Screenshot from ULA Webcast of NROL-91. Mikey. Close the Shed when you're done with the Torch
Mission Rundown: ULA - Delta IV Heavy - NROL-91
Written: December 7, 2022
Last one closes the giant shed
United Launch Alliance’s (ULA) Delta IV Heavy rocket made its last West Coast launch on Saturday, carrying out a mission for the National Reconnaissance Office, as Delta IV Heavy moves one flight closer to retirement. Liftoff of the NRO Launch 91 (NROL-91) mission from Space Launch Complex 6 — at the Vandenberg Space Force Base in California — took place at 3:25:30 PM PDT - 22:25:30 UTC.
NOTAM with three splashdown areas. Nearest is launch failure. Second is Delta side boosters. Third is the Delta core booster and fairing parts. DCSS splashdown area is the largest area
This was the 14th launch of a Delta IV Heavy from SLC-6 on September 24, 2022.
Saturday’s mission, NROL-91, is the final Delta IV launch from California’s Vandenberg Space Force Base, with the Delta IV Heavy’s remaining two missions to be executed from the East Coast at SLC-37 on Cape Canaveral Space Force Base.
The NROL-91 Payload
Due to the NRO being a government agency, there is no publicly available information regarding the parameters and function of the NROL-91 satellite. It is also difficult to speculate on the purposes, size, mass, and function of the satellite. The NRO has chosen ULA because of their successful and highly accurate launches for all customers.
Graphic speculation on what a KH-11 KENNEN spy satellite looks like. A small Huble telescope
Because of the chosen orbit, which is similar to the NROL-71 mission 3 years earlier, it’s speculated that NROL-91 is a Blok 5 KH-11 Kennen/Chrystal satellite now designated USA-338. With both NROL-91 and NROL-71 in similar orbits, it is speculated they are ‘sisterships’ who might be replacing an aging USA-xxx satellite launched on a previous NROL mission ten years earlier.
While the National Reconnaissance Office (NRO) keeps details of its satellites classified, the use of a Delta IV Heavy and the fact the launch is taking place from Vandenberg speak volumes. Delta IV Heavy missions carry satellites that have too great a mass for the most powerful Atlas V configurations to place into their destined orbits, indicating the satellite is very heavy, bound for a high orbit, or both.
From its location on the West Coast, Vandenberg is an ideal launch site for low Earth orbit (LEO) reconnaissance satellites operating in polar and near-polar orbits, as well as some signals intelligence satellites in elliptical orbits.
The Delta IV Heavy Launch
Saturday’s countdown saw the Delta IV rocket filled with cryogenic propellants while critical systems are powered up and tested as the count proceeds toward liftoff. The ignition sequence for the three RS-68A engines began seven seconds before liftoff with the starboard booster before the port and center cores ignited two seconds later. This staggered start helps mitigate the effects of hydrogen build-up around the base of the vehicle, which has scorched the rocket or set fire to insulation on previous missions.
Liftoff occurred at T0. After Delta IV cleared the tower, it began a series of pitch and yaw maneuvers to attain its planned orbit, with the first of these beginning about 10 seconds after liftoff. Flying downrange, Delta 387 throttled down its center core to its partial thrust setting. It passed through the area of maximum dynamic pressure, or Max-Q, 89.6 seconds into the mission and reached Mach 1, the speed of sound, about 1.4 seconds later.
With the side boosters firing at full thrust and the center core operating in partial thrust mode, the port and starboard cores depleted their propellant first. As they approached burnout, they began to throttle back before shutting down at the three-minute and 56.3-second mark in the mission. The spent boosters separated 2.2 seconds later, falling away from the center core as it throttled up to full thrust.
Booster Engine Cutoff (BECO), the end of first-stage flight, occurs five minutes and 37 seconds after liftoff. Six and a half seconds after BECO, the first stage separates and the DCSS begins preparations to ignite its RL10C-2-1 engine, including deployment of the extendible nozzle. RL10 ignition occurs under 13 seconds after stage separation. 10 seconds into the burn, Delta IV payload fairing separates, and the NROL-91 payload is exposed to space for the first time.
With fairing separation complete, NRO missions tend to enter a news blackout, with further mission details remaining classified other than a brief press release to confirm the successful deployment of the satellite.
The DCSS can be expected to continue firing its engine for about 12 minutes as it inserts the satellite directly into orbit. Spacecraft separation will occur shortly afterward, before the DCSS restarts its engine for a deorbit burn. DCSS will crash into the Pacific Ocean after less than one orbit. A short but sweet life of the Delta Cryogenic Second Stage.
The Delta IV Heavy rocket
Graphic showing ULA rocket Delta IV Heavy’s capacity to loft various payloads to different orbits
The Delta IV Heavy is a 725,7 ton reliable heavy lift launch vehicle, meaning that it can take bigger and heavier payloads into orbit. It can launch up to 28,000 kg (61,000 lbs) to a 90 degree - meaning compass course - inclination into Low Earth Orbit (LEO) and 14,000 kg (30,000 lbs) to a geostationary transfer orbit (GTO).
To accommodate payloads of all sizes, ULA offers two different payload fairing types with three heights both at 5 m (16 ft) in diameter. A 14 meter (47 ft) tall fairing and a 19.1 m (62.7 ft) tall fairing. As types go it's a three part fairing and a two part fairing.
