U.S. patent application number 15/348691 was filed with the patent office on 2017-03-02 for feed system and a method of suppressing the pogo effect.
This patent application is currently assigned to SNECMA. The applicant listed for this patent is CENTRE NATIONAL D'ETUDES SPATIALES CNES, SNECMA. Invention is credited to Ludivine BOULET, Alain KERNILIS, Serge LE GONIDEC, Nicolas LEMOINE.
Application Number | 20170058836 15/348691 |
Document ID | / |
Family ID | 46201731 |
Filed Date | 2017-03-02 |
United States Patent
Application |
20170058836 |
Kind Code |
A1 |
KERNILIS; Alain ; et
al. |
March 2, 2017 |
FEED SYSTEM AND A METHOD OF SUPPRESSING THE POGO EFFECT
Abstract
A feed system for feeding a rocket engine with a liquid
propellant includes a feed circuit, and a device to vary a volume
of gas in the feed circuit. The device is configured to cause a
volume of gas in the feed circuit to vary while the rocket engine
is in operation. The device to vary gas volume includes at least
one variable-flow-rate gas injector to inject gas into the liquid
propellant in the feed circuit. Methods of suppressing a POGO
effect are also provided.
Inventors: |
KERNILIS; Alain; (Freneuse,
FR) ; LEMOINE; Nicolas; (Vernon, FR) ; BOULET;
Ludivine; (Vernon, FR) ; LE GONIDEC; Serge;
(Vernon, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SNECMA
CENTRE NATIONAL D'ETUDES SPATIALES CNES |
Paris
Paris |
|
FR
FR |
|
|
Assignee: |
SNECMA
Paris
FR
CENTRE NATIONAL D'ETUDES SPATIALES CNES
Paris
FR
|
Family ID: |
46201731 |
Appl. No.: |
15/348691 |
Filed: |
November 10, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14118369 |
Jan 13, 2014 |
9528470 |
|
|
PCT/FR2012/051005 |
May 7, 2012 |
|
|
|
15348691 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F02K 9/50 20130101; F02K
9/52 20130101; F02K 9/566 20130101; F05D 2220/80 20130101; F05D
2270/54 20130101; B64G 1/401 20130101; F02K 9/56 20130101; F05D
2270/44 20130101; F02K 9/60 20130101; F02K 9/96 20130101 |
International
Class: |
F02K 9/56 20060101
F02K009/56; B64G 1/40 20060101 B64G001/40; F02K 9/96 20060101
F02K009/96; F02K 9/50 20060101 F02K009/50; F02K 9/52 20060101
F02K009/52 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2011 |
FR |
11 54298 |
Claims
1. A feed system for feeding a rocket engine with a liquid
propellant, the system comprising: a feed circuit; and a device to
vary a volume of gas in the feed circuit, which device is
configured to cause a volume of gas in the feed circuit to vary
while the rocket engine is in operation, wherein the device to vary
gas volume comprises at least one variable flow-rate gas injector
to inject gas into the liquid propellant in the feed circuit.
2. A feed system according to claim 1, further comprising a control
unit configured to control the device to vary gas volume.
3. A feed system according to claim 2, further comprising at least
one sensor connected to the control unit, and wherein the control
unit is configured to control variation in the gas volume as a
function of signals sensed by the at least one sensor.
4. A feed system according to claim 3, wherein the at least one
sensor comprises an accelerometer.
5. A feed system according to claim 3, wherein the at least one
sensor comprises a sensor to sense pressure of the propellant.
6. A feed system according to claim 2, wherein the control unit is
configured to control variation of the gas volume as a function of
time.
7. A method of suppressing a POGO effect, comprising: varying a
volume of gas in a feed circuit of a system to feed a rocket engine
with a liquid propellant while the rocket engine is in operation,
to control a difference between at least one hydraulic resonant
frequency of the feed circuit and at least one mechanical resonant
frequency of a structure coupled to the feed circuit, wherein the
gas volume is caused to vary by varying a rate at which gas is
injected into the liquid propellant in the feed circuit.
8. A method of suppressing the POGO effect according to claim 7,
wherein the gas volume varies to keep the difference above a
predetermined threshold.
