Planetary environments

Three primary candidate planets for aerobot missions (Venus, Mars and Titan) have very different environments (see Table 6.1).

The deep atmosphere of Venus exhibits extreme atmospheric parameters. The high temperature and pressure in the lower atmosphere strongly limit the lifetime of surface and near-surface vehicles: without nuclear power-driven refrigerators or high-temperature electronics, the lifetime would be to 3 h. High-temperature materials with good gas barrier and strength properties are needed for near-surface LTA vehicles. On the other hand, the environment of the higher troposphere is quite mild and comparable with the troposphere of the Earth. This region is the most favourable for aerobot missions (the VeGa balloons

Table 6.1. Planetary atmospheric environment parameters for Venus, Mars, Titan and Earth

Parameter

Venus

Mars

Titan

Earth

Acceleration of gravity (Earth g)

0.9

0.37

0.14

1

Main atmospheric gas

CO2

CO2

N2

N2

Surface temperature (K)

735

230

92

290

Surface pressure (atm)

92

0.0067

1.4

1.0

Surface air density (kg m~3)

64

0.015

4.9

1.2

Solar flux at the upper atmosphere (W m~2)

3200

700

13

1300

Solar flux near the surface (W m~2)

5

700

600

Altitude of tropopause (km)

-65

11

40

17

Pressure at tropopause (mbar)

97

2.7

200

90

Temperature at tropopause (K)

240

190

70

220

Diurnal temperature variations near the

<0.3

30-50

<1

<10

surface, 1T/T (%)

Winds at the tropopause (m s_1)

80-100

20-30

15

20-30

Winds in lower atmosphere (m s_1)

1-3

5-20

-1

5-20

flew at 54 km at 0.5 bar and -30 °C). The main challenge is the sulphuric acid clouds that cover 100% of Venus.

On Mars, the low density of the atmosphere in combination with large thermal variations requires light-weight and strong materials for long-duration aerobotic missions - a combination that is not easy to obtain. Although the proven balloon materials could be used for low-payload-mass aerobots, composite materials, new balloon designs ('pumpkin' shape), and advanced fabrication technology (so-called 3-DL or three-dimensional laminate technology, which is used for fabrication of sails for round-the-world yacht races) offer the most potential to improve the efficiency of aerobotic missions. The Martian troposphere is similar to the stratosphere of the Earth; this similarity provides the basis for terrestrial stratospheric flights to test Martian aerobot systems.

The combination of high density (4.4 times larger than on the Earth) with low gravity (7 of the Earth value) and low temperature contrasts makes Titan almost ideal for long-duration aerobot missions. The balloon materials become stronger at the extremely cold temperatures but adhesives that remain non-brittle at these temperatures are required.

Table 6.2 shows the typical parameters of aerobots to lift a payload of 10 kg on Venus, Mars, Titan and Earth. The values were calculated using the aerobot equations (including Equations 6.1 to 6.3). For the sake of comparison, the areal density of the balloon material is assumed to be —20g m~2 for all planets, reflecting current technology (the VeGa balloon material was -340 g m~2, which is comparatively heavy).

Table 6.2. Typical parameters of planetary aerobots

Parameter

Venus, 1km

Venus, 60 km

Mars, 5 km

Titan, 1 km

Earth, 1 km

Earth, 4km

Atmospheric density,

61.56

0.489

0.010

4.80

1.13

0.010

(kg m-3)

Temperature of

454

-10

-51

-181

-2

-33

atmosphere (°C)

Payload mass (kg)

10

10

10

10

10

10

Balloon diameter (m)

0.72

3.70

20.65

1.73

2.83

21.41

Balloon volume (m3)

0.2

26.5

4610

2.7

11.9

5140

Balloon mass (kg)

0.84

1.79

31.6

1.02

1.37

33.9

Mass of buoyant

1.16

1.25

4.46

1.97

1.94

7.44

gas (He) (kg)

Total floating mass (kg)

12.0

13.0

46.1

13.0

13.4

51.4

Payload mass as percentage

83.4

76.5

21.6

77.1

75.2

19.5

of floating mass (%)

Mass of entry vehicle (kg)

36

39

138

39

N/A

N/A

Atmospheric density dominates the balloon size: a Mars aerobot requires a balloon over 150 times larger (in volume) than the Venus aerobot at 60 km, and over 1500 times larger than the Titan aerobot near the surface. A mass efficiency (ratio of payload mass to the total floating mass that includes mass of payload, balloon and buoyant gas) is 75-80% for the Venus and Titan aerobots (it was ~30% for the VeGa balloons) and only ~20% for the Mars aerobot. Use of hydrogen instead of helium for the buoyant gas will increase the efficiency of the

Mars aerobot to 24%. The most radical way is to use lighter envelope materials:

an areal density of 12 g m will nearly double the mass efficiency.

Because of the dense atmospheres of Titan and Venus, payload mass is not as critical as on Mars. It is unlikely that in the immediate future Martian aerobots will be able to lift more than 20-30 kg of payload.

6.1.3 Deployment and inflation of planetary aerobots

Just as all lander and rover missions have many features in common, so it is with aerobots. The most common mission scenario would be: launch of an interplanetary bus with the aerobot system enclosed in an entry vehicle, cruise phase to the planet, targeting at a selected area on the planet, separation of the entry vehicle, entry and deceleration in the atmosphere, deployment and inflation of the aerobot, release of the heat shield and ascent (or descent) to the floating altitude where the active phase of the aerobot mission starts (Figure 6.2).

Entry 0-

Drogue parachute deployment

Backshell release and main parachute deployment

Parachute descent

Balloon deployment

Float altitude

Balloon inflation

' End of inflation and aeroshell release.

Parachute release

Figure 6.2. Typical entry, deployment and inflation (EDI) sequence for a Martian aerobot.

The launch of planetary balloons from the surface is impractical. Aerial deployment and inflation is more mass efficient and less risky since it does not require an additional (and costly) landing system, or the risky procedures of soft landing and the deployment and inflation of the balloon at the surface. However, aerial deployment and inflation is the most critical and least modelled part of the mission because of the complexity of the aerodynamic processes involved.

The feasibility of aerial deployment and inflation of balloons made of robust heavy material was demonstrated in the VeGa balloon mission. Future missions require much lighter and more efficient materials. Successful aerial deployment and inflation of a 3 m balloon (approximately VeGa balloon size) made of 17 times lighter material (12.5 mm Mylar film) was demonstrated in 1998 (Figure 6.3).

This test validated the concept of aerial deployment and inflation of the modern thin-film balloons, which are applicable to Venus and Titan missions.

The deployment and inflation of a Martian aerobot is even more challenging, because balloons are two orders of magnitude larger (in volume), descent velocities during deployment are 7-10 times faster, and balloon inflation should be completed very rapidly (usually in 150 to 250 s) to ensure that the balloon will start to rise before impact with the surface. Successive failures in flight tests of aerial deployment and inflation in the Russian-French Mars Aerostat project (1987-1995) show the complexity of the problem.

Only recently (in the summer of 2002) was the deployment and inflation of a Mars balloon prototype successfully demonstrated in the stratosphere (Figure 6.4).

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