After five years of double-digit growth, Rockwell Semiconductor Systems Inc.'s sales have dropped 10% in the last nine months and the once burgeoning company has fallen into the red, according to documents filed with the Securities and Exchange Commission. The struggling unit of Rockwell International Corp., which is expected to be spun off by Dec. 31, had a net loss of $38 million in the first nine months of the year, contrasted with a profit of $110 million in the year-earlier period. The division’s sales plummeted to $936 million from $1.04 billion in the same period. The filing is the first official step that Costa Mesa-based Rockwell International has taken to spin off its semiconductor division into a public company. Rockwell didn’t say how many shares of stock it planned to issue, the share price or when the stock would be distributed to existing Rockwell shareholders. The filing noted that the new company will have a new name, which has not yet been chosen. The company could not comment because it is in a “quiet period” imposed by the SEC while the deal is pending.