Today, we will be discussing the different methods — both proposed and tested, to reach space without the use of rocket propulsion in its entirety. The idea is to achieve orbit by a propulsion technique that is not subject to the limits of the rocket equation.
Is it possible? Well, let’s find out!
Early Concept: Space Tower
In reaching the edge of space (or above the Karman Line of 100 km), a great part of the rocket propulsion energy goes into coping with the atmospheric drag, which is higher at lower altitudes. Theoretically, we can have a long space tower of 100 km in height, through which objects can be released directly at the perigee point, and then the orbit can be raised. Even further, we can have a tower of a height of 36,000 km and directly release the objects in geostationary orbits.
This concept of Space Tower was first proposed by Konstantin Tsiolkovsky. It has been proven to be an impossible task because of the lack of materials strong enough to withstand compression for 100 km or higher height. Still, ideas for a tower of up to 20 km in height have often been proposed.
A skyhook is an orbiting platform that extends a tether down towards the Earth, enabling the transportation of payloads to orbit by cable car, instead of by rocket. In this, a cable that reaches down and toward the upper atmosphere is attached to a hefty orbiting station. As the cable passes, payloads, which are much lighter than the station, are linked to the end and are subsequently propelled into orbit by rotation of the cable around the center of mass.
Another famous concept — its principal element is a long cable or tether being attached to the surface and extending into space above the geosynchronous orbit. The upper end of the tether’s centrifugal force, which opposes gravity as the globe rotates, keeps the rope taut. After then, vehicles can ascend the tether and enter orbit without using rocket propulsion.
The high gravity of the earth makes this concept difficult because of the requirement of strong and light materials for making the long cable. However, carbon nanotube- or boron nitride nanotube-based materials have been proposed as the tensile element in the tether design. Their measured strengths are high compared to their linear densities. Also, the idea of combining Space Tower and Space Elevator has also been proposed.