Taking emerging Pentagon capabilities such as long-range conventional strike, cyber warfare and missile defenses into account, "are those all just additive or do we put a balance in here that acknowledges that the number of countries now that we have to deter has gone up from one to more than one?" the general said. "And the deterrence for one is not necessarily the deterrent for the next?"
He added: "We haven't really exercised the mental gymnastics, the intellectual capital, on that yet. It's starting. I'm pleased that it's starting. But I wouldn't be in favor of building too much [more military equipment] until we had that discussion."
In terms of modernizing today's nuclear-weapon platforms, Cartwright acknowledged that he has been skeptical of Air Force arguments that a future bomber aircraft must include a wide array of highly technological capabilities and include a human in the cockpit.
"I'm known as a bomber-hater, I guess," said the general, who went on to explain why he thinks the caricature is not quite accurate.
"I think you have to have a bomber," Cartwright said. "I'm questioning what it is we're building, and what attributes we're putting against it."
The Joint Chiefs vice chairman -- who heads the Pentagon's top-level review panel with authority to determine all of the military's major hardware requirements -- said the nation should buy an affordable bomber to replace its aging fleet of conventional-only B-1s and nuclear-capable B-52s and B-2s.
"What I'm trying to understand is: What is it we're going to build it for? Is it the most exquisite, high-end, penetrating, go-anyplace anytime weapon system?" Cartwright said. "Or is it a truck that has today's state-of-the-art survivability attributes, can incorporate the next-generation attributes in a way that makes sense -- [including] sensors and whatnot -- and carry a reasonable payloads?"
A cheaper aircraft would allow the Pentagon to build a larger fleet, Cartwright said.
"If we're going to go out and spend billions of dollars to build something less than 20, then I question the investment," Cartwright said.
He also said he would "throw down the gauntlet" by asking whether the bomber truly requires a human pilot, or if instead all of them could be remotely controlled. Air Force leaders have called for a new bomber that could be flown either manned or unmanned.
"Nobody's shown me anything that requires a person in that airplane. Nobody," said Cartwright, noting that "the manned part of this does not necessarily drive the cost."
However, a manned bomber is typically designed to be more survivable and human-friendly, features that could be modified or jettisoned if the aircraft is conceived to be remotely piloted from the start, he said.
While the Pentagon has already started planning how it would modernize the nation's fleet of nuclear-capable bombers and submarines, studies on how to update today's Minuteman 3 ICBMs are only just beginning. The Air Force was said to be completing an initial assessment of its future-ICBM options late last month.
"The land-based deterrent [is] the last one to be recapitalized," Cartwright said. "The challenge here is that we have to recapitalize all three [triad] legs and we don't have the money to do it.
"What I'm worried about is ... that the [funding] trough should not determine which one we have," Cartwright added. "So we ought to make that decision now, and we ought to engender the discussion about what does deterrence look like when we get out to 2020 [or] 2030."