The Indian Space Research Organisation (ISRO) is set to use GSLV Mk-III, a three-stage heavy-lift launch vehicle, for the Chandrayaan-2 mission. This powerful rocket is designed to carry satellites into different orbits with significant load capacity. However, there has been growing concern about the type of fuel used in this rocket, Unsymmetrical Di-Methyl Hydrazine (UDMH), along with an oxidiser named nitrogen Tetroxide. This combination is considered “dirty”, as space programmes worldwide are opting for cleaner alternatives like liquid methane or kerosene.
Understanding GSLV Mk-III and Its Fuel
The Geosynchronous Satellite Launch Vehicle Mark III (GSLV Mk-III) consists of two solid strap-ons, a core liquid booster, and a cryogenic upper stage. It is capable of carrying 4-ton class satellites into the Geosynchronous Transfer Orbit, or about 10 tons to Low Earth Orbit. This capability is twice that of GSLV Mk II.
The fuel used in this rocket’s core is UDMH, which is highly toxic and corrosive. This fuel is combined with an oxidiser called nitrogen tetroxide, creating a “dirty combination”. The shift towards cleaner fuels would require the implementation of a cryogenic engine, given that any gas must be maintained at extremely low temperature to remain liquefied.
The Nature of Propellant
Propellant, used to produce thrust in rockets, is a mix of fuel and an oxidiser. The fuel is the substance that burns when combined with the oxidiser for propulsion, while the oxidiser releases oxygen for the combustion process. The mixture ratio of oxidiser to fuel is vital, and propellants are classified according to their state: liquid, solid, or hybrid.
Liquid Propellant and Its Features
In a liquid propellant rocket, fuel and oxidiser are stored separately, and through the use of pipes, valves, and turbopumps, they are fed into a combustion chamber. Here, they combine and burn to produce thrust.
Liquid propellants offer several advantages. They allow control over the flow of propellant to the combustion chamber, enabling engine throttling, stopping, or restarting. However, they also come with their set of challenges. The storage of storable oxidisers such as nitric acid and nitrogen tetroxide is problematic due to their extreme toxicity and reactivity. Similarly, cryogenic propellants, stored at low temperatures, present their own handling issues.
The three types of liquid propellants used in rocketry are petroleum fuels, cryogens, and hypergolic. Petroleum fuels are refined from crude oil; cryogens are liquefied gases stored at very low temperature; while hypergolic propellants ignite spontaneously on contact with each other.
Solid Propellant: Simplicity and Efficiency
A solid propellant rocket is made up of a casing filled with a mix of solid compounds, typically fuel and oxidizer, which burns rapidly to produce thrust. Solid propellant rockets are easier to store and handle compared to their liquid counterparts. However, once ignited, they continue to burn until all the propellant is exhausted.
Hybrid Propellants: A Middle Ground
Hybrid propellant engines use both a solid substance (generally the fuel) and a liquid (usually the oxidizer). These engines offer high performance similar to solid propellants but also have the ability to moderate, stop, or even restart combustion.
Cryogenic Rockets: Efficiency at Low Temperatures
Cryogenic rocket engines use cryogenic fuel or oxidizer – substances stored at very low temperatures. They are highly efficient and provide more thrust for each kilogram of propellant burned compared to other types of rocket stages.
Despite being efficient, these engines require complex ground support systems due to the low storage temperatures. For instance, Oxygen liquefies at -183°C and Hydrogen at -253°C. Such temperatures demand specific storage and filling systems, engine and stage test facilities, transportation, and handling of cryogenic fluids.
Table of Rocket Propellants and Their Specific Impulse
| Propellant Type | Specific Impulse (sec) |
|---|---|
| Petroleum | 250-300 |
| Cryogens | 450 |
| Hypergolic | 200-260 |
| Solid | 210-238 |
| Hybrid | 240-270 |
The Importance of Specific Impulse
The efficiency of rocket propellants is rated by their specific impulse, measured in seconds. This value represents how much thrust is obtained by burning one kilogram of propellant in one second. The specific impulse is characteristic of the type of propellant, but it can vary depending on the operating conditions and design of the rocket engine.