What’s The Biggest Rocket You Could Build?
published during a waning crescent moon.
09/26/2016

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Introducing New Glenn: Reusable, vertical-landing booster, 3.85 million pounds thrust. Credit: Blue Origin

Ever since the fall of the Soviet Union, space advocates have been feverishly wishing for a new Space Race. Though most thought this would take the form of another international competition, it now seems that Silicon Valley tech billionaires are more than happy to take up the fight once occupied by communist and capitalist ideologies.

For the past few weeks, SpaceX dreamer-in-chief Elon Musk and Blue Origin bankroller Jeff Bezos have been playing the game of ‘show me your big rocket and I’ll show you my even bigger one.’ SpaceX has long planned to test their new 70-meter-high launch vehicle Falcon Heavy, though the company’s recent unrelated pre-flight accident has delayed the date until at least 2017.

In a surprise move earlier this month, Blue Origin unveiled designs for its New Glenn rocket, which in its largest configuration would be 95 meters tall, about the size of a 32-story building and almost as big as the Saturn V rocket that took astronauts to the moon.

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Official Artist Representation of Falcon Heavy on launchpad. Credit: SpaceX

Where does this rocket one-upmanship end?

Very soon, Musk is expected to announce plans for a launch vehicle that could allow humans to colonize Mars and beyond—whose latest nickname is the Interplanetary Transport System (or, more crudely, the Big Effing Rocket)—which is speculated to be a monstrous 180 meters tall. Not to be outdone, Bezos has teased that New Glenn will actually be the smallest of his company’s orbital launch vehicles and that a future rocket named New Armstrong will be even more gargantuan.

The question is: Where does this rocket one-upmanship end? Can Bezos and Musk just keep proposing bigger and bigger launch vehicles? Surely, there must be limits.

In some sense, not entirely. There is nothing theoretical stopping people from conceiving of ever larger rockets. At the same time, there are many practical things to consider. With any future designs, both billionaires must face the cold hard realities of physics, chemistry, and engineering. And eventually, they’ll likely come up against the colder and harder reality of economics.

“Tyranny is a human trait that we sometimes project onto Nature,” writes chemical engineer and NASA astronaut Don Pettit. “This projection is a form of rationalization, perhaps a means to cope with matters that we cannot control.”

Thus Pettit begins one of the most fascinating and informative NASA articles ever, titled “The Tyranny of the Rocket Equation.” The equation in question comes from rocketry pioneer and somewhat loony Russian mystic Konstantin Tsiolkovsky, who first derived it in the earliest years of the 20th century. It is deviously simple, containing at heart just three variables.

“Given any two of these,” Pettit writes, “The third becomes cast in stone. Hope, wishing, or tantrums cannot alter this result.”

On one side of the equation is the maximum change in velocity your rocket needs to experience—a proxy for how far your launch vehicle will take you. Want to get to the moon? You’ll need to travel at a certain speed. What about a little farther, to a near-Earth asteroid? Then you’ll need to go even faster. Most of the energy expenditure in this part of the equation simply gets you from the launch pad into the immediate environ of space about 400 kilometers up. You expend the same amount of energy getting from the Earth’s surface to low-Earth orbit as you do going from there all the way to Mars.

The second variable is the amount of kick you can get out of your fuel. Modern rockets direct the explosive reactions between oxygen and different combustive chemicals downward, providing thrust. Even the best reactions have their limits and we already use the more energetically favorable ones. Pettit calculates that if our planet was just 50 percent larger in diameter, its gravitational pull would be so powerful that we’d be unable to escape it using chemistry alone. Somewhere in the galaxy, there’s an alien civilization on a slightly-too-big home world whose rockets will not go to space today.

The final part of the rocket equation is the ratio between the amount of propellant you’re carrying versus the mass of your vehicle. This is one of the equation’s most dismaying sections because it points out that the heavier your rocket is, the more fuel it needs to carry, which makes the rocket heavier, which requires even more fuel, which—well, you see the problem. Pettit demonstrates just how out-of-proportion launch vehicles are compared to their Earth-bound counterparts: A car is comprised of roughly 4 percent fuel; a train, about 7 percent. A jet is somewhere between 30 and 40 percent fuel. After crunching some numbers, Petit reasons that a Molotov cocktail is around 52 percent fuel. But a rocket is at least 85 percent fuel (and therefore only 15 percent rocket).

This preposterous mass fraction means everything non-propellant-related about a rocket—its outer structure, tanks, pipes, living quarters, life support equipment, and cargo—must be as light as possible and engineered within extremely tight tolerances. Any modification has to be extensively tested, with the final results usually having only 10 percent room for error.

“Imagine driving your car at 60 mph and then drifting to 66 mph, only to have your car self-destruct,” writes Pettit. “This is life riding rockets, compliments of the rocket equation.”

The current biggest rocket around is United Launch Alliance’s (ULA) Delta IV Heavy, which stands 72 meters tall and can lift about 29 metric tons into low-Earth orbit. China’s Long March V, just coming online, will have a similar launch capacity. SpaceX’s Falcon Heavy is meant to smash these records. It will be capable of carrying almost double that weight into space at, quite importantly, a much lower price tag than ULA’s vehicle.

NASA is also working on a giant new rocket called the Space Launch System that should be capable of taking between 70 and 130 metric tons into orbit when it’s done. Blue Origin’s announcement of New Glenn didn’t come with any payload specifications, but its size approaches that of the Saturn V, which holds the record for largest cargo ever carried into space—a mammoth 140 metric tons.

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Composite of two NASA technical drawings, of the Saturn V rocket and the proposed Sea Dragon rocket, to the same scale. Credit: NASA

And what about the largest rocket ever conceived?

“There have been some whoppers proposed, though none has got very far,” says historian David Portree, who keeps a blog called Spaceflight History. The Soviet Nova launch vehicles were the Russian equivalent of the Saturn V, though they had a slightly lower payload capacity and a nasty tendency to explode.

In 1962, aerospace engineer Robert Traux drew up designs for a “big dumb booster” called Sea Dragon that would have been 150 meters tall and could have carried 550 metric tons to space. “Seems most of the really big rockets—including a Nova variant or two—were meant for sea launch,” says Portree. “That’s because if they launched from Cape Canaveral and one blew up, Orlando would suddenly become a seaport. After it was rebuilt, anyway.”

Probably the largest rocket ever seriously imagined was a 1959 project named Aldebaran from engineer Dandrige Cole. Eschewing chemical propulsion for nuclear engines, which can provide at least double the thrust, Aldebaran would have been 300 meters long and regularly brought a staggering 27,000 metric tons to orbit. Apparently, Cole thought this beast would be ready to fly by the 1980s.

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An artist’s rendition of NASA’s Space Launch System rocket in its latest configuration following its Critical Design Review. Credit: NASA

When concepts get to this stage—and maybe Musk and Bezos’ imaginations will one day—you have to start wondering if there’s any real need for them. Rockets are complex machines that need constant maintenance and a huge support team. Paying for all of that provides a limit of its own.

“In the end, cost and the market drive launch vehicle size as well,” says aerospace engineer Bobby Braun of the Georgia Institute of Technology. “A rocket with a very large payload capacity will likely have very few payloads. Given that the cost and complexity of this very large rocket will be high, there is a business case issue at play here as well.”