Within the next few days, the European Space Agency plans to fire up the current largest solid rocket motor built in one segment for the first time. The motor, called the P120C, is a key piece of hardware that ESA plans to use in its two next-generation rockets: the Vega-C and Ariane 6. This test-fire is a crucial step to see if the motor will be ready for spaceflight in the next couple of years.

The test is scheduled to take place at ESA’s South American spaceport, located in Kourou, French Guiana. There, the P120C will be held down on a test stand, and the motor will be fired for just over two minutes. The P120C needs a large test stand, too. The motor measures more than four stories tall and 11 feet wide.

this test-fire is a crucial step to see if the motor will be ready for spaceflight in the next couple of years

Solid rocket motors like the P120C are a relatively rare breed these days. Most major rockets rely on engines that run on cold liquid propellants, like cryogenic oxygen and hydrogen. These engines operate by feeding two types of liquids — a fuel and an oxidizer — into a chamber where they combust. This method makes the engines particularly efficient, meaning they use fuel more effectively to create thrust. Plus, you can manipulate the amount of thrust of a liquid engine, a concept known as throttling. It’s similar to how you change the acceleration of a car, depending on how you press down on the gas pedal. By changing the rate at which the liquids flow through the engine, you can hasten or slow a rocket’s acceleration. That’s useful for, say, landing a rocket back on Earth after launch.

However liquid engines have their downsides. Getting these propellants to combust in an engine requires a lot of complicated machinery, sensors, and valves, which make the vehicles complex and expensive. “Nothing is free, so this typically has to be paid off by extra costs, because it’s more complexity, more parts,” Nicola Ierardo, the Vega launcher stages engineering manager at ESA, tells The Verge. Meanwhile, storing propellants at such cold temperatures is difficult, and the liquids can get temperamental. For instance, the weird behavior of SpaceX’s liquid oxygen propellant led to one of the company’s Falcon 9 rockets exploding on a launchpad in 2016.

By comparison, solid rocket motors are much simpler than their liquid propellant counterparts. Instead of needing two liquids to mix inside a chamber, the propellants of a solid rocket motor are already mixed together in a big, solid chunk. When it comes time to launch, the solid propellants are ignited and simply burn away at a constant rate. That means these types of motors can’t be throttled, and there’s less flexibility in their design, but they don’t require as many valves and machinery, which makes them cheaper. And though they’re less efficient, solid rocket motors still pack a large amount of thrust, while being considered the safest kind of rockets around.

So if you need some cheap, reliable thrust, a solid rocket motor will do the trick. That’s why solid rocket motors are often used as add-ons to rockets, in order to provide extra power during takeoff. The ESA wants to use two P120Cs as strap-on boosters for the Ariane 6. However, the agency also wants the motor to serve as the main booster for the Vega-C.

“This is a result of different expertise all along Europe.”

Still, making a super large motor like the P120C in one piece is tough since large manufacturing plants and casings are needed to make all the large parts of the motor, and huge storage facilities are needed to house the solid propellants. “This is a result of different expertise all along Europe and the big facilities we have in Kourou for casting large quantities of solid rocket propellant,” says Ierardo.

If all goes according to plan with the firing, ESA will continue to test the motor as the agency works toward getting its next two rockets ready for flight. So far, the plan is to debut the Vega-C with its first flight in late 2019, and the Ariane 6 will follow sometime in 2020. This week’s test will help engineers know if they’re on track with those time frames. “This should prove several aspects of the design, including the performance, the thrust, the dynamic of the motor, and the mechanical integrity of the structure,” says Ierardo. “This is a very important test.”