The Kremlin “dropped” the ruble. How the Russians are paying for the bloody war in Ukraine

Against the backdrop of a full-scale war that Russia launched on the territory of sovereign Ukraine, the official dollar exchange rate fluctuates on average at the level of 115-118 rubles. This is the selling price. While the purchase of the dollar is officially banned by the Central Bank of Russia in response to Western sanctions.
Why the Russian authorities fail to keep the ruble exchange rate?
Is it true that economic collapse in Russia is unavoidable?