The Dune Encyclopedia offers an insight into the various methods that Frigates use to reach orbit. In short it boils down to one of three methods; "Steamships" (that use heated water (or ammonia) as a rocket), "Torches" (which use heated plasma to generate thrust) and "Brats" which use Project-Orion style fission explosions.
One of the more popular frigate designs was the "steamship," in which a fusion plant heated a reaction mass, usually water but sometimes ammonia or some other light compound. Various heat exchange techniques utilizing plasma fluxes and electromagnetic fields made the system reasonably efficient, and it was cheap to maintain. The same sort of energy fields around the launching pad absorbed much of the initial blast so that the major environmental problem was noise as the ship rose above the port.
A second widely used design was the "brat," which exploded small fission bombs under an ablative plate at its base. It was faster, more efficient, and lifted more pay- load than the steamship, but it was also much more expensive and left much short- term radioactivity in its wake.
Most efficient of all was the "torch," whose exhaust was plasma, but ruling Houses were often reluctant to allow what amounted to giant heat cannons to come and go overhead.
Because of the Guild monopoly, no frigate was capable of trans-light operation. In inter-stellar transit, frigates were mere cargo. In the confines of a planetary system, however, the frigate was dominant. No other class of ship was so flexible, with so favorable a combination of size, speed, and surface accessibility.
Obviously no single frigate could be wholly representative, but one that is broadly illustrative was Antiock, the personal spaceship of the Padishah Emperor Corrin XVIII (r. 6874-6892). Surely one of the largest frigates ever built, Antiock massed well over nine hundred thousand tonnes and was four hundred and eleven meters in length. Its torch engine gave it a maximum launch acceleration of approximately seven standard g's