Boeing has scrubbed Tuesday’s planned launch of their Starliner spacecraft on its second uncrewed Orbital Test Flight, called OFT-2, aboard United Launch Alliance’s Atlas V rocket. The issue at hand, according to Boeing, related to incorrect valve position indications in the propulsion system. The issues were first detected during checkouts after thunderstorms storms over launch site on Monday.
Later on Tuesday, Boeing confirmed that teams had cycled the Service Module propulsion system valves and had ruled out a number of potential causes, including software. Wednesday’s potential launch attempt will not proceed. No launch date has been set at this time.
Starliner overview
Orbital Flight Test 2 is a repeat of the OFT mission flown at the end of 2019. Due to issues encountered in the early stages of that mission, the OFT spacecraft was unable to complete several key test objectives – including rendezvous and docking with the International Space Station. The OFT-2 mission will aim to clear these final hurdles ahead of a Crewed Test Flight for Starliner no earlier than the end of 2021 according to current NASA estimates.
Starliner, also known as CST-100, is one of two spacecraft developed under NASA’s Commercial Crew Program to provide crew access to the International Space Station on American vehicles – a capability the agency had lost when the Space Shuttle retired in 2011. Boeing and SpaceX were selected to develop the new crewed spacecraft – with the latter’s Crew Dragon having already transported three crews: a demo mission and two operational rotation flights to the outpost.
With the advent of these new crewed spacecraft, NASA has been able to reduce its reliance on Russia’s Soyuz spacecraft, which had previously been the only way to transport astronauts to the ISS. Even during the Shuttle era, Soyuz was the only spacecraft that could remain on station to provide for crew return in the event of an emergency on a long-duration expedition. Both Starliner and Dragon are designed to remain docked for the duration of their crews’ stays.
The integrated Starliner spacecraft is 5.0 meters in length, and 4.6 meters in diameter. It consists of a pressurized capsule, which can seat up to seven astronauts; a service module housing thrusters; aft-mounted solar panels; propellant; and equipment to support a mission of up to 60 hours’ of free flight – although the spacecraft can remain on orbit for 220 days once docked to the International Space Station.
A NASA Docking Adaptor in the nose of the spacecraft can be used to connect it to one of the International Docking Adaptors located on the two Pressurised Mating Adaptors (PMAs) at the station’s Harmony module.
A total of 40 reaction control system (RCS) thrusters – 12 on the capsule and 28 on the service module – provide attitude control and fine positioning for Starliner, each generating 378 newtons of thrust. Twenty larger Orbital Maneuvering and Attitude Control (OMAC) thrusters on the service module generate 6.7 kilonewtons of thrust each and will handle most of the orbit adjustment maneuvers.
The most powerful thrusters are the four Launch Abort Engines (LAEs), based on Aerojet Rocketdyne’s RS-88, built into the aft of the service module. These liquid-propellant engines can generate 178 kilonewtons of thrust each and are there to push Starliner clear of the Atlas V rocket in the event an anomaly should occur during ascent.
At the end of its mission, Starliner will use the thrusters on its Service Module one last time for a deorbit burn. Once this is complete, the expendable service module will separate and burn up in the atmosphere while the capsule will be protected during re-entry by a heat shield comprised of Boeing Lightweight Ablator. Once re-entry is complete Starliner will deploy parachutes to slow its descent. Unlike all other US crew capsules, which have splashed down at sea, Starliner is designed to touch down on land with the aid of airbags to cushion its landing.
Each Starliner capsule is designed to fly at least 10 times, with Boeing alternating between two operational spacecraft: Spacecraft 2, which will be used for the OFT-2 mission, and Spacecraft 3 which flew OFT and will next be used for the Crew Flight Test. Spacecraft 3 has been named Calypso, with Spacecraft 2’s name due to be announced after the conclusion of the OFT-2 mission.
Why an OFT-2?
As part of the Commercial Crew project, NASA required each spacecraft to carry out two demonstration missions before operational flights could begin. The first of these called for an uncrewed test flight, with the second carrying astronauts. Starliner attempted to complete the first of these objectives with the OFT mission in December 2019.
Welcome back to SLC-41 where Atlas V N22 is again poised on the pad ahead of tomorrow’s scheduled launch of Starliner OFT-2. This will be the 145th mission for ULA and the 100th launch from SLC-41. #OFT2#Boeing#ULA
Myself for @NASASpaceflightpic.twitter.com/lZ6ppDtnlS
— Julia Bergeron (@julia_bergeron) August 2, 2021
Starliner’s launch profile sees the Atlas rocket place it on a suborbital trajectory close to but intentionally not quite reaching orbital velocity. Starliner then performs a short circularization burn to insert itself into low Earth orbit. During OFT, a problem with the Mission Elapsed Timer aboard the spacecraft – combined with communications issues that initially prevented a manual command being sent from the ground – meant that this burn did not take place at the correct time.
