The Indian Space Programme

by Gurbir Singh

  To mitigate the hazards of vibration and reduce the noise at launch, a water-based acoustic dampener was installed in 2013 in the SLP. The 80-m high water storage tower, which has a capacity of 50,000 tonnes of water, is emptied in 20 seconds at launch. The energy of the exhaust from the solid boosters is used to turn the water into steam, adding to the dramatic spectacle of the launch. This acoustic dampening system has produced the desired reduction in vibration with a tangible reduction of the noise at the moment of launch to one-quarter of the original (as engineers measure it, an attenuation of 6 dB).

  Figure 9‑8 Second Launch Pad Noise and Vibration Suppression System. Credit ISRO

  Sriharikota is located in a region where storms and cyclones are not uncommon. Unlike at FLP, the launch vehicle for the SLP is integrated on MLP within the VAB and then moved to the launch pad. This fixed VAB provides launch vehicle assembly and payload integration environment for all weather conditions. If required, perhaps under extreme weather conditions, the assembled launch vehicle can be wheeled back from the launch pad to the VAB for safety, maintenance or reconfiguration. This approach also reduces the time the pad is occupied. The FLP and SLP are supported by onsite storage, transfer and servicing facilities for all the propellants ISRO uses for its launchers, solid, liquid and cryogenic. Solid propellants are integrated into the solid stages at the time of manufacture, months in advance. Liquid and cryogenic propellant filling operations are conducted only at the launch pad during the hours leading up to the launch.

  Figure 9‑9 GSLV-D5 on the MLP moving from the VAB towards the SLP. Credit ISRO

  Fuel loading is managed remotely by the MCC only after the integrated launch vehicle stands alone and all personnel have been evacuated from the launch pad. Propellant loading is conducted by the Filling Control Centre (FCC). Propellant loading is a critical step in preparing for the launch. Most of the mass of a launch vehicle is from the propellants.

  The mass of the payload is typically less than 1% of the total mass of the fully loaded launch vehicle. The liquid fuel used by PSLV and GSLV is loaded in the final hours just before the launch. These fuels are delivered to the launch vehicle in colour-coded pipes, red for propellant (UH25) and yellow for the oxidant (N2O4). Currently, cryogenic propellants (LOX and LH2) are manufactured at IPRC in Mahendragiri and transported using specialised tankers by road to holding tanks at Sriharikota. ISRO’s cryogenic engine CE-20 has a capacity of 27 tonnes of combined LOX and hydrogen. While offsite manufacture and transport is practical at the present, Sriharikota will need onsite facilities to produce both as quantities rise in the future.

  Prior to launch, the FCC loads the GSLV’s third stage propellant from these holding tanks. To prevent high-pressure build-up in the hydrogen tank, some hydrogen is allowed to escape. However, to keep the propellant tank full, the hydrogen tank is continually topped up during the final seconds prior to the launch. During the launch, when the cryogenic stage kicks in, turbo-pumps running at around 40,000 rpm deliver the propellants from the onboard tanks to the combustion chamber.

  Solid Propellant Booster Plant

  The Solid Propellant Booster Plant (SPROB) was commissioned in March 1977 and is responsible for the manufacture of the solid propellant on an industrial scale. Vasant Gowariker (1933–2015) was influential in establishing SPROB at Sriharikota following his work in setting up the Propellant Fuel Complex at Thumba in 1972.[489] Gowariker was one of the scientists Sarabhai had brought back to India from the UK in the early days of Thumba. Some of the propellant processes and formulations used by ISRO today were first developed at Thumba. ISRO experimented and developed materials used in propellants, such as bonding agents, resins, plasticisers, inhibitors and insulators. In pursuit of self-sufficiency, ISRO acquired from foreign nations some of the plant equipment needed for high production volumes, typically 500 tonnes per year.[490]

  SPROB is one of the largest facilities within the Sriharikota complex comprising two independent plants for the production of the solid propellant to meet ISRO’s needs. It is charged with the production of Hydroxyl-Terminated Polybutadiene (HTPB), a propellant that ISRO has a long history of producing for its launchers, including the Rohini series of sounding rockets, SLV-3, ASLV, PSLV and the GSLV.

