Hello, friends! Welcome to Our Place in Space: A to Z! For this year’s A to Z Challenge, I’ll be taking you on a partly imaginative and highly optimistic tour of humanity’s future in outer space. If you don’t know what the A to Z Challenge is, click here to learn more. In today’s post, R is for…

**THE ROCKET EQUATION**

Are you bad at math? That’s okay. I’m bad at math too. I try to avoid talking about math on this blog because I know a lot of my readers are still traumatized by high school math classes, but also because I don’t feel I’m qualified to explain math anyway. So in today’s post, we’re going to talk about what the rocket equation means and why it’s so important without talking about what the rocket equation actually is or how it works.

As you know, you need fuel to go to space. If you’re a rocket scientist, the rocket equation tells you how much fuel you need to reach any specific destination in space. You want to travel from Earth to the Moon? Plug some numbers into the rocket equation, and the equation will tell you how much fuel you need. Want to go from the Moon to Jupiter? Plug new numbers into the equation, and it’ll tell you how much fuel you need for that trip. It always ends up being an absolutely ridiculous amount of fuel.

When you see space vehicles sitting on the launch pad, something like 85% to 90% of the mass of that space vehicle is fuel. The rocket equation demands that it be so. For the sake of comparison, fuel makes up about 30% to 40% of the mass of an airplane, or about 4% of the mass of a car. NASA famously refers to this as “the tyranny of the rocket equation,” because NASA is the American space agency, and whenever Americans don’t like something that call it tyranny.

With a little creative engineering, rocket scientists can make marginal improvements to a rocket’s fuel efficiency—a 1% or 2% improvement, perhaps! But that’s about it. The rocket equation is unforgiving, and it offers very little wiggle room. In other words, the rocket equation means that space exploration is super expensive, and it always will be, unless and until we invent some totally new Sci-Fi propulsion system that no longer requires rocket engines.

As a science fiction writer, I’m perfectly happy to dream up propulsion systems that ignore the rocket equation. But for the purposes of this “Our Place in Space” series, I’m trying to stick to more realistic science, which means that the distant future we’ve been exploring in these blog posts is still very much constrained by the rocket equation.

We humans can still do a lot under those constraints. We can get to the Moon (we’ve done it before!), and we can get to Mars and the asteroid belt as well. Most of the outer Solar System is within our reach—in time, perhaps the entire outer Solar System could be ours. But there are limits. So long as we’re still using rockets for space travel, there will always be limits on how far humans can go.

Want to Learn More?

Check out NASA’s “The Tyranny of the Rocket Equation” article, which goes into more detail about why the rocket equation matters. There’s also some colorful language in there about “revolting against tyranny.”

And for those of you who do want to see the math, here you go. Enjoy!