A successful space mission depends on flawless computing and fault-tolerant systems. So when an uncrewed space vehicle like the Dream Chaser®, now in development by Sierra Nevada Corporation (SNC), docks with the crewed International Space Station, everyone from ground mission control to the space station crew will be counting on the Draper-developed fault-tolerant flight computer system and flight software to get the craft to the station and back to Earth safely.

As part of the NASA mission to commercialize cargo resupply of the space station, SNC contracted with Draper to design the flight computer hardware and software guidance systems for the Dream Chaser, a reusable commercial spaceplane projected for a scheduled late 2020 mission launch. Model-based engineering (MBE) has been an important part of Draper’s best practices for the system development of the Dream Chaser.

With a lifting-body design similar to that of the Space Shuttle but only one-fourth of its size, the Dream Chaser will deliver more than 12,000 pounds of pressurized and unpressurized cargo to the station. On return to Earth, it will land like any commercial aircraft. While the Space Shuttle required more than 10,000 feet of runway, Dream Chaser has the ability to land at most commercial airports without needing a specific time window, and it can fit in a military cargo plane for the return trip to the launch pad.

The Dream Chaser fault-tolerant system – a set of four computers running the same software at the same time – needs to be perfectly in sync and reliable through the intricacies of launch, space flight, secure rendezvous and docking with the space station, as well as autonomous runway landings anywhere on Earth. From the beginning, more than 100 Draper engineers working on the project have been using System Modeling Language (SysML), an interconnected model that captures and quantifies all the aspects of system design in one place.

Draper’s Timeliner software, which automates many of the operations aboard the space station, will control and run autonomous sequences onboard the Dream Chaser commercial resupply flights. Dream Chaser also will use co-developed guidance, navigation and control (GN&C) algorithms, demonstrated in November 2017 during SNC’s atmospheric drop test. MBE ensures consistency and allows review of these complex systems, linking designs with low-level specifications and enabling ongoing review and easy incorporation of changes. MBE can show the impact of changes throughout the system before expensive prototypes are built, saving time and money.

The MBE applied on Dream Chaser is built on Draper experience and innovation going back to the Apollo space program. This includes modeling and rigorously testing systems adapted to the needs of specific spacecraft design, including trajectory adjustments in the gliding landing of the Space Shuttle; precision rendezvous and berthing of Northrop Grumman’s Cygnus automated cargo spacecraft, which currently transports supplies to the station; and guidance and navigation algorithms for the Orion deep space human exploration mission. MBE allows easy documentation and reuse of this deep knowledge of space avionics, especially in creating mission automation, GN&C systems and human-rated fault-tolerant flight computers.