Powered aerobots airships

A natural evolution of the balloon is the dirigible balloon, or airship. Rather than being at the whim of the wind, an airship offers the hope of traversing the surface in a desired direction. Planetary powered aerobots will provide the capability of global and targeted access to almost any location. For long-duration missions the power should be provided by non-expendable sources of energy - solar cells (Venus, Mars) or radioisotopes (Titan).

The volume (and thus the cube of the size) of an airship scales with its mass, while the drag area scales only as the square. Thus if propulsive power scales with mass, a larger airship is able to achieve higher forward speed. Airships are competitive with aircraft for large, slow payloads and missions. One advantage of an airship over an aeroplane is that the airship has a fail-safe condition, in that if its propulsion fails, it continues to fly. Further, for scientific investigations, an

Figure 6.3. Tropospheric deployment and inflation test of 3 m Mylar balloon (August 21, 1998, El Mirage dry lake, California).

airship can remain motionless over a region of interest, unless local winds exceed its forward speed capability.

The available power will determine the possible speed of a powered aerobot. If T is the thrust of the airship propulsion (propeller), then in steady flight thrust is equal to aerodynamic drag and

Figure 6.4. 10 m diameter Mars balloon prototype during inflation in the stratosphere at altitude 31 km (big island of Hawaii at left).

where Cd is the drag coefficient, S the reference area and U the airspeed of the airship. The effective mechanical power is

For illustrative purposes, Table 6.3 shows power requirements for aerodynamic shaped aerobots with the same diameter as in Table 6.2, for air speeds 3 m s-1 and 15 ms-1. It was assumed that the drag coefficient is ~0.2 (a conservative value) and the efficiency of transformation from electrical to thrust power is ~50%.

The required power grows very rapidly: theoretically as the cube of the speed. For relatively small aerobots, the available power can be of the order of tens to hundreds of watts, and their air speed would likely be 1 to 7 m s-1. It is not

Table 6.3. Power requirements for planetary aerobots

Parameter Venus Venus Mars Titan

Parameter Venus Venus Mars Titan

Table 6.3. Power requirements for planetary aerobots

Floating altitude (km)

1

1

60

60

5

5

1

1

Speed (m s-1)

3

15

3

15

3

15

3

15

Required thrust (N)

22.5

560

4.7

118

3.0

75

10.1

254

Required electrical

135

16000

28

3550

18

2260

61

enough to fly upstream in 10 to 20 m s—1 winds, but can be sufficient to steer across the wind to the desirable destination.

Empirically for Earth airships (Lorenz, 2001) the propulsive power for a given-mass airship to move at a given speed is given by

where n denotes how propulsive efficiency scales with density (a value between 0 and 1) - the exponent differs from 3, which would be expected from Equation 6.10, due to scaling effects in the propulsive efficiency, such as propeller size.

Another application of powered aerobots could be in situ surface studies and sample collection. When winds in the lower atmosphere are small (as in the case of Venus, Mars near noon and Titan), the powered aerobot can hover above the selected site; the surface instrument package can be winched down for the surface measurements or sample acquisition and winched up to the aerobot later. The hovering can be controlled by image processing in the horizontal plane and by pressure data or altimeters in the vertical direction. The aerobot would be used as a flying rover, covering a much greater area than the traditional surface rovers.

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