While controllers were able to regain communications and command a safe but not-as-planned insertion minutes later, the spacecraft burned more propellant than had been allocated, leaving it without enough to complete its journey to the station. After completing the tests that could be run in free flight, the spacecraft returned to Earth two days after it had lifted off.
Yet another software issue, identified and corrected while OFT was in orbit hours before it was set to return to Earth, could have jeopardized its safe return. The procedures for jettisoning the spacecraft’s service module after deorbiting had been configured incorrectly and if executed would have resulted in a collision between the capsule and service module after separation. Starliner landed two days after launch at White Sands Missile Range, New Mexico.
NASA and Boeing conducted a joint review of the mission, identifying the issues with the timer, software, service module separation, and space-to-ground communication as major concerns. In total, the review made 80 recommendations ranging from changes in testing to software and operational processes to ensure the safety of future missions. As OFT was aborted before it could reach the station, the OFT-2 mission was added to Starliner’s test program using the spacecraft and rocket that had been allocated to the next stage of testing, the Crew Flight Test ensure compliance with Commercial Crew Program goals and safety protocol.
In the absence of a crew, OFT-2 will carry Rosie the Rocketeer, an anthropometric test device that also flew aboard OFT, in the commander’s seat. On its previous mission, Rosie carried instruments to monitor the forces that astronauts might experience during flight, although for OFT-2 these sensors have been disconnected and its role will instead be that of a mass simulator.
As a secondary objective of the test flight, Starliner is carrying 245 kilograms of cargo – including 200 kilograms of crew supplies and provisions for NASA – both for the astronauts currently aboard the outpost and for the crew of the next Starliner mission, which could visit later this year. Boeing has placed 145 kilograms of commemorative memorabilia aboard the spacecraft, which they plan to distribute to employees and students once Starliner returns to Earth.
NASA will also use Starliner to bring 188 kilograms of cargo back to Earth from the ISS. This will include depleted Nitrogen/Oxygen Recharge System tanks used to transport pressurized gasses to replenish supplies aboard the station. Once returned to Earth these can be refilled and sent up again on a future cargo resupply mission.
Atlas V N22
Starliner will ride to space atop United Launch Alliance’s Atlas V rocket. A reliable workhorse of the US space industry for the last two decades, Atlas V was developed under the US Air Force’s Evolved Expendable Launch Vehicle program – now National Security Space Launch – and first flew in 2002. The OFT-2 launch is the 88th flight of an Atlas V, with the only blot on the rocket’s reliability record being the type’s 10th launch in June 2007, which delivered a National Reconnaissance Office payload to a lower orbit than had been planned after a second stage propellant leak. Despite this, the NRO satellites were still able to reach their planned orbit under their own power, and their overall mission did not appear to be adversely affected.
Atlas V was designed to fly in several different configurations, varying the type of payload fairing, number of solid rocket motors (SRMs) used to augment the first stage, and the number of engines on the Centaur upper stage to accommodate a variety of different payloads. Each configuration is denoted by a three-digit number, with the first digit indicating the diameter of the payload fairing – four or five meters – the second indicating the number of SRMs, and the third the number of engines on the Centaur.
For Starliner missions, the N22 configuration is used – with the N indicating a lack of payload fairing.
The rocket that will carry OFT-2 has the unique tail number AV-082. Its first stage is a Common Core Booster powered by a single RD-180 engine that is manufactured by Russia’s NPO Energomash. While the use of Russian hardware on Atlas has become controversial in recent years following deteriorating relations between the United States and Russia, the RD-180 has been an extremely reliable engine for the Atlas family, powering all 87 Atlas V missions to date – as well as six earlier Atlas III rockets – without any mission-affecting anomalies.
The RD-180 burns RP-1 kerosene propellant oxidized by liquid oxygen.
As we wait for our remote setup opportunity, heres a pic of Atlas V and #Starliner on SLC-41 from the press site. pic.twitter.com/lhMZ4aKUjT
— Stephen Marr (@spacecoast_stve) August 2, 2021
To provide additional thrust at liftoff, Atlas can fly with up to five solid rocket motors. In the N22 configuration, uses two Aerojet Rocketdyne AJ-60A boosters which contain hydroxyl-terminated polybutadiene propellant. These will burn during the early phases of flight as Atlas climbs through the lowest regions of the atmosphere.
The Centaur upper stage, mounted atop the Common Core Booster, uses a Dual-Engine Centaur (DEC) configuration for Starliner missions, with all of Atlas V’s other payloads using the single-engine variant; however, it is not a new innovation. Earlier versions of Centaur, used from the early Atlas-Centaur rockets of the 1960s through to the Atlas II in the 1990s, used dual engines as standard, with the single-engine Centaur only being introduced with the interim Atlas III in 2000.
