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Journal ArticleDOI

Fast Aquatic Escape With a Jet Thruster

01 Feb 2017-IEEE-ASME Transactions on Mechatronics (IEEE)-Vol. 22, Iss: 1, pp 217-226
TL;DR: In this article, a planar trajectory model was developed to predict aquatic escape trajectories using a CO $_2$ powered water jet to escape the water, actuated by a custom shape memory alloy gas release.
Abstract: The ability to collect water samples rapidly with aerial–aquatic robots would increase the safety and efficiency of water health monitoring and allow water sample collection from dangerous or inaccessible areas An aquatic micro air vehicle (AquaMAV) able to dive into the water offers a low cost and robust means of collecting samples However, small-scale flying vehicles generally do not have sufficient power for transition to flight from water In this paper, we present a novel jet propelled AquaMAV able to perform jumpgliding leaps from water and a planar trajectory model that is able to accurately predict aquatic escape trajectories Using this model, we are able to offer insights into the stability of aquatic takeoff to perturbations from surface waves and demonstrate that an impulsive leap is a robust method of flight transition The AquaMAV uses a CO $_2$ powered water jet to escape the water, actuated by a custom shape memory alloy gas release The 100 g robot leaps from beneath the surface, where it can deploy wings and glide over the water, achieving speeds above 11 m/s

Summary (3 min read)

I. INTRODUCTION

  • For an aerial robot, movement in water creates additional structural and propulsive design constraints that can be difficult to overcome [1] .
  • The authors are developing an AquaMAV capable of diving directly into the water and retaking flight using a high powered burst of thrust (Fig. 1 ).
  • The vehicle could then perform a short take-off (Fig. 1 ), and return to its launch site to submit collected samples and data.
  • This approach relies on accurate sensing and control to maintain position while a sample probe is lowered.
  • The speed and range of robotic aircraft may not always be required by a mission, but aerial-aquatic locomotion has broader advantages in robot mobility.

A. Principles for Aquatic Escape

  • Underwater locomotion is one of the most energetically efficient forms of locomotion in the animal kingdom [11] , principally because a neutrally buoyant animal is not required to resist its own gravity to swim.
  • Another drawback is that such a vehicle would require a calm surface to take-off, while the principle advantage would be that it allows multiple swimming and flying phases in a single mission.
  • This allows a vehicle to escape the water and accelerate when airborne, where drag is dramatically reduced compared to in water [11] .
  • Reconfigurable wings have also been shown to have advantages in jumpgliding [8] , and are features of almost all aerial aquatic animals [1] .
  • In the following sections, the authors introduce the physical principles behind water jet propulsion, and detail the key design features of the jet-propelled jumping robot.

II. WATER JET PROPULSION

  • 2, 3 and 4 to denote variables relating to the main gas tank, the gas within the water tank, the air-water interface, and nozzle outlet respectively (Fig. 2A ).the authors.
  • Decreasing nozzle diameter increases total efficiency, but reduces thrust production, and a minimum of diameter 2mm was set.
  • These equations remain valid until all water is expelled, after which the release of remaining gas produces a small amount of thrust.
  • When subsonic, the outlet will be at atmospheric pressure, but if the pressure ratio is greater than a critical value (equation 11, different to the valve-specific κ choke value), the flow is choked, M =1, and the nozzle outlet pressure will be greater than atmospheric.
  • In both cases, the mass flow out can be computed using a standard Mach number relation (equation 12).

A. Design Domain

  • For a given reservoir pressure and valve flow coefficient, the work extracted from the gas can be maximised by varying the water tank size and nozzle diameter.
  • Enlarging the water tank increases launch mass, and an optimum tank volume exists.
  • This decreases thrust (equation 1) and a very small nozzle will be insufficient to propel the vehicle.
  • The design domain was computed by numerical integration (Fig. 3 ), with the specific impulse calculated based on the mass of the thruster alone, excluding the electronics and airframe.
  • This gave an optimum tank length of 0.45m, which was fabricated.

III. PLANAR TRAJECTORY MODEL

  • The robot is fitted with fins and a collapsible wing for flight.
  • To investigate the robustness of the transition to flight from water, the authors implemented a planar trajectory model and simple estimation of the hydrodynamic forces during water exit (section III-C).
  • Subscript s refers to skin friction forces.
  • The authors also define position vectors, x, within a robot-fixed reference frame rotated by an angle θ about Ẑ from the inertial frame, with its origin at the robot nose and unit vectors x,ŷ,ẑ (Fig. 4A ).

B

  • The effect of increasing the depth of the robot beneath the water (50 o start angle), also known as A.
  • In order to estimate the viscous force on the wings and fins, the authors use a turbulent flow flat plate skin friction coefficient, using a Reynolds number (Re) based on retracted wing cord to estimate friction on the lifting surfaces (equation 25).
  • Neglecting Reynolds number changes, the only fluid specific variable in equations 15-26 is the fluid density.
  • The robot floats on the water surface prior to launch, so buoyancy must also be included.
  • The resulting equations of motion are integrated numerically using a Runge-Kutta solver in Matlab.

