Bouyancy Control

For propulsion within an atmosphere or a body of liquid (such as water) one may turn to buoyancy control. This method allows a vehicle to move up and down by changing its own density relative to the density of the gas or liquid around it. If the vehicle is less dense than the surround gas or liquid, it will rise and vice versa. Common forms of this propulsion method are hot air balloons (also known as lighter-than-air vehicles) and submarines. In fact, the first-time humans have attained flight was in 1783 using a hot-air balloon, way before the Wright brothers made their debut. These days, anybody who has ever tried SCUBA diving is also well aware of buoyancy control and how it works.

There are several ways of buoyancy control. One way is thermal variation: hot air is less dense than cold air so by heating up or cooling air in a large balloon we can control ascent or descent. Another way to control ascent in an atmosphere is to use a lighter than air gas by simply inflating a balloon with a gas of lower molecular weight then air (such as Hydrogen or Helium). Lastly, when operating underwater one can resort to a pressurized canister of air which can rapidly inflate a bladder allowing rapid ascent in emergency situations.

There is a benefit to this technology - it is efficient and requires little expenditure of propellant during ascent or descent. Vehicles can stay suspended at a particular altitude for years on end, requiring very little energy to maintain altitude. The biggest drawback is that the resulting vehicle is often large, slow and has negligible payload capacity. Ultimately, the vehicle still needs some form of conventional propulsion such as ducted fans or impellers simply to allow it to move forward and sideways (buoyancy controls only allows moving up and down).

With that said, that are still quite a lot of applications, but the killer app, in my opinion, is the exploration of planets. A balloon with a basic instrumentation package is an incredibly simple device. Dozens of such devices could be deployed on planetary bodies such as Jupiter, Saturn, and Venus, where winds will do all the work, allowing these simple explorers to cover vast territories while generating a lot of data. It is difficult to understand why NASA and other agencies have not been actively exploiting this technology in space exploration. There probably are some issues in deploying a lighter-than-air vehicle from orbit, but it seems to be straightforward task, after-all we are not actually trying to land. Nevertheless, for atmospheric missions where cost is the driving factor, lighter-than-air robotic explorers offer an excellent solution and will continue to do so for ages to come.

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