PORTSMOUTH — A group of fifth graders had a chance to meet their pen-pals on board the Rhode Island, a ballistic missile submarine, during a picnic lunch at Glen Park on Thursday.

The students, members of Cindy Jilling’s class at Elmhurst Elementary School in Portsmouth, have been writing letters to the crew all year, a tradition that began six years ago when Kati Machtley, the wife of former U.S. Rep. Ron Machtley, asked Jilling to adopt the sub’s crew.

The submarine’s “Blue Crew” is in Rhode Island to march in today’s Fourth of July Parade, the nation’s oldest Independence Day event and one that draws thousands of people from throughout New England.

Seven submariners, from the cook to the commander, fielded questions from about 15 students and their friends, who gathered in the sun-soaked park as thunder boomed in the distance.

The children learned that submariners spend almost three months on duty, a period when they have no access to phone calls from family or television. They also learned that submarines serve the best food in the armed forces, including prime rib and lobster.

Some fun facts about ballistic missile submarines:

•They are two football fields in length, the same size as a large cruiser.

•They carry 24 missiles (the crew can’t confirm or deny that they carry nuclear weapons; that’s classified).

•The crew sleep nine men to a room. Only the commander has his own bedroom.

“Are there any married couples on board?” a little girl asked.

“No,” Cmdr. Robert Clark said, “we don’t have any females. But eventually, there will be.”

“What does classified mean?” one parent asked.

“We try to keep some things secret,” Clark said. “If the entire world knew what our Navy was doing, they’d exploit that.”

“How long can submarines stay under water?” another asked.

“Indefinitely,” Clark said.

The Rhode Island is based in Kings Bay, Ga., and unfortunately, it couldn’t accompany the crew to Portsmouth to meet Jilling’s fifth-graders, who have also sent the crew art work and their favorite candy, Jolly Ranchers. Kati Machtley is the sub’s so-called guardian angel, a role she adopted after christening the ship in July 1993.

lborg@projo.com