The reason the PTAE-7 should not be used (or at least it won't be as efficient in the current form anyway) is because it has about 70% higher fuel usage per kilogram of mass it helps fly than what is need for an effective long range subsonic cruise missile. This makes the vehicle performance in range far less than desired (~550 km instead of 1000 km for the same fuel mass as a Tomahawk, for example)
Actually, this is a good point to do a parametric study of engine choice:
Consider the following:
a) Let's assume that the Nirbhay turns out like a stretched version of the Lakshya PTA and has similarities with the US Tomahawk missile
b) In assuming the same sizing, I will assume that the internal fuel carried is the same as that of the Tomahawk (1000 lbs/ ~454 Kg).
c) The tomahawk I am considering is using a Williams F107 Engine. (Check Google uncle for details on this engine if you like)
d) I would like to consider a choice of the F107, PTAE-7 and 36MT as possible engine choices
e) All three engines have the required thrust values needed to power the missile. Only difference is the fuel usage in cruise:
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---------Engine--------------------Fuel Usage (Kg/hr)------------------------Effective Range (km)----------
Williams F-107 154.26 2500.0
PTAE-7 260.10 1482.7
Saturn 36MT 160.60 2401.3
This is of course not a surprise. The PTAE-7 is effectively a turbojet engine when what is required is a turbofan-ski (there are alternatives to a pure Turbofan available, as outlined by the 36MT design, for example. There are others as well).
Now note that the effective sizes and masses of the three engines are also different (PTA and Saturn engines being heavier and having a higher thrust rating than required by the Tomahawk outer mold line), so the range numbers above will be more detrimental for the two replacements of the F107.
Looking at the thrust numbers for the three engines:
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---------Engine--------------------Thrust (Kg)-------------Launch Weight with Booster (Kg)------------Missile Length (m)
Tomahawk/Williams F-107 275.22 1600 6.25
Lakshya/PTAE-7 400.10 705 ~3
Lakshya/Saturn 36MT 450.00 - -
So the obvious question that comes to mind is why the Lakshya uses a clearly overpowered engine on a much smaller airframe? The answer is maneuverability. The Lakshya is designed to do typical 3g flight maneuvers and is rated for up to 6g. So the logical extension has always been whether a reduction in the maneuverability of the Lakshya could be traded for increased mass of fuel for a cruise missile application? The answer is yes. But not with the PTAE-7 engine for reasons described previously. However, note that for the type of application in mind for the Nirbhay (i.e. launch from perhaps sea level areas just south of the Himalayas for a flight through/above the peaks and then into terrain hugging mode for the plains beyond) it might be required to maintain a lower fuel mass (and hence overall mass and sizing) in return for higher maneuverability through the valleys and peaks, if required. Whether the decision to go for higher maneuverability has been made or not is going to decide the overall scaling beyond the Lakshya for the Nirbhay. However, the reason the range is less compared to the Tomahawk (i.e. about 1000 km) when it could have had a much higher range could be due exactly to this fact.
Now, the Lakshya even in its present form was always suspected of being easily converted to a cruise missile because of its engine performance and MTOW design for 705 kg. The thing is, the 705 kg is not accounted for in the typical flight profile masses (even if you are conservative) and the missile could easily add a 100 kg warhead and some additional fuel for a one way straight flight of >600 km. The only restrictions were the avionics and navigation required (and the will to do this!). The Nirbhay, then, being derived from the Lakshya has been long time coming from a systems standpoint.
The Nirbhay-M (For the Maneuverable version for the sake of this conversation) therefore, is likely to be a slightly shorter than Tomahawk with larger control fins like the Lakshya. We might also see the Nirbhay-LR with smaller control fins and a longer length very similar to the Tomahawk. But the PTAE-7 was always the achilles heel for converting the Lakshya into a cruise missile. Hence the Russian connection. The performance of the Nirbhay-M and -LR variations are highlighted below as a function of payload/warhead mass possibilities on a surface map of fuel mass and corresponding range of the missile choice.
What these maps will show is that there is a versatile design space available between the two original design points just waiting for inputs from other system requirements and user preference. But that said, the PTAE-77 performance will be lower compared with the US and Russian options. Is that necessarily a bad thing? If you look at the two plots, you can see that the Nirbhay-LR model created by lowering the maneuverability does afford very reasonable ranges. So Why go for the Russian option unless the need is for range and maneuverability. The US option in this regard is not usable even if it were available: it is too low on thrust capacity.
All in all, expect the Nirbhay to have either:
a) >2000 km range with lower maneuverability using the 36MT engine
b) ~1500 km range with lower maneuverability using either the PTAE-7 or some variant of it used for the Nirbhay (NE-7?)
c) ~1500 km range plus high maneuverability using the 36MT engine
d) >800 km range plus high maneuverability using the PTAE-7/NE-7 engine
P.S.: I have been wondering for quite some time about the Saturn 36MT. It would of course be better to develop a turbofan version around the PTAE-7 but that is a snake-pit in terms of risks involved. We cannot simply use the turbojet core of the PTAE-7 as the basis for a turbofan because of external diameter restrictions. Which means that the current core must be made even smaller. Not a trivial task under any conditions. It is only in this light that I find myself agreeing with a straight buy of the 36MT: it helps to ensure the product gets delivered on time for a change.
Anyway, JMT and all that.