This year opens the decade of hard choices for defense leaders, mainly highly selective prosperity for those aerospace companies dependent on government business.

Although boom and bust have long marked the topographic maps of the defense business, these highs and lows will become even more so in the 1980s because the Pentagon will spend most of its trillion-plus dollars on relatively few companies over the next five years.

One reason this must be the case, even under the Reagan administration which is taking a harder line on defense than the Carter administration it replaces, is that each super-weapon costs so much that buying a variety is out of the question.

Only General Dynamics and Tenneco Inc., for example, will make the big money on the Trident missile submarine and Nimitz nuclear aircraft carrier programs laid down for the 1980. Their respective shipyards, Electric Boat and Newport News, have those shipbuilding contracts locked up. At $1.5 billion for one Trident sub and $2 billion-plus for one Nimitz, the Pentagon is not about to go through the turmoil and expense of letting several more yards built those super-ships, even if they were capable of doing the work.

The same holds true for aircraft and, to a lesser extent, missiles.

The rich in the airplane business will get richer and the poor poorer. McDonnell-Douglas with the F-15 fighter, General Dynamics with the F16 and Northrop with the F18 fighter will continue to get the lion's share of the aircraft procurement dollar.

But Boeing, Fairchild, Grumman, Lockheed, Rockwell and Vought will be among those struggling for new military aircraft contracts. Although a new transport, the CX, is on horizon, it will be years before it becomes big bucks, unless the Pentagon does the unexpected and just orders an update of the Lockheed C5 transport. Boeing and Lockheed are prime contenders for the CX.

Fairchild is looking for business to take over from its A10 antitank plane nearing the end of production. Grumman needs a big order to supplement the low-level orders for its F14 fighter, while Rockwell has its hopes pinned on the Reagan administration making good on its campaign rhetoric about the need for a new bomber.

Vought, builder of the A7 attack plane, has diversified to the point that failure to get a follow-on contract would not put its aerospace program in a nosedive.

If Fairchild and Grumman fail to find new airplane business, they may well become examples of the poor getting poorer in the 1980s as far as prime defense contracts are concerned.

"There's a lot of stuff on the drawing board, but nothing close by," said one aerospace executive in surveying military aircraft potential business for the early 1980s.

Although it might sound from the campaign rhetoric of president-elect Reagan and his allies that billions of dollars for a new bomber is a sure bet for the early 1980s, 'tain't necessarily so.

The Air Force in this age of multibillion-dollar weapons can afford to finance only a few big ones at once. Bills are already piling up for the MX and will grow steadily higher in the 1980s, topping $34 billion in fiscal 1980 dollars if Reagan follows the course already laid down. On top of that are billions committed for the new A10, F15, F16 and rewinging of the C5 transport. Is 1981 the year to plunge full speed ahead on a new strategic bomber? Today's Pentagon executives do not think so. And some aerospace executives agree.

This is one reason the Air Force's Strategic Air Command commander and General Dynamics executives are allied in an effort to persuade the Air Force to build a stretched version of the General Dynamics F111 as a replacement for the B52 bomber.

"Over my dead body," said William J. Perry, Pentagon research director, when asked about this course. He favors building a more advanced plane, most likely one that would include the Stealth technology for making it virtually invisible to enemy radar. Perry would rather depend on the B52 a few years longer than rush into building a new bomber.

Since Reagan assailed Carter for canceling the B1 bomber, he may feel obliged to rush ahead with a new bomber. Perry warns this may cost the military services in readiness to fight, another big issue during the election campaign. He recommends that his successors space out their new starts in such superweaponry.

Readiness is the umbrella term for describing how ready soldiers, planes and ships are to go to war. The readiness accounts provide money for servicing weaponry, often by civilian firms, and buying spare parts. Critics complain that the military services have short-changed readiness to free more money for buying new weaponry. One result has been the finding that only about half the warplanes already purchased are ready to fly at any one time. Congress became so exercised about this short-changing of readiness that the fiscal 1982 Pentagon budget earmarks billions to reverse that trend. So much so, in fact, that Army leaders are complaining the new stress on emphasis will slow their modernization program.

For the defense industry, the big slice of the total Pentagon budget going into readiness does indeed mean fewer contracts for new weapons. While several aerospace firms will cash in on the new division of the money pie as they get more maintenance contracts, many of extra billions will not show up that way. Readiness, for example, requires extra training -- meaning millions more for fuel for ships and aircraft going on exercises.

Also, more than half the Pentagon's total budget, probably around 56 percent in the new fiscal year, goes to personnel accounts: paying, feeding, housing and training soldiers. With Reagan committed to making military pay comparable to civilian, the personnel percentage will increase. This is another reason the big Pentagon budgets for the 1980s will not mean boom times across the whole front of the defense industry.

"The near term future will not be as expansive as the dollars in the budget suggest," said one executive of an aerospace giant in the top 10 of Pentagon prime contractors. "With the bigger percentage going to operation and maintenance" (the readiness accounts), "I don't see the big bonanze for hardware. We're looking at a fairly flat situation in the early 1980s."

However, "fairly flat" still means prosperity for the aerospace industry, just not boom times for most of it. After all, a generous slice of a $200 billion military budget -- the total expected for fiscal 1982 -- still means millions of dollars in orders for hundreds of American companies.