Glide, without g – a systematic quantification of gliders 0-g flight capabilities

Abstract

With the ever democratizing access to Space, easier access to microgravity before launch is becoming critical as more and more space systems are being developed and require testing. On the other hand, science and research continuously benefit from experiments conducted in reduced gravity environments. Conventional solutions to reach weightlessness are today well established: sounding rockets, parabolic flights and drop towers, all provide an opportunity to reach weightlessness. The cost of that access to microgravity often scales up with the duration of the test window and the quality of the microgravity environment. Moreover, because of the increasing demand for microgravity solutions, long lead times are often to be expected. Recently, there has been some interest in the possible development of affordable microgravity solutions based on general aviation platforms, which could significantly increase the supply of microgravity test platforms and would create a new market opportunity. The team behind this paper, hereinafter referred to as LIDE, is a recent Belgian initiative which looks into the suitability of sailplanes to recreate quasi-microgravity conditions for research and technology development tests. This work further investigates the potential of sailplane-operated parabolic flights. Based on preliminary data collected during LIDE's ongoing flight test campaign, the characteristics of the parabolas are described with a focus on the segments of microgravity. The quality and repeatability of the weightlessness are characterized using statistics of the flights, showing that a quasi-0-g acceleration can be maintained for 5.5 seconds on average, with a measured standard deviation smaller than 0.1g. Operational considerations associated with glider parabolic flights are also discussed. Finally, sailplane and piston-engine flight data are compared to illustrate the negative effect of the engine-induced vibrations on the experienced acceleration. Sailplanes are only sensitive to atmospheric turbulence, and thus provide a good quality environment to support microgravity.

Full text

70th International Astronautical Congress (IAC) , Washington D .C., United States, 21-25 October 2019.
Copyright ©2019 by Denis-Gabriel Caprace, Camille Gontier, Mohammad Iranmanesh, Mehdi Scoubeau. Published by Eleven International
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IAC-19-A2.5.4x50685
Glide, without g – a systematic quantification of gliders 0-g flight capabilities
Denis-Gabriel Capracea, Camille Gontiera*, Mohammad Iranmanesha, Mehdi Scoubeaua
a Lide.SPACE, {dg,camille,mohammad,mehdi}@lide.space
* Corresponding Author
Abstract
With the ever democratizing access to Space, easier access to microgravity before launch is becoming critical
as more and more space systems are being developed and require testing. On the other hand, science and research
continuously benefit from experiments conducted in reduced gravity environments.
Conventional solutions to reach weightlessness are today well established: sounding rockets, parabolic flights
and drop towers, all provide an opportunity to reach weightlessness. The cost of that access to microgravity often scales
up with the duration of the test window and the quality of the microgravity environment. Moreover, because of the
increasing demand for microgravity solutions, long lead times are often to be expected. Recently, there has been some
interest in the possible development of affordable microgravity solutions based on general aviation platforms, which
could significantly increase the supply of microgravity test platforms and would create a new market opportunity. The
team behind this paper, hereinafter referred to as LIDE, is a recent Belgian initiative which looks into the suitability
of sailplanes to recreate quasi-microgravity conditions for research and technology development tests.
This work further investigates the potential of sailplane-operated parabolic flights. Based on preliminary data
collected during LIDE’s on-going flight test campaign, the characteristics of the parabolas are described with a focus
on the segments of microgravity. The quality and repeatability of the weightlessness are characterized using statistics
of the flights, showing that a quasi-0-g acceleration can be maintained for 5.5 seconds on average, with a measured
standard deviation smaller than 0.1g. Operational considerations associated with glider parabolic flights are also
discussed. Finally, sailplane and piston-engine flight data are compared to illustrate the negative effect of the engine-
induced vibrations on the experienced acceleration. Sailplanes are only sensitive to atmospheric turbulence, and thus
provide a good quality environment to support microgravity.
