Necessity, they say, is the mother of invention. So it's no surprise that, in today's rocky economy, the number of people who are launching their own businesses is growing.

Between 1972 and 1979 the number of self-employed Americans rose by more than 1.1 million, a trend that continued through the end of 1981, according to Labor Department statistics.

In prosperous times, says William Whiston, director of economic research for the Small Business Administration, "people make what they know how to make. But during periods of recession and high unemployment, people scratch around for new ideas. That's when your basement inventors and new-service providers get going."

This surge in self-employment occurs, says Whiston, "partly because someone who gets laid off has to find another way to make money. But it's also a psychological thing. Entrepreneurship is the American way. Making it on your own can be a measure of your personal ability to cope with a tough economic situation."

One of the greatest growth areas, says the SBA's Holly Medert, is "the small-service business. People today have less time, and new kinds of businesses are filling in the gap. There's a big increase in firms that do things like run errands and shop for busy people."

So, despite the grim statistics -- half of all new businesses fail in the first 38 months, eight out of nine don't make it past 10 years -- the pioneer spirit lives on.

Those who do succeed credit hard work, willingness to take risks and plain good luck. Among those who've taken the self-employment gamble and won are these three Washington entrepreneurs.

On the wall of Ray Mott's office hang two framed posters that sum up his personal philosophy: "Find a way or make a way," and "If you don't try you'll never know if you can do it."

"I believe in those sayings," says the 43-year-old president of Raven Systems and Research Inc., an information-processing firm in Southwest Washington that grossed $9 million last year. "Because according to all the rules, I shouldn't be where I am today."

Raised in foster homes in New Haven, Conn., Mott dropped out of high school at age 17 to join the air force. Although he became a skilled aircraft engineer and mechanic, he discovered when he got out of the service that he couldn't get a job without a high-school diploma.

He moved to the District in 1961 -- "some of my buddies lived here" -- and worked as a waiter and a model. "I'd chat with the customers to get bigger tips. One couple owned a data-processing firm and offered me a job. I was ecstatic, because I'd been thinking about going to school for data processing. It sounded like a future that would be forever."

Mott started as a $100-a-week coding clerk, and in five years worked his way up to a $13,000-a-year general manager. "But I'd gotten to where I'd learned as much as I could and couldn't move up any further. I wasn't building any retirement funds, and I wanted more than a gold watch in my old age.

"Starting my own business seemed the way to go. And I figured if it came down to it, I could always go back to waiting tables." So in 1972, when a client offered him a job, he asked for a contract instead.

"I hired five editing and coding clerks and four other temporaries. I rented a room for $50 a month from the guy who gave me the contract, bought some tables and chairs, pencils and paper and set up shop."

Since he had no start-up capital, Mott arranged to be paid for his company's work in installments. "I've never missed a payroll," he says proudly. After his first success, he began working with the Small Business Administration's program to help minority-owned firms get government contracts.

"That gave me the chance to develop the company, to gather experience as an entrepreneur and to compete with other firms."

Today, Raven employs nearly 400 people in five locations around the country. Mott works 50 to 70 hours a week, and enjoys it "immensely."

"When you own a business you throw 9-to-5 out the window. You have more control over your own life, but the business does tend to run you sometimes."

Starting Raven, he admits, "was the hardest thing I've ever done -- next to sky-diving. Both had similar aspects. You've got to get over the fear and the tendency to cling to security. But I told myself -- in both cases -- that hundreds of other people had done it, so I could do it too."

His advice: "Take the word 'can't' and throw it out of your vocabulary. It may take time to make it, and you've got to decide if you really want to do it. But if you're determined, and can conquer your fear, you can do anything."