North Korean Pukguksong series Part 4: Pukguksong-4 SLBM
The structure of the Pukguksong-4 SLBM
From the initial analysis of the Pukguksong-4, this new class of missile does not follow the main engine plus small graphite jet vane deflector design of the Soviet era R-27/4K10 submarine-launched ballistic missile, and it does not have the large grid fins of the Hwasong-10 missile.
It has most likely adopted the TVC (Thrust Vector Control) Actuator System, technology that has been fully verified on the Hwasong-15 intercontinental ballistic missile.
Shape analysis of the Pukguksong-4 SLBM
Judging from the shape and technical data of the missile body of the Pukguksong-4 currently disclosed, its performance is not weak at all, and its performance has even exceeded by a quantum leap the old vintage Soviet-era R-27/4K10.
Compared with the Pukguksong-1 SLBM which was first publicly tested in 2015 and the Pukguksong-2 GLBM which was first tested in 2017, the biggest difference is the shape of the Pukguksong-4's fairing and its aerodynamic.
The warhead's fairing of the Pukguksong-1 submarine-launched ballistic missile adopts a sharp cone design with a huge stable skirt, which can effectively reduce the drag when flying in the atmosphere. This is also a commonly used design for early submarine-launched ballistic missiles, such as the US Polaris-A1 and the Soviet Union's R-27 submarine-launched ballistic missiles, and also China's Julang-1 missile.
Judging from the currently disclosed photos of the Pukguksong-4 SLBM, this type of missile has changed the design of the conical warhead with a stabilizing skirt used by the Polaris-1 missile, and adopted the same design as the U.S. Trident-IID5 and the French M51. The same oval blunt head design. Compared with the conical warhead, this design has better underwater hydrodynamic flow performance, but greater flight resistance in the atmosphere.
According to the analysis of military experts, the Pukguksong-3/4 SLBM has removed the design flaws of the previous two generations, and its flight stability has been improved. It has shown a leap forward in the field of submarine-launched missiles, such as changing the pointed warhead of the Polaris-1 missile to a gentle arc, reducing underwater resistance and so on.
Secondly, the details of the high-definition picture of the missile body also show that the Pukguksong-3/4 SLBM has added bubble generating holes on both sides of the missile body's top, which are used to generate bubbles that wrap the missile body underwater, and isolate the missile body from contact with seawater.
This Active Cavitation Technology reduces the underwater resistance of the missile. This design can also be seen in the submarine missiles of other countries.
The principle of Active Cavitation Technology in water is similar to the supercavitation phenomenon. The missile is equipped with a cavitation generator, which can produce a large number of cavitations to wrap the missile body. At this time, the medium in contact with the missile body will change from sea water to gas. Greatly reducing the friction resistance of the missile. Active Cavitation Technology in water is commonly used in submarine-launched ballistic missiles in France and Russia, because the French and Russian missiles usually use underwater ignition technology.
Water friction resistance is much larger than un the air, which greatly consumes missile fuel and affects the maximum range. Therefore, Active Cavitation Technology is required to reduce water resistance.
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▲ 1. The Pukguksong-3/4 SLBM has added bubble generating holes on both sides of the missile body's top.
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▲ 2. Illustration of Active Cavitation Technology.
Launching ballistic missiles underwater is a very difficult technology. Overcoming this technology shows that North Korean submarines can launch ballistic missiles from underwater, improving the missile's attack capability and survivability.
The underwater launch of submarine-launched missiles has to go through three stages: underwater flight phase, water exit phase, and atmospheric flight phase. Among them, the underwater flight phase and the water exit phase are unique to the launch of submarine-launched missiles, and are also critical in determining the success or failure of the launch.
The most difficult part of launching a missile under water is to consider the evolution of the missile in two different mediums: underwater and atmospheric. Submarine-launched missiles must be agile in both water and the atmosphere. The missile is launched from the water into the atmosphere, and it has to work stably in two different media. This step takes the shortest time, especially when it breaks through the water. It is calculated in seconds, but it is the key to the success or failure of the submarine-launched missile launch. If a pointed warhead is used as the warhead shape, a wide truncated cone needs to be installed at the rear to stabilize the attitude of the missile in the water.
