MAN: Thank you to Brilliant for supporting PBS Digital Studios.

Hey, smart people.

Joe here.

I'm here with Don Pettit.

You probably recognize this guy.

He's my favorite astronaut that I know.

Hold on, Joe.

You only know one astronaut.

That's not important.

You're still my favorite.

Last time that I was here, we were hanging out.

We were talking about how to drink coffee in space and the cool invention that you made to do that.

And when we were done, I walked over here to this building to check out this thing.

This is a Saturn V. It made me think, though, when I was sitting in here looking at the size of this thing, because until you're standing up next to this thing, you just do not have a sense of how massive the Saturn V is, is that it took all of this to get just this little bit to the moon and back.

That's the command module.

So why did it take all of that to do this?

That's called the rocket equation.

Oh.

I was told there would be no math.

So there's a famous saying.

The dinosaurs went extinct because they didn't have a space program.

But we do.

And half a century ago, astronauts got in a rocket a lot like this one to send just this tiny little bit up here on a 384,000 kilometer trip to the moon and back.

And they were able to do that because a lot of extremely smart and dedicated people pushed engineering and chemistry to the limits to create this 36-story tower of carefully controlled space fire powerful enough to escape this.

This is Earth's gravity well.

This is a way to visualize how anything in the universe with mass causes spacetime itself to warp, bending or attracting any other thing with mass.

And the more massive the object, the deeper the gravity well.

And, well, if you don't expend enough energy, well, you're trapped inside the well, unable to escape.

Fortunately, rockets are excellent energy-expending gravitational well escape devices.

But the ability of a rocket to escape a gravitational trap or not depends on some basic rules of physics and chemistry.

And these rules are written down in the rocket equation.

Thanks.

The rocket equation deals with moving from point A to point B in a gravitational field.

It tells you how much propellant you need in order to do that compared with how much your total rocket weighs.

Let's explain this idea of mass fraction real quick.

Take a typical gas-burning car.

Actually, don't take a car.

That's illegal.

But you don't need very much gas in the tank compared to the total mass of the car to get from point A to point B down here on Earth.

I'm on my way to Houston to talk to Don, the astronaut.

But you already knew that because you're watching the video right now.

But the point is, this car, its total weight is only 3, 4, maybe 5% fuel.

But that airplane, that's 30% or 40% fuel.

What percentage is this thing fuel?

A rocket, per the rocket equation, is 85% to 90% propellant.

Everything you see here as this rocket is only 10% to 15% of the total mass of the vehicle.

And that 10% to 15% is the entire structure of the rocket.

The people, the life support, all that cool science stuff we want to carry into space, they're only 1% of the mass of the total rocket, propellant and all.

So it takes 99% of the mass of this thing to get the 1% of cool, important space stuff up there.

That's correct.

This is the rocket equation, a simplified version of it anyway.

It was figured out by a Russian rocket scientist named Constantine Tsiolkovsky.

Now don't be scared by how mathematical this looks.

It's actually pretty easy to understand.

Now this is the ratio of the mass of your rocket with a full gas tank to the mass of your rocket with an empty gas tank.

Now e is just a mathematical constant.

It's roughly 2.72 or so.

And what this means is that when there's an explosion here, how much of that energy is directed to the rocket going this way?

And we lose some of that explosion energy to things like friction, and heat, engine efficiency, and, most importantly, to gravity.

And since this is an exponent, it means that if we increase the strength of the gravity field we're in, this number goes up really quickly, like compound interest.

And that means the ratio of your rocket that has to be propellant goes up really quickly.

So the stronger the gravitational field, you pretty quickly find out that you need a lot of rocket to get a little bit of stuff up and out of your gravity well and up into space.

So if you're in the business of engineering rockets, what can you do?

To get off of planet Earth, you've got the gravity of Earth.

And we're not going to change that.

And then you have the energy in your rocket propellant.

And once you max out what is possible with chemistry, then there isn't anymore.

You max out the energy density and then you plug it in the rocket equation.

And you have to abide by what it says.

Think about that.

A rocket is basically a way to take the energy stored inside chemical bonds and use it to crawl out from the bottom of our gravity well.

So rocket science isn't just physics.

We have to fiddle with chemistry too.

And we have four, maybe five classes of rocket propellants to choose from these days, just a handful of chemical options to try to nudge the rocket equation in our favor.

So the universe has set the rules.

We're just playing the game.

That's one of the best ways of describing it.

I call it the tyranny of the rocket equation.

Now I love talking to Don because I really like how his brain works.

He understands the rocket equation in precise mathematical detail, but he's also able to engage his imagination and use this knowledge to answer unexpected questions, like what would our space program look like if we lived on a slightly different planet?

