Thanks primarily to Ilona Mask. It is believed that the appearance of colonies on Mars and the Moon is a matter of the next decade. Optimism is good. In fact, interplanetary ships capable of regularly transporting cargo even between the orbits of the Moon and the Earth, without landing on the surface are absent. Obviously, for a real expansion, it is necessary to conduct regular interplanetary transportation. We need reusable interplanetary, or rather interorbital tugs. This article is about such a system, the concept of which is taken from nature.
Natural orbital tugs.
While the inventors of the whole world are thinking about how we move in space. Nature has already invented everything. In fact, any periodic comet is a ready-made concept of an interorbital tug. We are looking at the basic scheme of the flight of such a comet.
Usually the comet is approaching the Sun and instead of moving to a circular orbit under the action of the Sun's attraction, it accelerates and goes to the ellipsoidal orbit. And this acceleration is due to the effect more known as "Gravitational maneuver".
This natural phenomenon was first discovered by astronomers of the past who realized that significant changes in the orbits of comets, their period (and, consequently, their orbital velocity) occur under the gravitational influence of planets. So, after the transition of short-period comets from the Kuiper belt to the inner part of the Solar System, a significant transformation of their orbits takes place just under the gravitational influence of massive planets, with the exchange of angular momentum with them, without any energy costs.
 Now gravitational maneuver is less often a perturbation maneuver, a purposeful change in the trajectory of the space vehicle flight under the influence of the gravitational fields of celestial bodies. It is often used to disperse automatic interplanetary stations sent to remote objects of the Solar System and beyond, in order to save fuel and reduce flight time. In this application is also known as the "gravitational slingshot" (from the English gravitational slingshot). 
The very idea of using gravitational maneuvers to achieve the target of space flight was developed by Michael Minovich in the 60s, when, as a student, he practiced at the NASA Jet Propulsion Laboratory. For the first time the idea of gravitational maneuver was realized in the trajectory of the automatic interplanetary station "Mariner 10" when the gravitational field of Venus was used to achieve Mercury. .
Further, the possibility of a gravitational maneuver near the Moon was proved in the article 'The gravitational maneuver near the moon during interplanetary flights of a low-speed spacecraft' Electronic journal 'Proceedings of the MAI'. Release N 50. Author: R.V. Elnikov 
So, this natural gravitational engine exists known is learned and already used. It is a matter of small to create on the basis of it a full-fledged reusable interorbital transport system that is simple and reliable as a locomotive. Let's imagine that some artificial body "Space Express" flies along an elongated ellipsoidal orbit around the Earth and the Moon. Then this body has two points where it can perform an accelerating "Gravitational maneuver"
It is enough once to bring this tug into this orbit, and it will fly from Earth to the Moon and vice versa, periodically accelerating at the points of gravitational maneuver practically forever. Further all is simple. We put into orbit by means of a rocket a module capable of landing on the Moon, for example, with passengers on board.
This module catches up with "Space Express" docked. And at the point of gravitational maneuver, they together perform a gravitational maneuver.
Having received additional acceleration, they together rush towards the Moon's orbit
When approaching the Moon, a landing module with cargo and people from docks and lands on the moon. Or it docks to a "space elevator" if one is built on the Moon.
Then a new rocket with a cargo for the Earth will start from the Moon.
Docked to the "Space Express", catching up with him. And together they make a "Gravitational maneuver" and they leave for the Earth.
Further, "Space Express" with the cargo are sent to orbit to the Earth.
When approaching the Earth, the load or the module from docks and lands on the Earth.
"Space Express" selects a new parcel from the Earth and after completing the "Gravitational maneuver" again go to the Moon.
Objections include the fact that the speed of the module accelerated by a rocket and the "sky express" will be too different. And accelerating the module to the speed of a 'sky express' flying at a greater speed to dock with it will lead to excessive fuel consumption, which may well be enough to send a cargo to the Moon without any 'express'. Yes, I agree that the "rocket" of the Earth drifting in orbit can have the first cosmic velocity equal to 7.91 km / s and the second one at 11.2 km / s from the "express". The difference is not so big and the two objects can easily catch each other, for example, using a cable system with an electromagnet at the end and grappling to get close and equalize the speed.
The magnet from the rocket accelerates to 11.2 km / s with the help of built-in rocket engines, its speed is equalized with the electromagnet which releases the 'express' and they are quite softly coupled. Further, adjusting the length of the rocket cables is attached to the 'express' tail. At this point, a jerk is possible, but it can be compensated for by the length of the cables, gradually drawing into the hulls sufficiently long cables made of some kind of heavy-duty material, for example, from nanotubes. To accelerate a relatively small electromagnet to 11.2 km / s in the direction of an approaching express using built-in rockets is far less energy-consuming than to accelerate, for example, a module with a weight of 100 tons.
Further, the calculations of some scientists involved in this problem show that it is not possible to provide a passive repetitive motion path between the Earth and the Moon. The fact is that during the execution of the gravitational maneuver of a celestial body, the spacecraft "takes" part of the kinetic energy of this celestial body. When performing multiple gravitational maneuvers, the spacecraft will recruit it time after time during the passage of the Moon or the Earth. As a result of this energy 'pumping', the device will be ejected from the Earth-Moon system and will switch to the heliocentric trajectory of motion, moving like a small artificial planet.
How to get rid of the "extra" energy? Unfortunately, it will be necessary to turn on the propulsion system and, burning fuel, to "slow down" the spacecraft. The cost of fuel will be very significant.
All this is true, but no one forbids installing engines on the 'express' system that will slow it down or accelerate it additionally, express and it should not be jet engines, but engines interacting with the solar magnetic field .
A magnetic field is a medium with which ions can interact, moving perpendicular to the lines of force of the Earth's magnetic field. The principle of its operation is very simple.
It's all about the power of Lorentz which pushes the charges out of the magnetic field. And in a place with them and the "turbine". This force depends on the magnitude of the magnetic field "H" on the particle charge "q", and on the speed of rotation of the ions in the external magnetic field "V" . We look at the formula: The force acting on the particles is determined by the formula: FL = q [V x B], in the CI system - in tesla (T)  Where: FL is the Lorentz force; q-- particle charge; v-- particle velocity; B-magnetic field induction.
Yes, this engine is low-power but in conjunction with the "gravitational maneuver" everything can turn out. Alas, jet engines are superfluous, they use too much fuel, everything should work without jet engines.