India’s new SSLV rocket will make its second attempt to reach orbit Friday, six months after its maiden flight ended in failure. Friday’s launch, which will carry a trio of satellites, is due for liftoff at 9:18 AM local time (03:48 UTC) from the Satish Dhawan Space Centre.
The Small Satellite Launch Vehicle, or SSLV, has been developed by India to enable lower-cost and more rapidly responsive access to space for small satellite missions. It flew for the first time last August, however, an issue during stage separation meant that it could not achieve a stable orbit. As a result of that failure, changes have been made to the rocket to give Friday’s launch a better chance of success.
As a demonstration flight, the principal objective of Friday’s launch will be to prove SSLV’s ability to reach orbit, although the rocket will be taking three satellites along for the ride. The largest of these is Earth Observing Satellite-07 (EOS-07) for the Indian Space Research Organisation (ISRO). It is joined within SSLV’s payload fairing by the smaller AzaadiSAT-2 and Janus-1 spacecraft.
EOS-07 is the latest mission in ISRO’s Earth Observing Satellite (EOS) series, a loose collection of missions dedicated to collecting scientific data about our planet. EOS includes satellites that were formerly part of multiple programs — including Cartosat, Oceansat, and RISAT — as well as technology demonstrators like EOS-07. When the series began, satellites already in orbit were not renamed, so the first EOS satellite was EOS-01 — formerly RISAT-2BR2 — which was deployed by a PSLV rocket in November 2020.
The EOS-07 satellite will serve to demonstrate new technologies for future Earth-observing missions. While ISRO has not published many details about the satellite, it appears to be based on an Indian Mini Satellite 1 (IMS-1) derived bus, and it has a mass of 156.3 kilograms with an expected on-orbit lifetime of about a year. EOS-07 carries two instruments: the Millimetre-wave Humidity Sounder (MHS) and Spectrum Monitoring Payload (SMP).
AzaadiSAT-2 is the second satellite ISRO has launched for Space Kidz India, an organization aimed at giving students at schools and colleges in India the experience of building and operating space hardware. AzaadiSAT-2 replaces the organization’s first satellite, which was aboard SSLV’s unsuccessful maiden flight. Once in orbit, it will broadcast telemetry to amateur radio users and provide a store-and-forward communication capability. The satellite is also equipped with a number of student-built experiments including a low-resolution camera and radiation detector whose output will also be made available to amateur radio enthusiasts.
With a mass of about 8.7 kilograms, AzaadiSAT-2 is a rare example of an eight-unit (8U) CubeSat. The CubeSat standard, which has become ubiquitous for small satellite missions, consists of a series of form factors based around a cube with sides of 10 centimeters — which is defined as one unit. The size of a CubeSat is typically given in terms of the number of units of which it is comprised, so an 8U satellite would take up eight units — in the case of AzaadiSAT-2, arranged two by two by two — to give the overall satellite a cube shape with sides of 20 centimeters.
Janus-1 is also a CubeSat, although built to the more common 6U configuration, with sides measuring 10, 20, and 30 centimeters and a mass of 11.5 kilograms. Operated by US company Antaris Inc, it is intended to serve as a proof-of-concept for the company’s software platform. The satellite was developed in ten months and carries a suite of five experiments, including radio and optical communication, internet of things (IoT) data relay, and on-orbit machine learning. In another aspect of the mission, a virtual clone of the satellite’s software is being operated in a simulated mission, which Antaris will be able to compare with the operation of the real satellite to help refine its simulation technology.
Friday’s mission, which is designated SSLV D2, is expected to inject the three satellites into a circular low Earth orbit at an altitude of 450 kilometers at an inclination of 37.2 degrees. The mission should last about fifteen minutes from liftoff to the final spacecraft separation.
SSLV is a four-stage vehicle, consisting of three solid-propellant stages designated SS1, SS2, and SS3, topped with a liquid-propellant fourth stage — called the Velocity Trimming Module (VTM) — which is responsible for ensuring an accurate insertion into the planned target orbit.
All three solid-fueled stages use a propellant formula based on hydroxyl-terminated polybutadiene (HTPB), with the motors designated S-85 for SS1, S-7 for SS2, and S-4 for SS3. The VTM burns monomethylhydrazine (MMH) and mixed oxides of nitrogen (MON3) — a mixture of 97% dinitrogen tetroxide and 3% nitric oxide.
The August 2022 launch of SSLV D1 — SSLV’s maiden flight. (Credit: ISRO)
After SSLV failed to reach orbit on its maiden flight last August, ISRO conducted a review of the mission to determine the cause of the failure and what lessons could be learned from it. This determined that although all three of the solid-propellant stages had operated as expected, a vibration during second stage separation disrupted the operation of the inertial navigation system (INS).