9. A method of suppressing the POGO effect according to claim 7,
wherein the gas volume is caused to vary as a function of at least
one mechanical oscillation value sensed on the structure.
10. A method of suppressing the POGO effect according to claim 9,
further comprising performing spectral analysis on at least one
mechanical oscillation to determine the at least one mechanical
resonant frequency of the structure.
11. A method of suppressing the POGO effect according to claim 10,
wherein a filter algorithm, or an unscented Kalman filter, is
applied to at least one sensed mechanical oscillation to determine
the at least one mechanical resonant frequency and/or to predict
its future variation.
12. A method of suppressing a POGO effect, comprising: performing
spectral analysis on at least one mechanical oscillation to
determine at least one mechanical resonant frequency of a structure
coupled to a feed circuit of a system to feed a rocket engine with
a liquid propellant; varying a volume of gas in the feed circuit
while the rocket engine is in operation, to control a difference
between at least one hydraulic resonant frequency of the feed
circuit and the at least one mechanical resonant frequency of the
structure coupled to the feed circuit.
13. A method of suppressing the POGO effect according to claim 12,
wherein a filter algorithm is applied to the at least one sensed
mechanical oscillation to determine the at least one mechanical
resonant frequency.
14. A method of suppressing the POGO effect according to claim 12,
wherein an unscented Kalman filter is applied to the at least one
sensed mechanical oscillation to determine the at least one
mechanical resonant frequency and predict its future variation.
15. A method of suppressing the POGO effect according to claim 12,
wherein the gas volume varies to keep the difference above a
predetermined threshold.
16. A method of suppressing the POGO effect according to claim 12,
wherein the variable gas volume is located at least in part in a
hydraulic accumulator connected to a duct of the feed circuit.
17. A method of suppressing the POGO effect according to claim 12,
wherein the gas volume is caused to vary by varying a rate at which
gas is injected into the propellant in the feed circuit.
18. A non-transitory computer readable medium including computer
executable instructions for performing a method of suppressing the
POGO effect according to claim 12.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S.
application Ser. No. 14/118,369, filed Jan. 13, 2014, which is a
National Stage Application of International Application No.
PCT/FR12/051005, filed May 7, 2012, and which claims the benefit of
foreign priority to French Application No. 11 54298, filed May 17,
2011. The entire contents of each of the above applications are
hereby incorporated by reference herein in entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a feed system for feeding a
rocket engine with at least one liquid propellant, the system
including at least one feed circuit.
[0003] In the field of liquid propellant rockets, the term "POGO
effect" is used to designate the liquid propellant in the feed
circuit of the rocket engine entering into resonance with
mechanical oscillations of the rocket. Since the thrust of a rocket
engine varies with the rate at which propellant is delivered by the
feed circuit, such an entry in resonance can give rise to rapidly
diverging oscillations and can thus give rise to difficulties in
guidance, and even to damage that may go as far as total loss of
the payload or even of the vehicle. The term "POGO effect" does not
come from an acronym, but rather from pogo stick toys comprising a
rod with a spring that bounces in a manner that reminded
technicians of the violent longitudinal oscillations of rockets, as
caused by this effect. From the beginning of the development of
liquid propellant rockets, it has therefore been very important to
take measures to suppress this POGO effect. In the context of the
present description, the term "suppress" is used to cover both
total elimination and partial reduction.
[0004] Two main types of POGO effect corrector systems are known to
the person skilled in the art: passive systems and active systems.
With passive systems, the hydraulic resonant frequencies are
changed so that they cannot coincide with the mechanical resonant
frequencies of the rocket. They can also be damped. This is done,
for example, by installing hydraulic accumulators in the propellant
feed circuit. Such a hydraulic accumulator is normally formed by a
pressurized volume containing both gas and liquid, which volume is
in communication with the feed circuit. The hydraulic accumulator
operates as a mass-spring-damper system in which the mass is the
mass of liquid in the accumulator, the spring is formed by the gas,
and the damping comes from the viscosity of the liquid entering and
leaving the accumulator via a narrow duct. French patent
application FR 2 499 641 discloses one example of such a hydraulic
accumulator that is adjustable in order to enable it to be adapted
to different rocket engines. Nevertheless, variation in the
compressibility and damping parameters of that accumulator cannot
be carried out while the rocket engine is in operation. Another
"passive" method of correcting the POGO effect consists in changing
the hydraulic resonant frequency of the feed circuit by injecting a
fixed flow rate of gas into the circuit so as to change the speed
of sound in the circuit. In contrast, with active systems, an
opposing oscillation of pressure and flow rate is established in
the feed circuit to counter the oscillations that are measured in
the circuit.
