Jeffrey LewisUnha Debris Revisited

Yesterday, I noted that images of the debris from the Unha launch might provide insights into the design of the missile.  We have such an observation already! The fuel tank appears to have four feed pipes coming out of the fuel tank.

This answers a long-standing question about whether North Korea might try to redesign the first stage of the Unha, which comprises a cluster of four Nodong engines, to reduce its weight.  North Korea might have, for example, designed a single turbo pump to service all four engines. Geoff Forden has written before on this blog about the weight savings (about 100 kilograms) from such a redesign:

As can be imagined, a missile’s engines constitute a major portion of the “dead” weight contribution outside of the payload. Engine weights have, therefore, been a major area of research with much work going into reducing the weight per ton of thrust.


Finally, a cluster of four Nodong engines—such as has been reportedly used in the first stage of the Tae’podong II—saves well over 100 kg in weight if it uses a single turbopump developed and optimized for that configuration. Is that enough to justify a proliferator developing a new turbopump? Perhaps we will only know for certain when we start seeing images of that stage appear in public.

Well, now we know!  Four feeds would imply four turbopumps, one for each engine. The answer, at least for North Korea, appears to be “no.”


  1. anon2 (History)

    And if it had a single turbopump, how exactly are you going to regulate the flow and pressure to each of the four engines which undoubtedly would have minor variations. Or would the engineering team accept a different thrust for each engine and use something else to keep the missile on course.

    I don’t know what the percentage of the fueled and empty weight of the first stage the turbo pumps are, but it seems to me that the engineering difficulties of regulating each engine reliably would exceed the weight gain payoff.

    • Jeffrey (History)

      I believe the early Soviet ICBM (R7) used a single turbopump to service four combustion chambers.

      As Geoff estimated, in an Unha, it might purchase 100kg of payload. More importamt is the possibility of wanting to build larger engines. My guess is that if you let German engineers take over the DPRK rocketry program, you’d see different technical choices.

    • Cameron (History)

      Without properly designed controls, and the money for multiple test launches, I’d guess keep it simple would be the order of the day. 100 extra kilos is nice, but not at the cost of a successful launch.

      And because I can’t resist, Jeffrey can an Unha even hit London from the DPRK?

    • John Schilling (History)

      The Unha is a fairly substantial vehicle with demonstrated orbital capability; it can almost certainly hit the southern United Kingdom but may lack the accuracy to hit London proper.

      Assuming quad Nodong engines on the first stage, a single Nodong for the second stage, and an upper stage using the same R-27 vernier scheme as the Iranian Safir, I get a best-guess payload of 1325 kg from Sohae to London. The relatively low thrust of the R-27 verniers is less than optimal for performance but good for accuracy, probably hitting within 10 km of the aimpoint using a pitch profile and integrating accelerometer cutoff scheme. Proper German engineers would no doubt insist on a more sophisticated inertial guidance scheme with dedicated post-boost propulsion 🙂

      If we assume a Taepodong-heritage upper stage, payload goes down to an estimated 1252 kg, and accuracy would be degraded by an unknown amount. And, as noted, shaving 100 kg from the first-stage weight using a common turbopump would add only 5 kg to the delivered payload. This engineer, at least, would stick with one turbopump per engine if I already had a tested pump/engine combo of the right size. But then, my ancestors left Germany about a century ago, so I may have fallen somewhat from the true faith 🙂

    • Jeffrey (History)

      This will teach me to be lazy around engineers. The point I was trying to make (inartfully) is that designing ever larger engines with separate combustion chambers (rather than merely clustering engines) is an alternate design approach that must have been considered but is clearly not being pursued in the context of the Unha launchers.

      Despite my crack about “German engineers,” my understanding is that the Chinese designed the DF-3 using clustered (maybe parallel is a better term?) engines, but for the the DF-5 used a single turbo pump with multiple chambers. I am not completely confident of the details; indeed, this is something I’ve been working on for a while in terms of understanding how the Chinese thought about the technical steps to an ICBM and how that might differ from, say, the choices the North Koreans are making.

    • John Schilling (History)

      Generally speaking, an ICBM wants one engine per stage – more engines just means more complexity, more cost, and more chance of an engine failure. If you see an ICBM-like missile with more than one “engine”, however defined, it tends to mean one of two things: someone had an engine that was one-half or one-quarter the right size and didn’t want to go through another engine development program, or someone ran into trouble scaling up an engine.

      And in the latter case, combustion chambers and turbopumps scale differently. Combustion chambers demonstrate new and exciting forms of instability as you increase the absolute scale. Pumps, mostly give you the same challenges at any reasonable scale, but those challenges are substantial. Which means, when the time comes to build a bigger missile than you ever have before:

      If you’re willing to design an engine, but you need one bigger than you know how to handle in terms of combustion stability, you design one pump of the right size and feed it to multiple chambers of a size you are comfortable with.

