Congress told the Navy to develop concepts for less expensive aircraft carriers than those being built at Newport News Shipbuilding after the USS Gerald R. Ford, which was commissioned in July as the first of a new class of warships, suffered repeated cost overruns and delays.

The federally funded Rand Corp. came up with four alternatives , released this week in an unclassified report. Two designs call for nuclear-powered carriers, while two call for much smaller, conventionally powered ships that could only launch aircraft capable of taking off and landing vertically.

The Navy sent copies of Rand’s report to congressional committees along with a letter warning that the designs for much smaller carriers wouldn’t meet current operational requirements and would require new aircraft types and alternate concepts of operations. The Navy said it would further study those concepts as it examines the design of its fleet of the future.

The designs closer in size to the Ford still would reduce the capabilities the Navy requires of its aircraft carriers for mission success, according to the Sept. 8 letter. The smaller of those two variants wouldn’t be cost-effective or feasible because of engineering challenges, according to the Navy.

“The Navy remains committed to studying the design of the next generation of nuclear-powered aircraft carrier while continuing to reduce the cost of the Ford-class program at every opportunity,” the letter said.

Here are the four concepts Rand developed, and the limitations it says each has for replacing Nimitz-class aircraft carriers, which the Ford class is supposed to do.

1. CVN 8X

The design: This would be a variant of the Ford, with the same size flight deck, a 1,090-foot length and 100,000-ton displacement. It would be built with two life-of-the-ship nuclear reactors instead of ones that have to be refueled after 25 years. But it would have a 40-year lifespan instead of the 50 years the Ford class has with a midlife refueling and complex overhaul, which takes the ship offline for 44 months.

The shortfalls: The Rand report says the CVN 8X offers similar warfighting capability as the Ford, but it would have one less catapult than the Ford’s four and, therefore, couldn’t launch as many aircraft as quickly . But even with one less catapult, this variant could launch more aircraft than Nimitz-class carriers, which also have four catapults but rely on steam instead of a new electromagnetic system used on the Ford.

The costs: The USS Gerald R. Ford cost about $12.9 billion. Rand estimates the CVN 8X would cost about $920 million less over the lifetime of the ship. The cost-savings would largely come from removing a catapult and eliminating mid-life refueling costs as well as working off an already in-place design and knowledge gained from its development.

2. CVN LX

The design: This would be about the size of the former USS Forrestal, the first of the “super carriers” built in the 1950s. It would be 1,040 feet long, displace 70,000 tons and have a maximum speed of 28 knots, compared with more than 30 knots for the Ford. It would rely on a hybrid propulsion system using a nuclear reactor and electric turbine generators. It would have a 50-year lifespan with a mid-life nuclear refueling and overhaul.

The shortfalls: This concept would only allow the launch of 80 aircraft per day – half the number of the Ford class. It also could carry less ammunition and may require more frequent refueling. It would be less survivable in some battle environments and have fewer backup systems than the Ford class.

The costs: Developing a new class of nuclear-powered carrier likely would be expensive up front. But follow-up expenses after that initial investment would mean those ships would cost about $5 billion less per ship than the Ford class over the lifetime of the ship.

3. CV

The design: A 43,000-ton variant of the USS America-class amphibious assault ships. It would be fossil fuel–powered and able to handle only short takeoff and vertical landing operations but at a higher tempo than the current America. It would be 850 feet long, have a maximum speed of 22 knots and be capable of carrying up to 35 aircraft – compared to the Ford’s 80 – and launching up to 55 aircraft a day, compared with 160 by the Ford. It would have a lifespan of 35 to 40 years.

The shortfalls: With a much smaller flight deck, this variant would only be capable of launching aircraft capable of short takeoffs and vertical landings. This version could launch the F-35B fighter aircraft but not airborne early-warning or electronic attack aircraft that are part of a current carrier air wing and so would require support from a traditional carrier or land-based planes. It can only operate in areas where air defense threats aren’t significant or as part of a battle force. It would require refueling and rearming a more dispersed fleet.

The costs: This variant would cost about $10 billion less over the lifetime of a ship than the Ford class, but it wouldn’t replace carriers on a one-for-one basis. The Navy still would need to deliver additional ships or other platforms to support it. There also would be costs associated with procuring more F-35Bs that can take off vertically than F-35Cs that use a longer runway as well as developing options for alternative electronic attack and early-warning aircraft.

4. CV EX

The design: A 20,000-ton variant just 800 feet long with a maximum speed of 28 knots and able to carry 10 aircraft. It would be conventionally powered and have a lifespan of 30 to 35 years. This version would resemble the Italian light aircraft carrier Cavour.

The shortfalls: This design could only launch 15 to 20 aircraft a day, and they would have to be capable of short takeoffs and vertical landings. It also carries less fuel and weapons and could only be used to respond to low-level contingencies or in conjunction with a legacy carrier.

The costs: There is no estimate of comparative costs of using this variant to replace the Ford class because it would require the Navy to completely revise its concept of operations much more so than the CV LX.