Design of The Hermes Orbital Flight Controller

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DESIGN OF THE HERMES ORBITAL FLIGHT CONTROLLER E. Gottzein*, K. Janschek*, W. Oesterlin* and J. Colrat** *,\I('.'I(' lvhlll ill-H iilko,,~HIIII/l1I (;lIIhH, Sp"(1' CII/lllllllllimlillll.' 1I1111 Pm/lld,ill ll SY.,II'III,', f) r/)(Irlllll'll I }\1'/ / , P .(). HIlX SO/ / (,1), SIJ()(! ,\llill(h" 11 so, FU(;

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Abstract: This paper gives a short overview of the HERMES Spaceplane Orbital Flight Control system as it is presently under development. After a short outline of system aspects, spaceplane configuration and Guidance- Navigation-Control (GNC) related mission aspects the HERMES Orbital Flight Controller (OFC) functional and operational architecture is presented with special emphasis on hierarchical and modular design aspects. A selected number of HERMES Spaceplane Orbital Flight Control topics are addressed and their technical solutions are outlined in principle. Simulation results for some standard control tasks show the performance capabilities of the current OFC definition. Keywords: HERMES Spaceplane, Orbital Flight Control , Attitude Control, Orbit Control , Hierarchical Control

1.

INTRODUCTION

HERMES is a European Space Agency (ESA) programme, as a component of European orbital infrastructure joined to the ARIANE 5 launcher and to the COLUMBUS programme consisting of orbital platforms[l]. HERMES Spaceplane as a component of the HERMES system is to be seen as the European entry into autonomous manned spaceflight. In this scenario it will perform servicing operations related to COLUMBUS elements (Man-Tended Free Flyer, MTFF) incorporating automatic rendezvous &docking [2]. The reusable part of the spaceplane will perform atmospheric reentry and horizontal landing. The HERMES Orbital Guidance-Navigation-Control (GNC) functions support all HERMES operations during exo-atmospheric flight. The development of the GNC functions are divided into separate development tasks concerning Guidance, Navigation and Control functional definition and relevant subsystems (e.g . Propulsion, Functional Electronic, On-Board Software etc.). The main emphasis of this paper is directed towards an overview of the ORBITAL FLIGHT CONTROL Function. Detailed studies on this subject started at MBB in 1986 during Phase B of the HERMES program investigating baseline concepts, e.g. [3]. In the frame of the current Phase C development, which started in June 1988, the Orbital Flight Controller (OFC) algorithms will be specified until the beginning of 1991.

The OFC development at the current status comprises • OFC architectural design • Control algorithm concepts • basic algorithmic solutions for specific topics and will result in • Detailed design of OFC algorithms • Detailed performance, failure and sensitivity analysis • Specification of related OFC software. In the course of qualification tests on system level MBB will be involved in validation activities concerning the OFC-function including related hardware and software components. Orbital Flight Control for comparable scenarios has been realized up to now only for the U.S. Space Shuttle (l-st launch 1981) and the U.S.S.R. Buran (1988). An important overview on the heterogeneity of problems arising in the course of realization of such a complex control system can be found in the Shuttle Orbital Flight Control lessons learned in [4].

One essential message from those lessons learned is also valid for the HERMES Spaceplane : "The Space Shuttle system has many complex and interacting pieces of hardware, each with its own set of performance specifications. In the Orbiter component development and test cycle, and some-times during in-flight tests, elements were found which did not perform according to original expectation. The software for orbit control is designed with predicted orbiter dynamics in mind, with some performance and safety margins to accomodate uncertainty. To maintain conservatively large margins, several significant software changes resulted from late revisions in expected