  ISRO has been gradually increasing the quality and types of propellants and the scale on which it manufactures them. During the 1960s, the solid propellant grain (a cylindrical unit of solid propellant produced by the plant) increased from the 75-mm diameter, weighing 4.5 kg for the RH-75 to the 2.8-m diameter, weighing 125 tonnes used by the PSLV.[491] HTPB is an established composite propellant used by space agencies of other nations, including the US, Russia and Japan, as well as an increasing number of commercial organisations.

  Once manufactured, HTPB is safer to handle, store and transport compared to liquid and cryogenic propellants. The process of HTPB manufacture is still hazardous. The HTPB propellant is a mixture containing oxidiser (ammonium perchlorate, 68%) and fuel (aluminium powder, 20%). Combining the oxidiser within the propellant permits combustion outside the atmosphere in the vacuum of space. HTPB is a paste when manufactured and is cured (hardened) at temperatures of up to 150°C. Although not an explosive, it is a highly combustive material that, once ignited, cannot be stopped.

  Worldwide, there have been many instances of fires and explosions in the production of propellant, resulting in damage and loss of life. Abdul Kalam recalls from his days at Thumba a particularly dramatic escape “trapped in this inferno, Sudhakar, however, did not lose his presence of mind. He broke the glass window with his bare hands and literally threw me out to safety before jumping out himself.”[492] The incident Kalam refers to did not involve HTPB, but an incident on 23 February 2004 at SPROB did and cost the lives of six staff, including three engineers. The incident occurred during the transport of a rocket motor from SPROB building 117 to another. Building 117 was completely destroyed.[493] For safety reasons, the SPROB facility is located away from other Sriharikota installations, and individual SPROB buildings are dispersed over a large 25 km2 SPROB complex isolated from each other for the same reason.

  Also, located on-site at Sriharikota is the Static Test and Evaluation Complex (STEX). Through a variety of tests at STEX, ISRO measures combustion characteristics. For every batch of propellant produced, a sample is test-fired to quantify its physical and propulsive characteristics. When used in external strap-on boosters, it is critically important that the propellant in each booster combusts at a known and consistent rate to meet the mission launch profile. Inconsistent combustion between strap-ons can result in differential forces leading to a loss of directional control. To analyse the structure of the solid booster casing, SPROB uses a high-power X-ray source. This meticulous level of inspection provides assurance on the physical integrity of the booster casing before it is used in flight. Further, while most rocket motors used for spacecraft manoeuvres in space use liquid propellant, some use a solid propellant. To establish baseline characteristics for space-borne firings, a sample of the solid propellant is tested within a thermo-vacuum chamber situated at Sriharikota.

  Local Propellant Facilities

  A fully loaded rocket is a precarious and dangerous entity. To minimise risk, liquid propellants are loaded only in the final few hours prior to the launch. Each of the launch pads has a local storage facility for liquid propellant UH25 (75% unsymmetrical Dimethylhydrazine or UDMH and 25% hydrazine hydrate) and N2O4 (dinitrogen tetroxide) from where the loading takes place. Sriharikota has a bulk storage facility for up to 400 tonnes located within the complex but at a safe distance from the launch pads.

  On 19 August 2013, the launch of the GSLV-D5 mission using a GSLV Mk-2 developed a propellant (UH25) leak in its second stage, 14 minutes prior to the scheduled launch at 16:50. The leak was triggered during the second-stage pressurisation phase that occurs near the end of the launch sequence. Once the leak was detected, the launch was immediately aborted, but by then 750 kg of the propellant had leak
ed, contaminating other parts of the launch vehicle and the immediate vicinity of the launch pad.[494]