      If you’re willing to design an engine but it doesn’t need to be too big, you design one pump and one chamber of the right size.

      If you’re not willing to design an engine, you cluster as many independent engines as you need of the biggest size you’ve currently got.

      North Korea seems to be very reluctant to design new engines, probably for good reason, so they are in the third category. China was as well, early in their program, but by the time the DF-5 came along were more willing to build the right engine for the job. Just not confident they could handle the combustion-instability issues for a single chamber producing 650,000 pounds of thrust; that’s more than the United States ever got out of a single liquid-propellant rocket chamber until we built the F-1 for the Apollo program, and there were issues with that one.

      Note that if you are designing a manned space launch system, or a high-end launch system for unmanned payloads, you sometimes go with multiple independent engines for redundancy. The Saturn V, Space Shuttle, and Falcon 9 have all demonstrated the capability to complete a mission with an engine failure, but it isn’t easy to implement that capability. For a missile, it’s probably cheaper to just buy a second single-engine missile.

    • BrunoG (History)

      Not to disagree since I do not have the tools to crunch the numbers but throw 1300+ kg at a 8000-9000 km range with that kind of system seems very optimistic.
      Clustering will augment the thrust of the first stage making it possible to lift off the complete mass of the propellant/inert (let’s say around 25tons), two more stage (20t), payload and equipment. The total would be around 45-50+ tons which is about the same as the DF-3A and a lot less than the DF-4 which ranges are far from the figure Sohae-London.
      Again I am not saying I totally disagree but I am surprised

    • John Schilling (History)

      A single Nodong engine has a sea level thrust of approximately 290 kN; a cluster of four would be 1160 kN or almost 120 metric tons. That is far more than is required or even appropriate for your hypothesized 45-50 ton launch vehicle; space launch vehicles typically have
      about 0.3G net acceleration as they leave the pad, and data from prior Unha launches suggest it was a bit slower than that. This is consistent with a vehicle of close to 100 tons launch weight; my model currently comes in at 92.8 tonnes excluding payload.

      This is also consistent with the observed size of the vehicle; there is room inside those stacked cylinders for about eighty tonnes of rocket propellant and there is nothing but rocket propellant, engines, and a bit of plumbing that belongs in there.

      And if that 0.3G launch acceleration seems puny to some, it is driven by the following: 0.3G vertical acceleration translates to 1.3G horizontal acceleration when the rocket pitches over – which happens surprisingly soon. There is another 15% increase as the rocket climbs out of the lower atmosphere and the engine becomes more efficient. And the big one: most of the rocket is rocket propellant; as that is burned off and the vehicle becomes lighter, acceleration increases enormously – to about 5G at first-stage burnout, if my estimates are correct.

      Try accelerating a bunch of now-empty fuel tanks at much more than 5G, and you risk the whole thing collapsing. Try running that acceleration profile starting at sea level, and you’ll reach unreasonably high airspeeds while you are still in the lower atmosphere – another way to tear your vehicle apart. As it is difficult to throttle a rocket engine over a wide range of thrust, and inefficient to carry rocket hardware you aren’t going to put to the full use, it does turn out to be best for a satellite launch vehicle to start slow.

      ICBMs will launch a bit faster, typically 1G net vertical acceleration (i.e. 2:1 thrust:weight ratio rather than 1.3:1), so as to rapidly depart from the launch site. Not generally considered safe to loiter around missile bases in an ongoing nuclear war, you see. But this does require that ICBMs be much sturdier and thus heavier vehicles; when pressed into service as satellite launchers they turn out to be somewhat inefficient. And vice versa, but if you really wanted to nuke London and had time to set it up, an Unha-3 could probably do the job.

    • BrunoG (History)

      Thank you for the details. I did not see it as a 90+tons total mass object and thus did not see it as loading so much propellant. Just to be sure, there is some inert mass that is not propellant and not plumbing, right (structures, engines..) ?
      Again thanks.

    • John Schilling (History)

      Inert mass: A typical first-generation satellite launch vehicle is about 90% propellant and 10% metal, the latter being mostly engines and tanks. One of the tricks to making space launch vehicles possible is to make your propellant tanks double as main structural elements, so there is very little structural dead weight.

      North Korea does seem to have got a handle on the tank = structure bit, but they still seem to be using steel tanks, separate bulkheads, and the like, so we generally assume something closer to 85% useful propellant and 15% inert mass on NORK vehicles.

  2. Markus Schiller (History)

    Just a short note: In a multi stage rocket, 100 kg weight savings in the first stage do not offer 100 kg additional payload. For Unha-3, saving 100 kg in the first stage actually offers just a single number digit of extra kg atop the third stage.

