The military has more than enough large transport planes, and the appropriation of any more in the next budget year will force some into premature retirement, Defense Department officials told a congressional panel July 13.

"We have enough C-17s," said Mike McCord, the principal deputy undersecretary of defense (comptroller). "Money spent on things we don't need takes away from those we do need."

Along with Mr. McCord, Maj. Gen. Susan Y. Desjardins, the director of strategic plans for Air Mobility Command, and Alan Estevez, the principal deputy assistant secretary of defense for logistical and materiel readiness, repeated Defense Secretary Robert M. Gates' position against the purchase of more C-17 Globemaster IIIs to the Senate Committee on Homeland Security and Government Affairs' federal financial management subcommittee.

All three defense officials agreed with the subcommittee's leaders, Sens. Thomas Carper and John McCain, that the C-17, in addition to the C-5 Galaxy, has been critical to airlift in and out of Iraq and Afghanistan. However, they said, the military's current fleet of 223 C-17s and 111 C-5s is more than enough airlift capability for years to come.

A department study that concluded in February was consistent with two other studies that found that the current fleet is sufficient "even in the most demanding environments" to take the military through 2016, Mr. McCord said.

The oldest plane in the transport fleet, Lockheed's C-5, will be viable until 2025, and the fleet as a whole should last until 2040, he said.

The department has not requested C-17s, built by Boeing, since the fiscal 2007 budget, yet Congress has added them every year since, spending about $1.25 billion on C-17s "that we don't want or need," said Mr. McCord, who was a 21-year staff member of the Senate Armed Services Committee before his current appointment.

Any additional appropriation for C-17s will have to be offset by retiring some of the military's older, but still viable, transport planes, the defense officials said.

And, adding force structure such as aircraft always entails additional costs in training, maintenance, and infrastructure, such as new hangars, bases and tooling, defense officials said.

The department spends about $50,000 per aircraft per year to store aircraft where spare parts are available, General Desjardins said.

"It's the gift that keeps on giving, because if you give it to us, we'll maintain it," Mr. Estevez said.

It would be more cost-effective, the defense officials said, to modify the C-5M for longer viability to continue to work in conjunction with the C-17.

Mr. Desjardins called the C-17 the "backbone" of the air mobility fleet, and said the C-5's combination of long range, high capacity and capability to carry outsize cargo is unequaled.

Together, she said, "they meet the needs for cargo and capacity anywhere in the world."

Retiring the least-capable C-5s would save about $320 million, General Desjardins said.

"Making tradeoffs of two types of aircraft when we already have more than enough of both is not going be cost effective," Mr. McCord said.

When asked what the department would cut to accommodate any new C-17s, Mr. McCord said that would depend on how many new C-17s were bought.

"You and Congress would decide that," he said, "because you would cut from our budget about $300 million for every C-17 added."

"We have a good mix right now," Mr. Estevez said. "Replacement is definitely not the most cost-effective way. Buying more to retire more is certainly not the way the department needs to balance its resources."

The defense secretary has made that case to Congress, and President Barack Obama has promised to veto any legislation that provides for more C-17s.