Introduction
Rockets that could reach near space altitudes have existed since World War 2 and have been developed greatly since then. This is a great improvement from the steam powered rocket of Archytas (4th Century BC) and the Chinese fireworks (7th Century AD). From then on rockets have carried satellites well beyond Earth’s gravity and to the edge of the solar system.
Rockets are used for anything concerning space or near space-like altitudes. This is because they are our only contact to space (except for the space shuttle which has been taken out of use due to it costing more than anticipated). We\’ve currently pretty much perfected the rocket design and have no significant improvements which can be made. We use rockets for sending research probes into other planets and putting various satellites into orbit, for example communications satellites, weather satellites, space stations and research satellites (Hubble Space Telescope).
So far, we\’ve only explored rockets and the space shuttle, but they have many disadvantages. The main ones are the very high costs (approx. $22,000 per kg1), extremely large fuel consumption, and fuel storage problems (as the fuel is either poisonous; or the oxidiser and fuel need temperatures close to 0 Kelvin). Most of the expenditure is still spent through getting the rockets out of Earth\’s gravity. Although due to their costs research is being done on alternative methods of getting into space. Other methods of launching objects into space include launch loops, space guns, laser propulsion, and space elevators.
If research is done into non-rocket launch systems then the cost of going into space is significantly reduced. So projects that have been so far hindered by cost will have a chance to flower, such as space colonisation; space based solar power; and the terraforming of Mars.
Aim
My aim is finding more efficient and economic ways of sending payloads into space instead of chemical rockets.
Different Methods of Launching
Launch Loops
Fig. 1: Launch Loop (fig. 1)
The red line shows the tube in which the rotor is. The blue lines are stabilization cables.
Copyright, http://en.wikipedia.org/wiki/File:LaunchLoop.svg.
This is a loop which is 2000km long from west to east and is 80km high in the middle. It starts from the one end on ground level it is then inclined up to a height of 80km and then goes straight for 2000km and then inclines down to the ground again. It then bends around itself and goes along the track that it initially took to end up where it started and connects to where it started (see Fig. 1). Several of these structures have been proposed to be built in equatorial Pacific. The loop is a tube known as a sheath and inside the sheath is a rotor, the rotor is the size of a wire and is also in the same shape as the loop and it is suspended in the sheath without touching the sheath. The rotor is also made out of a magnetic substance (e.g. iron).
When the rotor is not moving the whole structure is lying on the ground. When the rotor is gradually accelerated the structure lifts itself because of centripetal force. Wires are used to keep it in place. A constant power source is required to keep the structure in this way and more power is required to launch an object.
To launch an object it will be lifted to the westernmost point where it is 80km above ground, then the object will create an electromagnetic field. This will create eddy currents with the rotor which will both pull the object in the direction of the rotor and push it away from the rotor. When the object has reached the end of the 2000km track it would have reached escape velocity.
Some of the advantages about this is, that it can be made by materials available today; it can launch many launches per hour; it can reduce launch costs $300/kg to as low as $3/kg2 ; it gives a safe amount of acceleration; and has a very low chance of getting hit by space debris. Some of the main disadvantages include the fact that no research has been made into the weather systems of the Equatorial Pacific that is detailed enough for this. It has stored, in the rotor, the same amount of energy as the Hiroshima nuclear bomb in kinetic energy3 . So heating effects from weather can cause the rotor or sheath to expand, this will cause slack which will mean that, unless it is accounted for, it could cause the rotor and sheath to touch and explode. Also, the sheath must be absolutely airtight, otherwise air will leak into the sheath and will melt the rotor through friction.
Space Gun
Fig. 2: Space Gun (fig. 2)
A space gun with its end very high into the atmosphere
Copyright, http://gadgetfixx.com/wp-content/uploads/2013/01/space-gun.jpg.
A space gun is very literally a very large gun capable of shooting payloads into space. It is also featured in many early novels of getting into space including Jules Verne’s From the Earth to the Moon.
In the Project HARP a U.S. Navy 410 mm 100 calibre gun was used to fire a 180 kg slug at 12,960 km/h, reaching an apogee of 180 km, hence performing a suborbital spaceflight. However, a space gun has never been successfully used to launch an object into orbit4 . Incidentally another object was shot into space using nuclear power during Operation Plumbob by the U.S. it was part of the Pascal-B nuclear test where a 900 kg steel plate cap was accidently blasted off the top of a test shaft at more than 66km/s. The cap was never found though extremely rough calculations say that it reached six times the escape velocity. However, it is widely believed that it burned up in the atmosphere5 . Unlike a rocket a projectile fired in this way would continuously lose energy so huge amounts of energy is required amounting to around 64000g (62762.56N/kg) (assuming that there is constant acceleration which there isn’t so this is hugely conservative) to reach escape velocity with a muzzle of 100m5 . So to make it tolerable for humans (12.8g max.) a gun barrel of 500 km is required. This is not as simple as making the barrel longer because more and more problems are encountered.
