Screenshot from NASA Webcast of the OA-6 Cygnus launch. Honey. It’s dark outside. Have mercy
Mission Rundown: ULA - Atlas V 401 - OA-6 Cygnus
Written: January 2, 2023
Every time I don’t carry. People talk
Beginning its sixth flight late on Tuesday evening, Orbital ATK’s Cygnus spacecraft lifted off atop an Atlas rocket to begin a two-month mission to the International Space Station.
Liftoff of the OA-6 mission from SLC-41 at Cape Canaveral was on schedule at 23:05:51 local time (03:05 UTC on Wednesday), with berthing expected on Saturday.
Tuesday’s launch was Cygnus’ second flight atop United Launch Alliance’s Atlas V rocket – standing in for Orbital ATK’s own Antares while Orbital modified the latter’s first stage.
Antares is expected to return to flight – sporting new RD-181 engines in place of the AJ-26 used previously – in late June with the next Cygnus mission, its first flight since a launch failure in October 2014.
Atlas V 401 flight path with OA-6 Cygnus. Red zone is the booster crash site 2200 km downrange
Cygnus launched atop an Atlas V rocket, flying in the 401 configuration. This version of the rocket uses a four-meter payload fairing and a single-engine Centaur upper stage, with no solid rocket motors attached to the first stage. The rocket had the tail number AV-064, and was the sixty-second Atlas V to fly.
The OA-6 Cygnus Payload
Cygnus spacecraft are named after former astronauts. OA-6 was named SS Rick Husband, after the commander of Space Shuttle Columbia’s ill-fated STS-107 mission. Legacy
Cygnus arrival at the station is expected on Saturday, March 26, 2016, when astronauts will use the station’s CanadArm2 robotic arm to capture the spacecraft and berth it to the nadir port of the Unity module.
Cygnus is expected to remain at the station for close to two months, departing in late May ahead of the next Cygnus launch in June.
The OA-6 mission involves the enhanced Cygnus spacecraft, with an enlarged pressurized cargo module and redesigned fuel tanks and solar arrays.
Launching atop the Atlas V 401, instead of the smaller and less powerful Antares, also allows the spacecraft to carry a greater mass of cargo.
The Cygnus spacecraft will carry crew supplies and vehicle hardware to ISS - the orbital laboratory to support the Expedition 47 and 48 crews.
OA-6 is carrying 3,395 kilograms (7,485 lb) of cargo to the station; including 777 kg (1,713 lb) of scientific hardware, 1,139 kg (2,511 lb) of provisions for the crew, 1,108 kg (2,443 lb) of station hardware, 98 kilograms (216 lb) of computer equipment and 157 kg (346 lb) of hardware for EVAs. Packaging accounts for the remaining 116 kg (256 lb) of the payload.
Experiments being carried to the space station aboard Cygnus include Meteor, a high-resolution imaging payload intended to capture video and still images of meteors as they enter the Earth’s atmosphere, to allow better study of their properties and composition.
Strata-1 is a materials science payload aimed at studying how asteroid regolith – or loose surface material – behaves in microgravity.
The research will benefit future missions aimed at landing on asteroids and small planetary satellites such as Phobos.
The Additive Manufacturing Facility (AMF) is a 3D printer which will be used to construct tools and equipment in orbit by building them in layers.
Cygnus is also carrying “Gecko Gripper” adhesive devices to be tested aboard the station for mounting small items to the walls of the outpost.
Following Cygnus departure from the space station, a group of CubeSats will be deployed from a Nanoracks dispenser mounted externally to the vehicle.
These include the twenty-strong Flock-2d contingent of Planet Labs’ large constellation of Earth imaging satellites and further spacecraft of Spire’s Lemur-2 constellation.
Lemur-2, whose first four satellites were launched atop a PSLV last September, carry imaging payloads and AIS receivers to relay tracking information from ships at sea.
Once the CubeSats have separated, Cygnus will host the first of three Spacecraft Fire Safety Demonstration Project, or Saffire, experiments.