[0005] Nevertheless, both passive systems and active systems
present drawbacks. Passive systems are not appropriate for rockets
that present great variability in their mechanical frequencies
while they are in operation, since they do not damp modes outside a
narrow band around the frequencies for which they are designed. In
the event of there being a difference between the predicted dynamic
behavior and the real dynamic behavior of the flight of the rocket,
they are not in a position to correct their action. Active systems
are liable to have positive effects only locally and they can also
generate effects that are negative, whether local or global.
[0006] In Japanese patent application JP 03-287498 A, there is
proposed a POGO effect corrector system with an adaptive hydraulic
accumulator. In that adaptive hydraulic accumulator, the
compressibility can be varied by varying the pressure of the gas,
and above all the damping can be varied by varying a
flow-restriction device. Nevertheless, that presents several
drawbacks. Firstly, varying pressure serves to vary compressibility
in the hydraulic accumulator in only a very restricted manner. In
addition, although it is possible to control greater variation in
damping, the variable restriction device has moving parts in the
flow of propellant, and that can lead to problems of reliability,
particularly if the propellant is cryogenic.
OBJECT AND SUMMARY OF THE INVENTION
[0007] The present invention seeks to remedy those drawbacks.
[0008] This object is achieved by the fact that the feed system of
the present invention comprises at least one device for varying a
volume of gas in the feed circuit, which device is suitable for
causing the volume of gas in the circuit to vary while said rocket
engine is in operation.
[0009] In the context of the present disclosure, the term "varying"
is used to mean causing a magnitude to change successively through
a plurality of different values, whether gradually or stepwise.
Thus, said device for varying gas volume makes it possible to
obtain a plurality of different volumes of gas in the circuit in
succession.
[0010] By means of these provisions, it is thus possible to vary at
least one hydraulic resonant frequency of the circuit over a wide
range of frequencies while the rocket engine is in operation,
thereby avoiding the mechanical resonant frequencies of a support
structure even when they also vary while the rocket engine is in
operation. It is also easy to adapt a given hydraulic circuit to a
plurality of different structures having different mechanical
resonant frequencies.
[0011] In at least one embodiment of the invention, said device for
varying the volume of gas may comprise at least one hydraulic
accumulator with a variable liquid level. Amongst other things,
this presents the advantage of providing comparatively simple means
for varying the volume of gas, and thus for varying the
compressibility and the hydraulic resonant frequency in the
circuit. In particular, said hydraulic accumulator may have a gas
feed point, a connection to a duct of said propellant feed circuit,
and between said gas feed point and said connection, at least one
dip tube connecting said duct to said variable liquid level of the
hydraulic accumulator.
[0012] In a first variant, the hydraulic accumulator has a
plurality of dip tubes, each including a respective valve and
connecting said duct to a respective distinct liquid level. By
opening and closing the valves, it is possible to equalize the
pressure at the free surface of the liquid with the pressure of the
duct at various levels. It is thus possible to vary very
substantially the volume of gas in the hydraulic accumulator, and
thus to vary the compressibility of the hydraulic accumulator and
at least one hydraulic resonant frequency of the circuit.
[0013] In a second variant, the at least one dip tube is movable in
order to vary the liquid level that it connects to the duct. It is
thus possible to obtain continuous variation of at least one
hydraulic resonant frequency.
[0014] In at least one other embodiment, said device for varying
gas volume may advantageously comprise at least one
variable-flowrate gas injector. This also presents the advantage of
providing means for varying at least one hydraulic frequency, which
means are of complexity that is not significantly greater than the
complexity of prior art passive systems.
