Debris from North Korea’s Launcher: What It Shows

, former co-director | December 27, 2012, 1:20 am EDT
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Press reports now say South Korea has recovered four pieces of the first stage of the Unha-3 rocket that North Korea launched on December 11 (U.S. time). Since all these pieces were found in approximately the same area, they must all have come from the first stage.

The first and largest piece found was the oxidizer tank from the first stage, reported to be 7.6 m long by 2.4 m in diameter. Figure 1 shows on the left the tank that was recovered (being carried on the recovery ship, rotated by 90 degrees), scaled so it can be compared to a photo of the first stage of the launcher (compare markings) and a diagram showing the internal construction of the stage on the right. That diagram shows the oxidizer tank in blue sitting above the tan fuel tank, which is above the first-stage engines. The fuel and oxidizer are carried through pipes to the engines, where they ignite when combined in the combustion chamber to create thrust.

Figure 1. (click to enlarge)

Figure 2 shows the tank from the side, including the cables that run down the outside of the tanks from the guidance system in the front of the rocket to the engines in the rear.

Figure 2. (click to enlarge)

According to press reports, traces on the inner walls of the tank show that the first-stage oxidizer is a form of nitric acid called “red-fuming nitric acid,” which is the standard oxidizer used in Scud-type missiles. There had been some speculation that this stage might instead use a more advanced fuel with nitrogen tetroxide (NTO) as the oxidizer. Since the Nodong engines believed to power the first stage are scaled-up Scud engines, the use of RNFA is not a surprise.

There have also been claims that the stage uses a more advanced fuel called UDMH, but it appears instead to be the kerosene-based fuel used in Scuds. In his recent RAND study, Markus Schiller noted that a test Iraq performed using UDMH in a Scud engine gave poor performance, and that burning UDMH gives a transparent flame. The North Korean video of the launch instead shows an orange flame characteristic of Scud fuels (Figure 3 is an image from 12:44 into the video). These findings confirm that the stage is still Scud-level technology.

Figure 3. An image showing the flames coming from the first-stage engines several seconds after launch. (click to enlarge)

There have also been questions about what material was used to build the body of the first stage. The body of the Scud, and likely the Nodong, is made of steel, but reports say that the tank recovered is made from a lightweight aluminum-magnesium alloy, which is typically used for aircraft. This saves a significant amount of weight, which is important to allow its relatively low-thrust engines to get the launcher up to speed.

Examining this structure will give a good sense of the quality of construction and components used in building the rocket, and which components may have been imported. Early reports stated that “the eight panels from the first stage that were retrieved were crudely welded manually.”

South Korea released a picture of the inside of the tank (Figure 4). This shows the internal supports—called hoops and stringers—that give the tank structure and support its weight. These are then covered by thin metal sheets to form the tank walls. This kind of construction is fairly standard for liquid rockets.

Figure 4. (click to enlarge)

Because this launcher uses turbo pumps to feed fuel and oxidizer from the tanks to the engines, the tanks do not need to be kept at high pressure, which allows the walls to be thin. The tanks are typically kept at low pressure —10-50 psi—which improves the flow of the propellant to the turbo pump (see Rocket Propulsion Elements, p. 204). The pressure is maintained by an inert gas, like nitrogen or helium, that is stored in pressurized bottles and released into the tanks as the fuel is burned. A second piece of debris (Figure 5) appears to show two of these bottles. This piece probably sits between the fuel tank and the engines.

Figure 5. (click to enlarge)

A third piece recovered from the launcher is part of the fuel tank, shown in Figure 6. The blue “3” on its side can be seen in the launch photo in Figure 1.

Figure 6. (click to enlarge)

The final piece recovered appears to be a support ring from the first-stage body (Figure 7).

Figure 7. (click to enlarge)

No word yet if debris from the second stage has been found. It fell into the ocean off the coast of the Philippines.

Note added 1/2/13: Markus Schiller, who is an expert on rocket structure, believes that the bottles in Figure 5 are instead likely to hold the small amount of Tonka fuel that is used to ignite the engine. The Scud propellants, nitric acid and a version of kerosene (TM-185), are not hypergolic–they don’t spontaneously ignite when combined. So the Soviets designed the Scud to inject a small amount of Tonka fuel (called TG-02 by the Soviets) into the engine at launch since it would ignite when combined with nitric acid. The Scud used about 30 kg of TG-02 for this purpose.


