Atlantic Research Corp. of Alexandria is preparing a new public stock offering -- its second in a year -- to raise additional capital for its rapidly expanding aerospace and energy business.

The company announced yesterday it will seek to sell 400,000 shares of its common stock. The offer will be made as soon as a registration statement can be filed with the Securities and Exchange Commission.

About 524,000 shares of Atlantic Research are now owned by public investors as a result of an offering last June that sold out in a single day.

That initial offering, at $10 a share, raised about $5 million, most of which was used to pay off debts.

The offering price of the new shares will be determined at the time the stock is sold. In the over-the-counter market, Atlantic Research shares were equoted yesterday at $14.50 bid.

The company said the new stock offerings will include both shares sold by the company and some stock owned by "insiders" who hold stock that has not previously been available to the public.

Atlantic's announcement did not specify how many shares would be offered by the company and how many would be sold by persons affiliated with the firm.

Until last year's public offering, most of Atlantic Research's stock was owned by its employes. From 1967 to 1976, Atlantic Research was owned by Susquehanna Corp. At the time, the employes bought control from the troubled conglomerate.

For the year ended Dec. 31, Atlantic Research's profits increased 29 percent to $1.63 million ($1.25 a share) from $1.26 million ($1.19) the prior year. Sales for the year increased to $54.5 million from $45.2 million and its year end backlog of contracts totaled $42 million.

Atlantic Research's chief business is developing and manufacturing solid-fuel rocket motors and gas generators. It also makes data communication gear and air purification devices for submarines and does research on alternative energy sources.