By John Keller
TETERBORO, N.J. - The avionics of NASA`s next-generation manned spacecraft is to be a quad-redundant open-systems architecture that departs radically from traditional designs by locating the flight- and mission-control computers in the same card cage.
Avionics designers typically place flight control and mission computers in separate electronic boxes to minimize the risk of catastrophic failure in flight-control hardware, which could end a mission in disaster. "Normally if a mission computer hangs the flight-control data bus, the plane crashes," explains Anthony Cantasano, VMS product manager in the AlliedSignal Aerospace Guidance & Control Systems division in Teterboro, N.J.
Flight-control computers are critical components of today`s high-performance yet unstable aircraft because they automatically manipulate control surfaces to compensate for aerodynamic instability much faster than a human pilot could do manually.
AlliedSignal is chief subcontractor to Lockheed Martin Corp. of Bethesda, Md., on the project to build VentureStar, the next space shuttle design of the National Aeronautics and Space Administration. AlliedSignal is responsible for the avionics and other major subsystems in VentureStar, which is also called the Reusable Launch Vehicle, or RLV.
AlliedSignal avionics engineers are confident they can place flight control and mission computers side by side in the same box by using "watchdog" timers and by taking advantage of the system`s quad-redundant design, Cantasano explains.
VentureStar will have identical computer subsystems performing identical calculations configured in four separate boxes called Vehicle Mission Computers (VMCs). Each VMC enclosure will have single-board computers in its backplane that handle flight control, mission-related tasks, and utility management. Connecting each VMC independently to the spacecraft`s sensors and control subsystems will be 1553B 1 megabit-per-second data buses.
Connecting the VMC boxes to each other and synchronizing their operation, meanwhile, will be a high-speed cross-channel broadcast data link that is proprietary to AlliedSignal, as well as a real-time executive module.
The watchdog timers, which anticipate when certain operations should execute, are key to ensuring that each VMC functions correctly. If the timers detect that operations do not occur as scheduled in any of the four VMCs, "the box resets and goes offline, transparent to the user," Cantasano says. Meanwhile, the other three VMCs would take over control of the spacecraft.
In addition, a computer "voting" scheme compares processing results of the four VMCs before the data outputs to other systems. If data from one box does not agree with the other three, the deviating box resets and goes offline, Cantasano says.
Maximum use of COTS
Not only does this design enhance safety of the vehicle, but also it enables AlliedSignal engineers to make the maximum possible use of commercial off-the-shelf (COTS) components, rather than expensive specially designed ultra-reliable parts, Cantasano explains.
AlliedSignal designers are making extensive use of laboratory simulations to prove the worth of this architecture before installing it in VentureStar after 2001. Still, NASA and Lockheed Martin leaders will build an unmanned reusable launch vehicle prototype called the X-33, scheduled for flight testing in 1999.
The X-33 will be smaller than VentureStar, will have no cargo bay, and will have a triple-redundant avionics system rather than VentureStar`s quad-redundant approach.
Avionics on the X-33 is to be based on the PowerPC 603e microprocessor, 6U VME backplane data bus, 1553B serial data bus, C and C++ software, and the VX-Works real-time software kernel. The X-33 will not use radiation-hardened electronics, but VentureStar will, Cantasano says.
Upgrade paths
"We chose the PowerPC so we can rehost on the same target in the RLV," explains John Capeci, manager of advanced systems and avionics integration at AlliedSignal Guidance & Control. "It is broad based, it will be radiation hardened, and the software is transportable."
Providing the PowerPC single-board computers will be DY 4 Systems Inc. of Kanata, Ontario. The 1553 interface boards will come from Data Device Corp. of Bohemia, N.Y., while several I/O boards will come from Vista Controls Corp. of Santa Clarita, Calif. The VX Works real-time software kernel comes from Wind River Systems in Alameda, Calif.
Although 1553 will be adequate for the X-33, Cantasano has his doubts about its utility for VentureStar. "The 1553 has been a workhorse, but it is growing old," he says. "Whatever we choose for the future will be a deterministic, fault-tolerant, low-latency bus."
Several possible candidates come to mind, he says - including the asynchronous transfer mode bus known as ATM - but he makes clear that he and his colleagues are not yet ready to commit to any particular bus.
"ATM has the characteristics we need, but so do others, such as ARCnet and Firewire," Cantasano says.
The future VentureStar reusable launch vehicle, pictured above in an artist`s rendering, will have on-board computers based on the PowerPC microprocessor. Avionics designers have not settled on which high-speed data bus will be used.
