Geoff FordenWhats with the Goop?

I thought I’d try an experiment in blogging: a five part series going into the details of Iran’s liquid-propellant missile development program; something I’ve been thinking about a lot lately. (The over-all title of the series is “Scaling Up the SCUD” but I don’t have enough room to put that in every time.) Iran’s missiles are one of the main threats the US sees on the horizon and, if your believe General Obering, the Director of the Missile Defense Agency, could attack the United States as early as 2010. But that will require a considerable advance in Iranian technology! Just how reasonable is that? I hope that by the end of this series, you will have a good idea of the technological challenges Iran would have to overcome to justify that concern.

Today’s topic of discussion is motivated by a close-up of the nozzle of a Shahab missile, the same sort of engine that was used on the first stage of the Safir launch August 17th, 2008. (The paint scheme of desert tan seems to indicate it’s a deployed Shahab as opposed to one of the “civilian” launches. Note that you can just start to see two of the four jet vanes to the lower left and right.) Some goop is smeared on the inside of its smallest diameter, the throat of the nozzle: What’s with the Goop?

The goop doesn’t completely seal off the throat, nor even make an appreciable change in its area so I think we can rule out functions such as keeping dirt and dust out of the thrust chamber as the missile is driven around the desert or temporarily increasing the “stay” time of the fuel during ignition. My current best guess is that it’s a wax-like substance that is used to temporarily close holes intended to “film cool” the nozzle.

To understand a possible explanation, let’s consider the startup process in a missile. First, several squibs, or small pyrotechnic charges, are fired at several places in the fuel and oxidizer lines to open up seals that keep them in the tanks before ignition. Simultaneously, a larger charge is ignited to produce gas to start the turbopump turning and sucking fuel and oxidizer down out of their tanks. This pump pushes the fuel (in the case of a SCUD) through the regenerative cooling system. That too deserves some explanation.

The rocket engine’s combustion process produces gases that are very hot, roughly 2700o C for a SCUD, and the combustion chamber needs to be protected against that heat. (Steel, by the way, looses all its strength at about 1000o C.) To do this, the Soviet designers made the combustion chamber with an inner liner and an outer shell with a small gap between the two. Fuel is pumped through that gap before it enters the combustion chamber, both cooling the inner wall and preheating the fuel before it is burned. A highly efficient system! But if the chamber generates too much heat—or if there are problems with the regenerative cooling system, say if bubbles form at an undesirably low temperature—another method of cooling must be introduced. A natural thing is to take some of the fuel as it is circulated through the gap and spray it though little holes in the inner shell to create an additional cool-gas barrier between the hot center and the steel walls. I don’t remember seeing any such holes on SCUD engines and I cannot find any mention of them in my reference material. Do any of you readers know?

Here is where the policy implication comes in. If Iran is ever going to make a real ICBM, it needs to switch from SCUD-type fuels to higher energy fuels. But you get considerably more than just an increase in thrust from using higher energy fuels! You can reduce the volume of fuel used and, hence, the dead weight of the missile; the structure of the missile. The advantages don’t stop there! Lower missile weights means you don’t need as much structural strength possibly enabling you to switch to lighter materials, such as aluminum. You get the picture. A relatively modest increase in specific impulse by going to UDMH and nitric acid will increase the range of the missile much, much more than you might initially think. The problem is keeping the engine cool!

Iraq tried to advance to the next higher energy level of fuel by using UDMH and nitric acid but the problems associated with that higher energy fuel caused a burn-through in the SCUD engine’s radiatively-cooled nozzle skirt 14 seconds after the burn was started. (Would the combustion chamber have burned through if the test had gone on longer? That is the question! Note, however, that I don’t say “higher temperatures.” In fact, UDMH and nitric acid actually have only a slightly lower combustion temperature than standard SCUD propellants but there is a lot more involved in cooling than combustion temperature. That’s why it’s called rocket science!) Let’s assume—we certainly haven’t proved it!—that the goop is covering film-cooling holes until after ignition. Do those holes indicate that Iran has the capability of cooling its engines more than Iraq was able to? Will they be able to jump to higher energy fuels and quickly make an ICBM? And if so, why are they messing about with SCUD-type fuel for the Safir second stage? After all, doesn’t it makes more sense to use a higher energy fuel in the second stage where reductions in dead weight really payoff?

Part 2 of this discussion will consider what development path Iran is known to be following with the second stage of its Safir missile. (See the detailed post by Jochen Schischka for an alternative view of the propellant used by the Safir. I remain firmly of the opinion that it used propellants like RP-1 and nitric acid; SCUD-type fuels.) Until then, I look forward to reading your comments!


  1. Tal Inbar

    Hi Geoffrey

    Although the Pakistani Ghauri missile is basically identical to the Shahab 3 (both licensed copies of the NK No Dong), the picture is of a pakistani missile, not a Shahab.


  2. ak (History)

    Interesting except the photo you show is of a Pakistani Ghauri missile…While I understand the relavance given that it and the Shaheen are based on the same No Dong design/DPRK technology, I am a bit confused on the specifics. You seem to suggest in your earlier post that Iran was developing its own technology/modifications to missile systems…if so, does the linkage to the Ghauri stil hold for the “goop”?? If yes, then what can be learned from the Pakistani systems that would therefore also be true for the Iranian missiles??

  3. Geoff Forden (History)


    But it still, probably applies to Iran since both Pakistan and Iran got their “Nodong” plants from North Korea.

    So Im still interested in what the goop is all about. Does anyone have any ideas? I have to emphasize that I am *not* claiming it is the first step toward higher energy fuels, just that it might be.

  4. Jochen Schischka (History)

    I interpret this “goop” as some sort of smeared solder (indicating perhaps not the highest level of production quality) on the combustion-chamber-seal in the nozzle throat. The solder melts and the seal gets blown out when the engine is ignited by means of Tonka-prerun.

    Completely similar to the Isayev 9D21 of the Scud-family, only scaled up.

    I for my part see no signs of additional emergency transpiration-cooling, and i’m absolutely positively sure that there is no such thing on the Isayev 9D21. (Anyway, the only rocket-engine i can recall using the suggested form of emergency-cooling was that of the german A4/V-2 and its direct descendants like R-1…does anybody have knowledge of further examples, especially connected to the usage of UDMH?)

  5. bb (History)

    IMHO it’s just a dust cap with a lousy applied sealing. The SCUD-family IMO never had film cooling at the throat. They use intensively film cooling though by applying fuel rich injector swirler elements on the injector plate close to the chamber wall.

    I think the design based on the Scud family has its limits, regardless of the propellants you use. The nitric acid approach already has a quite high density ISP. You will find higher performing propellants but with maybe lower density and therefore bigger tanks. If they have a right mind then they switch to solid propellants at least for the first stage. You even can use the jet vanes from the SCUD for that to avoid gimbaled nozzles. The exhaust of solid motors is usually NOT hotter than the one of liquid engines. Jet vanes have been used widely in air-to-air (solid propelled) missiles… not really a technical challenge.