The Saffire experiment means a fire will be started within the spacecraft’s pressurized compartment and allowed to burn freely for up to six minutes while observed by a suite of thermocouples, radiometers and cameras.
Since the Cygnus spacecraft is not designed to be recovered, the end of its mission provides the opportunity to perform these experiments, which are intended to provide a better understanding of how a fire might spread inside a manned spacecraft.
After performing the Saffire experiment, the SS Rick Husband will be deorbited.
The Atlas V 401 Launch
The Atlas departed for the International Space Station - ISS from Space Launch Complex 41 (SLC-41) at the Cape Canaveral Air Force Station. Originally built for the Titan IIIC, the pad was also used by the Titan IIIE and Titan IV rockets before being torn down in 1999 to make way for the Atlas V.
At about 2.7 seconds before the countdown reached zero, the RD-180 engine ignited and began building up thrust. At about T+1.1 seconds, the thrust produced exceeded the weight of the rocket and liftoff occurred. The rocket initiated a series of pitch and yaw maneuvers 17.3 seconds later to put itself onto a 44.4-degree launch azimuth as it headed northwest over the Atlantic Ocean.
AV-064 reached Mach 1, the speed of sound, 82.6 seconds into her flight before passing through the area of maximum dynamic pressure, or Max-Q, 11.2 seconds later.
The first stage burned for four minutes and 15.5 seconds before its engine shut down. Six seconds later the spent stage separated, with the Centaur’s RL10 in its prestart phase.
Ten seconds after stage separation the Centaur ignited to begin a single burn with a duration of thirteen minutes and 38 seconds. However, the burn appeared to last over a minute more than previously advertised.
Eight seconds after Centaur ignition, the Atlas’ payload fairing was jettisoned.
For Tuesday’s mission the rocket flew with an Extra-Extended Payload Fairing (XEPF), which at 13.8 meters (45.3 feet) in length is the longest of the three four-meter (13-foot) diameter fairings available. Keep finding odd numbers on those fairings.
Following completion of the Centaur’s burn, Atlas coast for two minutes and 39 seconds before Cygnus separated, at twenty minutes and 58.5 seconds mission elapsed time over the Atlantic to the West of Ireland.
The target orbit for spacecraft separation is 229.8 by 230.0 kilometers (142.8 x 142.9 miles, 124.1 by 124.2 nautical miles) at an inclination of 51.6 degrees.
About two hours and six minutes after launch, Cygnus deployed its solar panels before beginning a series of thruster firings over the next few days to maneuver itself towards rendezvous with the International Space Station ISS.
A question of compared impulse.
Atlas V core booster couldn’t maintain thrust during the full burn time. It was 5 seconds short due to the g load restriction of 3.5 g which mustn’t be violated, so the Atlas V core booster shuts down 5.5 seconds early. That's a problem.
Centaur 2nd stage has to burn 77 seconds extra to make up for that lack of thrust in 5.5 seconds. It’s total impulse. Thrust x Time. The stages deliver thrust from two different engines; the RD-180 engine and the RL-10C-1 engine.
The RD-180 delivers 3.8 MegaNewtons at sea level, more in space, but it’s throttled down to the 3.5 g thrust level which must be too low for the engine to function properly. It seems counterintuitive but if Cygnus was heavier then the g load would be higher and the thrust from the RD-180 engine burned longer before the 3.5 g limit was reached.
The propellant ballast solution. I’ve had the thought that some rockets had too much fuel and oxidizer aka. propellant in their tanks. The thrust to weight ratio must not go over a certain g load in order to protect the payload from damage or buckling caused by high g loads under high engine thrust values.
22183 kg Atlas V + 23073 kg Centaur + 7492 kg Cygnus + 2687 kg Fairing = 55435 kg
Estimated thrust with ‘dry’ Atlas V: 55435 kg x 9.81 m/s2 x 3.5 g = 1.9 MegaNewtons
Centaur estimated thrust: 33252 kg x 9.81 m/s2 x 3.5 g = 1.14 MegaNewtons
The Centaur 2nd stage engine had to deliver the extra impulse, but with 15 times less thrust - 101.8 KiloNewtons in vacuum, so it had to burn that much longer.
