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Mass storage for military systems demands robust approaches to survive in harsh applications. Rotating media performs reliably in many applications if packaged appropriately, while flash media is required in the most extreme environments. While commercial commodity storage technology continues to improve in cost and density, most of it is practically useless in a typical military application beyond the lab or ground shelter without some proactive packaging innovation.
When looking at ground or air environments, systems can be divided into four main categories, each with their own set of environmental parameters: Labs, or fixed ground installations; C4ISR aircraft in-cabin; C4ISR aircraft out-of-cabin; and tactical combat platforms.
Storage technology is available for reliable operation in all those environmental classes. But that reliability comes at some price—mostly in supporting the required packaging. Figure 1 illustrates these costs as approximate multiples of commodity storage. Before looking deeper at these last three environments—progressing from harsh, through harsher, to harshest—it’s helpful to examine the potential problems with using hard disk drives in these environments without proper packaging.
Hard Disk Drive Dynamics
In most cases when deployed in labs or fixed sheltered ground installations, commercial storage technology does the job inexpensively. This commercial-grade technology typically includes commodity hard disk drives organized as JBODs (“just a bunch of disks”). Beyond those benign environments, three environmental hazards exist that most hard drive technology doesn’t get along with: shock and vibration, altitude and temperature.
Hard drive read/write heads float in very close proximity to the spinning media, close enough for an electromagnetic force to operate but not close enough for physical contact. Drives have improved in their capability to handle not-so-gentle bumps, but the margin for error is still small, and damage happens quickly at today’s high rotational speeds of 7200, 10,000 or 15,000 rpm. In moderate to high shock and vibration scenarios, heads subjected to those forces impinging on an inopportune axis can literally crash. In a hard disk crash, the head makes physical contact with the media, damaging itself or the media and rendering some or all of the data on the drive inaccessible.
At altitudes where insufficient air pressure exists, the head doesn’t have enough air pressure to float and the hard drive won’t spin up properly, with similar ramifications for potential damage. Meanwhile, most hard disk drives are rated for operation starting at 0°C, and many environments don’t offer such a warm climate. Packaging solutions are available to tackle all three of these problems in moderate shock and vibration environments, mitigating the forces a hard disk drive is subjected to and allowing it to operate reliably, avoiding crashes.
Harsh: C4ISR Aircraft In-Cabin
Many C4ISR aircraft are wide-body platforms designed to carry large numbers of crew and large amounts of sophisticated equipment on missions. Examples of aircraft in this class are the E-3 AWACS, Airborne Stand-off Radar (ASTOR), or C-130, which functions in the C4ISR role in several variants. These aircraft present moderate levels of shock and vibration such as that from air turbulence, jet or turboprop engines and their spinning blades or propellers, and landing gear contact with a runway. Cabins in these aircraft are pressurized, typically to an altitude of 8,000 to 10,000 feet above sea level, and temperature controlled to enable the human crew to work comfortably.
In-cabin use is a good example of an application where hard disk drives can be used. As long as the equipment is mounted inside the cabin, hard disk drives are able to operate because the required pressurization needed to float the drive head exists, and the ambient temperature is above freezing. With steps to isolate the hard disk drives from the moderate amounts of shock and vibration encountered during flight, they can function very reliably.
An example of a semi-rugged storage subsystem using commercial drives with semi-rugged packaging is shown in Figure 3. The VMETRO SANcab contains up to twelve 3.5-inch hard disk drives. Each drive is mounted on shock and vibration isolation pads within a shelf holding up to six drives, and two shelves can be installed in the unit, again each mounted with shock and vibration isolation. The unit also has an optional backplane with four 6U card slots for installation of VME and/or CompactPCI hardware. For these low-altitude, semi-rugged applications, the environment is somewhat harsh but hard disk drives with shock and vibration mitigation work well.