D. Take-off Robustness

  • The simple drag model was found to give a good prediction of the acceleration profile of the AquaMAV during aquatic take-offs (section VI).
  • The model was therefore used to evaluate the robustness of take-off to external perturbations which can occur in an outdoor environment.
  • Increasing depth has a strong effect on the final velocity, as drag greatly limits underwater speed (Fig. 5A ).
  • Simulating launches at several different angles and depths, the simulation indicates that the jet will be able to achieve its minimum flight velocity (8.5m/s) regardless of angle as long as it is not submerged more than 0.8BL beneath the surface (Fig. 5B ).
  • Launching the robot nearer to vertical results in lower speed (but higher altitude).

IV. PROTOTYPE

  • The fabricated thruster has an air and water tank, with sealed screw connections to a centrepiece containing a poppet valve (Fig. 6 ).
  • To contain and release the high pressure gas, an NiTi Shape Memory Alloy (SMA) actuator has been developed.
  • The gas pressure vessel is constructed from 7075 aluminium according to European standards [20] , with an extra safety factor of 2 applied to the wall thickness to increase safety.
  • The water tank is pressurised to less than 10bar (Fig. 2C ) and sustains pressure only briefly, so is instead made from a woven CRFP tube, bonded to an aluminium screw connection (Fig. 6B ) and plastic nozzle.
  • The system has a deliberate modular construction, with the centrepiece and valve actuation system entirely self-contained, so that both tanks can be changed according to final mission requirements.

A. Valve Actuation

  • The valve is opened by raising the valve stem 1.6mm.
  • This produced a force of 40N, which is near-constant over the stroke range.
  • To provide an electrical conduit into the pressurised container, the vessel wall is used as a negative earth and an insulated bolt was fastened through the tank end (Fig. 6 ).
  • This is passed through the wire from a 7.4V, 200mAh battery, sufficient for over 150 actuations, and controlled using an Arduino microcontroller.
  • While the water tank will fill gradually with the tail pointing upward in the water, it will not fill when nose up.

B. Flight Components

  • The AquaMAV is fitted with deployable wings for flight.
  • Of the six segments, only the leading edge is actuated, while the root segment is fixed to the fuselage.
  • The wing hinges are actuated by 4.5 gram servos.
  • Control electronics are contained in a separate fuselage section attached to the gas tank which allows the gas tank to be removed easily for charging.

V. STATIC PERFORMANCE

  • Static thrust was measured by mounting the robot vertically to a load cell, with force data recorded at 2500Hz (Fig. 7 ).
  • The sensor was zeroed with the jet water tank full, so weight reduction as water is expelled was also measured by the force sensor.
  • The model output has been modified to show this effect in the predicted force profile shown in Fig. 7 .
  • The measured thrust profiles show the expected features of a rapid rise in pressure before the water accelerates, followed Towards the end of the water expulsion, the velocity of the air-water interface increases rapidly as it passes through the nozzle contraction.
  • No significant variation in thrust when actuating underwater was expected, which was confirmed by the sixth static thrust test, conducted underwater.

VI. AQUATIC TAKEOFF PERFORMANCE

  • After thrust was recorded, the AquaMAV was launched from a water tank into flight.
  • To allow longer trajectories, the AquaMAV was also launched from a nearby lake and filmed, although tracking was not possible.
  • The use of separately actuated wings often led to asymmetry between the two wings, rolling the robot (Fig. 9B ).
  • This model indicates that the robot is able to take-off in the presence of perturbations from surface waves to both its launch angle and depth.
  • This includes batteries, electronics, a waterproof fuselage and large lifting surfaces.

