Why we should put rockets on the moon

I'm here with Don Pettit youprobably recognize this guy. He's myfavorite astronaut that I know. Hold onJoe, you only know one astronaut. That'snot important, you're still my favorite. Last time thatI was here, we were hanging out, we were talking about how to drink coffee inspace and the cool invention that you made to do that, and and when we weredone I walked over here to this building to check out this thing. This is a Saturnfive. It made me think when I wassitting in here looking at the size of this thing, because until you're standingup next to this thing, you just do not have a sense of how massive the Saturn 5is. It took all of this to getjust this little bit to the moon and back. That's the command module. So why did ittake all of that to do this. That'scalled the rocket equation. Oh I was toldthere would be no math. So there’s a famous saying: the dinosaurswent extinct because they didn’t have a space program. But we do! Half a century ago, astronauts got in a rocketa lot like this one to send this tiny little bit up here on a 384,000 km trip to the moonand back. And they were able to do it because a lotof extremely smart and dedicated people pushed engineering and chemistry to the limits tocreate 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 inthe universe with mass causes spacetime itself to warp, bending or attracting any other thingwith mass. The more massive the object, the deeper thegravity well, and… well, if you don’t expend enough energy, you’re trapped insidethe well, unable to escape. Fortunately, rockets are excellent energy-expending,gravitational well escape devices. But the ability of a rocket to escape a gravitationaltrap–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 withmoving from point A to point B in a gravitational field. And it tells you how much propellant you needin 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 Donthe astronaut… but you already knew that because you’re watching the video rightnow… 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, is85 to 90 percent propellant, which means everything you see here as this rocket, is only 10 to15% of the mass of the total vehicle. And that 10-15% is the entire structure ofthe rocket. The people, life support, and all the coolscience stuff we want to carry into space? They’re only 1% of the mass of the totalrocket, propellant and all. JH: So it takes 99% of the mass of this thingto get the 1% of cool important space stuff up there. DP: That’s correct. So this is the Rocket Equation, a simplifiedversion of it anyway. It was figured out by a Russian rocket scientistnamed Konstantin Tsiolkovsky. Don’t be scared by how mathematical thislooks. 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’san explosion here, how much of that energy is directed to the rocket going this way. We lose some of that explosion energy to thingslike friction, heat, engine efficiency and, most importantly, gravity. And since this is all an exponent, it meansthat if we increase the strength of the gravity field we’re in, this number goes up reallyquickly. Like compound interest. And that means the ratio of your rocket thathas to be propellant goes up really quickly. The stronger the gravitational field, youpretty quickly find that you need a lot of rocket to get a little bit of stuff out ofyour gravity well and up into space. So, if you’re in the business of engineeringrockets, what can you do? DP: To get off the planet Earth, you’vegot the gravity of Earth… and we’re not gonna change that. And then you have the energy in your rocketpropellant, 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 youplug 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 energystored 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 propellantsto choose from these days, just a handful of chemical options to try and nudge the rocketequation in our favor. So the universe has set the rules, andwe’re just playing the game. That’s one of the best ways of describingit. I call it the “tyranny of the rocket equation”. Now I love talking to Don because I like howhis brain works. He understands the rocket equation in precisemathematical detail. But he’s also able to engage his imagination,and use this knowledge to answer unexpected questions, like what would our space programlook like if we lived on a slightly different planet. Say you increase the size of Earth, soEarth’s gravitational constant increases. If Earth were about 10 percent, maybe 15 percentbigger, 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 youthink there could be alien planets, extraterrestrial civilizations, who just live on planets thatare too big for them to get off of? The sky’s not the limit! Whew. Gravity is. The tyranny of the rocket equation is alsothe main thing separating us from making x-wings and Enterprises in real life. As long as we’re using chemistry for ourrockets, we’re engineering rockets at the edge of what is possible in order to escapeEarth’s gravity well But what if we could find somewhere else nearby with a smallergravity well we could fuel up? Hmm… what could that be? There’s a lot of talk about going backto the moon. You wanna go? Oh, I’d go the moon in a nanosecond! It would take you a little bit longerthan a nanosecond. Yeah, it takes 3 to 5 days to get to themoon. But it’s an enabler for allowing humansto 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 difficultto go right out of the box. The moon, 3 to 5 days away, there are resourceswe can use, What kind of resources? Primarily propellant. Imagine if you could make your rocket propellantfrom resources you find on the moon. What can you make rocket fuel out of thatyou 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 wateron the moon. Water is found throughout the rocky planetswhere human beings would be interested in exploring. So if you make rocket propellant systems basedon hydrogen and oxygen, you will at least in concept be able to refuel your rocketsalmost anywhere you want to go in our solar system. So right now, would we have the ability to launch a rocket from Earth with peopleon 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 fromLow Earth Orbit straight to Mars and back again, would require 8-12 Saturn V launchesjust to stage one mission. Wow. That’s basically the whole Apollo programfor 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 requirea different fraction of propellant compared to their total mass to go from point A topoint B? A car is a few percent, an airplane is 30to 40 percent, and a rocket is more than 80 percent. This number is so high because… …Earth is a really hard gravity hole toget out of. But the moon is a much smaller gravity holeto escape from. Launch your rocket from lunar gravity, andaccording to the rocket equation it only has to be about 30 to 40 percent propellant… …and 30 to 40 percent propellant is lesslike the Saturn V, and more like the aviation industry here on Earth, and we’re alreadypretty good at engineering planes. The dinosaurs got stuck down here. To explore the rest of the solar system, likecenturies of explorers before us, we need to cross over this one tall hill so we cansee what’s on the other side. And we’ve got a much easier climb aheadof 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 rockswhile you’re up there too. You’ll see some cool rocks. Stay curious.