The questions that kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places that adults forget to explore. That is what inspired our series Science Question From A Toddler, which uses kids’ curiosity as a jumping-off point to investigate the scientific wonders that adults don’t even think to ask about. The answers are for adults, but they wouldn’t be possible without the wonder that only a child can bring. I want the toddlers in your life to be a part of it! Send me their science questions, and they may serve as the inspiration for a column. And now, our toddler …
How many cups of sugar would it take to get to the moon? — Jacob P., age 4.5
I’m going to be upfront here: I chose this particular question because I thought it would be kind of fun to troll some rocket scientists. I figured that asking them this question would be like asking a bunch of engineers about the number of cats that would be required to build the Verrazano-Narrows Bridge.
So you can imagine my surprise when Mason Peck, a professor of mechanical and aerospace engineering at Cornell University, started cheerfully telling me about the sugar-based fuel used to launch homemade rockets. “There are lots of amateur rocketry people around the world that are interested in wacky propellants,” Peck told me. “Anything with some kind of hydrocarbon in it works pretty well. So obviously sugar would work. Pepperoni works, and it smells delicious by the way.” Yes, seriously.
Rockets don’t fly on sugar (or pepperoni) alone, mind you. Instead, the sugar is mixed with potassium nitrate — a mineral substance that is also used to preserve food and make gunpowder — to create a pliable mass that can be molded into a shape to fit the rocket before it hardens. The sugar burns and gives the rocket its oomph, and the potassium nitrate feeds oxygen to the sugar that it needs to burn. If you’ve seen the movie “October Sky,” then you’ve watched fictionalized versions of future NASA rocket scientists build exactly this kind of “candy” powered rocket.1 (Since learning that sugar really is relevant to rocketry, we at FiveThirtyEight have begun to wonder whether we are, in fact, the ones being trolled and Jacob P. is secretly a 45-year-old amateur rocketry enthusiast.)
Regardless, this toddler’s question brings up some interesting issues. So, since sugar can launch a rocket, could it actually send an object to the moon? The answer to that question is no, said Katie Robinson, a postdoctoral fellow with the Lunar and Planetary Institute, a NASA-supported research center in Houston. Or, rather, it’s so impractical as to become functionally impossible. And the “why” behind that turns out to be deeply relevant — both to recent advances in space science and our dreams of interplanetary travel.
That’s because we decide how to power our rockets based not only on a fuel’s ability to burn, but also on how much of it is needed to accelerate a rocket out of Earth’s atmosphere. When we launch anything off the surface of the Earth, the vast majority of what we’re launching is the massive amount of fuel necessary to create the force to break the surly bonds of our planet’s strong gravitational field. “It’s grossly inefficient,” said Richard Vondrak, a consulting scientist for lunar science and exploration in NASA’s solar system exploration division. “You’re accelerating a lot of fuel, but burning it up in the process.”
Consider the Apollo program, the NASA project to put a man on the moon. When we sent humans from Earth to the moon, we launched them using the Saturn V — a skyscraper doubling as a flamethrower that is still the tallest, most powerful rocket to ever fly. When we launched humans from the moon to send them back to Earth, on the other hand, we did it with the lunar module — a scrappy little vehicle that is the functional (and near literal) equivalent of a wart on Saturn V’s face. The difference in the size of those two vehicles is the necessary result of the difference in the gravitational forces on Earth and the moon. Remember, the force of gravity on the moon is about one-sixth what it is on Earth. Astronauts bounce there. Rocketships soar.
The great misfortune of Earthling space travel is that we to have to start our travel from Earth. It means we need to carry a lot of fuel, which adds a lot of weight — and a lot of expense.. It costs about $10,000 to put one pound of payload — the people, machines, objects and supplies a rocket carries — into Earth’s orbit. For context, the biggest payload the space shuttle system ever carried was the 50,000-pound Chandra X-ray Observatory, in 1999. Vondrak estimates that it’s probably six times more expensive to send something to the moon. “So a quart bottle of water is $60,000 — a dollar for the water and $60,000 for the transportation,” he said.