E. GO llLein et af.

274

hardware performance or constraints" ... In some cases of hardware deficiencies "software modifications adequately overcame the hardware problems." A second source for late Shuttle software revisions was due to late changes and extensions of the Shuttle operational envelope, sometimes originated only from in-flight shortcomings and deficiencies. These experiences point out the importance of an efficient OFC development l ife cycl e beside the "pure" technical/scientific research & development tasks to be performed within such a challenging program, in order to cope efficiently with OFC software adaptation and redefinition. Having in mind those lessons and utilizing the experiences gained at MBB in the field of attitude and orbit control in the course of many space programs, some advanced technologies will be applied in the development of the HERMES Orbital Flight Controller. A main design objective i s directed to a strictly hierarchical and modular design of both • OFC architecture • OFC modes in order to facilitate transparency at all development stages, up to in-flight operation. Some advanced issues optimizing the technical performance of the OFC concern among others: • Optimum jet selection algorithms minimizing fuel consumption and jet duty cycles and supporting functional redundancy of jet assembly • Failure Detection, Isolation &Recovery (FDIR) techniques w.r.t. propulsion failures. To meet the requirements for an efficient OFC development life cycle an Integrated OFC Design is envisaged incorporating • Development of OFC algorithms ( design, analysis & simulation) in an on-board GNCcomputer like software environment based on ADA programming language • OFC software specification for on-board GNCsoftware design & integration by an ADA Pseudo Design Language approach based on the ADA simulation code of the OFC algorithms • Standardized and automatic procedures concerning OFC algorithm development (software tools, code generators, common data bases).

2. HERMES SYSTEM ASPECTS The HERMES spacep1ane is developed with Aerospatia1e as industrial prime with delegation to AMDBA (Av ions Marce1 Dassau1t - Breguet Aviation) for aeronautics. The spacep1ane components are divided up into a lot of subsystems whose responsibility is widely shared through European space oriented companies. Among system responsibilities, MBB has been awarded the function of In-Orbit Control during the orbital phase. To be realized, this function needs the involvement of some subsystems of the spacep1ane (see configuration). The main goal of this activity is: - to establish a functional scheme and algorithms needed to perform In-Orbit Control and to derive performances, - to settle requirements on subsystems to achieve the foreseen performances, - to establish the verification plan for the function, - to write algorithmic specifications for the Orbital Flight Controller (OFC) software, to test the functional adequacy of the software according to verification plan.

This task is embodied in the overall development of the system which can be illustrated by the following global features: - studies conducting to requirements on subsystems and to a verification logic for the system - development of subsystem and verification at subsystem level - integration and tests of subsystems at system level on specific benches and models - qualification tests (including flights) and achievement of verification process. The foreseen planning is sketched in Fig. 1 (in a simplified form): ACTlVTTY~

89

90

I ShJdies

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91

92

I 93

94

95

96

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97

98

99

;-~~~+'~IB

Su~ms Development

Integration & Validation Tests

Ground Qualification Tests Flight Qualification Tests

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FIG . 1

HERMES Space plane Development Planning

3. SPACEPLANE CONFIGURATION The HERMES Spacep1ane (S/P) configuration results from a wide optimization process, which takes into account: - The overall launched mass by Ariane 5 which is restricted (order of magnitude 23 t) although performance improvement - The crew limited to three astronauts: 1 commander, 1 pilot, 1 engineer (experiment specialist) - The up10ad and down10ad capacity which is different: some cargo might not be retrieved - The need for an independent safeguard means for the most hazardous phases: the crew escape module The design is mainly suited for the basic mission: to serve the MTFF (Man Tended Free Flyer, a component of European orbital infrastructure) with spares and scientific experiments. From that, after some iterations, concept appeared:

a two bodies

• a GLIDER especially designed for reentry, including elements to be retrieved: crew and the crew escape module, a part of payload, living quarters, reentry and expensive equipments. • a RESOURCE MODULE (MRH) abandoned before reentry including elements of less interest and useless for reentry, waste, empty tanks, worn-out racks, etc. This module (MRH) reuses the free volume (with some lengthening) of the adapter connecting the spacep1ane itself and Ariane 5. This concept is still subject of improvements, and Fig. 2 shows the status at the end 88.

HERi\l ES O rbita l

F li ~ ht

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,

I,

r--

23000 42000 = 315000 = 340000 =

=

[kg) [kgm ..... 2) [kgm ..... 2) [kgm ..... 2)

11. REENTRY Configuration

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o Glider o incl. fuel & payload

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m boc Iyy Izz

,",ERNES SPAC E PL ANE CCNF I GURAT iON

= 14500 27000 = 200000 = 215000

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[kg) [kgm ..... 2) [kgm ..... 2) [kgm ..... 2)

