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3rd Symposium on Space Educational Activities, September 16-18, 2019, Leicester, United Kingdom
Flight testing of parachute recovery systems aboard REXUS
E. Menting, L. Pepermans, T. Britting, E. Soliman, J. Klaassen, M. Rozemeijer, J. van ‘t Hof
Delft Aerospace Rocket Engineering (DARE)
The Netherlands
SPEAR@dare.tudelft.nl
The Supersonic Parachute Experiment Aboard REXUS or
SPEAR mission is a mission by the Parachute Research Group
(PRG) of Delft Aerospace Rocket Engineering (DARE). The
primary objective is to test the in-house developed Hemisflo
ribbon drogue parachute at supersonic conditions. To achieve
this a test vehicle is placed in the nose cone of the REXUS
sounding rocket and released near apogee (75-85 km). From
apogee, the test vehicle shall follow a ballistic trajectory and
deploy the drogue parachute above Mach 1.5.
Besides testing the drogue parachute at supersonic velocities, the
secondary goal of the mission is to gather a full validation data
set for the DARE ParSim and TumSim simulation tools.
The test vehicle is aerodynamically stabilized and centred around
the drogue parachute deployment system. The parachute system
is completed by two main parachutes for a safe landing. Data on
the parachute performance is stored onboard. All mission critical
data, including video, is sent down via telemetry.
The paper describes the SPEAR project, design and testing
envelope. Furthermore, an outlook is provided on diverse
experiment possibilities for parachute testing on the REXUS
sounding rocket.
Keywords— Hemisflo ribbon parachute, drogue parachute,
supersonic, SPEAR, REXUS, test flight.
I. INTRODUCTION
Within Delft Aerospace Rocket Engineering (DARE), the large
envelope advanced parachute system (LEAPS) is used as the
recovery systems of multiple flagship projects such as Aether,
Stratos III and Stratos IV [1]. The Hemisflo ribbon parachute is
chosen as the drogue parachute for this system due to its high
supersonic capabilities as it can theoretically function up to
Mach 3.
When looking at a parachutes deployment and flight, the two
most critical parameters are the dynamic pressure and the
velocity regimes encountered. The Parachute Research Group
(PRG) is able to test the parachutes extensively with wind
velocities up to 30 m/s and dynamic pressures up to 0.5 kPa in
the Open Jet Facility (OJF) at the TU Delft. In these tests the
drag and subsonic stability performance of the parachutes can
be determined. These conditions however, are not comparable
to the deployment conditions of the parachute in flight.
The team has designed a test vehicle, as part of the Parachute
Investigation Project (PIP), to test parachutes at dynamic
pressures up to 7 kPa. This is comparable to the main and
drogue parachute deployment conditions in larger DARE
missions.
It is however more difficult to test the supersonic capabilities
of parachutes. Large size supersonic wind tunnels exist, but are
very expensive [2]. Aside costs there is a difference in drag and
stability behaviour between parachute performance in a wind
tunnel (infinite mass scenario) or in flight (finite mass scenario)
[3]. Therefore, a supersonic flight test would be preferred to
validate the supersonic performance of this parachute.
Inspired by SuperMAX, flown on MAXUS-9 [2], PRG looked
into a sounding rocket piggyback mission for testing the
LEAPS drogue parachute. This led to the proposition of a new
DARE mission that was to fly on the REXUS sounding rocket
as part of the REXUS/BEXUS project; the Supersonic
Parachute Experiment Aboard REXUS (SPEAR).
This paper gives an overview of the mission and the
measurements to be performed. Furthermore, it gives an
overview of the electronic and mechanical design of the
experiment, and an outlook on future flight testing of
parachutes.
II. MEASUREMENTS
As the SPEAR mission aims to flight test the Hemisflo
parachute at supersonic conditions, it is essential to obtain
sufficient data in order to reach a post-flight conclusion on the
parachute performance.
