Screenshot from ULA Webcast of the launch of AFSPC-5 or OTV-4. I’m burning daylight right now
Mission Rundown: ULA - Atlas V 501 - OTV-4 - X-37B
Written: January 11, 2023
Flying even higher and longer
This launch of an Atlas V marks United Launch Alliance’s 96th mission. Atlas V is launched by ULA. It will ferry the X-37B orbital test vehicle (OTV-6) with the most payloads it has ever carried into a Low Earth Orbit (LEO).
For this mission an Atlas V in a 501 configuration will launch from Space Launch Complex 41 - SLC-41, Cape Canaveral Air Force Station. The primary customer for this launch is the U.S. Space Command.
Atlas V tail number AV-054, a unique designation assigned to each individual Atlas rocket which began with Atlas-Centaur rockets in the 1960s. Atlas-Centaur rocket tail numbers, beginning with AC-001, were continued by the Atlas I, Atlas II and Atlas III rockets which evolved the Atlas-Centaur design, before being replaced with the “AV” series for Atlas V, even if the Centaur III still was a part of the Atlas-Centaur rocket combination.
ULA’s Atlas V 501 was launched from SLC-41, CCAFS on May 20, 2015 at 11:05 EDT.
OTV-4 flight path in a 56 degree azimuth dropping the fairings off first about 800 kilometers and the core booster 2200 kilometers downrange ±100 km. Atlas V will ascent steeply into orbit
OTV-4 launched with X-37B aboard a United Launch Alliance’s Atlas V rocket, which flew in the 501 configuration for this launch. This was the fifty-fourth flight of an Atlas V rocket, one of the most proven and reliable rockets currently in service.
Atlas is a two-stage rocket, consisting of a Common Core Booster (CCB) and a Centaur upper stage. It can fly with up to five AJ-60A solid rocket boosters to provide additional performance for heavier payloads or where the rocket is targeting a higher-energy orbit.
However, with lightweight payloads like the X-37B Atlas V flies without these additional motors. In the 501 configuration there's a 5 meter fairing and a single Centaur engine.
This launch will use the shortest version of the five-meter fairing, which is 5.4 meters (17.7 feet) wide and measures 20.7 meters (68 feet) in length. The carbon composite structure is produced by Swiss manufacturer RUAG, who also makes a similar four part fairing for the European Ariane 5 rocket.
When Atlas V flies with a five-meter fairing, the fairing attaches to the interstage between the first and second stages, completely enclosing the Centaur upper stage as well as the payload. Because of this, the fairing must be jettisoned during the first-stage flight.
By around three minutes, 40 seconds mission elapsed time mark Atlas reached space, and the fairing was no longer needed, so it could be safely discarded. A few seconds later the forward load reactor also separated. This device, which attaches at the forward end of the Centaur, helps to spread some of the payload’s weight across the lower half of the fairing.
Screenshot of Atlas V 501 with X-37B as its payload. It's a tight fit for a space plane. The wings are far too asymmetrical so they will make the rocket veer of course during its atmospheric ascent.
This inclosure of the Centaur 2nd stage makes the fairing far larger than necessary, and also make the fairing look too big. The fairing half is actually made of two sections, top and bottom bolted together in the middle.
This design feature makes no sense, and increases the fairing mass. The Centaur upper stage needs this fairing design as an aerodynamic reinforcement during ascent.
Another reason for this design is the highly volatile Hydrogen gas ability to penetrate any material in fuel tanks, fuelpipes, valves and even the fairings themself, therefore a lot of gabs - square holes - is venting the inclosed Centaur upper stage.
The payload in the top compartment - ‘The Bullitt’ - is pressurized with Nitrox, a dry oxygen nitrogen gas mixture normally used by deep sea divers, and is preventing hydrogen gas from penetrating into the payload. With no hydrogen there can be no fire or explosion so the payload is secure from this source of destruction.
Two forward load reactor fittings act as a floor and a reinforcement of the fairings. They are jettisoned a few seconds after the fairing halfs fall away.
