The first three missions of NASA’s next-generation rocket, the Space Launch System, will all fly on the least powerful version of the vehicle that the space agency plans to build. NASA is moving forward with its plan to use a downgraded version of the SLS for its second and third flights, according to a memo from NASA headquarters obtained by The Verge. The original plan was to fly those two flights on a much more powerful upgrade of the rocket, but now, it seems that version won’t debut until 2024 at the earliest.

The SLS, meant to take humans into deep space, has been under development for the last decade, with its first three missions mostly set in stone. For its debut flight, called EM-1 and scheduled for 2020, the rocket will send an empty crew capsule called Orion on a three-week voyage around the Moon. Then a few years later, NASA plans to put a crew onboard: a second mission, called EM-2, will send two astronauts on a three-week-long trip around the Moon. Around that same time, NASA plans to use the SLS to launch a robotic spacecraft to fly by Jupiter’s icy moon Europa, called the Europa Clipper mission.

A memo directs the space agency’s contractors to start planning for the change

However, these three missions weren’t all supposed to fly on the same version of the SLS. NASA is planning to make two main variants of the vehicle: Block 1 and Block 1B. Block 1 is the less powerful form of the rocket, capable of getting 209,000 pounds (95 metric tons) to low Earth orbit. Block 1B is designed with a much more powerful upper stage, allowing it to carry about 287,000 pounds (130 metric tons). NASA’s plan was to fly Block 1 just once for the first SLS flight, and then fly Block 1B. But now NASA is going to fly all three missions — EM-1, EM-2, and Europa Clipper — on Block 1. The memo, signed by Bill Hill, NASA’s deputy associate administrator for exploration systems development, directs the space agency’s contractors to start planning for the change. A NASA spokesperson confirmed this change to The Verge.

In April, former NASA administrator Robert Lightfoot said that NASA might make this change — thanks to an unexpected influx of cash it received from Congress in March. The finalized spending bill for fiscal year 2018 gave NASA an extra $350 million to build a second launch platform for the SLS. And that gave NASA more options for how to move forward with the first SLS flights.

Right now, NASA only has one mobile launch platform — a slow-moving structure designed to carry the rocket to the launch pad where it’ll then take off. However, this one platform can only support flights of the Block 1 SLS at the moment. The platform would need significant upgrades to support the bigger and heavier Block 1B. And all that refurbishment would take time — at least 33 months to complete. During that time, nothing can take off from the platform. So under the original plan, NASA would first launch the inaugural mission of SLS on a Block 1 and then cease all flights of the rocket for nearly three years, while it upgraded the mobile launch platform to support the Block 1B. That meant the second flight of the SLS would be held hostage by however long those upgrades took.

To avoid this mess, Congress decided to give NASA the money to build another mobile launch platform, one specifically designed to fly the bulkier, more powerful Block 1B. This way, NASA could get started building the new platform now, to have it ready for the second flight of the SLS. But ironically, NASA is using this new money to shake its rockets up. Now the space agency is going to build the second, more robust platform while launching multiple Block 1s on the existing platform in the meantime. According to the memo, NASA will aim to have the second platform ready for a Block 1B launch in the beginning of 2024.

What’s still undecided is which mission will fly first — the Europa Clipper mission or the first crewed flight of Orion. Both are planned to occur around the same time, but Europa Clipper could fly before EM-2. It all depends on which one is ready to go the earliest, according to the NASA memos obtained by The Verge. NASA has set the date for this second flight to occur in mid-2022.

NASA will aim to have the second platform ready for a Block 1B launch in the beginning of 2024

The decision to fly multiple Block 1s may have to do with the fact that the powerful upper stage needed for the Block 1B, called the Exploration Upper Stage, is going to cost way more than originally planned, according to Ars Technica. Plus, the new stage is being designed from scratch, so it’s likely going to take many years to have the hardware ready for a Block 1B flight. Meanwhile, the first Block 1 flight is scheduled to happen around 2020. If it proves its chops by then, it’ll be easier and quicker to simply launch more Block 1 rockets over and over again. So this decision could mean the SLS will fly more rapidly in the future, instead of NASA waiting many years for the Block 1B to materialize.

Flying on these less powerful Block 1s will change the mission profile of at least the crewed flight of the SLS. It won’t be able to carry extra payloads as NASA had originally planned, but it will still get the astronauts around the Moon. It’s unclear exactly how the Europa Clipper’s mission will change; however, the Block 1 is still capable of sending the spacecraft on a direct path to Jupiter. NASA says that Europa Clipper has the opportunity to launch each year from 2022 to 2025. Other similar rockets, such as SpaceX’s Falcon Heavy, don’t have as much energy to do a straight shot and would need a gravity boost from another planet, according to Barry Goldstein, the project manager for Europa Clipper, as Space News reported.

Still, the fact that we won’t be seeing the Block 1B for a while doesn’t bode well for the SLS program. Critics of the rocket say it’s too costly to build and fly, especially when there are comparable vehicles like the Falcon Heavy, which can put similar amounts of weight into low Earth orbit and is cheaper to launch. NASA and Boeing, the manufacturer of the SLS, claim that the space agency needs the SLS because it’s so much more powerful than anything else on the market. But the longer the Block 1B takes to build, the harder it will be to make that argument.

Correction May 23rd, 1:30PM ET: A previous version of this article stated that Block 1 can carry 70 metric tons to low Earth orbit, but it is now designed to carry 95 metric tons.