Hello defence lovers! An old soviet era Mig-23 had a top speed of Mach 2.2 and a Mig 29 has a top speed of Mach 2.25, but modern fighters such as Dassault Rafale and Tejas have a top speed of only Mach 1.6. So why are fighter jets getting slower? We will try to find out the answer to this question in this article.
What is Mach Number?
In terms of fluid mechanics, the Mach number is the ratio of flow velocity to the velocity of sound in the prevailing conditions and environment. In simple words, it is the ratio of the speed of a body to the speed of sound in the surrounding medium.
The Jet Technology
In the initial days of aviation top speed was a critical aspect of a fighter aircraft. The designers tried to fit bigger and better engines (Piston engines) to make their fighters faster than those of their enemies. In the first world war, planes were used for reconnaissance purposes. Soon the pilots manning these planes began attacking each other with handguns. Having higher speed was advantageous as the faster aircraft could always attack or disengage from the slower ones. The average speed of planes was around 200 kilometres per hour back then.
During the Second world war, the fighter plane technology evolved further and the planes became even faster. The British specially built the de Havilland Mosquito to outrun the German fighters. Speed was such an important factor that this plane didn’t have any guns.
By the end of the Second World War, the jet fighter had taken to the sky. However, these new types of fighters had to wait till the Korean War to prove their worth. The Mig-15, F-86 sabre, Folland Gnat etc, the first generation of jet fighters had a top speed of Mach 0.9. The Supersonic fighters of the next generation like the F-4 Phantom, F-100 series, English Electric, and Mig 21 had top speeds roughly double the cruising speeds of the previous generations. However, the top speed was relevant as long as dog fights were restricted to cannons only. Air-to-air missiles changed everything.
Dog Fights
In modern fighter jets, during dog fights or within visual range engagements, the top speed of the aircraft barely matters. What matters most is the instantaneous turn rate and the sustained turn rate. The more manoeuvrable fighter will be able to lock its enemy first and fire its weapons. The typical speed of dog fights varies but the best speed for optimum turn rate is around 0.7 Mach. Some fighters like the Flankers are extremely manoeuvrable at low speeds. However, when these fighters do the insane manoeuvres, they bleed energy which can make them easy targets.
During combat engagements, fighters barely even come close to their top speeds. For example, during the Vietnam war, the F-4 phantom fleet spent not even a single moment at Mach 2.2 during combat. Moreover, the entire fleet’s aggregate flight time at Mach 1.4+ speed is barely a few minutes.
Efficiency Issues
In most modern jet fighters, the most efficient cruising speed is subsonic and as low as 50% of the top speed of the fighter. The top speed is achieved by using afterburners which are highly inefficient. Using afterburners can reduce the range by up to 70 per cent.
With the development of modern jet engines, some fighter jets have attained the capability of supercruise. Supercruise is the condition when a fighter jet flies with supersonic speed without using an afterburner. The Dassault Rafale in the Indian Airforce’s fleet is capable of supercruising at Mach 1.4.
When fighter jets are scrambled for interception roles, they rarely get supersonic as the sonic boom can cause inconvenience in the residential areas. The Sonic boom is sometimes so powerful that it can shatter the glass of the windows. It is created by the shockwaves when a jet crosses the sound barrier.
Escaping Enemy Fire
Some jet aircraft in the early days used speed as a defensive mechanism to escape enemy fire. During the second world war, the air defence batteries consisted of guns which fired air-bursting shrapnel rounds (For example – German Flak, Bofors 40 mm etc.). So the principle was to hit and run. During the cold war, Americans developed the SR-71 Blackbird which had a top speed of over Mach 3. At that time it could outrun any air-to-air missile fired at it. The Soviets developed the Mig-25 in response. However, these became obsolete with the development of Surface to Air missiles which can reach Mach 4 to 7.
The goal of the designers then shifted from making a faster aircraft to a more manoeuvrable aircraft which could dodge incoming missiles. For manoeuvrability, speed was compromised with designs like the Flying fuselage designs used in Su-27, mig-29, F-15 etc.
Going supersonic also increases the heat signature of the aircraft. The afterburner attracts infrared missiles like magnets. Moreover, the drag due to supersonic air results in the heating of the leading edge of the wings which further increases the infrared signature.
Reconnaissance Roles
In the early days, reconnaissance roles were performed by aircraft. Speed was a critical factor. The SR-71 Blackbird was specifically designed for that role. The Soviet counterparts like Mig-25 and Mig-31 were developed for supersonic interception. However later those were modified for reconnaissance roles as well. But now with the high-resolution satellite imagery, these aircraft are not required anymore. Now stealthier unmanned options like HALE and MALE UAVs are available for those roles.
So these are the reasons why the modern fighter jets like Dassault Rafale, F-35, and Tejas all have a top speed of Mach 1.6 whereas older fighters like Mig-21, Mig-23, etc have top speeds of Mach 2+. Follow DefenceXP for more such interesting articles.