One of the first problems you encounter is the fact that the air in front of the barrel cannot get out of the way, this can be solved by making it a vacuum with a seal on the top which is weak enough for the projectile to go through or lifting the barrel so much that there is very little atmosphere itself, each of them with their own problems. Another problem is that the propellants themselves cannot accelerate the projectile beyond the speed of sound. However, a light gas gun could be used to make the upper limit for acceleration much higher. The acceleration is not constant because the pressure drops. This could be remedied through timed explosions along the barrel and keeping the pressure high. Another way in which the pressure can be kept high is by making the projectile the shape of a ramjet and filling the barrel with fuel5 . Another disadvantage is that it will have to deal with re-entry style heat and friction on the ascent. This means that there will be severe energy losses on ascent and it would also mean that the trajectory would be difficult to control. One other disadvantage is that the only orbit that would not involve crashing into Earth or reaching escape velocity would be something that would essentially hit the launchers in the back. The only way this can be avoided is only when there is an active payload (rockets).
The main advantage for space guns is that small scale demonstrations of this have already been made, for example Project HARP, Project Babylon, and Project SHARP. Additionally, if it is made then it can produce a very cheap form of space travel. However unless the barrel is a few hundred kilometres long it can only be suitable for specially adapted spaceships and fuel.
Laser Propulsion
Fig. 3: Laser Propulsion (fig. 3)
The Lightcraft is the most advanced version of laser propulsion available today
Copyright, http://en.wikipedia.org/wiki/File:Lightcraft.jpg.
Laser propulsion involves a very high powered laser and specially designed crafts. It was first formally introduced by Arthur Kantrowitz in 19726 . There are several different ways in which energy can be transferred to the craft, through pressure from the radiation or by using the laser to burn some form of propellant. The last method is the only one possible to launch the object.
Most of the different methods involve a pulsed or continuous laser to shine on a parabolic mirror on the rear of the craft which focuses the beam on a gas or a solid which explodes. A model craft (known as the Lightcraft) uses this technique to heat air and has reached a record height of 72 metres (236ft)6 ; a different source7 says 71 metres (233ft)). Using this technique a megawatt laser could put a one kilogram satellite into orbit.
The other methods use a heat exchanger and a generator to convert the energy from the lasers into electrical energy which will power some form of propulsion, for example using the electricity to push plasma out of the rear.
Models of this technique have already been tested (Lightcraft), so it is quite advanced as a technology. If this technology is developed it could significantly reduce travel costs. It would also mean that, because most of the equipment is on the ground, there would be a lot more room inside the ‘rocket’. If a system of satellites is built, which can convert energy from the sun to high energy laser beams, then, not only interplanetary travel but interstellar travel as well will be possible. However building such a fleet would be enormously costly and the technology to build lasers that powerful, exists, but are expensive and very power-hungry. Although the developments of lasers can make them very available building a space port for this would still cost a lot head-on.
Space Elevator
Fig. 4: Space Elevator
This is the size of a space elevator compared to earth (all to scale)
Copyright, http://en.wikipedia.org/wiki/Space_elevator.
It is a tether like structure where there is an extremely long (taller than geostationary orbit) ribbon made out of, possibly, carbon nanotubes. The carbon nanotubes will be braided into a rope and coiled and sent up in a satellite; the satellite will then uncoil it, while keeping the centre of gravity on or just above geostationary orbit. When the cable has gone down far enough other craft will clamp onto it and pull it down to a base (probably in the sea). The end result will be an extremely long rope made of carbon nanotubes extending above geostationary orbit and pulled taut with a counter-weight made up of a captured asteroid or the space craft that took it into orbit in the first place.
A huge advantage of this is that it can send and bring all types of cargo with very little energy spent for the energy used to send the craft into space would be gained when that craft is brought back to Earth. This design is by far the most permanent link to space and would be crucial in colonising space. Once built it would make travel to space very cheap ($220 to $880 per kg8 The estimated costs for it to be built are $6 billion and one space shuttle flight costs 500 million, so it is within our scope. It will take a lot longer to go into space but the forces would be much more tolerable for a much wider amount of people. However, there is no known material strong enough that can support its own weight over such large distances. Theoretically though carbon nanotubes are strong enough for they have a theoretical tensile strength of 300GPa but the current maximum is 120GPa. The minimum estimated requirement of tensile strength is 130GPa. Another candidate is Boron Nitride. The tests are on strips of carbon nanotubes a few nanometres long and so far nothing has been built which can even compare to a space elevator. Vibrations from the sun and moon; climbers; weather; and meteoroids along the space elevator could literally split it open.
Building one on Earth can be very hard because of many factors like earthquakes, fluctuations in gravity and space junk which can have devastating effects on the cable and cause whip like motions or cause the cable to split. It will also require huge amounts of planning and funding. It also needs to be on the equator but many countries on the equator have political problems and the most powerful countries would not like to hand over all power to a small country.
