Electric Propulsion in GSAT-4

[Chota]

After a geostationary satellite like INSAT is placed in its orbit, it might experience orbital perturbations because of the combined gravitational attractions of the Sun and Moon. A satellite must remain within its prescribed boundaries to satisfy its mission requirements. To keep a satellite within its equatorial and longitude planes, “North-South" and "East-West" station-keeping maneuvers are performed weekly. "North-South" maneuvers accounts for 95% of total station-keeping propellant consumption.

A satellite's orientation can be maintained by momentum wheels supplemented by magnetic torquers and thrusters. Ion propulsion systems, are being used increasingly for station-keeping. For the first time, ISRO, is using electric propulsion for its GSAT-4 satellite. 2 indigenously developed and 2 imported SPT (stationary plasma thruster) will be 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) electrothermal propulsion, and (c) electromagnetic propulsion. GSAT-4 employs electromagnetic propulsion and uses Hall Effect thrusters or stationary plasma thruster (SPT) in particular.

Soviet Union has done pioneering research work on Hall thrusters. Soviet Union developed two types of Hall thrusters; stationary plasma thruster (SPT) and the anode layer thruster (ALT). They have been using SPT's on their satellites since 1972. A Hall effect thruster was also used by the European SMART-1 probe.

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.

GSAT-4, envisaged as a technology demonstrator, carries a communication payload consisting of a multi-beam Ka-band bent pipe, regenerative transponder and a navigation payload in C, L1 and L5 bands. GSAT-4 having propulsion system with four stationary plasma thrusters, Bus Management Unit (BMU), miniaturised dynamically tuned gyros, 36 AH Lithium ion battery, 70 V bus for Ka band TWTAs, on-board structural dynamic vibration beam accelerometer, are some of the new technologies developed for the mission. The satellite weighs around 2200 kg and has a payload power of 1600W. GSAT-4 will be positioned at 82 deg East longitude.

~Chota

 

[Rage]

Excellent article, Chota. Thank you.

A few questions:

• What inert propellant is planned to be used at the anodes and cathodes of the G-4's Hall effect thrusters? Is something like liquid bismuth being considered for its low partial pressure / high atomic mass?
• What would the approx. speed of average exhaust acceleration be? Consequently, what high specific impulse / low-thrust ratio, in terms of time frames are we looking at?
• Are technologies being developed to mitigate energy inefficiencies via non-thrust producing discharge ( I presume about 28-30% or more of the discharge is an electron current which produces zero thrust)?
• Have any particular issues been encountered with respect to large plume divergence?

 

[Chota]

Inert gas xenon is used as a propellant

Hall thrusters have very hight specific impulse between 1,200-1,800 seconds

However they produce very very low thrust of less than 1 pound. This means that they need to be fired for weeks and months to achieve the desired speed of 50K KM per sec

 

[Rage]

Inert gas xenon is used as a propellant

Hall thrusters have very hight specific impulse between 1,200-1,800 seconds

However they produce very very low thrust of less than 1 pound. This means that they need to be fired for weeks and months to achieve the desired speed of 50K KM per sec

Thanks Chota. I'm guessing the exact specifications and the other questions are aspects to which answers you cannot divulge.

My own understanding is that xenon is not an efficient propellant to use in Hall thrusters. The expense notwithstanding, large pump-throughputs are required to constantly remove the influx from an inert vacuum chamber.

Bismuth is much cheaper, and is as well condensible to free-flow ions on vacuum walls. Meaning that pump-thorughputs would only be required to be fast enough to keep pace with cathode mass-flow and other "chamber leaks".

 

[Akash]

chota, your article is very educational. I was wondering where you got the images and information from. I am not disputing it, I was just wondering if this could be added to wikipedia. This would give more people access to the work of ISRO. What say you?

 

[Chota]

The below image (In my article) is from ISRO website..The above one with 2 "Stationary Plasma Thruster" is from ISRO research paper. The article is prepared after refering lots of other articles on the subject

There is already a Wiki article "http://en.wikipedia.org/wiki/Electric_propulsion" with references

 

[plugwater]

The thruster that fell into sea

Amid the din of big rockets and cryogenics, a small but very significant step into space, which plunged into the sea along with the GSat-4 satellite, went completely unnoticed.

Even its mention in the brochures and other publicity literature was so unobtrusive and couched in jargon that perhaps an enthusiast would probably have glossed over its significance.

We are speaking of the 'plasma thrusters'.

Fuel is the key

First a little background. A satellite is put in a particular orbit the choice of which largely depends upon what the satellite is meant to do. In that orbit, it keeps circling Earth at a particular 'orbital velocity', which is a function of Earth's gravity acting upon it. But, since in the cosmos a body is acted upon by a number of gravitational forces, a body like the satellite, often swerves from its orbit and could either crash into Earth's atmosphere or slowly spin away into space.

In order to prevent a satellite from thus getting lost, small engines are fitted in and these can be fired by signals from Earth. When a satellite begins to go astray, its trackers on Earth fire one of these engines to nudge it back in line. But once these engines run out of fuel, the trackers can't do this and the satellite goes out of control. Therefore, the fuel these engines contained determines the life of the satellite.

Typically, in a satellite, half the space is occupied by these engines, leaving only the rest for equipment such as transponders or cameras.

The GSat-4 was a little different. Instead of conventional chemical engines, it had four 'plasma thrusters.' Because of this, the life of the satellite would have been seven years, instead of 4-5. At optimum use, these plasma thrusters could enhance the life of a satellite to even 15 years.

A plasma thruster is an engine that uses the discharge of plasma to propel an object. Plasma is a gas in which some electrons have been ripped off their atoms by the application of external energy. These electrons and the (remainder) ions co-exist, and this state is often referred to as the fourth state of matter, after solid, liquid and gas. Plasma exists everywhere. The sun, for instance, is a huge chunk of plasma.

Plasma, electrically charged gas, is influenced by magnetic field. In a plasma thruster (to put it in very simple terms), you create a magnetic field, with the help of which you can direct a jet of plasma out through a nozzle. A thrust, in the opposite direction, results.

Plasma thrusters are nothing new. They have been used off and on, even as early as the 1960s. There seems to be some resurgence in interest in them now. However, the GSat-4 was the first instance of them being used in India.

4 thrusters

The GSat-4 satellite had four of these thrusters – two made by Russians and two 'made in India'.

Satellites launched by ISRO in future may be expected to use plasma thrusters for maintaining 'attitude', or orientation. These thrusters last much longer than the chemical rockets used today, as they are powered by electricity that the solar panels generate from sunlight. Consequently, not only will the satellites live long. Also, more on-board space will be freed for instruments.

The GSat-4 would have been a good learning experience. Alas, it's gone!

http://www.thehindubusinessline.com/2010/04/25/stories/2010042551170400.htm