August 06th, 2022

Small is the new big in space industry. The space industry's current trend of going smaller is a result of technological advancements making processors and computers very small and capable of performing complicated tasks which were otherwise performed by bulky processors in past. Advancements in rocket propulsion technology made propellants that are lighter in weight. Throughout the world each and every space technology agency is on a race for making small launch vehicles. Satellites getting small and cheap have prompted many media companies to launch their own small satellites instead of seeking service from a heavy government owned satellite. Because of multiple competitors and heavy competition the prices are low and demand is high.

ISRO's Antrix Corporation had been able to generate a huge amount of revenue due to the relative cheapness in launch service. Problem however is ISRO’s current availability rate is lower than market's requirement. It takes around a month to build a PSLV or GSLV and also a huge amount of space in payload fairing is reserved for a Satellite from India. Elsewhere due to the availability of knowledge in public domain and lower investment costs Private companies are making foray into space launch services providing an alternative to the monopoly of Government funded space agencies. If these private companies take away the available customers ISRO would not just loose its revenue but also an opportunity to bring technological and commercial capability in India. Being quick and perfect in its efforts ISRO took no time designing a small launch vehicle that would satiate the growing demands of launch service with also providing its service in competitive prices. The prices charged by ISRO are as competitive as some private company.

DESIGN

Largely taking it's roots from PSLV's design the SSLV is a 4 stage launch vehicle with fourth stage being a Velocity Trimming stage. Velocity trimming is an interesting concept explained later in this article. The first three stages are largely common with PSLV. Where the first stage is a 3 segment solid booster and 2nd and 3rd stage having solid rocket motors derived from PSLV’s PS3 stage. The whole aim was to quickly design a Launch Vehicle that can be very quickly assembled by a small team and launched by a small team. The SSLV stages will be manufactured in modules totally by Private Industrial partners of ISRO once development is complete. The most interested in this project is L & T. Other companies like MTAR, Godrej Aerospace are major contributors in supplying critical components for both PSLV and GSLV.

The aim is ease of operation and reduction in costs’. Today prices of ISRO's launch services are high due to lower launch rates since a private company cannot be contracted to supply their components in large numbers it is unprofitable for them to supply smaller numbers. Since demand for launch services is high ISRO intends for increased number of launches hence increased orders for private companies which will drastically reduce prices. The ease of operation comes with large autonomy in assembly and launch procedures. Since solid rocket boosters once filled in with propellants can be stored for longer times and quickly assembled in launch ready position.

The first developmental launch is taking place from SHAR, but an alternate launch site in Thoothukudi in Tamil Nadu has been selected

​FIRST STAGE BOOSTER

The solid rocket propellant technology is widely used in India for Missile making purposes and also in initial stages of PSLV and GSLV. The most widely used fuel is HTBP. The first stage called SS1, the S-85 motor of SS1 will have 89 tonnes of propellant mass. The first stage will have 3 segments of motor casing. Since the casing has to be lined with insulation, ISRO has decided to use simple natural rubber based insulation.

A solid propellant rocket motor has a long combustion chamber concentrically filled with solid propellants. Once ignited electrically the propellant burn radially outward. The nozzle has to withstand a large amount of heat with no cooling and hence it is made of materials with high specific heats. In S-85 motor the propellant grain is circular in shape which intends to increase the thrust with respect to time. Please watch the video to understand what is grain and why it's shape matters.

The outer casings are made of 8mm thick sheets made of 15CDV6 sheets rolled into a cylindrical shape and then welded. The top dome is forged from the same material. The skirts and segment joints are also manufactured using the same material. The entire stage is 2 Metres in diameter and 21 Metres in height and would produce 2600 kN of thrust in Vacuum. To understand how solid rocket motor works do watch this amazing video.

Electro Mechanical Actuator of first stage.

