In his novel "From the earth to the moon" (1865) Jules Verne, maybe the first SF author described a giant cannon with a vertical barrel of 274m length which sent a manned projectile with 16.5km/s to the moon. Mr. Verne ignored maybe knowingly two problems of this endeavor:
1) The acceleration in the barrel would be 50 000g. A person of 80kg mass would be pressed down with a force of ca. 4000t. You may imagine the consequences yourself.
2) After leaving the cannon the projectile would move with 16.5km/s through the air and would behave like a meteorite.
To build a modern and well behaving space cannon, you have to overcome these 2 obstacles.
1) You have to reduce the acceleration by prolonging the barrel. To attain the necessary 8km/s (1st cosmic velocity) for reaching LEO (low orbit) with "moderate" 2.6g, it has to be 1250km long.
2) The barrel should end well outside the atmosphere, about 120km (70miles) high, so that the projectile wouldn't lose too much of its precious kinetic energy or even be incinerated. To prevent this to happen already in the barrel, you must evacuate it.
Point 2) implies the good news that the barrel can be laid nearly horizontally and relatively near to the surface most of its way instead of vertically, which makes its construction tremendously cheaper and easier. If the barrel starts horizontally and goes on in a straight line for 1250km, its muzzle will be 123km above ground because of its curvature. The angle to the surface at that point is about 11 degrees and in that direction the projectile will start rising to an orbit or farther. Running on the surface for too long and bending upwards only at the end would cause centrifugal forces too high for passengers. Another advantage of accelerating horizontally is, that the acceleration of 2.6g and gravity of 1g don't add up to 3.6g as with vertical starts, but only to ca. 2.8g (Pythagoras). It is not important to put a lid on the muzzle of the evacuated barrel constantly, because there is very little air up there to fall into it. To go to GEO, the moon or farther, you have to attain at least the real escape velocity/2nd cosmic velocity of ca. 11km/s, which exceeds the 1st cosmic velocity by the factor square root of 2. So the kinetic energy and the length of the tube has to be doubled (acceleration remaining the same). Of the 2500km runway the first 1250km can be on the surface. The vertical force will steadily go to zero, as the centrifugal force compensates the weight more and more. In return you could increase the acceleration a little bit. The rest of the way would be a straight line again.
In realizing this Space Catapult ("Space cat"), you have to meet 2 great challenges:
1) To develop an electromagnetic drive, linear motor, rail gun, mass driver... that can accelerate cargo of up to 10t or more to desirably far more than 11.2km/s (escape velocity) in a vacuum tube gliding by magnetic suspension (maglev).
2) A construction of several 1000km length going up from zero to 120km height at the end to carry the tube. From the thorough research on space elevator topics we know, that this is possible with existing materials and methods. It could consist for instance of a long row of pylons looking like the highest radio masts: slim lattice towers with guys that are anchored about as far sideways as the height of the tower. The danger of masts and guys for aircraft-traffic must be taken care of. Some advantages of the space catapult:
1) You can send spacecraft to any orbit from LEO, MEO to GEO (-stationary orbit) by varying the firing speed.
2) You can build the space cat on nearly every place on land where there is some 1000 km (=Mm) room. It can be built in your own country; it need not be placed at the equator and on the water. The start station can be situated in a region where many people live and can get there easily. E.g. Vandenberg Air Force Base near Los Angeles and the tube going to the New Orleans region. (Going eastward and nearer to the equator provides kinetic energy from earths rotation.)
3) You can use the space cat for return traffic from space to earth too. The projectiles have to hit exactly the muzzle of the tube and in the exact direction (with a little help from laser beacons, radar and computers). Then the linear motor acting as a generator would "softly" slow them down and win the kinetic energy back as electricity.
4) The travel time is relatively short: about 6 minutes accelerating in the tube and quite a few more for the ballistic trajectory going up to the orbit, where some steering jets do the exact positioning.
5) The transport capacity is unbelievably high. The container capsules could start in intervals of seconds or less, their distance when leaving the tube being several km's nevertheless.
6) The building material need not be brought up to space by rockets (costing ca. 10M$/t)
7) The vulnerability against assault is relatively little. Destroying one mast or one guy should not wreck the whole structure thanks to the redundancy of the construction. To hit a guy of several cm diameter from the distance is difficult. The masts are relatively thin too and guarded by cameras, radar and trained personnel. The tube could be hit in the first several 100 km's before it vanishes in the sky, but then air would flush into the tube, stopping the nearest vehicles. To hit a vehicle itself, going that fast and not to be seen from the outside is very improbable.
8) Maybe a much greater blessing than for space travel would be the "spin-off" for terrestrial transportation. Imagine maglev railways in vacuum tubes with speeds of thousands of km/h traveling in hours through whole continents. When the long planned bridge over the Bering Street is built, you could ride from London to New York on the ground faster than with an airplane. Maybe it will be the other way round: The development of maglev railgun vacuum tube transportation technology for terrestrial uses will make the space catapult possible.
One shortcoming of the space cat against rockets shall not be withheld: It can shoot only at one fixed angle with the equator. Maybe other countries will build space cats too, and if they choose other angles, they can use their space cats mutually by international cooperation. But there is a way to change the emitting angle of a space cat up to 90 degrees without additional energy: A tether is spanned from the muzzle of the tube to an anchor point on the surface in 1250km distance and in a direction at right angles to the tube. When the projectile leaves the tube, the tether is fastened to it, so that it swings in a huge circular arc. When the desired direction is attained, the tether is clicked off. A cord is fastened to the loose end of the tether to pull it back fast enough, that it won't touch ground. You can install 2 of these tethers on both sides of the tube.
The space catapult has been known for a long time for use on the moon (and maybe other planets) and there was never any doubt about its feasibility. But on the moon it would be much easier to build: You don't need a vacuum tube, the escape velocity is lower (only 2.4km/s) and the rail could be laid on the surface on the whole length. At the end the rail has to hold the projectile down against the centrifugal force. Maybe the great spaceships for future interplanetary or even interstellar travels will be produced or put together on the moon and in moon orbits, transportation up and down done by the rail gun. Even hydrogen and oxygen as rocket fuel can be produced on the moon, if we find water there. The energy stems from solar panels during the half-month-day of the moon.
A space elevator on the moon would be only suboptimal: The moon stationary orbit (MSO) is very high up because of the slow rotation of the moon. One could use the Lagrange point/libration point L1 between earth and moon instead, which is "selenosynchronous" too. But being 58 000km (58Mm) above it is even higher than the GEO.
Back to earth and a nearer future, when the space cat is jumping merrily (and cheaply) to and fro space. An ordinary family could do a trip to the orbit on its free day. They will relax for some hours in the LEO satellite hotel, which turns slowly around its axis to give the substitute of gravity and after going 1 or 2 times around our planet enjoying its beauty they return home contentedly.
Steffen Kummerow studied physics at the Universities of Hamburg and Karlsruhe, got his diploma at the Nuclear Research Center of Karlsruhe KFZK and worked in future research and in teaching. He has been a dedicated follower of serious and hard core Science Fiction all his life.
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