The Tejas Gun firing trials                                                                     Prof Prodyut Das.

 

The development of the Tejas has attracted a lot of followers from the Indian Aeronautical community. In the general disappointment with dates what has slipped under the radar is the fact that twenty one years after the first flight of the aircraft is not cleared to fire its gun. Jet engine/gun firing problem first came to attention in the 1950s . Today in terms of certification gun firing clearance should be  a “low hanging” fruit.

As someone pointed out on twitter without its gun the Tejas cannot be used even in peacetime to intercept and force a civil airliner to land at an Indian airport in case of suspicion or emergency. This is possibly one of the reasons that the aircraft is based at Sulur out of the way of major air routes where the possibility of the recent emergency with an Iranian airliner is less likely.

Our politicians, when accused of corruption never answer the charge. They point out that X, or Y or Z is also corrupt. Tejas diehards may not be political but they have done the same. They argued that the IAF Mig 21s had no guns. This is factually wrong. The MiG 21 F 13s of 1962 had 2x30 mm guns but for other reasons the IAF elected to go for the later FL sacrificing the guns for better radar. When they realized their mistake after 1965 they first got the GP 23 gun pods circa 1967 and by the next version license produced ,the 21 M the integral gun, a GSh 23 possibly specially designed for retro fitting, was back in place. It was evolutionary and all happened within years of each other. Here we are discussing a lapse of decades. 

The issue of the FOC without a gun firing clearance is both a lapse of procedure and an admission of guilt which devalues both organizations involved. It brings the administration of the project into question. This also raises suspicion that  ADA/DRDO itself knows that progress is unsatisfactory and feels the need to create an illusion of progress.

In this case an ostensibly independent Government Organization CEMILAC has given a FOC- a final operational clearance- to another organization -ADA- when an item of equipment specified in the OR by the customer has not been cleared- a confusion is being created between "installed" which may have been done and "cleared" which is the process by which the Gun can be used without danger.. Notably there was already a system of IOC-1, IOC-2, etc in place. Instead of an IOC-3 why was a FOC given when de facto the aircraft was NOT fully clearing its OR? Credibility is involved. To be sure the paperwork will be in order but paperwork does not protect borders.

Why the gun firing test has not been conducted in twenty one years is a matter only ADA will know. It is not a particularly hi-tech test but the following notes may be of interest to those following the Tejas.

Theory

There are three major areas of worry in clearing a gun firing test are: gun gas ingestion, blast suppression and recoil/structural problems; the rest- clearance/disposal of links and cartridges (note 1), gun gas accumulation and purging (note2), jamming of feeds etc are not hi-tech but do require a lot of “touchy- feelie” engineering and patient observation and testing to cure. You can if you insist, do it on a computer but if you know enough to visualize the problem on a computer you probably do not need the computer anyway.  

Gun Gas Ingestion

The propellant used in the shell generates large amounts of various combustible and corrosive gases. If they get sucked in to the intake they change the air fuel ratio- strictly the oxygen fuel ratio making the engine to “flame out”. The quantity of gun gas generated will depend on the calibre, propellant, burst duration and the amount ingested will depend on the position of the muzzle wrt the intake and the kind of design used. The problem is usually noticed at high altitudes at low altitudes the air is sufficiently dense to alter the equations. The usual cure is simple. The fuel supply to the engine is dipped when the trigger is pressed. This reduces the fuel supply thus maintaining the correct stoichiometric ratio. Without dipping the mixture would be too rich and the flame would go out.

Pressure pulsations

The breech pressure of the Aden was 18 tons/sq.in) which works out to about 2700 kg.cm². The muzzle pressure would be close to such a figure depending on the design of the propellant grain. When the gases emerge from the muzzle they are supersonic and form shock waves thus disturbing the air flow pattern ahead of the muzzle and the gases go far ahead - several meters in some cases.  Engines do not like these pulsations and depending on the ducting design will put up their protests at the most inconvenient moment during testing. Various simple devices are available to minimize the effect of blast. The Gnat is an extreme example where the muzzle is on the inlet lip and yet the problem was held within acceptable limits. If one works from fundamentals usually solutions are easy to find.

