The Design Of The Das D-9 Hawk Moth

 

 

The HLFT 42 Project                                                                          Prof. Prodyut Das Iss.6 ( 07/11/23)

I was approached by an Official (name withheld) who wanted to have my views on the HLFT 42/Tejas as if I were a “mentor”. In engineering design it is a little difficult to be a ”Tele-mentor” because one picks up a lot of information by putting one’s hand on the product but the idea was interesting. I have confined myself therefore to reliable sources of open information and my own experience which includes working on the Marut. I am sure there would be many more points of discussion- 12500 points if you go by the Jiro Horikoshi way of working- but that must await, if and when, one can run one’s  hands on “for real”. Views are personal.  

The HLFT 42 or the Hindustan Lead In Fighter Trainer 42 to give its full designation created a bit of surprise when it was announced; here was the Government, notoriously backward in taking any progressive steps, taking an excellent decision to resurrect one of the most brilliant non fifth generation airframes in jet fighter design history in order to serve the IAF’s urgent needs of fleet strength.

After the first above flurry there was a second one, more in character, about the Maruti image displayed on the tail fin of an exhibition model. It is a commentary on the state of affairs that the secularism of the ancient image of the God of the Airstreams (I don’t like to use the word Winds) caused much concern. If condign concern had been shown about the dwindling air strength of our AF we wouldn’t be where we are. There has been nary a cheep about what or when are the expectations are of this very important project.

This HLFT 42 project is of National Importance. Its criticality rests on several possible factors.

1.      The official raison de etre of the project is the IAF will need trainers as it expands back to its sanctioned strength. The Tejas will be too expensive and possibly too small in endurance for this LIFT role.

2.      An unstated but sagacious reason for this project could be that until 2025 i.e. only after physical confirmation of two years successive deliveries of the Tejas Mk1A can we be somewhat certain of the rate of deliveries and the utility of the product. As it is the locally designed portions of the Tejas Mk1 is 100% overweight and my personal belief is that none of the Mk1s are of any military use. At those weights it can’t be. The Mk1A is still by my estimate about 1000 kgs-one tonne- of excess weight and this cannot be doing much good for its effectiveness.

3.      With its American Engine and Japanese Composites, the Tejas supply chains are vulnerable to future pressure.

4.      The revival of the excellent Marut airframe with judicious introduction of modern systems and reasonable performance will give us a warplane that can be marketed worldwide without being Viggen-ed (see, later, Note 1).

Engines have been the bane of the Marut and allowed the import lobby to hamstring the original project. There are excellent foreign engines which can be directly fitted but usually the vendor causes difficulties when a particular foreign engine is selected and we do not have a local engine to switch back to e.g. HJT 36. To avoid a similar situation the HLFT 42 must look at 4 different engines at the start of project studies. These would be the following:

a.      The HLFT 42 /GE 414.

b.      The HLFT 42/ GE 404 IN 20

c.      The HLFT 42/Kaveri

d.      The HLFT 42 /Orpheus Upgrade (sic). 

I I put a ’sic” because this “stone age” engine by conventional wisdom, should not even be mentioned. It is not even the third best solution but it may be a low cost timely solution under certain circumstances and therefore should definitely be considered during the project stage. Below are some notes I made with regards to the possible configurations:

The HLFT 42/GE 414

By conventional thinking this project is the best because it offers the most. The GE 414 is in the news, the US has agreed to supply the technology and will give us a very powerful “trainer”. Its “weakness” lies in the fact that it will require the most amount of engineering development to be done. Everything will have to be new-systems, tests, much of the structure, flight trials and of course the cost & time. 

We have also to check out that we do not make exactly the same mistake that Northrop made when the developed the F 20 upward from the very successful F 5 series. The great power of the engine could be wasted on the airframe -something that happened with the F 404 powered Northrop F 20 Tigershark where the little wing (17.3 sq.mts- ex the little F 5A) could not do justice to the 60% increase in thrust. Yes the HF 24 has a bigger (28 sq. mts wing) but watch out we may fall in that same little rabbit hole.

The HLFT 42/GE F 404 IN 20

This F 404 engine is a better proposition. The strong points for this engine are that much of the systems developed for the Tejas can be directly utilized e.g. avionics, hydraulics, ECS, electrics etc thus cutting down the development work on systems. ( this is probably also true that the same F 404 systems can be used for the F 414 also) During production and in service the commonality with the Tejas will be welcome. The F 404’s a/b thrust of 79kN would be close to  the Marut’s original design required max. total a/b thrust of 77kN ( estimated to provide a max speed of Mach 1.9- 2.0 !)   the reduction of weight and drag of the single engine installation at the rear fuselage will actually improve performance to the extent that without afterburner the aircraft the aircraft may exceed M 1.3 at 5000 mts i. e. super cruise. The Marut could do M1,1 at on 2x22.3 of the Low Pressure Orpheus  It is another thing that super cruise is probably not a critical a requirement for India but try telling that to the specification wallahs.

