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First
Developmental Test Flight of Geosynchronous Satellite Launch Vehicle (GSLV)
Mission Objective
The Geosynchronous Satellite Launch Vehicle
(GSLV) project was
initiated in 1990 with the objective of acquiring launch capability for
Geo-synchronous satellites. The first flight test, GSLV-D1, is intended to
validate the various systems of the vehicle in an actual flight. Though each of
the subsystems has been tested on ground, it is only through a few developmental
flight tests that the launch vehicle, as a whole, and all the associated ground
systems can be validated. Several performance parameters of propulsion stages,
avionics, control and guidance system, the stage and spacecraft separation
system, will be monitored in flight. The design margins will be more
realistically estimated from the in-flight test of the vehicle.
In the first developmental test flight,
GSLV, will place a 1,540
kg experimental satellite, GSAT-1, in a Geo-synchronous Transfer Orbit (GTO).
GSLV Configuration
In its present configuration,
GSLV, is a three-stage vehicle. It
is 49 m tall and weighs about 401 tonne at lift-off. The vehicle configuration
makes use of several systems that have been flight proven through India's Polar
Satellite Launch Vehicle, PSLV. The first stage solid propellant motor and the
liquid propellant second stage of PSLV have been employed as the core first
stage motor and the second stage of GSLV. The first stage liquid strap-on stages
are also derived from the second stage of PSLV.
GSLV Stages
The first stage of GSLV comprises a solid propellant motor (S125)
and four liquid propellant strap-on motors (L40). S125 stage is 20.3 m long and
2.8 m in diameter. Its motor case is made of high strength steel. It carries 129
tonne of Hydroxyl Terminated Poly Butadiene (HTPB) based solid propellant. The
stage develops about 4700 kilo Newton thrust and burns for 100 seconds. The four
strap-on (L40) stages are 19.70 m long and 2.1 m in diameter and they are
fabricated using aluminum alloy. Each of them is loaded with 40 tonne of
hypergolic propellants, namely, Unsymmetrical Di-Methyl Hydrazine (UDMH) as fuel
and Nitrogen Tetroxide (N2O4) as oxidizer, stored in two tanks mounted in
tandem.
The second stage of GSLV is 11.6 m long and 2.8 m diameter. It is
loaded with 37.5 tonne of UDMH and N2O4 in two compartments of an aluminum alloy
tank separated by a thin metal sheet known as common bulkhead. The engines used
for the strap-on motors and the second stage are similar and employ a turbo-pump
fed engine producing a thrust of about 700 kilo Newton in vacuum. Due to the
larger propellant loading, the strap-on stages burn for about 160 seconds while
the second stage burns for 150 seconds.
The third stage of GSLV uses a Cryogenic Stage (CS) procured from
Glavkosmos, Russia. The stage, that employs liquid hydrogen and liquid oxygen as
fuel and oxidizer respectively, is 8.7 m long and 2.9 m in diameter. Liquid
hydrogen (LH) and liquid oxygen (LOX) are stored in two separate aluminum alloy
tanks connected by an inter-stage structure. With a propellant loading of 12.5
tonne, the stage can burn for a duration of about 750 second producing a nominal
thrust of 75 kilo Newton.
Auxiliary Systems
The different stages of GSLV are connected by inter-stage
structures, which also house the necessary avionics and control systems for
controlling the lower stage till it is separated. The vented inter-stage between
the first and second stage, enables the firing of the second stage even while
the first stage has just completed its thrusting action. This design avoids use
of additional systems needed to provide sufficient acceleration between the time
before the ignition of second stage takes place and sufficient reduction in
velocity of the first stage. The first stage, including the core and four strap
on motors after its function, is separated from the rest of the vehicle using a
flexible linear shaped charge (FLSC) that is severs the connection between first
and second stages.
The vehicle equipment bay housing the vehicle electronic systems
like processors, navigation system, control system, guidance system, telemetry
system, telecommand system, etc, is housed in a truss structure above the
cryogenic stage. The spacecraft is mounted above the equipment bay through a
payload adapter and separation system.
