The Soviet Union's mix of modernity and backwardness inspired the joke that, for Russian cosmonauts, the most dangerous part of the journey is the elevator ride to the top of the rocket.

It is also inspiring a new and so far virtually unpublicized attempt by the Defense Department and some of its big advanced-technology contractors to extend and stiffen the leaky embargo system that is supposed to deny the Soviets Western technology of strategic value. Following an 18-month study of recommendations drawn up last year by a special task force, the Defense Department has worked out a detailed plan that is to be presented this week to the State and Commerce departments. That sequence is in preparation for next spring's meeting of the Coordinating Committee of Nations - NATO minus Iceland, plus Japan - that have agreed to cooperate on restricting strategic exports to Communist-bloc nations.

The big difference in the current effort is that it isn't confined to hardware, such as the high-speed computers, miniaturized electronic components, an laser devices that have become the underpinnings of a new and frenetic arms competition. These are, of course, included, as was the case last month when the Commerce Department blocked the sale of Control Data's supercomputer, Cyber 76, to the Soviets. They said they wanted it for weather forecasting, but the U.S. figured it was easily applicable to nuclear weapons research and air-defense management.

The point of departure in the new embargo scheme is that it is aimed at technological know-how, the "recipes," so to speak, for advanced tecnology. An embargo based on that concept may appear to be little different from one based hardware. But, in fact, its enforcement would encompass - as the Defense planners candidly state - the entire span of advanced American technology, from university science and engineering departments to the industrial plants that produce the goods, from the foreign travels of Ameican technical representatives to commercial dealings between American firms and customers in friendly nations that tend to wink at the embargo. Hardware can be stopped at the loading dock, but know-how, being intangible or confined to paper, calls for different barricades.

As is often the case in bestirrings of the vast power of the U.S. government, the impetus for the new embargo comes from an obscure group of technocrats. In this instance it was a task force of the Defense Science Board, a blue-ribbon panel of senior academic scientists and industrial research executives that serves as one of the Department's lookouts on the frontiers of science and technology.

With out "smart bombs" and their surface-to-air missiles demonstrating in Vietnam that the electronics revolution had arrived in full force on the battlefield, the board in 1974 asked J. Fred Bucy, executive vice president of Texas Instruments, to head up a group that would identity "all technology areas in which maximum feasible protection is desirable."

Reporting back in April 1976, the task force stated that it had placed "overriding emphasis on mechanisms that transfer design and manufacturing know-how - the detail of how to do things," adding that "acquisition of know-how is currently being given the highest priority by the industrially advanced Communist nations."

And then it went on to lay out a plan that, in brief, called for the following:

The extension of export curbs to friendly nations, including "major allies," that permit sensitive goods to be transshipped to off-limits countries. Among many of these nations, it noted, "the degree of enforcement is thought to be sight," a view that is confirmed by frequent Commerce Department actions against firms in Western Europe that front for Soviet purchases of embargoed electronics goods. (KEY OFF)(KEYWORD) bigger role for the Defense Department in export-control administration, which is now handled almost exclusively by Commerce.

Ominously, the consideration of closer controls over exchanges of scientists, the employment of American citizens as technical representatives in Communist countries, and the "training of citzens from Communist countries at the more significant laboratories of U.S. technical institutes and universities."

The task-force recommendations were endorsed by the chairman of the Defense Science Board, Solomon J. Buchsbaum, a vice president of Bell Laboratories, and by the Pentagon's director of research and engineering, Malcolm Currie. Both have since left office. Meanwhile, the issue has been passed to a Defense study group, which has drawn up a list of some 200 technologies it would like to see embargoed.

The closed atmosphere and the cast of characters involved strongly suggest that the new embargo was formulated at the level of whether the settlers should sell muskets to hostile Indians. But the essential fact about the embargo is that virtually no one is interested in it except certain military and industrial elements in the United States. Furthermore, most for what we won't sell is easily available from sales-hungry producers elsewhere. And finally, there is ample evidence that when the inspiration is there, the Soviets, despite their many technological shortcomings, can home-produce whatever they deem necessary.

The proposed know-how embargo may retard them a bit, but the presence of Defense gumshoes in the American scientific and technological enterprise is a stiff price to pay for the delay.