[0015] Advantageously, the feed system further comprises a control
unit for controlling said device for varying gas volume. This
control unit makes it possible to associate a command for varying
the gas volume with parameters such as an estimated mechanical
resonant frequency, time since starting the rocket engine, etc.
[0016] Still more advantageously, the feed system further comprises
at least one sensor connected to said control unit, said control
unit is configured to control variation in the gas volume as a
function of signals sensed by said sensor. In particular, said at
least one sensor may comprise an accelerometer, thus enabling
mechanical oscillations to be detected and/or enabling at least one
mechanical resonant frequency to be estimated and/or enabling its
variation to be estimated. Said at least one sensor may also
comprise a pressure sensor for sensing the pressure of said
propellant, which may make it possible to estimate at least one
hydraulic resonant frequency of said feed circuit, and/or to
estimate how it will vary. By means of this provision, it is
possible to detect when at least one mechanical resonant frequency
comes close to at least one hydraulic resonant frequency, and to
vary the hydraulic resonant frequency so as to avoid the POGO
effect.
[0017] In at least one other embodiment, said control unit is
configured to control variation of the gas volume as function of
time. Thus, if it is known in advance how the behavior of at least
one mechanical resonant frequency of the structure coupled to the
feed circuit is going to vary over time, e.g. as a result of prior
testing and/or simulations or calculations, it is possible to
program the variation of at least one hydraulic resonant frequency
as a function of time so as to avoid coincidence between resonant
frequencies, and thus avoid the POGO effect, while the rocket
engine is in operation.
[0018] Although those two options may be considered as
alternatives, it is also possible to combine them, e.g. using a
base setpoint that is a function of time and a correction factor
that is a function of signals picked up by a sensor.
[0019] The present invention also provides a vehicle including at
least one rocket engine using at least one liquid propellant with a
feed system of the invention. In the context of the present
disclosure, the term "vehicle" should be understood broadly. Thus,
the invention may be applied to single- or multi-stage space
launchers, to individual stages of such space launchers, or to
space vehicles such as satellites, probes, capsules, or shuttles,
and also to single- or multi-stage guided or non-guided
projectiles, or to individual stages of such projectiles. In any
event, the invention presents the advantage of making the vehicle
more reliable so as to conserve its payload all the way to its
destination.
[0020] The present invention also provides a method of suppressing
the POGO effect, wherein a volume of gas in a feed circuit of a
system for feeding a rocket engine with at least one liquid
propellant is caused to vary while said rocket engine is in
operation so as to control a difference between at least one
hydraulic resonant frequency of the feed circuit and at least one
mechanical resonant frequency of a structure coupled to said feed
circuit.
[0021] The variation in the compressibility of the gas-and-liquid
fluid contained in the feed circuit thus enables at least one
hydraulic resonant frequency to be varied over a wide range. It is
thus possible to avoid the hydraulic and mechanical systems
entering into resonance, thereby leading to the POGO effect.
[0022] In at least one implementation, said gas volume varies so as
to keep said difference above a predetermined threshold. This
obtains a safety margin corresponding to the threshold.
[0023] Nevertheless, in particular when there are a plurality of
hydraulic resonant frequencies and/or a plurality of mechanical
resonant frequencies, it may be advantageous to cause at least one
hydraulic resonant frequency to vary in such a manner as to
maximize a function of at least one difference between a hydraulic
resonant frequency of the feed circuit and a mechanical resonant
frequency of the structure. In particular, if this function is a
function of a plurality of differences, each corresponding to a
different pair of respective hydraulic and mechanical resonant
frequencies, this function may be a weighted function, with an
individual coefficient for each pair.
[0024] The variable gas volume may be located at least in part in a
hydraulic accumulator connected to a duct of said feed circuit, so
as to cause it to vary by varying the volume of gas in the
accumulator. It may also be caused to vary by varying the rate at
which gas is injected into at least one propellant in said feed
circuit. Bubbles of gas in suspension in the propellant thus
provide a variable degree of compressibility to the gas-and-liquid
fluid contained in the feed circuit, with this being expressed in
particular by variations in the speed of sound in the circuit, and
in said hydraulic resonant frequency.
