developing a gliding spacecraft to flight over titan - Human ...

ABSTRACT
DEVELOPING A GLIDING SPACECRAFT TO FLIGHT OVER TITAN
Jose M. Giron-Sierra, Hector Garcia de Marina, Fernando Pereda
The target of our present research is to develop a
gliding spacecraft with several possible uses, including
the exploration of Titan. The vehicle has a profile
similar to a ray fish, and offers encouraging
performances for long gliding trajectories. The paper
describes several aspects of the research: application
scenarios, development of the on-board system for
autonomous flight and simultaneous data acquisition,
intelligent path planning, and preliminary mission
analysis for the next experiment: to be drop from a
balloon.
1. INTRODUCTION
A possible alternative for the exploration of Titan in
the 2020, by ESA, is to use a gliding spacecraft. This is
the subject of our project, recently started. The ratio
between atmosphere density and gravity near Titan
surface is high enough to favour the use of a glider.
Figure 1. Titan
Our project is conceived at this moment as an academic
research, with the help of students. The project has
been selected by ESA Education for the next BEXUS
experiment, autumn 2009. For the long-term
development effort we count with the support of Dr.
Guillermo Ortega and Eng. Nuno Felipe, ESA ESTEC.
To arrive to a candidate vehicle for Titan, we need to
proceed step by step. Our next step is to develop a
version of the glider for experimental testing in our
Dep. ACYA, Fac. Fisicas. Universidad Complutense de Madrid
28040 Madrid, Spain. gironsi@dacya.ucm.es
planet. The vehicle will be drop from a balloon, at 15
km high at Kiruna.
A development process is being executed to create the
glider for the BEXUS experiment. This paper describes
the most important aspects of the development.
2. THE GLIDER
___________________________________________________________________________________
Proc. ‘19th ESA Symposium on European Rocket and Balloon Programmes and Related Research,
Bad Reichenhall, Germany, 7–11 June 2009 (ESA SP-671, September 2009)
A special spacecraft has been selected, which has the
profile of a fish and behaves as a lifting body with little
wing span (Fig.2). This vehicle has low drag and very
good manoeuvrability at low (0.05 Match) and high
(0.8 Match) speeds. The designer of the vehicle, named
`SpaceFish’ is Eng. Koni Schafroth.
Figure 2, Smartfish
Figure 3 and 4 show aerodynamic characteristics of the
vehicle, which can be seen as a combination of glider
and subsonic aircraft. It owns great stability
performance by low wingspan.
Figure 3. Pitching moment of the vehicle

Figure 4. L/D relation of the vehicle
A particular version of the vehicle is under
development for the BEXUS experiment. Here are the
aircraft characteristics:
• Dimensions:
o Wingspan: 1m
o Length: 1.40m
o Height: 0.30m
• Weight:
o Empty plane: 3.5Kg
o Payload: 2-3Kg
• Material:
o Glass-fiber sandwich:
Outer layer: 2x150g/m 2 glass
fiber
Middle layer: 3mm of foam
Inner layer: 1x150g/m 2 glass
fiber
The moulds of the aircraft have been carved with
precision CNC. To gain confidence with the use of the
moulds, a first mock-up has been built. This mock-up
is now in use by our team for constructive decisions
about aircraft internals: where and how to fix the
servos, where to put the batteries, get an idea for
parachute placing, mind the location of the centre of
gravity, etc.
A second, lighter aircraft has been built: this time for
flying testing. Fig. 5 shows a photograph of the aircraft
being taken from the mould; some of the students
belonging to the development team appear in the
photograph.
A complete professional simulation study has been
already done by ESA, to check the performance of the
spacecraft. The results are encouraging. We expect
1km of fall for every 10km of advance. Fig. 6 shows a
screenshot of the flight simulation, over Kiruna zone.
Figure 5. An SpaceFish exemplar being taken
from its mould.
Figure 6. Screenshot of the 3D flight simulation
3. SCENARIOS
The development considers several scenarios for
gradual approach to be ready for missions.
There is a set of experimental tasks for testing. The
vehicle is put on top of a car, and driven at 120 km/h,
for the testing of the speed sensors, which are based on
air pressure measurement. The complete vehicle, with

