How small can low observable (stealth) combat aircraft designs get?

[red admiral]

Sure, but I suspect that the minimum acceptable internal weapons load is still going to be 2x 2000lb bombs and 2x BVRAAMs.

At which point you end up with an F-35 sized aircraft.

KF-21's 4 x AMRAAM or 4 x SDB + 2 x AMRAAM is probably more what I'd consider minimum for multi-role.

Or go back to the 60s and just have 2 x AMRAAMs?

 

[Prophet141]

There were accuracy issues with the Vigilante system. Bombs tended to get carried along in the wake and bounce around.

I read in another thread here that the accuracy issues with the tube-magazine weren't necessarily related to any aerodynamic effects, but rather the ballistic computer not being programmed with the tube in mind, leading to inaccurate drops (much less of an issue with a nuclear weapon, of course). And that's assuming that you're using an iron bomb at all (Which is of course preferable on cost grounds) and not a guided weapon.

The tubes extending to the back of the aircraft take up a lot of volume unless you design new weapons with folding fins etc. With multiple tubes then you'll also end up with a lot of rear fuselage volume which is bad for drag.

I think the volume is the key thing; for a small supersonic aircraft with internal weapon bays then you really need to minimise the numbers of weapons carried as this drives max cross sectional area, then length, mass etc.

That's true, this could only work half decently with a two engine design, and at that point you're immediately straying out of the "light" fighter category.

 

[Scott Kenny]

At which point you end up with an F-35 sized aircraft.

That was kinda my point. Most people are going to want large bays which means an F-35 sized aircraft.

KF-21's 4 x AMRAAM or 4 x SDB + 2 x AMRAAM is probably more what I'd consider minimum for multi-role.

I thought the KF21 was packing 6x AMRAAM/Meteor?

But in any case 4x-6x SDB and 2x AMRAAM is definitely the minimum anyone would expect. But I still think anyone wanting a 5th gen fighter would specify bays deep enough to hold 2000lb bombs, for those rare targets that need a Surprise Bunkerbuster.

 

[F-2]

What about something like Lampyidae? Or mako. In this loyal wingman era do you even need a weapons bay?

 

[Scott Kenny]

What about something like Lampyidae? Or mako. In this loyal wingman era do you even need a weapons bay?

Do you want to fly an unarmed aircraft into a shooting match? At the very least I'd require some defensive AAMs to be carried.

Any nuclear state will insist that their nukes be delivered by a manned plane for a long time. That means your minimum bay size is however big the national nuclear weapon is, IIRC the US/NATO B61 is roughly the length of a 2000lb bomb but is only about as wide as a 1000lb bomb.

While it's technically possible to make a nuclear warhead that would fit into the SDB form factor, it'd be limited to about 1kt based on reported yields of 155mm and 203mm cannon warheads. A warhead that shape, like a rugby ball, has two problems: one is that it'd be very inefficient and so need a lot more fissionables to go boom at all, and the second is that you wouldn't be able to pack nukes as tightly as the SDBs get carried. Packing warheads inches apart would cause a nasty neutron flux that would be unhealthy for pilots and ground crew, not to mention causing radiation effects in the electronics of the plane.

B-52 bomb "clips" and rotary racks show roughly how far apart you can safely stow nukes, and it looks to be a good 30" apart. So one bomb per bay regardless of yield, might as well use one that fills more of the bay and has greater flexibility.

And while I'd ordinarily say that any nation that would be interested in the "smallest" possible stealth fighter wouldn't be one of the nuclear states, sadly, the events of 2022 have proven that any given nation really needs nukes if they don't want one of their unfriendly neighbors to try something.

 

[paralay]

Make inconspicuous missiles/bombs and weapon bay will not be needed.
In addition to the weapon bay, the take-off weight is also affected by the mass of electronic equipment. The modern fighter has 4.5 - 5% of the maximum take-off weight.

 

[Shoes31]

Make inconspicuous missiles/bombs and weapon bay will not be needed.

Wouldn't the external ordinance make it hard to keep the jet stealthy, even if those missiles/bombs were shaped with LO qualities? I keep hearing that even if you make the airframe LO, and the missiles LO, and the pylons LO, there are a number of weird reflections and interactions that make it difficult to shape the items so everything is LO when together.

 

[F-2]

Wouldn't the external ordinance make it hard to keep the jet stealthy, even if those missiles/bombs were shaped with LO qualities? I keep hearing that even if you make the airframe LO, and the missiles LO, and the pylons LO, there are a number of weird reflections and interactions that make it difficult to shape the items so everything is LO when together.

Store them conformally and shape the not pylons be shaped. Not too impossible for two self defense missiles.

 

[Kat Tsun]

Wouldn't the external ordinance make it hard to keep the jet stealthy, even if those missiles/bombs were shaped with LO qualities?

HAVE DASH II addressed this in the 1980's: https://www.secretprojects.co.uk/threads/loral-ford-aeronutronics-have-dash-ii.14091/

Might be mass producible these days but cylinders are still cheap.

