LINTHICUM, MD. -- By day, they work in utmost secrecy designing radar systems and other sophisticated electromechanical equipment for the nation's defense.

By night, or whenever they have free time, they put their immense engineering and computer skills to work for the handicapped.

They call themselves the Volunteers for Medical Engineering Inc., and they are mostly aerospace wizards who derive satisfaction from using their technical know-how and inventive powers to help solve everyday problems of the disabled.

Their greatest contributions have been improving upon existing devices and equipment: The mobile standing frame, which allows the wheelchair-bound to move around in a standing rather than sitting position, is one of them.

Westinghouse engineer John Staehlin, 57, founded the group six years ago and is promoting its cause as energetically today as he did then.

Staehlin, a senior advisory radar antenna engineer at the company's Defense and Electronics Center, has filed 200 invention disclosures with the company and has at least 10 patents. He was the lead mechanical engineer on the antennas for the B 1B bomber and the Airborne Warning and Control System, known as AWACS. He's now working on advanced fighter radar.

Westinghouse now lets him spend eight hours a week on VME activities. He'd like it to be a full-time position, although he knows "it's asking a lot."

Nearly 100 employes, including some without scientific backgrounds, at Westinghouse's suburban Baltimore complex are active members of Volunteers for Medical Engineering Inc. Chapters have also been set up at five other companies around the country.

The organization, which Staehlin admits is only little more than a loosely knit network of volunteers, has reached a critical stage in its development.

Publicity about its endeavors has swamped VME, which has no paid staff, with calls for its products. An article in a May issue of Parade magazine about the group's inventions generated more than 1,300 letters and telephone calls.

"We were surprised by the magnitude of the response," Staehlin said.

Letters are going out soon to those who have inquired about VME's inventions, "telling them we are gearing up to satisfy their needs but that what they saw were prototypes," and they will be apprised of further developments, Staehlin said.

"That's not a good answer," he acknowledged, "but it's the only one we can give right now."

"We are having problems as an organization growing and organizing," said Westinghouse's Wilson Rivera, another volunteer. "We have to have a timetable in our minds that is reasonable to respond to the requests."

The group is developing some long-range plans, such as establishing a chapter in every aerospace industry, creating a research institute and setting up a company with both nonprofit and profit-making components.

It would be worse, Rivera said, if VME raised the hopes of the handicapped and didn't follow through than if the people never heard of the group's work at all.

Another Maryland corporation, Ridge Engineering, has built a prototype of a redesigned and less expensive mobile standing frame, and 30 units are under production. But there are at least 300 requests to meet.

Another invention in big demand is the servo-driven orthosis, a motorized brace. The device allows people with no use of their wrists or fingers to grasp an object and let it go. With the servo-driven orthosis in place, a twitch can send a signal from an electrocardiograph patch on the shoulder to a tiny electric motor attached to a flexible brace on the user's hand.

Although there is a pressing need for many of the kinds of devices VME builds, the markets for them are typically not large. That means they may never get built commercially -- or if they do, they may be too expensive for many people.

"The idea around here is to try to get something to these people without any cost, or at a low cost," Staehlin said. "That is the underlying goal. A lot of people we encounter could not otherwise afford what we're providing. You can't believe how nice it is to say, 'We're not selling them, we're loaning them to you.' "

By designing products in their spare time, VME members have been able to eliminate the large, nonrecurring cost of design that can be made up only by selling many units or charging high prices.

Donations of goods, services and cash have helped VME to build its prototypes. But Staehlin said the organization needs considerably more assistance from manufacturers.

VME works closely with medical advisers from hospitals, rehabilitation centers and universities. Gary Ward, a quadriplegic who has been the only user of the prototype of the servo-driven orthosis, also serves on the organization's board.

"He {Ward} tells us when we have crazy ideas that don't mean anything to the disabled community," Rivera said. "A lot of people have told us it's not good to go off and provide a solution if you don't have a problem."

Ward said he has been impressed by the group's work and its founder's energy and ambition.

"I don't think he had any idea VME was going to be as involved and grow as fast as it has," Ward said.

The organization has been involved in about 50 projects, about a dozen of which are being worked on actively. Computers and special peripheral devices have been used in a number of the projects to help people with various degrees of handicaps.

One team of VME volunteers is currently working with the University of Maryland School of Medicine on a microwave probe for the treatment of inoperable brain tumors. The probe is designed to literally "cook" a malignant tumor by microwave irradiation.

Engineers are also working to modify the controls of a tractor for paraplegics, so it can be operated by quadriplegics. A strap attached to the chin would enable quadriplegics to operate the controls.

"You could have a company cutting lawns or snow-blowing made up entirely of quadriplegics," Staehlin said.

As the volunteer organization grows, so do some of its worries. To date, the volunteers have been unable to get insurance. Staehlin recalls once having a restless night after installing a hoist-and-trolley arrangement in the home of a bedridden man. Fearing that something could go wrong despite its sturdy construction, he went to the home the next day with a steel cable to reinforce it.

The volunteers say they realize they will have to get their organization in better order to receive insurance.

"You can imagine what an insurance company's reaction is when you say, 'We have a bunch of engineers who volunteer their time, will you insure us?' " Rivera said.