A
heatshield, which is 7.8 m long and 3.4 m in diameter, protects
the vehicle electronics and the spacecraft from the hostile environment during
the ascent flight through the atmosphere. The heatshield is discarded at about
110 km during the second stage propulsion.
The spacecraft is separated by opening the band-clamp joint and
the springs attached within the separation system that provides the required
separation velocity to the satellite. The system is designed to ensure that no
collision occurs between the spent third stage and the spacecraft.
Mission Sequence
The launch of GSLV is conducted from SHAR
Centre, Sriharikota,
about 100 km north of Chennai. The vehicle is launched at an azimuth of 104O. It
takes about 1040 second for the flight from lift-off to the injection of
spacecraft into Geosynchronous Transfer Orbit. About 150 critical events have to
be gone through during the flight, before the satellite is placed in orbit. The
nominal mission sequence is as follows:
On the launch pad, the four liquid propellant (L-40) strap-on
stages are ignited first. The solid propellant core stage, S125, is ignited 4.6
seconds later, after confirming the normal operation of each of the L-40 stages.
The core solid propellant stage burns for 100 seconds and the four L-40
propulsion stages continue to burn upto 160 seconds by which time the vehicle
would have reached an altitude of about 73 km.
The liquid propulsion second stage (GS-2) ignites 1.6 seconds
before the separation of first stage. The second stage carries 37.5 tones of
propellant which it burns in about 150 second. About 100 seconds into the second
stage propulsion, the Heat shield, which protects the spacecraft and equipment
is separated. By this time, the vehicle would have reached an altitude of about
115 km. The separation of the second stage takes place at about 314 seconds from
lift-off at an altitude of about 127 km.
After the separation of GS2 stage, cryogenic stage ignites and
this stage burns for about 710 second. The spacecraft and the equipment bay are
separated at an altitude of 195 km. Before separation, it gives the spacecraft
the required injection velocity of 10.2 km per second to place it in the
Geo-synchronous Tranfer Orbit (GTO), which is a highly elliptical orbit with a
perigee (closest to the earth) of 180 km and an apogee (farthest to the earth)
of 35,975 km. After injection of the spacecraft, the Cryogenic Stage is
passivated by venting all onboard tanks and gas bottles and the stage is
re-oriented so as to avoid any possible collision with the spacecraft.
Avionics
The inertial navigation and guidance system Redundant Strap Down
Inertial Navigation System/Inertial Guidance System (RESINS/(IGS) which is
housed in the equipment bay computes the inertial position and velocity and
guides the vehicle from lift-off to spacecraft injection. The digital auto-pilot
and closed-loop guidance scheme resident in the on-board computer ensure the
required attitude maneuver and guided injection of the spacecraft to the
specified orbit.
The vehicle performance is monitored with extensive
instrumentation. The performance data is transmitted via telemetry systems to
the ground station. In addition to the performance parameters, the inertial
position of the vehicle and its orientation are computed by the vehicle inertial
system and computers which are also transmitted via the telemetry to the ground
stations. A telecommand system is used to terminate the flight, in case the
vehicle deviates from its flight path beyond the specified limits.
A C-band transponder on the vehicle helps in tracking it from
ground based radars. The complete telemetry and tracking coverage of the vehicle
from lift-off to satellite injection will be provided by four ground stations
located at SHAR Centre, Sriharikota, the down range stations at Port Blair,
Brunei and Biak in Indonesia. All these stations are networked with the SHAR
Centre during launch to provide data in real time. The vehicle undergoes checks
at every stage of integration, followed by checks on the integrated vehicle
along with the satellite and a launch rehearsal is also gone through.
New Elements in GSLV compared to PSLV
In addition to the cryogenic stage, the other major new elements
in GSLV, are the liquid strap-on stages, a heat shield with larger diameter than
PSLV (3.4 m compared to 3.2 m in PSLV) and the vented inter-stage between first
and second stage. Mission design and simulations, realization of test and launch
complex facilities including servicing of cryogenic stages, launch hold and
release mechanism, etc, were also involved in GSLV.
