A giant flies

Mark Broadbent reports on the unique Stratolaunch, the world’s largest aircraft by wingspan, which recently flew for the first time

The first air-to-air photo of Stratolaunch, flying during its April 13, 2019, first flight from Mojave Air and Space Port.
Scaled Composites

WITH ITS six engines, twin fuselages and twin tails, the Stratolaunch looks like no other aircraft. Fitting, perhaps, as it is designed to do something very different to the norm: dropping rockets that will then spear into space to put satellite payloads into low-Earth orbit.

On April 13, 2019, after years of development by its manufacturer Stratolaunch Systems Corporation and partner Scaled Composites, this giant aeroplane, registered N351SL (c/n 1), undertook its first flight from Mojave Air and Space Port in California.

In the hands of pilots Evan Thomas and Chris Guarente, the aircraft flew for 149 minutes, reaching a maximum speed of 165kts (305km/h) and an altitude of 17,000ft, Thomas and Guarente evaluating basic aircraft performance and handling qualities during the flight.

Air-launching rockets

Using a large aircraft to drop rockets that are then launched into space to deliver payloads into orbit is a long-standing idea.

Air-launching payloads has several advantages over the traditional method of launching into orbit vertically from a fixed launchpad on the ground.

The infrastructure and cost of a launchpad are not required and the rocket itself does not undergo the stresses of launch from the ground. A carrier aircraft is a fully reusable first stage for launch, able to be prepared for its next mission after recovering from a launch without the complex recovery and refurbishment required for a rocket used from a launchpad.

A carrier aircraft can operate from any location with a suitable runway and fly clear of populated areas. Above all, airlaunching gives flexibility over the launch point, ensuring an optimal trajectory for the desired orbit and taking account of changing weather.

Microsoft co-founder billionaire Paul G Allen decided to fund a project to develop a mobile launch system and make orbital access to space more convenient, reliable, affordable and routine, which led to the founding of Stratolaunch Systems Corporation in 2011.

Allen, who died late last year, previously funded the development of the Scaled Composites SpaceShipOne, the vehicle that was air-launched by its White Knight carrier aircraft and in 2004 won the Ansari X-Prize for undertaking the first successful suborbital flights undertaken by a private venture.

Stratolaunch Systems Corporation subsequently contracted Scaled Composites to build and deliver a carrier aircraft. Initially, SpaceX was also involved in the project and it was originally envisaged the carrier aircraft would transport that company’s Falcon rocket. However, SpaceX left the programme in 2012 and the following year Stratolaunch Systems Corp announced its new carrier aircraft would instead use the Northrop Grumman Innovation Systems Pegasus rocket as its payload

The ‘Roc’

As part of the R&D work to produce the carrier aircraft, Scaled Composites validated fabrication costs by building “demonstration pieces”. This work drove the aircraft’s design of twin flat-sided fuselages with a high-mounted, high-aspectratio wing, a configuration reminiscent of White Knight and WhiteKnightTwo, the respective carrier aircraft for SpaceShipOne and the subsequent SpaceShipTwo.

Stratolaunch is designed to drop rockets that will put payloads into low-Earth orbit.
Stratolaunch Systems Corporation
Stratolaunch’s flight crew reported good handling on the aircraft’s first flight.
Scaled Composites

Like WhiteKnightTwo with SpaceShipTwo, Stratolaunch will carry Pegasus in a central wing section, the payload dropping below the carrier aircraft’s wing on launch. The central wing section on Stratolaunch is fitted with a Mating and Integration System developed by Dynetics, a long-term partner in the programme, and is designed to be capable of handling a 550,000lb (249,475kg) payload. Stratolaunch Systems Corporation says the aircraft’s design allows for safe deployment of payloads and the reinforced centre wing, “provides lift, stability and pylons that can support multiple launch vehicles”. Stratolaunch is designed to carry up to three Pegasus rockets, with each Pegasus able to put an 816lb (370kg) payload into low-Earth orbit.

Stratolaunch has the widest wingspan of any aircraft ever built at 385ft (117m), which is 95ft (29m) wider than the 290ft (88.4m) wingspan of the sole Antonov An-225 Myria, for three decades the largest aircraft in the skies. Stratolaunch’s wingspan is also 65ft (19m) wider than the 320ft 11in (97.5m) wingspan on the one-off Hughes H-4 Hercules flying boat, aka the ‘Spruce Goose’, flown briefly in November 1947.

To put it another way, if the Wright brothers had begun their 120ft (37m) flight at Kitty Hawk at one of Stratolaunch’s wingtips, they would have completed their journey three times before they reached the other wingtip.

Despite its huge wingspan, at 238ft (73m) in length Stratolaunch is the same length as an Airbus A380, making it around 37ft (11m) shorter than the 84m (275ft 7in) An-225 and considerably shorter than the gigantic 245m (800ft) Hindenburg-class airships of the 1930s, by some margin the longest flying machines ever flown.

