What is a hybrid rocket motor? What advantages does it have over conventional liquid and solid propellant rocket motors? These questions point to an exciting breakthrough that occurred on December 13, 2018, when Virgin Galactic successfully launched VSS Unity on its first suborbital flight. VSS Unity, the newest of the company’s class of SpaceShipTwo vehicles, is capable of carrying up to six passengers into space.

SpaceShipTwo in flight on December 13, 2018. Credit: Virgin Galactic 

SpaceShipOne, developed by Microsoft co-founder Paul Allen and Burt Rutan's Scaled Composites, pioneered commercial spaceflight in October 2004. It is the first privately built vehicle to travel into space. 

  View of SpaceShipOne hanging in its new location in the Boeing Milestones of Flight Hall at the National Air and Space Museum's National Mall building, September 4, 2015. Credit: National Air and Space Museum

The recent launch of VSS Unity builds on the previous success of SpaceShipOne, and it is a major step forward as Virgin Galactic prepares to open up space travel to the public for the first time.

VSS Unity rode a specially designed carrier aircraft called WhiteKnightTwo before separating and igniting its hybrid rocket motor for the last leg of the trip into space. 

WhiteKnightTwo, Virgin Galactic’s carrier aircraft for the SpaceShipTwo vehicles. Credit: Virgin Galactic 

The motor, which is capable of producing 60,000 pounds of thrust, burned for 60 seconds and accelerated the vehicle to a maximum speed of Mach 2.9 (nearly three times the speed of sound). The two pilots on board reached a peak altitude of 51.4 miles.

RocketMotorTwo from the VSS Unity is one of the largest hybrid propulsion systems ever flown, and the National Air and Space Museum is delighted to have this historic artifact in its collection. 

The hybrid motor built by Virgin Galactic combines key advantages of solid and liquid propellant rocket engine technology to maximize efficiency and performance at a reasonable cost—all very important parameters in the nascent commercial spaceflight sector. Solid propellants, like those used in the booster rockets that helped launch NASA’s space shuttle toward orbit for three decades, are easy to handle and inexpensive to operate; they provide high thrust and power at reasonably low cost. The only drawback is that once ignited, solid propellant motors cannot be throttled to adjust thrust, nor can they be turned off and restarted. Such operational flexibility is the key advantage built into liquid propellant rocket engines, the only major drawback being that the super-cooled and highly volatile fuel and oxidizer require careful and complex handling. RocketMotor two combines the best of both of these engine technologies: The fuel is a solid propellant with the consistency of rubber, and the oxidizer consists of liquid oxygen. The thrust can be varied, and the engine can be turned off and restarted during flight.

RocketMotorTwo from the VSS Unity is one of the largest hybrid propulsion systems ever flown, and the National Air and Space Museum is delighted to have this historic artifact in its collection. The motor will go on temporary display at the Steven F. Udvar-Hazy Center in Chantilly Virginia until 2024, when the new “Future of Spaceflight” exhibition is slated to open at our National Mall building in Washington, DC. This new immersive gallery will explore the evolution of commercial spaceflight technologies over the past 35 years. It will also point to possible futures of human activity in space, ranging from such exotic possibilities as asteroid mining and travel to other planets, to the nearer term opportunities in space tourism that Virgin Galactic has pioneered.

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