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AQUAMAV: FAST AQUATIC ESCAPE 1
Fast Aquatic Escape with a Jet Thruster
R. Siddall* and M. Kovac*
AbstractThe ability to collect water samples rapidly
with aerial-aquatic robots would increase the safety and
efficiency of water health monitoring, and allow water
sample collection from dangerous or inaccessible areas.
An Aquatic Micro Air Vehicle (AquaMAV) able to dive
into the water offers a low-cost and robust means of
collecting samples. However, small scale flying vehicles
generally do not have sufficient power for transition to
flight from water. In this paper we present a novel jet
propelled AquaMAV able to perform jumpgliding leaps
from water, and a planar trajectory model that is able to
accurately predict aquatic escape trajectories. Using this
model, we are able to offer insights into the stability of
aquatic take-off to perturbations from surface waves, and
demonstrate that an impulsive leap is a robust method
of flight transition. The AquaMAV uses a CO
2
powered
water jet to escape the water, actuated by a custom shape
memory alloy gas release. The 100 gram robot leaps from
beneath the surface, where it can deploy wings and glide
over the water, achieving speeds above 11 m/s.
I. INTRODUCTION
Locomotion in unstructured terrain is a significant
challenge to robots operating in an outdoor environment,
often requiring multiple modes of operation. For an
aerial robot, movement in water creates additional
structural and propulsive design constraints that can be
difficult to overcome [
1
]. However, the ability to move
in air and water would allow unique robot operation in a
wide variety of oceanic, riverine or urban environments.
We are developing an AquaMAV capable of diving
directly into the water and retaking flight using a high
powered burst of thrust (Fig. 1).
An aerial-aquatic robot would find use in disaster
relief or water ecology, particularly where access is
limited such as flooded towns or littoral areas. In
these unstructured aquatic environments, obstacles
impede conventional aquatic vehicles, and prevent close
*Department of Aeronautics, Imperial College London
Manuscript received March 26, 2015; revised July 18, 2016. This
work was funded by the UK Engineering and Physical Sciences
Research Council.
observation by aerial robots. Flight allows targets to
be reached rapidly from outside hazardous zones, at
speeds that cannot be matched by man-portable aquatic
robots. During an emergency scenario such as a stricken
ship or a tsunami event, an AquaMAV could dive into
an isolated area of water, where it could collect water
samples and record environmental data. The vehicle
could then perform a short take-off (Fig. 1), and return to
its launch site to submit collected samples and data. This
would enable a fast, targeted response to emergencies
that could not be matched by current systems.
The efficacy of water sampling with aerial robots
using larger multirotor platforms has been demonstrated
[
2
]. This approach relies on accurate sensing and
control to maintain position while a sample probe is
lowered. However, a fixed wing vehicle provides greater
range and speed than hovering vehicles, and plunge
diving reduces the need for accurate control, allowing
platforms to be produced at lower cost and operated in
larger numbers.
The speed and range of robotic aircraft may not
always be required by a mission, but aerial-aquatic loco-
motion has broader advantages in robot mobility. Many
amphibious terrestrial robots have been implemented
[
3
], [
4
], but these robots are not able to cross large, sheer
obstacles, and often can only exit the water on gentle
inclines. Buoyant ‘floatplane UAVs [
1
] will be similarly
inhibited by obstacles or waves on the water, which will
prevent taxiing take-off in constrained spaces.
Several large (2-3m wingspan) unmanned seaplanes
are currently in operation [
1
], [
5
], and experimental
studies have shown the potential of an aerial-aquatic
robot propelled by adaptable flapping wings [
6
]. Other
work has demonstrated the efficacy of jumpgliding loco-
motion in terrestrial robots [
7
], [
8
], and fixed wing Micro
Air Vehicles (MAVs) have been implemented with terres-
trial mobility [
9
]. Aquatic locomotion by quadrotors has
been shown [
10
], but to the best of the authors knowl-
edge, no fixed wing AquaMAV has been realised to date.

AQUAMAV: FAST AQUATIC ESCAPE 2
Fig. 1: Outdoor testing of the presented prototype: An AquaMAV can return water samples and data from isolated
areas of water, using a powerful burst of water jet thrust to accelerate free of the water and transition to flight. A: The
AquaMAV launches itself out of water. B: Timelapse of a launch trajectory. Wings are deployed in the final snapshot.
A. Principles for Aquatic Escape
Underwater locomotion is one of the most energeti-
cally efficient forms of locomotion in the animal king-
dom [
11
], principally because a neutrally buoyant animal
is not required to resist its own gravity to swim. However,
neutral buoyancy is often opposed to the constraints of
flight. This becomes most apparent when attempting
to leave the water surface, where propulsive and lifting
surfaces must be kept out of water to develop forces,
made more difficult by motion of the water surface.
Without additional buoyancy control, water escape will
be extremely difficult for aerial-aquatic vehicles.
A buoyant quadcopter can allow itself sufficient
buoyancy to clear its propeller from the water for
take-off, or partially lift out of the water using additional
rotors [
10
]. Thrust from aerial propellers can be used for
swimming, but motors would have to operate off-design
at low speeds when underwater, greatly reducing
efficiency unless variable gearing is used. Another
drawback is that such a vehicle would require a calm
surface to take-off, while the principle advantage would
be that it allows multiple swimming and flying phases
in a single mission. However, we propose that for high
frequency single sample return missions, the most robust
system would be a short burst of thrust, launching a
fixed wing vehicle through the surface to return to base.
In nature, several species of squid are able to initiate
gliding leaps by expelling a pressurised jet of water
[
12
]. This jet propelled launch is uniquely applicable
to short take-offs by AquaMAVs. Jets of mass have a
very rapid thrust response, unlike swimming leaps, and
a jet continues to produce thrust in both air and water
because it does not rely on external reaction forces. This
allows a vehicle to escape the water and accelerate when
airborne, where drag is dramatically reduced compared
to in water [
11
]. While this could also be achieved
with combustible rockets, rocket propellants are often
hazardous, and many operating environments (such
as an oil spill) may preclude the use of combustion. A
water jet offers a clean and safe alternative.
When leaving the water, both flying squid [
12
] and
flying fish [
11
] keep their wings folded until they are
clear of the surface. There are large differences in fluid
forces between the two media, and doing this protects
wing structures from large hydrodynamic loads, reduces
drag, and may also have stability considerations (Section
III-B
). Reconfigurable wings have also been shown to
have advantages in jumpgliding [
8
], and are features of
almost all aerial aquatic animals [1].
In this paper we will present an AquaMAV capable of
gliding leaps from beneath the water. The robot launches
using a powerful water jet, powered by controlled
release of a 5ml tank of 57 bar CO
2
gas. The robot
uses a shape memory alloy actuated valve to control the
CO
2
release, and has deployable wings which allow it
to maintain stability and minimise drag when leaving
the water. These wings are then deployed in the air
for gliding. In the following sections, we introduce
the physical principles behind water jet propulsion,
and detail the key design features of the jet-propelled
jumping robot. We use a planar trajectory model to
examine the aquatic take-off process, and show that

