude of [math]mv[/math] for the ejected fuel.

From this, we can derive the rocket equation, also known as the Tsiolkovsky rocket equation:

That equation tells us that the change in velocity of a rocket is equal to the exhaust velocity of the burned fuel times the natural log of the original mass divided by the final mass. We change velocity by changing mass!

Now that we’ve gone through the explanation of how a spacecraft changes its motion, let’s return to your question. You asked “why don’t we use giant batteries to power rockets instead of fuel?

Since there is nothing to push against, in space, in order for us to get our equal and opposite reaction, we have to throw mass out of the back of the rocket. Using the electrical energy of a giant battery doesn’t allow us to manipulate the momentum. This is why chemical rockets are the only way we know how to get to space. If you don’t believe me, let’s ask Elon Musk:

Why don't we use giant batteries to power rockets instead of fuel?

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