Testing of Parachute systems
Testing is of a parachute system is critical for ensuring the system has the required reliability. Preferably the system should be tested in conditions as close to the actual flight conditions as possible. This is, however, not always in the technical or budgetary capabilities of a project. To still be able to test the system, one can test a parachute system in various ways ranging from frequently used to more experimental.
Wind tunnel Tests
Comparison of CFD and wind tunnel test of a Disk-Gap-Band at supersonic conditions
Multiple parachute models are built to mitigate the issue with the various test conditions in the different wind tunnels. This can be seen in the figures below. These parachutes are both Disk-Gap-Band parachutes being tested for the Curiosity landing. The large parachute is tested in a large wind subsonic wind tunnel. And on the right, a supersonic model is tested.
Disk-Gap-Band being tested at subsonic and at supersonic conditions
The standard way of testing a parachute in a controlled environment is by using a wind tunnel. In a wind tunnel, the test object (parachute) is stationary and the air (wind) is moved around the test object. Wind tunnels range from the very large, such as NASA's Ames Wind tunnel with a 24m x 36m test sections to tiny wind tunnels with a test section of 10cm x 10cm. The testing speeds can vary significantly from hypersonic speeds (>Mach 5) to no more than 30m/s. As wind tunnels are ground-based, they (usually) operate at sea level densities and operate with scaled models. This means that one can either recreate the dynamic pressure of the Mach number. Some tunnels are able to modify the density, but rarely can both conditions be reached for a re-entry mission. This poses an issue with wind tunnels that cannot be overcome. However, wind tunnels can assist in identifying the drag capabilities and stability parameters of a parachute and serve as a verification tool for computational fluid dynamics (CFD) results. The CFD can then be used to expand the knowledge of the parachute's behaviour. An example of a parachute by Vorticity Inc can be seen on the right.
Drop-tests
When wind tunnel testing does not allow one to verify the requirements, different methods are needed. One attaches the parachute system to something and drops it from a (large) height. Dependent on the body the parachute system is attached to, the speeds which can be achieved are still significantly high, even supersonic speeds. Drop tests can be performed at various heights ranging from large cranes to helicopters and aeroplanes to high altitude balloons. Examples of this can be seen in the Boeing Starliner drop tests, where both balloon and aircraft drops were done using both a blunt and aerodynamic forebody. A video can be seen showing various SpaceX drop tests from helicopters, aircraft, and balloons on the right. Furthermore, different test conditions can be seen, ranging from nominal flights to "one-parachute-out" flights.
A drop test can be highly repeatable, just like a wind tunnel test, simulating an environment closer to the actual flight conditions.
SpaceX testing parachutes by various drop tests
Drop testing the Orion parachute system
The effect of the forebody and drop platform can be seen in the figure on the left. This figure demonstrates the testing done to human rate the Orion parachute system. The systems compared are a high altitude balloon and a C-17 aircraft. The forebodies shown are an aerodynamic body for high-velocity testing and a blunt body representing the Orion capsule.
Flight Tests
When both the drop test and the wind tunnel test do not give the desired conditions, one can resort to flight testing. Flight testing can be seen as a general repetition of a parachute system. During these often costly tests, a system is exposed to the actual flight conditions.
A parachute system is tested on a sub-orbital rocket or put in/on the first stage on an orbital rocket. Here both supersonic as re-entry conditions can be achieved. This testing method was used for Vorticity's SuperMax experiment proving the supersonic deployment behaviour of a disk gap band parachute. As well as ESA's ARD experiment to test an entire re-entry capsule, including the parachute system. Both these missions were suborbital. A near orbital mission was the OREX mission performed by JAXA. This capsule orbited the earth once and then re-entered, validating the re-entry assumptions for HOPE. Final examples of flight testing are the SpaceX Demo-1 (uncrewed first flight of the Crew dragon), SpaceX Demo-2 (first crewed flight of the Crew dragon), and the comparable Boeing Starliner missions.
These tests are the most representative test for the EDL system and are favoured to verify multiple requirements in one test. They also give a very comprehensive data set for future missions and tests. However, organising and performing these tests take up a significant amount of resources. Furthermore, when one of these tests fails, it will be challenging to determine the root cause.
Innovative Tests
When none of the previous options fit the required criteria, innovative solutions need to be thought off to perform the test. One project that did try to reach both the desired Mach number and dynamic pressure was PEPP (Planetary Entry Parachute Program). In this program, a test object was lifted using a balloon to the desired air density. Then using rocket engines, the desired Mach number and dynamic pressure were reached. Other missions just used a two or three staged rocket. By allowing control over the velocity and altitude with two separate vehicles, engineers could recreate the parachute inflation conditions experienced on Mars. Research from this program is still used in Mars landers.
Another interesting test was that one of the stress tests NASA's Low-Density Supersonic Decelerator's parachutes dropped from a helicopter. However, since that didn't produce the required force for that test, the parachute was accelerated using a rocket sledge. Using the sledge, the parachute force was increased from 12 kN to 420 kN, ultimately ripping the parachute apart and giving the engineers enough data to improve the parachute and make sure this doesn't happen when entering the Martian atmosphere.