[0025] Advantageously, the gas volume may be caused to vary as a
function of time and/or of at least one mechanical oscillation
value sensed on said structure, in particular a mechanical
oscillation on which spectral analysis is performed in order to
determine at least one mechanical resonant frequency of said
structure. Still more advantageously, a filter algorithm, e.g. such
as an "unscented" Kalman filter (UKF), is applied to at least one
sensed mechanical oscillation in order to determine at least one
mechanical resonant frequency and/or to predict its future
variation, thus making it easier to avoid coincidence between the
at least one mechanical resonant frequency and the at least one
hydraulic resonant frequency.
[0026] The present invention also provides a computer data support
including instructions executable by a computer in order to perform
a method of the invention for suppressing the POGO effect. The term
"computer data support" is used to mean any support suitable for
storing data in durable and/or transient manner, and for allowing
the data subsequently to be read by a computer system. Thus, the
term "computer data support" covers, amongst other things: magnetic
tapes, magnetic and/or optical disks, and solid state electronic
memories which may be volatile or non-volatile.
[0027] The present invention may thus also be expressed in the form
of a computer programmed to perform the method of the invention for
suppressing the POGO effect, or even by software for use with a
computer to perform the method of the invention for suppressing the
POGO effect.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The invention can be well understood and its advantages
appear better on reading, the following detailed description of
embodiments given as non-limiting examples. The description refers
to the accompanying drawings, in which:
[0029] FIG. 1 is a diagram based on an analogy between hydraulic
circuits and electrical circuits, showing a rocket engine vehicle
with a liquid propellant feed system in an embodiment of the
invention;
[0030] FIGS. 2A and 2B are cross-sections through a variable-volume
accumulator installed in parallel with a feed circuit of the FIG. 1
system;
[0031] FIG. 3 is a cross-section of a variable-volume accumulator
in a second embodiment of the invention;
[0032] FIG. 4 is a diagram based on an analogy between hydraulic
circuits and electrical circuits, showing a rocket engine with a
liquid propellant feed circuit in a third embodiment of the
invention;
[0033] FIG. 5 is a diagram of a gas injection point connected to
the FIG. 4 circuit;
[0034] FIGS. 6A, 6B, 6C, and 6D are diagrams showing the operation
of an "unscented" Kalman filter algorithm; and
[0035] FIGS. 7A, 7B, 7C, and 7D are graphs showing three possible
ways in which the hydraulic resonant frequency of the feed circuit
can vary in response to a variation in a mechanical resonant
frequency of the FIG. 1 vehicle.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The vehicle 1 shown in FIG. 1 has a propulsion chamber 2
incorporating a combustion chamber and a convergent-divergent
nozzle. The vehicle 1 also has a feed circuit 3, 4 for each of two
liquid propellants that react chemically with each other and that
are fed to the propulsion chamber 2. The first feed circuit 3 is
shown in part only. Each feed circuit 3, 4 filled with fluid
represents a dynamic system that can be modeled like an electrical
circuit having resistors 5, inductors 6, and capacitors 7, and that
presents at least one resonant frequency.
[0037] In order to cause at least one resonant frequency of the
second feed circuit 4 to vary, the circuit includes in parallel
therewith a hydraulic accumulator 8 having a volume of gas that is
variable and thus presenting compressibility that is also variable.
This accumulator 8, shown in FIGS. 2A and 2B, comprises a tank 9
with a pressurized gas feed point 10 on one side and a connection
11 to a duct 15 of the second feed circuit 4 on an opposite side.
At various levels between the point 10 and the connection 11, dip
tubes 12, 13 connect the tank 9 with the duct 15. Each dip tube 12,
13 includes a respective valve 14, 16 that is interposed between
the tank 9 and the duct 15. Opening and closing the valves 14 and
16 thus makes it possible to vary the liquid level, and thus the
volume of gas 17, inside the tank 9, as shown in FIGS. 2A and 2B.
In FIG. 2A, the valve 14 of the shorter dip tube 12 is open, while
the valve 16 of the dip tube 13 is closed. The free surface of the
liquid is thus stabilized at the level of the inlet to the dip tube
12, and the volume of gas 17 together with its compressibility thus
remains comparatively limited. In contrast, in FIG. 2B, the valve
14 of the dip tube 12 is closed, and the valve 16 of the dip tube
13 is open. The free surface of the liquid is thus stabilized at
the lower level of the inlet to the dip tube 13, and the volume of
gas 17 and its compressibility is increased accordingly.
