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   David C. Wright 1 : "An Analysis of the North Korean Missile Program"

In this paper I discuss some technical issues relevant to the North Korean
missile program, which have implications for other missile development
programs as well.

The North Korean Missile Program

Short-range Scud-type missiles 2

In the early 1980s, North Korea reverse-engineered a Soviet Scud-B missile
and began producing its own version, the "Mod-A", with a reported
range/payload of 280-300 km/1000 kg. It then produced an improved version,
the "Mod-B", with a reported range of 320-340 km/1000 kg, which it sold to
Iran for its war with Iraq in the mid-1980s. If the reported range increase
is true, it likely resulted from increasing the specific impulse (Isp) of
the engine. My calculations 3 show that switching from kerosene and nitric
acid, which the Soviets used and which gave a sea-level Isp of 222 seconds,
to UDMH and IRFNA, which might raise the Isp to about 230 seconds, could
explain the range increase. These propellants are widely used and are
likely available to North Korea. Based on estimates for the Soviet Scud B,
I estimate a CEP of 750-1000 meters for this missile.

In the late 1980s North Korea developed the longer-range Scud Mod-C (500
km/700 kg). Using an engine with an Isp of 222 seconds (the value for the
Soviet Scud B), this range/payload can be achieved by stretching the
missile body by more than a meter and adding 25% more fuel, as Iraq did to
make the Al Husayn. My calculations show that this range/payload could also
be achieved with Isp = 230 and minimal stretching of the body, if fuel
replaces the volume freed by the reduced payload.

The Nodong Missile 4

Nodong missile development reportedly began in the late 1980s and the
missile is reported to have a range/payload of 1000-1300 km/1000 kg. My
analysis concludes that the missile is based on Scud technology and is
powered by a cluster of four Scud engines. Reasonable assumptions for the
fuel-fraction give the reported range/payload figures. Moreover, the Nodong
dimensions reported from satellite observations (15.2 m long, 1.2 m
diameter) are consistent with assuming the missile carries four times the
propellant as a Scud-B. Clustering simple Scud engines should be relatively
easy and clustering is exactly what other countries have done when they had
a working engine and wanted to achieve longer ranges. When the Soviets
clustered engines they fed the four combustion chambers with a single large
turbopump. North Korea likely instead followed the Chinese model and used a
smaller pump for each chamber, essentially using four Scud engines with
little or no modification. In addition to increasing the range, clustering
also has an advantage from a structural point of view, since the vibrations
of the four engines destructively interfere with one another, so that the
overall level of vibration is much smaller than you would get from a large
engine with four times the thrust. Clustering would, however, reduce the
reliability of the missile, possibly substantially, since a failure of any
one of the engines would severely limit the range. For example, if the
reliability of a single engine was 95%, the reliability of the cluster
would be 81%; a 90% single-engine reliability would give a 66% cluster
reliability.

North Korea has conducted only one flight test of the Nodong (in May 1993)
and only to partial range (500 km). As the fuel burns during boost phase,
the mass of the Nodong would change by about 75% and the center of mass of
the missile would shift dramatically, and this has important consequences
for the missile's operation. Thus, while some analysts argue that despite
the lack of testing, North Korea may see the Nodong as "useable" in an act
of desperation in a crisis, it seems unlikely the United States would
consider the missile operational without at least one full-range test.

The recent Pakistani flight test of the Ghauri missile, said to have flown
1,100 km with a 700 kg payload, may be relevant to this issue. Reports
state that the missile was not a Nodong, but was built with some level of
North Korean assistance and technology. 5 Based on data available on the
Ghauri in the Pakistani press, the missile appears to be smaller than the
Nodong (16 tonnes total mass with 13 tonnes of fuel for the Ghauri compared
to 18-19 tonnes total mass with 15 tonnes of fuel for the Nodong), although
it seems reasonable to expect it is a similar design and technology. It is
unclear whether the Ghauri design is close enough to that of the Nodong to
substitute for flight testing of the Nodong, but it would certainly provide
North Korea useful information.

Guidance issues

The Nodong and longer-range missiles would be expected to have a detachable
reentry vehicle (RV) since the reentry forces would tear a missile apart,
as they did the Al Husayns in the Gulf War. Assuming Scud-level guidance
and control, my analysis estimates a CEP of 2000-4000 meters for the
Nodong. This figure assumes relatively stable reentry; if it spirals or
tumbles due to asymmetries in the RV or problems with RV separation, it
could be much larger. Heating of the RV during reentry is proportional to
rv3 (where r is the atmospheric density and v is the speed), which
increases rapidly with the higher speeds of longer-range missiles. However,
heating on the Nodong RV could be reduced if necessary without using fancy
materials by making the RV blunt and increasing its atmospheric drag, thus
giving it a low weight-to-drag ratio, which would slow it relatively high
in the atmosphere. The slower reentry speed will tend to reduce the
accuracy, since atmospheric forces will have longer to act on the warhead.
However, the accuracy is already expected to be so large that it could only
be used against a soft target, so this increase in CEP will probably not
matter.