Keywords: microgravity experiments, gliders, parabolic flights
1. Introduction
Experiments in a reduced-weight environment
are a fundamental part of many branches of applied
sciences: material science, fundamental physics, fluid
dynamics, physiology and space medicine, plant and
cellular biology, combustion physics, all require to
conduct experiments in zero gravity [1]. Additionally, the
space industry produces a continuously growing number
of systems that often require to be tested in
weightlessness before they can actually be deployed. Yet,
the number of solutions able to reproduce microgravity
on Earth is limited and they all have their limitations.
Sounding rockets, parabolic flights and drop towers are
today’s most valued microgravity platforms. However,
they all suffer from low availability, low affordability
and long lead times. [2]
Considering the continuous increase in the
demand for microgravity solutions and the relatively
constant supply of test opportunities, there has been some
recent interest in alternative, less costly solutions for
microgravity testing. For instance, parabolic flights can
be performed with light single-engine piston aircraft, as
was demonstrated by flight test campaigns operated with
a Cessna 206 [3] and a CAP 10 [4,5]. Microgravity was
achieved for 8 seconds during each parabola.
Sailplanes are also potentially favourable
platforms for reaching weightlessness. A first
quantification of gliders 0-g flight capabilities has been
realized [6,7] using a Grob G-103 Twin II. The results
showed that:
- Weightlessness can be achieved continuously
for up to 6 seconds per parabola;
- A typical 20 to 25-minute flight allows to
perform between 3 and 21 parabolas, depending
on the pilot’s experience;
- The estimated level of achieved weightlessness
ranges between 10-2 and 10-1g.
The goal of the present study is to refine these
results and to provide a more complete and systematic
quantification of gliders 0-g flight capabilities. We
present the preliminary results of an ongoing flight test
campaign of parabolic flights realized with an ASK-21
(which has similar characteristics to the G-103). Data
were collected using custom made recorders and
accelerometers which allow us to precisely identify the

70th International Astronautical Congress (IAC) , Washington D .C., United States, 21-25 October 2019.
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level of 0-g achieved during parabolas, and their
duration.
In this article, we first expose the context of
research and tests in microgravity, and we elaborate on
the potential benefits of the development of low-cost
sailplane-based microgravity platforms. In section 3, we
introduce the flight test campaign that aims to better
quantify the capabilities of sailplane-operated parabolic
flights. We describe the sensors that are used to collect
data, and the additional systems involved. Section 4 is
dedicated to a first analysis of the preliminary flight test
results, with a specific focus on the microgravity phase.
Additional considerations specific to glider flights are
also discussed, including the comparison between the
vibration levels measured in a glider and in a light
airplane.
2. Technical and economic context
1.1 Applications of 0-g environments on Earth
The range of scientific fields that are related to
reduced-weight environment is extremely large. We will
focus on three main examples of experiments that
currently exist and are being run on ISS or other
platforms to illustrate how access to zero gravity is
fundamental to modern science.
FLUIDICS: Fluid dynamics are notoriously difficult to
model in zero gravity, which is a major impediment to
spacecraft control efficiency. Indeed, sloshing in tanks of
liquid propellants rockets or satellites leads to disturbing
torques that are hardly predictable and controllable. The
FLUIDICS (Fluid Dynamics in Space) experiment, run
on board the ISS, is an attempt to better understand liquid
sloshing and wave turbulence phenomena. [8]
ICE: The equivalence principle can be measured by
comparing the accelerations of two atoms having
different masses and structures using a two-species
atomic interferometer. On Earth, measured atoms are
subject to gravity and ultimately fall off the recording
area, while weightlessness allows for longer, and thus
more precise, recordings. The ICE (Coherent Source
Interferometry for space) is an experimental set-up
onboard Novespace’s 0-g Airbus, defined as « a first step
towards a spatializable system » [9].
SHS Capsule: The Material Science in Variable Gravity
Group of the University of Bremen [10,11] is especially
interested in seeing how reduced or high gravity levels
impact material synthesis and properties. To this end,
they developed the Self-Propagating High-Temperature
Combustion Synthesis Capsule (SHS) meant to be tested
within the ZARM drop tower. It is equipped with two
monitored reaction chambers, and allows to study how
material structures are affected by weightlessness.
