The Indian Space Programme

by Gurbir Singh

  ASLV was also ISRO’s first launch vehicle to be integrated vertically. Today, the PSLV and the GSLV are integrated vertically. By the end of the ASLV programme, ISRO had demonstrated all the key capabilities to build, launch vehicles, and place satellites in orbit with precision. In addition to operating boosters and getting two SROSS satellites into orbit to return scientific data, ISRO engineers had acquired critical experience and confidence by investigating problems and developing solutions for each of the first three ASLV flights that had not gone as planned. Had the ASLV programme been 100% successful it would not have generated the valuable learning experience that it did. In the process, the engineers developed a deeper conviction in their own abilities. A thorough investigation of the first ASLV flight failure probably could not have prevented the second, as the causes were unique. Lessons learnt from ASLV were implemented in PSLV, including an improved autopilot that ensured that the second stage kicked in once a deceleration was detected indicating that the first stage burn was coming to an end.

  ISRO, however, needed to design a launch vehicle with greater thrust as satellites for use in EO or communication were heavier and needed to get to higher altitudes than ASLV could muster. ASLV was still relying only on solid propellant technology. The next significant step for ISRO was to grapple with the technology of liquid engines.

  Polar Satellite Launch Vehicle (PSLV)

  The target in Sarabhai’s vision was to develop a launch vehicle by the end of the 1970s capable of putting a 1,200-kg satellite into GEO at 36,000 km.[545] He had given the task of developing more powerful rockets to the Space Science and Technology Centre, which was established close to Thumba and continues to flourish today. Even though the development and launch of four SLV-3 between 1979 and 1983 and four ASLV between 1987 and 1994 provided ISRO with the experience of placing progressively heavier launch payloads using larger launch vehicles, it could only manage small payloads (SLV-3 40 kg and ASLV 135 kg) to LEO.

  Eventually, ISRO fulfilled Sarabhai’s target of 1,200 kg with the third iteration of its satellite launch vehicle resulting in the PSLV, which has since been the mainstay of ISRO launch vehicles for over two decades.[546] One of the earliest references to the PSLV outside India was made in a US secret service report where it was referred to as the Polar Space Launch Vehicle. The evidence came from US spy satellite imagery of what looked like a static display of PSLV mock-up components on the ground of Sriharikota.[547] The PSLV stands 44.4 m high with a diameter of 2.8 m and depending on its configuration weighs either 229, 296 or 320 tonnes at launch. It was designed to take an Indian Remote Sensing (IRS) satellite, typically 1 tonne, to a 900-km SSPO from Sriharikota. The most powerful variant of the PSLV, the PSLV-XL, can deliver 1.4 tonnes to GTO.

  The key innovation in the PSLV, a four-stage launcher, was the introduction of liquid propellants. PSLV uses solid and liquid propellants in alternating stages. Not only are liquid propellants inherently more efficient than solid propellants, but unlike engines that use solid propellants, liquid propellant engines can be regulated, stopped and restarted multiple times. It was essential for ISRO to master this capability. The additional control and thrust available only through the technology of liquid engines were essential to achievingt its ambition for larger payloads to GTO and from there to Geosynchronous Equatorial Orbit (GEO) required for communication satellites.

  During the 1960s and early 1970s, France assisted ISRO engineers to gain key skills and experiences in solid rocket motor technology through the construction under licence of the Centaure rocket in India. However, ISRO’s total experience in using liquid fuel was negligible. Small amounts of liquid propellants were used for the operation of the SITVC and RCS systems that were deployed on SLV-3 and ASLV. What ISRO needed was to design, build and deploy complete stages using only liquid fuel, something that in the 1970s was completely outside ISRO’s capability. By chance, France made an offer ISRO could not refuse.

