BANGKOK — THE economy of the Soviet Union is now in a free fall. As it collapses, so it directly adds to the mounting problems confronted by each of the nations of Central and Eastern Europe as they struggle to move from communism to capitalism. As a group these countries have on average seen economic output fall fully 19 percent in the last 19 months, while inflation has rushed ahead at annual rates well over 100 percent.To strengthen the momentum of reform from Poland to Hungary to Czechoslovakia, while striving to prevent utter chaos in the Soviet Union, the West must aid Moscow. Western finance ministers, meeting this week in Bangkok at the annual meetings of the World Bank and International Monetary Fund (IMF) agree with this, but they are uncertain about the scale of their response. The collapse of the Soviet economy, involving massive shortages of foreign exchange, has produced huge cuts in Soviet imports. Traditional East European exporters to the Soviet Union have suffered. The IMF estimates that the decline in 1991 in Czech, Bulgarian, Hungarian, and Romanian trade with the USSR and traditional East European trading partners could be fully 65 percent; the fall in Polish exports could exceed 75 percent. At their Bangkok gathering, the seven major industrial democracies did agree with the Soviets to help devise a reform program, implying that aid would be forthcoming when the transition to free enterprise is under way. Deputy finance ministers of the seven are to go to Moscow in a few weeks in this regard. The caution on the part of Western finance ministers is understandable. Nobody has any hard facts. IMF experts are in Moscow trying to nail down the numbers. Daily matters seem to be getting worse. IMF chief Michel Camdessus admits that he just does not have any precise data on the scale of the country's economic deterioration, or the size of its foreign currency reserves. Since the London summit of the major Western industrial leaders in July, there has been quite a lot of action. The European Community has agreed to ready a $2.4 billion emergency aid program to get the Soviets through a harsh winter, and the Japanese have pledged $2.5 billion for the same purpose. United States emergency aid will also be generous. Then the IMF has signed a protocol with Soviet President Mikhail Gorbachev providing the Kremlin, as well as the leaders of each of the Soviet republics, with a full range of IMF experts on matters ranging from trade and monetary policy to basic fiscal policy approaches to restoring economic stability. Meanwhile, a World Bank mission is in Moscow offering to spend $30 million over two years to finance Western teams of experts to help the Soviets build private enterprise. But such initiatives do not appear to be enough. Desperate to react to the crisis, the Soviet monetary authorities are now printing money so fast that ahead, without doubt, lies monstrous inflation, still more chaos and still more hardship not just for Soviet citizens, but for the further 100 million people in much of Eastern Europe who continue to suffer when the Soviet economy sours (only the east Germans are protected, thanks to their new Western partners in united Germany). To aid the Soviets, the West must move fast to build a financial safety net. The Financial Times reported Tuesday that the Group of Seven finance ministers had agreed to bail out the Soviet Union in the event of an external payments crisis, such as being unable to service its roughly $70 billion foreign debt. It is time for very rapid action: for the Soviets to stop inflating and start reform, while also opening their books for Western scrutiny; and for the West to accept that a multibillion-dollar emergency finance pool has to be created like a line of credit for Moscow to not only build stability in the Soviet Union, but also to support the determined efforts of leaders in Eastern Europe to build democracy and free enterprise in their own countries.