Landing gear

Spacecraft that are intended to operate on a solid surface after landing require some sort of landing gear to allow the craft to come to rest undamaged in a stable position, ready for operations. The design should be able to cope with:

• the expected mass of the lander and the resulting impact overloads and weight

• the lander's expected motion and orientation on contacting the surface

• the expected range of terrains (topography and surface materials) that might be encountered

• any forces or torques to be reacted against after landing (e.g. due to anchoring, drilling, robotic arm operations, launch of an ascent stage)

• doing so with some suitable margin of safety

The landing gear may also carry sensors to detect surface proximity or actual touchdown (e.g. to trigger shutdown of the retro-rockets), and may even be a convenient platform to mount experiments that need to be in contact with the surface or within the field of view of the lander's cameras. The main configuration types are legged landers (usually three or four legs) and pod landers (ranging from near-spherical to egg-shaped or prolate).

In most cases the vehicle's vertical speed on landing will already be below some nominal value as a result of the deceleration systems employed during entry and descent, for example entry shields, parachutes and retro-rockets. These systems may on the one hand have been able to reduce the descent speed to nearly zero before contact with the surface, in which case the landing gear will need to cope with some nominal residual landing speed of only a few m s-1 at most (e.g. free-fall from the height at which the retros are shut down). On the other hand, the landing gear may have to perform a greater share of the task of decelerating the vehicle. In both cases the remaining kinetic energy has to be dissipated over some finite distance while minimising loads and mass. Approaches include the following:

• Damped elastic structures (e.g. piston-like landing legs)

• Plastic deformation: crushable material (e.g. honeycomb material, foam, balsa wood)

or structures (e.g. buckling of tubular struts, collapse of retro-rocket nozzles)

• Fluid damping: control of a fluid pressure or flow rate (e.g. airbags)

Note that landing loads can also be reduced by shedding mass (e.g. systems whose function has been completed, such as fuel tanks or spent rocket stages) prior to landing. Examples of this include all Luna and Surveyor landers.

Horizontal components of velocity also need to be addressed. These may be present due to the direction of the descent trajectory, atmospheric winds or the swinging of a vehicle under its parachute. While generally undesirable on impact8 and required to be minimised, some degree of horizontal motion (downrange or cross-range) may be necessary during targeting or hazard avoidance. Some rotation may also be present, due to attitude-control motion or, as in the case of Huygens, a slow spin about the vertical axis to scan instruments around during descent.

The performance of crushable materials may be characterised by their energy absorption capability per unit mass, or 'specific energy', Es, equal to the crushing strength a divided by the bulk density p. Vergnolle (1995) reviews a number of soft landing impact attenuation technologies and quotes Es values for aluminium and carbon of 16 and 100 kJ kg-1, respectively. Such materials are usually in the form of a honeycomb structure or foam. The total kinetic energy absorbed by a crushable component is the integral of force with respect to distance, i.e. the work done. The maximum allowable load, together with the kinetic energy, thus defines the minimum distance (stroke length) over which deceleration must occur. The maximum allowable load, together with the dynamic crushing stress of a candidate material, determines the cross-sectional area that will be required. The stroke length, cross-sectional area and density of the candidate material thus determine the mass that will be required - a value to be minimised.

8 An exception here might be aircraft, which require horizontal motion to gain lift to soften the landing.

The impact speed, deceleration and stroke length are related by v2

where v is the impact speed, a the deceleration and d the length of the deceleration stroke. The force required is represented by the product a X m, where m is the mass of the vehicle. Where the deceleration system is based on a crushable material with a crushing strength a and has a cross-section area S, then the deceleration force is S X a.

Crushable components may be incorporated into the struts of landing legs, the landing gear footpads and/or the base of the lander (e.g. Surveyor 1-7). In the case of pod landers the whole vehicle may be encased in crushable material (e.g. the balsa wood spherical capsules of Ranger 3-5, or the crushable shells of Mars 2, 3, 6, 7), since it may impinge the surface several times in different orientations after the initial impact, before coming to a final halt.

One can of course consider the surface of the target body to be a plastically deformable material itself, helping to cushion the landing with no mass penalty to the lander. However, the uncertainties and spatial variations in surface mechanical properties (e.g. from bedrock to windblown sand) of solid bodies of the Solar System are such that it is prudent to assume a non-deformable surface, dissipating none of the kinetic energy of landing. The damping system built into the lander can thus cope with any eventuality, rather than relying on a 'soft' landing. (Nevertheless, payload delivery penetrators do use deformation of the surface for braking, as described in the next Section 7.3.)