Besides fundamental research, weightlessness is
also used to test future equipment for the International
Space Station and other space systems. For instance, heat
dissipation systems and water supply systems, which
optimal operation envelope depends on applied
accelerations, are tested during numerous parabolic
flights campaigns before being installed (e.g. on the ISS).
With the additional and ever-growing interest in
CubeSat, and in space exploration in LEO and beyond in
general, there is nowadays a clear rise in the demand for
conducting experiments and testing components in
microgravity.
Finally, new fields are also looking into the
potential benefits of microgravity such as 3D printing,
medicine, etc.
Overall, this rise in demand comes mainly from
smaller research organisations or privately funded
commercial entities which are much more sensitive to the
cost and lead time of access to traditional platforms.
1.2 Existing solutions for ground-based microgravity
platforms and their limitations
Access to microgravity research is notoriously
expensive and complicated. The average cost and lead
time for the most popular solutions are summarized in
[6].
Only few laboratories and research
organizations can afford parabolic or suborbital flights,
not to mention the cost of sending an experiment to the
ISS [12]. Parabolic Flights are often used to conduct
experiments and train astronauts in microgravity. For
example, ESA and CNES conduct one campaign per year
with up to 30 experiments. Generally, parabolic flights
are performed with large aircraft, such as the A-310
operated by Novespace or the B-727 operated by the Zero
Gravity Corporation.
For Sounding Rockets, REXUS is the leading
initiative in Europe, conducting one campaign per year,
only for students.
Among ground based solutions, drop-towers are
used around the world (USA, Europe and Japan) to
achieve up to 9s of free fall, but these are often
overbooked. The highest in Europe is the ZARM tower
located in Bremen, Germany. In theory, the tower can be
used every day, but it requires the creation of vacuum
inside the whole tower to remove the impact of
aerodynamic drag.
To a lesser extent, clinostats, and random
position machines can be used to reproduce microgravity
from a statistical perspective, but these are limited to
niche research (e.g. to study cellular biology and plant
growth). [13]

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In Europe, all these platforms are being
currently used by laboratories and by the industry.
Students can also access some of these facilities
through ESA-financed program (Fly your thesis, Drop
your thesis, Spin your thesis). But these programs are
limited in scope and capacity and the lead times to get
access to one of the test slots are relatively long. While
these initiatives are very beneficial for microgravity
research in Europe, they also epitomize the demand for
more accessible test opportunities in microgravity
conditions.
1.3 Potential market benefits of sailplane-operated
parabolic flights
Sailplanes are commonly operated in many
places of the world. Moreover, a majority of the training
glider currently in use are suitable for parabolic flights,
as the structural loads endured during the parabola are
compatible with most of today’s glider flight envelopes
(see section 4). Besides, the cost of operation of a glider
are relatively small. These are all advantages to the use
of gliders for microgravity experiments.
In order to guarantee the safety and the quality
of the parabolic flight, one should however ensure that:
- pilots are properly trained for the maneuvers in
all phases of a parabolic flight;
- the subject of the experiment/test is correctly
installed and fastened in the glider.
For the latter, we propose the use of a
standardized container (still to be developed), which
could adapt to several types of gliders, and which could
potentially provide power supply and telemetry to the
experiment. Such a standard container could be sent to a
gliding club close to the customer, such that the
experiments can be performed there (with a qualified
pilot). It therefore offers less painful logistics to the
customer by reducing the cost and time of transporting
payloads, in addition to lowering the administrative
burden.
Provided that these two above conditions are
filled, sailplane parabolic flights could be used to endorse
a large number of microgravity experiments for research,
academic, and industrial purposes, provided that the
constraints on the quality of the weightlessness required
for the envisioned test is not too stringent.
Among potential beneficiaries, let us mention
universities and student projects, and laboratories
looking for preliminary results without discouraging
access fees, delays, experimental constraints, or
application procedures.
It is to be noted that the authors of [6] also
assessed the possibility to use a sailplane to generate a
high gravity environment (by flying high-bank turns) or
quickly changing g-loads (by flying aerobatics
maneuvers like chandelles). Results show that gliders can
achieve a g-load between 1.5 and 2g for 20 to 40s, and to
quickly vary the g-load between 0g and 4g. These
maneuvers are not within the scope of the present study,
but show that sailplanes may also be used to perform
high-g experiments.
