Planetary probes present a very diverse range of structural problems and solutions. This is in contrast to free-flying spacecraft (i.e. satellites and deep-space probes) which generally have a simple box or drum structure because there is only a single dominant loading (launch). On the other hand, landers and probes can range from resembling spiders to cannonballs, with the range generally being driven by thermal as well as structural requirements. Landers may be spidery open frames with equipment boxes bolted on, like the Surveyor landers; in contrast, entry probes for hot, deep atmospheres are constructed as pressure vessels and have thus been spherical in shape.

On most satellites the largest accelerations and thus structural loads are encountered during launch (typically 5-10 g): however, entry probes to Venus or Jupiter may encounter decelerations of 100-500 g. In such situations, load paths must be kept as short as possible to minimize the structural mass. The Pioneer Venus and Galileo probes (which had thermal constraints) used thick-walled pressure vessels supporting solid deck plates to which equipment was bolted. Spherical geometries are also appropriate where landing attitude is not initially controlled (e.g. Luna 9, 13; though note that the interiors of these spacecraft were pressurized, which also tended to favour a spherical design).

The Huygens probe did not need to exclude the atmosphere and therefore had an unsealed, thin-walled shell to preserve an aerodynamic shape and support light foam insulation. Huygens had three main sets of design loads (NB no impact or surface loads were considered). First are the launch loads, which are orthogonal to the probe axis since the probe is cantilevered sideways from the Cassini orbiter on its Titan launcher. These are transmitted through a support ring around the equator of the probe to a honeycomb platform onto which the units are attached. The same load path transmits the loads from the heat shield during entry (although these loads are in a different direction from the launch loads). Finally, parachute inflation loads - under Titan gravity, the probe weight and thus the parachute suspension load are quite modest - must be conveyed from the upper surface of the probe. This upper surface is also a honeycomb platform, and the loads are transmitted to the experiment platform via three stiff rods, as well as, in part, by the thin-walled (but stiffened) alloy shell.

The Soviet Luna 16, 20, etc. soft landers used an interesting structural design, with the large spherical propellant tanks towards the periphery of the vehicle, but presumably providing much of the required stiffness simply from the tank walls.

Structural and thermal design are intimately connected. The structure provides thermal leak paths from the outside to the equipment, and the designer might choose a more-or-less thermally conductive material to meet thermal needs, even when this might offer poorer strength to weight performance. The Pioneer Venus small probes used beryllium shelves for thermal reasons.

An interesting metric for an aerospace vehicle is its mass density. This parameter is directly relevant for capsules that may splash down on Earth or Titan, in that it determines whether they will float. Generally, probes tend to have densities

of the order of 200-400 kg m ; it is in fact difficult to attain much higher densities without explicitly adding ballast, largely due to the low volume-packing fraction associated with practical assemblies (adequate clearance must be maintained for access to connectors, for example). For a given shape of vehicle, there is also a direct relationship between the density, the size of the vehicle, and the ballistic coefficient.

The landed parts of the DS-2 microprobes were rather dense - indeed, being milled out of solid alloy, with a dense tungsten nose. The structure had to be stiff to withstand the very high impact loads; the tungsten nose was in fact not chosen for structural reasons, but to push the centre of mass as far forward as possible for aerodynamic stability. It should be noted that the difficulty, if subsystems grow or new equipment is added in an evolving design, is often a lack of volume in which to accommodate the growth, not a lack of mass.

It is often assumed that Venus and giant planet probes must have pressure vessels. In fact, deep sea instrumentation has been constructed and operated without using pressure vessels; simple plastic tubes containing the electronics are filled with oil and closed with bungs. The pressure is resisted by the incompressible oil which keeps the seawater out, but is transmitted to the electronic components. With the exception of a battery, which needed an additional vent hole, the components tolerated the pressure. While it is important to exclude hot, corrosive atmospheres, this exclusion requirement should not necessarily be interpreted as a requirement for a pressure vessel.

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