THRUSTERS L Ay - cu r

FIG. 2

HERMES Spaceplane Configuration

The design has been driven according to some rules to achieve a good level of reliability and a high level of safety. One of them is the FO/FS (Fail Operational/Fail Safe) concept: - for every failure configuration duely identif ied as catastrophic (e.g.entailing death for crew), duplicate items are mandatory. - for failure configuration leading to the loss of mission with a too high level of probability, it is requested to duplicate items . These rules have led to an appropriate architecture of subsystems. Two of them, "Functional Electronics" and "Propulsion", are concerned by the "In-Orbit Control" function of the spaceplane . • Functional Electronics: this subsystem provides means for: - getting data from outside environment : inertial platforms, star sensors, G.P.S., Rendez-vous specific sensors .•. - computing orders to actuators and managing information exchange. • Propulsion: this subsystem provides forces and torques for controlling the spaceplane. The means are multiple according to requirements of the mission: for large orbital transfers : two gimballed engines of 27.5 kN, on the resource module MRH - for rotation & translation control (during orbital phase) • in the forward part of the spaceplane: 8 bi propellant jets of 20 N each • in the rear part of the resource module: 8 bipropellant jets of 20 N each • in the rear part of the resource module: 8 cold gas jets of 20 N each, which are used in place of hot gas near the station to avoid pollution and thermal effect of plume impingement. - for attitude control (during early reentry): 8 jets of 400 N on the rear part of the spaceplane glider for delivering torques in pitch, yaw and roll until aerodynamic control surfaces are fully operant.

for redundancy of the 27.5 kN engines , two 400 N jets on the resource module to allow deorbitating in a safeguard mode. 4. MISSION ASPECTS HERMES mission is divided into three phases: • Launch Phase, which consists of the flight of Ariane 5 mated with the spaceplane on the top of cryogenic stage, and lasts until reaching a transfer orbit (perigee: 100 km altitude, apogee between 200 km and 450 km altitude). • Orbital Phase, which consists of flight in near vacuum to serve the space station (MTFF) and implies a rendez-vous with it. • Reentry Phase, which begins conventionally at the altitude of 120 km and leads the spaceplane onto a runway after encountering high thermal loads in the hypersonic flight. The first and the last phases are necessary phases, but the phase which justifies the use of HERMES is the second. It consists in a lot of subphases. For example, the basic design mission for MTFF servicing may include: - perigee raising, to avoid atmospher ic drag adjustment of semi -major axis, to overtake the station in due time - orbital corrections - Rendez-vous: this phase may be subdivided in more elementary trajectories: • homing, to reach the same orbit as the station and a waiting point (for crew's rest for instance), • station closing , without special sensor from 1000 m to 100 m, • proximity operations, with spec ific sensors down to docking, • docked phase, with MTFF and control of the composite (to be confirmed) , • undocking and removal. - phasing for reentry, to prepare the choice of the landing site - deorbiting and pre-reentry orbital arc until the altitude of 120 km is reached.

276

L C;ottze in 1'1 al.

The HERMES Mission Profile for servicing the MTFF is given in Table 1. Flight Subphase

Start Event End Event

Start Ti.., Ti.., End [h.m . s.)

Duration [h . m. s . )

Flight Compos i te

lift off H150 sep

0 . 00 . 0 0 . 09.42

00.09.42

Comp 1. Propulsion

H150 sep inj into orbit

0 . 09 . 42 0.14.47

00.05 . 05

Perigee Raising

1st apogee

0.59.10

sma 11

Orift Phase

1st apogee beg homi ng

1.00.00 28.29.00

27.29 . 00

Homing

1s t homo imp . waiting point

28 . 29.00 30 . 00 .00

00.91.00

30.00 . 00 44.56.46

14 .56.46

Wait Time R-V far Clos ing

1st c losi ng imp . lOOm target

44 . 56.46 46 .27 . 46

00 . 91.00

R-V Prox. Ops.

lOOm target dock ing

46 .27.46 47 . 00.00

00.32 . 24

Platform Servici ng

dock ing dedock ing

47 . 00.00 215.01.40

168.01.40

Return Phasing

dedock ing brack i ng imp .

