Pegasus XL rocket to carry out Tactically Responsive Launch demo for Space Force

Northrop Grumman’s air-launched Pegasus XL rocket will make a low-profile launch for the US Space Force Sunday, in a mission designed to demonstrate the ability to call up and conduct a launch at short notice. The launch is expected to occur off the coast of California, during a six-minute window opening at 01:09 PDT (08:09 UTC).

Sunday’s mission is designated Tactically Responsive Launch 2, or TacRL-2. The primary objective of the mission is to demonstrate an ability to call up a satellite to launch at short notice. The actual operation of the satellite – specifics of which have not been disclosed – is secondary to this demonstration.

While this is the first mission for Tactically Responsive Launch, a similar test was conducted with the Operationally Responsive Space Office’s Jumpstart mission in 2008. For Jumpstart, several payloads were prepared for launch with the final selection being made less than a month before the rocket was due to lift off.

Jumpstart – which eventually selected the Missile Defense Agency’s Trailblazer satellite as its payload – was launched on the third flight of SpaceX‘s Falcon 1 rocket but failed to reach orbit after an anomaly during first stage separation.

With TacRL-2, the Space Force have revived this concept. Planning is already underway for at least two further Tactically Responsive Launch missions to fly in the next few years.

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Very few details about the payload itself are known – other than that it was procured in less than a year, using off-the-shelf components, and will be used for technology demonstration relating to “space domain awareness” – typically a euphemism for monitoring the activities of other satellites.

The project is being managed by the “Space Safari” office, whose establishment within the Special Programs Directorate of the Space Force Space and Missile Systems Center was announced earlier this month. Space Safari mirrors the Air Force’s Big Safari program, which is dedicated to the rapid development of special mission aircraft using existing airframes and off-the-shelf technologies.

Pegasus is a three-stage rocket which was originally developed by Orbital Sciences Corporation (OSC). The US Navy had previously experimented with air-launched rockets in the 1950s as part of Project Pilot, but all six of its launch attempts ended in failure. When Pegasus made its maiden flight in April 1990, it became the first air-launched rocket to reach orbit.

Pegasus uses Orion 50 solid rocket motors for its first and second stages, with an Orion-38 third stage. The first stage is fitted with aerodynamic surfaces, including a triangular wing and a tail with movable fins, to provide stabilization and control during the early stages of flight. All three motors burn hydroxyl-terminated polybutadiene (HTPB) propellant.

The first five Pegasus rockets were dropped from NASA’s NB-52B aircraft, named “Balls 8”, before launches switched to Orbital’s own dedicated aircraft, Stargazer. This aircraft, which carries registration N140SC, is a Lockheed L1011-1 TriStar which first flew on 22 February 1974 and was delivered to Air Canada the following month as C-FTNJ.

In February 1982 the aircraft was briefly leased to Air Lanka, returning to Air Canada seventeen days later. Stargazer was acquired by Orbital Sciences in May 1992. Pegasus is carried and released using a special mount built into the bottom of the aircraft’s fuselage.

The name Stargazer was chosen as a nod to Star Trek: The Next Generation. It is named after the USS Stargazer, a ship which Captain Picard had commanded before becoming Captain of the USS Enterprise. His First Officer, Commander Riker, had previously served aboard the USS Pegasus.

The Lockheed TriStar was developed as a three-engine widebody airliner to compete with the McDonnel Douglas DC-10 and the larger four-engine Boeing 747. It has three Rolls-Royce RB211 turbofan engines, one mounted under each wing with the third at the rear of the fuselage and fed air via an s-duct from an intake built into the front of the tail. Despite being one of the most technologically-advanced aircraft of its era, the TriStar was commercially unsuccessful and only 250 were built. Today, Stargazer is the last airworthy example.

Orbital Sciences’ introduction of Stargazer coincided with the maiden flight of the upgraded Pegasus XL rocket, which featured stretched first and second stages compared to the original configuration and redesigned horizontal stabilizers to ensure adequate clearance for Stargazer’s landing gear to retract.

Stargazer, the last flying Lockheed L-1011 TriStar, took off from Mojave Air and Spaceport this morning. It’s headed to Vandenberg AFB to pick up a Pegasus XL rocket and @NASA’s ICON satellite ahead of their launch out of Cape Canaveral on October 10. #NASAICON @NASASpaceflight pic.twitter.com/JKuq3e1vuE

— Jack Beyer (@thejackbeyer) September 18, 2019

Orbital initially offered the Pegasus H – a modified version of the standard Pegasus retrofitted with the XL’s tail modifications – alongside Pegasus XL. The original Pegasus made one more launch in 1994, while the hybrid version was used for four flights between 1995 and 2000.

While Pegasus uses only solid rocket motors, a four-stage version was developed incorporating the liquid-propellant HAPS upper stage. This allowed payloads to be delivered to higher orbits which Pegasus could not reach on its own. HAPS was last flown on Pegasus in 2005, and although no announcement has been made, it is unlikely to be present on Sunday’s mission.

