While the platform of choice for wide-area surveys on the Earth is usually the aeroplane, for delivery of in situ instrumentation onto the planet's surface, a helicopter offers important advantages in that it can be placed at zero speed at a given location, regardless of whether that location is flat for a distance long enough to permit an aeroplane to land.
A helicopter achieves lift by propelling air downwards via a whirling rotor, which may be considered as a set of wings. The formidable control aspects of rotary-wing vehicles will not be discussed here, but the fundamental propulsive aspect deserves some comment. In order to hover, the vehicle must produce a downward momentum flux in the air that balances the weight of the vehicle. This momentum flux (in kg m s~2) may be generated with a large-area rotor pushing a lot of air down slowly (as in a helicopter), or a small amount of air down at high speed (as in a Harrier jump-jet). The momentum flux is Apv where A is the area of
the jet and v the induced velocity. The energy imparted to the air is Apv 12 - thus a large-area, low-velocity solution offers the lowest power consumption for a given thrust. Hence a large rotor is better, although (especially for a rotorcraft delivered in an entry shell) structural considerations are likely to be the limiting factor.
Note that a helicopter uses less power in modest forward flight than it does in hover. One expression for this power is given by actuator disc theory:
Note the dependence on rotor area and density, and an even stronger dependence on required thrust. In practice, rotor drag, blockage and other effects mean power is rather higher than this and an expression of the form
Submarines, hydrobots and cryobots
predicts the power that must be installed on the vehicle, where subscript e denotes the corresponding values for Earth-surface conditions.
Various imaginative concepts have been proposed for local exploration around a lander on Mars. Small vehicles in particular suffer from the aerodynamic inefficiencies posed by the thin atmosphere and the low Reynolds number, and thus some of the proposed solutions are inspired by insects, which confront similar Reynolds number regimes on Earth. One moving-wing concept is the entomopter (that is to say, looking like a dragonfly). Another approach is to use conventional rotorcraft designs, but with rotors adapted for the Reynolds number regime - a so-called 'mesicopter'.
A submarine is in essence a special case of airship, except in a rather more dense fluid. Submarine vehicles have been considered for Europa's sub-surface water ocean (where the term 'hydrobot', as an analogue of aerobot, has been used), although delivery to this ocean which lies under an ice crust that appears to be 10-20 km thick in most places presents significant challenges. The scientific goal would be to study the temperature structure and composition of the ocean, paying particular attention to searching for biota or biological molecules. The canonical illustration associated with this mission concept shows the vehicle inspecting the fauna around a seafloor hydrothermal vent, which might be expected to exist by analogy with Earth.
A submarine vehicle might also be contemplated to explore Titan's frigid seas of liquid hydrocarbons; these lakes and seas predominantly of ethane and methane at 94 K would have a bulk density of 450-650 kg m~3 and might be up to a few kilometres deep.
Thermal control and guidance in the presence of winds and currents are essentially equivalent for submersibles and for airships, although at significant depth the hydrostatic pressure may be formidable and structural design may need to take this into account. One bar of pressure corresponds to a depth increment of 10 m of water on Earth, or about 120 m of the hydrocarbon ocean on Titan, and a similar value for Europa (although the hydrostatic pressure beneath 10 km of ice already introduces 1 kbar of pressure).
For mobility, the empirical expressions given above for airship-power scale reasonably well for submarines. In fact, although the delivery of such a vehicle poses significant challenges, submersible explorers have been proposed for Europa's sub-surface ocean.
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