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 shapes

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.

Parachute Clusters: Advantages and Disadvantages

When it comes to bringing objects or people safely back to Earth, parachutes are one of the most reliable tools in the aerospace engineer's arsenal. While a single large parachute is often the go-to choice for many applications, parachute clusters have gained prominence in recent years. In this blog post, we will explore these parachute clusters, focusing on why they are used compared to a single large parachute and discussing their distinct advantages and disadvantages.

Advantages of parachute clusters

The primary reason for using parachute clusters is to address specific challenges that single large parachutes might struggle to overcome. These can be the following

3 - Redundancy and Safety

In high-stakes missions, like human spaceflight, redundancy is paramount. Parachute clusters offer this redundancy. If one parachute fails to deploy correctly (Apollo 15), the landing speed will be higher, but survivable.

2 - Load Distribution

By distributing the load among multiple parachutes, clusters can alleviate the stress placed on any one parachute. This results in a reduced risk of chute failure due to excessive strain. In scenarios where the payload is particularly heavy, such as the space shuttle boosters, a cluster's load-distributing capability can be a game-changer. An interesting cluster feature can be found on the booster parachutes for Ariane 5 where a cluster is done both in sequence and in parallel.

3 - Cost-Effective Solutions

For organisations working within budget constraints, parachute clusters offer a more economical alternative. Building and testing smaller parachutes is often more affordable than creating a single, massive one. This cost-effectiveness can free up resources for other mission-critical components.

Cost efficiency is particularly relevant for smaller space agencies, private companies, or research organizations operating with limited funding. The ability to use smaller parachutes and clusters provides a more budget-friendly approach without compromising safety and mission success. Alternatively using clusters can allow for some modes of standardization which means that different missions can share a parachute design, or use some parachute heritage.

The Disadvantages of Parachute Clusters

1 - Complexity

The deployment of multiple parachutes simultaneously adds complexity to the system. Engineers must carefully choreograph the sequence and timing of each chute's deployment to ensure a safe landing. This complexity can increase the risk of malfunctions.

The complexity involved in parachute clusters can introduce more points of failure. If the deployment sequence is not precisely timed, the parachutes may become entangled, leading to a potentially catastrophic failure. This necessitates meticulous design, rigorous testing, and sophisticated control systems.

2 - Mass and Volume

While parachute clusters can be more space-efficient than a single large parachute, they still consume a considerable amount of volume and mass. This can be a disadvantage in missions where payload space and weight are at a premium. 

In missions to other planets or celestial bodies, every kilogram of payload mass is carefully considered. The mass and volume of the parachutes, their deployment mechanisms, and associated hardware must be factored into the overall mission design. These considerations can sometimes limit the payload's scientific or operational capabilities.

3 - Potential Entanglement

Parachute lines from different chutes in a cluster can become tangled during deployment. This risk of entanglement can lead to chute malfunctions or failures, particularly in more complex cluster configurations. 

To mitigate this risk, engineers must design parachute clusters with mechanisms to prevent entanglement. This may include careful packing of the parachutes, separation systems, or methods to ensure the lines do not cross during deployment. Nevertheless, the potential for entanglement remains a challenge that engineers must address.

4 - Aerodynamic Instabilities

Multiple parachutes in close proximity can create aerodynamic instabilities during descent. These instabilities can lead to unpredictable behaviour, making it more challenging to achieve a controlled landing. 

Aerodynamic instabilities can occur when the airflow around the parachutes interacts in complex ways, causing fluctuations in descent speed and trajectory. These variations can complicate landing accuracy, especially in missions with stringent landing requirements.

Conclusion

Parachute clusters have become a critical tool in the field of aerospace engineering, offering solutions to challenges that single large parachutes cannot address effectively. The advantages of redundancy, load distribution, controlled deceleration, and cost efficiency make them an attractive choice for various missions. However, these advantages come with trade-offs, including added complexity, mass and volume constraints, entanglement risks, and potential aerodynamic instabilities.