3. Description of the experiments and collection of
data
Data presented hereafter were collected during
the first phase of a flight test campaign which consisted
of 7 flights, totalling 74 parabolas, conducted between
September 2018 and September 2019. All flights were
performed in smooth air conditions in order to minimize
the perturbations due to atmospheric turbulence.
Recordings were obtained onboard a Schleicher ASK-21
two-seat glider (see Fig.1). The manufacturer specifies a
g-limit of +6.5g and -4.0g for structural reasons. This
flight envelope largely covers the needs of parabolic
flights, and is similar to the limitations of other two-seat
plastic-made training gliders (e.g. DG-505, DG-1000, G-
103).
Fig. 1. The ASK-21 used for the flight test campaign
3.1 Data collection: sensors and means of measurement
During the flight tests, we used three
independent devices in order to record data for the
quantification of the parabolas.
The first flight data recorder (FDR-1) is based
on an Arduino-YUN rev2 board, equipped with a
microcontroller for data acquisition and processing, and
a Linux microprocessor which was used here for real-
time data display (see next section). The FDR-1 was
fitted with a DP0107 GNSS module for geo-localization
and with a MPU-9250 IMU providing measurements of
the acceleration, the rotation rate and the magnetic field
intensity in 3-D. Initially, we aimed at a data acquisition
rate of 10 Hz, which is the largest allowed by the GNSS
module. In practice, because of the high workload
demanded from the processing of both sensors data, and
also accounting for the communication between the

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microcontroller and the Linux processor, the actual
sampling rate varies between 3 and 5 Hz.
The second data recorder uses a Raspberry Pi 3
microcomputer, wired with a Bosch BNO055 absolute
orientation sensor.
The BNO055 is a smart 9-DOF which processes
data from an internal IMU (accelerometer, gyroscope and
magnetometer) and outputs the accelerations and
orientation of the sensor. Turning the IMU sensor data
into actual aircraft absolute orientation is difficult to
implement on low cost real-time systems [14]. This
BNO055 sensor was selected as the included high speed
ARM Cortex-M0 based processor automatically digests
all the sensors data and sends out directly usable
information in the form of quaternions, Euler angles or
vectors.
This abstraction level removed away concerns
about sensor fusion and real time requirements and the
bespoke algorithm developed by the authors could focus
on combining the orientation and acceleration data to get
meaningful insights for the pilots during post-flights
reviews. Overall, FDR-2 has a sampling and writing
frequency of about 25Hz.
Finally, the built-in accelerometer sensor of a
smartphone was used as a back-up sensor.
Advantageously, the sampling frequency could be
increased to 100Hz.
All sensors were installed in the backseat of the
glider, as shown in Fig.2, in order to minimize their
distance to the center of gravity (CG) of the sailplane.
Tape was used as the easiest and most efficient solution
to attach the recorders.
Notice that the distance between the sensor and
the CG have a small, but non negligible influence on the
recorded data. Indeed, during the parabola, the glider
pitch rate is of the order of 0.26 rad/s (as it goes from a
+45° to a -45° pitch angle in approximately 6s). If the
sensor is located e.g. 2m in front of the CG (ahead of the
front seat), the corresponding acceleration would be of
the order of 0.014g. In the backseat, the distance is closer
to 0.5m, and the spurious acceleration is down to less
than
!"# $ %&'()
.
Fig. 2. Mounting of the sensors in the backseat of the
glider (FDR-1 on the right, FDR-2 on the left).
3.2 Real time display
In 2018, a first series of exploratory flights were
conducted. From this early experience, it became clear
that specific training for the pilot was required to
guarantee the good quality of the parabolas. Meanwhile,
the pilots also identified the need for a real-time feedback
of the g-force experienced by the payload in order to fine-
tune the sailplane trajectory. Indeed, aviation
accelerometers exists but are not part of the mandatory
instruments for gliders (there were none in the glider
which was used for the present study).
