How stuff works: Rocket easily converted to missile

In mid-June, a Japanese newspaper reported something rather alarming. The North Korean military appeared to be preparing a Taepodong-2 missile for a July 4th firing. And the target appeared to be Hawaii. Since Hawaii is 4,350 miles away from North Korea, it is provocative.

The idea that a missile might actually launch, fly over Japan and then land near the headquarters of the U.S. Pacific fleet in Hawaii dredges up many bad memories.

It also raises the question – how does a ballistic missile like this work?

The first thing to understand is that any rocket is easily converted to a missile – it’s just a matter of sticking an explosive in the nose cone.

The Redstone rocket that took America’s first astronauts into space in the Mercury program was a re-purposed ballistic missile. Any rocket that can launch a satellite or an astronaut into space can easily turn into a missile with a few modifications.

That is why there was a great deal of concern when Iran launched its first satellite in February.

The second thing to understand is the ballistic nomenclature. It is called a ballistic missile because the rocket fires for only a small part of the flight.

The goal of the rocket is to get the bomb to a certain altitude with a certain forward speed. The bomb then flies to the target along a ballistic trajectory.

The bomb may have a guidance system and some limited ability to steer, but once the rocket engines use up their fuel and fall away, the bomb is a lot like an artillery shell. It coasts to the target.

The Taepodong-2 missile is a large rocket. It is thought to be about 110 feet tall and it has two or three stages.

Much of its technology began when North Korea purchased Russian SCUD missiles and took them apart to learn about the engines, guidance systems and fuel systems.

Those three parts are the foundation of any rocket. For example, the Mercury Redstone rocket had two large tanks to hold alcohol and liquid oxygen. These tanks make up most of the rocket and hold thousands of pounds of fuel. The rocket weighed about 60,000 pounds at launch.

Pumps move the oxygen and fuel to a single rocket engine that could produce about 80,000 pounds of thrust.

Small metal vanes in the exhaust stream of the engine, along with small fins, let the guidance system do its job.

At launch, the main thing the guidance system is doing is keeping the rocket from falling over.

Once the rocket gains some speed and loses a little weight, the guidance system can start pointing the missile in the direction of the target. In the Taepodong-2 missile, each stage is thought to burn for about 100 seconds.

Once the first stage runs out of fuel, explosive bolts fire to release it and the engine in the second stage ignites. Once the second stage burns out, it too falls away and the warhead is in ballistic flight toward the target.

The guidance system in a missile like this has traditionally been an inertial guidance system. A computer on the missile takes readings from several gyroscopes and accelerometers.

It calculates where it is in space second by second using the acceleration and heading readings that it sees. Modern bombs and missiles can now supplement the inertial system with GPS readings as well.

The obvious question for Americans is defensive: Is there anything we can do to eliminate the threat of a missile like this?

The United States has several systems able to knock out missiles in flight, including the Patriot missile system, the AEGIS missile system and THAAD.

All of these systems use kinetic energy, meaning that one missile runs into the other and destroys it.

There is also a Boeing 747 fitted with a huge laser that could disable the missile.

In theory, the United States could also launch a cruise missile or smart bomb and blow the rocket up as it sits on the launch pad.

So far the U.S. military has been disinclined to do that, probably for fear of provoking a bad response from China or the United Nations.

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