AQUAMAV: FAST AQUATIC ESCAPE 3
B
p
atm
T(t)
u
4
(t)
X
E
Z
E
h
1
p
1
m
1
m
1
1
h
2
p
2
m
2
u
3
(t)
A
2
3
4
0 100 200 300 400 500 600
0
1
2
3
4
5
6
Predicted Thrust Prole
Time (ms)
Propulsive force (N)
Water inertia allows
pressure to build
Water fully expelled
Reservoir gas sustains water
pressure during jetting
Remaining gas escapes
C
0 100 200 300 400 500 600
0
1
2
3
4
5
6
x 10
6
Tank Pressures
Time (ms)
Pressure (Pa)
Gas tank pressure
Water tank pressure
Time (ms)
0 100 200 300 400 500
Thrust (N)
0
1
2
3
4
5
Predicted Thrust Profile
Time (ms)
0 100 200 300 400 500
Pressure (Pa)
10
6
0
2
4
6
Tank Pressures
Gas tank pressure
Water tank pressure
Time (ms)
0 100 200 300 400 500
Energy (Joules)
0
50
100
150
Energy transfer during jetting
Gas tank energy
Water tank gas energy
pdV work to water
Total system energy
Water inertia allows
pressure to build
Water fully expelled
Reservoir gas sustains water
pressure during jetting
Remaining gas escapes
B
C
Fig. 2: (A) Jet propulsion principle: Gas released from a high pressure tank expels water, propelling the vehicle.
Circled numbers correspond to the locations indicated by equation subscripts. (B) Simulated thrust for water rocket
with separated chambers. (C) Gas pressures in water and gas tanks during jetting.
an impulsive jet is a robust means of flight transition.
Consistent static thrust from the fabricated device and
flight from beneath the water is then demonstrated.
II. WATER JET PROPULSION
In this section we use the subscripts 1, 2, 3 and 4
to denote variables relating to the main gas tank, the
gas within the water tank, the air-water interface, and
nozzle outlet respectively (Fig. 2A). The thrust produced
by a jet of mass flow
˙m
4
and velocity
u
4
is given by
equation 1. If a gas is used as propellant, its low density
means that thrust production is negligible without very
high exit velocities, and for efficient propulsion from
a limited reservoir, a heavier propellant is preferable.
For an AquaMAV, water can easily be collected before
launch, with compressed gas powering expulsion.
T = ˙m
4
u
4
(1)
Waters incompressibility means the expelled jet will
be at ambient pressure, and the gas expansion rate will
equal the water outflow. The water flow within the tank
is treated as quasi-1D by assuming uniform axial flow
[
13
]. By mass continuity, the local velocity is then a
function of cross-section area (equation 2). The unsteady
Bernoulli equation (equation 3) is used, integrating from
the air-water interface to the nozzle exit (Fig. 2). Total
pressure along a streamline running from 3 to 4 is equal
to the instantaneous gas pressure in the water tank.
A
3
(t)u
3
(t)=A
4
u
4
(t) (2)
Z
4
3
u
t
ds+
p
2
ρ
w
+
1
2
(u
2
4
u
2
3
)=0 (3)
Where
u
is the water velocity,
p
2
the gas pressure
in the water tank,
V
2
the gas volume,
A
n
the jet cross
sectional area and
ρ
w
the density of water. The pressure
acting on the water must be built up by the gas released
from the CO
2
tank. To compute the flow rate out of
the tank, we follow the valve flow equations given in
the European standard EN-60534 ([
14
], equations 4-7)
With the gas tank initially charged to 57 bar, the outflow
will be choked, and will remain so until the pressure
ratio (equation 5) falls below
κ
choke
(equation 6).
Υ
is
a compressibility correction factor (equation 7).
˙m
1
=K
v
Υ
κp
1
ρ
1
(4)
κ
0
=(p
1
p
2
)/p
1
(5)
κ=
κ
0
if κ
0
<κ
choke
κ
choke
if κ
0
κ
choke
(6)
Υ=1κ/3κ
choke
(7)