SECOND AND THIRD STAGE SOLID MOTORS

These stages use a unique Solid Booster derived from PSLV’s PS3 stage. The PS3 is of 2 m diameter and 4850 litre capacity. PS3 motor case weighing 325 kg was initially realised by filament winding process using Aramid/Epoxy for the construction for the shell and Carbon/Aramid/Epoxy hybrid construction for the skirt extension. A new improved version called High Performance motor case (HPS3) has been realised for PS3 motor case. An optimum design for this was evolved by selecting appropriate different profiles for both the domes, introducing dome reinforcements and adopting modified helical winding with wide band in multi axis filament winding machine.

The motor case is designed for a maximum operating pressure of 60.8 bar using Aramid/Epoxy for the construction of shell. The motor case consists of helical hoop layers and Aramid fabric for dome reinforcements. Carbon/Aramid/Epoxy hybrid construction for the skirt extension to attach with the fourth stage of PSLV is designed for taking the structural tensile load of 460 kN and a compressive load of 215 kN. A design factor of 1.25 is applied over the flight loads. Aluminium alloy end fittings are provided for the assembly of igniter and nozzle on either side. After rocasin insulation lay-ups, 7500 kg of solid propellant is cast.

Since a huge amount of technical know-how is readily available the SS2 and SS3 stage of SSLV also use the exact same technology. Here also Aluminium alloy is used for end bosses and skirt rings and Nozzle is made up of Carbon phenolic liners and graphite while motor casing is made up of filament case wound with medium strength carbon fibre. Both SS2 and SS3 uses HTBP as propellant. The SS2 uses 7.7 tonnes on propellant has a 2m diameter and 2m length and maximum thrust 250 kN while SS3 uses 4.5 tonnes on propellant has 1.7m and 1.6m as length with maximum thrust of 160 kN. The second and third stages of SSLV thus only have a size difference but technology wise are highly identical.

​VELOCITY TRIMMING MODULE (COOLEST STAGE)

This is the most interesting stage of SSLV and this one will be the only liquid propellant powered stage of the entire SLV. The 4th stage’s main function is to deliver multiple satellites into multiple orbits. ISRO has earlier modified PSLV's PS4 stage in such a way that it could deliver satellites in multiple orbits. In order to achieve multiple orbits, two different options, namely, having separate propulsion module or employing upper stage restart, are available. SSLV has selected the second option-restart of Velocity Trimming Module and having long coasting between two restarts.

EOS-02 being loaded into VTM

To launch satellites into multiple orbits the Velocity trimming module is essentially set into orbit itself and performs multiple burns to do roughly the same activity called Orbital Manoeuvre. Even though an orbit is determined by inclination, eccentricity and various other Keplarian elements, The change in orbit is performed by increasing or decreasing ∆V (Delta Vee). This is done by either firing or I should say re-firing the engines of a spacecraft to increase it's ∆V or turning a spacecraft around itself in 180° and then firing spacecraft to decrease it's ∆V. ∆V is the impulse per unit of spacecraft mass that is needed to perform an orbital manoeuvre.

The velocity trimming module’s design derived from PSLV’s upper stage PS4. This 4th stage of SSLV has Propulsion deck along with tankages positioned outside the central cylinder with 4x50N thrusters, also 4 thrusters just below each of the two tankages. This configuration thus has a total 12 Nos. 50N thrusters. PSLV has six of them for attitude control during its coast phase. On top of it there is a satellite adaptor which can hold multiple satellite deployment mechanisms based on launch requirements. Satellites can be stacked around the periphery on a Quadpack. The avionics packages are assembled on a deck peripherally around central cylinder. This configuration can accommodate the satellite protrusions below launch vehicle interface, APD was brought near the mid-section of central cylinder for better accessibility and for CG and MI balancing positioned in the remaining 240Deg. around the central cylinder. All components are sub-assembled as modules and integrated into each other during assembly this is done to reduce waiting time for a critical activity.