[0038] By varying the effective compressibility of the accumulator
8, it thus becomes possible, even while the rocket engine of the
vehicle 1 is in operation, to adapt the hydraulic resonant
frequency of the second feed circuit 4 so as to prevent it from
coinciding with a variable mechanical resonant frequency of a
support structure of the rocket engine. Naturally, in order to
achieve this result, it is necessary to have perceptible
acceleration in order to separate the heavier liquid from the
lighter gas. This hydraulic accumulator 8 of variable gas volume
therefore does not operate in the same manner under conditions of
microgravity.
[0039] In a second embodiment as shown in FIG. 3, the accumulator 8
likewise has a tank 9 with a pressurized gas feed point 10 on one
side and a connection 11 to a duct 15 of the second feed circuit 4
on an opposite side, but it has only one dip tube 12, which tube is
however movable in the depth direction of the tank 9 in order to
vary the level of the liquid, and thus the gas volume 17 inside the
tank 9. This embodiment makes it possible to vary the liquid level
continuously and thus to vary the gas volume continuously, and
hence varies the compressibility in the accumulator 8 and the
hydraulic resonant frequency of the second feed circuit 4.
[0040] A third embodiment is shown in FIG. 4. As in the embodiment
of FIG. 1, in this other embodiment, the vehicle 1 likewise has a
feed system with a feed circuit 3, 4 for feeding each of two liquid
propellants that react chemically with each other and that are fed
to a propulsion chamber 2.
[0041] Nevertheless, in this third embodiment, the at least one
hydraulic resonant frequency of the second feed circuit 4 is caused
to vary by injecting gas at a variable rate into the fluid of the
feed circuit 4 by means of a gas injection device 20 connected to
the second feed circuit 4. Downstream from this injection point 20,
the compressibility of the liquid/gas fluid in the circuit is
modified by the compressibility of the injected volume of gas.
Consequently, the at least one hydraulic resonant frequency of the
feed circuit 4 and also the speed of sound in the circuit 4 are
also varied.
[0042] The gas injection device 20 is shown in FIG. 5. It is
installed on a duct 15 of the second feed circuit 4 and it
comprises an annular chamber 21 around the duct 15, which chamber
is connected to a source of pressurized gas (not shown) via three
valves 22, 23, and 24, and communicates with the duct 15 via
injection orifices 25. The rate at which gas is injected into the
duct 15, and thus into the second feed circuit, can thus be varied
by opening and closing the valves 22, 23, and 24. Alternatively, or
in combination with the above arrangement, such a gas injection
device could include a variable-opening valve or a flow rate
regulator, thus making it possible to obtain continuous variation
in the volume flow rate of the gas that is injected into the duct
15, and thus of the at least one hydraulic resonant frequency.
[0043] Both the variable gas volume hydraulic accumulator 8 in the
first embodiment and the variable flow rate gas injection device 20
of the second embodiment can be connected equally well to a control
unit 30 for controlling them by means of a variable setpoint that
is issued by the control unit to the accumulator 8 and/or to the
gas injection device 20. If the way the mechanical resonant
frequency varies is known in advance, as a result of simulations
and/or tests that have already been performed, this setpoint may be
preprogrammed merely as a function of time. Nevertheless, it is
also possible, and indeed preferable under certain circumstances,
to cause this setpoint to vary in response to signals that are
received in real time or almost in real time. For example, as shown
in FIGS. 1 or 3, the vehicle 1 may include at least one
accelerometer 31 and a propellant pressure sensor 32 in the circuit
4. The accelerometer 31 is connected to the control unit 30 in
order to send signals thereto representative of the mechanical
behavior of the structure of the vehicle 1, and the pressure sensor
32 is also connected thereto in order to send signals
representative of the hydraulic behavior of the circuit 4.
