Have there been any tests or attempts at developing a propulsion system that uses variable thrust for attitude control (particularly during launch)?
Conventional systems often use fully gimballed mounts (eg. RS-25) or fixed main engines with smaller actuated vernier thrusters (eg. RD-107) to actively stabilise vehicles that are not passively stable. A single gimballed engine can control pitch and yaw moments and two or more can typically control roll moments as well.
Theoretically however, with the correct configuration of engines, attitude could be controlled by asymmetrically varying their thrusts by small amounts.
I can see several problem with this set-up which may explain if it has never been used:
- Thrust is difficult to vary smoothly and uniformly with good response times.
- It may not possible to control roll moments
Despite this, have any systems been developed or tested that implement this system?
Note: I'm mostly interested in active stabilisation during launch, but possibly this has been used for on-orbit attitude control also?