The HLFT 42/Kaveri

The strongest point about this engine is that it is politically correct. It is ours. The current status is that it has achieved slightly above its designed cold thrust but the afterburner is unstable. 

It is my belief that the instability of the afterburner which has delayed development has been caused by poor manufacturing and assembly.

Jet engine do not have continuous flow as is generally imagined but “shuttered” flow and “rough” manufacturing can cause disaster. Better build quality will reduce the reported unstable A/B and blade shedding problems.  The Government has very commendably ordered a number of Kaveris with Godrej. The Kaveri’s basic design is probably to international standards. DRDO labs tend to produce Engineers who shy away from “getting their hands dirty”. This is unfortunate because further refinement/development of the Kaveri is precisely in the area which requires “touch-y feel-y” skills. I will be disappointed if at the more dynamic ambience of Godrej the engine does not show further improvement in peak cold thrust and perhaps Godrej on their own make take initiative thereafter to get the A/B right. The engine is about a 100 kgs too heavy but the engine being politically correct the airframe people will have to put up with that cross at least for the present! That 100kg can be handled. The Kaveri needs more management to see it through and with better manufacturing standards at Godrej the engine may see the better days it deserves.

The crosses to bear 

Suppose we have these three powerful engines e.g F 414 ,F 404 & the Kaveri then why not proceed with one of them? The short answer is we are not on a picnic. The AF needs the squadrons and how to get the product going is the primary concern. Good as these engines these engines will come with a fair to considerable development burden some of the more obvious which I list below:

i)                 The F 414 will provide 25% more power than the HF 24 requires to achieve M 2.0 . Apart from re confirming the airframe for stress, the wing high lift systems will have to be bolstered if the great power of the engine is not to be wasted.

ii)               The customer will want (the need for such a high AoA value should be argued with the customer) a high AoA something like 24 degrees because that is the fashion. This will require keeping the ailerons “going” at low airspeeds which means LERX of a size the Marut does not have space for if it goes for the F5 style location. A possible solution would be to look at the M 2K’s vortex generators and its location on the top of the inlets. It is probably doing the same job in the M2K.

iii)             All the above engine are about 60 mm bigger in diameter than the existing Orpheus. The cheapest solution (paid for in drag and weight) will be to deepen the fuselage by that amount but that may be wasteful and unnecessary. Even at the existing dimensions the Marut has about 40% more gross fuselage volume than the Tejas Mk1 A. We may not need more and if we do the spine can be enlarged. Local bulges in the Centre & rear fuselage sections may be the lightest and best answer. 

iv)              The customer will also want “drop down” engine change (curse him!). The engine will have to slide back by almost a metre to clear the U/c and then drop down. That is not difficult but the entire stabilizer control run will have to be re-thought and naturally the rear fuselage section will be completely new.

v)               With “the Jiro Horikoshi” type mentoring, very judicious use of composites (< 15% and that too only for the various access panels & controls to begin with) and presuming that two of the Aden guns and the nose ballast of 134 kgs is removed, given that modern accessories are usually lighter and more compact than the Marut’s it should be possible to hold the landing weights to levels so that major redesign of the U/c is avoided. Otherwise, we will have another cycle of re- adjustments and redesign of struts, brakes wheels and tyres will be required..

vi)              The change to a single engine will change the area ruling particularly between the T.E of the wing and the L.E. of stabilizer. A beneficial (in terms of storage volume) bulge will have to go on to the dorsal surface.

vii)            If the IAF insists on the high AoA capability then the rear fuselage design as shown in the display model/HF 25 should be reviewed. The HF 24 had a “flat iron” rear fuselage bottom surface which would be very useful in generating lift at low speed high AoA and the stabilizer is well located  in the correct position for near and post stall behavior. It is noteworthy that the Northrop F5A/E series had a rear fuselage similar in configuration to the HF 24. When Northrop move on to the single engine F5G/F20 they kept a clever Ω shape on the rear fuselage section to eliminate a lot of uncertainties in handling behavior and the associated testing. Are we sure of our reasons for the choosing the “better” round shape or was it that Northrop is more cautious and if so why? One bonus of the Northrop shape would be the two “shelves” at the bottom of the fuselage will not only provide spigots for the stabilizer “axles” (with a drop through engine mounting the present arrangement of the HF 24 cannot be retained) but also provide a convenient location of split type differentially opening air brakes to get pitch free braking as in the F 16. As a bonus there will be, with “navy frigate” style sloping of the sides giving some natural ( i,e, w/o RAM ) reduction of the RCS when viewed from abeam. 

viii) If High M is required then some more fin area would be essential. The top speed needs healthy discussion.