In contrast, the deformability of the surface is important for sizing of the footpads (or equivalent structures). Too small a footpad and the lander may sink too deep into the surface, jeopardising the mission. This was of particular concern for the first lunar landings, since there was a risk that the surface may have turned out to have been of such a soft, deep regolith layer that the subsequent crewed landers would have disappeared into the surface. There is a similar element of risk for Philae, due to make the first soft landing on a comet nucleus in 2014. Footpads are thus sized to penetrate no deeper than a reasonable limit upon landing, calculated using soil mechanics models of the surface bearing strength. In the event, the Surveyor 1, 3, 5, 6, 7 landers penetrated only 2.1 to 10.5 cm into the lunar surface, for landing speeds ranging from 1.4 to 4.2m s~1 (e.g. Jones, 1971).

In many cases the landing gear will need to be deployed prior to landing. Landing legs are usually stowed for launch and cruise for reasons of space (e.g. accommodation within the launch vehicle or entry shield) and resistance to vibration. They are then opened out (downwards and outwards, or upwards and outwards) to provide a wide base for landing. Various configurations have been adopted over the years, many of which involve attaching the footpads to an inverted tripod of piston-like struts incorporating crushable material. The struts are jointed at their ends such that each landing leg assembly unfolds upon operation of a deployment actuator.

The higher the ratio between the width of the landing gear and the height of the centre of mass, the more extreme the landing scenario required to make the lander topple over. Knowledge of the surface topography at the scale of the lander also influences both the base width (steeper slopes require this ratio to be higher) and possibly the clearance required between the footpads and the underside of the lander (the underside should contact the surface either not at all or in a controlled fashion, e.g. via crushable components). Several models of landing stability criteria exist, e.g. Buslaev (1987).

Airbags need to be inflated shortly before landing, e.g. by gas tanks or chemical generators. Unlike legs, however, deployment too early during descent can be problematic, since loss of pressure due to leakage or cooling can reduce their performance. Conversely, too high a pressure can lead to unacceptably high impact loads and even rupture of the bags. After the lander has come to a final halt (which may be after much bouncing and rolling, of order 1 km for Pathfinder and MER), the airbags are either deflated and retracted by motor-pulled tendons (as for Pathfinder and MER), or are released and allowed to spring apart by means of their own elasticity (as for the two-bag system of Luna 9, 13 and the Mars 96 Small Stations, and the three bags of Beagle 2). Particular attention must be paid to the durability of airbag materials against impingement onto rocks - the pressure bladder being protected within outer abrasion layers.

The landing sequence of pod landers involves an additional manoeuvre after the vehicle has come to a halt on the surface, to bring it into its proper orientation ready for operations. Mechanisms to achieve this have taken the form of a system of three or four opening 'petals' - hinged flaps covering the upper surface of the lander, any one of which is strong enough to bring the lander upright (e.g. the near-spherical Luna 9, 13, Mars 2, 3, 6, 7 and Mars 96 Small Stations, and the tetrahedral Mars Pathfinder and Mars Exploration Rover landers) - or like a pocket-watch, where a disc- or lens-shaped lander has a single hinged lid to perform the same function (e.g. Beagle 2). Landing edge-on can be mitigated by providing an inflatable 'tyre' around the edge that can topple the lander one way or the other. Another possibility is to build a lander that can operate in any orientation, or one where only internal parts are brought upright (e.g. the Ranger 3, 4, 5 landers).

An apparently elegant solution, to achieve both landing and re-orientation with airbags alone, is to position the lander (and thus the centre of mass) off-centre inside the airbags, such that the lander rolls to a halt in the correct orientation. On ejection of the airbags the lander falls the short distance to the surface, remaining upright. Another approach, so far not implemented for planetary missions but under consideration for ESA's ExoMars lander/rover, is to use so-called 'dead beat' airbags on the underside of the landing platform. These are vented such that they provide a near-critically damped soft landing, i.e. without bouncing. Accelerometry can be used to govern the timing and rate of venting of separate cells of the bags, in order to cope with non-vertical motion such as horizontal velocity components and rotation, and to keep the landing overloads within acceptable limits.