Still, everything about Stratolaunch is very big, making the aircraft’s nickname, ‘Roc’ (a giant bird in Arabic mythology) very apt. The aircraft stands 50ft (15m) high at the tail, the twin fuselages are 238ft (73m) long and the separation between them where the aircraft’s payload will be carried is 95ft (29m) wide. The aircraft has 28 wheels and weighs 500,000lb (226,796kg) empty. It is the largest aircraft ever built from composite materials and its six Pratt & Whitney PW4056 turbofan engines, each producing 56,750lb (252.4kN) of thrust, mean it is the most powerful aircraft ever flown. Its maximum take-off weight of 1.3 million lb (589,760kg) is another record.

The PW4056 engines, landing gear, flying controls and avionics have all been adopted from the Boeing 747-400, and the aircraft’s manufacturer says the allcomposite structure maximises range and lift capacity.

Ground tests and flight

Stratolaunch Systems Corporation built a facility at Mojave Air and Space Port to produce its giant aircraft, including an 88,000ft2 (8,200m2) facility to build the composites for the wing and fuselage.

May 2017 saw N351SL roll out at Mojave. This was followed by a range of ground tests, beginning with static runs of the six PW4056 engines at different power levels and fuelling tests, followed by initial assessments of the flying control surfaces on the wing and vertical stabilisers and checks on electrical, pneumatic and fire detection systems and the flight control system.

An initial low-speed taxi took place in December 2017 before further steering, braking and higher-speed taxi tests were carried out in 2018 and into the first few weeks of this year in the build-up to the mid-April maiden sortie.

Stratolaunch characteristics

A huge wingspan, twin fuselages, six engines, 28 wheels; Stratolaunch truly makes an impression.
Stratolaunch Systems Corporation
At 385ft (117m), Stratolaunch’s wingspan is the largest of any aircraft ever built. Stratolaunch
Systems Corporation

Speaking after Stratolaunch’s first flight, Thomas said: “The flight itself was smooth, which is exactly what you want the first flight to be, and for the most part the aircraft flew as predicted.

“We got running down the runway, did our rotation and the aircraft very nicely and smoothly rotated. It climbed out quickly and that was great to see. Then we turned out and basically went right into our test cards, climbing up to 15,000[ft]. For the first part of the flight, we checked out the handling qualities of the aircraft.

“It flew very much like we had simulated and predicted. We saw a few little things that were off nominal, but really for a first flight it was spot on.

We checked out the handling qualities, brought down the flaps and then repeated our handling qualities, taking a look at what it was going to be like to land, checking the pitch response and then practising some rolls and yaws to make sure we’d be able to line up with the runway like we wanted to. That all felt pretty good.

“The systems ran like a watch.

It is a very complex aircraft – the propulsion, the pneumatic system, the hydraulics; they all ran perfectly, which was great. We were able to concentrate on practising some simulated approaches and getting ready to come back to land.

“The aircraft felt really nice on the touchdown [and the] gear felt good. We had a couple of corrections to line up in the slow down and ended up rolling to a stop pretty much where we wanted to. I honestly could not have hoped for more on a first flight, especially of an aircraft of this complexity and this uniqueness.”

Future

Stratolaunch is undeniably distinctive, and the engineering to create it is readily apparent, but it is not the sole platform out there for air-launching satellite payloads.

Northrop Grumman Innovation Systems offers the same Pegasus rocket Stratolaunch will use on its own platform, the company’s L-1101 Tristar N140SC (c/n 1067) named ‘Starliner’. The Pegasus already has an operational history; it has now been launched more than 40 times since its first launch in 1990.

Separately, Virgin Orbit is planning to use a Boeing 747-400 N744VG (c/n 32745) named Cosmic Girl to lift its two-stage rocket called LauncherOne. The company is preparing for the first test launch of LauncherOne following a captive-carriage test flight late last year.

It remains to be seen if there is enough room in the air-launch sector for all these platforms given how putting payloads into space is, ultimately, a niche market.

Stratolaunch Systems Corporation says because its aircraft is able to release one or more launch vehicles, multiple payloads can be deployed to different inclinations during the same mission. This would be a differentiator from the other two systems, which can only carry one payload at once.

However, Stratolaunch Systems Corporation has rowed back on plans announced last year to develop a family of three proprietary launch vehicles: a medium launch vehicle with a 7,495lb (3,400kg) payload, a heavy launch vehicle with a 13,227lb (6,000kg) payload and a reusable spaceplane.

Although these projects were still listed on the company’s website at the time of writing, when AIR International asked Stratolaunch Systems Corporation for an update on these systems earlier this year, the company said it had halted development on them.

A statement said: “Stratolaunch is ending the development of their family of launch vehicles and rocket engine. We are streamlining operations, focusing on the aircraft and our ability to support a demonstration launch of the Northrop Grumman Pegasus.”

Stratolaunch Systems Corporation did not respond to a request for information about the next steps in the aircraft’s flight test programme towards this demonstration launch, but information released after the first flight said this launch is planned for 2020, with operational launches from 2022.