215.01.40 238 . 56.40

23 . 55 . 00

Oeorbiting

braking imp . reentry

238.56 . 40 239.20 . 00

00.23 .20

Atmospheric Phase

pass. 120 km land ing

239 . 20.00 240 . 15.00

00.55.00

TABLE1

;' •.. ORBrTAL '~Ll~~f: ;.CONTROLL~1:i. · OF. .c .•

SIP DYNAMICS & KINEMATICS

FIG . 3 - HERMES GNC ARCHITECTURE

. HERMES MISSION PROFILE FOR SERVICING MTFF

MISSION MANAGEMENT GUIDANCE

MMI

5. OFC FUNCTIONAL ARCHITECTURE The HERMES Orbital Flight Controller (OFC) has to perform orbit and attitude control of the HERMES spaceplane for the complete orbital phase. The OFC is imbedded within the complete HERMES GNC System as sketched in Fig . 3. The main OFC operational interfaces are existent between OFC and • • • • •

Guidance Man-Machine Interface (HMI) Mission Management Navigation Propulsion

Two basic types of propulsion systems are available • Main Propulsion System (MPS) consisting of two 27.5 kN gimballed engines located on the separable HERMES Resources Module (MRH) on the rear spaceplane mainframe (used for orbit injection, orbit correction and deorbit maneuver) . • Auxiliary Propulsion System (APS) consisting of a set of 400 N, 20 N bipropellant and 20 N cold gas jets, (used for on-orbit operations, back-up deorbit and reentry) To meet all the requirements for the complex HERMES scenario a modular and hierarchical architecture is developed for the OFC . A simplified blockdiagram of the OFC is given in Figure 4.

PROPULSION

NAVlGATlON

NAVlGATlON

FIG.4 - HERMES OFC ARCHITECTURE

Due to the different types of propulsion systems (MPS, APS) two respective control systems are required : • MPS - Thrust Vector Control System (MPS - TVCS) • APS - Control System (APS - CS)

Ht::R:'.IES Orbit ,d Flif{ht COlltroller

Each of these propulsion related control is hierarchically subdivided into a

systems

'2ii

As an example the following Figure 5 points out in principle the APS Control System (APS-CS) architecture for the HERMES orbital rotation and translation control.

• medium-level Maneuver Control System (MCS) operating at low sampling rates as far as possible • low-level Feedback Control System (FBS) operating at high sampling rates Actual SIP rotation & translation control will be performed by the low-level FBS's whereas optimal maneuver related operations will be performed by the medium-level MCS's. The overall coordination, mode sequencing, failure management, switching and blending between APS and MPS propulsion etc. will be executed at the highest OFC-level, called

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,

• OFC-Maneuver Management System (OFC-MMS)

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Thus it is possible to define very transparent interface levels to OFC external world: - maneuver control interface located at the OFC-MMS level will perform all I/O to Mission Management • Man-Machine Interface (pilots) . Guidance

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• &

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FIG. 5 - APS CONTROL SYSTEM ARCHITECTURE

- sensor/actuator interface located (functionally) at the feedback control levels will perform all fast I/O for closed loop control, i.e. I/O to Navigation (Inertial Measurement Unit, RVD sensor assembly etc.) Propulsion (main engine, gimbal mechanism, jets) The concentration of all external command & control I/O exclusively at the highest OFC level (MMS) enables an effective management for automatic (autonomous) on-orbit operations. The related tasks of those 3 different OFC levels mentioned above can be briefly summarized as fo llows: • Maneuver Management Level - command & control I/O handling - mode sequencing - failure management - switching and blending of APS/MPS

The main modules of the APS - Control System clude:

in-

• APS Maneuver Control System (APS - MCS) - I/O - data handling with the OFC-Maneuver Management System (OFC - MMS) - Determination of optimized APS maneuver trajectories including optimum feedforward jet selection - automatic maneuver management, generation and computation of the essential reference state variables for optimum tracking capability (feedforward control) - OFC related Failure Detection/Isolation & Recovery (FDIR)

• Maneuver Control Level - maneuver optimization - coordinate transformation - feedforward control - Failure Detection, Isolation &Recovery (FDIR)

• APS Feedback Control System (APS - FBS) - APS rotation/translation feedback controller including related dynamic filters taking into account fuel sloshing, flexible structure dynamics etc.

• Feedback Control Level - feedback SIP - stabilization - reduction of disturbance effects - baseline dynamic performance

- feedback jet selection module selecting on-line the optimum jets w.r.t. minimum fuel consumption.

The main emphasis on OFC architectural design has been directed toward - maximum commonality between the different control systems (APS, MPS) - modular structure - transparent definition of interfaces to OFC external world (pilots, guidance, mission management, navigation, actuators) in order to facilitate an easy OFC software plementation, check out and reconfiguration.