The data set gathered will include force data from parachute
inflation which will be used to validate the “small parachute,
supersonic conditions” inflation models in ParSim. Three load
cells are used to ensure the parachute inflation load can be
measured, even when the parachute force does not act in line
with the longitudinal axis of the test vehicle.
Next to this, the inflation behaviour and stability of the
parachute will be observed with an HD, high speed camera
which is mounted in an upward looking configuration towards
the parachutes.
The final onboard sensor to study the drogue parachute is the
inertial measurement unit (IMU). This sensor will measure the
acceleration in three axes as well as rotational rates.
The inflation of the parachutes can be directly observed from
the video images of the on-board camera. The loading
conditions during parachute inflation and flight will be
obtained during post processing of the load cells and IMU
measurements.
Next to parachute related measurements, more data will be
gathered from the SPEAR flight in order to validate other
assumptions and simulation programs within DARE.
Because SPEAR is expected to experience aerothermal
heating effects during re-entry, the heat fluxes will be
measured to validate the thermal models currently
implemented in ParSim. Finally, a camera is mounted on the
REXUS rocket in order to confirm that the separation of the
vehicle occurred without any issues and a pressure sensor is
used for main parachute deployment.
III. FLIGHT SIMULATIONS
The SPEAR trajectory consists of four major flight phases,
these can be seen in Figure 1. In the first phase, ascent,
SPEAR is attached to REXUS. The test vehicle will be
separated shortly before apogee.
After this SPEAR and the REXUS rocket will go their separate
ways. From around 60km, SPEAR will undergo a re-entry
phase. During this second phase, SPEAR is stabilized by a
ballute stabilizer, which ensures the vehicle remains close to a
zero-degree angle of attack. This is needed to increase the
terminal velocity of the vehicle at drogue parachute inflation.
Furthermore, as ParSim assumes a constant angle of attack [5],
it is preferred to keep the vehicle as stable as possible.
After re-entry, the vehicle enters the third phases of its descent.
In this phase the Hemisflo parachute will be deployed, at
supersonic conditions over Mach 1.5. This will be at
approximately 25km and is dependent on the conditions at the
launch site, the performance of the REXUS rocket and the final
parameters of the SPEAR vehicle. The team also performs a
sensitivity analysis on the velocity at drogue deployment to
ensure this requirement is met [4]. Figure 2 Shows the
minimum deployment altitude as a function of the apogee
altitude and vehicle mass of SPEAR.
When SPEAR reaches an altitude of about 1 km, the main
parachutes are deployed to ensure it lands safely with a landing
velocity lower than 10 m/s.
Figure 1 Flight Sequence of the SPEAR mission
Figure 2 The cross-over point for SPEAR deployment at M=1.5
Other than the deployment, the expected forces from the
parachute on the vehicle as well as the decelerations during the
flight are essential for designing the test vehicle. The
constraints lead to the flight envelope of the vehicle which can
be seen in Figure 3. Here the maximum parachute inflation
loads as well as the maximum dynamic pressure the parachute
can handle are plotted in one figure. The envelope shows that
the limiting factor in this mission is not the drogue parachute
but the main parachutes. This can be seen as the main
parachute inflation occurs on the edge of the flight envelope.
Figure 3 Flight Envelope of the SPEAR Flight
Figure 4 SPEAR vehicle – front view
IV. MECHANICAL DESIGN
The mechanical design of SPEAR includes the parachutes,
deployment and separation systems. A mechanical overview
can be seen in Figures 4 and 5. These are bound to a set of
strict constraints, given that the vehicle will fly within the
REXUS nose cone. The limited available volume and mass
formed driving requirements for the design. The use of design
tools such as CAD-software were of large value during the
design process and greatly helped the team determine the
feasibility of the design. Finding a suitable location for all
subsystems without interfering with each other was arguably
one of the most difficult challenges faced throughout the entire
design. Because the parachutes and electronics already occupy
a significant amount of the available volume, the team had to
be creative and flexible to allocate all components to a suitable
location, while keeping an eye on the feasibility of assembling
the vehicle. Several design changes followed from the initial
assembly procedures to facilitate integration. Using Dassault
Systems CATIA v5 as a 3D design tool was a very convenient
way to arrange and visualise all components inside the vehicle.