In order to allow for real-time acceleration
display, the Arduino-based FDR hosts a HTTP server
embarking a Javascript runtime environment. The server
has access to the measurements and is programmed to
pack them inside a graphical user interface and return
them on HTTP requests. We use the Arduino’s capability
of creating a Wi-Fi hotspot for handling wireless
connections between the FDR and external devices. As a
result, a smartphone with a standard internet browser can
be used as a monitor for our digital accelerometer. Fig.3
shows an example of the view displayed to the pilot.

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Fig. 3. Example view of the pilot display generated by
FDR-1, showing the current load factor.
A satisfactory latency of less than 0.25s is
observed between the time of measurement and the time
of display, and the refresh rate of the display is close to
the sampling rate of FDR-1. This shows that the entire
data processing and communication chain only
introduces a moderate lag. However, feedback from the
pilots indicates that a higher refresh rate would allow
them to refine the parabolic trajectory and smoothen their
actions on the glider flight controls. For the second part
of the flight tests, a software solution is envisioned to
improve the performances of FDR-1, but this might
eventually also require hardware changes.
4. Systematic quantification of gliders 0-g flight
capabilities
4.1 Parabola from an instantaneous perspective
Unpowered aircraft can hardly reach the same
quality of weightlessness as conventional aircraft used
for parabolic flights, nor will they be able to sustain
parabolas nearly as long. Indeed, at first order, the
duration of the parabola is proportional to the injection
velocity (i.e. the velocity at which the parabolic trajectory
is initiated), which gives a clear advantage to fast flying
airplanes for maximizing the duration of the free fall.
Nevertheless, some aspects of the 0-g flight capabilities
of a sailplane are here further investigated.
The precision of the trajectory is quantified, also
attesting the reproducibility of microgravity conditions.
The influence of the aerodynamic drag and the
environmental noise are also assessed.
In practice, the flight procedure consists in
performing several parabolas in sequence, starting from
an initial dive used to build up speed and reach the initial
velocity. In order to maximize the duration of the
parabolas, the injection velocity should be as high as
possible. However, in addition to the never exceed
velocity (VNE = 250 km/h for the ASK-21), an even
more restrictive constraint stems from the flight
envelope.
One must verify that the initial velocity will not
lead to load factors exceeding the threshold during the
pull-up between two parabolas. In this case, we
intentionally set this limit to 4g in order to reduce the
loads on the glider, and therefore, we selected a target
initial velocity of 200km/h which gives an injection
velocity of 150km/h.
In this document, all accelerations are given in
the body axis frame, with the x-axis pointing forward out
of the nose (i.e. in the axial direction), the z-axis pointing
downward (i.e. in the vertical direction) and the y-axis to
form an orthonormal frame, as illustrated in Fig.4.
Fig. 4. Body frame axes.
Fig.5 shows the time history of the vertical
acceleration
*+,
divided by the gravitational acceleration
g (i.e., the load factor
- . *+/)
) and the altitude during
a sequence of parabolas.
The sequence of parabola is made of several
periods, and it can be clearly seen from the acceleration
profile that each period can be decomposed in three
phases. Starting when the initial velocity is reached, the
first phase is the pull-up where the glider transitions from
a dive to a pitch angle of +45° (yellow shaded area). The
second phase starts at the injection: the pilot promptly
pushes the stick forward until the 0g is reached, and by
doing so he initiates the parabola. This phase lasts for
about 6s (green shaded area) and will be further analyzed
here after. In the third phase which lasts for about 3s, the
glider is maintained in a nose down attitude in order to
recover the initial velocity required for the next period in
the sequence (blue shaded area).

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Fig. 5. Time history of the vertical acceleration (load
factor) and the altitude during a portion of flight #3,
with the initial dive and seven subsequent parabolas.
4.2 Parabola from a statistical perspective
In order to better characterize the 0-g phase, the
accelerations measured by the back-up high frequency
sensor was used to derive statistics over a set of 30
parabolas. The results are shown on Fig.6. We stress that,
because of the small number of parabolas involved, the
statistics are not fully converged for this preliminary
study. However, the ensemble size should be sufficient
to identify the main trends.