I keep hearing that even if you make the airframe LO, and the missiles LO, and the pylons LO, there are a number of weird reflections and interactions that make it difficult to shape the items so everything is LO when together.

It would require the external whatever to be designed to fit a particular airframe. This isn't a big deal if you use pods or something.

 

[stealthflanker]

Problem with conformal is it takes away range of weapons that can be mounted there. Also necessitating some large procurements of the weapons and platform. Otherwise one may have a Zummwalt.

 

[quellish]

Wouldn't the external ordinance make it hard to keep the jet stealthy, even if those missiles/bombs were shaped with LO qualities? I keep hearing that even if you make the airframe LO, and the missiles LO, and the pylons LO, there are a number of weird reflections and interactions that make it difficult to shape the items so everything is LO when together.

Yes. This is also why summing the RCS of aircraft components does not work. Also why “Low RCS tanker” doesn’t make sense

 

[stealthflanker]

Wouldn't the external ordinance make it hard to keep the jet stealthy, even if those missiles/bombs were shaped with LO qualities? I keep hearing that even if you make the airframe LO, and the missiles LO, and the pylons LO, there are a number of weird reflections and interactions that make it difficult to shape the items so everything is LO when together.

Yeah. as therre are multiple reflection phenomenon here, where EM Wave bounce from say a missile to other missile. It creates constructive interference where the reflection strengthen eachother thus you ended up with strong and large reflection. External load is just better be avoided whenever possible.

For example this is my estimates on F-16 with Have Glass-like treatment and how external load affect the RCS

https://twitter.com/x/status/1751611023214473318

View: https://x.com/Flankerchan/status/1751611023214473318




One interesting thing however is about "stealthy external weapon bay" Unfortunately currently i only make single concept for my Su-35 model. This is the external bay, the Su-35 with extenal bay have clean wing also with wingtip ECM's.





Then we compare that one with full counter air suite here. This Su-35 model has 4x R-37, 2 R-77-1 and 2 R-73 also a jammer pod. also with an armed F-16, with 4x Amraam and 2x Sidewinders

Su-35 Counter Air	F-16 Counter Air

The result is as follows. SImulated using Ansys HFSS and data processed in MS Excel.



One may notice the apparent benefit of external weapon bay. Still however it would need some optimizations. I havent test the case for wing mounted bay.

 

[F-2]

Yeah. as therre are multiple reflection phenomenon here, where EM Wave bounce from say a missile to other missile. It creates constructive interference where the reflection strengthen eachother thus you ended up with strong and large reflection. External load is just better be avoided whenever possible.

For example this is my estimates on F-16 with Have Glass-like treatment and how external load affect the RCS

https://twitter.com/x/status/1751611023214473318

View: https://x.com/Flankerchan/status/1751611023214473318




One interesting thing however is about "stealthy external weapon bay" Unfortunately currently i only make single concept for my Su-35 model. This is the external bay, the Su-35 with extenal bay have clean wing also with wingtip ECM's.



View attachment 731212

Then we compare that one with full counter air suite here. This Su-35 model has 4x R-37, 2 R-77-1 and 2 R-73 also a jammer pod. also with an armed F-16, with 4x Amraam and 2x Sidewinders

Su-35 Counter Air	F-16 Counter Air
View attachment 731211	View attachment 731216

The result is as follows. SImulated using Ansys HFSS and data processed in MS Excel.

View attachment 731217

One may notice the apparent benefit of external weapon bay. Still however it would need some optimizations. I havent test the case for wing mounted bay.

What do you think a DSI might do to help?

Here is a paper on the J-10B/C DSI.

 

[stealthflanker]

What do you think a DSI might do to help?

Here is a paper on the J-10B/C DSI.

It's more like into the shape which to break down the radar reflection into smaller less harmful lobes, while at the same time expanded the velocity range where the inlet can be efficient. The bump can also serve as additional volume where something can be placed there e.g Fuel.

The signature benefit however is remains to be seen. Might be easier to apply thick absorber composite due to nonmoving parts.

 

[Scott Kenny]

It's more like into the shape which to break down the radar reflection into smaller less harmful lobes, while at the same time expanded the velocity range where the inlet can be efficient. The bump can also serve as additional volume where something can be placed there e.g Fuel.

The signature benefit however is remains to be seen. Might be easier to apply thick absorber composite due to nonmoving parts.

Also has no discontinuities that like to massively increase signature.

 

[stealthflanker]

Also has no discontinuities that like to massively increase signature.

The bump can be considered discontinuities tho, that's why it needs to be treated with absorbers, and maybe that careful shaping to make use of the discontinuities to break down the lobes into less harmful thing.

 

[Scott Kenny]

The bump can be considered discontinuities tho, that's why it needs to be treated with absorbers, and maybe that careful shaping to make use of the discontinuities to break down the lobes into less harmful thing.

Even an untreated bump is better than the discontinuities in an F-15 inlet.