The nuclear saltwater missile is possible

Departure to the stars


Part 3: Propulsion for space travel

Rockets can be propelled by a wide variety of techniques - which are the best?

Electric motors

Nuclear drives

alternative drives

Heavenly elevators

We want to leave the earth with a rocket, whereby we can pursue different goals. It is conceivable, for example, to transport a payload into an orbit, i.e. into an orbit around the earth. This could be satellites for weather observation, communication, navigation, research and many other purposes. However, we can also take astronauts to a space station, take them to the moon or even to Mars. Or unmanned space probes to planets, asteroids or comets. It is also important to consider whether you want to or have to take off directly from Earth, or whether you can take off from an orbit.

There are therefore completely different motivations for using a rocket, which then also determine whether you can use high acceleration forces or not, whether high or lower top speeds are considered or whether you can Swing-by- want to carry out a maneuver (with such a maneuver, one can let the gravitational field capture a planet by cleverly choosing the orbit of a spacecraft, which increases or decreases speed).

The drive types common today, which are based on chemical (exotherms) Reactions are based, we have already got to know. Here a solid or liquid fuel, e.g. cellulose nitrate or hydrogen, is burned with an oxygen supplier and possible catalysts. The hot exhaust gases are passed through a nozzle, which increases the outflow speed even further. Chemical drives achieve very high thrust performance, but their outflow speeds are relatively low. They can be used to transport larger payloads directly from Earth into space.
But if we one day undertake interplanetary flights and do not want to travel unreasonably long, we have to look for alternatives. We'll look at some of them in a moment. But before that, I would like to briefly discuss another key parameter of a rocket engine, the Specific impulse:

The (mass) specific momentum of a rocket is the change in momentum (= Mass times speed) per unit mass of fuel. It is expressed using the following formula:

Therein mean:

  • : Burn time
  • : Fuel mass
  • : medium thrust
  • : Thrust course

This results as a unit, i.e. it is the outflow speed of the combustion gases from the rocket motor.

In general, however, the weight-specific momentum of a rocket is used today, which is corrected for the acceleration of gravity. Its unit is thus reduced to the second and is calculated according to:

But now to the various drive methods:

Electric motors

At first glance it may sound surprising that one can navigate spaceships through space with electrical energy. However, if you consider that it can be used to heat up selected support masses or even ionize them, its use is no longer so absurd. However, electric drives deliver only a very weak thrust and are therefore not suitable for launching a rocket from a celestial body. However, they are used, for example, to carry out path corrections. If the spacecraft is to be relatively close to the sun, electrical energy can be provided inexpensively by solar cells. For large energies, however, you need generators that are powered by nuclear energy. Here is a brief overview of the types of drive that are already in use:

TypedescriptionapplicationthrustSpecific impulse [s]example
Fuel is heated by an electrical resistorPosition and path control150 [mN]1000 
Arc engine Fuel is heated up to 5000 [K] by an arc between 2 electrodesPosition and path control, drive3,5 9342000  
Field emission engineA liquid metal (cesium) flows between two positively charged, closely spaced anodes to a tip near which a cathode is located. The electric field between the two electrodes ionizes and accelerates the fuel.Position corrections1 [mN]12 000 
Kaufmann engine An arc creates a plasma (xenon, mercury), which is accelerated through a grid and then electrically neutralized.Position and path control, drive90 [mN]3100 "NSTAR" from NASA:
Magnetoplasmic dynamic drive The support mass located in a funnel shaped as an anode is ionized by means of a rod cathode, as a result of which a current flows in the plasma, which generates a magnetic field. The plasma particles (argon, lithium, hydrogen) are strongly accelerated by interaction with an external magnetic field.Path correction 300 [mN] 4000
Radio Frequency Ion Thruster (RIT) Electromagnetic waves create a plasma (xenon), positive particles are accelerated through a grid, then neutralized and ejected. Position and path control, drive 600 [mN]up to over 9000 "HiPEP", NASA, RIT from Germany:
Hall ion engine (HET)A plasma of xenon or bismuth is generated by an arc. Electrons move in a circle in the discharge chamber, which is designed as a ring, the positive ions are generated by the one known from electrical engineering Hall effect pushed out. A neutralizer is used for the electrons that remain.Position and path control, drive 68 [mN] 1640 SMART I:
Pulsed plasma engine Resembles one Railgun, in which a projectile is accelerated as a live slide between two rails. Here, however, there is a block made of Teflon at the end of the rails, which is evaporated slice by slice. The repelled plasma creates the thrust. Large capacitors provide the electricity required for evaporation.Position control 1 [mN]2200 

