April 9 (Reuters) - The U.S. Space Force and a

Boeing-Lockheed joint venture sent a secret reconnaissance

payload to orbit on Tuesday atop a Delta IV Heavy rocket, the

last flight of a workhorse launch vehicle brand that has logged

nearly 400 missions dating back to 1960.

The United Launch Alliance-owned rocket, standing roughly 23

stories tall, blasted off from Cape Canaveral Space Force

Station in Florida around 1 p.m. EDT (1700 GMT), 12 days after a

previous launch attempt was scrubbed at the last minute due to a

technical glitch.

A live ULA webcast showed the rocket ascending from the

launch tower through partly cloudy skies in a thunder of flames

and billowing clouds of exhaust and water vapor.

The flight was intended to deploy a satellite for the

National Reconnaissance Office (NRO), a U.S. defense

intelligence agency, on a classified mission designated as

NROL-70. The Space Force field command confirmed hours later

that the launch had successfully delivered the payload to orbit.

It marked the 16th and final flight of a Delta IV Heavy and

the last of any of the Delta family of rockets, a space launch

dynasty that originated from a modified intermediate-range

ballistic missile and grew to include about two dozen

increasingly powerful variants.

Since the Thor-Delta rocket was introduced in 1960 at the

dawn of the Space Age, the Delta brand has logged 389 launches,

with payloads ranging from the world's first weather and GPS

satellites to NASA science missions including eight spacecraft

to Mars.

The world's first passive communications satellite, Echo 1A,

and first active communications satellite, Telstar 1, which

enabled transatlantic television transmission, were launched by

Thor-Deltas in 1960 and 1962, respectively.

Among other space science missions, Delta II rockets

launched the twin Mars rovers Spirit and Opportunity in 2003,

and a Delta IV sent the Parker Solar Probe to space in 2018.

"The Delta rocket played a pivotal role in the evolution of

space flight since the 1960s," said Tony Bruno, ULA's president

and chief executive officer.

ULA, a partnership of aerospace giants Boeing ( BA ) and

Lockheed Martin ( LMT ), is retiring Delta and Atlas rockets in

favor of its newly developed Vulcan rocket, which made its

inaugural flight in January carrying a privately funded moon

lander.

The payload malfunctioned before reaching the moon, but the

Vulcan launch from Florida was a success. The Atlas V had 17

more missions booked before it was due to go out of service.

The Delta IV rocket, weighing 1.6 million pounds (725,748

kg) when fully fueled, consists of a triple-booster lower stage

that produces 2 million pounds of thrust at launch, and a

single-engine upper stage that carries the vehicle's payload to

orbit.

About four minutes in to Tuesday's flight, after reaching

speeds 15 times faster than sound travels, the two side boosters

of the rocket's lower stage separated and fell away, followed by

separation and ignition of the upper stage two minutes later.

Around 6-1/2 minutes after launch, the cargo panels

protecting the NROL-70 payload during its ascent were jettisoned

as the rocket's upper stage climbed above the edge of space.

Live video coverage of the flight was cut off at that point at

the government's request, ULA said.

The precise nature and purpose of the NROL-70 mission has

been kept secret.

In a vaguely worded statement before launch, the government

said the mission would "strengthen the NRO's ability to provide

a wide range of timely intelligence information to national

decision makers, warfighters and intelligence analysts."