General
GSLV consists of hundreds of sub-systems, which are designed,
manufactured, tested and qualified before the integration for launch. Most of
the vehicle hardware like motor cases, inter-stages, heat shield, engine
components, electronic modules are manufactured by the Indian industry. About
150 industries, both public and private sector are involved. The subsystems are
integrated at the various facilities of ISRO and tested before transportation to
SHAR Centre.
Launch Complex
The launch complex at SHAR Centre has facilities for storage,
testing and integration of the various stage elements. The launch complex houses
a 75 m tall Mobile Service Tower inside which the vehicle is integrated. In
addition, the launch complex has extensive network of tracking Radars, the
launch and mission control center, the facilities for spacecraft checkout and
integration.
The launch facilities at SHAR Centre have been suitably modified
to launch both PSLV and GSLV. A Second launch Pad is also now under construction
to enable more frequent launches.
Operationalisation of GSLV
GSLV will be declared operational after two successful
developmental flights. Efforts are already on to improve the payload in GTO up
to 2000 kg and beyond in about 2 or 3 years. Other than GTO missions, GSLV can
also perform mission to LEO and polar missions.
Conclusion
GSLV is the most technologically challenging mission undertaken so
far under the Indian space programme. It is the culmination of efforts of a
large number of scientists, engineers and technicians, over the last ten years.
The mission will herald a significant milestone towards the establishment of
indigenous capability for launching communication satellites like INSAT. Having
already established indigenous capability for launching IRS class of remote
sensing satellites through PSLV, the launch of GSLV will fulfill the vision of
Dr Vikram Sarabhai to make the Indian space programme a self-reliant one, while
tuning it towards national development.
GSAT-1
While the first developmental test flight is primarily intended
for validating the vehicle design and its performance parameters as well as the
associated ground infrastructure, the flight opportunity is also made use of to
place an experimental satellite GSAT-1 weighing about 1540 kg.. GSAT-1 will be
used to prove new spacecraft elements like ten Newton Reaction Control
Thrusters, Fast Recovery Star Sensors and Heat Pipe Radiator Panels to validate
them before using them in the ISRO operational ISRO satellites like IRS and
INSATs. GSAT-1 will also carry two C-band transponders employing 10W Solid State
Power Amplifiers (SSPAs), one C-band transponder using 50 W Travelling Wave Tube
Amplifier (TWTA) and two S-band transponders using 70W TWTA.
GSAT-1 will be used for demonstrating digital audio broadcast,
internet services, compressed digital TV experiments and developmental
communication.
Cryogenic Stage
GSLV employs a cryogenic stage. It is for the first time that a
cryogenic stage is being employed in an ISRO vehicle. The cryogenic stage is
much more efficient and provides more thrust for every kilogram of propellant it
burns compared to solid and earth-storable liquid propellants. Specific impulse
achievable with cryo fluids (liquid hydrogen and liquid oxygen) is of the order
of 450 sec compared to 300 sec of earth storable and solid fuels, giving a
substantial payload advantage; for every one second increase in the specific
impulse, the payload gain is of the order of 10 kg.
However, cryogenic stage is technically very complex system
compared to solid or earth-storable liquid propellant systems due to the use of
propellants at extremely low temperatures and the associated thermal and
material problems. The temperature of Liquid Hydrogen is -253 deg C and that of
liquid oxygen is -195 deg C. The propellants, at these low temperatures, are to
be pumped using turbo pumps running at 42,000 rpm. It also entails complex
ground support systems like propellant storage and fill systems, cryo engine and
stage test facilities, transportation and handling of the cryo fluids and
related safety aspects.
While the initial flights of GSLV will use Russian supplied
cryogenic stage, a project CUSP has already been initiated to develop the stage
indigenously. The first in a series of tests of an indigenous engine developed
under this project was conducted in February 2000 for 15 seconds. Further tests
are planned in the coming months.
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