Thread by @Space_Baguette

Today, it's time for me to explain how the F-117 Nighthawk stealth works !
Welcome to a [THREAD] invisible to radars 👀
👇🏻
#AvGeek #Engineering

Let's review the basic history of the aircraft. The F-117 was developed by Lockheed Martin in 1978, flew for the first time in 1981 and was revealed to the public in 1988.
This weird looking jet was the world’s first operational stealth aircraft.
The US Air Force flew 59 of them.

The main goal of the F-117 was to drop laser guided bombs in a threat-rich environment, like Baghdad during Iraqi War, full of Anti Aircraft Artillery (AAA) and Surface to Air Missiles (SAMs).
It operated at night using special flight profiles to be as stealthy as possible.

In order to understand how stealth works, let's go back to how radars work. To be very simple, a radar will send an electromagnetic wave that will be reflected (bounce) on any object it hits. The radar will then "listen" to the return of the wave, listen to the target wave bounce

Then, the radar will process the signal. Using Doppler effect, you determine the speed of the contact, with basic maths, you determine the range of the contact, and with the radar antenna rotation, you determine the heading of the contact.

Let's understand how to escape these radars !
A usual plane is not stealthy and will reflect the wave to the antenna, but a stealth one will try to reflect as less energy as possible to the antenna.
The strength of the reflection is called Radar Cross Section (RCS).

There are several ways to reduce the RCS of your aircraft.
I will talk about these three :
- Passive stealth
- Active stealth
- Tactical stealth

Please note that plane stealth is different to ship stealth or submarine stealth. We only talk about aircraft stealth now !

The F-117 is a good example of passive stealth. The geometry of an aircraft is a big factor of how big its RCS is. There are two main ways : the rounder the better, like the B2, diffusing energy, or straight planes reflecting the energy in another direction like the F-117 !

The jet engine compressors are big mirrors to radar waves, this is why a stealth aircraft will never show its compressor blades to a radar wave. You can compare the F-16 and the F-22, the latter one using a S duct to hide the engines from the radars.

But as you probably guess, the reflected wave may have enough energy to reach the radar and reveal our aircraft. The second massive factor is to use a Radar Absorbing Coating, that will absorb and reduce the energy of the incoming wave as much as possible.

Yet, this coating has a main con : absorbing energy means heating up, for example the F-22 Raptor has to be left with vent hatches open for a long time after each mission because all the absorbed energy was stored as heat inside the plane.

The F-117 Nighthawk used both techniques to absorb a part of the incoming energy and deflect the remaining part in another direction.
Fun fact, its straight planes shape comes from the 70's computer not being performant enough to make a round plane like the B-2 Spirit !

For those wondering, it still had correct aerodynamics at the end, since it could fly at Mach 0.92 at 45,000 ft, able to cover up to 930 nautical miles (1.070 miles, 1 720 km)

Another fun fact before we move on, the F in F-117 means Fighter, but this plane is only a Bomber ! It should have been named A-117 (Attacker 117) but was finally named F-117A. Probably some tricks to confuse intelligence ?

But in order to strike Baghdad, the F-117 also had to hide itself, its heat (infrared signature) and sound, so it would be perfectly stealthy.
Hiding the plane is simple : operate during night time.
But how to hide the heat and noise now ?

To hide the heat of the exhaust gases of the jet engines (you can learn how jet engines work by reading my articles here : spaceandscience.fr/en/blog/compre…), Lockheed Martin engineers did mix the exhaust gases and the cold air, creating a less hot exhaust signature.

On the Northrop B-2 Spirit, the engines exhaust are placed on top of the plane to hide the heat from the ground !

Hiding the noise was made by not adding afterburners to the plane to keep it subsonic (afterburners and supersonic booms are pretty loud, you know) and by flying at high altitudes.

The F-117 was a good passive stealth aircraft, just like the B-2, F-22, F-35 or Su-57 for example. But there are other ways to hide from radars !

We now have to talk about active stealth and tactical stealth.

Active stealth is more about confusing and blinding the radar than be invisible to it. Specialised planes like the EA-18G Growler uses jammers, decoys and 1/2 phase radar wave emission to either saturate the enemy radar or make him receive incorrect data

Some high-tech stealth is being developed, called "Plasma stealth", using a ionized gas around the aircraft absorbing the radar waves.
Even if surrounding a jet with plasma looks cool, this technology is yet to be developed and proven.

Finally, tactical stealth means "fly where the radar won't detect you". For example you can hide behind hills, fly super low near the ground or even fly so slow that Doppler based-only radar can't make the difference between you and a slow, not interesting object.

There are many others factors about stealth : hiding weapons in bays, having sharp teeth shaped doors, changing radar frequency over time, etc
I'd like to finish this thread by showing you that a B-2 Spirit, almost as big as a B-52 has a 1000 times smaller RCS !

Stealth is nowadays big topic for planes, helicopters, ships and submarines, but it has many cons. Costs, weight, space required (less weapons carried), and of course the main issue is maintenance. The coating on the F-22 is very expensive and fragile !

I hope you liked this [THREAD] about Stealth and more precisely the awesome looking F-117 !
If you did, please like, retweet and share it, this is how you can make me want to do more of them.

My space/planes/engineering articles : spaceandscience.fr/en