In engineering, variable-buoyancy propulsion is the use of a

buoyancy engine A buoyancy engine is a device that alters the buoyancy of a vehicle or object in order to either move it vertically, as in the case of underwater profiling floats and stealth buoys, or provide forward motion (therefore providing variable-buoyanc ...

to provide

propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...

for a vehicle. The concept was first explored in the 1960s for use with

underwater glider An underwater glider is a type of autonomous underwater vehicle (AUV) that employs variable-buoyancy propulsion instead of traditional propellers or thrusters. It employs variable buoyancy in a similar way to a profiling float, but unlike a flo ...

s, but has since been applied to

autonomous aircraft An autonomous aircraft is an aircraft which flies under the control of automatic systems and needs no intervention from a human pilot. Most autonomous aircraft are unmanned aerial vehicle or drones. However, autonomous control systems are reachi ...

as well.

Principle

Variable-buoyancy propulsion is based on the ability of a vehicle to change its

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...

from negative to positive and vice versa (for aircraft, this means alternating between being heavier and

lighter than air A lifting gas or lighter-than-air gas is a gas that has a density lower than normal atmospheric gases and rises above them as a result. It is required for aerostats to create buoyancy, particularly in lighter-than-air aircraft, which include free ...

). While positively buoyant, the vehicle trims bow up and uses its

hydrofoil A hydrofoil is a lifting surface, or foil, that operates in water. They are similar in appearance and purpose to aerofoils used by aeroplanes. Boats that use hydrofoil technology are also simply termed hydrofoils. As a hydrofoil craft gains sp ...

s or

wing A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expres ...

s to glide forward while rising, using buoyancy as the driving force. At the top of the climb, buoyancy is made negative and the vehicle trims bow down and

glide Glide may refer to: * Gliding flight, to fly without thrust Computing *Glide API, a 3D graphics interface *Glide OS, a web desktop *Glide (software), an instant video messenger *Glide, a molecular docking software by Schrödinger (company), Schr� ...

s forward while descending, using gravity as the driving force. The process can be repeated for as long as the

can operate, and allows for highly energy-efficient albeit generally slow propulsion. The vehicle's trajectory typically presents a sawtooth-like profile. Various methods may be used to alter the buoyancy.

References

Buoyancy Fluid mechanics {{Engineering-stub