Here Rothbard deals with the pure rate of interest, as determined by time preference. It's a ratio of prices between present and future goods. The interest rate is equal to the rate of price spread in the various stages of production. It pervades all time markets.
It is not true that capital is an independent productive factor. The capitalists' function is thus a time function. Many economists have made the great mistake of believing that the interest rate determines the time-preference schedule and rate of savings, rather than vice versa. Capitalists' savings and investing are necessary simply to keep the production structure intact. Consumption does not dominate business prosperity. Civilization advances by virtue of additional capital, which lengthens production processes.
An Alice J. Lillie Seminar. This lecture covers pp. 367-451 in the Scholar's Edition of Rothbard's Man, Economy, and State.