  Sarabhai had established the tradition of sending Indian engineers and scientists to reputable institutions in the US (Wallops Island, MIT, Goddard Space Flight Centre) to gain hands-on experience in launching rockets when Thumba was first established. The tradition continued throughout the period of the Cold War and even into the early 1990s. Indian engineers went to French Guyana Space Centre in South America for training in handling launch vehicles, range safety and radar tracking; to Deutsche Forschungs-und Versuchsanstalt für Luft-und Raumfart in West Germany for training in high-altitude test facilities, pulse code modulation telemetry and wind tunnel testing; to France for developing the Centaure rockets programme with Sud-Aviation[548] and to Russia for training in cryogenic engine technology.[549]

  Sarabhai had first engaged France through Jacques Blamont to supply the sodium payload for India’s first sounding rocket in 1963. Dhawan continued that relationship and invited CNES President J. F. Denisse to visit ISRO in January 1973. A seven-member delegation visited Ahmedabad and Trivandrum, and a joint commission set up two working groups, one on TV satellites and the other on launchers.[550] In the following year, Dhawan visited France and concluded a unique agreement that would allow ISRO to acquire liquid Viking engine technology in exchange for hardware and labour rather than cash. This agreement was a product of the working groups established during the CNES President’s visit. In July 1973, France had led the replacement of the European launcher programme called Europa with Ariane. As its primary backer, France had to develop Ariane on a tight budget and timeline and needed the support of a large number of technically skilled personnel.[551] The agreement involved India helping France in developing Ariane in exchange for liquid engine technology transfer.[552]

  The Viking engine for the Ariane launcher was being developed by the French company Société Européenne de Propulsion (SEP). ISRO would eventually customise and build Viking in India and rename it as the Vikas engine for use on the PSLV. The unusual agreement required ISRO to provide 100 person-years to work at SEP. This unique arrangement was the product of India’s inability to make payments in hard currency and the supportive relationship that had been cultivated with France over many years. SEP would decide how and where 75 of the 100 person-years would be deployed, and ISRO would direct the remaining 25. Thirty-Five Indian engineers were sent to the SEP facility in Vernon to work on system development, quality control and programme management. In parallel, 25 engineers based at VSSC worked on design fabrication, integration and testing of the Viking engine technology.

  The license fee for the Viking technology was included in the deal. No money changed hands. The difference was met by ISRO providing CNES 10,000 space-qualified pressure transducers.[553] The agreement ended in March 1978. By then, the Viking technology designs and documentation had been transferred to ISRO, and the engineers sent to France had returned having gained considerable experience.[554] The Vikas engine used by PSLV’s second stage evolved from this experience. The second stage uses a single Vikas engine, 40 tonnes of UDMH as fuel and N2O4 as an oxidiser. The fourth stage deploys two L-25 engines using liquid fuel, Mono-Methyl Hydrazine (MMH)) as fuel and mixed oxides of nitrogen (MON) as the oxidiser. This use of liquid engine technology in the second and fourth stages is a key innovation in PSLV. The larger masses of the PSLV stages also required a more active stage separation system. To ensure collision-free stage separation, the first stage employed eight and the second stage four retro rockets.[555] In addition to the larger physical size and the mix of solid- and liquid-fuel stages, the PSLV incorporated a complex network of electronic systems that required 40,000 connections with 20 km of cable. Between the four stages, PSLV had 450 avionics packages and subsystems, which included navigation, guidance, pyro system and telemetry.

  Originally, the PSLV was designed as a three-stage launch vehicle, stage 1 and stage 3 using solid fuel and stage 2 based on liquid fuel. The final stage of a launch vehicle is responsible for precisely inserting the payload into its designated orbit. To achieve that precision, the final-stage engine h
as to have the capacity to regulate its power, stop completely and restart. This level of control is only possible with liquid engines. ISRO recognised that the PSLV needed an additional fourth stage and it had to be powered by a liquid engine. The fourth stage was added after the project had been approved.