This not only increases the length of the submarine-launched missile, but also causes a sharp decrease in resistance when the missile crosses the water's surface into the atmosphere.
This results in the missile's attitude to be instable, which in turn leads to the launch failure.
Most of the early submarine-launched missiles in many countries all over the world also used pointed designs, such as China’s Julang-1, the U.S. Polaris-A1, Russia’s R-27, etc., but the launch success rate is actually not high.
The U.S. submarine-launched ballistic missile Polaris-A1 to Trident-II D5 changed gradually from a cone to a blunt nosecone.
Therefore, in order to ensure the stability in the water, with the gradual research of submarine-launched technology, the warheads of submarine-launched missiles have been changed to a blunt shape, which is different from ordinary intercontinental missiles.
In the process of developing submarine-launched ballistic missiles, the Americans gradually improved from the Polaris-A1 and A2 with a cone-shaped head design to the Trident series use of an oval blunt head.
In addition, Russia's Bulava, China's Julang-2, France's M51 and other submarine-launched missiles also use a blunt body shape.
The reason why the blunt shape is favored for underwater launch is its cavitation effect.
Since the top surface area of the warhead of this shape is large, a larger water area can be opened during the propulsion process, and the missile body can move forward in the cavity supported by the warhead. If a bubble generator is added to the warhead, it is basically the same as traveling in the air, which can minimize the resistance in the water. In addition, the use of blunt warheads can also increase the internal space and be equipped with more multiple reentry vehicle (MRV) to improve the missile penetration and attack capabilities.
General land-based missiles and air-based missiles only need to consider air resistance. In order to reduce air resistance, missiles often use a slender body and a sharp conical warhead or arc design. However, this kind of missiles are not intended to be used underwater, and more cavitation will be produced when the missile exits the water.
These cavitation bubbles will quickly collapse at the moment the missile exits the water, which will cause a large load on the warhead and the missile body and damage the missile structure. For sea water to flow out steadily, it is best to have a blunt tip, and it is best to generate bubbles by itself to reduce water resistance and maintain a stable hydrodynamic flow.
However, the shortcomings of the oval blunt design warhead are also very obvious. When the submarine-launched missile flies in the atmosphere at high speed after exiting the water, the blunt tip has a large air resistance, which severely reduces the speed and range of the missile. Flying in the air requires the least resistance arc head.
Two ways to resolve this contradiction have been developed among the major powers.
One is the blunt-headed Trident of the United States and the M51 of France that pop up the drag reducing rod called aerospike after exiting the water.
Missiles or launch vehicles will encounter huge air resistance when flying in the air, and a large part of the engine energy is used to overcome air resistance. The head resistance of the spacecraft mainly comes from the bow shock wave near the head. The faster the missile or rocket, the higher the pressure and temperature in this area, and the resistance will increase sharply. It is generally believed that in order to overcome the bow shock, a missile needs at least a quarter of the thrust of the engine, which can be said to be very lossy. The drag reduction aerospike is designed to overcome the bow shock wave. Its principle is very simple. It pierces the bow shock wave and turns it into an oblique shock wave. The pressure and resistance of the latter are much smaller than the bow shock wave. This is equivalent to increased engine thrust and improved missile range.
The American Trident-IC4 uses a retractable drag reduction aerospike for the first time on a submarine missile, which is usually retracted in the fairing.
When the missile's exits the water, the rocket engine is ignited after 4 seconds, the drag reduction aerospike extends out of the fairing and pierces the air.
According to relevant data, the drag reduction effect of the aerospike can reach more than 30%.
In addition to the use of aerospike, a double-head design can also be used to solve the problem of the contradiction between the movement of the missile in the water and the atmosphere, which is to add a blunt fairing outside the missile cone fairing.
After the missile exits out of the water and enters the atmosphere, the blunt-headed outer cover is separated, and the missile exposes its cone fairing ready to start its atmospheric flight.