  UH25 and N2O4are known as hypergolic fuels. In addition to being efficient propellants, they exhibit a very desirable attribute in that they require no ignition mechanism. They spontaneously ignite upon coming into contact with each other. Had there also been a leak of the oxidant N2O4 at the same time, it could have resulted in a devastating explosion on the launch pad? The launch vehicle, payload and the launch pad itself could have been destroyed.[495] This potentially disastrous incident with far-reaching consequences was averted by the prompt implementation of the procedure established to address such a scenario. In addition to safely draining the nearly 350 tonnes of propellant from the problematic second stage, the liquid fuel from the four strap-ons and the cryogenic fuel from the third stage was also removed. This had to be done while ensuring that the small pyrotechnic devices used for stage separation were not triggered. The situation was managed professionally, and SLP was made safe. It was “more than defusing a bomb”, recalled the ISRO chairman.[496] The SLP was back in normal use after six days. Eventually, all four of the strap-ons and the problematic second stage were replaced, and the GSLV-D5 Mission was launched successfully on 5 January 2014.

  Electric Propulsion

  A typical two tonne communication satellite requires typically around 25 kg of chemical propellant for station keeping (maintain its precise place in orbit) for each year of its operational life which is typically around 12 to 15 years. On arrival at its orbital slot, a communication satellite will have about 300 kg of propellant. It is the depletion of this propellant that ultimately marks the end of the satellite’s mission. Electric propulsion has the potential to reduce or replace this propellant.

  Traditional engines use combustion to create thrust (accelerate and expel particles at high speed). Electric Propulsion System (EPS) creates thrust by using electricity from solar cells to create thrust (accelerate and expel particles at high speed using electrostatic or electromagnetic force). EPS produce very low thrust compared to chemical propellants but is very much more efficient with an Isp of around 1500 to 3000 compared with an Isp of 450 of Cryogenic engines. The low thrust requires EPS to be active continuously for days whereas chemical engines operate for seconds or minutes. For interplanetary spacecraft, EPS has to operate continuously for weeks, months or years. One of the earliest spacecraft fitted with an operational EPS using the inert gas xenon as fuel was NASA’s Dawn mission launched in 2007 to explore the asteroids Ceres, Vesta and others in the solar system. Dawn’s EPS accelerates the spacecraft from 0-60 mph in 4 days but uses only about one kg of fuel.[497] Although not suitable for use at launch, EPS is a remarkably attractive propulsion mechanism once in space.

  Instead of EPS, ISRO uses the terminology, Stationary Plasma Thrusters (SPT). ISRO first attempted to test SPT on GSAT-4 in 2010 but the launch was not successful. The 2,230 kg GSAT-9 launched on 6 May 2017 is configured with the chemical propellants for station keeping. In addition, it has 80 kg supply of xenon and 4 SPTs as a technology demonstrator.[498] ISRO engineers will test the SPT technology on GSAT-9 and acquire operational experience with view to increasing the use of SPT in the future. Whereas GSAT-9 SPT is a technology demonstrator, chemical thrusters provide the primary source for station keeping, ISRO is designing GSAT-20 where only SPTs are used for station keeping. GSAT-20 is scheduled for a launch in 2018. Electric propulsion has been used for many years by other space agencies. Hundreds of the current operational spacecraft use electric propulsion. It is expected that all new spacecraft from 2020 onwards will be equipped with electric propulsion systems.

  Launch Dynamics

  Any launch site selected is a compromise of some or all the above factors. India’s first launch site, Thumba, was not entirely unpopulated at the outset but it was on the coast. Further, Thumba was designed for sounding rockets and launched only to sub-orbital (go up and then down again) flight over the Arabian Sea.

  Figure 9‑10 Launch Trajectories from Sriharikota. Credit Bhushan Hadkar

  The ground track of the sub-orbital flights did not cover the Indian land mass. A special road was built when Thumba was commissioned, and it has rail links, too.[499] The trajectories of all launches from Thumba took the rockets away from land and over to the Arabian sea. Thumba’s location, near the south-western tip of India, launching to a polar orbit is not an issue but it is not suited for equatorial (easterly) launches, as the launch vehicle would have to fly over populated areas.

