Parachute types

The design of parachutes and related systems is a somewhat arcane science (e.g. Knacke, 1992; Murrow and McFall, 1968) of sufficient complexity that empirical testing remains the only trustworthy design tool.

Different parachute geometries are available with different inflation performance, drag coefficient, stability, manufacturing cost and so on. The lowest cost type of parachute is the cruciform - this is easily manufactured as two strips of fabric sewn at an orthogonal intersection. These are used widely in retarded bombs and submunitions, but are not usually used on planetary probes due to their generally poorer stability.

A key feature of a parachute is its porosity, at both the macroscopic (gaps in a ribbon parachute or ringsail, or in a disk-gap-band chute) and microscopic (porosity in the fabric) scales. The porosity allows some part of the flow to go through, rather than around, the parachute, and is crucial in controlling its inflation characteristics and its stability in operation - a chute without adequate porosity will exhibit undesirable oscillations. The microscopic porosity in particular is sensitive to the Reynolds number, so particular caution is required in applying test data to different flight conditions.

Circular (i.e. 'flat') canopies typically have wider oscillations than conical types and are therefore rarely used. Conical parachutes have triangular gores, and so form a conical shape (although in inflated operation, the cone tends to be rather rounded). Conical ribbon chutes have good supersonic characteristics and are strong since, generally, materials can be made stronger as ribbons than as broadloom fabric. Ribbon-type parachutes can have high porosities.

Disk-gap-band (DGB) chutes are a variant of circular canopies that have better stability characteristics: the gap allows a through-flow which stays better attached to the canopy, avoiding asymmetric flow separation which can cause oscillations. Each gore is approximately triangular, with a rectangular segment to form the band. Even though the DGB chute for the Mars Exploration Rovers was derived from the previous Viking and Pathfinder designs, its different size and operating conditions were such that testing found that the chute would not reliably inflate. A modification (e.g. Steltzner et al., 2003) that enabled successful operation was to increase the size of the gap.

Ringsail parachutes are something of an intermediate between a ribbon chute and a hemispherical chute, with one side of the panels in each gore being free, allowing a flow through the canopy. These chutes offer good drag performance and were used on Beagle 2 (Northey, 2003) and on Apollo. However, there has been less testing and experience with this type of chute, which is not well-suited to supersonic inflation. Note that the drag coefficient (and the drag area) of a parachute is referred to its constructed size (i.e. the gores laid flat), since the inflated diameter is less well-known (but is usually a factor ~ smaller), and indeed can vary with time. Drag coefficients for most parachutes of the order of 0.5-0.6 are typical for subsonic conditions.

Most planetary probes have broadly similar parachute-inflation conditions (Mach number and dynamic pressure) which restrict the choice of design. Conical parachutes and disk-gap-band types are essentially the only types used on planetary spacecraft, in part due to the base of experience obtained with them and the significant costs of qualifying new designs in extreme environments.

Parawings are rectangular parachutes, with cells that inflate in the ram-air flow to create a lifting surface (these systems are therefore also termed ram-air parachutes). These are being considered for precision-landing applications on Mars, and for sample-return on Earth, one being used on the Genesis capsule, for example.

A descent system contemplated for the Gemini manned capsules was the Rogallo wing. This is essentially the original hang-glider, with a kite-like diamond flexible wing whose span and chord are maintained by a rigid frame comprising a keel and a spar. (Modern hang-gliders have more sophisticated aerofoils and shorter chords - achieving rather better glide performance at the cost of complexity and somewhat reduced stability.)

A variety of other parachute types are available, including variants with stiffening battens (guide-surface parachutes), ringsails, etc. Only a parachute expert would have particular reasons for using these systems, and given the large testing background and demonstrated reliability of conical ribbon and DGB parachutes, they are unlikely to be used.

4.4.1 Parachute components and manufacture

The fabric elements of a parachute canopy are usually referred to as gores (Figure 4.2). The lines between the gores also extend down as suspension lines to convey the forces to the payload. Generally, these lines meet at an apex and a single line carries the load. This single line, which is typically driven by the need to avoid payload wake effects on the parachute, is called a strop, and may also include some shock-absorbing elements to alleviate the peak loads during parachute inflation. (Sometimes the term 'riser' is applied to the strop, although this can also be applied to the suspension lines.) Finally, the riser usually splits into several lines for attachment to the payload to improve damping of attitude oscillations of the payload. This split line is termed a bridle. Note that the aerospace industry, generally driven by military requirements, often terms the payload a 'store'.

