ISRO’s ‘dirty fuel’ dilemma

For ISRO, the key challenge is to break free from the philosophy that is often expressed in this American aphorism: ‘If it ain’t broke, don’t fix it’.

Perhaps nobody knows better than ISRO that legacy issues are hard to shake off. Take, for instance, the fuel that will power the core (or the middle unit of the lower part) of the GSLV Mk-III, the rocket that will be used in the Chandrayaan-2 mission on Monday.

There are two chemicals at play — UDMH (Unsymmetrical Di-Methyl Hydrazine), which is the fuel, and Nitrogen tetroxide, the oxidiser. This is called a “dirty combination”. UDMH is highly toxic and corrosive if it comes into contact with the oxidiser, an explosion will result.

Elsewhere in the world, space programmes have moved to a cleaner and greener fuel — liquid methane or kerosene.

Experts point out that it is not that ISRO does not know how to shift to these better fuels — only it can’t. Shifting to liquid methane would mean bringing in another cryogenic engine, because any liquefied gas would need to be kept in extremely low temperatures to stay liquefied.

ISRO can use kerosene as the fuel — it can be kept at room temperature — and liquid oxygen as the oxidiser. Indeed, that is the semi-cryogenic engine the space agency has been working on for years.

For now, ISRO is stuck with the dirty fuel because it carries the burden of its legacy. The PSLV, a trusted workhorse, and the GSLV are working fine with the dirty fuel, so why change it?

Legacy is also pretty much the reason why ISRO is still not ready with a semi-cryogenic engine, which could have used cleaner fuels. Having got seven cryogenic engines from Russian, ISRO had no option but to go into developing similar fully cryogenic engines to get the GSLVs going.

Otherwise, it would have developed the simpler semi-cryogenic engine first, and gone fully cryogenic later. In a semi-cryo, the fuel is at normal temperatures, only the oxidiser (liquid oxygen) has to be kept extremely cold. In a full cryo, both the fuel and the oxidiser need to be ultra cold. The GSLV Mk III’s cryogenic engine will have liquid hydrogen (fuel) at -250 degrees C and liquid oxygen at -150 degrees C.

Like a student attempting a more difficult question first in an examination, ISRO has chosen to develop the cryogenic engine first. This should make it easier for it to develop the semi-cryo,. A new engine essentially means a different rocket. Straight-jacketed with a tight schedule of launches, ISRO, with its limited bandwidth, has little headroom to try out a new rocket. Hence, the semi-cryo has remained a low priority even though a rocket with a semi-cryo in the lower stage and a full cryo in the upper would be far more powerful.

If bringing in a different ‘model’ or rocket is difficult, what to say of developing an entirely new family of rockets?

World-over, research is on into air-breathing rockets, which have only the fuel and take oxygen from the atmosphere. ISRO, too, has the ‘scramjet’ technology on its development sheet; when it flight-tested a scramjet engine in August 2016, it was only the fourth country to do so. But incorporating it into a rocket would be to build an entirely new launch vehicle.

That is difficult when the existing rockets, regardless of their dirty fuel and lower power, are working well — that is exactly the problem.

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