Could a gun fired rocket be used to send satellites in earth’s orbit? project HARP

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space gun

The common concept of sending any object in space is by blasting method, which is used in rockets. But can you think of using a giant gun for this purpose? The idea seems to be hypothetical and has many limitations but in 1960s a joint project between the U.S and Canada was started to work on this concept. It was named project HARP “high altitude research project” was headed up by Gerald Bull. They used such a giant gun to fire objects into space to test their re-entry characteristics.

Though the idea seems to be quite interesting and have many more advantages like without carrying the massive amount of fuel like a rocket object could be sent into space and also it will take much less time and cost-effective too.

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One of the major challenge this technique face was already shown over 300 years ago by Isaac Newton. In Newton’s cannon experiment a cannon is fired horizontally from a very high mountain, so high in fact that it would be above the atmosphere, this was to demonstrate that the gravity is universal and if you got the speed of a cannonball just right it would end up in orbit around the earth. Newton himself showed that just like any other parabolic, hyperbolic or elliptical orbit if you accelerate an object during the orbit you will always return to the point where the acceleration took place.

 

project harpAs our gun is on the surface of the earth then the starting point or the firing of a gun is where the acceleration will take place and this is where our projectile, assuming it’s a dumb unguided shell will return within one orbit. Basically it will crash back to earth where it was fired from. It can be avoided only if it could be fired with such a high velocity that the projectile can break free of Earth’s gravity completely and end up in deep space or once it’s reached a suitable altitude it can correct its course itself and enter a stable orbit as a rocket does.

harp guns strctureThe HARP guns were made from 16 inch US Navy guns that were modified by connecting two barrels together to create a total length of 41 meters, one was located in Barbados and the other in Yuma, Arizona. The key feature of these tests was that the gun was oversized and the projectile, a Martlet rocket which was held in a Sabot was undersized.

This allowed the test to have the high-speed acceleration to withstand the huge amount of g-forces during the firing of the gun, which at its peak reached 25,000 G when they pumped out the air in the barrel and then sealed it. The sensitive electronics were potted into a sand and resin mix which proved to be highly effective in protecting them from the forces of the launch.

During tests in 1966, the Yuma based gun fired a Martlet rocket around a 180kg to a height of 180 kilometers around 590,000 ft. easily into what is considered space which officially starts at a hundred kilometers and is a record which still stands today.

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Although this might seem like a lot, the velocity was only around 2.1 kilometers per second that’s still a fraction of the 7.8 km/s required to reach low Earth orbit. The traditional guns relying upon an explosive charge can only ever reach around two kilometers per second as our projector can’t accelerate faster than two pressure wave from the exploding charge that’s pushing it along the barrel.

Again in 1980’s the Lawrence Livermore Laboratory as part of Ronald Reagan’s Star Wars initiative to shoot down incoming missiles conducted tests on what they called the sharp or the Super HARP gun. This used a two-stage propulsion technique in an L-shaped barrel. Firstly in a chamber at right angles and at the bottom of the main barrel a methane-air mix is ignited, this pushes a pump piston to compress hydrogen which then propels the projectile along the main barrel. You know hydrogen is the lightest gas thus it has the capacity to achieve high speed. Thus, in tests, small projectiles reached 10 km/s enough to achieve orbit.

But moving at such a high speed in the atmosphere will create a heating problem. Well, the increase in temperature can be as high as 1000 degrees C and cause damage to the projectile itself. As the altitude increases the air becomes less dense and the heat buildup becomes less till it gets to space where it stops altogether.

Although if we see in a rocket, it starts off slowly in the dense atmosphere and builds up speed through the thinning atmosphere to space, but a gun accelerates the object to its full speed during the length of the barrel and right into the dense atmosphere where the air compression and heat will be at its greatest.

Now before the Soviets put Sputnik satellite into orbit there was a rumor that the US had accidentally launched an object into space during an atomic bomb test. During the Pascal B nuclear test of 1957 a 100 millimeter thick, 900-kilogram armored steel end cap was used to seal the 152-meter deep test shaft and it was welded into place to see if it could contain the radioactive material.

When the bomb was detonated at the bottom of a shaft the end cap was blasting upwards for what was thought to be around 66 km/h and a 150,000 miles an hour. Now this was actually expected and as part of a test high-speed cameras were trained on the cap but it was only ever caught on one frame of film so exact figures could not be calculated but it was said to be traveling like a bat and probably out of hell.

In this way it became the first man-made object to be blasted into orbit, but Dr. Robert Brownlee who worked on the test believed that it was vaporized by the intense compression heating of traveling through the atmosphere at such a great velocity but either way the cap was never seen again.

Though the heating problem can be fixed by shielding, due to the speed, there is also another related issue in that the gun’s projectile has an acceleration that’s somewhere around 20,000 g that would turn any delicate satellite into scrap metal. The space shuttle gives a maximum acceleration of 3g to its passengers and we could not allow it to be more than 7g for space gun.

If we allow the acceleration to increase safely from zero to 7.8km/s the size of the barrel would need to be greater than 50 kilometers long, depending upon the level of G going to inflict on passengers. This pretty much rules out any conventional gun powered by an explosive charge but an electromagnetic railgun could start at a much lower speed and reach top speeds high enough to reach orbital velocities.

This idea has actually been proposed to launch payloads into orbit which are not affected by the high g-forces such as food, water and fuel but even limiting the acceleration to a 1000g it would still require a track over a kilometer long. With current technologies, this would be very difficult, not impossible the biggest challenge is the arcing of the electrically conducting track in the Sabot and the atmospheric heating of the projectile at hypersonic speeds.

Coil guns which use high-power electromagnets to propel the projectile with no physical contact would eliminate the wear on tracks but aren’t currently capable of reaching 7km/s. The equator could be best launch site as it has the advantage of the Earth’s rotational speed and 2 to 3 kilometers up the side of a mountain in less dense air. This would still rule out transporting delicate satellites and humans but could dramatically reduce the cost of getting raw materials into orbit.

But currently, it is not possible so Rockets are the only option we have, till we come up with any other stunning idea.

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