The sheer number of vent holes in the lower part of the fairings tells me that hydrogen gas is present and abundant enough to cause a major accident. Liquid hydrogen tanks are in their supercooled state even more prone to hydrogen gas penetration and therefore even more dangerous around oxygen and electric spark sources.
The X-37B Payload
This launching of the X-37B is designated United States Air Force Space Command 5 – AFSPC-5 – part of a series of generic designations that are increasingly being used to identify US military space launch missions. This X-37B mission is designated OTV-4.
Now in orbit, the X-37B will acquire another public designation, under the USA series that is used for American military satellites. Each X-37B receives a new USA designation each time it enters space. USA designations have been assigned sequentially since 2006, so AFSPC-5 is expected to become USA-261 on orbit.
As well as flying on Atlas V, OTV spacecraft can be deployed by SpaceX’s Falcon 9 rocket, a capability that will be demonstrated with the next OTV-5 launch.
On its AFSPC-5 flight, the X-37B also known as Orbital Test Vehicle 4 (OTV-4), the mission is believed to be demonstrating technology for future programs.
One of the experiments aboard the spacecraft which has been published by the US Air Force consists of a Hall effect thruster evaluation.
The Hall thruster that will fly on the X-37B experiment is a modified version of the units that have propelled SMC's first three Advanced Extremely High Frequency military communications spacecraft.
A Hall thruster is a type of electric propulsion device that produces thrust by ionizing and accelerating a noble gas, usually xenon. While producing comparatively low thrust relative to conventional rocket engines, Hall thrusters provide significantly greater specific impulse, or fuel economy.
This results in increased payload carrying capacity and a greater number of on-orbit maneuvers for a spacecraft using Hall thrusters rather than traditional rocket engines.
This experiment will enable in-space characterization of Hall thruster design modifications that are intended to improve performance relative to the state-of-the-art units onboard AEHF. The experiment will include collection of telemetry from the Hall thruster operating in the space environment as well as measurement of the thrust imparted on the vehicle.
The resulting data will be used to validate and improve Hall thruster and environmental modeling capabilities, which enhance the ability to extrapolate ground test results to actual on-orbit performance.
The on-orbit test plans are being developed by AFRL and administered by RCO.
The X-37 is also carrying NASA’s Materials Exposure and Technology Innovation in Space, or METIS, payload; building on research conducted aboard the International Space Station, METIS will expose an array of material samples to the space environment before returning them to Earth for study.
Developed by Boeing, the X-37B is an evolution of NASA’s proposed X-37A ‘flying body’ that was designed to be deployed from the Space Shuttle to conduct robotic satellite repair and research missions before returning to Earth independently.
The X-37A underwent a series of glide tests in 2006, however the program was canceled. The US Air Force and Defense Advanced Research Projects Agency (DARPA) opted to continue development as a military program, with the first launch coming in April 2010 atop an Atlas V.
OTV-1 or USA-212, lasted over seven months, ending with the successful recovery of the spacecraft via a runway landing at Vandenberg Air Force Base in California. Vandenberg has been the landing site for all X-37B missions to date and it is likely that AFSPC-5 will also touch down there once it reaches its conclusion.
Boeing constructed two X-37B vehicles, with the second taking flight in March 2011 and remaining in orbit for over a year before landing in June 2012. The third and most recent mission, marking the second flight of the first vehicle, began in December 2012 and concluded in October 2014 after almost two years in space.
In addition to deploying the X-37B, Wednesday’s launch also carried the UltraSAT payload consisting of ten small satellites for NASA, the US military and educational institutions.
X-37B flying with 10 CubeSat
Mounted on an Aft Bulkhead Carrier (ABC) attached to the rocket’s upper stage, UltraSAT is a Naval Postgraduate School CubeSat Launcher (NPSCuL) equipped with eight Poly-Pico Orbital Deployers (PPODs).