Discussion
Because rockets have such a huge cost, they are on the decline. Less and less rockets are sent out but each one of them is crammed with as many satellites as possible. Research has been carried out to find a method that significantly reduces these costs and open space up for everyone not just governments.
Space elevators are the most permanent link to space. They also have the potential to make a trip to the moon as common as a holiday to Spain. However, building one on Earth with our current technology is out of the question, as we don’t have any material that is even remotely strong enough so it is impractical. Even if it is completed then there will be some major complications to consider. For example, space debris and meteoroids may collide with the space elevator and destroy it. So some type of method that actively destroys space junk needs to be introduced and made and the space around Earth completely cleared of space junk before building a space elevator can be considered. Vibrations that can occur from the gravity of the sun, moon and other planets may produce whiplash movements which could be dangerous for both itself and anyone on it. There are also some political problems with this. A space elevator is a very easy target for a terrorist for you cannot expect there to be a very high security along the entire elevator because it is so big. One other political problem is the placing of the space elevator. Many of the most powerful countries are nowhere near the equator and are unlikely to give the power of controlling all space travel to any other country. So it very likely that it would be built in the middle of the ocean (probably the Pacific).
Of all the technologies looked at so far space guns are the most developed. They can form a very cheap form of transport. However, the ‘bullet’ must have an active component (something that directly changes the orbit of the ‘bullet’) if it is to achieve a stable orbit. This is explained by Isaac Newton in his book Philosophiae Naturalis Principia Mathematica which explains the three possible orbits that can be achieved from his example: cannon shooting a cannonball. The three possible orbits are: 1) the cannonball falls onto the Earth; 2) the cannonball makes an orbit and essentially hits the cannoneers in the back; and 3) the cannonball reaches escape velocity and shoots out of Earth’s gravitational field. It is very likely that the space gun will be restricted to fuel and specially adapted spacecraft because the acceleration would be too high for humans to survive. The only way that humans can travel on it is when the ‘gun’ is a few thousand kilometres long. Extending the barrel however will not solve this for the pressure behind the ‘bullet’ drops so the acceleration is not constant. So, even more imaginative methods must be used. For example the blast wave accelerator in which the ‘bullet’ has a tapered end and a series of sequenced explosions push against this and keep the acceleration high; another method to keep the acceleration high is filling the barrel with a combustible mixture and making the projectile the shape of a ramjet. another problem with long barrels is that the air in front of the ‘bullet’ does not have time to get out of the way so the methods proposed that would solve this involve either a gun barrel so long that it ends several kilometres high in the air where there is practically no atmosphere; or having a cover on the nozzle of the barrel that is strong enough to stop the atmosphere getting in yet weak enough for the projectile to get through.
Launch loops can send a large volume of materials into space (estimates are for it to be able to launch up to 6 million tons per year2 . It can also dramatically reduce costs to maybe as low as $300 to $3 per kg2 . It also has a much higher launch rate than any conventional rocket. However, it requires high precision control of the rotor because if the rotor touches the sheath at any point this will cause a release of 1.5 petajoules of energy roughly equivalent to a detonation of a nuclear bomb. Weather effects can cause it to heat up and cause slack, this slack has to be then accounted for otherwise it could touch the sheath and explode.
Laser propulsion can not only make travel to the planets seem feasible but also makes interstellar travel likely. It can also revolutionise aircraft. The main problem is the fact that lasers powerful enough to send a manned mission are, at the moment, very expensive (the rule of thumb is that it takes a 1MW laser to send 1kg into low Earth orbit) and needs a lot of electrical energy. However, running costs, compared to traditional chemical rockets, are much lower. To use this method for interplanetary travel having all the lasers based on Earth is not feasible. So an array of satellites which convert sunlight into lasers and then direct them to the satellites will be needed.
Conclusion
Of all the methods discussed so far, space guns are the most readily available and, because I think that an alternative to traditional rockets must also be readily available, space guns should be incorporated in our design of non-rocket launching until a time when a more efficient method become readily available. Although space guns cannot participate in sending humans they can send huge volumes of raw materials into space this way space manufactories which will build many instruments too delicate to survive the trip to space may be built. Shipping the raw materials with low costs will mean that projects such space colonies and the terraforming of Mars may have a chance to blossom.
Because sending projectiles into low Earth orbit is impossible with just a space gun, I propose that laser propulsion to be used to alter the trajectory and deliver the projectiles back to Earth.
Whatever the type of space port created they must be as close to the equator as it is possible and send projectiles in an easterly direction. This way as much velocity as possible is gained through the earth’s rotation.
As a fuel electricity could be used as it is highly versatile and can be generated in a number of different ways. The space gun could be a light gas gun with a ram-jet shaped projectile which will combust specially tuned gas mixtures in the gun barrel to reach as high a velocity that is possible.