Someone mentioned the 4D10 (the rocket engine in the R-27/SS-N-6) on Twitter.
The 4D10 is one of the reasons I got into rocket propulsion. It's also a ridiculously complex engine.
(I put this together in a hurry, so it's a bit short of pictures.)
2/18 As Dr. Postol mentioned, the 4D10 is a submerged engine. On the vast majority of liquid-propellant missiles and rockets, the engine(s) are mounted below the propellant tanks.
3/18 But as a submarine-launched ballistic missile (SLBM), the R-27 needed to be very compact, with the largest possible propellant tanks to maximize range. Thus, the 4D10 was submerged in the R-27's fuel tank, with only the tip of the nozzle protruding from the missile's bottom.
4/18 This required several technological innovations. Since Postol impugned on the CNS team's technical skill, I will now describe two of them.
5/18 Firstly, because the engine is submerged in fuel, it must be completely leak-proof. Every single connection in the engine was either welded or brazed (the opposite of US practice at the time). New quality control methods were also developed to ensure the welds were perfect.
6/18 Secondly, the engine was attached to the airframe at the nozzle. However, the nozzle was made of steel (12Kh18N10T), while the airframe was aluminum (AMg6 alloy). Fusion welding could not produce an acceptable join between the two metals...
(This is a training mock-up)
7/18 So a method to produce a bi- (actually tri-) metallic bushing was developed. 12Kh18N10T and AMg6 were (presumably hot-) rolled, with a layer of AD1 aluminum between them for ductility. The resulting part could be welded to aluminum on one side, and steel on the other.
8/18 (The 4D10 is actually two engines. The main engine is submerged, while another, smaller, simpler engine powers 2 small nozzles for steering. The turbopump for this engine is not submerged, being in a capsule protruding up into the fuel tank. That's beside the point, though)
9/18 As Dr. Postol notes, the complexity of the 4D10 contributed significantly to the failure of the Hwasong-10 program. This complexity did not, however, prevent North Korea from planning a series of missiles based on this engine.
10/18 On March 9, 2016, a North Korean press release included an image of Kim Jong Un and a KN-08. The first stage appeared to be powered by two 4D10s submerged in its base.
11/18 Experts thought the idea was technically outlandish - Rocket nozzles vibrate, and submerged engines are rigidly mounted in the airframe by their nozzles. The vibrations from two nozzles, both rigidly mounted in the airframe, could tear the missile apart.
12/18 One month later, in April 2016, North Korea released photographs of a rocket engine test showing what seemed to be that very same engine - a twin 4D10.
13/18 @Armscontrolwonk wrote a blog post about it at the time: armscontrolwonk.com/archive/120127…
14/18 This is the only test of this engine that we were ever shown, and it was made obsolete a year later when North Korea showed us its (probably RD-250-derived) March 18th engine.
Kim was so excited by the March 18th success that he started giving out piggy back rides.
15/18 Still, even though the paraded missiles appeared to be mock-ups, North Korea gave us good reason to believe that the KN-08 and KN-14 were active development programs intended to produce a long-range ballistic missile.
16/18 This point was reinforced by pictures displayed at the celebration for the first successful Hwasong-14 ICBM launch. North Korea viewed the KN-08 and KN-14 as critical steps in their ICBM program. The missiles were not a deception. They were just suboptimal.
17/18 All of this is to say that, while technical expertise is valuable for missile analysis, it must be accompanied by domain expertise to explain the technical, bureaucratic, and political reasons that different design decisions were taken in the creation of a specific missile.
18/18 In his criticism of Jeffrey's analysis, Dr. Postol demonstrates that he either lacks or has forgotten the necessary domain knowledge to understand the KN-08's and KN-14's role in the North Korean ballistic missile program.
George adds some valuable context to the story of analyzing the Hwasong-10 (Musudan) and KN-08/14.
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