Centaur was developed as a high-energy upper stage for the Atlas and Saturn rockets in the 1960s and evolved from this original design into a series of larger and more capable versions – including variants for use in conjunction with the larger Titan IIIE and Titan IV rockets, as well as two unflown models which could have been carried in the Space Shuttle’s payload bay. All Centaur stages use of liquid hydrogen and liquid oxygen propellant, burned in Aerojet Rocketdyne (originally Pratt and Whitney) RL10 engines.
The dual-engine Centaur also sees the return of the RL10A-4-2 engine. While these engines were used on all Atlas V launches prior to 2014, they have been phased out on single-engine Centaur missions in favour of the RL10C-1 variant which offers greater commonality with the RL10B engines used on the Delta IV rocket. The RL10C engine has a wider nozzle than the RL10A, which prevents it being used in Centaur’s dual-engine configuration, so all Starliner missions are expected to make use of the older model engines.
While Atlas V does not use a payload fairing when launching Starliner, the spacecraft includes a feature to provide protection from the atmosphere and aid the aerodynamic profile of the rocket: an aeroskirt attached to the aft of the service module. The aeroskirt extends back from the spacecraft around the forward end of Centaur, helping to counter loads that would otherwise be placed on the forward end of Centaur due to the spacecraft’s larger diameter than that of its carrier rocket.
The launch
Atlas V was assembled atop a mobile launch platform in the Vertical Integration Facility (VIF), about 550 meters south of the pad. Once secure at the launch pad, fueling of the first stage’s RP-1 tanks commenced, ahead of the start of an 11-hour countdown. Liquid oxygen and liquid hydrogen loading began on time approximately 5 and a half hours before the planned liftoff.
At T-2.7 seconds, Atlas V’s RD-180 engine ignites, followed at T+1.1 seconds with liftoff as the twin solid rocket motors light and the thrust generated by the vehicle become greater than the mass of the rocket. Five seconds later, Atlas will begin a pitch and yaw maneuver to place itself onto a 50.4 degree azimuth, heading on a northeasterly track out over the Atlantic Ocean.
Atlas V will quickly reach the point in its flight where the combination of velocity and atmospheric density combine to produce the greatest aerodynamic loads on the rocket. Known as Max-Q, or Maximum Dynamic Pressure, this is expected to occur around 41.8 seconds into the mission.
The AJ-60A boosters carry enough propellant to burn for approximately 90 seconds. Once they have depleted this, they burn out and stop producing thrust. Atlas V, however, will continue to carry the spent cases for more than half a minute after burnout to reach an altitude where the air pressure is low enough that the boosters can separate cleanly without striking the aft end of the Atlas booster. The boosters will separate at the T+2 minute 22 second mark in the flight.
With the boosters separated, the RD-180 engine will continue firing alone. The first stage is expected to burn for 4 minutes and 29 seconds, with stage separation coming six seconds after its cutoff. There will be a 10-second coast between staging and second stage ignition, during which time the Centaur engines will complete their pre-start sequence.
Centaur’s two RL10 engines will ignite at the T+ 4 minute 45 second mark in the flight, beginning their 7 minute 9.5-second burn. Twenty seconds after Centaur ignition, the aeroskirt will separate from the aft of the Starliner service module.
Main Engine Cutoff (MECO) – the end of Centaur burn – will occur at T+11 minutes 54.5 seconds mission elapsed time. Three minutes after cutoff, Starliner will separate from Centaur in a 72.8 by 181.5 kilometer suborbital trajectory. This is not a stable orbit, as the perigee – or lowest point – lies well within Earth’s atmosphere. Starliner will use its OMAC engines to complete orbital insertion with a burn 31 minutes into the flight as it reaches apogee – highest point in its orbit.
Starliner approaches the ISS for docking as Crew Dragon Endeavour watches from a neighboring docking port. (Credit: Mack Crawford for NSF/L2)
Centaur will remain in the deployment orbit, performing a propellant blowdown to safe the stage before it passively reenters over the Indian Ocean west of Australia.
Following orbital insertion, Starliner will make a series thruster burns as it begins maneuvering towards the International Space Station, raising its orbit and changing its plane to match the outposts.
Starliner is slated to dock with the forward port of the station’s Harmony module, via Pressurized Mating Adaptor 2 (PMA-2) and International Docking Adaptor 2 (IDA-2), approximately 24 hours after launch.
(Lead image: The Atlas V booster separates on the OFT-2 launch, exposing the dual-engine configuration of the Centaur upper stage. Credit: Mack Crawford for NSF/L2)