:

~ FEEDBACK CONTROL SYSTEM ~;,"_

<

im-

- modulator system module calculating pulse width/frequency for each activated jet From the modulator system the Propulsion System is fed with on/off commands for the selected jets. The dynamic/kinematic response of the spaceplane due to control and disturbance torques and forces is measured by the sensors of the Navigation System which also provides estimates for the control variables. Attitude control is performed by a feedback of SIP rates and attitude in quaternion representation, translation control is performed open-loop or in particular during RVD-phase by feedback of SIP velocity and (relative) position.

278

E. Gutlzeill rt al.

6. OFC OPERATIONAL ARCHITECTURE From the functional point of view the in-orbit control is performed by activation of a certain number of required modules of the APS-CS and the MPS-TVCS. Which of those modules are activated, in which sequence, and at which frequency they are required, depends on the individual control task to be performed at that time. The proper definition of mission related standard control tasks enables a direct assignment of those tasks to specific OFC modes. So, from the operational point of view the in-orbit handling of the HERMES spaceplane will be performed by activation of specific OFC modes which - individually are well designed for the performance of a specific task, as rate damping, attitude hold, attitude change, rendez-vous, manual control by side-sticks, etc. - in total cover the complete HERMES mission scenario as orbit injection, phasing, HERMES/ MTFF rendez-vous/docking/de-docking & servicing scenario, de-orbit, etc. and where safety and back-up modes comply with the stringent requirements of manned space flight. Taking into account the HERMES specific aspects of - complex mission scenario with . automatic/manual operation . large number of possible/necessary modes - long term program evolution with a stringent need of adaptability and reconfiguration of OFC algorithms, parameters and modes a straightforward extension of the strictly hierarchical OFC functional architecture is also applied to the operational architecture defining the OFC modes. Three operational levels of OFC-modes have been established: • Elementary Modes (ELM) representing basic modes, which activate certain APS-CS (MPS-TVCS) modules for the purpose of basic control tasks, e .g. attitude hold, rotation, velocity control, etc. The ELM's are subdivided into rotational, translational, and main engine control modes. • Medium Level Modes (MLM) which actually are invoked by the pilots, Guidance and Mission Management. They consist only of certain combinations/sequences of ELM's (rotation, translation, main engine control) and can easily be redefined by changing the combinations and/or sequences of ELM's. • High Level Modes (HLM) which is the highest level of mode hierarchy where the decision is made between three basic types of spaceplane piloting. - Manual Mode (HAM) where the spaceplane is controlled directly by the pilots using side sticks related to all six degrees of freedom (translation and rotation) - Se.i Automatic Mode (SAM) where the spaceplane is controlled indirectly by the pilots selecting Medium Level Modes on the Flight Control Panel, but where the according maneuvers are performed automatically. After mode activation flight crew engagement is mainly related to supervision with full override capability.

- Auto Guidance Mode (AGM) where the spaceplane is controlled in a fully automatic way from the Guidance System with regard to long term spaceplane operation according to the mission flight plan. Flight crew engagement is reduced to supervision with partial override capability. Table 2 gives a rough summary of the OFC hierarchy and architecture . The proposed hierarchy incorporates the following advantages: • simplification for definition of more complex modes (MLM) because only clearly defined basic modules (ELM) can be used • transparency of individual mode architecture • definition/development of ELM's can be performed at an early program stage, no precise knowledge of exact mission scenario is needed • final definition/development of MLM's can be performed at a later program stage, with already tested/qualified ELM's • easy redefinition/reconfiguration of MLM's, which actually determine the SIP operational envelope • modular structure, important for S/W and system development, test and qualification A summary of the current OFC Mode definition, structured into ELM, MLM, and HLM's is given in Table 3.

HIERARCHY

I

H L M

Mission Management'j ( Orbital Guidance

High level Mod..

AcnON I INFORM.

