Defense Secretary Caspar W. Weinberger's decision last week to spend $25 billion to $40 billion on a combination helicopter and airplane for the Marine Corps followed warnings about the project from Pentagon analysts and the House Armed Services Committee.

Critics contend that the aircraft, called the V22 Osprey, is entering full-scale development at a time when there is not enough money to finance it, when existing helicopters could do the job and when uncertainties hang over the technology envisioned.

Supporters say the Marines must build the Osprey because their helicopter force is growing obsolete. The aircraft's enthusiasts also say the Pentagon will monitor cost and performance during production and that three years of testing have resolved the technical questions.

In a routine announcement last Friday, the Pentagon said it was awarding the team of Bell Helicopter Textron Inc. of Fort Worth, Tex., and Boeing Vertol Co. of Philadelphia a down payment of $497.3 million for full-scale development of the Osprey, formerly called the JVX. Full-scale development is the last step before full production and, if completed, will result in 12 flying prototypes.

Marine Col. H.W. Blot, Osprey program manager, said yesterday that the "tilt-rotor" aircraft will be able to hover like a helicopter or fly straight ahead like a conventional plane, with better speed and range than anything of its kind, providing the military with new capabilities beginning in 1991. He put the price for 913 Ospreys at $25 billion in fiscal 1984 dollars. Other Pentagon officials said the cost will be $40 billion when inflation and other increases are counted.

The biggest technical advances envisioned for the Osprey are engines and propellers, which are mounted on the ends of the wings and can be tilted up for vertical flight. The engines are locked in the conventional airplane position for straight-ahead flight.

Some critics contend that the wing, to be built of new composite materials rather than metal, will not be able to take the strain of this rotation while carrying a combat load of Marines and equipment. Blot, however, said scale-model prototypes of the Osprey had demonstrated the feasibility of the new technology.

The Pentagon's office of program analysis and evaluation, however, concluded three years ago that the Osprey concept was seriously flawed. The analysis, a copy of which was obtained by The Washington Post yesterday, said a combination of existing helicopters -- specifically the CH53E Super Stallion and CH60 Blackhawk -- could do the job for $9 billion less.

Mark F. Cancian, assistant director of national security programs at Harvard University's Kennedy School of Government, confirmed yesterday that he had written the report in 1983 while on the Pentagon analysis staff and said he stands by the conclusions. But Cancian declined to discuss them further on the ground that his report was an internal document.

The Pentagon study says the successor to the jeep -- the High Mobility Multipurpose Wheeled Vehicle, known as Hummer -- is too big for the Super Stallion to lift, and should be slimmed down or replaced by jeeps. The study also says that the Navy, which buys planes for the Marine Corps, can barely afford to buy its basic aircraft, much less start on a "very expensive transport."

The House Armed Services Committee staff last month issued a report stating that "the Marine assault mission does not appear to require" the Osprey's longer range because military tactics would require the aircraft to remain relatively close to accompanying assault boats and helicopters, and within the range of naval gunfire.

The Pentagon said the Osprey "represents a major technological breakthrough in aviation" for military and civilian purposes.

Deputy Defense Secretary William H. Taft IV last week said that "affordability" of the Osprey "still presents substantial concern" and would be closely monitored and formally reviewed by Pentagon executives by Dec. 15