WASHINGTON — U.S. forces in Afghanistan get nervous when they have to reel down one of their tethered, video­camera-equipped aerostats for maintenance.

“They want it up as long as they can have it — that persistent surveillance stare,” said Army Lt. Col. Robert Helms, who oversees work on the aerostats, called the Persistent Threat Detection System (PTDS).

Last year, the Army accepted 28 PTDS aerostats, adding to the nine it already owned. Now commanders want airships that stay up even longer, scan more terrain with radars and cameras, and serve as communications relays.

That means they will have to be bigger.

The Army, the Defense Advanced Research Projects Agency (DARPA) and their contractors are moving forward with imminent plans to inflate and fly giant airships that would set new aviation standards for size, endurance and presence. The airships would provide detailed pictures of battlefield conditions for combat troops who need the information immediately.

One, the Army’s Long Endurance Multi-Intelligence Vehicle (LEMV), could be in Afghanistan by the end of 2011. Others are being prepared for key tests.

In June or July, Lockheed Martin plans to test-fly its High Altitude Long Endurance-Demonstrator (HALE-D) airship at the company’s Mission Systems and Sensors division airship facility in Akron, Ohio. About 21 meters in diameter, the test airship will carry a 23-kilogram payload — a camera and a communications repeater — at an altitude of between 18 and 21 kilometers for 10 to 14 days. HALE-D is big, but it is a scaled-down version of the airship originally envisioned under the dormant High Altitude Airship program.

“This is very Wright Brothers-ish,” said Eric Hofstatter, the program manager for another Lockheed airship effort, the Integrated Sensor is Structure (ISIS) airship project. Like HALE-D, ISIS is designed to fly extremely high, providing a wide-area view. “There are no stratospheric airships flying today, at all,” Hofstatter said.

Lockheed Martin Skunk Works, the company’s Advanced Development Programs Division, is developing ISIS for DARPA.

Developers and researchers want to use HALE-D to compare real performance to simulations conducted with models on the ground prior to launch and gather data on wind and air pressure. Though Hofstatter said HALE-D and ISIS are not directly linked, the results garnered from this summer’s test flight could help develop ISIS in time for its projected 90-day flight test from Akron to Key West, Fla., beginning in April 2013.

“Once we validate all the technology required for the stratospheric airship, whether from HALE-D or ISIS, it [will provide] a laundry list of capabilities everybody can use — from communications nodes to electro-optical [infrared] sensors,” Hofstatter said.

During its test flight, the 152-meter-long ISIS demonstrator craft will stay on station at the same altitude range as HALE-D, carrying a smaller and more modest radar and communications package than developers plan to place on the final product. It will have a top cruise speed of about 30 meters per second — roughly twice as fast as the Goodyear blimp.

Plans call for the construction of a second ISIS airship, incorporating changes as determined by the test demonstrator, for use by the Air Force and U.S. Northern Command. Ultimately, ISIS will carry very large, dual-band active electronically scanned array radars, capable of detecting air-, water- or land-borne moving targets at ranges from 25 to 600 kilometers, detect and track ballistic missiles at ranges greater than 1,500 kilometers, and penetrate foliage at ranges up to 170 kilometers.

The second ISIS, with the complete intelligence, surveillance and reconnaissance package and more-powerful regenerative fuel-cell and electric propulsion, is projected for a 2018 deployment and should stay in operation for 10 years.

At the extremely cold temperatures in which it will operate, ISIS will employ state-of-the-art thermal control units built by ATK of Beltsville, Md., comparable to those the company has long built for satellites and spacecraft, but adapted for the airship platform. The units will be able to transport heat through the craft with passive systems, eschewing heavy and complex mechanical pumps and valves.

The inflated skin, or hull, of ISIS — made of an extremely strong composite of woven materials with the thickness of two sheets of paper — will serve as the radome for the radar, powerful enough and with sufficient aperture to provide detailed information to front-line troops.

“The only way to take advantage of high-resolution tracking and targeting is to increase apertures by orders of magnitude,” Hofstatter said. “That provides insight as to why DARPA came up with ISIS.” Beyond that, he said, details on the system are classified.

While ISIS would patrol the stratosphere for one long deployment, another large airship set for debut will provide comparable capabilities at lower altitudes and shorter durations, but with greater payload flexibility. Northrop Grumman’s LEMV, about 91 meters long, will face its first inflation test in June. The goal is to deploy the airship to Afghanistan by December for an operational demonstration.

Once on station, it will fly at 6 kilometers and provide a continuous intelligence, surveillance and reconnaissance presence for three weeks at a time. The LEMV will be a hybrid airship, combining the lift of helium with aerodynamic lift and control.

While larger airships carry a bigger wow factor, proponents of lighter­than-air surveillance insist that smaller craft like PTDS are doing more than merely filling the breach until ISIS and LEMV enter service.

In fact, while Lockheed’s Mission Systems and Sensors division is preparing HALE-D for inflation, the same group is working on an upgrade to PTDS, including installation of new L-3 Wescam MX-20 video cameras and Northrop Grumman STARLite radars. Ground control stations and gondolas will be upgraded as well. The Defense Department awarded Lockheed the $85.3 million contract Feb. 25, with an anticipated completion date of Nov. 23.

Another company, Mav6 of Alexandria, Va., is working under a Defense Department contract to develop and deploy its Blue Devil 2 multisensor aerostats to Afghanistan by early next year.

Once on station, the aerostats will provide warfighters in Afghanistan with two tools they do not have, according to Dave Bither, the project’s program manager. “One is the persistent stare and collection of data for long periods of time,” Bither said. “The other is tailorable sensors, which the [Defense] Department does not have now.”

Once inflated, the aerostats are roughly 100 meters long and 19 meters high and will carry a payload of up to 1,134 kilograms. Because the aerostats are not filled to high pressure, they can resist small-arms fire. And because they rely on what Bither refers to as tried-and-true aviation-grade engines and possess vertical takeoff capability, they require neither extensive retraining for maintenance personnel nor long and smooth runways.

Still, experts wonder how practical they would be in a conflict against a foe with better capability to shoot them down.