The science and commerce of PSLV

Print edition : June 05, 1999

The latest PSLV configuration with simple jettisoning systems meant for passenger payloads in the micro-satellites category will make it a versatile and flexible commercial launcher.

THE launch of the Polar Satellite Launch Vehicle (PSLV-C2) on May 26 was significantly different from the previous four PSLV launches in that it carried, for the first time, more than one satellite on board. Besides the primary payload of the Indian Rem ote Sensing (IRS) satellite, IRS-P4 or Oceansat-I, it launched two auxiliary payloads which would fall in the category of micro-satellites (MS): the 110-kg engineering test satellite KITSAT-3 of the Satellite Technology Research Centre (SaTReC), Korea Ad vanced Institute of Science and Technology (KAIST), Republic of Korea, and the 45-kg earth observation satellite DLR-TUBSAT, jointly developed by the Technical University of Berlin (TUB) and the German Aerospace Centre (DLR). It was not only a multi-sate llite launch but PSLV's first commercial launch; however, as a promotional measure, KAIST and DLR-TUB were charged only nominal sums - about $1 million each. The market rate for an MS (100-150 kg) launch is about $15,000 (about Rs.63 lakhs) a kg of paylo ad mass.

The question is: is PSLV now a proven launch vehicle and is the Indian Space Research Organisation (ISRO) ready to offer the PSLV in the world launch market? As the name implies, PSLV has been configured to put satellites in the Sun Synchronous Polar Orb it (SSPO) and specifically tailored to launch one-tonne class IRS satellites into a 900 km SSPO. In order to be a commercial launcher, a vehicle has to be reliable and configurable to launch a range of payload masses in a variety of orbits besides the SS PO. All the five launches of PSLV (three of them developmental, of which the first, D1, failed, and two operational or continuation launches, of which C1 resulted in sub-optimal injection of IRS-1D, and the other is the present one) have been configured for an SSPO launch. Only C2's was a multi-satellite launch.

Given the success of C2, it would be reasonable to assume that PSLV is now in a position to offer commercial launches for piggyback rides of auxiliary payloads in the MS category. In fact, the Vehicle Equipment Bay (VEB) housed in the fourth stage of the vehicle has been reconfigured to accept two MS category auxiliary payloads in all launches. Antrix Corporation Ltd., ISRO's marketing arm, has already signed an agreement with VERHAERT Design and Development N.V., Belgium, for launching its MS, Proba, w hich weighs about 100 kg, on board a forthcoming PSLV launch.

It is also significant that ISRO has signed a memorandum of understanding (MoU) with the French launch service company Arianespace, to evolve a launch vehicle interface common to both Ariane 5 and PSLV. Depending on Ariane's and PSLV's launch schedules o f primary payloads, this would enable auxiliary payload launches to be shared between Arianespace and ISRO. Under this cooperative agreement, the two agencies have formed a Committee for Auxiliary Payload Policy Execution (CAPE) to encourage and enlarge the opportunities for small satellites. Although there is a latent (albeit small) demand for MS launches primarily for research purposes, not many launchers are available for launching these. This means that if PSLV (or Ariane) has to secure MS launch co ntracts, an aggressive promotional campaign has to be initiated now. The MoU is essentially aimed at this.

Some ISRO scientists, however, feel that the commercial potential of PSLV, even in the MS category, may be limited. ISRO may be able to get to launch at best four MS, they feel. Satellites from the United States are certainly not going to come the PSLV w ay. The Europeans may be looking at the cheaper Chinese or Russian option. A couple of years ago, Professor Martin Sweeting, managing director and chief executive officer of Surrey Satellite Technology Ltd. (SSTL), a wing of the University of Surrey whic h specialises in MS, opined that an MS launch by PSLV may not be cost-effective, but did not spell out the reasons. However, one reason for his opinion could be Russia's plan to convert its Inter-Continental Ballistic Missiles (ICBMs) into satellite lau nch vehicles. These missiles seem to be ideal for launching mini- and micro-satellites in SSPO as well as low-earth orbit (LEO) satellites in the sub-tonne category. In short, with the huge number of submarine-based and land-based missiles due for disman tling under the arms control agreements, Russia will be able to launch satellites in any category at a very low rate.

Last July a Russian submarine launched two environmental micro-satellites, TUBSAT-N and TUBSAT-N1 of the Technical University of Berlin, on an ICBM from the Barents Sea Base. This was the first such satellite launch in the world. The Russian Navy is said to have an order for three launches for eight MS. Similar missile launches, both from land and sea, have been contracted up to 2001. On April 21, SSTL launched its UoSAT, a 350 kg satellite, aboard a Russian SS-18 ICBM. Missile launches have also been c ontracted for 10 launches of Motorola's Iridium satellites. Given this emerging scenario, it is not clear whether ISRO has carried out suitable market surveys even for MS.