EXAMPLE

ARCHrrECTURE

MANUAL MODE

Long Term

_ SEMI - AUTOMAnc MODE _ AUTO - GUIDANCE MODE

--------+ --------- ------------------------ ---------- ----- --I - MANUAL CONTROL

MLM

-

I

( ?FC -

Medium level

MMS

~

Mod ••

JI

BASIC AUTOCONTROl

Medium Term

-ATTITUDE CHANGE - GUIDED CONTROL

--------- ---------L------------------------l--------------- ---

If"PS - MCS J I -

ELM

_

Elementary Mod ••

(

APS _I FBS

JI

UNCO'lIl10llED TRANSLAT

-

RATE DAMPING ATTITUDE HOLD

--

SlEWlNG MANEUVER GIMBALLED ENGINE CONTRO

Short Term

I TABLE 2

Hierarchy and Architecture of OFC Modes

El_nhry Modes (EU4)

Med i u. level Modes (1t.M)

rCR! • Bas i c Autocontrol

• Uncontrolled Rotat i on • Rite Duping RaP

• Attitude Change

• Rotet ion • Slewtng Maneuver • Attitude Kold

• Gravity Gradient • Tracking

ROT SLM "TH

r! BARBC

• Barbecue

High level Modes (HU4)

TCD • Manual Mode ATCHG • Ss; Auto.t 1c Mode • ALlto-Gu idance Mode

GRAV I TRACX

liE!

• Att i tude' Position Change

(APCHG)

• Uncontrolled Translation

• RVO-Autocontrol

• Gu i ded Control

~R.ATCO~

GU ICD

• Velocity Control • Gu ided Trans lat ion • Trans lat ion Maneuver • Position Hold

!m! !"'APSOC STVl PDH • APS Orbit Ch.nge

• lII in Eng ine COntro l

(HEC) • 8erthing Control

• fif>S-Thrust Vector Control

• Docked Contro 1

• 6hlballed Eng i ne control

(GEC)

TABLE

3

• Senicing Cont rol

!OOCCD! BERCD SERCD

• Safe JItode Control

(SAFCO)

• Aeentry Contro 1

(REECD)

• Mlnua 1 COntro 1

(MAIICD)

SUMMARY

OF

HERMES

OFC

MODES

HER,\IES Orhital Flight Controller

7. CONTROL TOPICS

(b)

Among a lot of topics, which are important and critical for a successful development of the OFC, some of them, which are specific to control system design aspects, shall be addressed in detai 1. 7.1

FAIL SAFE JET CONFIGURATION

• the SIP should be operational with single fai lure • the SIP should be safe with a double failure This criterion has to be applied also to the Propulsion System. Due to the restrictive mass constraints it is not possible to realize a redundant jet configuration similar to that of the Space Shuttle [4] (clustering in groups of two or three parallel jets). The current baseline for phase Cl considers only 8 x 20 N bipropellant front jets and 8 x 20 N bipropellant aft jets (or exclusively 8 x 20 N cold gas aft jets during docking) for 6 DOF on orbit maneuvers (rendez-vous). For the reentry currently 8 x 400 N jets are to be used for attitude control only. Given this limited number of available jets it is only possible to realize functional redundancy. This can be achieved by skewing the jets in such a manner, that the force & torque capability meets the operational requirements for any combination of two failed jets. Analysis of force & torque capability is performed by application of linear programming [8] whereas the optimization of the jet alignment is based on a stochastic optimization algorithm[10]. This algorithm has proved to be very robust for the present problem which is characterized by strong unsteadinesses of the optimization criterion and restrictive jet alignment constraints due to plume impingement.

7.2 FEEDFORWARD JET SELECTION The optimization of OFC performance w.r.t. orbital maneuvers for given final conditions e.g. - large angle slew

(APS-rotation loops closed) - velocity increment (APC-translation loops open, guidance loop closed) will be performed by application of appropriate jet selection algorithms for generation of feedforward jet switching commands. The criteria to be optimized (minimized) are - fuel consumption - jet duty cycle

(a)

Supervised pseudo-eigenaxis rotation divided into different subphases and on line adaptation of jet burn-time at discrete maneuver instants (analytical solutions applicable for inertially fixed coordinate frames; iterative solution of a single scalar equation for coordinate frames rotating with constant rate; [7]).

