MiG-21 Fishbed showing its round nose intake and green shock cone

MiG-21 Nose Cone Explained: How the Shock Cone Feeds a Mach 2 Jet

by Max Grünwald | Oct 3, 2026 | Aviazione militare | 0 comments

Look a MiG-21 in the face and the first thing you see is a hole with a cone in it. The whole nose of the aircraft is one big round air intake, and sitting in the middle of it, like the stopper in a bottle, is a pointed cone.

That cone is not decoration. It is the reason a 1950s fighter with a single engine could fly at twice the speed of sound, and it does three jobs at once: it shapes the shock waves in front of the engine, it moves to keep them in the right place, and in most versions it carries the radar.

Informazioni rapide

Aeromobili: Mikoyan-Gurevich MiG-21 Fishbed
Intake: Round nose intake with a central conical body (inlet cone or shock cone)
Early control: Three cone positions: aft up to Mach 1.5, middle from Mach 1.5 to 1.9, forward above Mach 1.9
Later control: From the MiG-21PF, the cone moved continuously according to speed, controlled by the UVD-2M system
Radar: Most versions carry their radar inside the cone, such as the RP-21 Sapfir
Velocità massima: About Mach 2 at altitude
Costruito: 11,496 in total, according to Wikipedia

Why a jet engine hates supersonic air

A jet engine's compressor cannot swallow air arriving at supersonic speed. The air has to be slowed down to well below the speed of sound before it reaches the first compressor blades, and it has to be slowed in a way that wastes as little of its energy, its pressure, as possible.

Slow it down badly, with one big, strong shock wave right at the mouth of the intake, and the engine loses a large part of the pressure it needs to make thrust. Slow it down gradually, in steps, and the engine gets far more of that pressure back. Engineers call this pressure recovery.

What the cone actually does

At supersonic speed, the tip of the cone sets up a conical oblique shock wave that spreads out from the point like the wake of a boat. Air passing through an oblique shock slows down and compresses, but loses much less energy than it would through a single head-on shock.

The trick is to place that cone shock exactly at the lip of the intake. Too far forward and air spills around the outside, adding drag. Too far back and the shock is swallowed into the duct, which can make the flow unstable. At the right position, the intake catches all the air it needs and compresses it efficiently before a final, much weaker shock takes it subsonic inside the duct.

Front view of a MiG-21 showing the round intake and nose cone — The intake and shock cone of a MiG-21 on display in Kyiv. The gap around the cone is where all the engine's air enters. Photo: Jorge Láscar / CC BY 2.0

Why the cone has to move

The angle of the shock wave changes with speed. As the aircraft goes faster, the shock folds back closer to the cone. A fixed cone can only be perfect at one speed, so on the MiG-21 the cone moves forward and back to keep the shock where it belongs.

On the early MiG-21s, according to Wikipedia, the cone had three positions: fully retracted up to Mach 1.5, a middle position between Mach 1.5 and Mach 1.9, and fully forward above Mach 1.9. From the MiG-21PF, the cone moved continuously according to the actual speed, controlled by a system known as UVD-2M that used air pressures measured ahead of and behind the engine's compressor.

For take-off and low-speed flight, when the engine wants more air than the nose can supply, the MiG-21 also has auxiliary intake doors on the sides of the forward fuselage.

A radar in the way

The cone also solved a packaging problem. With the whole nose taken up by the intake, there was nowhere else to put a radar. So from the MiG-21PF onwards the radar, such as the RP-21 Sapfir, was mounted inside the cone itself, while earlier versions carried only a small ranging radar there.

It worked, but it came at a price. The cone could only be so big without choking the intake, and that limited the size of the radar antenna and therefore its range. Most later fighters, from the F-4 Phantom to the MiG-23, put their intakes on the sides of the fuselage instead, leaving the nose free for a much bigger radar.

The MiG-21 was not alone in using a centre-body intake. Britain's English Electric Lightning carried its radar in a fixed nose cone, the Sukhoi Su-7 used a similar layout, and the SR-71 Blackbird had translating spikes in each of its engine nacelles. But few aircraft made the idea as famous as the Fishbed, with more than 11,000 built.

Feel it work for yourself

The best way to understand that cone is to feel it do its job. MiGFlug offers a supersonic MiG-21 flight in Florida, the 40-minute flight that takes you through the sound barrier, typically to around Mach 1.3, with aerobatics and time at the controls. As the jet accelerates past Mach 1, the cone in front of you is quietly doing exactly what it was designed to do in the 1950s.

MiGFlug customer in the cockpit of a MiG-21 over the sea — A MiGFlug customer in the cockpit of the Florida MiG-21 during a supersonic flight. Photo: MiGFlug

Sources: Wikipedia (Mikoyan-Gurevich MiG-21; Inlet cone), Vietnam Conflict Aviation Resource Center, MiGFlug

Domande frequenti

What is the cone in the nose of the MiG-21?

The cone in the MiG-21's nose intake is an inlet cone, also called a shock cone. At supersonic speed it creates a conical oblique shock wave that slows and compresses the incoming air efficiently before it reaches the engine. In most MiG-21 versions it also houses the radar.

Does the MiG-21 nose cone move?

Yes. On early MiG-21s the cone had three positions: fully aft up to Mach 1.5, a middle position between Mach 1.5 and Mach 1.9, and fully forward above Mach 1.9. From the MiG-21PF onwards, it moved continuously according to speed, controlled by the UVD-2M system.

Why do supersonic jets need an inlet cone?

A jet engine's compressor cannot accept supersonic air. An inlet cone uses oblique shock waves to slow the air down in steps with much less pressure loss than a single strong shock, which gives the engine more thrust and better efficiency at supersonic speed.

Is there a radar in the MiG-21 nose cone?

Yes. From the MiG-21PF onwards, the radar, such as the RP-21 Sapfir, is mounted inside the intake cone. Earlier versions carried only a small ranging radar there. The cone's limited size restricted the radar's antenna and therefore its range.

A che velocità può volare un MiG-21?

The MiG-21 can fly at about Mach 2 at high altitude, roughly twice the speed of sound, using a single afterburning engine and its moving inlet cone.

Can I fly in a MiG-21?

Yes. MiGFlug offers a supersonic MiG-21 flight from Florida, USA, for a limited time. The 40-minute flight includes breaking the sound barrier, typically at around Mach 1.3, aerobatics and stick time, at altitudes up to 28,000 feet.