The third NPSCuL to launch, UltraSAT follows the OutSAT mission that launched with NROL-36 in 2012 and the GemSAT mission that flew with NROL-39 in 2013.
The 10 CubeSats will be deployed from a dispenser placed on the Centaur aft bulkhead. Seen here among two green pressure vessels, one gray Hydrazine tank, batteries, avionic boxes and RL-10C-1 rocket engine with its bell. A timer opens the lids and springs deploys them
The LightSail-A satellite, a three-unit CubeSat which will be operated by The Planetary Society, is a prototype solar sail. The primary objective of the LightSail-A mission is to demonstrate deployment of the spacecraft’s sail, with a later mission – LightSail-B – intended to demonstrate the use of the sail when it launches in 2016.
The LightSail missions build on technology developed for 2005’s Cosmos 1 solar sail experiment which was lost in a launch failure of Russia’s Volna rocket.
The launch of LightSail-A has been sponsored by NASA under its Educational Launch of Nanosatellites (ELaNa) program, with the flight designation ELaNa XI. The remainder of the payloads were sponsored by the National Reconnaissance Office.
The AeroCube-8 mission, consisting of two 1.5-unit CubeSats built by The Aerospace Corporation, will test the use of carbon nanotubes in spacecraft construction and radiation protection and investigate electric propulsion technologies. Also known as IMPACT, the two satellites are identical and will be deployed together from a single PPOD.
The US Naval Academy’s Ballistically Reinforced Communication Satellite Propulsion Test Unit, or BRICSat-P, is a 1.5-unit satellite which will be used to demonstrate the use of plasma thrusters for attitude control and orbital maneuvering. It will share a PPOD with ParkinsonSat-A (PSAT-A), also developed by the US Naval Academy, which carries a communications experiment.
Equipped with a bidirectional transponder, the satellite will collect and relay data from buoys and remote stations in support of the Ocean Data Telemetry Microsatellite Lint (ODTML) as well as acting as a relay for amateur operators.
A further payload was carried for the Naval Academy is the USS Langley, or Unix Space Server Langley. A three-unit CubeSat, Langley is intended to demonstrate the use of off-the-shelf components to operate a Linux-based web server in space. This will be connected to the internet via the satellite’s ground stations.
The Globalstar Experiment and Risk Reduction Satellite 2 (GEARRS-2) is a three-unit CubeSat intended to demonstrate whether the Globalstar satellite constellation can be used to relay commands and telemetry for a small satellite mission. It follows on from the GEARRSAT spacecraft deployed from the International Space Station in March after its launch aboard a Cygnus spacecraft last September.
Three Optical CubeSats, or OptiCubes, were also carried for the California Polytechnic University (CalPoly) to serve as tracking and calibration targets for studying small satellites and debris in orbit.
The Atlas V OTV-4 launch
Wednesday’s launch began with the ignition of AV-054’s RD-180 engine at the T-2.7 second mark in the countdown. The engine built up to full thrust, achieving launch readiness at the zero point in the countdown. Liftoff occurred about a tenth of a second later, with the thrust generated by the RD-180 exceeding the weight of the vehicle.
The RD-180 engine is manufactured by Russia’s NPO Energomash and is derived from the RD-170 series of engines developed originally for the Zenit and Energia family of rockets. A twin-chamber engine, it burns RP-1 propellant in liquid oxygen to provide thrust to the Atlas during the early stages of its mission.
The first stage, or Common Core Booster, has a single RD-180 engine and can also support up to five Aerojet AJ-60A solid rocket motors – although none of these were used for Wednesday’s flight.
Around 18.3 seconds after launch, the Atlas executed a pitch and yaw maneuver to attain the correct attitude for its journey to orbit, flying on an azimuth of 61 degrees.
The rocket passed through the area of maximum dynamic pressure, or Max-Q, seventy and a half seconds later at the T+88.8 second mark, experiencing peak aerodynamic forces as it accelerated through the atmosphere.
