ISRO launches PSLV with Singaporean satellites

by William Graham

The Indian Space Research Organisation used its PSLV rocket to deploy a pair of Singaporean satellites Saturday, in a commercial launch contracted by its subsidiary NewSpace India Limited. The PSLV-CA rocket lifted off at 08:49 UTC (2:19 p.m. local time) from the Satish Dhawan Space Centre.

Saturday’s launch carried the TeLEOS-2 radar-imaging satellite, to be operated in partnership between the government of Singapore and ST Electronics. The smaller Lumelite-4 satellite was also deployed, while seven additional payloads rode attached to PSLV’s upper stage, which will remain in orbit as an experimental platform after completing the deployment of the two satellites.

TeLEOS-2 carries a synthetic aperture radar (SAR) payload, capable of imaging the Earth without being affected by darkness or cloud cover. Constructed by ST Electronics, it is based on the same SS-400 bus as the earlier TeLEOS-1 satellite, which was launched in December 2015, but with the SAR payload installed instead of the electro-optical system aboard the earlier satellite.

The satellite, which has a mass of 741 kilograms, is powered by four solar panels which will be deployed once it is on orbit and will be capable of producing images with a resolution of one meter.

Lumelite-4 is a 12-unit CubeSat that is being carried for the Agency for Science Technology and Research (A*STAR), the National University of Singapore (NUS), and other industrial and government partners. With dimensions of 20 by 20 by 30 centimeters and a mass of 16 kilograms, it will be used for an on-orbit demonstration of a space-based VHF Data Exchange System (VDES) for maritime users.

The benefits of using VDES in this way are seen as improving safety and communications for ships at sea, with the system a potential long-term replacement for the Automated Identification System (AIS) which ships currently use to broadcast position and status information.

After deploying TeLEOS-2 and Lumelite-4, PSLV’s upper stage will remain in orbit to host an array of experiments. Named PSLV Orbital Experimental Module 2 (POEM-2) in this role, the stage is equipped with solar panels and seven non-deployable experiment packages. It is expected to operate in this role for around a month.

POEM-2 carries seven attached payloads for multiple users. The Advanced Retarding Potential Analyser for Ionospheric Studies (ARIS-2) will be used by the Indian Institute of Science and Technology (IIST) to study the density and distribution of ions and electrons in Earth’s ionosphere to help with space weather monitoring. It follows on from the first ARIS, which was carried aboard PSLV C45 in April 2019.

PSLV In-orbital Onboard Computer (OBC) and Thermals, or PiLOT, which is also being flown for IIST, will validate the operation of an onboard computer system and associated software in the flight environment, paving the way for future missions. This is the second PiLOT experiment to be flown using the POEM platform, with another PiLOT experiment having flown aboard POEM-1 in June 2022.

An ARKA200 electric propulsion system is being carried for on-orbit testing on behalf of its developer, Bellatrix Aerospace. Three payloads for Dhruva Space will test out its Dhruva Satellite Orbital Deployer (DSOD) 3U and 6U CubeSat deployment mechanisms and the DOSL-Transceiver communications system.

TeLEOS-2 during integration with PSLV (Credit: ISRO)

POEM-2’s final payload, Starberry-Sense, is being flown for the Indian Institute of Astrophysics (IAP). This will test a low-cost star tracker that has been developed for future small satellite missions. The Starberry-Sense payload uses off-the-shelf components and is based around a Raspberry Pi Zero miniature computer, with attached camera and lens assemblies.

The PSLV, or Polar Satellite Launch Vehicle, is a four-stage rocket built and operated by the Indian Space Research Organisation (ISRO). Introduced in September 1993, it has made 56 flights prior to the TeLEOS-2 mission, with 53 of these completed successfully, two failing and one resulting in a partial failure with an off-nominal orbit attained. Saturday’s launch, designated PSLV C55, will aim to continue the rocket’s current run of 15 consecutive successful missions.

Depending on mission requirements – particularly payload mass and target orbit – PSLV can fly in several different configurations, varying the number of boosters attached to the first stage to adjust performance to mission requirements. The PSLV-CA, DL, QL, and XL configurations have zero, two, four, and six boosters respectively. The PSLV-G configuration, the original version of the rocket which has not flown since 2016, used six boosters of a smaller type than the other variants.

For Saturday’s mission, PSLV C55 flew in the CA, or “Core Alone” configuration, with no additional boosters. It launched from the First Launch Pad (FLP) at the Satish Dhawan Space Centre (SDSC), formerly known as the Sriharikota High Altitude Range, on Sriharikota island in the Bay of Bengal. SDSC is India’s primary launch site and has been the venue for all of the country’s orbital launches.