The average vertical acceleration shows that the
pilot is indeed able to maintain the glider in a fairly
constant reduced gravity environment. The microgravity
phase, which is here arbitrarily defined as the portion of
the parabola with
*+/)
< 0.2, is 5.5s long on average,
with a standard deviation of about 1s.
Because it is quite short as compared to
conventional parabolic flights, it is hard to further
decompose the microgravity phase as was proposed in
[15].
Importantly, the standard deviation of the
vertical acceleration is lower than 0.1 g. This mainly
characterizes the repeatability of the maneuver and the
level of smoothness acquired in piloting the trajectory of
the glider. Those two characteristics are mostly
dependent on the pilot and on his experience. Notice that
the cause of the standard deviation is not to be found in
measurement noise or vibrations, as will be shown in
section 4.4.
Fig. 6. Average vertical (top) and axial (bottom)
acceleration measured during a parabola, and their
standard deviations (shaded areas).
The average vertical acceleration during the
parabola is almost zero. The small residual offset,
together with the standard deviation itself, could be both
further decreased with an improved real-time display
system (or a standard instrument), and will be also
reduced with the increasing experience and practice of
the pilots.
It is worth mentioning that the targeted mean
level of weightlessness can also be adapted to simulate
Moon (0.165g) or Mars (0.374g) surface gravities, just as
in traditional parabolic flights [4].
Strictly speaking, because the glider has no
engine, perfect zero-gravity conditions cannot be reached
as opposed to propelled aircraft, for which the residual
drag force can be compensated by the engine thrust. The
parasitic drag experienced by the glider on the parabolic
trajectory results in an axial acceleration which cannot be
zero. It is here measured to be smaller than 0.02g on
average (see Fig.6), which is compatible with a rough
estimation based on the glider technical specifications.
Indeed, if one considers that the parasitic drag is
50% of the total drag at the best glide angle bga
(assumption stemming from the Prandtl Lifting Line

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theory), and based on the technical specifications of the
glider, we find a drag coefficient of
012.%
3,%
4)*
5)
%
3,6,7
89:
;,<
. &"&%3
with S the reference surface,
7
89: ,
the optimal velocity
and ρ the air density.
With a glider mass M=550kg, this should
correspond to an acceleration on the order of 0.025g
under the conditions of our flight tests.
This axial acceleration is however small
compared to the variations in vertical acceleration shown
above.
4.3 Number of parabolas per flight/day/year
In this section, we determine the average
number of parabolas achievable over one flight which
obviously depends on the release altitude of the
glider. Using the characteristics of every flight of the
flight test campaign, we have determined that the average
altitude loss per parabola is 50m. This is confirmed by
the altitude profile obtained from GNSS data.
Operationally speaking, the glider is preferably
towed up to a height of about 1000m. Accounting for a
reserve height of 300m necessary for maneuvering and
for the execution of the standard landing pattern, the
remaining 700m can be used for parabolic flight.
Therefore, one can expect to perform 14 to 15 parabolas
per flight. Obviously, several flights can be performed in
a row to increase the number of parabolas performed in a
day, as the duration of one of these typical flights is
around 20 minutes.
Generally, the operation of sailplanes and
general aviation aircraft is more dependent on weather
conditions than standard commercial aircraft. Rain,
strong winds, low ceiling or reduced visibility typically
prevent them from taking off.
It is difficult to assess the number of days per
year where flying is possible, especially because the
go/no go threshold can be subjective and because there
are many criteria to take into account: the local weather,
the type of flight envisioned, the airspace, the safety
margins... Within Europe, there are regions where the
conditions make it possible to fly gliders all year long and
even in less favorable locations, there are always several
“good” days per month. It must be stressed that, for the
type of parabolic flights described above, the weather
minima are simply the same as in VMC (allowing for
VFR flights). This is way less stringent than the weather
conditions awaited by the sport sailplane pilot, in which
the formation of thermals and cumulus clouds are
expected to enable long duration or long distance flights.
Therefore, while it cannot be guaranteed that
sailplane parabolic flights can happen on any day, it can
be postulated that the waiting time before the next flying
opportunity is shorter than the typical lead time of current
microgravity platforms.