Three minutes and 38.4 seconds after lifting off, AV-054 jettisoned the payload fairing which encapsulated the X-37B for its ascent through the atmosphere.
Photo of X-37B in its old and new fairings. The silver and blue tiles are soundproofing. The silver padding makes me think of ‘TARDIS’ in Doctor WHO and the blue reminds me of a padded cell. What there is missing could be an extra ‘in orbit’ service module providing extra mission time
Although the X-37 is designed to function in the atmosphere – and was at one point scheduled for launch atop a Delta II rocket without a fairing – engineers determined it would be too difficult to predict the aerodynamic properties of the unencapsulated spaceplane.
First stage powered flight ended with Booster Engine Cutoff, or BECO, at the four-minute, 23.5-second point in the mission. Six seconds later the spent first stage was separated with the Centaur beginning its pre-start sequence.
The first ignition of the Centaur’s RL10C-1 engine came ten seconds after staging.
Mission events after second stage ignition have not been published, however the Centaur is likely to make a single burn of around fourteen minutes to reach Low Earth orbit, followed by separation of the X-37.
The Centaur will then make at least one further burn to achieve a more inclined orbit for CubeSat separation, before a final deorbit burn will cause the Centaur to reenter the atmosphere over the South Indian Ocean.
Orbital parameters for the X-37 have not been made public, however the launch azimuth suggests an inclination of approximately 39 degrees and apogee and perigee altitudes of 300 to 400 kilometers (186 to 249 miles, 162 to 216 nautical miles) would be consistent with previous flights.
A much more highly-inclined orbit has been given in documentation for some of the secondary payloads, suggesting that the Centaur will be called upon to make a significant maneuver – although one well within its capability once it has shed the mass of the X-37.
ULA declared mission success via a release later in the day on Wednesday.
AeroCube-8’s stated target orbit is 350 by 700 kilometers (189 by 435 miles; 217 by 378 nautical miles) at 57 degrees, while the orbit of LightSail-A is given with a perigee of 389 kilometers (242 miles or 210 nautical miles) – again with an apogee of 700 kilometers – and inclination of 58 degrees.
The X-37B spacecraft incorporates a payload bay that can be opened in orbit to expose experiments to space. A solar panel, deployed from the bay, provides power to the spacecraft and its experiments.
When it is time for the X-37B to return to Earth it will fire its engine for a deorbit burn, lowering the perigee – or lowest point – of its orbit into the Earth’s atmosphere. Following re-entry into the atmosphere the spacecraft will glide down to a runway landing at one of its three designated landing sites.
It is not clear whether the new service module will remain attached to the X-37B for the duration of its mission, or whether it will be jettisoned partway through. However it’s clear that it is enabling X-37B to change orbit altitude and inclination during the mission.
The Atlas V 501 rocket
Wednesday launch of United Launch Alliance’s Atlas V was flying in the 501 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 50 launches with no complete failures.
The Atlas V 501 rocket, tail no. AV-054 is standing 58.22 meters - 206 feet tall on SLC-41.
The Atlas V, tail number AV-054, consists of 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 RL10C-1 engine and is burning liquid hydrogen (LH2) with liquid oxygen (LOX).