There would be many such points on detail examination, some minor and some critical but all would be important because attention to small details finally adds up whether the project will be a success or a lemon. Crucially the ability to visualize and anticipate problems save enormous time if worked on whilst waiting for funds to be released. 

The reason for listing some of these points is to illustrate that even before the first line is drawn the it is possible for the Designers to have a clear idea of what problem they are likely to face and in tranquility think out solutions to the same.  Each of the above is not too difficult to solve and engineer. One does not need funds to dream and to sketch alternative solutions which save time and panic. Consultants are a help but they help most as a second opinion.

The powerplants

Any one dealing with turbo machinery would not have failed to notice that the creatures are very sensitive to resistance. That means a fresh re- appraisal of its fit finish tolerances and study to reduce expansion and contraction losses the power could go up substantially and the engine behaviour be more docile. Coupled with a study of heat leakage i.e. ΔQ ≠ ΔW quite significant increase in thrust can be expected. The beauty of this approach is only a few sub-assemblies need to be looked at and results can be quickly confirmed. (Note 2) One can expect thrust to go up by as much as 7 to 15% with only “revenue” expenses.

Reprising the Orpheus

I will begin this section by repeating that there are 4 modern engines ( Honeywell F 124/Adour/ AI 222/ HTFE 25 which can be considered for the HLFT 42 as a twin engine warplane and the thrusts of the F 404/414/Kaveri are also acceptably close  So why am I even considering the obsolete Orpheus as a 5th “Plan B”? The reason is the dirty politics suppliers play when they realize you are committed to them without a fall back. The second reason is modern i.e. Western engines are technically wonderful and are essential for their requirements. We can do with much much less and yet not feel the difference. The HF 24 was visualized by Kurt Tank to be powered by the Orpheus B.Or.12 with ratings of 29.3 kN dry and a wet thrust of 38.9 kN wet that is a total installed thrust of 59kN and 78Kn dry and wet thrust i.e. close to the Kaveri and the F 404.  If you think from fundamentals, you will realize that jet engines operate “off design” most of the time and need complications to reduce the ensuing waste. For example, compression ratio improves efficiency as the reciprocal of the ratio so beyond a figure they mean increasing problems with surge margins. If like the US we needed global ferry capabilities that high CR despite its problems was mandatory but we really don’t need that kind or range to begin. Western engines aim for CR of 25-30 for that last one percent and then they have to pour in complex technology to have combustion & flow stability of the engines So can we avoid the waste ab initio? I feel that we are quite ok with much lower CRs may be even less than 10 ad we may develop alternate methods once we understand exactly how the CR functions to contributes to fuel efficiency, carefully trading reliability and costs against unnecessary performance. The Orpheus is dirt cheap and easy to develop and its proven ruggedness and simplicity may be much appreciated in 3^rd world countries where economy is a greater fact than the last wrung drops of performance.

Remembering the Orpheus

  By today’s standard the Orpheus is a joke. Designed and developed within in two years by Hooker after he joined Bristol Engines in 1951 its statistics were a mass flow of 39 kg/sec, a spool rpm 10,000 rpm, a continuous TET of 640 ⁰ C and a compression of only 4.4;1. It is in its laughable “crudeness” (by modern standards) lies its hidden strength. It is highly-and easily- developable even with “low “technology and still meet our modest needs.  The current Orpheus engine for which we have engineering data the thrust is 22 kN but this is limited by the “B” pump fuel limit and with a “C” pump the present design can develop 23.6 kN. Our requirement is approximately 30 (dry) and 39 (wet) kNs. By a combination of:

i)                 working to tighter tolerances within the existing given tolerance limits,

ii)               ii) improved finish of the flow passages

iii)             iii) improved metallurgy of the turbine stages and some judicious use of ceramic coatings in critical areas.

 This will give us the dry thrust and with more stable performance and the relatively low compressor pressure it should be possible to meet the wet thrust required fairly easily. This will transform the engine to a remarkably suitable, politically free “appropriate Technology” inexpensive engine. 

The original Marut as a contender?

What we need to see is how feasible is the original Marut HF 24 platform suitably upgraded be as a economically and commercially viable prospect for non -major air-forces and indeed as a second string arrow for our own depleted IAF. To discuss and compare I have tabulated some figures in the Table 1 below.

Table 1

Note:

1, Structural weight below is the bare airframe less undercarriage.

2. Systems weight is all other equipment , undercarriage & systems that goes into making a flyable aircraft including built in armament but less the weight of the engine.

3. Dry weight of engine.

4. Clean Combat weight includes Empty weight plus weight of all pilot (s), various fluids, parachutes. 2x 100 rounds of 30 mm ammunition, 2x CCMs and 100% internal fuel. Usually T/W is given at 50% fuel but I want to stick to published figures as far as possible.