IRS-P4, or Oceansat-I, undergoes tests at the SHAR centre.-BY SPECIAL ARRANGEMENT

THIS limited market notwithstanding, ISRO is working towards making PSLV a commercial launcher until such time that the Geosynchronous Launch Vehicle (GSLV) becomes operational, which is at least five years away. In fact, PSLV's performance has steadily improved over the years through improvements in the vehicle design. At present it is capable of orbiting a 1,200 kg satellite in an 800 km SSPO. Since IRS-P4, on-board PSLV-C2, weighs only 1,050 kg, the two MS, KITSAT and DLR-TUBSAT, could easily be acco mmodated as passenger payloads. PSLV has accomplished three successful missions. Its payload capability increased from 820 kg to the present 1,200 kg in the period from September 1993, when the first developmental flight PSLV-D1 took place, to September 1997, when the first operational and successful launch that placed the satellite IRS-1D in the SSPO took place. PSLV, with an overall height of 44.4 metres and a gross lift-off mass of 292 tonnes, is configured as a four-stage vehicle with alternate soli d and liquid propulsion modules. The first stage motor (PS-1) carries 138 tonnes of solid propellant and measures 2.8m in diameter, and is one of the largest solid fuel stages in the world. The PS1 is augmented by six 1-metre solid strap-on booster rocke ts (PS-0s). Either two or four of the PS-0s are ignited on ground at lift-off and the remaining ones are ignited in air depending on the mission requirements. The PS-1 stage is controlled in pitch and yaw by the Secondary Injection Thrust Vector Control (SITVC) system and the roll control by two externally mounted swivellable liquid engines.

In view of the south-eastwardly launch instead of an ideal due south launch, which is decided on to avoid flight over Sri Lanka, PSLV has a complex pitch/yaw control system to carry out the complicated dog-leg manoeuvre after it passes over Sri Lanka and continues due south. This in effect entails a loss of payload capability by at least half a tonne, according to ISRO scientists. In the absence of another launch site, this cannot be avoided. It is for this reason that at one point of time, Balasore was explored as a suitable launch site. However, the idea was abandoned owing to other logistical considerations.

The first developmental launch, PSLV-D1, took place on September 20, 1993, but it failed to inject IRS-1E into orbit owing to a software error in the third-stage autopilot which caused the vehicle to tumble. However, ISRO claimed that almost all subsyste ms of the vehicle associated with propulsion, structures, aerodynamics, control and guidance, navigation and staging functions, were validated. The subsequent developmental flights - D2 (October 15, 1994) and D3 (March 21, 1996) - were successful.

PSLV-D2 incorporated some major corrective measures after D1's failure and succeeded in putting the IRS-P2 satellite, weighing 804 kg, into a polar orbit of 880 km x 805 km. The aim of the PSLV-D3 launch, which took place on March 21, 1996, was to demons trate the repeatable performance of the vehicle. Changes in PSLV-D3 were essentially limited to correcting certain minor anomalies observed in the D2 flight. But a significant improvement in PS-3 was also implemented; this included a reduction in the ine rt parts and an increase in propellant loading in order, to improve the vehicle's performance. By this, a 920 kg IRS-P3 satellite was injected into an 844 km x 810 km SSPO. The near-nominal orbital parameters that were achieved proved the basic soundness of the vehicle design and the performance of the systems. Given the two perfect developmental flights, the PSLV continuation programme was put in place in 1996 and the next operational remote-sensing satellite, IRS-1D, was committed to a PSLV launch. Al l the earlier operational IRS satellites were launched aboard Russian launchers.

But launching an operational satellite required increasing the payload capability to 1,200 kg from the existing 900 kg.

The increase achieved in payload capability by altering the strap-on ignition and separation sequences can only be marginal. In PSLV-C1, one required a 30 per cent increase in performance. So, not only the strap-on burn sequence was optimised - the earli er sequence of two ground-lit strap-ons were modified to four - but the propellant loading in the first and second stages was immensely increased without any hardware changes. The PS-1 improvement was significant and it increased propellant loading from 128 tonnes to the present 139 tonnes of solid fuel. PS-2 propellant loading was increased from 37.5 tonnes to the present 40 tonnes. Additional deliverable payload capability was achieved by a major reduction in the upper stage by using composite structu res and by the miniaturisation of the avionics package in the VEB. ISRO's confidence in handling solid fuel was so great that only one static firing of the 139-tonne motor was carried out.

The 45-kg German micro-satellite, TUBSAT, which rode piggyback on PSLV-C2.-S. THANTHONI

THE PSLV-C1 flight took place on September 29, 1997 and it injected IRS-1D, weighing 1,203 kg, into an elliptical 823 km x 301 km orbit against a targeted 817 + or - 35 km circular orbit. The improper orbit injection was due to a shortfall of 14 0 metres/second in the injection velocity after the PS-4 thrust was cut off. Although subsequently IRS-1D's on-board thrusters were used to raise the orbit to an 821 km x 737 km orbit, the sub-optimal performance of the launch vehicle had to be investiga ted. The satellite, however, is fully operational and has been transmitting high-quality data and images.

