Military hard-drive manufacturers balance the benefits of spinning-media and solid-state data storage

By John Keller

There was a time not long ago when military hard-drive manufacturers believed the military data storage industry was moving exclusively to solid-state data storage. Traditional military hard drives made from spinning magnetic media were thought to be too delicate for punishing military systems in the field, where extremes in shock, vibration, temperature, and other harsh-environment operating conditions are the norm.

Military hard-disk makers these days, however, have a whole new outlook. Spinning-media hard drives are no longer the shrinking violets they once were, and today are appropriate for the vast majority of military applications, even in milspec environments. Their primary advantages are low cost and high data capacity compared to solid-state data storage. Reliability often is not an issue.

Today's spinning-media hard disks "are certainly more rugged" then they used to be, points out Amos Deacon III, president of military data storage specialist Phoenix International in Orange, Calif. "The error correction capabilities of the drives have increased, and technically I have seen the published specifications of 3.5-inch devices go from a spec of eight or 10 Gs to 40 Gs in an operational environment; the same with vibration. Now you can throw those things against the wall, and they'll still work."

The increase in ruggedness of military hard drives comes from the reduction in mass of the head assemblies and in the data storage media itself, Deacon explains. Not only are standard-sized hard disks becoming more rugged, but also are their 2.5-inch notebook computer drives, which add to the size, weight, and power (SWaP) advantages of spinning-media hard disks.

Perhaps the most significant advantage of spinning-media disks is their cost relative to solid-state storage. Take a 600-gigabyte data storage system, Deacon says. A conventional spinning-media hard disk costs perhaps less than $1,000. Ruggedize and seal that same hard disk and the cost goes up to about $2,500. Yet use solid-state memory for 600-gigabyte drive and the cost can go to more than $10,000, Deacon says.

Military hard-drive manufacturers also are becoming much more innovative in packaging than they once were, points out Vladymir Rogov, vice president and director of marketing and design at military data storage maker Z Microsystems Inc. in La Jolla, Calif.

Often it is the need for spinning-media hard disks to operate at high altitudes aboard aircraft as part of military avionics systems that drives manufacturers to hermetically seal their disks.

The reason for this is how a spinning-media disk works. The read-write heads essentially travel over the disks on a cushion of air. At altitudes higher than about 10,000 feet, the air cushions get thin and the disks can cease to function. Seal in the air, however, and the hard disks can function to altitudes as high as 30,000 feet in unpressurized areas.

"We had to hermetically seal our hard drives to go at very high altitudes," Rogov explains. When they did that, his engineers opened many opportunities for aircraft applications where high capacities are necessary.

"With the increase in storing imagery, very high capacities are required," Rogov says. Z Micro offers a hermetically sealed, ruggedized RAID hard-disk system with four removable, hot-swap "brick" modules–each module with six terabytes of hard-disk storage. The system is 7.25 inches high and fits in a standard electronics rack. "This is 24 terabytes in a 19-inch rack; that is a big deal," Rogov says.

For the most environmentally demanding applications, such as the intense vibration of a helicopter, solid-state hard drives often are the only alternative, Deacon and Rogov agree. Extremes in heat and cold also can rule out spinning-mass drives.

"What a solid-state device buys you is significantly higher environmental specifications beyond shock and vibration," Deacon says. "In a rugged or deployed environment, rotating devices have problems with temperature; you can't go below 5 degrees Celsius or the hard drive will not spin up. With hot temperatures, it is not so much the mechanical operation, but the cooling of the devices that run the hard drives that is important. The high end is roughly 55 degrees Celsius. It won't burn up the drive, but it will significantly reduce their MTBF [mean times between failures]."

Many industry experts still believe that solid-state memory eventually will become the dominant storage media for rugged hard disks, but that time is in the future.

"If only there were equal price and equal capacity," Rogov says. "Solid state is the future. It is very cost-driven, but it will go there one day."

Military & Aerospace Electronics June, 2009