AQUAMAV: FAST AQUATIC ESCAPE 4
0
0.2
0.4
0.6
0.8
1
2
3
4
0
5
10
15
20
Nozzle exit diameter (mm)
6
8
10
12
14
16
18
Water tank length (m)
Nozzle exit diameter (mm)
0.2 0.4 0.6 0.8
2
2.5
3
3.5
4
Water tank length (m)
Fabricated geometry
Design Domain: Specific Total Impulse (Ns/kg)
Specific Total Impulse (Ns/kg)
Fig. 3: Design domain: Variation of specific total impulse, showing the existence of an optimal water tank volume
for a given gas tank. Decreasing nozzle diameter increases total efficiency, but reduces thrust production, and a
minimum of diameter 2mm was set. The prototyped geometry is marked with a
L
.
Gas flow depends on the valve flow coefficient,
K
v
, and the limiting pressure ratio,
κ
choke
, the point
at which the valve flow becomes sonic. Liquids and
gases behave similarly at low pressures [
15
], so
K
v
was
measured by fixing the valve in the open position and
logging the discharged volume against time of a 0.5m
tall, 4cm diameter column of water through the valve,
and fitting
K
v
according to the EN-60534 equations for
incompressible fluids.
κ
choke
is a compressible property
which cannot be measured from water flow, and so was
inferred from manufacturer data [
16
] for air flow at 7
bar based on the measured
K
v
value, and corrected for
the different properties of CO
2
(an ideal CO
2
nozzle
chokes at an upstream pressure of 1.8 bar, so we assume
sonic conditions for the data at 7 bar).
To determine the variation of gas conditions in the
two tanks, a first law energy balance is used. The gas
exchange is treated as a quasi-equilibrium, adiabatic pro-
cess, as jetting takes place over too short a timescale for
significant heat transfer to occur. This gives an equation
in which the stagnation enthalpy flux from the gas tank
(
˙m
1
h
01
) is equivalent to the increase in enthalpy and
kinetic energy of gas in the water tank (
m
2
(h
2
+u
2
3
/2)
),
less the
pdV
work done against water pressure (equation
8). Gases obey the ideal gas equation of state throughout.
˙m
1
h
01
=
d
dt
m
2
h
2
+
u
2
3
2

p
2
˙
V
2
(8)
Where
h
is specific enthalpy (subscript
0
denotes
a stagnation quantity). Combining equations 1-8 leads
to a system of four first and second order differential
equations in
V
2
(t)
,
˙
V
2
(t)
,
h
1
(t)
,
h
2
(t)
and
m
1
(t)
.
These equations remain valid until all water is expelled,
after which the release of remaining gas produces a
small amount of thrust. At this stage, total gas mass
inside the thruster (
m
1
+
m
2
) is no longer conserved,
equation 8 does not hold, and the mass flow out of the
nozzle must also be included in the thermodynamic
calculation (equation 9). To calculate this gas mass flow,
the outlet Mach number,
M
, is calculated based on the
outlet stagnation pressure ratio (equation 10).
˙m
1
h
01
=
d
dt
m
2
h
2
+
u
2
3
2

˙m
4
h
02
(9)
p
4
p
02
=
1+
γ1
2
M
2
γ
γ1
(10)
Where
γ
is the gas adiabatic index. The conical
water nozzle has no diverging section so
M 1
. When
subsonic, the outlet will be at atmospheric pressure,
but if the pressure ratio is greater than a critical value
(equation 11, different to the valve-specific
κ
choke
value), the flow is choked,
M =1
, and the nozzle outlet
pressure will be greater than atmospheric. In both cases,
the mass flow out can be computed using a standard
Mach number relation (equation 12).
p
atm
p
2
=
2
γ+1
γ
γ1
(11)
˙m
4
p
c
p
T
02
A
4
p
02
=
γM
γ1
1+
γ1
2
M
2
1
2
γ+1
γ1
(12)

