On October 20th, the Capsule and Recovery team spend 14 hours in the Open Jet Facility (OJF) of the TU Delft. The aim for the team was to get a more thorough understanding of the stability of cross-shaped parachutes and the shock loads that occur during deployment. To test this, the team worked tirelessly in the week before the test to create a test bench and produce 5 different parachutes.
The test bench was largely borrowed from the hybrid propulsion team and a special adapter was fitted, containing a canister for the parachute and a load cell to measure the drag and shock loads. This test bench ensured that the parachutes would be raised from the test table by 1.5m to avoid interference during testing.
Test setup during Capsule wind tunnel test
The parachutes that have been tested consisted of one drogue parachute and 4 main parachutes. Two of the main parachutes were standard cross parachutes, one made out of kite material and one made out of human-rated parachute fabric. This distinction was made to compare the quality of the different fabrics. The third main parachute was shaped as a trapezoid, which meant that the outer edges were larger in size than the middle square. The final main parachute was a small adaption to the regular cross parachute but the ends of the sides were stitched together to form a box shape which vents near the middle. It was suggested that this type of configuration would be more stable than a regular cross-shaped parachute. Both the third and fourth parachutes were also constructed out of the human-rated parachute fabric.
A completely different kind of parachute shape was used for the drogue parachute. Since Aether, and afterwards Stratos III, will descend at supersonic speeds, a regular shaped parachute will not suffice. For this reason, the design of the drogue parachute was chosen to be a hemisflo ribbon parachutes. This design consists of ribbons (bands) of fabric, of horizontal and vertical direction, producing a semi-sphere when constructed. This design was made up from almost 200 individual pieces of ribbon that were cut to exact lengths and stitched together.
Hemisflo ribbon drogue parachute deployed in the wind tunnel
After several nights of hard work, the team gathered at the OJF at 7 am to start preparations. This went very smoothly and within an hour the first test run of the wind tunnel was conducted. After this successful trial run, the rest of the day was filled with a grand total of 42 parachute tests. All five parachute types were tested multiple times under different wind speeds (and thus different dynamic pressures) to get an overview of the behaviour of the parachute as complete as possible. There were some problems with the deployment of some parachutes in the start. This was due to the parachute bag closing on itself because the rim got tangled up with the parachute lines. This was easily resolved by cutting the rim of the parachute bag away.
It was observed that the two regular parachutes and the trapezoidal parachute were very unstable and moved all around the wind tunnel. This also caused the measurement of the drag to become complex. In the end, post-processing of the data did provide some measures of the forces involved. However, due to the instability, these designs were scrapped as viable for Aether. The cross parachute with ends attached (box parachute) did prove to be very stable, as predicted by theory, which can be also seen in the picture below.
The biggest problem that was spotted during the test series was the instability of the drogue parachute. This parachute started turning as soon as it deployed and this caused the lines to entangle which ultimately led to the collapse of the drogue. It was established that the swivel attaching the suspension line to the load cell didn’t rotate with the drogue parachute. This problem couldn’t be fixed on the spot but different ideas will be tested during the second series of wind tunnel testing on January 3rd, 2017.
Capsule team present at of OJF
The shock loads produced by all parachutes were higher than the team initially expected. It was on average twice as high as the steady drag after deployment which is due to the infinite mass scenario that occurs during the wind tunnel test. On the actual flight, the rocket will already decelerate during the deployment of the parachute and the shock will be lower in that case. Nevertheless, the shock load will still be higher than initially estimated. One test was performed with a slider attached to the parachute lines. This slider should ensure that the parachute will deploy less quickly and the shock load will be lowered in this way. This test gave the team enough confidence to keep on studying sliders in the near future.
The tests were on average successful. Some problems were identified during the testing itself, of which a few could be fixed on the spot. However, some problems required a more thorough analysis. The team will complete a second parachute test campaign in the OJF on January 3rd, 2017 during which the main parachute shape will be optimised and a new drogue parachute will be tested, to make sure that the final design will be more stable.