V0.1 - 22-06-2020

Parachute Deployment

The deployment subsystem in a parachute system has the objectives to keep the parachute inside the capsule when needed and deploy the parachute upon command. The choice of the deployment mechanism is dependent on the trade-off between four key requirements. Gernally speaking there are five broad categories of deployment mechanisms which can be chosen from. These are discussed on this page. 

• Deployment Time/Ejection Velocity – Choosing the right ejection velocity is critical in the design of a deployment system. A high ejection velocity usually leads to higher reaction loads and higher system mass, but it may also increase system reliability. A high ejection velocity is preferred because:
  • The riser and suspension lines should be fully stretched before inflation of the parachute to avoid high snatch loads.
  • The parachute bag may need to avoid sharp rocket parts such as fins during deployment.
  • It prevents entanglement of wires around a spinning rocket during deployment.
  • It provides predictability during deployment at high dynamic pressures.
• Reaction Loads – Reaction loads are proportional to the parachute mass and ejection velocity. A high reaction load requires additional structure to transfer the load and increases system mass.
• System Mass – System mass is a sum of the structural mass and components of the deployment system. A low system mass is always desired to increase efficiency.
• Reliability – Reliability is a function of the number of moving parts and flight heritage. Low number of moving parts increases reliability as less parts can fail. Flight heritage increases reliability as risks have likely been identified and mitigated.

Aerodynamic deployment

It uses the drag force of a pilot chute to pull out the parachute. The pilot chute can also be the drogue parachute. Release mechanisms such as servos, explosive bolts, wire cutters, etc can be used to trigger the deployment. This deployment system is often used in conjunction with one of the other mechanisms as the pilot chute still needs to be deployed. The key advantage of the system is that the pilot chute needs to be relatively small (<1/50) compared to the main parachute in drag area. This significantly reduces the parachute mass that needs to be deployed. Aerodynamic deployment has low ejection velocity, low reaction load and low system weight. It has widely been used in amateur sounding rockets.

Springs

Springs are one of the oldest and most reliable parachute deployment mechanisms. A variety of release mechanisms can trigger them. They are limited to low ejection velocities and therefore have low reaction loads. Their system weight is medium but can increase exponentially with higher ejection velocity and parachute mass. Therefore, they are ideally suited for small parachutes and low velocities.

Mortar

A mortar system uses the force of compressed gas to eject the parachute at a high velocity. Mortars can use cold gas (Carbon dioxide, Nitrogen) or hot gas from combustion. They provide high ejection velocity which results in a high reaction load. This results in a high overall system weight. Mortars have high reliability and therefore are the default choice for large parachutes. They have been used in numerous space mission such as the Apollo and Mars 2020. A mortar system can be combined with aerodynamic deployment for extracting large main parachutes. This can be seen in most crewed main parachute systems where the  pilot chute is ejected with a mortar, which pulled the main parachute from the container. 

Slug Gun

A slug gun is like a mortar system, as it also uses compressed gas to eject mass out of the rocket at high velocity. However, a slug gun ejects a dense slug which pulls the parachute. As the slug is usually lighter than the parachute, the reaction loads are lower than a mortar. Slug guns are only suitable for small parachutes as the added mass of the slug becomes inefficient with larger parachutes. Therefore, slug guns have high ejection velocity, medium reaction loads and high system weight. They are used in spin recovery of planes and aircraft ejection speeds.

Tractor Rockets

A tractor rocket uses a solid propellant motor to pull out the parachute at high ejection velocity. As the tractor rocket provides its own thrust, the reaction loads are low, and the system mass is on the lower side as well. The mass of the propellant can increase system mass for big parachutes. As most solid propellants require pressure for ignition, its use at high altitudes requires the system to be pressurised. Additionally, it is necessary to shield the parachute and other rocket components from the exhaust of the tractor rocket. They are used in aircraft ejection seats.