The Delta IV Heavy first stage consists of three nearly identical 40.8 meter - 170 foot boosters strapped together. The Hydrogen and Oxygen tanks hold together 470 000 gallon of liquid propellant in 6 tanks measuring about 1 792 m3 in needed tank volume.
Each booster has one RS-68A engine also manufactured by Aerojet Rocketdyne. Together with DCSS and fairing they stand 71,6 meters - 235 feet tall on the launch pad.
The Hydrogen tanks hold 330 000 gallon of liquid Hydrogen chilled to -252,8 0C Celsius or -423 0F Fahrenheit in 3 tanks measuring about 1 254 m3 in estimated tank volume. The three Hydrogen tanks each hold at least 418 m3 cubic meter liquid Hydrogen.
The Oxygen tanks hold 120 000 gallon of liquid Oxygen chilled to below -182,96 0C Celsius or -297,33 0F Fahrenheit in 3 tanks measuring about 454,2 m3 in estimated tank volume. The three Oxygen tanks each hold at least 151,4 m3 cubic meter liquid Oxygen.
NROL-82 states that 120 000 gallons of liquid Oxygen is loaded. NROL-44 gave me these numbers. Second source found. Is it 470 000 gallon of liquid propellant with DCSS?
Using a calculator dividing 33 with 15 gets you a ratio of 2,75 in hydrogen to oxygen. Doing the same with DCSS gets you a ratio of 1,33. 13,750 gallon LH2 divided with 5,000 gallon LOX gives you the same ratio of 2,75. Ergo 10,000 gallon LH2 and 6,000 gallon LOX are both wrong numbers. That is if the limit to DCSS volume is 20,000 gallon liquid propellant.
The first stage is infamously known for lighting itself on fire just before launch to burn off extra hydrogen. It does this because it needs to get rid of any hydrogen so it does not explode unintentionally during liftoff.
The hydrogen comes from the purging or chilling of the engines prior to ignition. The engine can’t handle the freezing chock of liquid Hydrogen and Oxygen and will split itself apart especially in the turbopump bearings. They will become brittle and shatter.
Each RS-68A engine has the capability to produce 3,160 kN (705,000 lbf) of thrust for a combined 9,420 kN of total thrust. The RS-68A engine has a specific impulse of 362 seconds and uses a combination of liquid hydrogen (LH2) and liquid oxygen (LOx).
During the flight, the center booster burned at a slightly slower throttle setting - 80% - than the two side boosters. This is because the Delta IV Heavy needs all three boosters in order to get enough velocity to pass through the thick parts of the atmosphere. However, after that, they are expended and jettisoned as to not carry any extra weight.
As the vacuum optimized Delta second stage is very efficient, but not very powerful, the Delta IV Heavy burns its center booster longer than other rockets so the second stage will be able to put its payload into orbit.
Continuing up the rocket comes the second stage. The Delta Cryogenic Second Stage (DCSS) is powered by a single, vacuum optimized RL10B-2 engine. For its fuel, the DCSS too uses liquid hydrogen (LH2) and for the oxidizer, liquid oxygen (LOX).
The LH2 tank - volume of 38 m3 holding 10 000 gallon LH2 - being on top, it has the job of supporting the payload and the payload fairing and is structurally separated from the other ‘half’ of DCSS. The clearly smaller LOX tank - volume of 23 m3 holding 6 000 gallon LOX - is suspended below it and is responsible for structurally supporting the RL10B-2 engine.
The LH2 tank - volume of 52.0 m3 holding 13 750 gallon LH2 - being on top, it has the job of supporting the payload and the payload fairing and is structurally separated from the other ‘half’ of DCSS. The clearly smaller LOX tank - volume of 18.9 m3 holding 5 000 gallon LOX - is suspended below it and is responsible for structurally supporting the RL10B-2 engine.
With 20,000 gallons of liquid propellant reserved for the DCSS there should be room for 1,250 gallons of Hydrazine and pressurized Helium gas for orbit maneuvers and back filling of the empty propellant tanks.
The numbers of DCSS are found to be wrong. It must be double checked first.
The RL10B-2 was originally built by Aerojet Rocketdyne and first flew in 1998. It has the capability to produce 110 kN (24,700 lbf) of thrust in a vacuum and has a specific impulse of 462 seconds. It will light up at least four times with 18 minutes 15 seconds of burn time and an unknown throttle setting during the mission.
In order to save costs and weight, the gimbal system uses electromagnetic actuators over normal hydraulics; this also increases reliability.
Facts on this Delta IV Heavy launch vehicle
Height of Delta IV Heavy: 233 feet (71 meters)
Fuel onboard: 470,000 gallons of cryogenic liquid propellant
3 x Core stage Delta IV Heavy: 110,000 gallon LH2 - 40,000 gallon LOX
Second Stage 5 meter DCSS: 10,000 gallon LH2 - 6,000 gallon LOX
150 kg (340 lb) of Hydrazine is stored - Unknown so far
Helium storage tanks: Unknown so far
Mass at liftoff: 1,6 million pounds (725,750 kg)
Thrust at liftoff: 2.1 million pounds (9.4 mega-Newtons)
Orbit: Low Earth Semi-Polar Orbit - 226 x 257 miles (364 x 414 km)
