While it's obvious how a single gimbaling engine can control a rockets attitude, it is less clear how it could control roll.
Nonetheless many rockets use just a single main engine with no fins or control vanes. Some examples include Ariane 5, and Delta 4 (pictured).
How do these single engine rockets use the main engine to control roll?
If they don't use the engine, what systems do they use?
The Ariane 5 launches with two strap-on solid rocket boosters. The thrust vector control on these solid rocket boosters provides the necessarily roll authority during the first 130 seconds of flight while the solid rocket engines are attached. After solid separation, roll control is performed by a set of 400 newton hydrazine thrusters located on the Vehicle Equipment Bay, which sits atop the cryogenic main stage.
The RS-68 engine on the Delta 4 uses a gas-generator cycle. A portion of the fuel and oxidizer is diverted from the main engine to a preburner driving a gas turbine.
The primary purpose of the turbine is to power the fuel pumps. The exhaust gas from the turbine is put to a secondary use, attitude control. The exhaust is vented separately from the exhaust from the main engine. The vented turbine exhaust is thrust vector controlled and provides the requisite roll authority to keep the vehicle stable.
References:
Ariane 5 Attitude Control System
The functions of the 400 N ACS include roll and pitch control of the launcher after jettisoning of the solid rocket boosters. Thereafter, the same 400 N thrusters are used for fine control manoeuvres and precision upper stage orientation before separation of one or more payloads.
HTPB solid propellant of 68% ammonium perchlorate, 18% aluminium and 14% liner produced and cast in casings in Kourou; 3.4 m-long forward section shipped already loaded by BPD from Italy. Nozzle steering by two hydraulic actuators using flexbearing for 6° deflection.
The above are very explicit on roll control after solid rocket separation. Before? It's not so clear. Finding anything technical at ESA is a tooth-pulling chore. I'll have to add more on roll control while the solids are still attached later. On thing that is clear (but is never said): were it not for that thrust vectoring, the slightest misalignment on those solid rockets would send the vehicle spinning out of control.
B. K. Wood, Propulsion for the 21st Century—RS-68, AIAA 2002-4324
Engine Specifics. The new RS-68 is capable of operating in—and transitioning between—full power level and minimum power level upon command from the vehicle. It also supplies pressurization gasses to vehicle fuel and oxidizer propellant tanks and thrust vector and roll control by gimbaling the thrust chamber assembly and the fuel turbine exhaust roll control nozzle.
