This is Tor Double #35, of a series of 36 double books published from 1988 to 1991 by Tor Books. It contains two novellas. Unlike most of the volumes in the series, this one is not bound tête-bêche (back-to-back and inverted). There is only one cover. The novellas are listed here alphabetically by author; I feel neither should be considered “primary.”
Universe, by Robert A. Heinlein (1941) ****
This novella was originally published in the May 1941 issue of Astounding Science Fiction. Heinlein later combined it with its 1941 sequel, "Common Sense", in his 1963 novel Orphans of the Sky.
It is a classic generation ship story, one of the first such. The population of the ship has forgotten its origin and its mission has become a primitive fable, not literally believed. Hugh Hoyland, a young apprentice Scientist, rediscovers the truth.
Silent Thunder, by Dean Ing (1991) ***
This is the original and only publication of the short novel, 50 years after its companion in this Tor Double. What the two works have in common is a plot that involves rediscovery of hidden truth.
In 1967, amongst information gathered from Germany in the aftermath of World War 2, Walter Kalvin finds out about a powerful secret Nazi weapon – the Donnersprache device. Now America is adopting a new fascism, and powerful forces within the government intend to stop him from learning the truth, and to control the weapon. A thriller, but not a particularly memorable story.