on-board electronics is put into a research refrigerator,
able to reach -70ºC since this is the temperature we
expect in the BEXUS experiment.
The first flyable SpaceFish is being equipped with a
ducted fan propeller. We need it to be able to take off
from a runway. A particular runway, property of a R/C
society, has been located 50 Km from Madrid, in a
place with no buildings nor roads in 10 Km around.
Batteries have been added to the vehicle, for the ducted
fan, so weight increases. An experimental plan has
been designed, for separate testing of the main parts of
the BEXUS flight: straight gliding, turns, final
approach and landing. The tests are first made by R/C
control, to take data. Initial trimming of the vehicle will
be done in this manner. Afterwards, this is substituted
by autonomous control. Fig. 7 shows a photograph of
the runway
Figure 7. The runway for flight testing
We got a 3m diameter balloon, and we made a
gondola. The plan is to be able to test the initial part of
the BEXUS experiment, which is to let the vehicle fall
and recover stable flight. Several types of ropes and
wires will be used to ensure that the balloon will not
escape.
Another SpaceFish is under construction, since we
want to have two exemplars when at Kiruna.
After the testing campaign, it is expected to conduct
this autumn the BEXUS experiment from 15Km high
using a balloon, as shown in fig. 8. The purpose of the
experiment is to let the vehicle glide along a
programmed trajectory, for the testing of flying
qualities, and for taking data: evolution of pressures,
altitude, attitude, speed, cog motion.
Figure 8. The BEXUS balloon
The research will continue in several fronts: more
testing of the vehicle, with possible design changes;
improvements of the on board system for flight control
and for data acquisition, improvements in the path
planning methodology, advances in the simulation and
analysis tools, advances in the external support from
ground. Let us insist in that the vehicle has autonomous
control; the ground support is for telemetry, getting
data from the vehicle.
Main aspects of the Titan scenario are that the vehicle
will behave as a re-entry vehicle at the beginning, and
gradually will transition to gliding for high altitude
exploration; and that an intelligent real-time path
planner, we denote `SmartPath’, will be used to adapt
the glider trajectory to targets being recognized as
interesting during the flight. Other functions of
SpacePath are optimization of power consumption, and
improvements of exploration area coverage.
The glider is able to recover altitude by using, from
time to time, a propeller. We think that in an
exploration mission it would be interesting to approach
places of interest, or recover attitude in zones with less
interest. So we are starting to develop an on-board
optimizer for this task. The optimizer combines a rule
base with the on-board control, taking into account
operability restrictions. In addition we plan to be able
to drop from the vehicle probes/transponders.

4. ON-BOARD SYSTEM
As said before, the spacecraft will have autonomous
control during the BEXUS experiments and in next
applications, but for initial experiments an external
supervisory system is also advisable. Therefore, in
addition to a radio data link from the spacecraft to the
ground station, there is a conventional R/C channel
from the ground station to the spacecraft, being able to
override the autonomous flight control, so a human
operator can intervene in exceptional situations.
The on-board system includes three subsystems.
a) Navigation subsystem:
Figure 9. Block diagram of
the control system
This subsystem employs pressure sensors, an
inertial unit with accelerometers and
magnetometers, and GPS. Kalman filtering is
used for data fusion. The World Magnetic
Model is also used.
The subsystem control the spacecraft rudders,
and in some experiments the ducted fan.
The flight control software has a script that
describes the mission plan, a script interpreter,
and a set of flight control primitives.
b) Data acquisition subsystem:
All data from sensors are recorded with a time
stamp. A video camera is included. The data
are saved in a on-board card, similar to the
cards used in digital cameras.
Selected data are transmitted to the ground
station via digital radio link.
c) Safety subsystem:
It may happen that dropping from the balloon
the vehicle could not reach controlled flight.
Therefore a free-fall detection system is
provided, using accelerometer.
Other navigation problems may occur during
the experiments, like electronic faults in the
on-board system. A supervisory circuit is
provided, acting as watch dog.
Faults in the communication system could
also appear. They are detected in the ground
station.
A parachute is embodied into the vehicle as a
safety measure. Any crucial problem will
result in the parachute to be deployed. The
safety subsystem is in charge of that.
The safety subsystem is independent from the
flight and data acquisition subsystems, with
independent power supply.

The ground station has two functions. One is to
monitor and display the data being received from the
flying vehicle. The second includes a panic button, and
manual R/C control.
Fig. 9 shows a block diagram of the complete system.
5. DEVELOPMENT ASPECTS
An important part of the research is to develop an
animated 3D simulation environment, and the software
for the on-board control system and for the external
supervisory system. All these software units share
common modules.
The simulation can include several terrains, using
Google Earth. The simulation environment is
experimentally validated.
Testing along development steps required a lot of
imagination. For instance, the artificial horizon that
will be part of the supervisory ground station has been
tested with a WII racquet. Several conventional R/C
airplanes, of the Cessna type, have been used for
telemetry and autopilots testing, etc, some of them
were lost in the battle.
6. PLANNING OF THE BEXUS MISSION
The BEXUS experiment has been analyzed in order to
establish main phases, and a description in terms of onboard
script.
The central part of the flight will be devoted to repeat a
trajectory with the form of an 8. The arc of the 8 has
500m radius, and the straight branches of the 8 are 2km
long. Along an 8 the glider may descend about 600m.
The main phases of the experiment are drop and stable
control taking, approaching to a delimited zone for the
8’s, point to the landing site and approaching along 20
km, final landing with the help of the parachute.
The on-board control for speed, pitch and roll, would
be based on rules, for a sort of gain-scheduling
according with the changes of air density along the
descent.
Fig. 10 shows the footprint of the landing site.
Figure 10. Footprint of the landing site
The experiment will be monitored from the ground
station by digital radio link able to reach 50 km
distance.
6. CONCLUSION
This paper briefly described main traits of our research
on a glider, in order to examine its performances as a
candidate for planetary exploration.
The research uses a spacecraft with the form of a fish,
enjoying good properties as a glider and as re-entry
vehicle.
Main parts of the system, and steps of the development
has been described.
The near future has a clear target, to investigate in
detail the flying qualities of the vehicle in several
atmospheric and environmental conditions, and to
develop adequate autonomous control, with intelligent
real time path planning, for non Earth scenarios.