Of 44 flights prior to the TacRL-2 mission, Pegasus has completed 39 successfully, with three failures and two partial failures. All of these incidents occurred on early missions: the partial failures came on the second and fifth flights of the standard Pegasus in 1991 and 1994, which reached lower than planned orbits because of issues with the HAPS upper stage. The first two Pegasus XL launches in 1994 and 1995 both failed to achieve orbit.

The type’s most recent failure occurred in November 1996 when a third-stage electrical fault prevented the payload of a Pegasus XL vehicle from separating. This resulted in the loss of NASA’s HETE satellite and Argentina’s SAC-B, which had shared the ride into orbit. Since then, Pegasus has achieved a run of 30 consecutive successful launches.

Sunday’s launch will be the first for Pegasus since October 2019, and only the fifth launch of the veteran rocket in the last twelve years. Its last mission was the successful – though much-delayed – deployment of NASA’s ICON ionospheric research satellite.

Although Pegasus was offered for commercial launches, and a small number of commercial missions were flown for companies including Orbcomm, Orbimage, and the national space agencies of Brazil and Canada, the majority of its flights have been for the US Government, including the Air Force and NASA. TacRL-2 is its first mission for the United States Space Force, which was formed in late 2019 from the former US Air Force Space Command.

A ground-launched version of Pegasus has also been developed. Initially called Taurus, and later renamed Minotaur-C, this removes the rocket’s aerodynamic surfaces and mounts it atop a TU-903 or Castor 120 booster. This can carry a payload two to three times greater than Pegasus inside a larger payload fairing.

The Orbital Boost Vehicle used by the Missile Defense Agency’s Ground Based Interceptor anti-ballistic missile (ABM) program uses the same three stages as Pegasus without aerodynamic surfaces. A two-stage version without the third stage can be deployed from the rear cargo door of a C-17 Globemaster transport aircraft to serve as a target for ABM tests.

The larger four-stage Minotaur I rocket uses the two upper stages of Pegasus, and on some launches the payload fairing, mounted atop the first two stages of a decommissioned Minuteman II missile, for orbital launches.

Northrop Grumman acquired Pegasus along with the rest of OSC’s fleet of rockets in its 2018 merger with Orbital ATK – which had itself been formed by the merger of Orbital Sciences and Alliant Techsystems three years earlier. The former Orbital ATK operated as Northrop Grumman Innovation Systems until the start of 2020, when the company was restructured, and space vehicle operations transferred to Northrop Grumman Space Systems.

The Pegasus XL vehicle which will fly Sunday’s mission was one of two originally built for Paul Allen’s Stratolaunch project, which were to have been launched from that company’s giant Roc aircraft – built by Northrop Grumman’s Scaled Composites subsidiary – while it worked on a new, larger rocket.

Following Allen’s death in 2018, Stratolaunch has refocused its near-term plans on hypersonic research and abandoned its plans to fly Pegasus. Northrop Grumman subsequently bought back the rocket hardware that they had produced for the project.

The Space Force reportedly paid around $28.1 million for Northrop Grumman’s launch services on Sunday’s mission, around half of what NASA originally paid for the Pegasus launch of their ICON satellite which was deployed in 2019. The low price is almost certainly down to the spare rockets that Northrop Grumman had on hand and a desire to launch them rather than continue paying for their storage and upkeep.

Although the agreement to launch Sunday’s mission on Pegasus was secured last July under the Orbital/Suborbital Program 4 (OSP-4) contract, as part of the rapid response aspect of the TacRL-2 mission, Northrop Grumman and the Space Force’s Space Launch Delta 30 (the former 30th Space Launch Wing) were given only three weeks notice to prepare for the launch when it was actually due.

In this time, the company has had to take delivery of the satellite, encapsulate it within the rocket’s payload fairing and mate with the Pegasus, attach the rocket to the Stargazer carrier aircraft, and carry out all of the usual steps and checkouts that make up the launch campaign. These activities have taken place at Vandenberg Space Force Base in California.

Drop-zone for #TacRL2 on the #Pegasus rocket — air launched from underneath the #Stargazer L1011. Plane take off will be from Vandenberg Space Force Base. Given night launch at 01:11 local (08:11 UTC) Sunday, if June gloom is not present, you should be able to see some of it. https://t.co/by7bFyFaKJ

— Chris G – NSF (@ChrisG_NSF) June 12, 2021

At around midnight local time on Sunday, Stargazer will take off from Vandenberg and begin a climb to the drop altitude, likely to be around 39,000 to 40,000 feet (about 12 kilometers). Once at altitude, Stargazer normally flies an oval “racetrack” pattern, with one side of the oval lined up with the launch corridor, allowing it to come back around for another attempt should an issue delay launch on the first attempt. The relatively short six-minute launch window for Sunday’s launch means that there will only be time for one attempt.

