President Carter's blueprint for a rapid deployment force came under its first congressional challenge yesterday as senators questioned whether it made sense to spend $6 billion on cargo planes that could carry only one tank at a time to the Persian Gulf.

The plane at issue, still on the drawing board, is called the CX. Carter wants to build from 130 to 200 of the planes to carry the Army's big stuff, like tanks, to distant trouble spots, where it would be used by the rapid deployment force.

Democratic Sens. Sam Nunn of Georgia, home of Lockheed, which would compete for the CX contract, and Gary Hart of Colorado, questioned at a Senate Armed Services committee hearing yesterday whether a CX that could carry only one XM1 Army tank was worth buying at all.

"There would be no way to get enough XM1 tanks by airlift to do much good in a rapidly developing situation," Nunn complained to Pentagon witnesses.

The better idea, he said, might be to build lighter armored vehicles that could be carried in quantity to world hotspots inside cargo planes the Air Force already has flying.

These planes are the Lockheed C130, C141 and C5A. Although the C5A is the giant, it can only fly one Army XM1 tank at a time. An Air Force spokesman said yesterday that it costs $6,793 to operate a C5A for one hour carrying an XM1 tank inside. Cruising at 500 mph, this would mean spending $135,860 to deliver one XM1 tank from the United States to the Persian Gulf.

Robert Komer, under secretary of defense for policy, said, "I've been asking for 15 years" why the Air Force builds planes big enough to accommodate Army vehicles and other equipment rather than hold down the size of the weaponry so it would fit into smaller aircraft.

"I have never gotten a satisfactory answer," he told the committee.

The army's XM1 tank weighs about 65 tons, and costs $1 million a copy.

Komer said that the Marines, who would spearhead the rapid deployment force, would not depend on aerial delivery of tanks, artillery and other weaponry. Instead, he said, that would be stored ahead of time in a fleet of 14 cargo ships anchored near likely trouble spots around the world.

If neither the C5 nor CX could deliver big tanks in volume, but ships could store gear for the rapid deployment force, why spend $6 billion on the CX? Hart asked.

"We need both," Komer replied, stressing that the rapid deployment force now being organized should be supported by both ships and new long-distance cargo planes.

After the hearing, Nunn said, "I'm dubious" about the current Air Force concept for the CX plane. The senator said, however, that he favored building more long-distance airlift planes, but not necessarily the version of the CX being planned.

Lt. Gen. Kelly H. Burke, Air Force research chief, told a group of reporters yesterday at a breakfast meeting that the CX would be smaller than the C5 so it could land on airstrips near battle zones. The C5A, he continued, is so large that only international airports are big enough to handle it. Then the equipment inside the C5A has to be airlifted by another type of plane to field nearer the trouble spot.

The newly appointed commander of Carter's rapid deployment force Marine Lt. Gen. P. X. Kelley, said that the Marine Corps is attracted to lighter armored vehicles and is investigating various ones with an eye to buying them. Nunn said he was delighted that the Marines are looking in this direction, given the need for deploying light but hard-hitting forces to distant points in a hurry.

"What we need," Nunn joked in an interview after the Senate hearing, "is a flying tank."