SpaceX got the OK from the Federal Aviation Administration and looked to launch what would have been the most powerful rocket to ever blast off from Earth on a suborbital test flight today, but pressure issues scrubbed the attempt.

The combined Starship and Super Heavy rocket was set to fly from SpaceX’s launch facility in Boca Chica, Texas, with capacity to generate more than 17 million pounds of thrust on liftoff.

Propellant loading began before 8 a.m. with an announced initial target launch time of 9:20 a.m. While weather was a go, a booster issue arose with less than 20 minutes before the planned liftoff.

The flight director announced the scrub of the launch attempt, but the countdown continued until T-40 seconds with the company treating the day as a wet dress rehearsal. Potential retries won’t come until at least Wednesday because of the time it takes to recycle the massive rocket’s propellants.

“On the one hand it’s a little bittersweet,” said SpaceX principal integration engineer and commentator John Insprucker. “On the other hand, we watched them go all the way through first stage propellant load, just now wrapping up the second stage ... Everything was looking good. Sadly, we just have the one issue.”

SpaceX CEO Elon Musk said on Twitter a valve appeared to frozen.

“Learned a lot today, now offloading propellant, retrying in a few days,” he posted.

The FAA on Friday gave Elon Musk’s company the OK for the flight after several years of assessment.

“After a comprehensive license evaluation process, the FAA determined SpaceX met all safety, environmental, policy, payload, airspace integration and financial responsibility requirements. The license is valid for five years,” the FAA made in a statement. “We carefully analyzed the public safety risks during every stage of the mission and required SpaceX to mitigate those risks.”

The mission aims to fly the combination of Starship 24, as in the 24th prototype of the company’s next-generation spacecraft, and the Super Heavy 7 booster, which have been on the pad since early April.

At launch, Super Heavy 7′s thrust comes from 33 Raptor engines with a goal to send the Starship up over the Gulf of Mexico before falling away for a hard splashdown without a signature booster recovery for this test flight. The Starship spacecraft with six of its own Raptor engines will then attempt to continue its suborbital flight making it into space at altitudes between 93 and 155 miles, then making it 2/3 the way around the Earth before reentering the atmosphere for its own had splashdown landing near Hawaii.

The Starship spacecraft is made of stainless steel and covered with 18,000 hexagonal-shaped heat tiles to protect reentry that can reach 2,600 degrees Fahrenheit.

Starship and booster fuel is different than the Falcon 9 and Falcon Heavy using a liquid methane propellant mixed with liquid oxygen. On liftoff, the booster will have 10 million pounds of propellant loaded, which is 10 times the fuel of a Falcon 9.

“Today’s tests will be the first of many as we work toward transitioning Starship from a developmental to an operational program,” said SpaceX operations engineer Siva Bharadvaj during coverage of the attempt. “Our primary objective today is to gather as much data as we can around the fully integrated vehicles — that means the booster and the Starship and the ground systems.”

A full test flight could have lasted around 90 minutes.

“Whether we end the test with a hard landing or a rapid unscheduled disassembly, or some combination of both, excitement is pretty much guaranteed,” said SpaceX quality systems engineer and commentator Kate Tice.

Even if it had simply made it off the ground, it would have become the most powerful rocket launch ever attempted, breaking the 10.2 million pounds of thrust generated during four attempts by the Soviet Union of its N-1 rocket from 1969-1972. Those rockets all suffered failure midflight and never made it to space.

NASA’s Space Launch System rocket that flew on Artemis I last November is the current title holder for rocket power to make it to space when it sent the Orion capsule on its lunar mission through the heft of 8.8 million pounds of thrust. The SLS bested the power of the Saturn V rockets during the Apollo era.

The goal is to eventually be able to recover both booster and Starship with vertical landings as it does with its current stable of Falcon 9 and Falcon Heavy boosters. But for Starship, the company recovers both booster and spacecraft.

“Starship is a fully reusable transportation system designed to carry both crew and cargo to Earth orbit, help humanity return to the moon, and travel to Mars and beyond,” the company posted on its website. “With a test such as this, success is measured by how much we can learn, which will inform and improve the probability of success in the future as SpaceX rapidly advances development of Starship.”

NASA has high hopes for the program’s success as it awaits a version of Starship to act as the Human Landing System on the Artemis III mission to the moon that will bring back humans, including the first woman, to the lunar surface for the first time since 1972.

That mission is slated for no earlier than December 2025, and Starship has to make a test landing on the moon one time before it flies with NASA astronauts on board.

Starship has its own spate of human flights for Starship planned as well, but first it has to get what’s predicted to be hundred of flights under its belt flying payloads such as its Starlink internet satellites.

That starts with this combined booster and spacecraft flight,. the first of what the company has said will be a steady lineup of test flights to get the launch system ready for commercial use.

So far the company has only flown test versions of Starship without the booster to low altitudes.

“These flight tests helped validate the vehicle’s design, proving Starship can fly through the subsonic phase of entry before re-lighting its engines and flipping itself to a vertical configuration for landing.” the company stated.