The toroidal landing gear of the late Venera/VeGa landers employed a cunning combination of techniques. While acting essentially like a single, annular crushable footpad, the torus was hollow, with vent holes around the top. Upon landing, additional damping was thus provided as the dense atmosphere (which had found its way into the torus during the lander's descent) was expelled through the vent holes, thus avoiding the need for on-board provision of a working fluid.

For missions involving surface mobility, the roles of landing gear and locomotion may be combined. For instance, the Mars Science Laboratory, due for launch in 2009, is proposed to be lowered to the surface from a hovering 'sky crane' descent system, the rover's wheels also acting as landing gear for the initial touchdown. An earlier example was the proposal to equip Viking-derived Mars landers with caterpillar tracks instead of footpads.

Other systems sometimes used in landing-gear designs-include hold-down thrusters (used particularly in low surface gravity to prevent rebound or toppling over, e.g. the LK lunar lander, Phobos DAS and Philae) and harpoon anchors (to prevent rebound and/or later ejection by outgassing in the case of cometary nuclei, e.g. Phobos DAS, Philae (Thiel et al., 2001)). Philae also incorporates a 'cardanic joint' damping mechanism in its landing gear, at the interface between the tripod legs and the main body of the lander.

Although it is useful in the first instance to consider the one-dimensional case of a vertical landing onto a flat, uniform surface, a real design has to take into account the full three-dimensional nature of the problem. For example, there may be some transverse motion, rotation and tilt, and on impact there may be sliding, rolling and bouncing to consider. Models have been published for a number of previous landers, representing a range of configurations.

The landing dynamics of the Surveyor, Apollo Lunar Module and Viking landers are particularly well covered (e.g. Sperling and Galba, 1967; Jones, 1971; Zupp and Doiron, 2001; Doiron and Zupp, 2000). The impact and subsequent motion (slipping, then slipping and rolling, then rolling) of the near-spherical Venera 7, 8 entry probes was discussed by Perminov (1990), while the particular case of the later Venera landers was discussed by Buslaev et al. (1983), Grigor'ev and Ermakov (1983) and Avduevskii et al. (1983). The airbag-assisted landing of Mars Pathfinder is described by Spencer et al. (1999) and Cadogan et al (2002).

The impact of the Huygens probe on Titan was discussed prior to launch by Lorenz (1994) and was measured by several accelerometers (Zarnecki et al., 2005).

The set of forces to be taken into account in modelling landing dynamics depends on the specifics of each case and the constitutive model assumed for the mechanical behaviour of the surface. The failure stress of the surface material when a normal force is applied over the contact area is usually expressed as a bearing strength. A dynamic component may also be included, being dependent on the bulk density of the material and the landing speed. The resistance offered by the ground may also be considered to increase with penetration depth. Sliding friction will also need to be taken into account if significant horizontal components of velocity are expected.

Although computer models can be employed to test possible landing gear configurations under many different situations (e.g. Zupp and Doiron, 2001 for the Apollo LM; Hilchenbach et al., 2000, 2004 for the comet lander Philae), full-scale or sub-scale drop tests may still be employed to validate a landing gear design (e.g. Bazhenov and Osin, 1978). For low-gravity environments such as asteroids and comets, tests can be performed horizontally with the lander suspended sideways on a long pendulum, encountering a vertical surface. Alternatively a system of counterweights can be employed (e.g. such a rig is described for the PrOP-F Phobos hopper by Kemurdzhian et al., 1993).

Sub-scale drop tests involve constructing a scaled-down model of the lander, performing drop tests and scaling the results to determine the behaviour of the full-scale lander design. Such sub-scale drop tests were performed for several landing vehicles, and for Huygens (e.g. Seiff et al., 2005). Table 7.1 gives the formulae for scaling key parameters for sub-scale tests, for a linear scale factor ,.

Table 7.1. Scaling factors and scale model values for physical quantities

Quantity

Full-scale value

Scale factor

Model value

Length

l

Area

A

,2

,2A

Mass

m

,3

,m

Moment of inertia

I

,5

,5I

Time

t

,0.5t

Velocity

v

,0.5

,v

Linear acceleration

a

1

a

Angular acceleration

a

Force

F

,3

,3F

Pressure

p

,

,p

Spring constant

k

,2

,2k

0 0

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