AQUAMAV: FAST AQUATIC ESCAPE 5
0 0.1 0.2 0.3 0.4
Position (mm)
-500
-400
-300
-200
-100
0
Take off Longitudinal Stability
CG Location
Aerodynamic Centre (Wings Deployed)
Aerodynamic Centre (Wings Folded)
Wings Deployed
Wings Folded
Ŷ
X
Ŷ
Ŷ
Z
A
WET
T
A
z
B
B
F
W
θ
mg
β
cg
D
F
F
v
f
x
ŷ
x
cg
x
cb
x
w
x
f
ŷ
ŷ
Z
e
X
e
A
WET
T
A
x
CG
x
F
x
W
x
o
B
x
CB
B
F
W
D
W
α
I
YY
θ,ω
mg
D
B
D
F
F
F
v
cg
Time (s)
Fig. 4: A: Nomenclature for the equations of motion. B: The AquaMAV centre of gravity as water is expelled
during jetting, with the aerodynamic centre of the wings in both configuration shown. Folding the wings moves
the aerodynamic centre backward, ensuring stability until all water has been expelled.
Where
c
p
the gas heat capacity and
p
02
is the
stagnation pressure of gas in the water tank). Thrust is
given by equation 13 with an additional term to account
for the outflow being above atmospheric pressure.
T = ˙m
4
u
4
A
4
(p
4
p
atm
) (13)
This system of equations is solved in Matlab with a
variable order implicit solver. A conditional statement
links the regimes; integration of the water jetting equa-
tions is halted once all water is expelled, and final values
provide initial conditions to the gas-only equations.
The simulated results for the prototyped thruster are
shown in Fig. 2. Initially, the waters inertia limits flow
rate, and allows pressure to be built up in the water tank.
A small amount of gas thrust can be seen after all water
is expelled at 0.3s, reducing rapidly. Due to the high
pressures, gas flow through the valve and nozzle are
choked throughout jetting.
A. Design Domain
For a given reservoir pressure and valve flow
coefficient, the work extracted from the gas can be
maximised by varying the water tank size and nozzle
diameter. Enlarging the water tank increases launch
mass, and an optimum tank volume exists. To obtain
this optimum, the specific total impulse (
I
sp
, equation
14) is used as an objective, maximising the momentum
imparted to the robot.
I
sp
=
Z
Tdt/m
total
(14)
During jetting, pressure in the water tank is main-
tained by reservoir gas with a limited flow rate, so a
smaller nozzle allows a higher water pressure to be main-
tained, increasing performance. However, this decreases
thrust (equation 1) and a very small nozzle will be
insufficient to propel the vehicle. It was decided to target
a thrust to weight ratio greater than 5, or 5N of peak
thrust, giving a 2mm minimum nozzle exit diameter,
rounded for manufacturing. The design domain was
computed by numerical integration (Fig. 3), with the spe-
cific impulse calculated based on the mass of the thruster
alone, excluding the electronics and airframe. This gave
an optimum tank length of 0.45m, which was fabricated.
III. PLANAR TRAJECTORY MODEL
The robot is fitted with fins and a collapsible wing
for flight. To investigate the robustness of the transition
to flight from water, we implemented a planar trajectory
model and simple estimation of the hydrodynamic
forces during water exit (section
III-C
). Here, we use
the subscripts
w
,
f
,
b
,
cg
and
cb
to refer to the robot
wing, fins, body and centres of gravity and buoyancy
respectively. Subscript
s
refers to skin friction forces.
The trajectory is defined by velocity and acceleration
vectors,
~a
and
~v
, in earth fixed inertial axes with unit
vectors
ˆ
X
,
ˆ
Y
,
ˆ
Z
. We also define position vectors,
~x
,
within a robot-fixed reference frame rotated by an angle

Citations
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Journal ArticleDOI
TL;DR: A unique Aquatic Micro Air Vehicle (AquaMAV), which uses a reconfigurable wing to dive into the water from flight, inspired by the plunge diving strategy of water diving birds in the family Sulidae is presented.
Abstract: Aerial robots capable of locomotion in both air and water would enable novel mission profiles in complex environments, such as water sampling after floods or underwater structural inspections. The design of such a vehicle is challenging because it implies significant propulsive and structural design trade-offs for operation in both fluids. In this paper, we present a unique Aquatic Micro Air Vehicle (AquaMAV), which uses a reconfigurable wing to dive into the water from flight, inspired by the plunge diving strategy of water diving birds in the family Sulidae. The vehicle's performance is investigated in wind and water tunnel experiments, from which we develop a planar trajectory model. This model is used to predict the dive behaviour of the AquaMAV, and investigate the efficacy of passive dives initiated by wing folding as a means of water entry. The paper also includes first field tests of the AquaMAV prototype where the folding wings are used to initiate a plunge dive.