Production methods such as 3D printing and wood laser cutting
enabled the team to produce a low-cost mock up vehicle.
A. Vehicle shape
Throughout the conceptual design, it was found that a conical
vehicle shape was unstable due to the centre of pressure being
in front of the centre of mass. The SPEAR team investigated
different stabilisation options including deployable grid fins,
small wings, and a stabilisation parachute. The latter was
chosen to stabilise the vehicle during its descent in the upper
atmosphere. The parachute was sized such that it provides
adequate stability whilst allowing the vehicle to reach
supersonic conditions at drogue deployment. This led to a 19
cm diameter ballute parachute. Due to the small size, this
Ballute is difficult to produce and test.
Figure 5 SPEAR vehicle – side view
As the vehicle was now stabilised, the available volume could
be increased by adjusting the shape of the shell from being
conical to cylindrical with a dome.
B. Parachutes
A total of six parachutes are required for the mission, and thus
have to fit inside the vehicle, which in its turn should not
conflict with the internals of the REXUS nose cone. These six
parachutes consist of the stabilizing ballute, one Hemisflo
ribbon drogue parachute and two main parachutes with one
pilot chute each. The drogue parachute is connected to the
main structure via three load cells. The two main parachutes
are used to decelerate the vehicle to a safe landing velocity.
Disk-Gap-Band parachutes are chosen as main parachutes as
these have proven to be very stable during wind tunnel tests.
This type is used to land the Stratos nose cone as well, for
which SPEAR can provide useful parachute flight data. The
main parachutes are attached to the main internal structure at
the bottom of the main parachute canisters.
The drogue parachute is deployed using a hot gas deployment
device (HGDD). The system works by igniting 0.5 grams of
nitrocellulose, generating a rapidly increasing amount of gas in
a confined volume, generating a pressure of 36 bar. The gas
pushes the parachute and the sabot, shearing off a set of nylon
bolts that keep the system closed [6]. The HGDD is used for
the drogue deployment since it can deploy a parachute with a
high velocity of 25 m/s, decreasing the chance of entanglement
between the drogue parachute and the test vehicle. One of the
main challenges encountered during the HGDD design was to
figure out a suitable amount of nitrocellulose. Not using an
adequate amount of nitrocellulose will not shear off the set of
nylon bolts and eject the parachute with a sufficient velocity,
whereas using too much will generate too much pressure that
cannot be taken by the canister, leading to structural failure.
Furthermore, as the drogue parachute is deployed around
25km, the HGDD has to function in low ambient pressures and
air density. Pyrotechnic actuation in low air density is difficult,
which is why the entire pyrotechnic section is sealed. Despite
the seal, ignition in near vacuum is being thoroughly tested for
redundancy.
A simple spring system is used to deploy the main parachutes
and stabiliser. This system works by compressing a spring and
keeping it in place using a wire. The wires are cut using wire
cutters which releases the springs, ejecting the parachutes out
of the canisters [6].
Most of the mechanisms inside SPEAR are triggered using
Cypress wire cutters. These are lighter solutions than servo-
actuators and are very reliable as they are bought commercially
off-the-shelf. Three sets of two wire cutters are used to trigger
the deployment of the stabilisation parachute and both main
parachutes. A set of two is used for redundancy reasons.
C. Separation system
The separation system holding SPEAR during the ascent which
releases the vehicle on command was part of the design as
well. As SPEAR has to cope with the qualification level
vibrational loads up to 12g RMS, it requires a strong and stiff
separation system [7]. The selection of this system was
challenging as there was a large variety of design options and
changing requirements which made it difficult to perform a
trade-off. For all hold down and release mechanisms, springs
were chosen as ejection system.
