Our Place in Space: The Rocket Equation

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…


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!

9 thoughts on “Our Place in Space: The Rocket Equation

  1. I like the phrase “the tyranny of the rocket equation”. It gets the point across that no one really likes these facts, but they to be dealt with. It’s why the staged system, despite having few fans, isn’t going away anytime soon. (Although SpaceX is providing important new innovations.) As sci-fi writers, we can ignore or hand wave our way through these issues, but real engineering is stuck with them. At least for the foreseeable future.

    Liked by 1 person

    1. I do appreciate it, though, when Sci-Fi writers acknowledge the issue in some way. I forget where I heard this (probably Writing Excuses), but there’s a rule some Sci-Fi writers follow that any FTL technology you create should come at a cost: a really high or really weird cost. I try to keep that in mind in my own writing.

      Liked by 1 person

      1. I appreciate it too, although if seems rare to me. For example, The Expanse will occasionally mention fuel, but I don’t recall it ever being a plot point. Ships seem able to fly for several months without refueling.

        Orson Scott Card, in his book on sci-fi writing, mentioned that the cost of space travel is always an important background element in any space story. If help is only a couple of hours away, as opposed to days, months, or decades, or only through some perilous or agonizing process, it makes a big difference to any local conflict and the character’s attitudes about it.

        Liked by 1 person

  2. I heard about the fuel rockets eat up….I saw a movie where they wanna reduce wt of rocket for some reason but they can’t change the amount of fuel it consumes….i wish in actual math also we had automated equations which can calculate for us in even our day to day math….I dread math too😄

    Dropping by from a to z “The Pensive”

    Liked by 1 person

    1. It’s doable for small space probes, but for larger spacecraft with people inside them… along with food, life support, and all the other stuff people need to stay alive… it gets absurd real fast.


  3. My other half has demonstrated how important an understanding of physics is, but before I can study that, I have to do something about my abysmal maths, and it would all take so long I’d be even older & greyer than I am now. So, I’m bowing out of that arena gracefully (or as gracefully as my bad back allows). That said, I get there are certain immovables we have to accept & work with, until & unless there’s a massive scientific breakthrough. I had a good laugh at your description of Americans and their application of the word tyranny 😀 We all have own little foibles – amusing to think of humans being lumped into one characteristic group by Martians 🙂

    Debs visiting this year from
    Making Yourself Relationship Ready

    Liked by 1 person

    1. NASA’s article kind of leans into the metaphor with its talk about “revolution” and “overthrowing the tyrant.” They knew what they were doing.

      I wish I had a better grasp of math, but it’s still possible to learn a lot about science without any math skills. For my purposes as a science fiction writer, it’s enough for me to know there’s a mathematical relationship of some kind at work, and I can skip over the details.

      Liked by 1 person

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