7.3 FEEDBACK JET SELECTION

In general the HERMES spaceplane is designed to meet the Fail Operation/Fail Safe (FO/FS) criterion, which means

The optimization algorithm for large angle maneuvers is based on a nonlinear rigid model (gyroscopic coupling, kinematics) results in suboptimal continuous jet burn phases. Two approaches are currently investigation:

279

slew body and time under

Linear Programming jet selection and burn time optimization by a Hamilton-Jacobi approach (iterative, requires numerical integration [6])

A continuous feedback control operation has to be realized by both APS rotation and translation control loops for 2 reasons - compensation of OFC disturbance torques - compensation of OFC model uncertainties control laws

external environmental &forces internal spaceplane from feedforward

Due to the limited number of jets available, which have moreover to meet the FO/FS requirements, it is not possible to arrange the jets in well matched axes decoupled pairs. In contrary each single jet will couple in each axis and the generation of 6 DOF control forces and torques requires an efficient online jet decoupling algorithm (3 DOF torque decoupling in all flight phases, additional 3 DOF force decoupling during rendez-vous and orbit maneuvers). Depending on the mission phase 8 to 16 jets are available for 3/6 DOF control. A tradeoff study [5] has been performed investigating in detail - Linear Programming - Quadratic Optimization (pseudoinverse/singular value decomposition/orthogonal correction) The tradeoff has shown some computational advantages of Quadratic Optimization, but important operational advantages of Linear Programming Jet Selection, which are: - minimum number of active jets used - smaller jet duty cycles - better jet efficiency - smaller fuel consumption (25 % reduction w.r.t. Quadratic Optimization) - advanced flexibility for torque generation options w.r.t. residual or nonresidual forces - advanced flexibility for handling of variable jet configuration (due to jet fa ilures) Therefore the Linear Programming approach has been selected as baseline for implementation in the OFC and a realtime realization is currently under development [9].

7.4 FDIR - FAILURE DETECTION, ISOLATION & RECOVERY The HERMES GNC architecture requires a distributed FDIR concept, where Navigation is responsible for sensor related FDIR, and the OFC is responsible mainly for support to the propulsion related FDIR. The jet and gimballed main engine failure detection and identification will be performed by - propulsion related sensors monitoring the propulsion status - additional OFC based dynamic/static observer models The establishment of observer models with regard to sensor noise, model uncertainties, propulsion deficiencies etc. is currently under progress.

E. (;ott lc illl'l fl l.

8.2 GUIDED CONTROL (GUICO)

The recovery of propulsion failures has a strong interrelation with the feedforward/feedback jet selection algorithms, which have to deal with on-line reconfiguration of jet sets.

This automatic mode is invoked by the Guidance for orbit correction and rendez-vous maneuvers. This flight phase is a dimensioning one for OFC, because during Av-generation the "FINE" control accuracy level is required. One mode option foresees open loop velocity control combined with closed loop attitude control using a set of 10 N jets both for attitude & translation control . This mode will be invoked during rendez-vous homing phase for closing to the station as given in Fig. 7 (from [11]) .

One operational interface between OFC and Navigation is given by the OFC delivery of the estimated control torque/force profile to Navigation, in order to improve sensor failure detection capabil it ies.

8. SIMULATION RESULTS Some simulation results shall demonstrate the dynamic performance capabilities of the current OFC status under various operational conditions . The simulations represent the HERMES Phase B3 Spaceplane status (10N/400N jets for all orbital phases incl . deorbit), as referenced in [3], somewhat different from current Phase Cl baseline in geometry and propulsion configuration . The selected simulation examples include one 3-axes slew maneuver demonstrating the performance of the feedforward jet selection algor ithm and one rendez-vous based transfer orbit maneuver. The simulation models are based on nonlinear rigid body spaceplane dynamics/kinematics , simplified Navigat ion and 100 ms sampling per iod for OFC-algorithms.

,6.v1 = ,6. v2 FIG. 7 -

The feedforward jet selection algorithm used was the one based on Hamilton-Jacobi approach [6] resulting in 6 continuous jet firings to perform the maneuver. It should be noted , that for this maneuver the influence of gyroscopic coupling due to i nertia unsymmetry is very strong, which results in a non constant angular rate during the coast phase for this fuel optimal solution. The relative high fuel consumption is due to the required short maneuver duration (e.g. 1 kg fuel required for a 90 deg slew within 150 sec).

""'B '.0

'.5 '.0

LS LD

0.5 -0.2203

0

A

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0.' 0""1 0.'