The simplest hold down and release system was determined to
be one where the vehicle is bolted down on REXUS and the
bolts are disintegrated upon actuation. This can for instance be
done using explosive bolts, explosive nuts, or Frangibolts®.
The main advantage of the latter system is that it can be reused
and does not use pyrotechnics which increases testability and
safety [8]. Unfortunately, these parts are expensive and
procuring them was not possible for the SPEAR project.
Another concept, used by the REXUS 25/26 BESPIN
experiment [9], was considered as separation system as well.
Here three tensioned steel cables hold down the experiment in
the longitudinal axis supported by columns which prevent
lateral movement. TRW Pyro cutters cut the steel cables and
springs eject the experiment. For the SPEAR mission this
design would need 3mm thick steel cables, which are not
possible to cut with the TRW cutters. Aside this there were
doubts on the repeatability of assembly, as the pretention in
these cables is very difficult to determine.
The last design option, a clamp band, was selected, see Figure
6. This design originally did not score best in the trade-off, due
to a relative high mass and production effort. As there is more
experience within DARE with this system [10] and it has a
high stiffness, vibration resistance and reliability there is a high
confidence in its success. The system consists of two half
bands, connected by an actuation and tensioning mechanism.
The actuation is done by cutting an M3 bolt with a Cypres wire
cutter®.
Figure 6 Separation mechanism – clamp band
V. ELECTRONICS DESIGN
The SPEAR electronics system contains three processing units
and is responsible for actuation, recording flight and sensor
data, handling video inputs, and telemetry. The three
processing units are stored in two compartments in the SPEAR
structure: the power subsystem container and the avionics
subsystem container, see Figure 5.
The power subsystem container holds the lithium-ion batteries
used to power the entire electronics system as well as the
battery management system (BMS) processor. This processor
controls the battery charging process, distributes and monitors
the power throughout the electronics system and measures the
environmental conditions inside the power subsystem
container.
The avionics subsystem container houses two processors: the
supervisor (SUP) processor and the Raspberry Pi Compute
Model 3 (RPI) processor. The SUP processor controls
actuation of pyrotechnical devices and handles the interface
between SPEAR and the REXUS Service Module. The SUP
processor also handles data storage in on-board black boxes as
well as the retrieval subsystem, which includes a GPS module,
Iridium modem and a VHF beacon for locating purposes.
The RPI processor is used for video processing and handling
the high data-rate telemetry downlink. This downlink is
achieved through an on-board software defined radio along
with a power amplifier. Aside from telemetry handling the RPI
processor also handles and stores the sensor data of the sensors
used in SPEAR, including a heat flux sensor and load cells.
The use of a high data-rate telemetry downlink allows for live
digital video streaming and uncompressed live data output,
effectively providing the SPEAR team with data during flight
through a telemetry receiver in the SPEAR electronics ground
station setup. Part of this data can be used to monitor and
predict the trajectory of the free-falling unit(FFU) in case the
retrieval subsystem fails to provide the location of touchdown.
Aside from retrieval purposes this direct data stream is
captured directly by the ground station and can be used for post
processing purposes. It therefore provides the SPEAR team
with a great deal of data in case of unsuccessful retrieval of the
vehicle.
The data provided by the SPEAR electronics system will be
essential for validating calculations on parachute, structural and
thermal design.
VI. OUTLOOK
Within the REXUS program, multiple experiments have been
performed in the re-entry research area [11]. However, SPEAR
is the first mission aiming to flight test a parachute recovery
system. It could be considered to continue parachute research
with new student missions that participate in the
REXUS/BEXUS program. As can be seen in the SPEAR
envelope, supersonic velocities can be obtained with a test
vehicle. Testing a larger ballute stabiliser or the Disk-Gap-
Band main parachutes supersonically are interesting subjects
for future missions. These test vehicles should accommodate
for the new requirements of these missions.
