Stratolaunch conducts high-speed taxi tests on giant carrier aircraft

by Michael Baylor

Last weekend, Stratolaunch Systems performed high-speed taxi tests of the world’s largest aircraft by wingspan. The tests represent continued progress towards the maiden flight of the plane. The Stratolaunch carrier aircraft will serve as a mobile launch platform for rockets including Orbital ATK’s Pegasus XL.

Stratolaunch was founded in 2011 by Microsoft co-founder Paul G. Allen, with the goal of increasing access to Low Earth Orbit. The system uses an aircraft to launch rockets instead of the traditional launch pad.

Air-launch comes with a few key advantages. For example, the launch vehicle has to fly through significantly less atmosphere. Also, weather delays are not as common, as the rocket is released from an altitude above most weather systems.

Initially, Stratolaunch planned to work with SpaceX to develop a launch vehicle. The goal was to build a four or five engine variant of the Falcon rocket designed for air-launch. However, work between the companies ended in 2012, after SpaceX determined that too many modifications were required.

As a result, Orbital Sciences (now Orbital ATK) was selected to replace SpaceX. Unlike SpaceX, Orbital already had a proven air-launch vehicle in the Pegasus – a rocket with 29 successful launches in a row.

Orbital ATK’s Stargazer carrier aircraft releases a Pegasus XL rocket carrying NASA’s Cyclone Global Navigation Satellite System. Image by NASA.

While Orbital ATK prepares to begin operations with Stratolaunch, the fate is unclear for launches from Orbital’s Stargazer aircraft – the current carrier for Pegasus. There is only one Stargazer based launch remaining on the Pegasus manifest – the Ionospheric Connection Explorer (ICON) mission for NASA. Therefore, it is possible that all Pegasus missions will transition to Stratolaunch after ICON launches in 2018.

Stratolaunch will carry the Pegasus from a carbon fiber aircraft designed by Scaled Composites – the same company that built WhiteKnightTwo (WK2) for Virgin Galactic. Like WK2, the Stratolaunch plane features a dual fuselage design.

While both fuselages on the carrier have windows, the three-person crew will fly in the right fuselage. The left fuselage is not human rated and thus will only house electronics.

The plane is propelled by six jet engines which have been salvaged from two 747s. Additional components from the 747s were also utilized, including landing gear, windows, avionics, and actuators.

The Stratolaunch aircraft features three 747 jet engines on each side. Photo by Stratolaunch.

The wingspan of the carrier aircraft is a world record breaking 385 feet (117 meters) – long enough for the Wright brothers to complete their historic flight three times. Additionally, it also makes the carrier wider than the Saturn V rocket is tall.

In addition to the massive wingspan, Stratolaunch has a carrying capacity of 549,290 pounds (249,153 kilograms) and a 1,000 nautical mile mission radius. These metrics will enable up to three Pegasus rockets to be launched in a single flight. They also create the potential for larger launch vehicles to be launched from the aircraft, as the plane is not exclusively designed for Pegasus.

A Stratolaunch visualization of their aircraft carrying three Pegasus rockets.

In May 2017, the Stratolaunch plane rolled out of its hangar at the Mojave Air and Space Port for the first time to begin fuel tests.

In the fall, Stratolaunch moved into stationary engine tests. These were incremental, with the engine’s power gradually increasing over time.

Around the same period, Stratolaunch also announced that it had conducted “prerequisite testing of the electrical, pneumatic, and fire detection systems.”

Finally, in December Stratolaunch capped off the year with a successful low-speed taxi test. During the taxi, the vehicle reached a top speed of 28 miles per hour (45 kilometers per hour) as it headed down the runway.

Following the test, Aircraft Program Manager George Brugg stated, “This was another exciting milestone for our team and the program. Our crew was able to demonstrate ground directional control with nose gear steering, and our brake systems were exercised successfully on the runway. Our first low-speed taxi test is a very important step toward first flight.”

Last weekend, Sratolaunch kicked off 2018 with two days of additional taxi tests. Most notably, the tests included reaching the maximum taxi speed of 40 knots  (46 miles per hour). According to Allen, these tests allowed the team to “verify control responses.”

After that, the path remains unclear. Stratolaunch likes to keep a low-profile in terms of scheduling. However, as of last year 2019 was the target for the first launch.

During commercial operations, the aircraft will be capable of performing approximately one mission per week. Stratolaunch will primarily fly out of its base at the Mojave Air and Space Port. However, other airports can be utilized, as long as they can handle the large wingspan.

Initially, Stratolaunch will carry one launch vehicle at a time. Flights with multiple rockets will occur once operations ramp up. Release will occur at approximately 35,000 feet (10,668 meters).

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