However the Centaur 2nd stage engine didn’t have to push the Atlas V with its unused propellant reserve and the boosters 22 183 kg ‘dry’ mass. Plus the fairing was jettisoned a little later so that helped with the thrust to weight ratio.
The Centaur 2nd stage had to burn its ‘tail feathers’ meaning it had to angle itself rather steeply in order to maintain its current flight altitude.
The Centaur 2nd stage deployed Cygnus successfully and did a short 3 second deorbit burn which meant it came down slightly OUTSIDE of the intended box south of Australia.
These facts was known:
Centaur burn was extended with - 77 seconds
Propellant burned in those 77 seconds was 1,730 kg
Propellant margin on Centaur for this mission ~1,800 kg
That’s 8.6% of propellant includes the margin for used to deorbit
Delta V before gravity losses ~ 814m/s
In order for the Atlas V core stage to have such a high DV loss its RD-180 must have under performed ISP wise by > 2%. The estimate is from 2% to 2.5%. This would cause a thrust decrease such that during throttle limiting G loads the throttle would have been opened more consuming more than normal fuel or oxidizer for less thrust resulting in running out of one or both propellants 5.5 seconds earlier.
Speculation as to what can cause this is a prop mixture ratio error caused by bad sensors, leaks, or flow restrictions. Other items also related to flow restrictions can be something as simple as bad bearings which cause low spin rates which cause low ISP.
Another item is an error in the engine control software parameters for engine operation that operated the engine with incorrect calibration/coefficients settings.
As to the root cause it may not be a simple matter to determine so be patient.
The Atlas V 401 rocket
Tuesday's launch of United Launch Alliance’s Atlas V was flying in the 401 configuration.
The Atlas V, tail number AV-064, consists of a Common Core Booster (CCB) first stage, with a single-engine Centaur (SEC) upper stage and a four-meter payload fairing fitted atop the Centaur. Although the Atlas V can fly with up to five Aerojet AJ-60A solid rocket motors boosting the first stage, AV-064 will use none.
Atlas V 401 split in its major parts. This is a generic non mission specific graphic configuration
The Atlas V is an expendable medium lift launch system and member of the Atlas rocket family. The rocket is one of the most reliable in the world, having more than 60 launches with no complete failures.
The Atlas V 401 rocket, tail no. AV-064 is standing 58.22 meters - 191 feet tall on SLC-41.
The rocket has two stages. The first is a Common Core Booster (CCB), which is powered by an RD-180 engine with two bells and burns kerosene (RP-1) and liquid oxygen (LOX). This is accompanied by up to five strap-on solid rocket boosters. The second stage is the Centaur upper stage, which is powered by one or two RL10 engines and burns liquid hydrogen (LH2) and liquid oxygen (LOX).
Atlas V rocket is filled with 344 472 liter - 91 000 gallons of RP-1, liquid oxygen and liquid hydrogen. Question is now how much goes to fill each stage and the four tanks. Together they can contain 344,47 m3 RP-1, cryogenic oxygen and cryogenic hydrogen.
The Common Core Booster holds 184 728 liter - 48 800 gallon liquid oxygen chilled to below -182,96 0C Celsius or -297,33 0F Fahrenheit and can fit in a 184,73 m3 oxygen tank.
The Common Core Booster holds 94 635 liter - 25 000 gallon RP-1 highly refined kerosine at room temperature that can fit in a 94,64 m3 fuel tank.
The Centaur upper stage holds about 15 709 liter - 4 150 gallons of liquid oxygen chilled to below -182,96 0C Celsius or -297,33 0F Fahrenheit that can fit in a 15,71 m3 fuel tank.
The Centaur upper stage holds about 48 075 liter - 12 700 gallons of liquid hydrogen chilled to -252,8 0C Celsius or -423 0F Fahrenheit that can fit in a 48,07 m3 hydrogen tank.