  PSLV’s liquid fourth stage used two engines, but unlike stage 2, they were not the new Vikas engines. Instead, they were a modified reuse of the existing RCS used for roll control of the first stage. The redesign involved increasing the thrust from 6.5 kN to 7.5 kN and extending the operational time from 110 seconds to 420 seconds. Since the fourth stage would kick in once the launch vehicle was in space, it would also need to ignite and operate in a vacuum. ISRO’s comprehensive testing over five years made PSLV’s fourth stage the most tested stage at that time.[556] Further innovations since the 1990s when the PSLV first became operational have increased the maximum payload to near 1,850 kg from the initial 1,200 kg. PSLV is now capable of placing payloads in GEO and not just the SSPO.[557] The PSLV was responsible for launching all seven of the IRNSS satellites, and with the aid of gravity assist, it was used to get spacecraft to the Moon and Mars.

  PSLV-XL

  Stage 1

  Solid

  Stage 2

  Liquid

  Stage 3

  Solid

  Stage 4

  Liquid

  Burn Time (second)

  103

  148

  112

  525 (max)

  Propellant Type and Mass (tonne)

  S138

  HTPB

  L40.7

  UH25+ N2O4

  S6.7

  HTPB

  L2

  Liquid MMH+ MON[558]

  Length × Width (m)

  20.3 × 2.8

  12.8 × 2.8

  3.6 × 2

  2.7 × 2

  Lift-off Weight (tonne)

  138

  42

  7.2

  2.5

  Directional Control (Pitch and Yaw)[559]

  SITVC

  Engine Gimbal

  Flex Nozzle

  Engine Gimbal

  Table 10‑7 Typical PSLV-XL Specification

  ISRO has developed a shorthand to describe the propellant type and the quantity in each stage for all its launch vehicles, not just the PSLV. A letter denotes the propellant type, solid (S), liquid (L), semi-cryogenic (SC) and cryogenic (C), and a number the quantity in tonnes. For example, an S40 stage has 40 tonnes of solid propellant, an L110 stage has 110 tonnes of liquid propellant and so on. The first PSLV launch on 19 September 1993 failed. It failed immediately after the ignition of the third stage.

  Following its investigation, the FAC identified three causes:[560] (i) during the second-stage separation, two of the retro rockets did not function as commanded indicating a break in the electrical circuit, so the separation did not go as planned; (ii) during the second stage, yaw (side to side) error grew and was not corrected by the autopilot; and (iii) the pitch (pointing up and down) error had increased because of an error in the autopilot’s software that failed to detect an overflow condition. Despite the failure, most of the telemetry indicated that many of the subsystems operated as designed. All PSLV launches since then have been successful.

  Through the 1990s, a further four launches placed IRS satellites in orbit, which is what the PSLV had been designed for. During its more than two decades of active use, the PSLV has evolved into three, increasingly powerful, configurations: (i) PSLV Regular, also known as PSLV Generic (PSLV G); (ii) PSLV Core Alone (PSLV CA), without strap-ons and (iii) PSLV XL. Three key developments have increased PSLV’s capacity to deliver 1,600 kg to SSPO of 600 km or 1,850 kg to GTO.

  Figure 10‑5 Three Configurations of PSLV. Regular, Core Alone and XL (What looks like strap-ons in the Core Alone configuration are SITVC fuel tanks, which are present in all three configurations). Credit ISRO

  The first-stage solid motor capacity has increased from 125 tonnes to 139 tonnes, the second-stage has increased from 40 tonnes to 42.5 tonnes, and the longer strap-ons used for the XL version has increased the strap-on capacity from 10 tonnes to 12.8 tonnes. By mid-2016, the PSLV XL’s higher payload capacity had made it the most used variant, PSLV XL 13 flights, PSLV CA 11 flights and PSLV G 12 flights. The first stage of PSLV is made up of five segments (each 2.8 m in diameter and 3.4 m long) and six strap-on boosters that use HTPB propellant. Each strap-on is 1 m in diameter and either 10 m or, in the XL variant, 13.5 m in length. At launch, the first stage and two of the strap-ons (or four depending on the payload) ignite within the first 0.6 second. The first stage is steered using SITVC for pitch and yaw and RCS for roll.