Sources for figures:  Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7

Posted in: Space and Satellites Tags: , , , , ,

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  • Brian W

    I think it’s also important to point out that red fuming nitric acid (RFNA) is also used as an oxidizer in the Russian Kosmos-3M space launch vehicle:

    The Kosmos family is derived from a ballistic missile and RFNA is a storable liquid oxidizer. While the use of RFNA is not conclusive proof that what was launched by North Korea was an ICBM, it does show that they are pursuing dual use technology in much the same manner as the US and Soviet Union and could result in development of either an SLV or ICBM.

    • David Wright

      Yes, that’s a good point to make since a number of press reports state that the use of RFNA implies the rocket was an ICBM. Using storable fuels that can be used at room temperature simplifies the design and construction of a rocket compared to using cryogenic fuels, which need to be kept at very low temperatures.

  • Steve

    I am curious if they would have bothered to include a flight termination system on this vehicle. If so, I wonder what precautions the South Koreans would have taken when picking up the pieces.

  • John

    Thanks for the analysis on the rocket.

    I have one question about the fallen parts of the rocket.
    Under the outer space treaty, isn’t N. Korea
    entitled to the return of the fallen parts?

    • David Wright

      I don’t know if international law covers debris from a launch, but I would guess that in any case, due to the UN resolution 1718, countries are not required to return debris to North Korea.

  • Has anyone considered that North Korea may have some capable secret service people who, for example, might stage rocket launches with somewhat outdated gear to later catch anyone by surprise? After all, such shenanigans have a long tradition going back to the cold war and North Korea is the last cold warrior state left.

  • James W. Barnard

    N2O4 is an allotropic form of NO2. N2O4 is liquid under its own vapor pressure at approximately room temperature, which means it can be stored, and, indeed the Titan II ICBM/Launch Vehicle utilized this as an oxidizer. If N2O4 is allowed to dissociate in the pressence of water, it will form, depending on the percentage of water RFNA or with more water, White Fuming Nitric Acid. The only reason I can see for using RFNA versus N2O4 would be storing it. Perhaps the analysis of “RFNA” doesn’t take into account the reaction of N2O4 with sea water. RFNA gives lower specific impulse than Nitrogen Tetraoxide (N2O4), and I can’t see why anyone in this day and age would use RFNA instead. As to the fuel, I’m not sure what hyrocarbon would give decent performance, instead of UDMH, although analine, furfuryl acohol, and several others were used during WWII and through the 1950’s, including my own RFNA/analine static test engines I design and built in the late 1950’s (during high school). Can’t get at my old texts to see if kerosene would work well.

    • David Wright

      Interesting about N2O4 becoming RFNA in the presence of water. I assume the people examining the tank understood that. Since they have found part of the fuel tank as well, it will be interesting to see if they can find a trace of the fuel, to verify that it isn’t UDMH. For more on the propellant, see my comment below.

      • Thikinng like that shows an expert’s touch

  • David Wright

    The use of RFNA in this engine comes from the decision by the Soviets to design the Scud engine to use it, in a form called AK-27 (which has 27% N2O4 mixed with nitric acid, compared to about 13% for RFNA). It does give a lower specific impulse than using N2O4 itself (about 5% lower), and I assume they used it because it has a considerably higher boiling point (120C vs 22C for N2O4), which could be useful for a battlefield missile like the Scud that was intended to be fueled and then kept ready for use. The Scud fuel is called TM-185, which I believe is a mixture of 80% kerosene and 20% gasoline. I don’t know why the Soviets did not use UDMH in the Scud since they seemed to have been developing UDMH in the late 1940s. The Nodong engine is a scaled-up Scud engine, so it was presumably designed to use the same propellant combination. When China developed its DF-3 missile in the 1960s, which is similar to the first stage of the Unha, it instead used UDMH and N2O4.

    • David Wright

      The more I think about it, I suspect the reason for using kerosene for the propellant rather than UDMH was availability. It’s one thing if you want to use an optimum fuel for an occasional space launch, but the Soviets were designing a battlefield missile they planned to build in large numbers, and it probably made sense to design them to use a readily available fuel.