V0.1 - 22-06-2020

Parachutes

Parachutes are part of the descent stage of most EDL operations. They are a cheap, lightweight, reliable option with flight heritage as opposed to auto-rotation or powered Descent. The main function of a parachute is to decelerate the vehicle or payload. Other functions can include stabilisation, control and parachute extraction. Depending on their function, parachutes can be called main parachute, drogue parachute, programmer chute or pilot chute. Parachutes also provide an object to grab onto for mid-air recovery. 

Canopy

Parachutes can generally be  divided into five different types:

• Solid Textile: Used for the final stage of Descent at low subsonic speeds. They have a high drag coefficient and are relatively easier to manufacture
• Slotted: Used for both Drogue and Descent stage at a broad velocity range. They have high stability and their manufacturing is labour intensive.
• Rotating: Used for small Descent stage parachutes. They have a high drag coefficient but their scalability is unproven.
  
• Gliding: Used for final Descent stage at low subsonic speeds. They are manoeuvrable and have a lot of flight heritage from sky-diving.
• Ballute: Used as a drogue at high supersonic speeds (up to Mach 8). They can be inflated at extremely low dynamic pressures.

For the solid and slotted parachutes, there are several sub-divisions and combinations possible. These are to focus either on a specific regime during flight or to improve on previous designs. For the solid textile parachutes, the following sub-designs can be distinguished.

• Round: This is the arche-typical design for a solid cloth parachute. A large dome-shaped piece of cloth usually with a hole in the middle for stability. These parachutes can be steered when cuts are made in the side of the parachute.
• Cruciform: In principle a very simple design where two rectangles are placed over each over and with suspension lines at least at every corner. Although this is only the basic design alterations are frequently made to improve the design. Variations are usually made by adjusting the aspect ratio. Or the width-to-length ratio of the rectangles. Increasing the aspect ratio increases drag but decreases stability which leads to a trade-off. Finally, the ends of the rectangles can be attached to the next corner each other to improve the design. A cruciform parachute with the ends attached is currently used by the US Airforce (T-11 parachute) to drop material and men.
• Annular: The annular parachute is a half-doughnut-shaped parachute. To increase the pendulum stability and to decrease the sensitivity to wind due to its lower side profile. Annular parachutes also have a lower shock load factor than a round parachute, meaning that during inflation the loads experienced are also lower.

For the slotted parachutes mostly four variations are used:

• Ringsail: The Ringsail parachute was created during the ’60s and ’70s for crewed space flight. It is a dome-shaped parachute with a lot of slits to prevent overinflation. The parachute is currently used on most US spaceflights that require human-rated parachutes. This is because of its opening reliability, damage tolerance and low opening
• Ringslot: Parachute consisting of many concentric rings attached to each other through the suspension lines. Similar in its advantages to the ringsail it has the disadvantage of a lower drag coefficient.
• Hemisflo-ribbon: The Hemisflo parachute has exhibited satisfactory performance characteristics at supersonic speeds and its ribbon construction provides a weight-efficient canopy surface that will withstand the large canopy stresses generated at high dynamic pressures[1]. It is usually used either as a drogue parachute for supersonic speed or as a braking parachute on aircraft.
• Disk-gap-band: As the name suggested is nothing more than a disk a gap and a band. This is one of the most common parachutes for high-altitude deployment. The usage goes back to the Viking program in the 1970s and has been used ever since on various Mars missions like Perseverance. But also other missions like Huygens.

Parachutes are used widely for landing men and materials, both here on Earth and to land vehicles on Mars and other extra-terrestrial places like Saturn’s moon Titan. They are usually cheap and simple to produce. They are lightweight and do not have to require complex control systems.

However, parachutes also come with downsides. Most of them have to do with the fact that parachutes have to deploy and inflate. This is a very chaotic moment and is very hard to model. This inflation besides being chaotic can also create huge loads which need to be absorbed by the rest of the vehicle it is attached to.