While Stargazer gets into position, systems aboard Pegasus will undergo final testing and preparation for launch, including checkouts of the vehicle’s flight controls. In the event a problem is detected while it is still attached to Stargazer, the launch can be aborted and the rocket brought back to Vandenberg for investigation or repair. In the event of a delay, a backup launch opportunity is available on Monday.

The drop zone for the TacRL-2 launch is over the Pacific Ocean, to the west of Vandenberg. The launch corridor runs to the south, lining up with a likely sun-synchronous orbit. When Stargazer is flying along this heading within the drop zone, if all systems remain go for launch, Pegasus will be released from the belly of Stargazer.

For the first five seconds of free flight, the rocket will fall, unpowered, before its first stage engine ignites. For a Pegasus launch, T-0 is timed as the moment the rocket is dropped from the carrier aircraft.

Controlled by the three fins at the rear of its first stage, Pegasus will pitch up to being its climb reaching its maximum angle of attack eleven seconds after ignition. The rocket will pass through Max-Q, the area of maximum dynamic pressure, about 30 seconds into the first stage burn. After this, the vehicle will then begin to reduce its angle of attack, building up more horizontal velocity.

The first stage is powered by an Orion 50S XL solid rocket motor. This is a variant of the Orion 50 that is both stretched and equipped with the Pegasus wing and aft control surfaces. By contrast, the second stage, an Orion 50 XL, has the same stretched motor but without the wing or tail. Instead, the second and third stages are guided using thrust-vectored engine nozzles and reaction control thrusters.

Pegasus will fire its first stage for 68.6 seconds. After this burns out, a short coast will follow before first stage separation and second stage ignition occur in quick succession at about T+89 seconds.

The second stage Orion 50 XL will fire for 69.4 seconds, raising the apogee – or highest point – of the rocket’s trajectory to roughly the altitude of the planned orbit. Pegasus will reach space during the second stage burn, with the payload fairing separating from the nose of the rocket about 121 seconds into the flight, once it is no longer needed to protect the satellite from Earth’s atmosphere.

After the second stage burns out, Pegasus will enter a coast phase where the second and third stages will remain attached as the rocket ascends towards its apogee. The duration of this coast will depend largely on the altitude of the target orbit – which has not been disclosed – but on a typical launch the coast will last a little over seven minutes.

At its conclusion, the spent second stage will be jettisoned and the third stage will ignite for a 68.5-second burn to inject itself and its payload into orbit. The third stage uses an Orion 38 motor for this purpose.

Spacecraft separation will occur shortly after third stage burnout, completing the forty-fifth mission for Northrop Grumman’s Pegasus rocket.

Following the TacRL-2 mission, Pegasus faces an uncertain future. A second Pegasus XL rocket that was produced for the Stratolaunch project still exists. It is likely that Northrop Grumman will offer this at a reduced price to attract a customer – it is also possible that it could already be assigned to a similar mission to Sunday’s launch which has not yet been announced.

In 2016, prior to its merger with Northrop Grumman, Orbital ATK indicated that they planned to continue offering the rocket with additional vehicles being manufactured if the need arose. Keeping the production lines open is unlikely to be a major problem for Northrop Grumman, due to the commonality with its other rockets.

For most of its career, Pegasus has occupied a niche in the market. Because it is air-launched, it needs very little ground infrastructure once the rocket has been attached to the carrier aircraft, and integration can take place at Vandenberg before a ferry flight to another airstrip. This means that launches can take place from multiple sites around the world, targeting any orbital inclination.

As well as from Vandenberg, past Pegasus launches have taken place from Edwards Air Force Base in California, the Cape Canaveral Air Force Station (now Space Force Station) and Kennedy Space Center in Florida, Wallops Island in Virginia, Kwajalein Atoll in the Marshall Islands and Gran Canaria in the Canaries. In January, Virgin Orbit’s LauncherOne rocket became only the second air-launched rocket to reach orbit, ending Pegasus’ monopoly on this capability.

With increasing competition from new rockets in its weight class and also from larger rockets like SpaceX’s Falcon 9, fewer satellites are looking to Pegasus for their ride into orbit. This was exemplified by NASA’s decision in 2019 to award the launch contract for the IXPE satellite to SpaceX.

IXPE had been designed with Pegasus in mind, with its planned equatorial orbit requiring a launch from Kwajalein. SpaceX bid a flight-proven Falcon 9 at a lower cost than a typical Pegasus mission, and although this will be tied to a launch from Florida, Falcon has enough reserve performance to reach equatorial orbit through a plane change burn.

Sunday’s launch is the first of two in quick succession for Northrop Grumman, with the company also expected to carry out a Minotaur I mission from the Mid-Atlantic Regional Spaceport, on Wallops Island, Virginia, on Tuesday. That flight is expected to deploy a classified national security payload – potentially a technology demonstrator – as part of the NROL-111 mission for the National Reconnaissance Office.

(Lead photo of Pegasus XL and Stargazer prior to the launch of ICON – via NASA)

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