79 citations

Journal ArticleDOI
TL;DR: This paper looks to the natural world for solutions to many of the challenges associated with the design of fixed-wing cross-domain vehicles, inspired by multiple animals that cross between aerial and underwater domains, including the common murre.
Abstract: This paper looks to the natural world for solutions to many of the challenges associated with the design of fixed-wing cross-domain vehicles. One example is the common murre, a seabird that flies from nesting locations to feeding areas, dives underwater to catch prey and returns. This hunting expedition provides an outline of a possible mission for a cross-domain vehicle. While the challenges of cross-domain vehicles are many, the focus of this paper was on buoyancy management and propulsion. Potential solutions to each challenge, inspired by multiple animals that cross between aerial and underwater domains, are investigated. From these solutions, three design concepts are considered, a quadrotor/fixed-wing hybrid, a vertical takeoff and landing (VTOL) tailsitter aircraft, and a waterjet-assisted takeoff vehicle. A comparison was made between the capability of each concept to complete two missions based on the common murres' hunting expedition. As a result of this comparison, the VTOL tailsitter design was selected for further study. In-depth design was conducted and a prototype vehicle was built. The completed vehicle prototype successfully conducted submerged operation as well as four air flights. Flights consisted of egress from water, flight in air, ingress into water in each flight, and water locomotion. A total of 11 min, 23 s of flight time was recorded as well as underwater swims down to 12 ft (3.7 m) below the surface.

44 citations

Journal ArticleDOI
11 Sep 2019
TL;DR: This paper investigates the use of solid reactants as a combustion gas source for consecutive aquatic jump-gliding sequences and presents an untethered robot that is capable of multiple launches from the water surface and of transitioning from jetting to a glide.
Abstract: Robotic vehicles that are capable of autonomously transitioning between various terrains and fluids have received notable attention in the past decade due to their potential to navigate previously unexplored and/or unpredictable environments. Specifically, aerial-aquatic mobility will enable robots to operate in cluttered aquatic environments and carry out a variety of sensing tasks. One of the principal challenges in the development of such vehicles is that the transition from water to flight is a power-intensive process. At a small scale, this is made more difficult by the limitations of electromechanical actuation and the unfavorable scaling of the physics involved. This paper investigates the use of solid reactants as a combustion gas source for consecutive aquatic jump-gliding sequences. We present an untethered robot that is capable of multiple launches from the water surface and of transitioning from jetting to a glide. The power required for aquatic jump-gliding is obtained by reacting calcium carbide powder with the available environmental water to produce combustible acetylene gas, allowing the robot to rapidly reach flight speed from water. The 160-gram robot could achieve a flight distance of 26 meters using 0.2 gram of calcium carbide. Here, the combustion process, jetting phase, and glide were modeled numerically and compared with experimental results. Combustion pressure and inertial measurements were collected on board during flight, and the vehicle trajectory and speed were analyzed using external tracking data. The proposed propulsion approach offers a promising solution for future high-power density aerial-aquatic propulsion in robotics.

42 citations


Cites background from "Fast Aquatic Escape With a Jet Thru..."

  • ...These included friction of the fluid against the internal walls of the chamber, the thrust produced by the jetting air once the water was evacuated, the irregularity of the water-air interface during jetting, and the effect of water-air spray caused by fluid instabilities at the end of the water jetting phase (35)....

    [...]

  • ...S9 the estimated jumping height of the robot at 90o compared with impulsive jumping animals in (15), squid flight in (22), and water-jumping robots (15, 16, 27, 35, 37)....

    [...]

Journal ArticleDOI
TL;DR: A nonlinear dynamic controller, otherwise known as the adaptive dynamic surface control (ADSC) scheme, which effectively deals with the challenges aroused by the nonlinearities, uncertainties, and time-varying parameters of the system.
Abstract: A hybrid aerial underwater vehicle (HAUV) that could operate in the air and underwater might provide tremendous potential for ocean monitoring and search and rescue as well as underwater exploration. Differences between the aerial and underwater environments, along with common disturbances in wind or oceanic currents, present key challenges when designing a robust global controller. This paper presents a nonlinear dynamic controller, otherwise known as the adaptive dynamic surface control (ADSC) scheme, which effectively deals with the challenges aroused by the nonlinearities, uncertainties, and time-varying parameters of the system. First, the mathematical model of the HAUV is developed by means of the Newton–Euler formalism, highlighting the influence of the environment change on the vehicle dynamics. Second, the variations of added mass and damping during the water/air transition are estimated. Finally, the ADSC scheme is used to control and provided robust transition between distinct mediums for the vehicle in simulations, compared with the gain-scheduled proportional–integral–derivative scheme. The simulation results validate the good tracking performance and strong robustness of the presented scheme.

36 citations


Cites background from "Fast Aquatic Escape With a Jet Thru..."

  • ...Available: http://blog. modernmechanix.com/denmarks-amazing-submarine-plane/#mmGal [14] R. Siddall and M. Kovač, “Fast aquatic escape with a jet thruster,” IEEE/ASME Trans....

    [...]

