ULA Delta IV Heavy successfully launches NROL-44

by William Graham

In August 2020, the U.S. Space Force announced that Vulcan was chosen to receive contracts under NSSL Phase 2, the replacement for the current NSSL program, which begins in 2022. Vulcan was awarded 60% of the contracts while SpaceX will carry out the remaining 40% of the launches with their Falcon 9 and Falcon Heavy rockets.

Vulcan is currently expected to make its first flight in 2021, carrying out the first of two certification missions before being cleared to carry out critical launches for the Space Force.

ULA included the Atlas V as a backup vehicle for those NSSL Phase 2 missions should Vulcan encounter an issue and not receive certification in time for them.

Like the Atlas V, the Delta IV was designed as a modular rocket, capable of flying in different configurations depending on mission requirements. This Delta IV design centered around the Common Booster Core (CBC), which serves as the first stage of the rocket. Powered by a single RS-68A engine – originally an RS-68 – it is fuelled by liquid hydrogen and liquid oxygen.

The rocket’s smallest configuration, the Delta IV Medium, consisted of a single CBC, with a four-meter diameter Delta Cryogenic Second Stage (DCSS) atop it. The DCSS, powered by an RL10B-2 engine, burns the same cryogenic propellants as the CBC.

Several intermediate “Medium+” configurations added two or four solid rocket boosters to the first stage and optionally replaced the upper stage with a five-meter DCSS – and a corresponding enlarged payload fairing. It was a Medium+ configuration that made the Delta IV’s first launch in November 2002, carrying the Eutelsat W5 (later Eutelsat 33B) communications satellite into orbit.

The Delta IV Medium made three launches between 2003 and 2006, but was subsequently discontinued. Delta IV Medium+ launches continued until August 2019.

The first Delta IV Heavy flew on 21 December 2004, carrying out a demonstration mission for the U.S. Air Force. This configuration uses three Common Booster Cores burning together at liftoff, with the two cores strapped to the sides of the rocket separating ahead of the center core. A five-meter DCSS upper stage completes the stack and is capable of making multiple burns to inject its payload directly into geostationary orbit if required.

A Delta IV Heavy lifts slowly from SLC-37B to being the launch sequence of NASA’s Parker Solar Probe, August 2018. (Credit: Nathan Barker for NSF/L2)

During the 2004 demo mission, cavitation (small vapor-filled cavities) in the rocket’s fuel lines as the CBCs began to run out of propellant led to the premature cutoff of their engines. The upper stage was still able to reach orbit, although a significantly lower one than had been planned.

This remains Delta IV’s only launch failure to date. Three years later, the next Delta IV Heavy successfully orbited the DSP-23 missile detection satellite. The Heavy has primarily been used for military launches, although it has also carried out two key missions for NASA – with the EFT-1 test flight of NASA’s Orion spacecraft in 2014 and deployment of the Parker Solar Probe in 2018.

NROL-44 was the 41st flight of Delta IV and the 12th of the Heavy configuration.  It used vehicle Delta 385. The launch took place from Space Launch Complex 37B (SLC-37B) at the Cape Canaveral Air Force Station.

SLC-37B is the East Coast home of the Delta IV, serving alongside the rocket’s West Coast pad – Space Launch Complex 6 (SLC-6) at Vandenberg Air Force Base. SLC-37B was built on the site of the Apollo-era Launch Complex 37B (LC-37B), which was used in the mid-1960s for uncrewed orbital test flights of the Saturn I and IB rockets – culminating in 1968’s Apollo 5 mission which tested the Lunar Module in low Earth orbit.

Complex 37 originally consisted of two pads – 37A and B, sharing a common Mobile Service Tower (MST) – although LC-37A was never used for a launch. Following Apollo 5, the complex was mothballed ahead of an expected role in the Apollo Applications program after the race to the Moon had been won. After this project was scaled back, the complex was demolished and abandoned until the 1990s.

