An idiot’s guide to aircraft design, part 5: Stealth comes at a cost

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Lockheed Have Blue

Radar stealth, the use of shaping and materials to minimise an aircraft’s visibility to hostile radars comes at a cost. Jim Smith, a man with significant technical roles in the development of the UK’s leading military aviation programmes, explains. 
Much depends what you are trying to achieve, and how far you are willing to compromise performance and cost to minimise signature. As noted in my earlier Hush-Kit discussion on aircraft for BVR combat, there is a difference for offensive aircraft, designed to be sent ‘in harm’s way’ in penetrating and seeking to dominate defended airspace, and defensive aircraft, particularly if armed with long-range systems and supported by an integrated air defence system. For the latter, the fact that the opposition is aware of their presence may not be a bad thing, as they are there to deter and defend, rather than to penetrate and secure.

Striking heavily-defended targets deep in the opposition’s territory, or seeking to achieve air superiority by controlling and denying the opposition’s own airspace, is a high-stakes, dangerous game. And is likely to require the highest degree of signature minimisation, currently apparently delivered by the F-22 Raptor and the B-2 Spirit. Similar characteristics are claimed for the F-35 Lightning II, alongside supreme situational awareness, through the use of shared multi-platform sensor data.

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Sukhoi S-70 ‘Okhotnik’

What are the penalties and possible impacts of achieving the lowest possible signature, not just in the radar domain, but also in the infrared? A few potential challenges are listed below:
Cost – of design, of security, of special protective systems, and of materials. Cost of maintenance will also increase, due not only due to the requirements of specialist materials, but to the need to make good the signature every time the surface has been disturbed. There is also potentially additional cost, due to a larger and heavier airframe being needed than might otherwise be the case.
Size and weight – the necessity of carrying stores internally means that the fuselage volume has to be greater, increasing weight and drag. The propulsion system will need to be screened or shielded, probably reducing propulsive efficiency and increasing weight due to intake treatments. In some cases, the exhaust system will also require treatment, which will decrease efficiency and increase weight.
Wave drag – this is likely to be increased due to the increased fuselage volume, necessitating larger engines to meet performance requirements.
Control system – The US view is that European, Chinese and Russian aircraft equipped with canards are at a signature disadvantage compared to the F-22 and F-35. The alternative view is that canards offer significant aerodynamic advantages in lift-dependent and wave drag.
Weapons requirements – Low signature may result in highly successful ‘one shot, one-kill’ engagements, where the opposition is unaware of the low signature aircraft’s presence until it is engaged. In general, however, stealth aircraft weapons bays are relatively small, and may carry fewer weapons than opposing air defence fighters.
Through the use of tailored electromagnetic materials and coatings, it seems likely at present that low signature can be achieved without great compromise to the configuration, apart from the measures required to reduce signatures from the engine intake duct and exhaust system, and the need to provide internal weapons bays. The Chinese J-20 and Russian Su-57 are beginning to be operationally deployed, but the F-22 Raptor remains a stand-out as a capable manoeuvring fighter aircraft with low signature.

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Chengdu J-20

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