Frontier Airlines announced today that it plans to double the frequency of flights it offers between Des Moines and its Denver hub, beginning in June. The airline began offering six weekly round-trip flights between the two cities last October. With the change, Frontier will have two daily departures to Denver from Des Moines six days a week. The airline also announced that it will begin new nonstop flights between its Kansas City hub and Minneapolis-St. Paul and San Antonio this summer. Frontier also said it will add Knoxville, Tenn., Provo, Utah, and Sioux Falls, S.D., to its route map with the addition of nonstop service from its Denver hub, bringing the number of nonstop destinations it serves from Denver to 63.