SERA4 - The design of the skirt

Why use a skirt?

The SERA3 and SERA4 pave way for the development of a demonstrator that uses bi-liquid propulsion. The presence of tanks aboard this future demonstrator has seen a need to optimize the rocket’s structure and consequently a need to increase the overall diameter of the rocket from 160mm (the usual diameter) to 250mm. In order to adapt to this new configuration without having to completely reconstruct the whole structure, the upper part that contains the avionics was left unchanged (at a diameter of 160mm). This change in diameter allows the use of three engines (bundle assembly) instead of one. The skirt is the entity that links the propulsive segment to that which carries the electronics and the payload.

A complex element to make

It is important to note that the rocket shall be a supersonic one, hence its maximum velocity will be greater than 1.2 times the speed of sound through air. In this supersonic flight regime, the skirt is exposed to extreme forces due to the pressure exerted by the air through which it moves. These forces become dissymmetric owing to the shearing wind stresses that give rise to an aerodynamic incidence, thus giving rise to a flexion moment. Another source of forces is that linked to the acceleration of the rocket. In fact, the inertial forces will be fairly huge because the rocket must have acceleration greater than 10G. The sizing of the skirt (i.e its thickness and the chosen material) is thus complex. In fact, the forces applied to it vary largely in function of its velocity, wind speed and the acceleration of the rocket during the course of the flight. The keystone of this design process lies in the correct sizing of the skirt as it impacts the rocket as a whole.

Criteria of sizing

The sizing of the skirt commences with the determination of its shape and the forces to which it is exposed. The Systems assessment team provides the magnitude of loads applied to the skirt’s body i.e. the total forces acting on the skirt during the flight. These forces are:

  • Compression /Traction (P) along the rocket’s axis;
  • Shearing forces (T) applied radially;
  • Flexion moments (M’).
Critical cases P (N) M’ (Nm) T (N)
Compression 6001 40 76
Moments 3045 444 444

Chargements appliqués

Load applied to the skirt

The dimensions of the skirt were defined by the ISAE-Supaero students’ team which is in charge of mechanical designs.

The SERA3 skirt

For the SERA3 rocket a simple structure was employed due to the fact that the conic structure concept was not yet justified
.The upper tube of 160mm in diameter rests on the lower tube of 250mm in diameter via a metallic ring. 5 radial brackets ensure the verticality of the whole structure. In order to ensure a proper aerodynamic flow around the structure, two conic half shells enrobe the structure.

This whole structure was fabricated at the University of Rennes and at INSA in the city of Rennes.

Structural and sizing verifications were accomplished through ground tests. The skirt was fixed onto a test bench and the desired loads were applied to one end so as to verify the overall resistance of the structure. This verification on proto-flight apparatus was carried out by the University of Évry.

The SERA4 skirt

For the SERA 4 rocket, the skirt shall be similar to that of SERA 3 in shape but the structure will be different. A conic metallic structure shall be realized. Grade 5 titanium is the material of choice. The structure shall be printed in 3D so as to test this new printing technology.
In order to size the overall thickness, the ISAE-Supaero students found analytical and numeric models that enabled them to calculate the linear forces and bowing of the structure. This allowed them to calculate the minimum thickness of the shell. This shall be multiplied by a coefficient of security (1.4). If the structure fails, different thicknesses shall be tested or structural modifications shall be made (e.g. adding stiffeners). This sizing is theoretic as it does not take into consideration the holes and breaks on the surface of the structure. Evidently, these areas will be exposed to forces. A second coefficient of security shall thus be applied to the obtained results.
Having completed the analytic calculations, the verification of results shall be done through numerical simulations. The software ABAQUS, will calculate the 3D forces acting on the skirt. Comparing the two solutions will allow the team to validate the results. The influence of the holes and breaks on the surface is evident.

Contraintes Von Mises pour l'analyse analytique à différents φ / modèle 2D ( épaisseur de 5mm).

Von Mises stresses for comparative analysis of different φ / 2D model ( thickness of 5mm)

In order to verify deformation in bolt/screw holes and on surface breaks a highly demanding method was put to use i.e the finite elements method. The most drastic case of forces acting on the skirt is with flexural stress. The following results were obtained:

Coupe de la jupe pour la simulation 3D en flexion. Champ de contraintes Von Mises en MPa

3D cross section analysis of flexion Von Mises stresses in MPa

The maximum stress is still lesser than 800MPa. This is the physical limit expected for the material used. The skirt thus resists the stresses to which it is exposed!
Studying the effects of bowing is a bit particular in this case. A comparison was made between the simulation carried out on ABAQUS and those made at NASA on cylindrical and conical elements that are synonymous to the SERA 4 skirt. New coefficients of security related to bowing were thus obtained.

Calcul de flambage pour le cas de compression Bowing due to compression Calcul de flambage pour le cas de flexion Bowing due to flexural forces

A margin on 2.47 was deducted for the worst possible case (flexion), hence the skirt may support up to 2.5 times the maximum load in flight!
This also brings about the conclusion that even with the smallest possible thickness, the SERA 4 skirt may withstand all forces applied to it in flight tests.
This work as a whole was monitored by experts from the PERSEUS project. Verification tests of stress resistance were made by two students from the university of Évry,Djess Biyoro and Hachim Chanfi. The sizing of the SERA4 skirt was done by Gonçalo Mendonça and finished by Adrien Gimonnet (both of whom are members of the Supaero Space Section of ISAE Supaero.

Author : Cyril Dosne (S3)