Nuclear drives

To get really big thrusts, we have to consider nuclear power. Several variants are available here, for example nuclear fission or nuclear fusion. However, such drives are more of a theoretical nature, because on the one hand it is far from clear how the crew could be protected from the radiation. On the other hand, for the time being the Treaty banning nuclear weapons tests in the atmosphere, in space and underwater the use of such technology. However, it cannot be completely ruled out that such concepts will be used for peaceful purposes in the future. Let's take a quick look at the variants in tabular form:

drivedescriptionthrustSpecific pulse [s]example
Solid core reactor In a nuclear reactor, a gas (hydrogen, ammonia) passed through is heated to high temperatures (3000 [° C]) by nuclear fission and then ejected through a nozzle under high pressure. 1000 [kN] 1000 NASA's NERVA project
Gas nuclear reactor A solid core reactor is limited by its melting point. But the higher the temperature, the higher the thrust. So one thinks of gaseous reactors, whereby a spec. Impulse up to 5000 [s] is possible. The problem here, however, is that the reaction mass would also be expelled.up to 1000 [kN] 5000  
Radionuclide reactor A gas (hydrogen, helium) flows over a radionuclide, which releases heat through natural decay. This is transferred to the gas, which then exits through a nozzle. 10 934 800 Project Poodle, USA
Nuclear pulse driveIt's hard to believe: With this drive, a nuclear explosion is to be triggered every few minutes at the stern of the rocket, which would then catapult the ship forward. This would be technically feasible, the payload or crew would be protected from excessive acceleration values ​​by "shock absorbers". Although the pressure wave of the atomic bomb explosion is supposed to be supported on a baffle plate, the extent to which this would protect the crew from radiation is unclear. Advantage of such a drive: high thrust and high spec. Pulse.up to 10,000 [kN] up to 10,000Orion, Daedalus project
Propulsion by nuclear fusionMuch more energy than nuclear fission could be released by a nuclear fusion reactor. Unfortunately, this technology is not yet available to us. But if it is ever possible, one could do so with a so-called Buzzard collector capture the hydrogen present everywhere in space and feed it into the reactor. The collector would be nothing more than a huge magnetic field in front of the spaceship to capture as much of the extremely thin gas as possible. The hot combustion products (e.g. helium) could be expelled through a nozzle. The advantage of such a drive would be a practically unlimited range, since only very little fuel would have to be carried.30 000 934 47 000

alternative drives

With the drives presented so far, the end of the story has not yet been reached. Resourceful minds have come up with other ways to make rockets fly through space. Let's take a look at some of these partly exotic alternatives:

drivedescriptionthrustSpecific pulse [s]example
Photon rocketIt was already conceived by Eugen Sänger. It is driven by ejected photons. At the end of the rocket, a screen made of a heat-resistant material is installed, which is heated as high as possible. The emitted black body radiation provides the (vanishingly small) thrust. Disadvantage: Very high energies are required for heating with only the lowest degree of efficiency. Advantage: Very high speeds up to the relativistic range are conceivable.300 [mW]?? 
Antimatter PropulsionAlready described many times in science fiction, this concept brings matter into contact with antimatter. Protons and antiprotons, for example, annihilate (annihilate) each other and completely release their rest energy. In principle a simple process, one could introduce small amounts of antiprotons (anti-hydrogen) into a cloud of protons (or e.g. hydrogen), the pair annihilation would start immediately. However, the provision of the antimatter is extremely problematic. We are technically not able to produce larger quantities (a few grams!). Only single particles can be produced in the accelerators. There is also no technical solution for storage.100 [kN] 400 000  
Fission sails A (largest possible) area is coated with a radioactive material, which represents an α-emitter. The helium nuclei hurrying away in only one direction cause the recoil. Disadvantage: "refueling" is difficult.10 [N / km2]40 000  
Saltwater nuclear missile Radioactive salts of uranium or plutonium are dissolved in water. The solution is stored in separate tanks so that the critical mass is always below the limit. The walls of the tanks also serve to absorb neutrons. The water is metered into the "combustion chamber" in such a way that the critical mass is reached there and the nuclear chain reaction begins.10 [MN] 10 000  

"Heavenly Elevators" & Co.

So far we've looked at how to propel a body in space. But there are also - sometimes exotic - methods of how to rise from a celestial body without a conventional rocket propulsion system.

Space elevator In this concept, which is being seriously discussed, a kind of rope is anchored to the ground while a weight (a counter station) is a little beyond the geostationary orbit. The centripetal force would stabilize the rope in such a way that a transport basket could be carried upwards. The question of materials is problematic. The necessary strength could, however, be achieved using modern materials such as carbon nanotubes. However, there are many other problems that need to be solved, e.g. power supply for the elevator, fall protection, emergency systems, etc.
Space cannon Jules Verne already chose the hammer-hard method to get away from the earth. A payload is accelerated in a kind of cannon like a projectile so much that it overcomes gravity. Not suitable for living beings, but it could shoot material into orbit. Serious attempts were made to this end in the 1960s and heights of 180 [km] have already been achieved. Today the company Quicklaunch tries to develop the process further.
Mass driver In principle, mass drivers work like a linear motor. Erected on the moon or Mars, for example, such a rail-like, electromagnetic catapult also serves the purpose of accelerating a payload (cabin) so far that it reaches orbit. This project is also being seriously researched (SSI). In contrast to the space cannon, mass drivers could also transport passengers with careful acceleration.

As we have seen, there are a number of ways to put spacecraft into space and move them there. Many of these methods can be viewed as realistic and technically feasible based on current knowledge, even if they sometimes sound quite exotic. Travel by means of warp drives or through wormholes is not listed, nor are anti-gravity lifts or beaming. We'll leave that to SF literature and future supercivilizations for the time being.