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Rocket Flight Profiles- by GU Rocketry

GU Rocketry
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This article serves as a summary of the Sutton Program in that all of the principles the readers have learned across the Sutton Program educational series can be applied to an actual model rocket launch, using GU Rocketry’s Saltire-1 as a reference. Written by Sean Clark & Kieran Webb. Graphics by Ellie Thomson.

Introduction

Over the summer, our Sutton Program covered each of the individual stages of preparation one must undergo prior to launching a small-scale rocket. The aim of the initial articles was to provide a solid background- the first example of a firework as a weapon, the founding fathers of rocketry and the inception of the basic laws which are essential to the launches of the modern era.

As the program continued, we also covered more advanced aspects of rocketry, such as complex aerodynamics principles, hybrid rocket propulsion methods and rocket avionics. This article will tie everything together- applying some of the principles we were taught to an actual profile of a rocket’s flight path, such as GU Rocketry’s Saltire-1.

A video showcasing the successful launch of Saltire-1 can be found seen below.

SALTIRE-1 Launch profile

Ground Control to Captain Tim?

Before the rocket can even be launched, on the day, the weather needs to be assessed – adverse conditions will greatly affect the launch and flight and so GU Rocketry’s ground infrastructure team are currently building a weather station to do just that and determine if the launch can be given the go-ahead. The first stage of rocket flight is, of course, the launch. It is essential that the rocket have a successful take-off phase to ensure that the correct flight path is taken.

Rocket on the launchpad at ignition

Rocket on the launchpad at ignition

This is the job of the ground infrastructure team- using a launch rail, the rocket is aligned correctly so that the thrust can propel it skyward as efficiently and smoothly as possible. The launch is electrically initiated via a launch box, operated from a safe distance. The motor is fired, and the thrust launches the rocket safely to begin the flight.

Post-Partum Propulsion

In the case of Saltire-1, an L-class solid rocket motor was used, of which the total impulse ranges from around 2,500 to 5,000 Newton seconds (Ns). For now, total impulse can be explained simply as a measure of engine performance (thrust produced over time).

As we recall from our propulsion article, it is difficult to stop a solid rocket motor from consuming its fuel after it ignites. However, this lack of control also comes with the benefit of the motor having very few moving parts and less chance of failure during flight. A solid rocket motor was the ideal choice for Saltire-1 as they are relatively simple to set up, cheap in comparison to other engine types and reliable.

Rocket at burnout point, still travelling upwards

Rocket at burnout point, still travelling upwards

As the rocket accelerates upward from the ground, it burns fuel steadily along its grain until it the fuel burns out. After the fuel burns up, the rocket’s momentum carries it further into the air, a phenomenon which is known as ‘coasting’, where the rocket still gains altitude despite it not having any more fuel to burn. In the case of Saltire-1, the maximum altitude was approximately 5,500ft, so the burnout altitude would have been slightly below this.

Smooth Sailing

The points in the above paragraph have covered what the Propulsion team at GU Rocketry would have a part to play during a real launch. However, the aerodynamics of the model rocket also has a critical role to play during the flight. The main points of focus that our aerodynamics article covered are the points which effect the rocket in flight, which all must be determined well in advance of launch day, as there is little in the way of last-minute tuning to the rocket body which can be carried out on launch day.

Some of the basic factors which are important to rocket aerodynamics are as follows- the centre of pressure, centre of gravity, weight, lift, thrust and drag. For an introduction to how each of these affect the flight path of our rocket, check out our article on aerodynamics. We can use simulation software to determine the fight path of the rocket, along with how the body of the rocket will respond to an oncoming fluid flow of air, i.e., during flight.

For simulation of the flight path, we can use OpenRocket. This software can determine the path of the rocket under different conditions, such as various wind speeds/directions and turbulence intensity. For simulating flow over the nose cone, body, and fins of the rocket (which determine the rocket’s stability, a term introduced previously), we can use CFD software or Computational Fluid Dynamics. This tells us how skin friction, profile drag and induced drag will affect our overall flight.

On the day, the aerodynamics team can check the rocket body for any protruding materials, cracks or inconsistencies within the rocket body prior to launch, as this can have a substantial negative affect on the rocket’s stability and aerodynamic profile during flight.

Free Falling

After the rocket’s motor has consumed all of its fuel, it will continue to increase in altitude until the force of gravity counteracts its upward momentum (this period is known as coasting) and then begins its descent to earth – this is the apogee stage of flight when the rocket reaches its highest point. Through the use of accelerometers and gyroscopes, the rocket’s microcontroller will detect this change of direction and can initiate any protocols, such as the deployment of the parachute once the rocket has reached a certain altitude. From our recovery video, we know that the drogue chute will be deployed first to limit lateral movement during descent and ensure the rocket does not reach too high of a velocity such that the main chute cannot be deployed.

The altitude can be computed by the microcontroller using a simple equation and a barometer, which measures the ambient air pressure. Early deployment of the main parachute can result in the rocket covering large distances laterally during descent, thus making it very difficult to predict the landing zone, and so accurately calculating the current altitude is essential. To deploy the parachute, the flight computer will activate the corresponding pyrochannel and cause a section of the rocket to open and release the parachute. This helps to return the rocket back to the ground safely and with minimal damage.

Successful deployment of parachute post-apogee

Successful deployment of parachute post-apogee

The on-board flight computer will also record flight data, such as altitude, velocity and direction. This data can be analysed once the rocket has been recovered to better understand the kinematics of the flight and any faults that may have occurred to identify any mistakes or improvements that can be made for the next launch.

Once the rocket has been recovered, the structure needs to be carefully checked to detect any damage and determine any repairs that may need to be made before another launch – this would be the case for a multiple-use rocket. All that needs to be done before the next launch would be to replace the motor, igniter, re-pack the parachute and reset any charges that were used up by the pyroevents.

Conclusion

This article has described some of the important aspects of rocketry that are applied on flight day. To get a better understanding of the individual principles behind rocketry, we have a series of articles in the Sutton Program which go into further detail on the principles discussed above. This is the first in a series of four articles released over the course of this year which will cover other interesting and exciting areas within rocketry. Happy learning!

The Sutton Program: Rocketry Summer School- by GU Rocketry

We are a University of Glasgow based engineering project, focused on launching Scotland's first student-lead, high-powered rocket.

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