Because of the expense, it’s important that any rocket fuel we use to leave Earth pack as much punch-per-pound as it can. And that’s why NASA doesn’t launch sugar rockets. Sugar may work fine for amateur rocketry, but compared with professional rocket fuel, it has much less energy in the same amount of space.
Robinson helpfully did the math for me, comparing the energy in white sugar with the energy in the liquid hydrogen and kerosene used in the Saturn V. There are a lot of different ways you could calculate this, she told me, depending on the assumptions you make about the way the different fuels burn. But Robinson said we’d need, oh, 4,753,787 cups of sugar to launch the Saturn V to the moon. And because that sugar doesn’t pack the same energy-to-weight ratio punch as actual rocket fuel, this hypothetical Saturn V would have to carry 285 metric tons more fuel than the real one. “Basically sugar has a lot of energy, but it’s still a lousy rocket fuel,” Robinson told me. “Liquid hydrogen is a really good rocket fuel; it only accounts for 13 percent of the fuel by mass on the Saturn V, but accounts for nearly half the total energy produced when that fuel is burned.”
Even with the right fuel, long-distance space travel makes an already inefficient system even more so. Imagine trying to blast off from Earth, travel to Mars, land, launch from there, and travel back again. Now imagine the extra weight and expense of carrying all the fuel necessary for that trip. And that’s why you hear about research into things like space elevators — a so-far-still-sci-fi system that would carry people, parts, and (yes) fuel into Earth’s orbit by moving a vehicle along a tether between the planet and a geosynchronous space station. A space elevator wouldn’t need rockets, or rocket fuel, and would be 100 percent reusable. But, for now, there’s no material strong enough to build that kind of tether.
Instead, in the near term, our best bet for planet-hopping is to perfect a system that would allow us to make fuel someplace other than Earth. Once you can get your space fuel in space, you wouldn’t need to weigh down an Earth launch with extra gas, and then all kinds of possibilities open up, Peck said. Some ideas for how to do this involve building spaceships that can create their own fuel on board. Peck is part of a team working on one of these projects. The team’s rocket engine uses solar power to create an electric current that can separate a molecule of water into hydrogen and oxygen. Split from their H2O tangle, these elements burn very well. In fact, the combination of liquid hydrogen and oxygen is a common choice for rocket fuel.
Peck’s team has developed a briefcase-sized craft that can fly on their engine. The catch: It has only enough power to fly in space, not launch on its own from Earth. He told me that NASA has agreed to take it into orbit for a test flight, though — probably sometime in the next couple of years. Even if it can’t launch itself, a ship like this would make it more feasible and a lot cheaper to traverse the solar system.
Another option, along the same lines, would be to harvest hydrogen and oxygen from another object in space — the moon, for instance. Not very long ago, Robinson said, this would have sounded insane. When she was an undergrad — around 2007 — the idea of water on the moon was rejected out of hand. By the time she was working on her Ph.D., just four years later, water on the moon was a proven fact. How easily this water could be turned into rocket fuel is an open question, Robinson said. Most of the tangled hydrogen and oxygen that has been found is not in a form that most of us would recognize as water. Instead, it’s bound up with the mineral apatite — microscopic crystals making up 1 percent or 2 percent of a lunar soil sample. And researchers still aren’t sure whether there’s enough of it on the moon to be a fuel source.
That said, NASA has also found evidence suggesting that there is ice in some of the shadowed craters on the moon’s surface. If that’s true, splitting lunar hydrogen and oxygen into rocket fuel would be much easier. That would be the key to a real, permanent moon base, Robinson said — a source of not only fuel, but also drinkable water and breathable oxygen.
But, for now, we’re still struggling with the limitations imposed by what we can carry, and that first step from the ground to the sky remains our biggest hurdle. As long as rocket fuel weighs a lot and takes up a lot of space, anything that makes it easier and cheaper to get into space is a breakthrough.