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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 and when we were done I walked over here to this building
to check out this thing.
This is a Saturn five.
It made me think 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 5 is.
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!
Half a century ago, astronauts got in a rocket a lot like this one to send this tiny little
bit up here on a 384,000 km trip to the moon and back.
And they were able to do it because a lot of extremely smart and dedicated people pushed
engineering and chemistry to the limits to create a 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.
The more massive the object, the deeper the gravity well, and… well, if you don’t
expend enough energy, 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.
The rocket equation deals with moving from point A to point B in a gravitational
field.
And 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.
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 3-4-5% fuel.
But that airplane… that’s 30-40% fuel.
JH: What percentage is this thing fuel?
DP: A rocket, per the rocket equation, is 85 to 90 percent propellant, which means everything
you see here as this rocket, is only 10 to 15% of the mass of the total vehicle.
And that 10-15% is the entire structure of the rocket.
The people, life support, and all the cool science stuff we want to carry into space?
They’re only 1% of the mass of the total rocket, propellant and all.
JH: So it takes 99% of the mass of this thing to get the 1% of cool important space stuff
up there.
DP: That’s correct.
So this is the Rocket Equation, a simplified version of it anyway.
It was figured out by a Russian rocket scientist named Konstantin Tsiolkovsky.
Don’t be scared by how mathematical this looks.
It’s actually pretty easy to understand.
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.
We lose some of that explosion energy to things like friction, heat, engine efficiency and,
most importantly, gravity.
And since this is all 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.
The stronger the gravitational field, you pretty quickly find that you need a lot of
rocket to get a little bit of stuff out of your gravity well and up into space.
So, if you’re in the business of engineering rockets, what can you do?
DP: To get off the planet Earth, you’ve got the gravity of Earth… and we’re not
gonna 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.
That’s it?
That’s it.
You max out the energy density, and 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.
We have 4, maybe 5 classes of rocket propellants to choose from these days, just a handful
of chemical options to try and nudge the rocket equation in our favor.
So the universe has set the rules, and 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 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 Earth’s gravitational constant increases.
If Earth were about 10 percent, maybe 15 percent 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.
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?
The sky’s not the limit!
Whew.
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 rockets at 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 we could fuel up?
Hmm… what could that be?
There’s a lot of talk about going back to the moon.
You wanna go?
Oh, I’d go the moon in a nanosecond!
It would take you a little bit longer than a nanosecond.
Yeah, it takes 3 to 5 days to get to the moon.
But it’s an enabler for allowing humans to expand into other places in our solar system.
A rocket scientist named Krafft Ehricke
made one of my favorite quotes: “If god intended man to be a spacefaring
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, 3 to 5 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?
You can’t make it out of rocks.
Water!
There’s water on the moon?
There’s water on the moon!
We didn’t know this during the Apollo era, but now we have verified there is water on
the moon, significant quantities of water on the moon.
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.
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 back
again, would require 8-12 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 to 40 percent, and a rocket is more than 80
percent.
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 about 30 to 40 percent propellant…
…and 30 to 40 percent propellant is less like the Saturn V, and more like the aviation
industry here on Earth, and we’re already pretty good at engineering planes.
The dinosaurs 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 so we can see what’s on the other side.
And 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 even stay for a while.
And you’ll get to see some cool rocks while you’re up there too.
You’ll see some cool rocks.
Stay curious.