In the beginning of the SPEAR project, the location of the
experiment within the REXUS rocket was still unsure, and
multiple options were considered. These different
configurations could be considered for future parachute
research in the REXUS program.
The REXUS rocket offers three payload bays: two modules
and one spot within the ejectable nose cone, see Figure 7.
Figure 7 The REXUS Soundi ng Rocket [ 7]
An FFU can be deployed from all these locations. The module
however has restrictions on the maximum size of a cut-out in
the structure to maintain structural integrity, which limits the
maximum size of an FFU [7]. This could be considered a
feasible option when the experiment and vehicle are scaled
down in complexity and size significantly. It would be
recommended to only include one parachute, the test article, in
this case. As the weight distribution of this FFU can lie more
towards the dome end of the vehicle, it can be designed as such
that it is inherently stable during descent.
Another possibility is to integrate the experiment in the
REXUS nose cone. This has been done in some REXUS
experiments such as Aquasonic [12]. This implementation
removes the design of a main outer structure and separation
system from the team’s workload as the REXUS nose cone and
nose cone ejection system are used for this. It also increases the
area and volume that one can work with immensely. It does
however require intense collaboration with REXUS as there are
much more interfaces with the rocket compared with using the
nose cone adapter plate as the single physical interface.
Lastly, one could opt to apply for the unconventional option of
recovering the REXUS engine section. An experiment module
could be placed between the yoyo de-spin and REXUS
recovery module. This does have the downside of a large
entanglement risk with the fins, and that the orientation of the
engine under the parachute has to be switched. This is alike the
Aether mission by DARE, and radial deployment should be
considered [6].
CONCLUSION
The SPEAR mission provides the Parachute Research Group
with a vehicle capable of fulfilling the need of supersonic
parachute testing. The combination of IMU, load cells and
cameras can provide a full overview of the supersonic
capabilities of the LEAPS Hemisflo drogue parachute. The
addition of a heat flux sensor to the sensor suite also allows
for verification of the in-house developed simulation tools
ParSim and TumSim. Based on first simulations, the vehicle is
capable of reaching the velocity conditions required. However,
further sensitivity analysis will be performed to ensure
mission success.
Finally, it can be concluded that supersonic parachute testing
is possible on board the REXUS sounding rocket and within
student capabilities and experience.
REFERENCES
[1] Menting, E. et al. (2019). Evolution and evaluation of the DARE Large
Envelope Advanced Parachute System, FAR, Capitolo, Italy , 2019
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[2] Lingard, J.S. (2017). Supersonic Parachute Testing Using a MAXUS
Sounding Rocket Piggy-Back Payload, AIAA, Indianapolis, USA.
[3] Knacke, T. W. (1992). Parachute recovery systems: design manual.
Santa Barbara, CA: Para Pub.
[4] Pepermans, L. et al. (2019). Trajectory simulations and sensitivity for
the SPEAR parachute test vehicle, IAC , Washington, USA , 2019
[unpublished]
[5] Pepermans, L. et al. (201 8). Flight Simulati ons of th e Stratos III
Parachute Recov ery Syste m, IAC, Bremen, German y, 2018
[6] Pepermans, L. et al. (2019). Comparison of various parachute
deployment systemsfor sounding rockets, EUCASS, Madrid, Spain,
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[7] REXUS/BEXU S, EUROLAUNCH. (2018). REXUS user manual V7.16,
retrieved from http://rexusbexus.net/rexus/rexus-user-manual/
[8] TiNi Aerospace, INC. (2019), Frangibolt. Retrieved from
https://tiniaerospace.com/products/space-frangibolt/
[9] BeSpin. (2019). REXUS – BESPIN experiment. Retrieved from
https://www.rexus-bespin.com/start
[10] DARE. (2019) Stratos IV structural design, retrieved from
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[11] REXUS/BEXUS. (2019). REXU S/BEXUS experiments. Retrieved from
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