[0044] These signals are processed in the control unit 30 in order
to extract the mechanical and hydraulic resonant frequencies by
spectrum analysis. Filter algorithms, such as for example the
"unscented" Kalman filter algorithm as described in "The unscented
Kalman filter for nonlinear estimation", Proceedings of Symposium
2000 on Adaptive Systems for Signal Processing, Communication and
Control (AS-SPCC), IEEE, Lake Louise, Alberta, Canada, October
2000, may be used, not only to filter noise from the signals, but
even in predictive manner in order to forecast short-term variation
in the resonant frequencies of the modes of oscillation, and to
anticipate them in the way the hydraulic resonant frequency is
controlled. The control unit may be programmed to initialize such a
filter algorithm close to an expected mechanical resonant
frequency, thereby making it possible subsequently to track this
frequency in flight.
[0045] In a dynamic system such as a vehicle 1, it can be assumed
that there exists a Markov sequence of latent states x.sub.t that
vary in time in application of a function F. These latent states
are observed indirectly by sensors giving measured states y.sub.t
as obtained via a measurement function G. Thus, x.sub.t and y.sub.t
can be expressed by the following formulas:
x.sub.t=F(x.sub.t-1)+.epsilon.
y.sub.t=G(x.sub.t)+.nu.
[0046] The values .epsilon. and .nu. represent respectively the
noise inherent to the system and measurement noise, and both of
them present Gaussian distributions.
[0047] The object of a filter algorithm is to infer the state of
the dynamic system from noisy values as measured by sensors. A
Kalman system provides an inference that is fast and accurate for
systems that are linear. It is nevertheless not directly applicable
to systems that are non-linear, and the present application is
potentially classifiable as a non-linear system. Among various
alternatives for adapting the Kalman filter algorithm to non-linear
systems, there is known in particular the "unscented" Kalman filter
(UKF). This algorithm propagates several estimates of x.sub.t
through the functions F and G and reconstructs a Gaussian
distribution, assuming that the propagated values come from a
linear system. The positions of these estimates for x.sub.t are
referred to as "sigma points", and they are calculated from an
initial average and variance with an approximation scheme referred
to as an unscented transformation.
[0048] In FIG. 6A, a first step is shown in which the initial sigma
points x.sub.0.sup.0, x.sub.0.sup.1, x.sub.0.sup.2, x.sub.0.sup.3,
x.sub.0.sup.4 are calculated by such an unscented transformation
starting from a mean m.sub.0 and a variance P.sub.0 taken into
consideration for the latent state x.sub.0 based on a first set of
measurements y.sub.0 at the initial moment t=t.sub.0. Thereafter,
in a prediction step, shown in FIG. 6B, estimated positions
X'.sub.0.sup.0, X'.sub.0.sup.1, X'.sub.0.sup.2, X'.sub.0.sup.3,
X'.sub.0.sup.4 for the sigma points corresponding to the following
sampling instant (t=t.sub.1) are predicted by applying the
prediction step of the Kalman filter algorithm to the initial
signal points X.sub.0.sup.0, X.sub.0.sup.1, X.sub.0.sup.2,
X.sub.0.sup.3, X.sub.0.sup.4. In the following step of updating, as
shown in FIG. 6C, the actual sigma points X.sub.1.sup.0,
X.sub.1.sup.1, X.sub.1.sup.2, X.sub.1.sup.3, X.sub.1.sup.4 are
calculated on the basis of the previous sampling at t=t.sub.1. The
differences between the positions X'.sub.0.sup.0, X'.sub.0.sup.1,
X'.sub.0.sup.2, X'.sub.0.sup.3, X'.sub.0.sup.4 as predicted on the
basis of the initial sigma points X.sub.0.sup.0, X.sub.0.sup.1,
X.sub.0.sup.2, X.sub.0.sup.3, X.sub.0.sup.4 and the positions
X.sub.1.sup.0, X.sub.1.sup.1, X.sub.1.sup.2, X.sub.1.sup.3,
X.sub.1.sup.4 as actually calculated on the basis of the new
sampling make it possible to obtain information about the function
f representing variation of the latent state x.sub.t over time. In
the following step, shown in FIG. 6D, the new mean m.sub.1 and the
new variance P.sub.1 are calculated on the basis of the new sigma
points X.sub.1.sup.0, X.sub.1.sup.1, X.sub.1.sup.2, X.sub.1.sup.3,
X.sub.1.sup.4. This algorithm is recursive, and each step starting
with the prediction step is repeated for each new sampling.
