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A long
way from the fire-cracker |
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On September 14, ISRO
took a long stride in its progress to master the technology of ‘cryogenic’
rocket propulsion, a step in aid of our own launching of geocentric
satellites. ISRO has been going it
alone since the promised transfer of know-how from Russia was stalled by the
US in the early nineties. The successful test firing, last month, of an
indigenous engine for a full 1,000 seconds is thus a significant milestone. Rocket technology has
come a long way from the simple fire-cracker rocket where it started. The
simple rocket carries both fuel and oxygen for combustion, in the form of
gunpowder, which has sulphur and carbon to burn and potassium chlorate to
provide the oxygen. The more sophisticated ‘jet engine’ squeezes the air that
the engine is whizzing through, to help burn the hydrocarbon fuel it uses.
But, as the craft go to higher altitudes or into space, where the air itself
is rare, the engine needs to carry its own oxygen along. The economics of rocket
propulsion are that for a craft to be placed in orbit, the fuel load makes up
95 per cent of the total take off load. By the time the article is in orbit,
this 95 per cent load has been burnt or jettisoned, most of it having been
used to propel the fuel, not the payload. To save on the waste of carrying
empty fuel tanks the full way, rockets are built in stages, each stage being
jettisoned as the rocket goes higher. In this way of working out
the loads, a very great concern is the fuel load of the last stage, because
this decides how much ‘slack’ there is for the ‘payload’! For this ‘last stage’
economy, by far the best fuel material is pure hydrogen, to burn in pure
oxygen, and to be carried highly compressed, or better, liquefied. Liquid
gases not only occupy less space but also need not be under high pressure,
which requires heavy cylinders. This is where the problem
becomes tricky. One way of liquefying gases is to compress them so that their
boiling point increases — like what happens in a pressure cooker — till the
gas is ‘below’ its boiling point and so it liquefies. But this rising boiling
point has a limit and, for hydrogen, this limit is well below the freezing
point of water! Hence, to carry hydrogen as a liquid fuel for the last stage
rocket, the gas needs to be cooled well below this temperature and kept that
way till it is used! And there’s the reason for calling the technology
‘cryogenic’. But thanks to cryogenic
propulsion, a launch vehicle can place in orbit a payload two times heavier
than otherwise. According to U.R. Rao, the former chairman of ISRO, ‘Without
cryogenic technology there is no way we could think of geo-synchronous
satellite launching.’ One can imagine the need
to maintain such low temperatures would put great constraints on the design
of the engine, the fuel tanks and what have you. And rightly, there is
complex engineering, control systems, electronics, insulation, to keep the
fuel cool and finally, during the burn, to pump it so that the hydrogen and
oxygen can reach a temperature of thousands of degrees centigrade, to propel
the craft into orbit. There are 12 tonnes of liquid gases and the engine runs
at 46,000 revolutions a minute. According to ISRO, the
behaviour of materials at cryogenic temperatures of less than 250 degree
below zero, the turbo pump operating at very high speeds, the elaborate
chilling process for preparing the ground and on-board systems, the interplay
of critical engine parameters and a host of technical aspects make the
development quite challenging. Since 1945, engineers the
world over have been working at the technology, and barely a handful of
countries now have the capability of putting a satellite into orbit. Apart
from this being a billion dollar opportunity in itself, the capability is of
such strategic value that one can understand the despatch with which the US
shushed cash-strapped Russia from fouling the market! |
