WASHINGTON — SpaceX is set to perform the third flight of its Falcon Heavy rocket June 24, a mission that will effectively be a test flight demonstrating the capabilities of the vehicle for the U.S. Air Force.

The Falcon Heavy is scheduled to lift off from Kennedy Space Center’s Launch Complex 39A at 11:30 p.m. Eastern, at the beginning of an unusually long four-hour launch window. Forecasts are favorable for the launch, with a 70 percent chance of acceptable weather.

The rocket is flying a mission designated Space Test Program 2 (STP-2) for the Air Force. Its payload includes two dozen satellites, primarily technology demonstrations for the Air Force, NASA, NOAA and other customers, with a total mass, including various adapters, of 3,700 kilograms.

The mission is rather complex. The Falcon Heavy’s second stage will make four burns to place spacecraft into three different orbits. The first spacecraft will be released from the upper stage less than 13 minutes after liftoff, but the last one won’t be deployed until more than three and a half hours after liftoff.

“STP-2 is a remarkable achievement for the entire team,” said Air Force Col. Tim Sejba, director of the innovation and prototyping directorate at the Air Force’s Space and Missile Systems Center (SMC), in a June 20 statement. “In one launch, we’ll deliver 24 spacecraft to a variety of orbits. Each of these missions will advance civil and military objectives by demonstrating next generation space technologies.”

“This will be our most difficult launch ever,” tweeted Elon Musk, chief executive of SpaceX, June 19.

The purpose of the STP-2 mission is less to place the satellites — none of which have critical national security roles — into orbit than to demonstrate the performance of the Falcon Heavy. That includes the multi-burn profile for the rocket’s upper stage as well as the reuse of side boosters that first flew on the previously Falcon Heavy mission, launching the Arabsat-6A satellite in April.

“The STP-2 multi-manifest (rideshare) launch will demonstrate the capabilities of the SpaceX Falcon Heavy launch vehicle and provide critical data supporting certification for future National Security Space Launch (NSSL) missions,” SpaceX notes on a dedicated website for the mission. “In addition, SMC will use this mission as a pathfinder for the development of mission assurance policies and procedures related to the reuse of launch vehicle boosters.”

The launch will be the third for the Falcon Heavy, developed by SpaceX for payloads too large to be accommodated on its Falcon 9 rocket. Prior to the Arabsat-6A mission two and a half months ago, the rocket made its debut in February 2018, launching a Tesla Roadster sports car into a heliocentric orbit.

While the launch is principally a demonstration of the vehicle’s capabilities for the Air Force, it is highly anticipated by the operators of the satellites on board, who have been waiting patiently while its launch was postponed by years because of delays in the development of the Falcon Heavy. The Air Force procured the STP-2 launch from SpaceX in 2012 for a launch originally scheduled for 2015.

The STP-2 payload includes six COSMIC-2 satellites, developed by NOAA and Taiwan’s National Space Organization to collect GPS radio occultation data for weather forecasting. The mission also carries several NASA technology demonstrations, including a satellite intended to test a non-toxic “green” propellant and a payload on another satellite that will demonstrate an atomic clock to aid in deep space navigation.

One of the smaller, yet high-profile, satellites on STP-2 is LightSail 2, a cubesat-class satellite developed by The Planetary Society to demonstrate solar sail technology. LightSail 2 is contained within another satellite on the mission, Prox-1, and will be deployed about a week after launch into a 720-kilometer orbit.

LightSail 2 will deploy a solar sail that, using force exerted on it by sunlight, will allow the spacecraft to change its orbit. The organization flew a similar spacecraft, LightSail 1, in 2015 that tested the deployment of the sail but did not do any maneuvering.

“Through solar sailing, we should be able to increase our orbit altitude on the order of a half kilometer per day,” David Spencer, project manager for the mission and a Purdue University professor, said at a June 20 media teleconference about the mission. “If we get a measurable increase in altitude on a regular basis and see the apogee, the far point in the orbit, increasing over time, that’s a win for us.”

The Planetary Society, which used donations to fund the $7 million mission, wants to demonstrate the viability of the technology for use on future exploration missions, although the organization said it had no plans for a follow-on mission of its own.

“We’re going to see what really happens as we try to increase orbital altitude,” Bill Nye, chief executive of The Planetary Society, said at the teleconference. “We’ve done all sorts of mathematical models, but we’re going to see how it really works.”