The article in the middle of the front page of today's Sankei Shimbun asks whether or not the Taepodong II test yesterday really can be classified as a failure. The article offers some good reasons why the DPRK military might be satisfied with the test flight.
The article, however, does not speculate on what happened.
According to Occkam's Razor, the best answer must be the simplest one:
They never fueled the missile's second stage.
The bottom stage of Taepodong is thought to be a variant of either the Chinese CSS-2 or CSS-3 land-based missile, or a Russian SS-N-6 submarine-based missile. The second stage is thought to be Nodong.
In terms of a minimal testing requirement, it was necessary only to test the first stage boost phase flight characteristics of a Taepodong II. Data from an upper stage burn would have been a bonus—-but sufficient Nodong performance data already exists to venture a guess about what a full missile firing could do.
For the purposes of a boost phase test, the upper stage could be filled with an inert substance, preferably a liquid in order to maintain similarity of flight behavior--simulating the mass of a fully fueled rocket.
An unfueled upper stage would answer one of the conundrums of mid-June:
U.S. ships to monitor N. Korea launch
By Thomas E. Ricks and Joohee Cho - June 21, 2006 – Seoul – The U.S. military Tuesday moved ships into position off the coast of North Korea to detect the launch of any long-range ballistic missiles and prepared its new, unproven missile-interception system.
It is apparently the first time that the U.S. government has readied its rudimentary missile-defense system other than to test it. But officials played down the possibility that the interceptors might be used against a North Korean missile, and the South Korean government expressed doubt that Pyongyang is even preparing a test launch of its first intercontinental missile. It suggested that the government of Kim Jong Il might be only preparing to send a satellite into space.
A South Korean parliamentary panel concluded that North Korea "does not seem" to have completed injecting fuel into the missile, citing information from South Korea's National Intelligence Service.
"The NIS reported that it is hard to believe the missiles have been fully fueled already," Rep. Chung Hyung Keun, secretary of the National Assembly's Intelligence Committee, told reporters in Seoul. The lawmaker made the remark after emerging from a briefing by the NIS. "The 40 fuel tanks spotted at the site do not contain enough to launch a missile that needs 65 tons of liquid fuel," the lawmaker said.
U.S officials have examined intelligence that they say suggests Pyongyang may be preparing to test a Taepodong-2 missile from a remote village on North Korea's northeast coast. They have said U.S. satellites have observed liquid fuel canisters placed near the missile, but officials said there was no confirmation that the missile had been fueled.
Why not fuel the second stage?
Several months ago, some of the more militant voices among the Japanese political classes declared that undeclared missile overflights of the Japanese main islands should be considered acts of war.
Best ways to avoid some really, really serious political unpleasantness then?
1) point the damn thing at Russia, and
2) don't give it enough gas to make landfall.