The number of active duty and reserve component members enrolled in the federal Thrift Savings Plan will hit a half-million this month. Recent improvements could encourage many more to join, say plan administrators.

In July, TSP began year-round enrollment, in place of twice-yearly "open seasons." The open seasons didn't always coincide with periods of transition, such as boot camp or reassignment, when members are dealing with personnel offices and opening a TSP account is more convenient.

A second, more important change was the introduction in August of Life Cycle, or "L" Fund, options. Designed by experts, the funds will provide TSP savers "all of the investment return they should get for the risk they're taking," said Gary Amelio, executive director of the Federal Retirement Thrift Investment Board.

The board administers TSP for 1.9 million federal civilian employees and, since March 2002, for the military. For service members, it's an opportunity to save and invest in quality funds at minimum expense. And though TSP is intended to boost retirement wealth, participants can borrow against accounts at attractive rates to buy a car, finance college or make a down payment on a home.

The Navy leads the services in touting TSP. Through September, 41.1 percent of active duty sailors had TSP accounts, compared with 30 percent of Marines, 26.6 percent of Air Force members, 25.6 percent of Coast Guard personnel and 18.7 percent of soldiers.

Carl Witschonke, the uniformed services representative to a TSP employee advisory council, provided examples of how the money can grow.

A new recruit who contributes 5 percent of basic pay each month, about $57, and earns a return of 7.5 percent annually, will have $83,000 in a TSP after 20 years. If the member retires, making no more contributions, there will still be $440,000 by age 60.

If the same member serves another 10 years, still contributing 5 percent of basic pay and drawing a 7.5 percent return, the account would grow to $257,000 at the 30-year mark and to $658,000 by age 60.

A typical officer will accumulate $163,000 in 20 years, which would grow to $643,000 by age 60. If the officer stayed 10 more years, TSP would climb to $483,000 by the 30-year mark and reach $927,000 by age 60.

Until August, TSP offered five investment funds, listed here with average annual returns over the past 10 years: Government Securities Investment,or "G" Fund (5.75 percent); Fixed Income Index Investment, or "F" Fund (7.72 percent); Common Stock Index Investment, or "C" Fund (11.99 percent); Small Capitalization Stock Index Investment, or "S" Fund (11.84 percent); and International Stock Index Investment, or "I" Fund (5.45 percent).

Amelio, who became executive director in June 2003, created the new Life Cycle, or "L," Fund. It is a mix of the five traditional TSP funds in combinations that maximize returns and minimize risk based on when investors expect to retire and start to draw down TSP. L Funds have been explained in marketing materials and at the TSP Web site, www.tsp.gov.