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April 1 (Reuters) - A long-range rocket North Korea is expected to launch within days appears to have a bulb-shaped nose cone consistent with a satellite payload, rather than a warhead, U.S. defence officials said on Tuesday.

North Korea insists it is putting a communications satellite into orbit, but it is still expected to be accused of testing a ballistic missile in violation of U.N. sanctions.

Following are some facts about the North's long-range rocket, usually called the Taepodong-2, and whether in this case there is any difference between a satellite launch and a missile test.

SATELLITE LAUNCH OR MISSILE TEST?

The rocket is the same, the only difference is what sits on top -- either a warhead-like projectile or a satellite.

In both cases, the rocket's first stage booster is expected to splash down in the Sea of Japan and the second stage in the Pacific Ocean on the other side of Japan during the North's planned test between April 4-8.

A missile test would send the projectile from the rocket, and a complete test would have this object return to Earth in a parabolic arch to a target.

Experts will be closely monitoring the path and speed of what is fired in the third stage. Satellites need to be sent from rockets at specific speeds and trajectories so they can go into orbit. There is no need for that precision in a missile test. But experts say that since it appears North Korea has no advanced infrastructure for satellite development, testing or monitoring, there is little reason to believe this will be anything other than a missile test. The bulb-shaped nose cone sitting on the rocket may be a nonfunctioning object designed to leave the atmosphere or a rudimentary satellite that experts said could be based on an early Soviet design.

The United States, Japan and South Korea say they see no difference between a satellite and a missile launch because they would both use the Taepodong-2.

MISSILE SPECIFICATIONS

The Taepodong-2 is a two- or three-stage missile with a designed range of 6,700 km (4,160 miles), which means it could hit Alaska. It has a possible warhead payload of 650 kg to 1,000 kg (1,450 to 2,205 lb) in its short-range configuration. One study has said that, with a reduced payload, the missile could travel 10,000 km (6,200 miles), which would theoretically put the U.S. West Coast within range.

Research reports say a two-stage Taepodong-2 is about 35 metres long and has a diameter of 2.2 metres.

PURPOSE

North Korea says the rocket that will be launched is at the centre of its peaceful space programme.

Proliferation and military experts said the Taepodong-2 was developed by North Korea to eventually use against the United States to carry nuclear weapons.

But many proliferation experts believe the North, whose only nuclear test in 2006 was seen as a partial success, does not have the technology to miniaturise a nuclear device for a warhead. It might be able to place a biological or a dirty bomb, where radiation is spread through conventional explosives.

Missiles and their technology are a lucrative source of income for the impoverished state, and research reports say Iran has shown interest in buying the Taepodong-2.

TECHNICAL PROBLEMS

The Taepodong-2 has suffered from problems brought on by merging a Chinese design with the North's own medium-range Rodong ballistic missiles. This has led to structural flight problems that decreased the missile's intended range, experts said.

The Taepodong-2 has never flown successfully. Its only test launch in 2006 failed when the missile fizzled and destructed about 40 seconds into flight because of what is generally believed to be a structural failure in its airframe, propulsion system or fuel tank. (Source: U.S. Congressional Research Service, South Korea's Defence Ministry, GlobalSecurity.org, Reuters) (Reporting by Jon Herskovitz and Jack Kim, Editing by Dean Yates)

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