The CX cargo plane President Carter wants to build for the Rapid Deployment Force is in trouble again, this time with the General Accounting Office.

The GAO, which minitors government contracting for Congress, complains in a new report that the Air Force is taking off on the $10-billion CX program without knowing wher it is going.

Without waiting until it has sketched out what kind of new cargo plane the military needs for the 1990s, lamented the GAO, the Air Force is formally asking contractors to design the CX. A copy of the report was sent to Secretary of Defense Harold Brown.

Rather than specifying what the CX should look like and how much it should carry, the Air Force took the unusual step last week of asking defense contractors to recommend the best way to move various loads of cargo specific distances with a mix of aircraft -- existing C5A, C141 and C130 transports and the new CX.

One of the specific challenges given to the airplane companies was to move 130,000 pounds in a CX 2,400 miles without refueling in mid-air. But industry sources said the Air Force is not getting into specifics of design, as is usually the case.

While defense executives said they welcome such freedom in designing the CX, the GAO took a dim view of this approach. It criticized the Air Force for going into the full-scale development phase on the CX before finishing the mobility studies ordered by Congress earlier this year.

This lack of preparation, warned GAO, may result in an airplane that can't do the job at hand: carrying tanks and other heavy stuff to such distant places as the Persian Gulf where the Rapid Deployment Force may have to go to protect oil fields.

Already, the watchdog agency added, the Army's new XM1 tank is gaining so much weight that the CX the Air Force lobbied for early this year could not carry it the required distances.

Several congressional committees lambasted the Air Force for trying to get money for the CX without knowing just want kind of airplanes it needs. The House Appropriations Committee, for example, in slashing the CX request from $81.3 million to $20 million last month, said:

"No one is able to say what a CX will look like, what its capacity will be or what it will cost. No one is able to say how the CX fits with the other airlift, sealift and prepositioned assets already available or planned. Even the secretary of the Air Force has said that the Air Force has failed to make a convincing case for the CX."

Besides that, said the House Appropriations Committee in its report on the Pentagon's Fiscal 1981 money bill, the Air Force is trying to launch the CX program without fixing up the C5A transports it already has.

"The Air Force admits that it has underfunded spare parts for the C5A fleet [by] an astonishing $511 million," said the report. "The committee believes that the Air Force should be caring for its existing assets before embarking on a new effort variously estimated at costing between $10 billion and $15 billion."

The GAO weighed in with its complaints about the CX program in a report addressed to Brown dated Oct. 10. The Air Force went ahead with its requests to industry on Oct. 15. The service either brushed aside GAO's objections or was not aware of them. The Pentagon's spokesman promised to look into the matter yesterday.

"We believe," said GAO in its report, that asking industry for designs before the studies are in hand "is both premature and contrary to the sound acquisition principles of Office of Management and Budget Circular A109."

The Army, GAO continued, is making changes in its XM1 tank that are expected to make it weight more than 130,000 pounds. This means the CX the Air Force described in rough terms to Congress could not carry the XM1 the desired 2,400 miles without mid-air refueling.

Since the Air Force already is short of tankers for such long-distance refueling, the GAO said, "adequate aerial refueling may not always be available."

Aggravating that problem, the report said, is the likelihood of countries refusing to let the CX land on their fields for refueling en route to the Persian Gulf.

In its report signed by Acting Director W. H. Sheley Jr., the GAO urged Brown to stop the Air Force from putting out the call for designs. Now that the Air Force has gone ahead, the next step would be for the service to choose one of the designs submitted. The selection process is scheduled to begin in January 1981.

Production of the fleet of 200 CX aircraft could begin in 1987.But the CX project already has provoked so much controversy that the Air Force timetable is likely to slip.

There also is a chance that the administration or the Congress will give up on the Idea of building a new CX and insist that an existing plane, such as the C5A or the 747 commercial airliner, be modified to meet the extraordinary requirements of the Indian Ocean theater considered so critical these days.