The vibration temporarily overloaded the six accelerometers that feed data to the INS to determine the rocket’s trajectory and orientation, causing them to provide differing and incorrect data. Although the problem was transient, it continued for long enough for the INS to determine that the sensors were faulty and that their input should be disregarded, and for the vehicle to enter its contingency “salvage mode.” Without input from the accelerometers, the rocket attempted to continue the mission using the expected mission parameters and timings.
Because data from the accelerometers was being ignored, the vehicle had no way to determine its acceleration or to ensure accurate orientation, which resulted in a shortfall of about 56 meters per second in its final velocity. This corresponded to an orbital perigee of 75.66 kilometers — within the atmosphere — instead of the planned 356 kilometers.
The report on SSLV D1’s failure made five recommendations that have been implemented for Friday’s SSLV D2 mission. The separation system between the second and third stages has been changed from an expanding bellow type to a marman band with springs to push the stages apart. Changes have also been made to the equipment bay and satellite deck to adjust their resonant frequencies.
The INS has been modified to be more resilient in the face of transient events and to implement a longer delay before implementing salvage mode when sensor failures are detected. A new contingency mode will allow the rocket to be guided using position data from the Navigation with Indian Constellation (NavIC – formerly IRNSS) satellite navigation system before resorting to open-loop control. Finally, if the rocket is forced to fall back to salvage mode, it will include the capabilities of the VTM stage when calculating available performance.
ISRO plans to build dedicated launch facilities for SSLV, including a pad at a new spaceport near the town of Kulasekharapatnam that will allow direct access to polar orbits without the need to fly complex dog-leg trajectories that can limit a rocket’s payload capacity. For its initial test launches, however, SSLV is flying from the First Launch Pad (FLP) at the Satish Dhawan Space Centre. The FLP was built in the early 1990s for ISRO’s Polar Satellite Launch Vehicle (PSLV), with which SSLV shares the launch pad.
Integration of SSLV D1 at the First Launch Pad ahead of its August 2022 launch (Credit: ISRO)
The FLP is one of two active launch pads at the Satish Dhawan Space Centre, along with the Second Launch Pad (SLP), which is used by PSLV and Geosynchronous Satellite Launch Vehicle (GSLV) rockets. The center — formerly known as the Sriharikota High Altitude Range — has been the site of all of India’s orbital launches to date.
As Friday’s countdown reaches zero, SSLV’s first stage will ignite and the vehicle will lift off. Burning for about 114.6 seconds, SS1 provides the thrust that will power the vehicle through Earth’s atmosphere. Just over nine seconds after burnout, with the rocket at an altitude of 94 kilometers, the stage separation sequence will begin. This will see the SS2 stage ignite, followed three-tenths of a second later by the jettison of the spent first stage. Finally, five seconds after first stage separation, the second stage will discard its interstage.
The second stage burn is expected to last 124 seconds. SSLV’s payload fairing will separate about 34 seconds into the burn, exposing the satellites to space for the first time. After the end of the burn, Friday’s mission will enter a brief coast phase as the rocket climbs, before second stage separation takes place at six minutes, 24.2 seconds mission elapsed time. The third stage will ignite 9.8 seconds later for its 103.9-second burn.
After the third stage burns out, there will be another 177-second coast phase before the third stage separates from the fourth. The fourth stage — the VTM — will ignite eight and a half seconds later to complete orbital insertion. ISRO has not published the planned burn time for the VTM prior to Friday’s launch, however, it should be a fairly short firing as EOS-07 is scheduled to separate from the upper stage at the 13-minute, five-second mark in the mission — 102 seconds after the burn is due to start.
Mission timeline for the SSLV D2 flight (Credit: ISRO)
Janus-1 will be the next satellite to be deployed, separating 95 seconds after EOS-07. AzaadiSAT-2 will follow 20 seconds later at fifteen minutes mission elapsed time.
Friday’s launch marks India’s first of 2023 and the country’s first orbital mission since November’s successful launch of a PSLV rocket with the EOS-06 satellite. Success for the SSLV D2 mission would be a big step forwards for ISRO, as it would prove the rocket that the organization hopes will help it to capture a larger proportion of the commercial launch industry. SSLV is expected to fly at least one more demonstration mission this year before it can be declared operational.
ISRO’s next launch is currently expected to take place in March. This could be the second OneWeb mission to launch aboard a GSLV Mk.III rocket, which was expected to occur early this year. A GSLV Mk.II mission is also scheduled, carrying NVS-01 — a replacement satellite for the NavIC navigation system. These missions require the same launch pad, so it is unclear if both will be able to take place in the same month; one of them will likely take place at a later date.
(Lead photo: SSLV D2 at the launch pad ahead of Friday’s launch. Credit: ISRO)
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