VTM configuration top view showing tankages, thruster blocks, APD, ISP and quad-pack location, (b) Isometric view of module showing tankages, axial, tangential and radial thrusters, (c) cut section view.

sub-assembly 1: Propulsion deck with 50N thruster 2: thrust block assembly with 50N thruster, sub-assembly 3: avionics package deck 4: structure.

mounting system called ISRO Ball Lock or IBL of various sizes is used to stack multiple small satellites circumferentially around it. It uses a Pyro actuated ball lock separation system. This provides a debris free release and very less shock during pushing out a satellite into orbit. SSLV would be using IBL-296 and IBL-230. IBL stands for ISRO Ball Lock and the number stands for the interface diameter in mm. There are several of these IBLs available with ISRO and used on multiple launch vehicles. It is beyond the scope of the article to explain all separation mechanisms in detail but We have provided links here for whoever wanted a further reading.

ISRO Satellite Separation

Similarly there are multiple satellite separation systems used in SSLV upper stage. Satellites can be stacked one above other, 3 satellites can be mounted circumferentially and so on. Multiple satellite mounting options are shown in pictures below

SATELLITE SEPARATION SYSTEM

The satellite separation system has the ultimate job of releasing the sensitive payload away from the terminal stage of the Launch Vehicle. ISRO has developed various satellite separation systems for the array of its launch vehicles based on mission requirements. A typical separation system has 3 main sequences 1. Actuation 2. Release 3. Separation Impulse.

ISRO Ball Lock as mentioned before functions by releasing a preloaded ball locked joint between two rings by rotating a ball retainer ring using pyro assisted thrusters. This system is characterised by good joint stiffness, lightweight construction, tuneable jettisoning velocity, debris free actuation and redundancy in initiation. The system generates low release shock. The pyro thrusters are the source of energy for release function. It is powered by ISRO standard cartridge with squib based electrical initiation. The firing of the cartridge generates pressure inside the thruster, which moves a piston and rotates the retainer ring.

For the sake of understanding we will see the example of IBL-298 used on PSLV. The configuration of the separation system is shown in images below. The system consists of three rings. The fore-end ring interfaces with the satellite. The aft end ring is attached to the launch vehicle deck. These rings are held together using a ball lock and pre-loaded radially using a retainer ring which provides the required joint stiffness. The retainer ring is locked in position using two shear screws attached to the stopper bracket. For releasing the system, the retainer ring is to be rotated overcoming the redial pre-load and friction and shearing the pins through a fixed angle. The required angle is reached when the lug of the retainer ring makes contact with the stopper mounted on the aft ring. The holes on the retainer ring align with the balls in the aft ring and jettisoning springs cause the separation of the forend ring. The configuration of the piston and the retainer lug, which is of interest in this model, is shown in Fig:4. There are two pyro thrusters mounted on a bracket attached to the aft ring keeping a small gap from the retainer ring lug. The pyro thrusters are mounted so as not to make contact with the stopper lug and retainer ring at the end of the stroke. This is to prevent the impact of the piston on the lug at the end of the stroke. There are eight sets of compressed springs providing jettisoning energy for the spacecraft. The springs react on the fore end ring, which forms a part of the spacecraft after separation.

​LAUNCH ON DEMAND

Their won’t be any point in developing such a launch vehicle if it uses a bulk of large no. of systems to operate. All the sub-assemblies and components made for separation systems are modular. The onboard computer has been kept simple and quickly reconfigurable to suit mission launch requirements. The avionics are said to be miniaturised for low cost using readily available industrial off the shelf. New Mission Management Computer (MMC) and Sequencing Execution Module (SEM) has been developed. The miniaturized telemetry system has been developed for SSLV has achieved 70 % mass reduction in the telemetry package.

​COMPETITION FROM HOME AND ABROAD.

ISRO has earned 1000s of crore Rupees in revenue by launching foreign satellites and is about to make a huge dent in commercial space market by being able to launch satellites at competitive prices. But Space Industry's trend going small and cost effective has given birth to many space start-ups both in India and abroad. The SSLV was quickly made by ISRO to have a matching launch vehicle that can compete with local and foreign commercial space launch companies. Here I would list out some rockets that would give a tough competition to SSLV.

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1 Kuizhou 1 China
2 Rocket Lab Electron.
3 Agnikul Agnibaan
4 Skyroot Aerospace Vikram 1
5 CAS Space ZK-1A
6 Virgin Orbit Launcher One.

GALLERY