Oct. 15 (UPI) -- SpaceX on Wednesday launched 21 satellites from California for the U.S. military's Space Development Agency.

The Falcon 9 rocket lifted off at 4:06 p.m. PDT from Vandenberg Space Force Base's Pad 4E. About 8 1/2 minutes later, the first stage landed on the drone ship Of Course I Still Love You in the Pacific Ocean. This booster was used seven times previously, including the first Tranche 1 Transport launch on Sept. 10 with 21 satellites.

The satellites are part of the SDA's Tranche 1 Transport Layer of eventually 126 oritals in low-Earth orbit that "will provide global communications access and deliver persistent regional encrypted connectivity in support of warfighter missions around the globe," according to SDA.

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The network will use high-speed optical and radio frequency links on board the Transport SVs "to achieve high data rates at extremely low latencies."

"The Transport Layer will be a proliferated constellation of hundreds of space vehicles (SVs), in low-Earth orbit (LEO)," the agency said. "Additional Transport Layer SVs are deployed to add resilience and targeted technological enhancements."

The Trench 2 is scheduled to begin launches in fiscal year 2027 to "both replenish and enhance the capability provided by Tranche 1 by increasing the overall number of SVs and by introducing new capabilities," including new tactical data links and new waveforms, it said.

The next launch from Vandenberg is scheduled for Friday night of 28 Starlink satellites.

SpaceX also plans to launch 28 Starlink satellites at 5:27 a.m. EDT Thursday from Cape Canaveral Space Force Station's Pad 40 in Florida.

The private company's last launch from Florida was Monday night of 24 Project Kuiper satellites for Amazon's broadcast Internet. The Falcon 9 lifted off at 11:27 p.m. from Cape Canaveral Space Force Station.