The impulse engines are small engines seen on a portion of the outer perimeter of the aft end of the saucer hull as shown below. A primary use of the impulse engines is to provide high thrust for short periods to help the Enterprise break out of the orbit of a planet or moon. Since the three ion propulsion engines give low thrust, the only way they can cause the Enterprise to leave orbit is by causing the ship to make larger and larger diameter spiral orbits until it can finally break free. It might take two weeks, for example, for the Enterprise to break free from Earth’s gravity if the ion propulsion engines run constantly at full power. But if the impulse engines are turned on for less than an hour, the Enterprise could break out of orbit in less than a day.
Impulse engines can also be used in emergency situations. When used, they apply stabilizing forces to the saucer hull, with high thrust if needed, to protect the humans inside this hull and to make sure the saucer hull does not break up or become damaged. For example, if a main engine hull or one of the aux engine hulls must suddenly be jettisoned away from the ship because a meteor strike caused a radiation leak, the impulse engines kick on to keep the Enterprise from going into a dangerous spin or roll. In fact, the main engine hull and aux engine hulls could suddenly all be jettisoned away, and the impulse engines will make sure that the saucer hull remains stable and intact.
The impulse engines can also provide thrust for short periods to alter the flight path of the Enterprise in the case where the ship might be on a collision course with an object in space, such as space debris. And the impulse engines can be used if there are problems with all three ion propulsion engines. The impulse engine can maneuver the ship out of harm’s way before it’s near some object that it might impact. For example, if a disabled Enterprise was on a collision course with Mars, but Mars was still hours away, a slight change in direction of the Enterprise would be more than enough to prevent the Enterprise from crashing into Mars.
The impulse engines, unlike the ion propulsion engines in the three engine hulls, have a need for short term high thrust. To achieve this, it’s assumed the impulse engines will be liquid-propellant rockets. They will use liquid hydrogen and liquid oxygen. This type of engine is proven, for example this technology was used for the space shuttle’s main engine. Also, hydrogen has other uses on the Gen1 Enterprise including as propellant for the ion propulsion engines and as fuel for the backup power systems based on hydrogen fuel cells. It’s efficient and practical for many of diverse systems on board the Gen1 Enterprise to utilize the same material from storage tanks on the ship.