The original parachute material, silk, is still used in terrestrial applications. For planetary probes, temperature considerations and more importantly outgassing issues, force the use of synthetic materials like polyester.

Kevlar, having a very high strength-to-weight ratio is often used for risers but is an awkward material to sew and is therefore rarely used for the canopy itself. Polyethylene ('Spectra') is used in similar areas. Polyfluoroethylene (Teflon) has

Beagle Parachute

Figure 4.2. Construction of a conical ribbon and a disc-gap-band parachute. Gores are manufactured flat but form a curved surface when the chute is inflated. Note that the inflated diameter is a factor of smaller than the constructed diameter.

inadequate strength for load-bearing applications, but is useful in ancillary components e.g. parachute bags, because of its low friction.

Polyesters are probably the most widely used materials for planetary parachutes. Dacron is a common polyester material. It has good strength properties, and can tolerate a wide temperature range. Polyester material was initially selected as the material for the Huygens probe parachutes, but appropriate lightweight fabric was not available and so nylon was used.

Nylon is another common terrestrial parachute material, but it has poorer outgassing properties than polyester. Furthermore, it is much less tolerant of temperature extremes. Note that in applications where planetary protection is a concern, the parachute canopy may represent the largest surface area of the probe system, and may require extensive cleaning treatments to bring the total probe bioload down to permitted levels (e.g. although nylon or other materials might work well at cold Martian temperatures, they could not survive the >100 °C sterilization procedures needed during the development programme).

Table 4.2 lists some key features of some planetary descent vehicle parachutes.

Table 4.2. Features of some descent vehicle parachutes; NB: care must be taken to interpret parachute system masses; the canopy and lines themselves may weigh rather less than the container and deployment mortar

Quoted

diameter

Comments : Mach number,

Parachute

(Do) or area

Mass

dynamic pressure

Mercury drogue

6 ft

6.4 lb

30d conical ribbon

Mercury main

63 ft

56 lb

ringsail (+4.2 lb lines)

Apollo drogue

2 x 16.5 ft

50 lb

25d conical ribbon, 10-204 psf

Apollo main

3 x 83 ft

135 lb

ringsail, 30-90 psf

Venera 4 brake

2.2 m2

?

Venera 4 main

55 m2

?

Venera 5,6 brake

1.9 m2

?

Venera 5,6 main

12 m2

?

Venera 7 main

?

?

'glass nitron'; reefed by cord designed to melt at 200 °C to slow descent before landing

Venera 8 main

2.5 m

?

Mars 2,3 auxiliary

2m

?

Developed by N. A. Lobanov et al. at NII PDS

Mars 2,3 main

6.7 m

?

Viking main

16.2m

56 kg incl. mortar

Dacron DGB 100-500 Pa

44 Descent through an atmosphere

Quoted

diameter

Comments : Mach number,

Parachute

(Do) or area

Mass

Dynamic pressure

Pioneer Venus pilot

0.76 m

?

M- 0.8 3300 Pa

Pioneer Venus main

4.94 m

?

20d conical ribbon polyester

VeGa 1,2 pilot

1.4 m2

?

(D = 1.3 m)

VeGa 1,2 brake

24 m2

?

(D = 5.5 m)

VeGa 1,2 main

3 canopies

?

60 m2 each

(D = 8.7 m)

Galileo pilot

1.14m

0.36 kg

20d conical ribbon dacron

M- 0.9-1, - 6000 Pa

Galileo main

3.8 m

3.7 kg

20d conical ribbon dacron

Pathfinder

12.7 m

17.5 kg

+7 kg bridle; large mass

deployed at 700 Pa

Huygens pilot

2.59 m

0.7 kg

nylon DGB M - 1.5,

q - 400 Pa

Huygens main

8.31 m

4.6 kg

nylon DGB (Total system

incl. mortar, pilot,

stabilizer etc. -12.1 kg)

Huygens stabilizer

3.03 m

- 0.76 kg

nylon DGB

MPL, Phoenix

8.4 m

?

polyester DGB

MER

14.1 m

26.4 kg

nylon/polyester DGB

total

M - 1.8 - 1.9, 730-750 Pa

Beagle 2 pilot

1.92 m

0.5 kg

nylon DGB

Beagle 2 main

10m

2.764kg

nylon 28 gore ringsail

+2 0

Responses

  • Emanuele DeRose
    Do band gap parachutes increase stability?
    8 years ago
  • Roxanne
    Why parachute is having gap?
    2 years ago

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