Propfan engine for DRDO's Nirbhay cruise missile? (India)

IBNLIVE Blogs ^ | March 09, 2012 | Saurav Jha

[sukhoi-30mki]

Propfan engine for DRDO's Nirbhay cruise missile?

It is often remarked that the only thing missing in India's missile portfolio is a subsonic cruise missile akin to the BGM-109G Tomahawk ground launched cruise missile (GLCM) deployed by the Americans in the eighties. Fortunately, 2012 seems to be the year when this is set to change with several reports of DRDO's Nirbhay being shortly unveiled. One report is particularly noteworthy - TS Subramanian's 'Nirbhay likely to be testfired in April' in this Wednesday's The Hindu.

The story talks about the Nirbhay being a two-stage missile with the second stage powered by a turboprop engine. While the first part is simple enough - the two stages are obviously a reference to the solid booster (which is the 'first stage') used by the Nirbhay when it is launched from the ground; it is the second 'salient feature' that bothered me, i.e the part about the Nirbhay being powered by a turboprop engine during the cruise phase.

Cruise missiles in their 80-year history (considering the WW2 Luftwaffe V-I flying bomb as the first true cruise missile) have been powered by pulsejets, ramjets, turbojets and turbofans with the last two being the propulsive configuration of choice for missiles in Nirbhay's category, but never really by turboprops, unless of course you consider the MQ-9 Reaper to be a cruise missile, during a one-way suicide mission!

Assuming that the turboprop reference is not a typo there is one possibility that may be the real source of this reported feature. Certain experimental designs in the past have used propfans (also known as open flux rotor jet engines) to propel cruise

[Vinnie]

Several yrs. ago there was talk of commercial jets using propfans. I recall seeing pics of them. Ultra high bypass.

For one reason or another the concept hasn’t taken off.

[ZOOKER]

I also recall experiments with propfan engines. There were several problems which they apparently never solved - noise, efficiency, and durability.

Turboprops use reductions gears to drive the propeller so the blades don't exceed the speed of sound. IIRC, the propjet's blades run directly so there is a continuous sonic boom as the blades break the sound barrier. It would be quite a feat to pass Federal noise regs, but the Indian Navy doesn't have to, I suppose.

No one had ever designed a propeller that was efficient above the speed of sound. Various airfoil shapes were to be tested, but I don't know if they found the right ones.

The blades ran unshielded, on the outside of the engine (like a jet engine with the cowling off, and the turbine blades running at mach speed naked to the world). A catastrophic failure could be deadly to anything nearby.

[Vinnie]

Yeah, I do remember thinking the blades reminded me of a blender w/o the container.
I can see where the noise , danger would be a concern.
Looked neat though.

[Yo-Yo]

For one reason or another the concept hasn’t taken off.

Often referred to as Unducted Fans, or UDFs.

One reason why they have yet to be adopted: Extremely noisy, and noise abatement laws have gotten much stiffer around airports as suburbia encroaches on the world's airport locations.

Another reason why they have yet to be adopted: No way to control a lost fan blade from potentially penetrating the cabin. Since the 1990s, all commercial jet engines must be able to contain lost blades for certification.

Neither of these problems are insurmountable, nor is the additional problem of mechanical complexity required for the UDF's additional gear reduction and fan blade pitch control. One proposal is to put the engines at the back of the airplane, and above the fuselage so that the horizontal tail blocks noise from reaching the ground.

[James C. Bennett]

Interesting stuff.

Thanks!

[hattend]

I think I just found the subject for my next RC airplane. Sexy!