1000 500

0. 1

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RVD HOMING REFERENCE

)

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IH(RH(S I

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0.23 m/s

The velocity increment of AV z =0.23 m/s will be generated by a 30 Nz-thrust applied during 147 sec directly as input to the Feedback Jet Selection Algorithm in addition to the control torque commands generated by the closed APS-RC (the detailed OFC definition foresees for Av-generation optimal feedforward jet selection commands, which have not been applied for this simulation example). To show the robustness of the APS-CS a center of gravity (c.o.g.) offset of 0.1 m in all axes w.r.t . real c.o.g. has been used within the jet selection algorithm. The respective time histories are given in Fig. 8. Due to c.o.g. offset a constant disturbance torque is produced during Av-generat ion (constant attitude error after a short transient phase) .

An automatic 90 deg 3-axes slew (spheric diagonal) within a fixed maneuver time of 30 sec using the 400 N jet package is shown in Fig . 6.

:-"::=,,,,- "'j

=

CONFIGURATION

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RVD HOMING MANEUVER 11. REFERENCES

9. CONCLUSION The status presented concerning the definition of the HERMES Orbital Flight Controller shows the available operational and functional capabilities utilizing the OFC hierarchical and modular architecture. This current status incorporates a baseline architectural design, functional definition and well defined low (feedback) level control algorithms, allowing the derivation of baseline performances. Future development steps are directed towards a detailed design of the complete OFC (including performance, failure and sensitivity analysis) in the frame of an Integrated OFC Design approach resulting in ADA-specified OFC algorithms for on-board GNC software design and integration .

10. ABBREVIATIONS Auxiliary Propulsion Subsystem APS Control System APS Feedback Control System APS Maneuver Control System APS Rotation Controller APS Translation Controller Center of Gravity Degree of Freedom Elementary Mode Failure Detection, Isolation &Recovery Fail Operational/Fail Safe High Level Mode Maneuver Control System Medium Level Mode Man Machine Interface MMI Maneuver Management System t-t1S Main Propulsion Subsystem MPS MPS-FBS MPS-Feedback Control System MPS-MCS MPS-Maneuver Control System MPS-TVCS MPS-Thrust Vector Control System HERMES Resources Module MRH Man Tended Free Flyer MTFF Orbital Flight Controller OFC Rendez-vous and Docking RVD Spaceplane SIP Technical Note TN with respect to w.r.t.

APS APS-CS APS-FBS APS-MCS APS-RC APS-TC c.o.g. DOF ELM FDIR FO/FS HLM MCS MLM

[1] Couillard,Ph., Le Programme HERMES. Proceedings of International Symposium on Europe in Space - The Manned Space System-, 25.-29. April 1988, Strasbourg, pp. 55-58, 1988 [2] Marechal,L., DiMauro,F . , Hoge,D., Fehse,W., HERMES-MTFF Rendezvous Strategy and Composite Operations. Proceedings of International Symposium on Europe in Space - The Manned Space System-, 25-29 April 1988, Strasbourg, pp.185-190, 1988 [3] Gottzein,E., Janschek,K . , Oesterlin,W., HERMES Orbital Control: Design to Robustness and Automation. Preprints of SITEF'87, Toulouse, 1987 [4]

Cox,K.J., Hattis,P.D., Shuttle Orbit Flight Control Design Lessons: Direction for Space Station. Proceedings of the IEEE, Vol.75, No.3, pp.336-355, 1987

[5]

HERMES OACS (Orbital Attitude Control System) Final Report Phase B3, MBB-Tech.Rep., H-ST-1-0004-MBB, 1988

[6]

Sieber,G., Metzger.R., Theoretical Baselines for the HERMES Large Angle Rotation Maneuver Function. MBB-Tech.Rep . TN-KTll-78/88, 1988

[7] Oesterlin,W., An Analytical Solution for Time-Variant Eigenaxis Slew Maneuvers. MBB-Tech.Rep. TN-KTll-63/88, 1988 [8]

Janschek,K., Application of Linear Programming for Systematic Jet Selection Problems. MBB-Tech.Rep. AN-RR51-63/87, 1987

[9] Chemnitz,J., Ein Simplex-Algorithmus zur verbrauchsoptimalen Dusenansteuerung. MBB-Tech.Rep. TN-KTll-82/88, 1988 [10] Ruppert,M., Reglersynthese mit Hilfe der mehrgliedrigen Evolutionsstrategie. VDI-Verlag, Dusseldorf, 1982 [11] Colrat,J.,Reference Attitudes of Spaceplane during Main Mission. SE/HTno 388/87,Aerospatiale, 1987