5. the T/W ratio at MTO is given as it is a first approximation of field performance under Hot Indian condition.

6. The HLFT 42/ F 414 has the same equipment fit particularly avionics similar to the  latest F 16D block 80. The Orpheus powered HLFT has the equipment fit similar to the F 16B block 30 and the F 404 version has the equipment fit of the Block 50 series.

 

Sl. No.	Parameter	HLFT 42/ F 414	HLFT 42 F 404	HLFT 42 Kaveri	HLFT 42 Orpheus NG	Northrop F 20 F 404
1	Structure weight	2660	2660	2660	2860	N/A
2	Systems Weight	4940	3230	3230	3230	N/A
3	Powerplant weight	1116	1036	1235	900 ( 450x2)	1036
4	Dry Empty weight	8716	7163	7263	7228	5357
5	Internal fuel kgs	3500	3500	3500	3500	2628*
6	Clean combat weight	12916	11363	11463	11,428	8581**
7	Engine Thrust	98kN	78kN	78kN	77kN	78kN
8	MTO	17500	14000	14000	14000	12747
9	W/L kgs/sq, mts @MTO wt	625	500	500	500	736
10	Thrust/Weight (MTO/Clean)	0.56/0,77	0,56/0,77	0,56/0,77	0.56/0.77	0.623/0.92
11	External Load	4700	2700	2600	2600	4166

 

 The Table shows that prima facie the fears of a F 20 happening is probably unlikely and  all the versions of the HLFT 42 are viable. To select the winner the decision will depend on the urgency of the requirement. If the HLFT 42  is urgently required it would be better to focus on the F 404 and the Orpheus versions the exact weightage can be arrived at after the studies are completed and the politics of engine supply ascertained. The best method would be to sponsor studies of about 10,000 hours each on the four proposals and a separate study for the upgraded manufacture Orpheus engines. The HLFT42 Orpheus version would be the ideal project to induct the private sector and the Orpheus & Kaveri upgrade studies should go to companies such as Godrej and Kalyani Bharat Forge, the cost of each project study would be about 3 crores each i.e. a total for around 18-20 crores for the  6 studies and the studies should be over in 6 to 8 months. The results would assist in taking the final decision which in essence will be political. By the way the old Pegasus of the Navy has a powerful Orpheus as its "gas generator" and could fit the bill as a direct "Chinese Copy". 

Given the low technology of the platform and the relatively light weight  HLFT 42 /Orpheus would probably cost about half to one third that of the Tejas LIFT. but with a comparable /F 16D block 30/50 standard of equipment.

Note 1

In 1976 the SAAB Viggen was one of the contenders for the DPSA. The Viggen never made it because one of the objections was the RM 8 engine was of US origin and was embargoed by the US . This despite the fact the Swedish engineers at Volvo Flygmotor had started with a civilian JT8D engine used on the Boeing 737, added an A/B, tinkered with the compressor stages and had done 4000 hours of testing in 4 years and cleared it by 1971. Later when the JA 37 needed more thrust they increased the  fan stage by 1, reduced the compressor stage by one and redesigned many of the hot parts of the engine. The point of my Litany about how much effort the Swedes put in  is that in spite of substantial re-design/contribution SAAB/VOLVO did not have “sovereignty” over the engine. This an illustration of getting the politics right and a small flag about not getting too excited about the F 414. Oh, we will get to use all the F 414 we need but any visions about sweeping 3^rd world markets with F 414 powered Tejas Mk2 or AMCA is a large assumption because Boeing would like to sell the T 7. 

Note 2

I am narrating this as an illustration of the possibilities of applying the “heat leakage” approach in improving the performance in heat engines. A mass-produced diesel engine which was rated at 25 hp would produce no better than about 22.3 HP. The design/technical experts in the design office would swear that only by producing every component 100% to drawing would the engine produce the designed power. This was of course a ploy because everyone knew that the Works could never do that so the Design Office went about as wronged geniuses -victims of the cruel Works who butchered their masterpieces. This ploy is or at least was in the License Permit Raj quite common- even in the Private sector.

When the problem came to me I studied the problem from the angle of “heat leakage” and “breathing” and made small changes to the cylinder head and two of its dimensions, checked some dimensions and surfaces and after ensuring careful planned assembly the engine gave 24.5 HP DIN . an increase of almost 10%.  For the curious the other 0.5 HP was recovered by designing the internal aerodynamics of the oil bath air cleaner.

Turbo machinery I later found are even more sensitive to fit finish tolerance. Going by the work culture it is almost certain that in India engines whether production or experimental is assembled it is assembled to “within tolerance” by technicians rather than by a planned exercise under the direct shop floor presence of the Chief designer to work to limits to get higher outputs. We will be in for a pleasant surprise when this is done.