Say you increase the size of Earth so the Earth's gravitational constant increases.

If Earth were about 10%, maybe 15% bigger, we would not be able to make a rocket to carry any useful payload into space.

In essence, we could not get off this planet.

JOE: This is shocking news.

Huge new developments.

This makes me think of something.

Do you think there could be alien planets, extraterrestrial civilizations who just live on planets that are too big for them to get off of?

Yeah.

The sky's not the limit.

Gravity is.

The tyranny of the rocket equation is also the main thing separating us from making X-wings and Enterprises in real life.

As long as we're using chemistry for our rockets, we're engineering those rockets to the edge of what is possible in order to escape Earth's gravity well.

But what if we could find somewhere else nearby with a smaller gravity well where we could fuel up?

Just can't think of where that would be.

There's a lot of talk about going back to the moon.

You want to go?

Oh, I'd go to the moon in a nanosecond.

It would take you a little bit longer than a nanosecond.

Yeah.

It takes three to five days to get to the moon.

It's an enabler for allowing human beings to expand into other places in our solar system.

A rocket scientist by the name of Krafft Ehricke made one of my favorite quotes.

"If God intended man to be a space-faring species, he would have given us a moon."

If Earth had no moon, next stop past Earth would be Venus or Mars, both very difficult to go right out of the box.

The moon three to five days away.

There are resources we can use.

What kind of resources?

Primarily propellant.

Imagine if you could make your rocket propellant from resources you find on the moon.

What can you make rocket fuel out of that you can find on the moon?

Water.

In addition to rocks.

There's water on the moon?

There's water on the moon.

JOE: In the polar regions of the moon, some craters are deep enough to be in permanent shadow, protected from the sun's rays.

And recent lunar observations have confirmed that frozen H2O persists inside these dark craters.

Using electricity, this water could be split into hydrogen and oxygen for rocket propellant.

Water is found throughout the rocky planets where human beings would be interested in exploring.

So if you make rocket propellant systems based on hydrogen and oxygen, you will, at least in concept, be able to refuel your rockets almost anywhere you want to go in our solar system.

JOE: So right now, would we have the ability to launch a rocket from Earth with people on it and point it directly at Mars, or is that just really, really hard?

Yeah.

It's tough to do that.

It would take a lot of propellant.

To go from low Earth orbit straight to Mars and come back again would require 8 to 12 Saturn V launches just to stage one mission.

Wow.

That's basically the whole Apollo program for just one mission to Mars.

And here's where a little bit of imagination, combined with the science we've just learned, can show us a solution to another interesting problem.

Now remember how different vehicles require a different fraction of propellant compared to their total mass to go from point A to point B?

A car is a few percent.

An airplane is 30% or 40%.

And a rocket is more than 80%.

This number is so high because Earth is a really hard gravity hole to get out of.

But the moon is a much smaller gravity hole to escape from.

Launch your rocket from lunar gravity and, according to the rocket equation, it only has to be 30% to 40% propellant.

And 30% to 40% propellant, that's less like a Saturn V and more like the aviation industry here on Earth.

And we're already pretty good at engineering planes.

See, the dinosaurs, they got stuck down here.

To explore the rest of the solar system like centuries of explorers before us, we need to cross over this one tall hill to see what's on the other side.

We've got a much easier climb ahead of us if we start from the moon.

Sounds like a pretty good reason to go back and maybe even stay for a while.

And you'll see some cool rocks while you're up there too.

You'll see some cool rocks.

Stay curious.

As always, thank you to our patrons for being part of our community and help making episodes like this possible.

We couldn't do it without you.

And a big thank you to Brilliant for supporting PBS Digital Studios.

Making YouTube videos is not rocket science, but rocket science is rocket science, and that means having some serious physics and math and computer science chops.

But becoming awesome at those subjects doesn't have to be dull-- not with Brilliant.

It's a website and app built for ambitious and curious people who want to excel at problem-solving and understanding the world.

Brilliant has over 50 interactive courses with a hands-on approach that includes storytelling and code-writing, interactive challenges-- a whole bunch of different ways to learn.

Like, in Brilliant's Scientific Thinking course, you'll develop a solid foundation in physics while playing with puzzles.

With Brilliant, you'll unravel concepts bit by bit and build up to an interesting conclusion while discovering deep truths in unexpected places.

Check out the link in the description to learn more.

And a special thanks to these Galaxy Brain patrons.

Hey, are you about to make a flat Earth joke down in the comments?

I'm here to tell you to just say no to flat Earth, kids.

The only buzz I get is from real science.

We've been to space.

I can't believe we even have to do this!