The PSLV C55 mission marked the first use of a new integration process, taking advantage of a new mobile launch pedestal (MLP), to support a PSLV mission from the First Launch Pad. Previously, PSLV rockets launched from the FLP have been built up at the launch pad, but with the new pedestal their lower stages can be pre-assembled at a nearby PSLV Integration Facility (PIF) before they are transferred to the pad.

ISRO hopes that this will allow future launch campaigns to be run in parallel, with the build-up of the next PSLV able to commence at the PIF while the preceding rocket is still on the launch pad, ultimately increasing the number of launches that can be made from the FLP.

The FLP is one of two active orbital launch complexes at SDSC. It is primarily used for PSLV launches and has also recently supported the new Small Satellite Launch Vehicle (SSLV) — which made its first successful launch in February after a failure last August. The Second Launch Pad, located about 1.6 kilometers to the south, can also be used by PSLV as well as the larger Geosynchronous Satellite Launch Vehicle (GSLV) Mk.II and Mk.III.

The first and second stages of PSLV C55 leave the PSLV Integration Building for the launch pad (credit: ISRO)

Following Saturday’s countdown, the PSLV C55 mission began with the ignition of the first stage roll control thrusters (RCTs) at the T-3 second mark in the countdown, followed by the first stage main engine at T0. The first stage, or PS1, is powered by an S139 solid rocket motor, burning hydroxyl-terminated polybutadiene (HTPB) propellant. The RCTs are liquid-fueled vernier thrusters, which help to steer the rocket during ascent.

The first stage burned for a little under 110 seconds, propelling PSLV to an altitude of 50 kilometers and a velocity of 1.74 kilometers per second before it burned out and jettisoned. About two-tenths of a second after first stage separation, the second stage – known as PS2 or PL40 – ignited to continue the climb towards orbit.

The liquid-fueled PS2 is powered by a single Vikas engine, burning 41 tons of UH25 propellant – a mixture of three parts unsymmetrical dimethylhydrazine and one part hydrazine hydrate – and dinitrogen tetroxide oxidizer. This burn lasted just over 150 seconds and encompasses several significant mission milestones. Five seconds into the burn, the rocket transitioned to closed-loop guidance. As the ascent continued, it crossed the Kármán line – the internationally-recognized edge of space, at an altitude of 100 kilometers – and shortly afterwards the payload fairing separated.

The fairing, termed a “heat shield” by ISRO, protects the payload from the Earth’s atmosphere during ascent, while also preserving the aerodynamic characteristics of the rocket. Once the vehicle reached space it was no longer needed and is discarded to reduce the rocket’s weight. Fairing separation was around the three-minute, four-second mark in the flight, or about 74 seconds into the second stage burn.

Second stage shutdown and separation occurred just over four minutes and 20 seconds after liftoff. About 1.2 seconds later, the HPS3 third stage ignited for an approximately-70-second firing of its S-7 solid rocket motor. With this complete, the mission entered a coast phase as the vehicle climbs towards the apogee – or highest point – of its trajectory. The spent third stage was jettisoned partway through this coast, at around nine minutes, 46 seconds mission elapsed time.

PSLV C55’s mission profile (credit: ISRO/NSIL)

PSLV’s fourth stage, or PS4, made a single burn beginning shortly after the 16-minute, 22-second mark in the launch. The PS4 has a pair of engines burning monomethylhydrazine and MON-3 – the latter, mixed oxides of nitrogen (MON) is a mixture of 3% nitric oxide and 97% dinitrogen tetroxide. The two-minute, 11.6-second burn of the PS4’s engines completed PSLV’s insertion into low Earth orbit and its conclusion marked the end of powered flight for Saturday’s mission.

PSLV was targeting a circular orbit at an altitude of 586 kilometers and an inclination of ten degrees to the equator. TeLEOS-2 was the first satellite to separate, deploying 57 seconds after fourth stage cutoff. Lumelite-4 followed 50 seconds later.

With its deployable payloads separated, the fourth stage will begin passivation to ensure it can remain on orbit safely in its extended role as POEM-2.

Saturday’s launch was India’s third of 2023, following successful missions for the smaller SSLV in February and the larger GSLV Mk.III in March. India’s next launch is expected to be a GSLV Mk.II mission in late May, deploying a new satellite for the NavIC – or IRNSS – navigation system.

(Lead image: PSLV C55 at the First Launch Pad ahead of the TeLEOS-2 mission. Credit: ISRO)

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