Rocket Recovery - Sutton Program Article 7

This article will accompany the seventh and final video in our series made possible by RS Components Grass Roots aiming to introduce key rocketry concepts and principles. This article analyses the different methods of safe rocket recovery. Written by Cameron Johnson

Introduction

Recovery is an essential element of rocketry that has seen significant changes in recent decades. Rocketry has become safer, more sustainable and more financially viable for a wider range of projects as a direct consequence of focusing on and improving recovery systems.

Rocket recovery has two aims:

• preservation of the descending sections of the rocket
• protection of the payload or crew inside.

The way this is achieved varies greatly based on the mission parameters and proposed capabilities of the rocket. If the rocket contains human crew, the recovery system is more complex with many more elements. If the rocket is a model flown by amateur hobbyists, then the recovery system may be no more than a parachute connected to a shock cord.

Parachutes and their subtypes

For decades, parachutes have been the foundation of almost all recovery systems. They are lightweight and have a small surface area when packed, along with their long history in slowing objects down when falling- the first recorded successful use of a parachute dates back as far back as the 15^th century. This makes them ideal elements in all recovery systems used for the first manned missions to space. The use of parachutes has continued to the present day, where they still serve as the primary method of bringing a rocket or module back down safely.

Parachutes work by atmospheric drag working against gravity. In this case, the parachute canopy creates a drag force on the falling object which is usually connected to the parachute with paracord or roping of some kind. The larger the drag force the parachute generates, the slower the object will fall back to the ground. This mechanism is simple, not prone to failure and cheap to implement.

For our Saltire-02 rocket, parachutes make up a significant portion of the recovery system, housing a drogue chute and a main chute. Drogue chutes are required for launches above a certain altitude as they reduce the drift a rocket can experience when descending through the atmosphere. Drift is caused by lateral drag.

The main chute has a large surface area and a large canopy ‘plume’, which is the dome shape that most non-steering parachutes take. This shape and size mean it has a very large lateral drag; therefore, wind can blow the rocket horizontally. This, combined with the slow descent speed caused by the main chute, means that a rocket which deploys its main chute at high altitude could cover vast distances before reaching the ground. Because of this, finding the rocket’s landing zone becomes much more difficult.

Parachutes in a modern context

In the early days of human orbital flight (check out Article 1 for information on the History of Rocketry), both American astronauts and Russian cosmonauts had to prepare for the possibility of landing in an unfamiliar territory- this was a far more likely prospect before the development of GPS technology. Soviet cosmonauts were even sent into their capsules with shotguns to protect them from wild animals if they happened to land in the Siberian wilderness!

Instead of using a main parachute initially, our Saltire-02 rocket deploys a drogue chute first and then deploys its main chute at a specified altitude. In our rocket, the parachutes are deployed when explosive charges detonate, breaking sections of the rocket open, which allows the parachutes inside to unfurl and open. This is the most common method of parachute deployment on a rocket, due to its simplicity and the redundancies that can be implemented, creating a reliable system.

However, with larger rocket modules, parachutes alone are sometimes not enough to slow the module down for a safe landing, especially if human passengers are involved. As parachutes can only slow a rocket down so much, we must employ additional measures to fulfil the recovery system’s aims to preserve the rocket and protect the payload or crew.

When Chutes Aren’t Enough

Most additional measures are implemented either at touchdown or moments before. These can change the landing environment for the module, or soften the force exerted on the payload by touchdown. During the Apollo Program, the heavy recovery capsules used landed in the Pacific Ocean to soften the landing for the crew inside. Alternatively, the Soviet Program would land their capsules with small solid fuel rocket boosters, or ‘retrorockets’, that ignite just before touchdown to slow the recovery module before landing.

Rocket modules have used many types of recovery systems throughout history. Boeing’s Starliner module uses air bags to cushion the landing for the crew, while SpaceX’s Dragon capsule uses retro boosters similar to the Soviet Soyuz module. The Space Shuttle would glide down using its aerodynamic body and wings to land on a runway, deploying a parachute to slow it down further.

Typically, non-crewed rocket modules that are sent into orbit are not recovered, which means a large waste of expensive resources. Currently, almost all unmanned systems sent into orbit are de-orbited and burned up in the atmosphere. In the future, we could see recovery systems that make it more economically viable to bring satellites and orbiting telescopes back to Earth in one piece. This leap in technology has been accomplished before, recently by the introduction of first stage recovery.

How can we improve reusability?

For decades, it was commonly thought that the first stage, although expensive and complex to build, was a disposable part, due to the size and speed of the stage at the point of stage separation. Engineers in the 1960’s would have thought it near impossible to recover a first stage booster that had been detached at high speed and high altitude.

However, with today’s tracking systems and on-board computing power, private companies have been able to land the first stage back successfully,  ready to be used in a future flight. This huge step in rocket capability has allowed a dramatic reduction of the cost of getting cargo into orbit.

It is all made possible through a combination of avionic computing power, aerodynamic knowledge and control of the rocket’s nozzle, or thrust vector control (TVC). The exponential increase in computing power over the past few decades has meant that flight computers are able to give feedback to the overall systems more quickly and precisely. In this case, the flight computer would be sending signals based on position, orientation and speed to actuators that will adjust the rocket’s current trajectory to match the desired one input into the system, like autopilot in aircraft.

The difference lies in the way the rocket reduces its speed to land gently. On an aircraft, this is done by reducing throttle, using ground friction and brakes to slow the aircraft to a halt. The opposite is true for rockets. Once the rocket is in the correct position to land, it relights its engines and increases throttle to slow the falling rocket.

During the descent, the thrust of the engine can be directed by the rocket nozzle to adjust the rocket’s orientation as it descends. This can be done in a multitude of ways, such as putting the nozzle on a gimbal, or contracting and expanding the nozzle asymmetrically.

Beyond our atmosphere

So far, we have only mentioned recovery with regards to Earth and its atmosphere. As we move further into the solar system, the different environments we experience will give us new challenges. Mars has a far less dense atmosphere, so parachutes are less useful in slowing down modules. The Moon has no atmosphere at all, so propulsive landing is the only option.

How and when we can successfully launch and carry out recovery from other planetary bodies has yet to be developed but scientists and engineers are working on it right now, such as synthesising rocket fuel from the Moon’s ice caps to orbital tethers and space elevators.

Conclusion

Rocket recovery, be it at an amateur level or on an orbit-reaching scale, is an essential part of rocketry. In the future, this field will only become more essential as sustainability, responsible exploration and construction in space become more pressing issues in the future.

The people that will make these breakthroughs are the people who are inspired in schools today who go on to choose a career in science and engineering. We hope that this program has inspired you to try out rocketry, be it here at GU Rocketry or at your own university teams and clubs.

Parts in this series:

• An Introduction to GU Rocketry
• The History of Rocketry - Sutton Program Article 1
• Rocket Dynamics - Sutton Program Article 2
• The Rocket Equation - Sutton Program Article 3
• Rocket Propulsion - Sutton Program Article 4
• Rocket Aerodynamics - Sutton Program Article 5
• Rocket Avionics - Sutton Program Article 6
• Rocket Recovery - Sutton Program Article 7