The Pentagon acknowledged last night that the 50 Lockheed C5B Galaxies it wants for strategic airlift of military equipment will cost $2.1 billion more than Congress was told, but it heatedly rejected charges of a cost overrun.

Rep. Norman Dicks (D-Wash.), who is leading a fight to substitute modified Boeing wide-bodied airliners for the C5Bs, said that Air Force Assistant Secretary Russell D. Hale confirmed a new cost estimate of $10.9 billion and charged that the Air Force has "a very serious credibility problem."

But Hale told a reporter that Dicks had "misconstrued" data, saying that the $2.1 billion--a tentative "planning" figure--is not a true increase over the $8.8 billion in the fiscal 1982 budget. That budget, he emphasized, presumed that a C5B contract would be signed last April, but Congress denied funding. Now, with a delay to November expected, Air Force programmers have had to recalculate the numbers, taking new inflation projections into account.

Hale also said that the new, $10.9 billion estimate includes an unrealistic $2.1 billion for spare parts. The original amount was $681.8 million. Hale, Sen. Sam Nunn (D-Ga.) and Lockheed all insisted that there has been no growth in the price of the C5Bs as expressed in 1980 dollars and proposed by the company in 1981.

They blamed the seven-month contract delay for a slippage in Galaxy deliveries. Originally, Lockheed was to deliver four C5Bs in 1985, 11 in 1986, 12 in 1987 and the final 23 in 1988. The Air Force now expects none in 1985, seven in 1986, 12 in 1987, 21 in 1988 and 10 in 1989.

The Pentagon has said expansion of strategic airlift capacity is urgent. Lockheed said last night that there will be no change in the time needed for deliveries once a contract is signed, but Dicks disputed this, saying that a key Lockheed subcontractor has warned of delays.

The disclosure of the new data came on the eve of a House-Senate conference on how to deal with the conflict between the Senate, which embraced the Boeing 747s in May, and the House, which voted July 21 to fund the Galaxies.

Boeing's original estimate of comparative costs, in 1982 dollars, claimed that used 747s could be bought for at least $5 billion less than the C5Bs. But the Air Force said last month that over a 20-year life cycle the saving would be a relatively modest $900 million and would not be worthwhile because the Galaxies better suit its airlift needs.