4.4 Jitter analysis and comparison with aircraft
capabilities
Gliders do not suffer from the increased level of
vibrations generated by piston engines, which had
previously been identified as one of the main drawbacks
of using light aircraft for weightlessness studies [3,4]. In
order to verify this assertion, we compare the high
frequency acceleration data collected during our glider
flights with that measured in a General Aviation (GA)
aircraft. The latter recordings were obtained on board a
Diamond DA40-TDI, a four-seat single-engine piston
light aircraft equipped with a 135Hp Thierlet Centurion
Diesel-injected engine.
The data post-processing is similar to what is
proposed in [16], and involves the following on the norm
of the measured acceleration vector:
- Data segmentation. As we are interested in the
vibration level in both aircraft types, we
compare data collected during similar flight
conditions: constant speed straight flight for the
glider, versus cruise condition flight for the GA
plane. Data of interest was selected prior to the
analysis based on the pilot’s flight log.
- Filtering. A high-pass filter is applied on data to
remove the low frequency component from the
jitter analysis (see Fig.7). A 5th order digital
Butterworth filter with a cutoff frequency of
1Hz was designed using the Scipy Signal
Processing toolbox in Python [17] and applied
to data forwards and backwards.
- Sigma computation. The variable of interest to
be compared among recordings is finally the
standard deviation of the filtered data.
- Power Spectral Density computation. For
further analysis, the PSD of recordings are
computed plotted using the Matplotlib library
[18], see Fig.8. The number of data points used
in each block for the Fast Fourier Transform is
NFFT = 512.

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IAC-19-A2.5.4x50685 Page 8 of 10
Fig. 7. Measured and filtered accelerations for glider
(top) and GA aircraft (bottom).
Fig. 8. Power spectral density of the filtered acceleration
for glider (top) and GA aircraft (bottom).
The standard deviation of the filtered
acceleration is finally 0.0358g for the glider, and 0.0452g
for the aircraft. As seen on the plots of Power Spectral
Densities (Fig.8), no striking frequency appears in the
recordings. We would however expect that the engine
induces peaks associated to the engine operation. They
are not visible here because the nominal regime for a
light aircraft engine is typically above 2000 RPM (i.e.
approximately 33Hz), higher than the Nyquist frequency
for our recorder (50/2 = 25Hz).
In order to systematically compare the level of
jitter for sailplanes and engine aircraft, and their
capacities in terms of achieved weightlessness, we should
not only have used data collected during similar flight
conditions (constant speed straight flight for the
sailplane, and cruise condition flight for the engine
aircraft, both operated in a stable atmosphere), but also
data recorded during parabolas, which are the phases of
flight during which the level of jitter is critical. However,
there are several risks associated with sustained 0-gravity
with Single Engine Piston aircraft. Often, the oil system
of GA engines relies on gravity for the proper feeding of
the lubrication pump. Prolonged microgravity could thus
lead to oil starvation of the engine, causing the risk of
engine damage due to the absence of lubrication.
Depending on the engine design, this remark could also
apply to the fuel system.
4.5 Additional comments: payload size and weight
The backseat of the glider should offer to
accommodate a large range of experiments, without
imposing too severe constraints on their mass and their
volume. Indeed, the ASK-21 empty mass and MTOW
are respectively 360kg and 600kg, which yields a total
useful mass of 240kg for the pilot in the front seat and the
payload. The maximum mass specified by the constructor
for the back seat is 110kg, which will be the maximum
mass of our payload (and can be used as a rule of thumb
for other types of sailplanes).
Payload limits, in terms of size and weight, are
thus of the same order of magnitude as for a Cessna 206
previously studied for weightlessness purposes (1000L in
volume and 150 kg in mass) [3].
5. Conclusions
5.1 Why gliders are good for 0-g, and what we did to
show it
In this work, we showed that using gliders to
reach weightlessness would represent a step further in the
evolution towards providing more accessible and flexible
microgravity platforms. A microgravity platform based

70th International Astronautical Congress (IAC) , Washington D .C., United States, 21-25 October 2019.