Atlas V 501 split in its major parts. This is a non specific mission configuration with known data points
Facts usable on the Atlas V 501 launch vehicle
Height of Atlas V 501: 59.7 meters - 195.87 feet - with a 5.4 meter Ruag fairing 206 feet
Mass at liftoff: 348,087 kilograms - 767,400 pounds - calculated 757000 pounds
Thrust at liftoff: 3.8 mega-Newtons - 0.86 million lbf
Fuel onboard: 91,000 gallons of liquid propellant
LOX+LH2 = 66,000 gallon of cryogenic liquid propellant in three tanks
Core stage Atlas: 25,000 gallon RP-1 or 94,64 m3 - 48,800 gallon LOX or 184,73 m3
Core stage weighs fully fueled 306,271.7 kilograms - 675,213.5 pounds
Core stage measures 35.63 meters - 116,9 feet tall and 3.81 meters - 12,5 feet wide
Core stage 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
Upper Stage Centaur: 13,050 gallon LH2 or 48,07 m3 - 4,150 gallon LOX or 15,71 m3
Upper Stage Centaur weighs fully fueled 23,073 kilograms - 50,867.3 pounds
Upper Stage Centaur measures 12.68 meters - 41,6 feet tall - 3.05 meters - 10 feet wide
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
Centaur has 150 kg (340 lb) of Hydrazine - N2H4 is stored in a tank
Centaur has 2-3 Helium 100-150 gallon pressure vessel storage tanks
Atlas V 501 5.4 meter fairing exact weight is unknown - kilograms - pounds
Atlas V 501 Payload Fairing measures 20.7 meter - 68 feet in length
Boeing X-37B weighs 8 113 kg ~ 17 848 pounds
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.
The aft RL-10C-1 engine section of the Centaur upper stage seen here in a graphic format
This technical image of the aft bulkhead with the RL-10C-1 vacuum engine depicts two green pressure vessels and a gray composite wrapped Hydrazine propellant tank used to feed the attitude Reaction Control System.
In the 501 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.
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.
The Boeing X-37B spacecraft weighs 8,113 kilograms - 17,848 pounds on its own.
Atlas V 501 fairings weight was estimated to be 10,629 kilos - 23,432.93 pounds.
Doing the math: 306272 kg CCB + 23073 kg Centaur + 8113 kg X-37B = 337458 kg.
Rocket maximum mass plus fairing estimated mass = 348087 kg
The boat tail, the short fairing and the two forward reaktor segments were calculated to weigh a total of 10,629 kg. Now all that’s missing is to determine the weight of said parts and the weight of the two other fairing lengths on the Atlas V rocket.
The Atlas V 501 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 5.4 meter fairing. This launch will use the 20.7 meter long (68 ft) fairing.
The fairing manufactured by RUAG of Switzerland, comes in three different lengths, with this launch using the shortest of these with a diameter of 5.4 meters (17.7 feet).
The fairing is a Carbon Composite sandwich, made with two layers of Graphite-epoxy face sheets over a middle aluminum honeycomb core. Silver aluminum plates mounted on the inside walls are temperature, sound and vibration dampening pads made of cotton.
The shortest measures 20.70 meters (68 feet) in length, the medium measures 23.45 meters (77 feet) in length and the longest measures 26.49 meters (87 feet) in length but is usually only used on Delta IV Heavy launches.
The four part fairing sections rest structurally on the ‘Boattail’ and extension rings on the Atlas V core booster. The two bottom halves encapsulates the Centaur stage completely and is preventing it from carrying the full load of the payload and fairings. There is a structural maximum of 9,050 kg on the Centaur stage itself during launch.
The two top halves encapsulates the MUOS-4 payload and stands on the Centaur upper stage that is reinforced by two forward reaktor load segments. They act as a floor that keeps Hydrogen gas from seeping into the top fairing compartment, and stops the fairings from wobbling and buckling during maximum aerodynamic pressure loads.
The Centaur bottom fairing half is filled with venting holes to allow the Hydrogen gas to escape during propellant loading. This is seen in photos of the 5 meter fairing.
The ‘Bullitt’ or top fairing payload compartment is air conditioned with dry NOx gasses using a mix of pure Nitrogen and Oxygen gas under pressure to prevent water vapors and hydrogen gas from entering the payload compartment.
Deep Sea divers use NOx gas mixtures to prevent getting the ‘bends’, it’s nitrogen bubbles in their blood and they also mix in Helium gas on long duration deep sea dives.
Reaching space the fairing separation splits the two fairing halves lengthwise, and a few seconds later the forward reaktor segments are jettisoned as well.
The halves now fall in a ballistic curve back to Earth to be incinerated as shooting stars and the remains will sink beneath the Ocean waves some 1000-1600 kilometers down range depending on when and where the fairings were released.
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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