  The SITVC system consists of four sets of six valves (24 in total) located symmetrically around the nozzle of the first stage injecting strontium perchlorate into the flow from the nozzle. This provides the differential pressure to manipulate pitch and yaw sufficient for the PSLV to steer away from Sriharikota to its destination. The launch trajectory is designed to minimise the hazard to life and property along the ground track. For polar orbit the initial launch angle of around 135°S is gradually increased to 180°S to avoid Sri Lanka. For equatorial/GTO orbit launch angle is around 105°S and the rocket stages are discarded in the Indian ocean and the final stage in the Pacific Ocean.

  Figure 10‑6 Launch Profile of the PSLV-C27 IRNSS-1D. Credit ISRO

  The precise launch trajectory is dependent on the payload and its final destination. As an example, PSLV-C27 took nearly twenty minutes to place IRNSS-1D in orbit. The journey started at T=0, with first stage ignition and lift-off occurred 0.6 seconds later when four strap-ons ignited. At T+25 seconds the two-remaining strap-ons ignite. Within 90 seconds after ignition, all strap-ons were exhausted, and drop away. This approach of staggered ignition of strap-ons ensured that the PSLV’s speed was not high and the dynamic pressure on PSLV and its payload was within safe limits while it traversed the densest part of the Earth’s atmosphere.

  At T+1 minute and 51 seconds at an altitude of 56 km, the second stage is ignited. This stage was powered by the ISRO-built Vikas engine using the hypergolic liquid propellant UDMH and N2O4. Vikas, the Indian variant of SEP’s) Viking engine, was rated to burn for a longer period for the stretched second stage. During the second-stage burn, the payload fairing, which included the heat shield, completed its job of protecting the payload as it ascended through the atmosphere and separates at an altitude of 113 km. Stage two is steered using its engine gimbaling control (EGC) systems until the propellant is exhausted, and discarded at T+4 minutes and 22 seconds, and one second later the third stage is ignited.

  The third stage took about six minutes to burn its almost seven tonnes of solid HTPB propellant before it also separated and burned up during re-entry through the Earth’s atmosphere. The fourth stage had the critical role of placing the payload precisely in the required orbit. It used a more efficient liquid propellant, MMH with MON-3, and had two identical engines instead of one. Immediately after the third-stage separation, the twin engines in the fourth stage were ignited. Following a burn of about 10 minutes, the payload was ejected. The PSLV successfully delivered its payload IRNSS-1D to the planned transfer orbit and the PSLV has completed its mission. IRNSS-1D uses its onboard engine to get to the final orbit.

  Through its collaboration with SEP in France, ISRO had obtained the know-how to build the critical systems that comprise the liquid-fuel engine, the gas generator, turbine, turbo pump and the combustion chamber. Many of the materials, components and tools required to make them, along with the manufacturing technique, were part of the technology transfer agreement. The PSLV programme took over a decade from the initial design before it demonstrated its first successful flight. In the ISRO infrastructure, the facility at Valiamala (about 30 km from Trivandrum), originally a firing range for the police service, became a key centre for designing, building, testing and developing the PSLV. SLV-3 and ASLV enabled ISRO for the first time to launch
satellites to Earth orbit. However, mastering the liquid engine technology was the single technical advance that brought India to a point where it could claim to be self-sufficient in space.

  With the PSLV, India had all the elements launch vehicles, satellites, infrastructure and professional expertise to deliver the space-based services as Sarabhai had imagined decades earlier. The liquid engine capability of the PSLV and its larger capacity was essential for ISRO to reliably place payloads for national development in designated orbits with high precision. The capability was then quickly expanded to include payloads with commercial potential and wider needs, such as space re-entry vehicles, science and interplanetary exploration. Satisfied by its competence in solid and liquid engines, ISRO then turned to the epitome of rocket science, the cryogenic engine technology.

  Geosynchronous Satellite Launch Vehicle