Harsher: C4ISR Aircraft Out-of-Cabin
Some C4ISR aircraft don’t have large crew complements, and carry their equipment outside of the crew cabin. The newest C4ISR platforms, UAVs, dispatch with the need for pressurization entirely since they are unmanned. Good examples of this application are payload on the WB-57, NASA’s high altitude weather research aircraft, or equipment onboard the Global Hawk ISR UAV. On such platforms, data-gathering equipment is carried in an external pod, or on a pallet in an unpressurized bay. Shock and vibration mounting is required just as in the in-cabin applications, but attention to pressurization and temperature is also required.
In that scenario, pressurization is not fully controlled or does not exist where the equipment is located, and equipment may be subject to temperatures as low as -40°C at altitude. These conditions allow use of hard disk drives only with a self-contained pressurized chamber and heating elements to bring them within operational range.
An example of a sealed, ruggedized storage subsystem using hard disk drive technology is the VMETRO SANbric. Six drives are mounted in a small rugged sealed canister with total capacity up to 1.8 Terabytes, which is in turn installed in a shock isolation unit for further protection. In Figure 4, note the black rubber isolation bumpers on the corners of the canister, and the rubber shock mounts on the side of the isolation unit. To address the problem of possible low temperature extremes, 300W of heating elements that can be cycled in any combination are provided in the canister to bring the hard drives into operational range above 0°C before the drives are spun up.
The SANbric platform is very rugged, able to operate in environments harsher than what one would normally associate with using hard drives. It has been tested to RTCA/DO-160D specifications, including 20g 11 msec operational shock, and has been shown to operate at an altitude of 72,000 ft. BAE Systems has recently performed operational flight testing of the SANbric, collecting imaging systems data on a pallet within an aircraft cargo bay. Each SANbric canister weighs 22 lbs and measures 11.5 x 5.1 x 7.5 inches. This type of high-altitude, rugged solution takes hard drives into places where they have not been able to operate before, with two levels of shock and vibration mitigation, pressurization and heating to deal with a harsher environment.
Harshest: Tactical Combat Platforms
Even with packaging augmentation, hard disk drives have their limits. When most engineers think of rugged applications, fighter aircraft such as the F-22 Raptor, helicopters like the AH-64D Apache Longbow, or tanks such as the M1A2 Abrams immediately come to mind. In most cases, extreme conditions of shock, vibration, temperature, dust, humidity and other conditions these platforms are subjected to rule out use of hard disk drives.
For extreme conditions like these, the best alternative is use of solid-state drives with no moving parts. Solid-state drives, like most ruggedized electronics, can withstand relatively high shock and vibration forces and don’t require pressurization to operate. Also, drives can be mounted into a conduction-cooled package, making cooling simpler and further reducing SWaP requirements.
An example of a solid-state ruggedized storage subsystem using flash technology is the VMETRO VMDRIVE. Based on a 6U card in either VME or CompactPCI, the VMDRIVE storage capacity is 40 or 80 Gbytes using flash-based drives. Versions are available in either air-cooled or conduction-cooled, as shown in Figure 5. The conduction-cooled units are operationally rated for 40g shock, -40° to +75°C temperature and up to 75,000 feet altitude. The unit weighs 2.2 lbs and consumes less than 2.5W total. Tests to MIL-STD-810F show these units suitable for helicopter environments. While not offering the capacity of hard disk drive solutions, solid-state drives can definitely deal with the harshest environments, and offer advantages in SWaP with their compact size and lower power consumption.
Packaging Makes it Possible
Storage for off-the-shelf platforms has taken exciting new directions, with higher capacity, better survivability and improved SWaP characteristics. Most of the breakthrough is creative packaging for commodity disk technology. Taking commodity hard disk drives and packaging them to survive in harsh and harsher environments is a robust, cost-effective solution. Hard drives with the right shock and vibration isolation can operate at low altitudes and temperatures above freezing. Adding pressurization and heating takes hard drives into higher altitudes and out-of-cabin applications. For the harshest environments, flash technology in solid-state drives has reached capacities making their use feasible, and eliminates moving parts making them extremely rugged. As flash continues to improve in density and cost, the solid-state solution becomes even more attractive.