Tuesday, September 28, 2021

Satellite Launch? Yes it is actually Rocket Science

Satellite Launch? Yes it is actually Rocket Science

Satellites have formed an integral part of the communication   broadcasting, navigation, weather forecasting and remote sensing. Positioning a satellite in its orbit is a highly skilled and technical activity.  This job definitely entails Rocket Science and we detail here the engineering involved in the process. Indian Space Research Organisation has done a commendable job in desigining the launch mission for the various payloads delivered and positioned in their designated orbits. Since 2001, ISRO has commissioned 14GSLV Launch missions of which 10 have been successful. This itself speaks a volume about the engineering capabilities developed by India. We detail the various stages of the launch for an insite into the operation.

The Geosynchronous Satellite Launch Vehicle

Geosynchronous Satellite Launch Vehicle Mark II (GSLV Mk II) is satellite launch vehicle developed by India, which is currently in operation. This fourth generation launch vehicle is a three stage vehicle with four liquid strap-ons. The indigenously developed cryogenic Upper Stage (CUS), which is flight proven, forms the third stage of GSLV Mk II. From January 2014, the vehicle has achieved four consecutive successes.

Vehicle Specifications

Height : 49.13 m
Number of Stages : 3
Lift Off Mass : 414.75 tonnes
First Flight : April 18, 2001

Technical Specifications

  • Payload to GTO: 2,500 kg, GSLV’s primary payloads are INSAT class of communication satellites that operate from Geostationary orbits and hence are placed in Geosynchronous Transfer Orbits by GSLV.
  • Payload to LEO: 5,000 kg, Further, GSLV’s capability of placing up to 5 tonnes in Low Earth Orbits broadens the scope of payloads from heavy satellites to multiple smaller satellites.

Figure : Stages in the launch, Image courtesy ISRO

Strap-on Motors

The four liquid engine strap-ons used in GSLV are heavier derivatives of PSLV’s PS2, and use one Vikas engine each.

Fuel : UDMH + N2O4
Max. Thrust : 680 kN
Burntime : 160 sec

UDMH fuel

UDMH is a derivative of hydrazine and is sometimes referred to as a hydrazine. As a fuel, it is described in specification MIL-PRF-25604 in the United States. UDMH is stable and can be kept loaded in rocket fuel systems for long periods, which makes it appealing for use in many liquid rocket engines, despite its cost.

Unsymmetrical dimethylhydrazine is a chemical compound with the formula H2NN(CH3)2 that is used as a rocket propellant. It is a colorless liquid, with a sharp, fishy, ammonia-like smell typical for organic amines. Samples turn yellowish on exposure to air and absorb oxygen and carbon dioxide. It is miscible with water, ethanol, and kerosene. In concentration between 2.5% and 95% in air, its vapors are flammable. It is not sensitive to shock. Symmetrical dimethylhydrazine, 1,2-dimethylhydrazine is also known but is not as useful.

First Stage: GS1

The first stage of GSLV was also derived from the PSLV’s PS1. The 138 tonne solid rocket motor is augmented by 4 liquid strap-ons.

Engine : S139
Fuel : HTPB
Max. Thrust : 4700 kN
Burntime : 100 sec

HTPB Fuel

Hydroxyl-terminated polybutadiene (HTPB)isanoligomer of butadiene terminated at each end with a hydroxyl functional group. It reacts with isocyanates to form polyurethane polymers.

HTPB is a translucent liquid with a color similar to wax paper and a viscosity similar to corn syrup. The properties vary because HTPB is a mixture rather than a pure compound, and it is manufactured to meet customers’ specific requirements. A typical HTPB is R-45HTLO. This product consists of oligomeric units typically containing 40–50 butadiene molecules bonded together, with each end of the chain terminated with a hydroxyl [OH] group:

Second Stage: GS2

One Vikas engine is used in the second stage of GSLV. The stage was derived from the PS2 of PSLV where the Vikas engine has proved its reliability.

Engine : Vikas
Fuel : UDMH + N2O4
Max. Thrust : 800 kN
Burntime : 150 sec

Third Stage: CUS

Developed under the Cryogenic Upper Stage Project (CUSP), the CE-7.5 is India’s first cryogenic engine, developed by the Liquid Propulsion Systems Centre. CE-7.5 has a staged combustion operating cycle.

Fuel : LOX + LH2
Max. Thrust : 75 kN
Burn-time : 720 sec

 Indigenous Cryogenic Engine and Stage

A Cryogenic rocket stage is more efficient and provides more thrust for every kilogram of propellant it burns compared to solid and earth-storable liquid propellant rocket stages. Specific impulse (a measure of the efficiency) achievable with cryogenic propellants (liquid Hydrogen and liquid Oxygen) is much higher compared to earth storable liquid and solid propellants, giving it a substantial payload advantage.

Oxygen liquifies at -183 deg C and Hydrogen at -253 deg C. The propellants, at these low temperatures are to be pumped using turbo pumps running at around 40,000 rpm. It also entails complex ground support systems like propellant storage and filling systems, cryo engine and stage test facilities, transportation and handling of cryo fluids and related safety aspects.

ISRO’s Cryogenic Upper Stage Project (CUSP) envisaged the design and development of the indigenous Cryogenic Upper Stage to replace the stage procured from Russia and used in GSLV flights. The main engine and two smaller steering engines of CUS together develop a nominal thrust of 73.55 kN in vacuum. During the flight, CUS fires for a nominal duration of 720 seconds.

Liquid Oxygen (LOX) and Liquid Hydrogen (LH2) from the respective tanks are fed by individual booster pumps to the main turbopump to ensure a high flow rate of propellants into the combustion chamber. Thrust control and mixture ratio control are achieved by two independent regulators. Two gimballed steering engines provide for control of the stage during its thrusting phase.

Advantages of Cryogenic engine

Fuel density: – The fuel use in this engine is oxygen and hydrogen in liquid form so More fuel can be stored in the tank due to more density of liquid fuel stored at low temperatures.

Cooling: – The fuel temperature is about 273°C which can be used to cool the engine. So, no additional cooling circuits is required to cool the engine.

Power density: – Due to higher density, the energy per unit volume of fuel is higher. So it can deliver more power per unit volume of the fuel.

Every mission has its risk and learning.  So is the case with the current mission.

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