  • ...Siddall and Kovač [14], [15] built an aquatic micro air vehicle capable of launching from the water like a flying squid by expelling a pressurized jet of water....

    [...]

  • ...[21] Y. H. Tan, R. Siddall, and M. Kovac, “Efficient aerial–aquatic locomotion with a single propulsion system,” IEEE Robot....

    [...]

  • ...Siddall and Kovač [14], [15] built an aquatic micro air vehicle capable of launching from the water like a flying squid by expelling a pressurized jet of water....

    [...]

  • ...[15] R. Siddall and M. Kovač, “Launching the AquaMAV: Bioinspired design for aerial-aquatic robotic platforms,” Bioinspiration Biomimetics, vol. 9, no. 3, 2014, Art. no. 031001....

    [...]

Journal ArticleDOI
TL;DR: An improved design of a multimodal HAUV capable of level and vertical flight, hovering, and underwater glide is presented, which includes a newly developed lightweight pneumatic buoyancy system and excludes the linear actuator commonly used for pitch control of gliders.

35 citations

References
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Book
01 Jun 1965
TL;DR: Fluid-dynamic drag: practical information on aerodynamic drag and hydrodynamic resistance, Fluid-dynamics drag as discussed by the authors, Fluid dynamic drag: real-time information about aerodynamic and hydrodyynamic resistance.
Abstract: Fluid-dynamic drag: practical information on aerodynamic drag and hydrodynamic resistance , Fluid-dynamic drag: practical information on aerodynamic drag and hydrodynamic resistance , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

759 citations


"Fast Aquatic Escape With a Jet Thru..." refers methods in this paper

  • ...To compute body drag, this coefficient is modified based on the ratio of the body’s maximum width and length [BW/BL, (26)] [18]....

    [...]

01 Dec 1966
TL;DR: In this article, a concept for the calculation of the vortex lift of sharp-edge delta wings is presented and compared with experimental data, based on an analogy between vortex lift and the leading-edge suction associated with the potential flow about the leading edge.
Abstract: Polhamus Langley Research Center SUMMARY A concept for the calculation of the vortex lift of sharp-edge delta wings is pre­sented and compared with experimental data. The concept is based on an analogy between the vortex lift and the leading-edge suction associated with the potential flow about the leading edge. This concept, when combined with potential-flow theory modified to include the nonlinearities associated with the exact boundary condition and the loss of the

401 citations

Journal ArticleDOI
TL;DR: Comparisons with animal data are presented, and the results show striking similarities with the gaits observed with real salamanders, in particular concerning the timing of the body’s and limbs’ movements and the relative speed of locomotion.
Abstract: In this paper, we present Salamandra robotica II: an amphibious salamander robot that is able to walk and swim. The robot has four legs and an actuated spine that allow it to perform anguilliform swimming in water and walking on the ground. The paper first presents the new robot hardware design, which is an improved version of Salamandra robotica I. We then address several questions related to body–limb coordination in robots and animals that have a sprawling posture like salamanders and lizards, as opposed to the erect posture of mammals (e.g., in cats and dogs). In particular, we investigate how the speed of locomotion and curvature of turning motions depend on various gait parameters such as the body–limb coordination, the type of body undulation (offset, amplitude, and phase lag of body oscillations), and the frequency. Comparisons with animal data are presented, and our results show striking similarities with the gaits observed with real salamanders, in particular concerning the timing of the body’s and limbs’ movements and the relative speed of locomotion.

187 citations


"Fast Aquatic Escape With a Jet Thru..." refers background in this paper

  • ...Many amphibious terrestrial robots have been implemented [3], [4], but these robots are not able to cross large, sheer obstacles, and often can only exit the water on gentle inclines....

    [...]

Journal ArticleDOI
TL;DR: The primary aim of this research is to lay the foundation for a generation of vehicles capable of multi-modal locomotion, allowing ambulatory abilities in more than one media, surpassing current capabilities.
Abstract: The majority of robotic vehicles that can be found today are bound to operations within a single media (i.e. land, air or water). This is very rarely the case when considering locomotive capabilities in natural systems. Utility for small robots often reflects the exact same problem domain as small animals, hence providing numerous avenues for biological inspiration. This paper begins to investigate the various modes of locomotion adopted by different genus groups in multiple media as an initial attempt to determine the compromise in ability adopted by the animals when achieving multi-modal locomotion. A review of current biologically inspired multi-modal robots is also presented. The primary aim of this research is to lay the foundation for a generation of vehicles capable of multi-modal locomotion, allowing ambulatory abilities in more than one media, surpassing current capabilities. By identifying and understanding when natural systems use specific locomotion mechanisms, when they opt for disparate mechanisms for each mode of locomotion rather than using a synergized singular mechanism, and how this affects their capability in each medium, similar combinations can be used as inspiration for future multi-modal biologically inspired robotic platforms.

167 citations