Improving the guidance and control of missiles is easier than it used to be
given the computing power available today in small packages. However, it is
important to remember that CEP has two main contributions: (1) guidance and
control errors during boost phase, which result in errors in the speed,
position, and direction of the missile at burnout--essentially errors in
"aiming" the missile--and (2) errors that result from unpredictable
atmospheric forces during reentry. Even if you had a perfect guidance
system and could eliminate entirely the first contribution, the reentry
errors are very substantial. My calculations of CEP for the Nodong show
that reentry errors are expected to be the dominant source of error. 6
Advanced missiles attempt to reduce reentry errors by having the warhead
reenter at high speeds (by increasing the ballistic coefficient of the
warhead) so it spends less time in the atmosphere, but doing his means one
has to be able to handle the much higher heating rates by, for example,
developing ablative coatings for the RV.

Reaching significantly longer ranges requires multiple stages and/or
developing a more powerful engine.

Staging:

Staging is commonly assumed to be a significant technical hurdle, and it
clearly requires a more complicated guidance and control system. However,
it is unlikely to be a big problem if it is done, for example, as China
does in its multi-stage missiles. Stages are joined by a structure that
leaves an open space between them. The upper stage is then ignited while
the previous stage is burning. Igniting the engine under acceleration
eliminates problems such as bubble formation in the fuel for a liquid
engine. Moreover, thrust is available to keep the missile stable during
separation. A Chinese missile engineer told me that when China developed
its first multiple-stage missile (the DF-4) they expected staging to be
difficult but found it was not.

The Taepodong-1 Missile

In early 1994, press reports stated that US satellites had seen two missile
mockups at a missile research center in North Korea. 7 The Taepodong-1 has
an estimated range/payload of 1500-2000 km /1000 kg and looks like a Scud
second stage on a Nodong first stage. If North Korea had an operational
Nodong, building a longer-range missile by using the Nodong as a first
stage with a Scud Mod-C second stage might appear to be an obvious step if
North Korea were able to master staging technology. Calculations show that
such a Nodong/Mod-C missile combination would have a range compatible with
the official estimates given above--1,500-2,000 kilometers with a one-ton
warhead--depending on the structural weight of the missile body. This will
depend on the materials North Korea might use, for example, steel versus an
aluminum alloy.

However, this combination of stages would result in a missile that is very
long relative its diameter, and according to an experienced missile
engineer familiar with the Taepodong models, this is a poor design that
could lead to structural problems during boost phase. 8 If North Korea
wanted to build a two-stage missile using Scud engines, a different design
would give greater range and a stronger structure, and would be no more
difficult to build. Such a design would use Scud engines but would match
diameters of the two stages by making the diameter of the second stage
equal to that of the Nodong (1.2 meters) rather than that of the Mod-C
(0.88 meters). This would make the body stronger and decrease the overall
length by three to four meters. The fact that the reported mockup does not
have such a design calls into question whether the mockup represents a
serious attempt to design a two-stage missile or whether the object seen
was even a mockup of a real missile.

The Taepodong-2 Missile

The Taepodong-2 is described as having a Nodong for a second stage on top
of a large first-stage booster having roughly the dimensions of the Chinese
DF-3 (CSS-2) missile. 9 Official US estimates give the range as 3,500 to
6,000 kilometers. The shorter range probably assumes a one-ton warhead, but
the longer range almost certainly implies a reduced payload. 10

The development of a missile with the range reported for the Taepodong-2
would present a number of significant technical hurdles for North Korea, in
particular developing new engines with a thrust double that of the Scud B.
However, my analysis of the reported design of the Taepodong-2 itself
raises questions about the missile.

If North Korea were able to build or acquire a DF-3-like missile there are
three routes to adding a second stage: (1) use a Nodong missile, if it
became operational, (2) use a Scud Mod-C missile, or (3) develop a second
stage powered by one of the engines developed for the first stage. All
three options would be comparably difficult to build. Of these three, using
a Nodong makes the least sense, for several reasons.

First, the Nodong missile is so heavy that using it as the second stage
would limit the missile's range relative to the other options. The Nodong
mass is roughly 20 tons. In contrast, a Scud Mod-C has a mass of roughly 6
tons. Calculations show that because of the smaller mass, a two-stage
missile using the Mod-C as the second stage would have a range several
hundred kilometers longer than one using a Nodong (for the same payload),
even though the Mod-C has only one-quarter the thrust. Moreover, the first
stage of the Taepodong-2 would require engines that are much more powerful
than Scud engines. If North Korea had developed these new, more powerful
engines, it could achieve a greater range by using one of those engines for
the second stage as well.

Second, using a missile as long as the Nodong for the second stage would
give the Taepodong-2 a large length-to-diameter ratio, which could create
structural problems during boost phase. Using a Mod-C as a second stage
would result in a Taepodong-2 that was several meters shorter than if a
Nodong were used. In the third option, the second stage would be designed
to have the same diameter as the first stage (2.25 meters for the DF-3,
compared with 1.2 meters for the Nodong), which would make it much shorter
than the other two options.

