This week's space
mission, GSAT-9 or the South Asia Satellite, will carry a new feature that will
eventually make advanced Indian spacecraft far lighter. It will even lower the
cost of launches tangibly in the near future.
The 2,195-kg GSAT-9,
due to take off on a GSLV rocket on May 5, carries an electric propulsion or EP
system. The hardware is a first on an Indian spacecraft.
M.Annadurai, Director
of the ISRO Satellite Centre, Bengaluru, explained its immediate and potential
benefits: the satellite will be flying with around 80 kg of chemical fuel - or
just about 25% of what it would have otherwise carried. Managing it for more
than a decade in orbit will become cost efficient.
In the long run, with
the crucial weight factor coming down later even for sophisticated satellites,
Indian Space Research Organisation can launch them on its upcoming heavy
rockets instead of sending them to space on costly foreign boosters. Shortly,
its own vehicle GSLV MkIII is due for its full test flight.
Dr. Annadurai told The
Hindu that GSAT-9's EPS would be used to keep its functions going when
it reaches its final slot - which is roughly about two weeks after launch - and
throughout its lifetime.
Normally the 2,000-kg
class INSAT/GSAT communication satellites take 200-300 kg of chemical
propellants with them to space. The fuel is needed to keep them working in
space, 36,000 km away, for 12 to 15 years.
Dr. Annadurai said,
"In this mission, we are trying EPS in a small way as a technology
demonstrator. Now we have put a xenon-based EP primarily for in-orbit functions
of the spacecraft. In the long run, it will be very efficient in correcting the
[initial] transfer orbit after launch."
He said that the space
agency normally uses up 25-30 kg of fuel on the satellite each year to maintain
its functions and orbit position. An EP system would vastly bring this amount
down.
Next big trend
A xenon based EPS can
be five to six times more efficient than chemical-based propulsion on
spacecraft and has many uses, according to Dr Annadurai, whose centre assembles
all Indian spacecraft. A 3,500-kg EPS-based satellite, for example, can do the
work of a conventional spacecraft weighing 5,000 kg, but cost far less.
"One day, we
should be able to launch a 5-tonne equivalent spacecraft - but weighing less
than it - on our own GSLV [MkIII.] We are not yet there," he said.
All this is on the
way, may be in around three years. GSAT-20 is planned as the first fully
EPS-enabled satellite; its features were not immediately available. ISAC and
the Kerala-based Liquid Propulsion Systems Centre are lead centres in developing
it.
A trend that started
about four years back, EPS is expected to drive half of all new spacecraft by
2020. For Space-dependent sectors across the globe, the economic benefits of EP
systems are said to be immense. Currently government-owned and private space
players agencies are said to be scrambling to make space missions 30 per cent
cheaper than now - by lowering the per-kg cost of lifting payloads to specific
distances.
A satellite's
orientation can be maintained by momentum wheels supplemented by magnetic
torques and thrusters. Ion propulsion systems, are being used increasingly for
station-keeping. For the first time, ISRO, had used electric propulsion for its
GSAT-4 satellite. 2 indigenously developed and 2 imported SPT (stationary
plasma thruster) has flown on board GSAT-4 to cater for "North South"
station keeping operations. Since less than 5 m/s per year delta velocity needs
to be imparted for east–west station keeping (EWSK), EPS system usage is not
advantageous for EWSK, as overheads will negate the benefits. Similarly, use of
EPS systems for orbit raising involves months of continuous operation and a
very long wait to reach GSO, nullifying the advantage. However, this could be a
backup option for conventional chemical propulsion.
Electric propulsion (EP)
offers a cost effective and sound engineering solution for space applications.
Use of high performance electric propulsion system (EPS) will result into
reduced chemical propellant and tankage requirements, in exchange for
significant usage of power. Chemical rocket engines, like those on the lower
stages of GSLV and PSLV, work by burning two gases to create heat, which causes
the gases to expand and exit the engine through a nozzle. These exiting gases
produce thrust which lifts the rocket. Instead of relying only on the energy stored in the
propellants, if we add external energy using electricity, we can increase the
temperature of the gases and thus create more thrust per pound of fuel.
This is the basic concept of an electric propulsion or EP. EP provides much
lower thrust compared to a chemical rockets but they provide very high specific
impulse. This in effect means that though EP must burn for longer durations
compared to a chemical rocket to achieve desired thrust, it consumes very less
fuel because of higher specific impulse.
EP systems fall into three
major categories: (a) electrostatic propulsion, (b) electro thermal propulsion,
and (c) electromagnetic propulsion. Four components are needed to make a
complete electric propulsion system: a power source, a power processing unit
(PPU), a propellant management system (PMS), and a control computer. The power
source can be any source of electrical power, but solar and nuclear are the
primary options. A solar electric propulsion system (SEP) uses sunlight and
solar cells for power generation. A nuclear electric propulsion system (NEP)
uses a nuclear heat source coupled to an electric generator. The PPU converts
the electrical power generated by the power source into the power required by
each component of the Hall thruster. It generates the high voltages required by
the Hall thruster channel and the high currents required for the hollow
cathode. The PMS controls the propellant flow from the propellant tank to the
thruster and hollow cathode. Modern PMS units have evolved to a level of
sophisticated design that no longer requires moving parts. The control computer
controls and monitors system performance. The Hall thruster then processes the
propellant and power to perform work. Hall thrusters use inert gas as
propellant. The thrust is generated from the force that the propellant ions
impart to the electron cloud inside the thruster.
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