ASRW and the ASENKA Rocket
27 September 2026
ASRW (Airbus Sloshing Rocket Workshop) is a workshop organized by Airbus in which student teams design and fly a rocket powered by pressurized water. A water rocket sounds like a simple school experiment; but the challenge here is not getting the rocket off the ground, it is keeping the water inside it under control.
The problem: sloshing
Sloshing is the motion of a liquid's free surface in a partially filled tank. Sudden accelerations such as launch or vibration displace the liquid; this places unexpected loads on the structure, shifts the center of gravity and misleads the control system. The result can be a deviation from the trajectory or loss of control. The attitude control issues on the Gemini missions and the instabilities observed on Ariane 5 are well-known examples.
At our scale the problem is even sharper: since there is no chemical propulsion, our only thrust source burns for 0.06 seconds. All performance depends on how cleanly the 875 N peak thrust produced in that very short moment is transferred — and the 0.7 kg of water in the tank starts sloshing at exactly that moment.
The solution: a helical baffle
The classic approach is to place horizontal perforated plates inside the tank and try to stop the liquid. At high accelerations the liquid spills over these plates. Instead of stopping the liquid, we redirected it: a 234 mm long, Ø80 mm helical PLA baffle inspired by the lignin helix turns axial oscillation into circular flow, dissipating the energy through the liquid's own internal friction.
We compared two models, with and without the baffle, in ANSYS Fluent using the VOF method (24,074 poly-hexcore cells, 0.05 s time step, 2.5 s in total). We used the same mesh; the only difference was the addition of the baffle. Results:
- The peak cell Courant number dropped by 40% and its average by 35%.
- The standard deviation of the tank wall loads was roughly halved.
- Wave breaking was largely prevented and the liquid's settling time was shortened.
Even if the mean load stays the same, halving the fluctuation directly lowers the peak load the structure has to be designed for. The comparison of the flow with and without the baffle is shown in the two images in the analysis folder.
The rocket
We developed three concepts and weighted them against the ASRW scoring criteria. The heaviest criterion was horizontal distance (35 points); the glider concept led in flight time but was eliminated because it scored lowest on range. The concept we chose was a two-stage rocket with active fin control (3.95 / 5).
| Property | Value |
|---|---|
| Structure | Two-stage, PET body, carbon tube frame |
| Tank | 1.5 L PET · 23.3% full (0.350 L water + 1.150 L air) |
| Operating pressure | 10 bar (competition limit 10 atm) |
| Peak / average thrust | 875 N / 438 N · 0.06 s · E438 equivalent |
| Nose cone | Elliptical, ABS, 100 mm × Ø115 mm — dry bay for avionics |
| Stability | ≈ 2 calibers static, semi-active canard control |
| Launch | 2 m aluminum sigma profile rail |
An elliptical nose cone was chosen because short, blunt noses give the lowest drag on rockets that fly strictly at subsonic speeds. The movable fins are there directly for points: when the rocket is held at a non-zero angle of attack, the body and fins generate lift together, turning part of the vertical descent into horizontal distance.
Staging is triggered in two different ways. The first stage is triggered manually from a safe distance at 10 bar by a mechanical pin-release system. The second stage fires in flight when the avionics, made up of an MPU-6050 and an Arduino Uno, actuate the servo.
Safety and outcome
Since the body is a drink bottle, safety had to be built into the design. The PET bottles were reinforced with multiple layers of packing tape, the bottle–nozzle joint was secured with two-stage epoxy, and each bottle was surrounded by a glass fiber wrap and a 3D-printed PLA cage. A pressure relief valve and a manual water drain port were added to the system; a leak check was performed before flight on every fill.
The design was validated in three ways: laboratory sloshing tests, field launches and risk matrices. Finite element analysis showed that the tape-wrapped body and the carbon tube frame stay above the safety factor under operating loads. Thanks to the PLA ring-lock system, the tanks can be removed in under two minutes; the refill cycle takes 15 minutes, which allows back-to-back flights.
ASENKA was a six-person team — split into mechanical and avionics — and the project ran on a 14-week schedule. I served as the structural design lead.