The 350 gallon of extra tank capacity found in the LOX tank could be used by Helium gas, Nitrogen gas and Hydrazine pressure vessels. Seven 50 gallon tanks.
Still to find is data on Helium gas, Nitrogen gas, pressures levels used and number of tanks - Carbon Overwrapped Pressure Vessels - COPV to store it. And there are tanks to store Hydrazine N2H4 propellant used to maneuver during launch and in orbit.
HAZ GAS operations are completed when the hydrazine is loaded. The RCS thrusters on the Centaur stage are using hydrazine as a monopropellant during orbit insertion.
The reaction control system (RCS) includes the ullage pressure thrust from the tanks and consists of twenty hydrazine monopropellant engines located around the stage in two 2-thruster pods and four 4-thruster pods.
For propellant, 150 kg (340 lb) of Hydrazine is stored in a pair of bladder tanks and fed to the RCS engines with pressurized helium gas, which is also used to accomplish some of the Centaur RL-10C-1 engine start up functions.
In the 401 configuration, the Atlas V is capable of carrying a structural maximum of 9,050 kg to Low Earth Orbit - LEO, 8,200 kg to the International Space Station - ISS and 4,950 kg to Geostationary Transfer Orbit - GTO.
The Common Core Booster contains a total of 284,089 kilograms - 626,309 pounds of RP-1 kerosene and liquid oxygen, weighs 306,271.7 kilograms - 675,213.5 pounds fully fueled, and is 35.63 meters - 116,9 feet tall and 3.81 meters - 12,5 feet wide.
Its RD-180 main engine produces 3,826.9 kilonewtons - 860,321.35 pounds of thrust at sea level while the thrust level increases to 933,406.73 pounds in space.
The Centaur V1 upper stage contains 20,830 kilograms - 45,922.3 pounds of liquid hydrogen and liquid oxygen, weighs 23,073 kilograms - 50,867.3 pounds fully fueled, and is 12.68 meters - 41,6 feet tall & 3.05 meters - 10 feet wide.
Its RL-10C-1 engine is optimized for vacuum usage with a big nozzle - engine bell, so it only produces 101.8 kilonewtons - 22,885.55 pounds in space.
Atlas V 401 weighs an estimated 336,368.7 kilograms - 741,556.0 pounds, including the OA-6 Cygnus spacecraft; and is 58.22 meters - 191 feet tall and 4.2 meters - 13,8 feet wide.
The OA-6 Cygnus spacecraft weigh 7,492.0 kilograms - 16,517 pounds on their own, that’s with the fairings weight excluded.
The Atlas V 401 LPF fairings weigh 2,487.0 kilograms - 5,482.9 pounds. The weight of a 3 foot fairing extension is estimated to be a small part of the LPF fairing. 100 kg at most.
Doing the math: 306272 kg + 23073 kg + 7492 kg Cygnus + 2687 kg = 339524 kg.
The Atlas V 401 rocket has a three number configuration code.
The first number represents the fairing diameter size in 4 or 5 meters, so in this instance there is a 4 meter fairing. This launch will use the 14.0 meter long (46 ft) XLPF.
The standard four-meter fairing, named the Long Payload Fairing (LPF), measures 12.2 meter (40 feet) in length and was first introduced as a larger fairing for the Atlas I rocket that was used in 1990.
One or two 90-centimeter (3-foot) cylindrical segments can be added to the fairing to form an Extended Payload Fairing (EPF) 13.1 meters (43 feet) or Extra-Extended Payload Fairing (XEPF) 14.0 meters (46 feet) respectively for payloads that require the additional space.
The second number denotes the number of solid rocket boosters (SRBs), which attach to the base of the rocket. The number of SRBs for a 5 meter fairing can range from 0 – 5. In this case there will be no SRB’s attached to the center core.
The third number shows the number of engines on the Centaur Upper Stage, which is 1 in this configuration. So this means that this rocket will have a 4 meter fairing, no solid rocket boosters, and 1 engine bell on the Centaur Upper Stage.
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