  Satellites orbit the Earth in two distinct orientations, pole to pole (north-south) known as a polar orbit and west to east in line with the Earth’s rotation known as an equatorial orbit. Each category, polar and equatorial, has multiple variations and there are other orbit types too. A polar orbit requires launch in a north or south direction, whereas a due east launch is required for an equatorial orbit. An ideal due east launch from the Earth’s equator can take advantage of the Earth’s maximum rotation and pick up a ‘free’ 465-m/s velocity (or a lower value if the launch site is further away from the equator[500]). Establishing an actual launch site is subject to a number of safety, logistical and technical considerations.

  The launch site and the area immediately around it must be located away from populated zones and yet offer convenient transport links for the supply of launch vehicle parts, components, propellants, as well as staff who work there.

  An optimum polar orbit requires a clear due north/south launch corridor, and an optimum equatorial launch requires an unhindered due east/west launch corridor, ideally on the equator.

  Launch vehicles are tracked (optically and by radio) for safety, real-time flight data collection (to help understand unexpected events) and taking measurements that help calculate the eventual orbit. The ground stations collecting these data from ascending launch vehicles must be located along its ground track and be in the line of sight of the transmitters onboard the launch vehicle.

  Staging (where each rocket stage is discarded after the launch, once its propellant has been consumed) is a well-established feature of a rocket launch. It is critical that the ground track of the launch trajectory is unpopulated to ensure that people and property are safe from falling used rocket stages. Equatorial launches (PSLV/GSLV) discard the first stages prior to the ground track encountering the Malay peninsula. The final stage is discarded over the Pacific ocean.

  The launch trajectory should not interfere with the sovereign airspace of neighbouring countries. India is a signatory to the International Liability Convention and thus legally liable for any loss of life or damage in neighbouring countries resulting from its space programme.

  The barrier island topography of Sriharikota provides a natural separation from the larger populated towns nearby, such as Chennai. It has, over the years, developed its infrastructure to meet its annual operational goals, although the key transport link is a single road leading from the mainland to Pulicat Lake. However, Sriharikota is located at 13°N on India’s eastern coastline, and local geography enforces launch constraints. From Sriharikota, Sri Lanka in the south gets in the way for a polar orbit and Malaysia and Indonesia (including the busy shipping lanes and offshore oilfield) in the east for an equatorial/geostationary orbit.

  Figure 9‑11 International Launch Sites around India. Credit Adapted from Federal Aviation Administration Compendium 2016

  Thumba was conceived and built between 1962 and 1963 to explore the EEJ at a time when India had no rocket launch experience, and the capability to launch satellites into orbit was a distant aspiration. Although not suited for equatorial orbits, Thumba fulfilled its objectives at the time. To place payloads into equatorial orbit, India needed a launch site on the East coast.

  To overcome these constraints, a typical polar orbit trajectory for a PSLV launch is not due south at 180° (the most efficient) but 135° with a course correction after launch vehicle to manoeuvres around Sri Lanka. Similarly, a typical (each mission has its unique r
equirements) GSLV launch trajectory for an equatorial orbit is not due east at 90° but 108° with a course correction once the launch vehicle is beyond Malaysia. These enforced initial launch trajectories and course corrections decrease the efficiency of any launch from Sriharikota. To compensate, additional propellant is required which results in a reduced payload mass or decreases the satellites orbital altitude.

  Figure 9‑12 Proposed Second Vehicle Assembly Building. Credit Adapted from ISRO

  At present, between the two launch pads, ISRO can schedule up to eight launches every year. While the VAB is occupied for several weeks for each launch, the launch pad is engaged for only around ten days. To cater for the expected increase in launch operations additional VABs and launch pads are required. A formal ministerial announcement in the Rajya Sabha on 12 March 2015 indicated that a Third Launch Pad (TLP) would be built at Sriharikota, but no firm timescales have been established. In the meantime, construction of the second VAB (SVAB), finally got underway in early 2015 when funds dedicated to the project of Rs.130 crore ($20 million) was announced.[501] It will have its rail track to the SLP. SVAB will be used to prepare existing and future launch vehicles.