An emerging class of powerful rockets is supposed to start flying in the next couple of years. They’re known as heavy-lift launch vehicles. These rockets are capable of getting a whole lot of stuff into space at once — and everyone seems to be making one. SpaceX has been promising that its Falcon Heavy, a larger variant of the Falcon 9 rocket, will fly for the first time this summer. The United Launch Alliance is working on a brand-new vehicle called the Vulcan that’s supposed to fly in 2019. And spaceflight company Blue Origin is the latest to throw its hat in the ring, recently claiming its next big rocket, the New Glenn, will be able to deliver 100,000 pounds of cargo — and eventually people — to lower Earth orbit.

At the same time, NASA is developing a monster rocket of its own; it’s called the Space Launch System, and it’s being touted as the most powerful rocket ever created. Similar in shape and size to the Saturn V rocket that took astronauts to the Moon, the Space Launch System, or SLS, will be capable of carrying between 150,000 and 290,000 pounds to lower Earth orbit (or up to the weight of nine school buses). In capability, the vehicle dwarfs the other rockets the private space industry is working on.

Bigger isn’t necessarily always better

But when it comes to comparing rockets, bigger isn’t necessarily always better. The SLS may dwarf the other commercial rockets in capability, but in other key areas, the giant vehicle falls short. For one thing, it’s expensive to launch — around $1 billion per mission. And it’s not going to launch very often either, probably only once or twice a year. Some experts argue that it’s these numbers we should use to measure a rocket’s merit: not how much it can carry, but how much it costs and how frequently the vehicle is expected to launch. If those are the standards, then the SLS isn’t necessarily the best vehicle to pull off ambitious goals in space.

If we ever want to send people to the Moon or Mars, NASA needs to get the most value for its money, especially when it comes to actual launches. NASA’s budget is already pretty limited at roughly $19 billion a year, or about 0.5 percent of the overall federal budget. And that funding isn’t expected to dramatically increase anytime soon. Meanwhile, NASA estimates that a crewed mission to Mars could cost upwards of $400 billion over the next 30 years, and advisors to the space agency have suggested that NASA look for ways to cut those costs. The new batch of smaller but efficient commercial rockets presents a possible way to do that.

“Having New Glenn, having Falcon Heavy, having SLS — once they’re flying, putting them next to each other will be the first time we’ve had that many capabilities and that many choices,” Phil Larson, a former space advisor to President Obama and a former representative for SpaceX, tells The Verge. “And the ultimate benefactor of that is the American taxpayer and NASA.”

In that case, does NASA need the SLS? And how do you measure the value of a rocket?

Why you can’t take everything with you

Right now in the US, there is already a diverse landscape of commercial rockets that can launch satellites and cargo into orbit. You have SpaceX’s Falcon 9 rocket, for instance, the United Launch Alliance’s Atlas V, and Orbital ATK’s Antares. These vehicles aren’t what you would call “heavy lift” though — they can launch between 20,000 to 50,000 pounds to lower Earth orbit. To be considered a heavy-lift vehicle, a rocket should be capable of launching upwards of 50,000 to about 100,000 pounds of cargo into lower Earth orbit, according to NASA.

Heavy-lift launchers are usually considered crucial for doing more ambitious space missions

Heavy-lift launchers are usually considered crucial for doing more ambitious space missions, such as sending humans into deep space or to the surface of another world. A lot of heavy cargo will be needed to do a Mars mission or to start a lunar base: people will have to ride inside a spacecraft that can keep them alive, landers will be needed to take passengers and cargo to the surface of another planet, and a propulsion system will be needed to move all of this hardware through space. Throw in habitats and food supplies, and you’ve got a lot of stuff you need to break free of Earth’s gravity.

All of these pieces may weigh a lot combined, but no one says they have to be launched together. In fact, to do any of these complex missions in space, it’s almost certain that multiple launches will be needed. That’s because there simply isn’t anything big enough to take all the pieces together at once. Take NASA’s Saturn V rocket, which launched the Apollo missions to the Moon in the 1960s and ‘70s. The Saturn V is still considered perhaps the most powerful rocket ever made, capable of getting more than 260,000 pounds to lower Earth orbit. And in the end, it was still only able to get about 100,000 pounds to the Moon’s surface, according to NASA.

The main problem revolves around the fuel, or propellant, you need for the trip. Earth has a pretty sizable gravity well, so a vehicle has to burn lots of propellant to reach a speed of thousands of miles per hour needed to break free of the planet’s pull. For particularly heavy cargo or human crews going into deep space, even more propellant is needed; the engine has to burn longer to compensate for the extra weight and distance. Propellant is heavy and takes up a lot of space — that’s why you need a huge rocket to house it all. And the more propellant you add, the more propellant you need to lift it all.

no ambitious mission to Mars or colony on the Moon is going to be done in one launch alone

If you plan on taking all your propellant with you for a deep-space mission, then you definitely need a huge rocket to house it all. But there’s also the option of “going up dry” — just bringing the fuel needed for launching to orbit and having empty tanks when you get there. Then a separate rocket can be sent up to refuel your vehicle. SpaceX CEO Elon Musk, who’s had his eye on a Mars colony for years, doesn’t expect to send everything to the planet’s surface in one flight. His proposed Interplanetary Transport System, which is supposed to take up to 100 colonists to and from the Martian surface, is designed to launch into lower Earth orbit and then wait to get fueled up by another vehicle launched separately. There’s also the option of setting up fuel outposts in space — think “space gas stations” — where vehicles can make pit stops on their way from Earth. “You might fill it up in lower Earth orbit and go to lunar orbit and fill it up there, if you do it that way,” Charles Miller, president of NexGen Space LLC, a space consulting firm, and a former member of the Trump administration’s NASA transition team, tells The Verge. “You minimize the size of the hardware you need.”