India’s K15 submarine-launched missile uses this design. The most ideal shape of the fairing to deal with seawater resistance is a blunt head shape. The blunt head shape fairing has little resistance underwater, but after exiting out of the water, the resistance increases. The sharp cone fairing is the opposite, so the most ideal fairing shape for atmospheric resistance is a sharp cone. The purpose of adopting the double hood design is to complement each other and have the best shape underwater and in the air.
But the disadvantage is that it increases the complexity of the launch process, mainly due to the addition of a low-altitude fairing ejection step. At the same time, the blunt fairing and ejection pyrotechnic will increase the length and weight of the missile, and make the internal structure of the missile more complicated, which will reduce the missile's reliability.
The most eye-catching part of the Pukguksong-3 missile test is that the missile exits directly out of water without the need for an additional protective cover, thus effectively reducing the length and weight of the missile. As an economically and technologically underdeveloped country, North Korea’s ability to acquire such mature technology is indeed impressive.
https://archive.fo/93Tek/64bc5a4fa5077d0911e4f7ef06cfcd3f4131ba92.jpg ; https://archive.fo/93Tek/69c97d0437947fb9550b088c85d72e24c3e5595f/scr.png ; http://web.archive.org/web/20191025.../10/multi_photo_2019-10-24_dn99517_image1.jpg ; http://www.uriminzokkiri.com/index.php?ptype=photo&no=8201 ; https://www.dprktoday.com/index.php?type=42&s=1#/42&s=1/
▲ 3. Korean-developed Singijon, at the origin of multistage rockets, in the early 15th century, and fitted with aerospikes to extend the range.
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▲ 4. M-51 SLBM nose cone section with an aerospike.
Further development of this concept should soon be demonstrated with air-spike. This is formed by concentrated energy, from a pulsed laser, projected forwards from the body, which produces a region of low density hot air ahead of the body. This has the advantage over a structural aerospike that the air density is lower than that behind a shock wave providing increased drag reduction.
http://web.archive.org/web/20190319013327/https://c1.staticflickr.com/5/4885/31487803767_7c98fddf26_b.jpg ; http://web.archive.org/web/20190810170025/https://www.flickr.com/photos/arirangmeari/31487803767/
▲ 5. Further development of this concept should soon be demonstrated with the DPRK's pulsed laser air-spike. Uploaded on December 23, 2018.
Regarding speed, it is estimated that the maximum speed of Pukguksong-3 SLBM is about 10 Mach. This is mainly because according to the available information, the Pukguksong-2 GLBM previously tested by North Korea has a similar design to the Pukguksong-3 SLBM and it is said that its maximum speed is Mach 10. However, because North Korea has the potential to continue to promote engine development, it cannot be ruled out that the missile has a higher speed. It is estimated that the potential maximum speed is about 12 Mach.
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Initial calculations on the Pukguksong-4:
Weight: ~23t (without gas generator)
Payload: ~500kg
Range: ~6300km (short range ICBM)
Length: 9,8m (with gas generator)
Diameter: 1,8m
Features:
Flexnozzle TVC 1st stage
2 Filament wound composite motor stages
Compact nozzle design
8:16 PM · Oct 16, 2020
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The structural ratios applied here are according to the motor technology used, reasonable estimates:
1st stage 8,75%
2nd stage 10,5%
In a worst case scenario, the range would be reduced to 5000km and the structural ratios very unfavorable at:
1st stage 12,5%
2nd stage 14,5%
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With an effective aerospike the author gets ~7300km max. range
There is no space for jet vane TVC on the 1st stage and if they have flexnozzle tech. Then its certainly also applied to the 2nd stage.
Thats why the structural ratios are:
1st stage 8,75%
2nd stage 10,5%
Quite optimistic ones
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https://twitter.com/Pataramesh/status/1317167509993328640
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▲ 4. Differences between the DPRK Pukguksong prototype and the mature looking PK-4 from the recent parade. This is the author's interpretation of it and there are still open issues like the TVC actuator of the first stage. 2020.