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on sailplane gliders would be especially suitable for
compact experiments and equipment that do not require
a long-lasting microgravity test conditions and which do
not require the very large payload capacities of
conventional aircraft.
Many experiments could therefore benefit
greatly from the use of a glider, provided that they can fit
in the backseat volume and that they comply with the
maximum 100kg mass limit requirement.
Typically, a single sailplane parabolic flight will
provide approximately 15 test windows of 6s duration
each. Several flights can easily be performed in a row
allowing similar or longer combined total test flight time
in quasi microgravity than zero-g flights performed by
wide-body aircraft.
From the preliminary results of our ongoing
flight test campaign, we measured that the excursion on
the acceleration during the microgravity phase remains
below 0.1g. We showed that these low excursion levels
are reproduced from one flight to another, and thus that
the operation is well repeatable. These variations,
together with the absolute level of acceleration reached,
are highly dependent on the pilot experience. Better
controlled parabolas will be obtained when a visual
feedback on the acceleration is provided to the pilot, and
the hardware solution that was presented here is currently
being improved to eventually compensate for the
potential absence of an approved g-meter in the cockpit.
Alternatively, general aviation aircraft have also
been proposed as microgravity platform in the past, and
could provide even longer duration parabolas. However,
because of the piston engine, the level of vibration is
higher in a light aircraft than in a glider, as confirmed by
the measurements that were here presented and
compared. The augmented risk inherent to the operation
of internal combustion engine in microgravity also
hampers the development of such solutions.
5.2 Further technical developments
Our approach encompasses the three main
trends that drive the design of microgravity experiments,
as defined in [4]: miniaturization, automation, and
interface standardization.
While the glider solution requires a pilot and
proper training, one alternative solution would be to use
Unmanned Aircraft Systems (UAS) for longer test
campaigns thus increasing safety and robustness of test
results. These Autonomous fixed-wing drones could fly
at higher altitudes. The higher altitude (and higher
velocity) would provide two more additional benefits: an
increase in the duration of the ballistic segment of the
flight path as well as a decrease of aerodynamic jitter due
to less turbulent atmosphere at higher altitudes. The
avionics of such glider UAS could be partly based on the
IP and algorithms already developed for FDR-1 and
FDR-2.
In the long term, this solution would also
potentially reduce the operational costs.
5.3 Economic perspectives
Entering the 2020’s, ISS operations are soon
coming to an end. While several other solutions are being
assessed in Europe and in the world and that many
endeavors beyond LEO are foreseen, there might be an
opportunity to revolutionize the field by offering an
alternative lost-cost platform to conduct microgravity
research and testing.
An independent study that we conducted has
shown that a market exists for the proposed innovative
platform. This new solution would primarily focus on
industrial actors active in any field related to space and
research institutions from multiple disciplines, with a
need for fast and cheap access to microgravity. Thanks to
the flexibility of the platform, repeated test windows can
be offered within short delays, with high value for the
customer as it allows them to shorten their process and
facilitate their developments.
In addition, gliders might open the door for a
whole new segment of the market by enabling
educational parabolic flight activities to promote Science
in schools. The affordability of the service and the limited
number of requirements and procedural constraints will
also allow many enthusiasts to experience microgravity
for fun and artist to express their art in microgravity.
While the technical details are currently being
tested and validated, the next step would be to assess the
financial viability and come up with a realistic business
model that will be leveraged as a selling argument with
investors and incubators.
Acknowledgements
The authors thank Dr. V. Pletser for his decisive role in
the instigation of the present project and for insightful
discussions. They acknowledge the contribution of N.
Frischauf, P. Billuart and H. Delattre in collecting
valuable flight test data, and the collaboration with
Aeroclub Universitaire de Louvain through which the
airframe was made available. They also express their
gratitude to the personnel of UCLouvain-iMMC for
technical advising and sharing of resources.
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70th International Astronautical Congress (IAC) , Washington D .C., United States, 21-25 October 2019.
Copyright ©2019 by Denis-Gabriel Caprace, Camille Gontier, Mohammad Iranmanesh, Mehdi Scoubeau. Published by Eleven International
Publishing, with permission.
IAC-19-A2.5.4x50685 Page 10 of 10
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