The PSLV-C1 post-flight analysis confirmed that all the performance improvement modifications had performed well and therefore could be incorporated into a standard vehicle configuration in future flights. The shortfall in injection velocity was primaril y due to an improper thrust delivery by PS-4's twin engines, caused by abnormal depletion of the pressurant helium gas, which is used to force the propellant into the combustion chamber. This was found to be because of the malfunctioning of the pressure regulator system. The anomaly observed was re-created on ground and was traced to a see-saw pressure coupling between the primary and secondary pressure regulators leading to an anomalous increase in pressure and the subsequent venting of gas. For the PS LV-C2 launch, a modification was introduced in the pressure regulator system by providing a buffer volume in order to prevent the dynamic coupling. This system has been validated, according to ISRO scientists.

The present PSLV configuration (circa 1998) can, therefore, be taken as the baseline configuration for a 1,200 kg launch in a 800 km SSPO. One simple modification that is part of the baseline configuration is the change in the VEB layout, with simple jet tisoning systems, for accommodating passenger payloads in the MS category (around 100 kg). KITSAT and DLR-TUBSAT are the first of such MS to be launched, marking PSLV's entry as a commercial launcher. The PSLV continuation project is planned for realisin g six launch vehicles (including C1 and C2) and carries a total sanctioned cost of Rs.665.93 crores. However, the operational vehicle and the associated mission planning infrastructure are versatile and flexible and the PSLV can therefore be configured t o meet a variety of spacecraft and orbit parameters expected of a commercial launcher. Typically, a 1,500 kg satellite can be put into a 500 km SSPO. It can, for example, also be used to place a 3,500 kg payload in an LEO or place a 800 kg payload, at an 18 degree inclination, in a 200 km x 36,000 km Geosynchronous Transfer Orbit (GTO).

Alternatively, the vehicle can carry four satellites of 300 kg each into SSPO. PSLV-C3 and C4 will carry the IRS satellites Cartosat and Resourcesat, which are roughly of the same weight as Oceansat-I. So these launches can at best carry two MS each. Oc eansat-II, which is also of the same class, will also be launched by one of the PSLV-C launches. Also, the need will arise to replace the two in-orbit IRS satellites, 1B and 1C, for continuity of service after their life ends, which will happen very soon . So, to accommodate any other launch profile, commercial or otherwise, would require a plan for additional launch vehicles and funding. Although ISRO claims that it now has a launch rate capacity of between two and four launches a year, domestic demand is only about one launch a year. The question is whether on an external demand Antrix can come up with the requisite money. More important, ISRO has to compete with the Russian, Chinese and French launchers, by having quick turnaround times. It does have a cost advantage but it has also to contend with the Russian undercutting strategy.

Notwithstanding the above, ISRO is investigating various improvements in the launch vehicle configuration. The limitation in PSLV's performance comes from the fact that the vehicle is volume-limited. So any modification has to be within the existing volu me constraint. One important modification is the high performance PS-3 motor by further reducing inert mass and increasing the propellant loading. Another key modification being investigated is to reconfigure PS-4's twin engines as a single engine with o ne-tonne propellant loading. This would enable an enormous gain in payload volume and would be useful for multiple satellite launches (two to three tonnes) in LEO missions. According to Dr.K. Kasturirangan, ISRO Chairman, this configuration called PS-4L1 will be ready in a year.

Another possible improvement is replacing PS-1's SITVC mechanism by flex nozzle for pitch and yaw control although it is not clear how much of payload advantage this can yield. In all, according to Kasturirangan, the total payload capability to SSPO can go up to 1,450 kg. Yet another interesting possibility being investigated by ISRO is a PSLV with a core-alone configuration - with no strap-ons - which is being viewed as a simpler and cost-effective option for medium payload mass to LEO. Studies have es tablished this to be a viable variant of PSLV.

Clearly, with PSLV's different proven stages, playing around with various combinations would give it a payload capability over a wide range and orbit profiles, making it a potentially versatile and flexible commercial launcher. One possibility which ISRO is at present not seriously considering, although it was advocated long ago by some space scientists, is to use the Vikas L40 engine as the booster rocket instead of the solid motor akin to the GSLV design. This modification will arm PSLV with many comb inatorial possibilities - for example, using solid and liquid boosters like Ariane - and enable a GTO launch capability of more than 1,600 kg. Such a capability would be better than the GSLV, with the Russian cryogenic stage that is currently being built and would, in fact, be a far cheaper option. So, until the indigenous GSLV with its two-tonne-plus capability is developed, PSLV itself can become ISRO's work horse and obviate the intermediate route of developing a GSLV with the Russian stage.

But thanks to a lack of vision and strategy, good money has already been paid to the Russians. This is what is actually driving the Russian stage-based GSLV development and not any real advantage in terms of launch capability. But that's another story.

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