[0049] In the control unit, the mechanical and hydraulic resonant
frequencies are compared, and by way of example if their difference
approaches or crosses a certain threshold, the control unit 30
varies the setpoint that is transmitted to the accumulator 8 and/or
to the gas injection device 20.
[0050] FIGS. 7A, 7B, 7C, and 7D show four examples of how a
hydraulic resonant frequency 50 can be controlled in response to an
increasing mechanical resonant frequency 51. In the first example,
shown in FIG. 7A, the hydraulic resonant frequency 50 may be varied
continuously so as to maintain a constant difference relative to
the mechanical resonant frequency 51. In the second example, shown
in FIG. 7B, the hydraulic resonant frequency 50 is varied stepwise
so that the difference between the two frequencies 50 and 51 is not
less than a given threshold. It may also happen that the hydraulic
resonant frequency 50 cannot be varied over a range of frequencies
that is great as the range over which the mechanical resonant
frequency 51 can be varied. Under such circumstances, it is also
possible, as shown in FIG. 7C, to implement an almost instantaneous
change from a hydraulic resonant frequency 50 that is well above
the mechanical resonant frequency 51 to a hydraulic resonant
frequency 50 that is well below the mechanical resonant frequency
51 (or vice versa). Coincidence between the resonant frequencies
takes place only momentarily and does not lead to dangerous
resonance. Finally, it is also possible to combine gradual
variations in the hydraulic resonant frequency 50 with abrupt
changes, as shown in FIG. 7D.
[0051] The support structure of the rocket engine may also present
a plurality of variable mechanical resonant frequencies, just as
each feed circuit may present a plurality of hydraulic resonant
frequencies. Under such circumstances, controlling the volume of
gas in the feed circuit solely for the purpose of maintaining the
difference between the hydraulic resonant frequency and the
mechanical resonant frequency to a value greater than a
predetermined threshold might not be adequate. In at least one
alternative, the volume of gas may be controlled so as to maximize
a function of differences between a plurality of pairs respectively
of a hydraulic resonant frequency of the feed circuit and of a
mechanical resonant frequency of the structure.
[0052] Thus, in a first example in which the feed circuit has two
variable hydraulic resonant frequencies, namely a higher hydraulic
resonant frequency f.sub.h,high and a lower hydraulic resonant
frequency f.sub.h,low, and the structure presents a variable
mechanical resonant frequency f.sub.s, the function that is to be
maximized R.sub.opt may satisfy the following equation:
R opt = min ( f h , high - f s f s , f h , low - f s f s )
##EQU00001##
[0053] This function may be a function that is weighted with one or
more weighting coefficients. Thus, in a second example in which the
feed circuit presents two variable hydraulic resonant frequencies,
namely a high hydraulic resonant frequency f.sub.h,high and a low
hydraulic resonant frequency f.sub.h,low, and the structure
presents two mechanical resonant modes, with a first mode
mechanical resonant frequency f.sub.s,1 and a second mode
mechanical resonant frequency f.sub.s,2, the function R.sub.opt for
maximizing may satisfy the following equations:
R opt , 1 = min ( f h , high - f s , 1 f s , 1 , f h , low - f s ,
1 f s , 1 ) ##EQU00002## R opt , 2 = min ( f h , high - f s , 2 f s
, 2 , f h , low - f s , 2 f s , 2 ) ##EQU00002.2## R opt = min ( R
opt , 1 , x 1 , 2 R opt , 2 ) ##EQU00002.3##
in which x.sub.1,2 represents a weighting coefficient for the
second mechanical resonance mode of the structure.
[0054] Although the present invention is described above with
reference to specific embodiments, it is clear that other
modifications and changes may be made to those embodiments without
going beyond the general scope of the invention as defined by the
claims. In particular, individual characteristics of the various
embodiments shown may be combined in additional embodiments.
Consequently, the description and the drawings should be considered
in an illustrative sense rather than in a restrictive sense.
* * * * *