Structural problems

The recoil load of the guns will again depend on the design. The Aden had about 2 tons recoil and a moment of 1 ton metre. The GSh 23 had less recoil because of the twin barrel GAST configuration. Nevertheless these loads can locally buckle the mainly sheet metal structure which takes these loads- the Marut being a well -known case. (Note 3) 

Integrating

The net result of all these effects is a situation that can be quite puzzling if a strictly analytical approach is relied on. The MiG 15 with an enormous 37 mm NR cannon- about the size of a Bofors 40mm-had no problem thanks to a centrifugal engine and a long duct to teh compressor face. The Hunter F 1, for example with the Avon engine, had severe engine problems when the 4x 30 was fired but the identical F 2 with the Sapphire engine had no issues from the word go. In the case of its contemporary, the Swift –again with the same Avon as the Hunter- the problem was so severe that there was compressor damage and Rolls Royce and Supermarine spent time blaming dud airframes and dud engines respectively. Later it was found that a particular batch of engines was defective and gave rise to the unnecessary problem. I mention this to emphasise that diligence at the  textbooks- or computers-alone- will not give the solutions. getting hands dirty may.

The Tejas situation

The best time to fix the gun location is during the project studies. Unfortunately the Tejas gun position appears to be an afterthought. It is located close to the inlet lip –about 60 cms- and is thus prone to many of the problems mentioned above. What worked for the Su 24 –a similar layout but on a very much larger aircraft- may not work in a smaller aircraft like the Tejas and the entire problem – gun gas ingestion, vibration and engine running problems may be of an order higher of challenge though the challenge would be rated interesting rather than impossible.

The gun firing problem should be easy to solve if one works from first principles. Why the Tejas has not got its gun cleared by now is worrying. The conjectures are:

i)                 That is the way ADA works. For example the prototype was ready for flight by early 2000. BAC consultants had by June told ADA to “screw up your courage and fly it” but ADA continued to sit on the prototype. It was finally was an Air Force Officer who stuck out his neck and said that the prototype would fly by such and such date. Had the prototype crashed I am sure he would have been blamed for being hasty and not “qualified” ! Since the flight went off incident free it was probably hailed as  “a joint decision”.  The Services term –LMF- made worse by Babu giri on part of ADA leadership is suspected; one year was wasted. This gun firing lapse could be a symptom of the same disease.

ii)               The ground tests have been done and the results are – to ADA leadership – quite alarming. No major relocation is now possible. So we have another round of “sitzkreig”- you sit on teh problem till it becomes irrelevant. Excuses by ADA should not be tolerated because it is now 21 years since first flight there cannot be any issue with the Mk1 still. If there are issues then ADA is not up to the job. Mr/Dr. Prasenjit Das has very perceptively pointed out on twitter that a possible cause could be that the Composite structure is not able to take the recoil loads and we may not see the Gun  being cleared on the Mk1/1a. If true it will mean significant  delays because the centre fuselage design AND tooling will need change.

 

Notes

1.      1. The Gnat’s system design tended to remind one of the ingenuity of the Japanese mechanical toys of the 1950s. In the Gnat the cartridge was ejected in the usual manner but the links were fed across the fuselage into the ammunition box on the opposite side. The links took up the space vacated by the cartridges. Fine in theory but if one gun jammed/stopped for any reason whatsoever the other gun stopped soon thereafter because its links had nowhere to go. This faulty system was reportedly finally rectified in the Ajeet by TV Vardarjan.  (notes anyone?)

2.      2.The gun gases are “oxygen deficient” to reduce barrel wear which is why one sees the muzzle flash. The gases are explosive and corrosive. In the Marut the accumulation of the gun gases during initial flight trial led to one of the gun bay door (there were two) being blown off. The aircraft was recovered, an electrically operated purge scoop was introduced which purged the gun bay during firing. I cannot resist adding that the Gnat’s solution was delightful in its lightness and simplicity. The Aden had are recoil of 19 mm. This movement was used to kick open a purge scoop. A simple “door closer” unit allowed the scoop to shut slowly after firing ceased! No electrics, no cables, no actuators! “Simplificate and add lightness”  philosophy at its best!.

3.      3. Read , if interested , The HF 24 –an Inquest ( Vayu III/2021) or https://prodyut7.blogspot.com/ for some details of the gun vibration problems of the Marut.