January 6, 2014
Congratulations, ISRO, for successfully launching the GSLV-D5 (and the GSAT-14 satellite with it) on January 5. Even as I write this, ISRO has put out an update on its website: “First orbit raising operation of GSAT-14 is successfully completed by firing the Apogee Motor for 3,134 seconds on Jan 06, 2014.”
With this launch comes the third success in eight launches of the GSLV program since 2001, and the first success with the indigenously developed cryogenic rocket-engine. As The Hindu reported, use of this technology widens India’s launch capability to include 2-2.5 tonne satellites. This propels India into becoming a cost-effective port for launching heavier satellites, not just lighter ones as before.
The GSLV-D5 (which stands for ‘developmental flight 5′) is a variant of the GSLV Mark II rocket, the successor to the GSLV Mark I. Both these rockets have three stages: solid, liquid and cryogenic. The solid stage possesses the design heritage of the American Nike-Apache engine; the liquid stage, of the French Vulcain engine. The third cryogenic upper stage was developed at the Liquid Propulsion Systems Centre, Tamil Nadu—ISRO’s counterpart of NASA’s JPL.
There is a significant difference of capability based on which engines are used. ISRO’s other more successful launch vehicle, the Polar Satellite Launch Vehicle (PSLV), uses four stages: alternating solid and liquid ones. Its payload capacity to the geostationary transfer orbit (GTO), from which the Mars Orbiter Mission was launched, is 1,410 kg. With the cryogenic engine, the GSLV’s capacity to the same orbit is 2,500 kg. By being able to lift more equipment, the GSLV hypothetically foretells our ability to launch more sophisticated instruments in the future.
The better engine
The cryogenic engine’s complexity resides in its ability to enhance the fuel’s flow through the engine.
An engine’s thrust—its propulsive force—is higher if the fuel flows faster through it. Solid fuels don’t flow, but they let off more energy when burnt than liquid fuels. Gaseous fuels barely flow and have to be stored in heavy, pressurised containers.
Liquid fuels flow, have higher energy density than gases, and they can be stored in light tanks that don’t weigh the rocket down as much. The volume they occupy can be further reduced by pressurising them. Recall that the previous launch attempt of the GSLV-D5, in August 2013, was called off 74 minutes before take-off because fuel had leaked from the liquid stage during the pre-pressurisation phase.
Even so, there seems no reason to use gaseous fuels. However, when hydrogen burns in the presence of oxygen, both gases at normal pressure and temperature, the energy released provides an effective exhaust velocity of 4.4 km/s—one of the highest (p. 23, ‘Cosmic Perspectives in Space Physics’, S. Biswas, 2000). It was to use them more effectively that cryogenic engines were developed.
In a cryogenic engine, the gases are cooled to very low temperatures, at which point they become liquids—acquiring the benefits of liquid fuels also. However, not all gases are considered for use. Consider this excerpt from a NASA report written in the 1960s:
A gas is considered to be cryogen if it can be changed to a liquid by the removal of heat and by subsequent temperature reduction to a very low value. The temperature range that is of interest in cryogenics is not defined precisely; however, most researchers consider a gas to be cryogenic if it can be liquefied at or below -240 degrees fahrenheit [-151.11 degrees celsius]. The most common cryogenic fluids are air, argon, helium, hydrogen, methane, neon, nitrogen and oxygen.
The difficulties arose from accommodating tanks of super-cold liquid propellants—which includes both the fuel and the oxidiser—inside a rocket engine. The liquefaction temperature for hydrogen is 20 kelvin, just above absolute zero; for oxygen, 89 kelvin.
Chain of problems
For starters, cryopumps are used to trap the gases and cool them. Then, special pumps called turbopumps are required to move the propellants into the combustion chamber at higher flow-rates and pressures. Next, relatively expensive igniters are required to set off combustion, which also has to be controlled with computers to prevent them from burning off too soon. And so forth.
Because using cryogenic technology drove advancements in one area of a propulsion system, other areas also required commensurate upgrades. Space engineers learnt many lessons from the American Saturn launch vehicles, whose advanced engines (for the time) were born of using cryogenic technology. They flew between 1961 and 1975.
In the book ‘Rocket Propulsion Elements’ (2010) by George Sutton and Oscar Biblarz, some other disadvantages of using cryogenic propellants are described (p. 697):
Cryogenic propellants cannot be used for long periods except when tanks are well insulated and escaping vapours are recondensed. Propellant loading occurs at the launch stand or test facility and requires cryogenic propellant storage facilities.
With cryogenic liquid propellants there is a start delay caused by the time needed to cool the system flow passage hardware to cryogenic temperatures. Cryogenically cooled fluids also continuously vaporise. Moreover, any moisture in the same tank could condense as ice, adulterating the fluid.
It was in simultaneously overcoming all these issues, with no help from other space-faring agencies, that ISRO took time. Now that the Mark II has been successfully launched, the organisation can set its eyes on loftier goals—such as successfully launching the next, mostly different variant of the GSLV: the Mark III, which is projected to have a payload capacity of 4,500-5,000 kg to GTO.
While we are some way off from considering the GSLV for manned missions, which requires mastery of reentry technology and spaceflight survival, the GSLV Mark III, if successful, could make India an invaluable hub for launching heavier satellites at costs lesser than ESA’s Ariane program, which India used in lieu of the GSLV.
Good luck, ISRO!