SCRAMJET: Another Upward Stride by ISRO
*Dilip Ghosh
Indian Space Research Organisation, ISRO successfully tested two indigenously
developed scramjet engines on the 28th August. The test of the
engines was conducted on board a Rohini sounding rocket, also called Advanced
Technology Vehicle, ATV which was launched from the Satish Dhawan Space Centre
at Sriharikota in Andhra Pradesh. The successful technology demonstration
of scramjet engines in flight is a modest yet important milestone in ISRO’s
endeavour to design and develop such engines. With this, India has now joined
the elite club of nations that have designed, developed and successfully tested
scramjet engines. The United States first successfully tested scramjet engines
in July 2002. It was followed by Russia, European Agency, Japan and then China.
Significantly, the Indian space agency achieved this feat in its maiden
attempt.
The 3277 kg ATV which carried the
scramjet engines touched down in the Bay of Bengal approximately 320 km from
Sriharikota after a flight of 300 seconds. ISRO later informed that when the ATV reached a
height of 11 km, the on board scramjet engines started breathing air
directly from the atmosphere. ISRO’s main concern was igniting the engines
in the air and then sustaining the flame at supersonic speed. However both the
engines on board the ATV ran for six seconds and they enabled the rocket to fly
at Mach 6 speed which is at about 7200 km per hour. Prime
Minister, Narendra Modi in his congratulatory message said that the successful
testing of scramjet rocket engines is a testimony to the hard work and
excellence of Indian scientists.
What is a scramjet engine?
A scramjet engine means a
supersonic combusting ramjet engine. Both scramjet and ramjet engines use the space craft’s forward motion to compress incoming
air without an axial compressor. Since scramjets cannot produce thrust at
zero airspeed, they cannot move a space craft from a standstill. A
scramjet-powered vehicle, therefore, requires an assisted take off by a rocket to accelerate it to a speed where it begins to produce thrust.
It has been found that scramjet engines work most efficiently
at supersonic speeds between Mach 3 and Mach 6. A
ramjet engine on the other hand can work at subsonic speed. Both ramjet and
scramjet engines use atmospheric oxygen as oxidizer. While exit flow from the
inlet of a ramjet engine is subsonic that from a scramjet engine is
supersonic. The word Mach comes from Ernst Mach, a brilliant 19th century
scientist whose most famous contribution was in the area of the speed of
sound. Mach 1 means the speed of sound that is 1195 km/hr in air. A rocket flying at
Mach 1 speed means it is going at the speed of sound in a particular medium say
air. Mach 2 means twice the speed of sound.
Scramjet
vital for ISRO’s future plans
Scramjets are very important for ISRO’s future
plans. The space agency currently uses rocket
launch vehicles like the PSLV to deliver satellites into orbit. PSLVs are
expendable, meaning that they can only be used once. In future, ISRO wants to use rockets fitted with
scramjets because their launching cost iwill be much less than that of the
conventional rockets. The difference between a rocket fitted with a
scramjet engine and a conventional rocket is that while the former
carries on board only liquid Hydrogen as fuel and uses Oxygen
from atmosphere for combustion to produce thrust, the latter carries both
liquid Hydrogen as fuel and liquid Oxygen. Since the rocket
fitted with scramjet engine does not have to carry Oxygen as oxidiser, it is
lighter and can carry an extra payload equal to the weight of
Oxygen. So, ISRO’s rockets in future, fitted with scramjet engines will be
able to carry heavier satellites. Currently the cost of lifting one kg of
payload by ISRO’s conventional rockets ranges between 12,000 to 15000 US
dollars. So, when this cost comes down substantially, the ISRO already
launching satellites of other nations at competitive rates, is likely to be
flooded with orders for launching not only from foreign governments but also
from many organizations who need specialized services.
ISRO is now working on a
launch vehicle platform called Avatar with which the space agency plans
to launch re-useable space crafts. This reusable launch vehicle platform
will be capable of carrying out satellite launches – takes off vertically and
lands back on a runway. The rockets launched from Avatar will use both
ramjets and scramjets for thrust and, flight and also cryogenic engines for
landing. Each of these engines will be used in different stages of the flight
–ramjet at lower speeds, scramjet at hypersonic speeds and cryogenic engines
when the craft reaches the edge of the atmosphere. Incidentally, both these
engines are different from turbojets. While in turbojets there are moving
parts, in ramjet and scramjet there is no moving part.
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*Dilip Ghosh is a Freelance writer. Regularly writes on science
subjects
Disclaimer: The views expressed by the author in this
feature are entirely his own.