After arriving at Cape Canaveral aboard ULA’s Rocketship – formerly the MV Delta Mariner – Delta IV was taken to the Horizontal Integration Facility at SLC-37 to begin processing for its launch. Here, the three Common Booster Cores and the DCSS were mated before the combined vehicle was transported to the launch pad and raised into place. The NROL-44 payload, encapsulated in its payload fairing, was mated to the rocket vertically using the pad’s MST.

The Delta IV Heavy for NROL-44 is rolled out to SLC-37B in November 2019. (Credit: ULA)

Five seconds before the scheduled launch, Delta’s three RS-68A engines ignited. At this point, a fireball formed around the base of the rocket. This is caused by the engines igniting residual hydrogen that has boiled off from the rocket. The process is well understood and harmless but has charred or set fire to the insulation on several previous flights.

Once the three engines have built up to full thrust, Delta IV Heavy lifted off to begin its mission. Liftoff occurred at the T-0 mark in the countdown when the thrust the rocket’s engines are generating exceeds the weight of the vehicle. For the first 9.4 seconds of flight, Delta climbed straight up, before initiating a pitch and yaw maneuver to place it on an easterly trajectory for the climb into orbit.

Shortly after this, the center core throttled down into partial-thrust mode, limiting loads on the vehicle early in the mission and conserving fuel so it can continue to burn after the side boosters separated.

One minute and 18.4 seconds into the mission, Delta reached Mach 1, the speed of sound. A second and a half later it passed through the area of maximum dynamic pressure – Max-Q – where it experienced peak mechanical stress from aerodynamic forces.

Three minutes and 56 minutes after liftoff, the two side boosters engines shut down, with the spent CBCs separating from the vehicle two seconds later. Around this time, the center core throttled back up to full thrust as it continues the boost phase of the mission. Its role in the flight ended with Booster Engine Cutoff, or BECO, at five minutes, 42.8 seconds mission elapsed time.

About six and a half seconds after BECO, the first and second stages separated, with the final CBC falling away from the rocket. The second stage RL10B-2 engine extended its deployable nozzle and initiate its pre-start sequence – with ignition coming 13 seconds after stage separation.

About 42 seconds after the second stage ignites, Delta’s payload fairing separated. The fairing is the nose cone of the rocket which protects the payload during ascent through Earth’s atmosphere and gives the rocket a consistent aerodynamic profile. Once the rocket reaches space, the fairing is no longer needed and can be discarded to reduce mass.

There are two different payload fairings that can be used on the Delta IV Heavy – a composite fairing which was designed for the Delta, and a metallic fairing made of aluminum which was inherited from the Titan IV. The launch will use the latter, which measures 19.8 meters (65 feet) in length. This is a trisector fairing, meaning that when it falls away from the rocket it separates into three segments, not two as with most contemporary fairings. The metallic fairing was first used on Delta IV for the DSP-23 launch in 2007, and has subsequently been used for all of the NRO’s geostationary launches on the Heavy.

Northrop Grumman builds the payload fairings for the Delta IV as well as all composite structures and the RS-68A engine nozzles on the Delta IV.  In short, all the white parts visible on the rocket are built by Northrop Grumman.

With fairing separation, the mission will enter a media blackout – as is typical for NRO missions. The only likely official updates after this point will be a confirmation of mission success once the NROL-44 payload has separated from Delta IV Heavy. Given that the launch is targeting a geostationary orbit, this will not occur until six or seven hours after liftoff.

In this time, the DCSS upper stage can be expected to perform three burns. The first, which began after separation from the first stage, will continue for about seven minutes. This will establish the upper stage and its payload in their initial parking orbit. Based on the published flight profile of Delta IV Heavy’s initial demo mission – which was rumored to be simulating deployment of an Orion satellite – after coasting for a little under eight minutes, the rocket will fire its RL10 engine again for another eight-minute burn.

Now in geostationary transfer orbit, DCSS will coast for about five hours before commencing its final burn. This will last for about 3 minutes and 15 seconds, increasing the orbit’s perigee and decreasing its inclination to deploy its cargo directly into a circular geostationary orbit. Following spacecraft separation, the DCSS will perform a collision avoidance maneuver to take itself out of the geostationary belt and minimize the risks of a future collision with a satellite.

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