These are just a few ways to get complex hardware into space, but one thing remains the same: no ambitious mission to Mars or colony on the Moon is going to be done in one launch alone. And that means you don’t necessarily need a monster rocket to pull such a mission off.

How to measure a rocket

Heavy-lift vehicles do have some benefits, and that’s why they’re still favored for crewed missions into deep space. Generally it's good to get as much up in one launch as you can. That way you save money on doing additional launches — which are easily hundreds of millions of dollars and represent a significant chunk of a mission’s overall budget. “There’s a thing called the tyranny of small launch vehicle economics,” says Miller. “A larger launch vehicle may have a larger price, but the dollars per pound delivered to Earth orbit is lower cost.”

Plus, if you send up different parts on separate rockets, they will still need to be assembled together in space — and that’s expensive. This in-space assembly has to be done either robotically or by people on spacewalks, which is much more costly and more difficult than manufacturing and assembling space vehicles on the ground. So in some aspects, a rocket that can lift the most weight can save on costs. But this economic benefit depends on two key things: how often the vehicle flies and how much it costs to launch just one vehicle.

And this is where the SLS fails. By the mid-2020s, NASA only expects to launch its upcoming vehicle once a year — maybe twice at best. And each launch is expected to cost about $1 billion, according to NASA’s Bill Gerstenmaier. Compare that to the other heavy-lift rockets that are expected to be available by the end of the decade. SpaceX’s Falcon Heavy will be able to carry more than 100,000 pounds to lower Earth orbit. That’s about half of the capability of SLS, but SpaceX says the price tag per flight will start at only $90 million. It’s possible that the price may increase somewhat depending on each mission, but if the vehicle does just three flights a year, it can get more into space than the SLS for a fraction of the cost. Meanwhile, Blue Origin’s New Glenn and a heavy-lift version of ULA’s Vulcan are expected to lift about the same. ULA and Blue Origin haven’t said what it will cost to launch these vehicles. But even if the rockets each cost $500 million to launch and take off at least twice a year, they have already matched the SLS for a similar or lower cost.

And experts say there is incentive to keep the launch costs down, since these commercial companies are also driven by profit. SpaceX, for instance, has been working to slash launch costs by saving parts of its rockets after each launch to be used in future missions. Blue Origin and ULA also plan to make their rockets partially reusable as a cost-saving measure.

“These private sector options, just by their nature, will be more cost-effective than SLS.”

“The question is cost per pound to lower Earth orbit, cost per pound to the Moon, cost per pound to Mars, and I think it’s indisputable that these private sector options, just by their nature, will be more cost-effective than SLS,” says Larson. “[It’s for] a number of reasons: flight rate, procurement method, and just different ways of doing business.”

The only reason a rocket the size of the SLS would be absolutely needed is if NASA had something huge that could only be launched up in one piece by the rocket. It’s a problem that a few space experts refer to as the “tyranny of the fairing” — referring to the nose cone at the top of a rocket. The size of the nose cone ultimately limits the size of what you can launch into space. And if what you want to launch is particularly large and can be sent only in one piece, then a larger rocket is the way to go.

But right now, there’s nothing that NASA wants to send into space that only the SLS is capable of launching. “To my knowledge there is not yet a single piece of anything that’s been proposed to launch by SLS that’s more than the capacity of New Glenn or Falcon Heavy,” Jim Muncy, founder of PoliSpace, a space policy consulting agency, tells The Verge.

Pros versus cons

If NASA hopes to achieve its ambitions of going to deep space and onto Mars, the agency is going to have to find ways to cut costs. One of NASA’s acting chief scientists warned that the agency’s budget isn’t expected to increase, not even to keep up with inflation, for the next five years, Space News reported. Meanwhile, NASA is looking for other ways to cut costs, such as retiring the International Space Station, in order to free up the funds needed to make other vehicles for deep-space missions — like habitats and interplanetary transport vehicles. But rather than throw away expensive investments, another option could include using more commercial launches in NASA’s long-term plans, and relying less on SLS. “Ultimately the question is money,” says Muncy. “Can we afford to launch people to Mars or not? And if more of the launches can be done on smaller, cheaper launch vehicles, obviously we should do that to save the money and actually enable more people to go to Mars.”

Of course, there are problems with having too many launches for a mission. More in-space assembly of parts adds complexity to an already complex task. Plus it creates more opportunity for a launch to go wrong, and one flight’s failure can throw off the entire operation. Of course there are benefits to this model, too. For instance, the more a vehicle flies, the more reliable it becomes. And the more frequently rockets fly, the faster you can get hardware into space.

Yet even NASA doesn’t expect to do a Mars mission with just one launch of the SLS. Last month, Gerstenmaier presented a tentative outline of all the launches the space agency hopes to do before sending people to Mars. The plan involves building an outpost around the Moon, where future astronauts can live and train for deep-space missions. Eventually, astronauts will leave from this site to go onto Mars. But in order to build this station near the Moon, pieces of it will have to launch on at least four flights of the SLS, and then even more launches will be needed to deliver the transport vehicle that will take astronauts to the Red Planet.

break those launches up even further and use cheaper rockets

But for NASA, a more efficient solution may reveal itself in the coming years: break those launches up even further and use cheaper rockets. “As the private industry shows itself to be more and more capable, it may become obvious that NASA... will save so much money launching [hardware] in smaller pieces and putting everything together in space,” says Muncy.