Scaled Composites Model 351 Stratolaunch “Roc”

On Saturday, 13 April 2019 an aviation record was broken.  Howard Hughes’ Spruce Goose held the record of the largest aircraft to have ever flown for a whopping 71 years.  That record was broken by Scaled Composites’ Model 351 Stratolaunch “Roc”.  With a wingspan of 385 feet (117 meters), its wings are 64 feet (19.5 meters) longer than those of the Spruce Goose. The Antonov An-225 still holds the record for the heaviest aircraft to ever fly.  Film of the flight here:
Stratolaunch was funded by Microsoft Billionaire Paul Allen to perform aerial launches of orbital rockets.  The aircraft was constructed by Burt Rutan’s Scaled Composites.  It is intended to be the “first stage” of a launch vehicle and is designed to carry up to a 550,000 lb (250,000 kg) external payload between the twin fuselages.
A number of orbital insertion vehicles were in design to be carried by the Roc, but all of the designs have ceased development except the Pegasus II.  These are the missile-like objects pictured on the left in the graphic above.  Three can be carried at a time.  The manned Dream Chaser spaceplane is another proposed payload.
To reduce developmental time and costs, Stratolaunch incorporated several proven systems used on Boeing 747s.  The engines are the same Pratt & Whitney PW4056 turbofans, six in total.  These provide the Roc with a maximum speed of 530 mph (853 kph).
The Roc carries a crew of three, all in the starboard fuselage.  The flight deck of the port fuselage is occupied by flight test instruments and avionics.  The cockpit instrumentation is off the shelf, sharing many of the same systems as the 747.
The landing gear components are also from the 747.  Twelve main wheels and two nose wheels are housed in each fuselage, for a total of twenty eight wheels in all.  Empty weight is 500,000 pounds, payload is 550,000 pounds, maximum take-off weight is 1,300,000 pounds (226,800 kg, 250,000 kg, and 589,700 kg, respectively).
The test flight lasted two and a half hours and was successful in all respects.  The current goal is to achieve an operational capability by 2022 for launching Pegasus orbital insertion vehicles.