    Would have been too much of an effort for that gain, without even addressing the Nodong engine’s problem of combustion instabilities.

    Robert & Markus
    German engineers

    • Jeffrey (History)

      Ha! I suppose you are German engineers!

  3. A Complete Stranger (History)

    I think much of this conversation has missed the important difference between surmising something from engineering principles and knowing that it is true. Before seeing the four intake openings, we could surmise that North Korea might not develop a new cluster of engines that used a single turbopump but now we know that to be the case. There is actually a world of difference between the two situations.

  4. BrunoG (History)

    A single turbopomp to feed 4 Nodong type engine would have been a real engineering feat. Not impossible but highly improbable for the NORKs without someone to help with the design and production.

    • A Complete Stranger (History)

      That question of help with the design and production is exactly the point everyone is arguing about regarding North Korea’s missile program.

  5. Cthippo (History)

    It definitely seems like the North Koreans are following a conservative design arc building on components that are known to work. Now that they’ve demonstrated the whole system they can start tweaking it and making improvements.

    One of the failings of the US aerospace industry is that they like to re-invent the wheel every time. They tend to reject good ideas that work because they are “old”. one of the best examples of this comes from the space race in the late 50s and early 60s. Realizing that regular pens wouldn’t function in a microgravity environment, the US spent millions to develop a pressurized ballpoint. The Russians sent up pencils.

    In the west we tend to disparage anything that isn’t cutting edge, but then we end up with massively expensive hardware that is the best in the world, when it works. Unfortunately, even when it does work, we can only afford to buy 10 of them.

    There’s an old soldier’s saying that “If it’s stupid but it works, it isn’t stupid”

    • Tim (History)

      Please don’t post false urban legends here.

    • joshua (History)

      But they’re the best kind!

    • John Schilling (History)

      As for reinventing the wheel every time, part of my job involves technical oversight of people who are even now building the Marquardt R-4D engines first developed as attitude-control thrusters for Apollo and still being used as main propulsion on e.g. top-of-the-line communications satellites. When we want a newer design with higher performance, we go with the Leros series developed by Royal Ordnance in the 1970s. Technology so far from the cutting edge that we run into problems when the last guy who really understood how to build the thing finally retires.

      In rocketry, at least, people responsible for gigabuck missions with no second chances really, really like to stick with what they know will work because it always worked before.

    • Cthippo (History)

      You’re correct, I did have some of the details wrong on the space pen bit.

      Originally both the US and USSR used pencils, but the danger of having the leads break and go floating around the cabin made them sub-optimal. The so called “Space pen” was developed in response to this problem by a private company at a cost of over a million dollars, and these pens were later used by both NASA and the USSR space program.

  6. David (History)

    In spite of all the possible deficiencies in the design, the North Koreans managed to out-spook the spooks in picking a launch window. go on over to and read for yourself.

    • Allen Thomson (History)

      It would be interesting to learn what the weather was at the launch site. If there were substantial cloud cover, only the radar satellites would be relevant.

  7. unha (History)

    Nice try of North Korea.

    But I think these images are so damning. The welding is sub par, the design of the fuel tank is soo wrong. What strikes me,is that two flexible pipes are still attached to the tank. Two others have disrupted the tank. Which leads me to question tank stability.

    The internal supports, visible on the image, are soo poorly arranged to the bolts. (which are used to fix the fuel/oxidizer tank to the lower section)

    All in all, I would like to compare this to merely upscaled German V2 technology, nothing new (apart from the teflon cabling, that is)

    • Cthippo (History)

      Yes, but it worked, and the rest is just details.

      To quote Comrade Stalin, “Better is the enemy of good enough”, and the North Korean effort was clearly good enough.

    • unha (History)

      Yes, it worked. But did you have a look at the actual images, showing the recovered fuel tank? I doubt, but be my guest.

      What struck me, was the observation that several random welds were visible on one of the images. They were on the lower semi-cylindrical part of the fuel tank, the most critical part in sense of pressure. This part is normally without welds, forged from a single piece of steel. Two long welds can be seen on images. The course of the welds, their quality, suggest that this was a haste job to salvage the mission. No such weld would have qualified under my supervision.

  8. unha (History)

    Dear Dr Schilling,

    Is it that you are trying to tell us that one has to fire more missiles in order to get acquainted with their properties?

    Would you like to address the following;

    Israel’s Iron Dome went on air more than 1000 times.

    What did we (they) learn from this?

  9. David Watson (History)

    This video is interesting – particularly from 2.00 (where we see Kim Jong Un declaiming in front of what’s presumably the satellite) and from 2.23 (where we see presumably the rocket itself in a disasembelled state) And lots of angles of the rocket launch from 12.30.