Third, since the Nodong has four engines, it would give a less reliable
second stage than the single-engine Mod-C or a single-engine stage based on
the engine used for the first stage.

Thus, even if North Korea wanted to use an existing missile as a second
stage, it would make sense to use a Scud Mod-C rather than a Nodong since
it would result in a missile with greater range, better structural
strength, and better reliability. However, if North Korea had developed new
engines for the first stage, the Taepodong-2 design that would make the
most sense, in terms of range, structural strength, and reliability would
have a second stage that used one of the engines developed for the first
stage, had a diameter equal to the first stage, and had the amount of fuel
chosen to maximize the range. This is exactly what China did when it used
the DF-3 as a first-stage booster to build its first two-stage missile, the
DF-4 (CSS-3).

What does this analysis imply about the Taepodong-2 development program? If
the Taepodong-2 mockup seen represents a real missile under development, as
is generally assumed, it means that North Korea is developing a missile
that does not make technical sense. This would raise serious questions
about the technical competence of North Korean missile engineers and would
suggest that the program is not as advanced and the personnel not as
capable as is often asserted.

A second possibility is that the mockup seen in 1994 did represent a real
system in the early stages of development, but that North Korea has since
improved the design. This possibility has not been raised publicly.

A third possibility is that the mockup seen was not a prototype of a real
system and that North Korea does not have a serious program to develop such
a missile. If North Korea was not developing a missile like the Taepodong-2
so that no one had thought carefully about its design, it would not be
surprising that the mockup did not have a sensible design. This explanation
may be compelling for several reasons, especially in the context of the
other North Korean actions.

The mockups seen in early 1994 were displayed in the open, almost certainly
with the intention of having them seen. Why? North Korea learned during its
nuclear negotiations that it was being watched carefully by US satellites.
It thus seems plausible that North Korea decided to use that fact to help
manipulate Western perceptions of its missile program. The mockups may well
have been dummies intended to suggest significant progress in missile
development. Their appearance may have been intended to influence the
negotiations over North Korea's nuclear program, which were then in a
contentious phase. For that reason, some in the US intelligence community
apparently believed at the time that the mockups did not represent a real
missile.

Conclusions

From my analysis of the North Korean missile program I conclude that the
Nodong missile appears to be a technically sound design and the kind of
evolutionary step one would expect to see in a program that is attempting
to reach longer ranges. Why final flight testing of this missile has not
occurred is not known, but there are reasons to believe it may reflect
political decisions rather than technical difficulties. 11 On the other
hand, the Taepodong structures that have been described in press reports
raise serious questions about how serious a development program North Korea
might have for these missiles. This does not mean that North Korea could
not with time develop intermediate-range missiles if it had the inclination
and resources, but it is an important consideration in assessing the
existing and potential near- to moderate-term missile capability of North
Korea.

Missile Comparison

Circles 1,000 Kilometer Range From North Korea

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1. Dr. David Wright is a Senior Staff Scientist, Union of Concerned
Scientists, and Research Fellow, Security Studies Program, Massachusetts
Institute of Technology. Has written extensively about ballistic missile
defense, threat assessment, treaty compliance, and China's and North
Korea's missile programs.

2. For information about North Korea's short-range missiles, see J.
Bermudez, "New Developments in the North Korean Missile Programme," Jane's
Soviet Intelligence Review, August 1990, p. 343 and J. Bermudez, "Ballistic
Ambitions Ascendant," Jane's Defense Weekly, 10 April 1993, p. 20.

3. My technical analysis is based on open-source information. A key part of
this analysis is reverse-engineering North Korea's missiles based on what
is known about Soviet and Chinese missiles (especially the Scud-B) and the
development histories of those programs, and building computer models of
the missiles that can be used to calculate range-payload curves and compare
the effects of different assumptions about the missiles.

4. For a detailed study of the Nodong, see D. Wright and T. Kadyshev, "An
Analysis of the North Korean Missile Program," Science and Global Security,
vol. 4, 1994, 129-160.

5. Tim Weiner, "US Says North Korea Helped Develop New Pakistani Missile,"
New York Times, 11 April 1998, p. A3.

6. Wright and Kadyshev, "An Analysis of the North Korean Missile Program."

7. "N Korea Casts Longer Shadow with TD 2," Jane's Defense Weekly, 12 March
1994, p. 12.

8. Personal communication, April 1997 and December 1997.

9. "N Korea Casts Longer Shadow."

10. To put these ranges in context, North Korea lies about 4,500 kilometers
from the tip of the Aleutian Islands and 5,000 kilometers from the closest
point of the Alaskan mainland. The main, inhabited Hawaiian islands are
more than 6,700 km from North Korea, although the far western end of the
Hawaiian chain (Kure Atoll, a circular reef with no permanent residents) is
about 4,500 kilometers from North Korea. The closest point of the 48
contiguous states is roughly 8,000 kilometers from North Korea. Guam and
the Marshall Islands are about 4,000 and 5,000 km from North Korea,
respectively.

11. For a detailed discussion of this point, see David Wright, "Will North
Korea Negotiate Away Its Missiles?", Union of Concerned Scientists
Technical Report, 2 April 1998.

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