Tomorrow morning, in a corner of Utah wedged between the Great Salt Lake and Idaho border, NASA and Orbital ATK are test-firing a solid rocket booster. The 54-meter rocket—about the length of 12 sedans—will burn for two minutes, torching a nearby hillside. This is the first of two flight-like qualification tests to validate the booster’s performance before its brethren are attached to NASA’s new rocket, the Space Launch System.

It’ll be a good show. But the better display of smoke and fire will come during the rocket’s first flight, currently scheduled for 2018.

A few seconds before liftoff, four liquid-fueled RS-25 engines—best known for their decades of space shuttle service—will crackle to life. During the shuttle program, this caused the orbiter, boosters and external fuel tank to sway like a Florida palm tree in a motion known as the twang. The shuttle stack twanged because its unique design offset the orbiter’s liquid engines from the vehicle’s center of gravity.

SLS won’t twang. But just like the shuttle, the vehicle's onboard computers will make sure the liquid engines are all performing well before the solid rocket boosters ignite. You can’t turn off SRBs. Like backyard bottle rockets, they burn to depletion.

As it was for the shuttle, the aft SRB skirts will bear the entire weight of the launch vehicle. Each skirt slides over four posts to hold the vehicle steady. To keep the shuttle steady while it twanged, these posts used to contain explosive bolts that fired when the vehicle pitched back to center—right at the moment of SRB ignition. For SLS, there are no bolts. Even under the thrust of four mighty RS-25 engines, SLS won’t budge an inch.

But when the solids ignite, look out. SLS will leap through its posts and rocket into the sky, with its two boosters providing 75 percent of the vehicle’s liftoff thrust. One of these boosters now sits on its side in Promontory, Utah, held in place by a block of concrete for tomorrow’s test.