Safe Parachute Opening: CLES-FACIL uses Strain Gauges from HBM on board an experimental Rocket

Every year a team from the aeronautical association "CLES-FACIL" develops an experimental Rocket in order to research key scientific questions. In a project to calculate the force that is exerted upon a rocket through the opening of a parachute, "CLES-FACIL" uses HBM strain gauges.

Not all questions about air and space travel have been explained. Therefore Institutions worldwide have conducted many projects with the aim to research further into areas that are scientifically questioned. In 1967, Alain Juge founded the club Lyonnais d'Experimentation Spatiales (CLES-FACIL), the oldest aeronautical research association in France. A strong connection between the club and the well renowned INSA University in Lyon means that research projects are continually carried out at a high level.

The CLES-FACIL team with the "Detrona" Rocket

Around 20-30 members of the CLES-FACIL continuously work on different research projects, for example the development of experimental rockets, stratospheric balloons and miniature rockets. Every year, CLES-FACIL also builds a research rocket ("Fusex"), with the exact goal to research and find an answer to particular scientific questions.

Detrona Project: Measuring the force from a parachute

<i>Successful start of the "Detrona"</i>

The task of the 2011 rocket "Detrona" from CLES-FACIL was, amongst other things, to find out the force that the opening of a parachute exerts on a rocket. This value before now was only ever formally calculated and never actually measured. Therefore the exact knowledge of this value is of great importance to Rocket makers because: The opening of a parachute causes a "shock" to the Rocket, which, in this case, could cause irreparable damage to the complete Rocket construction.

The Fusex Detrona is just short of 2 meters in length, with a diameter of 120mm and a weight of 10Kg. Parts of the rocket are transparent Polycarbonate in order for the installation of an on board camera to shoot 360°. In addition to this, a parachute is mounted, from which the force measurement will be taken.

Semi-analytical Force Measurement

Altogether the force, upon the opening of a parachute, can be estimated through a semi-analytical formula: F=0,65*S*V2, whereby S represents the surface area of the parachute and V is the speed at the moment of opening. The speed of V can not be estimated and the size of the parachutes surface area really depends on the form that it opens in. Therefore the Team from CLES-FACIL decided to use HBM Strain Gauges to calculate the force of the Rocket Detrona.

The Strain Gauges were installed on the Parachute mounting in opposing directions, two in the direction of tension and two in the direction of compression. This layout allows Torsion and Strain flows to be blended out. In addition, there is an amplifier and a form of calculating software to record the measurement data.

Application of Strain Gauges

Application of Strain Gauges on the Parachute mounting of the Detrona

Successful test flight and acquirement of measurement data

<i>The Detrona Rocket</i>

The test flight, within which the Detrona Rocket was equipped with the strain gauges and telemetric system, was very successful. The Rocket reached a height of over 1000 meters. The opening of the parachute proved reliable and precise. Through the telemetric system the team collected all the relevant measurement data, and showed that the maximal force, which was exerted at the opening of the parachute, was 1210 Newtons.

The graph shows the development of force, which was measured during the Rocket flight. The different stages from the start of the flight to the expansion of the parachute are clearly identifiable.

Force measurement graph

The measured value from the flight helped the CLES-FACIL team in further developments and was also recognised in the field of aerospace. So much so that, due to the project Detrona, the CLES-FACIL won the first prize in the 2011 "Planète Sciences" C'Space competition for the third time. Moreover, a further prize was won the "Thalès Alenia Space" hosted by the French Air and Space-Travel association GIFAS.

Click here for reports of the Detrona project (In French)

Follow up projects with Strain Gauges

Further measurement technology projects are already in place. As a follow up project from Detrona, is the investigation into Stress Analysis of an Aileron on a rocket. Here again the team will use Strain Gauges from HBM.

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