  10. Anon2 (History)

    Is it just me, or did the exhaust plume when shown against the blue sky appear reddish brown. Is it already known that they use UDMH with N2O4 ?

    • anon2 (History)

      Or Red Fuming Nitric Acid / Kerosene ? The hot exhaust clearly was yellow; the smoke appeared reddish/brown against the blue sky.

  11. Gridlock (History)

    Nice 3 render of the launch tower underway here BTW,14912.msg171096

  12. Anony (History)

    South Korea recently announced that the analysis of the debris suggested that a Unha-3 derived missile would have a range of 10,000km. How credible is this assessment?

    It also seems more debris was recovered.
    “a fuel tank, a combustion chamber and an engine connection rod ”

    • Jeffrey (History)

      I am planning another post but am trying to make tentative IDs of he structures first.

    • anon2 (History)

      The South Korean Ministry of Defense is confirming my guess of Red Fuming Nitric Acid oxidizer.

      However, I don’t think that says too much other than the North Koreans used a proven Nodong engine rather than re-engineer the engine for Lox. Of course Nodong is I[C or R]BM technology, what else do they have available.

    • George William Herbert (History)

      Ahh… If it has non-stainless steel steel tanks, it’s a new structure system for a LOX conversion. LOX is cold enough that regular steel goes brittle. Not so with stainless or aluminum tanks / airframe, though.

      I do not have particular insight into this, yet, but would love a chance to get down and dirty with the recovered stage with calipers and tape measure and a good camera and…

    • George William Herbert (History)

      Serves me right for posting quick. The article says the tank was aluminum-magnesium alloy. Not steel.

      In US alloy terms, it would be a 5-series alloy of some sort…

      Five quatloos for the first alloy ID.

    • George William Herbert (History)

      Press reports saying AlMg6, which isn’t specific enough. There’s an AlMg6 legacy Soviet engineering alloy for sheet/plate use, but I can’t find good documentation on its standard. The western 5059 alloy reaches that Mg content at the high end of its spec (5.5-6.0 %), and has really interesting performance.

      Ping is out to the South Koreans via their website to see if they can clarify.

    • George William Herbert (History)

      Courtesy of Alexander Ponomarenko: AMg6 alloy specifications:

      275-305 MPa ultimate tensile strength (39,875 to 44,225 PSI)
      130-145 MPa yield strength (18,850-21,050)

      Roughly analagous to 5086 aluminum in performance, though it has much more Mg in the alloy (more like 5059). It’s clearly not as strong as modern 5059 is though.

  13. Jim Oberg (History)

    Another argument they made in north korea, about how the rocket was non-weaponized, dealt with the type of inertial guidance hardware it carried — free-floating versus strap-down gyros. But the interpretors were baffled by most of the jargon and I never felt I had gotten the explanation straight. Has anyone else heard this particular argumwent?

    • Anon2 (History)

      I’m no expert in solid state vs. the older mechanical gyros. I am guessing that solid state gyros, although they are probably used in all kinds of cheap navigation systems by now; are a controlled export item. So, one hypothesis is that North Korea went with the older technology because they could not obtain the newer technology.

      Wikipedia has a good explanation of so called “strap down” gyros. I am guessing that “free floating” means “fluid suspended” gyros which is the older mechanical technology compared to the solid state laser gyros which I believe are called “strap on”. The North Korean propaganda on the suitability of one system over the other for peaceful purposes would likely be just that — propaganda.

  14. Anon2 (History)

    Can anybody do better on interpreting this image:

    Looks like an accordion fuel or oxidizer line going in; and a accumulator tank perhaps to average out impulse pressure from something upstream. The rest of it I can’t figure out.

  15. Anon2 (History)

    This guy has GOOD images:

    I still can’t make out the debris in the VOA photo.

  16. Hairs (History)

    Assuming that Iran and North Korea are cooperating on the development of rocket technology, what (if anything) does North Korea’s successful launch imply for the Iranian rocket programme?

  17. Chess (History)

    Now that North Korea successfully launched a long range satellite or rocket, instead of looking at the technical side of the rocket itself, more attention needs to be paid to the implications and ramifications of the long range rocket with the US’s dwindling global hegemony. We need to take off the US tinted glass and become more objective on what has been transpiring all around us. I was astonished by an article written by an “expert” who claimed the long range rockets displayed in North Korean military parade several months ago was fake and made of paper.

    In light of North Korea’s rocket advancement that can annihilate the US, what would the US reaction be when North Korea forcefully move into South Korea and unify both North and South Korea? Would the military alliance between South Korea and the US is still valid? Would the US risk her territory being annihilated to save South Korea? There are many questions that are raised and need to be answered.

    • Jeffrey (History)

      Wrong blog, dude.