In mid-2014, a group of researchers from NASA’s Johnson Space Center announced that a new type of engine devised in 2000 by the British Roger Shawyer could be possible: the EmDrive, also called a radiofrequency resonant cavity thruster . It is a motor that uses the microwaves that are generated in a magnetron to propel itself; it does not need any fuel, only an electrical source to generate microwaves.
The problem with Shawyer’s design, his critics point out, is that it appears to violate one of the most sacrosanct principles of physics, that of the conservation of linear momentum , the product of mass times velocity. Since this engine does not expel propellant like conventional rockets, there is nothing to compensate for the change in linear momentum of the ship when it starts to accelerate. The laws of physics say that if a ship starts from rest, the product of its mass times its speed must be equal to the product of the mass times the speed of the gases expelled by the engine. But if in this case nothing is expelled, how will it move?
The Johnson Space Center seemed to show that this criticism was unfounded and that an EmDrive motor did meet this conservation principle, since they successfully tested this motor in a vacuum. In 2021 scientists from the Dresden University of Technology repeated NASA’s test observing similar apparent impulses. Now the German scientists reported that all published results showing thrust had been false positives caused by external forces . His conclusion was a jug of cold water: “Our measurements refute all of EmDrive’s claims by at least 3 orders of magnitude.”
However, its defenders remain inaccessible to discouragement .
We must understand this illusion: since the 1980s few proposals for propulsion to stars have seen the light of day. That is why in 2011 the Pentagon ‘s Advanced Research Projects Agency (DARPA) announced that it was going to invest half a million dollars to promote the technologies necessary for interstellar travel .
One of them -and the most proven- It is called solar navigation, that is, using photons of starlight or very powerful lasers like the sailors used the wind. In essence, this type of propulsion consists of using what surrounds the ship to propel it. With this idea in mind, Robert Bussard, while working at Los Alamos National Laboratory, conceived his Ramjet booster in the 1960s. The principle is simple: with an electromagnetic field, the spacecraft traps the hydrogen from the interstellar medium needed to make operate a nuclear fusion reactor. Assuming a collection zone of 4,000 km, a thousand-tonne spacecraft could achieve an acceleration of one g in a typical region of interstellar gas, where there are a million hydrogen atoms per cubic meter.
The only problem -and it seems that there is always a ‘but’- is that the magnetic field needed to collect the hydrogen would have to be 100 times more powerful than those obtained today . In 1975 Daniel Whitmire modified Bussard’s design by introducing catalytic fusion, using carbon and neon as catalysts so that the nuclear reaction is a zillion! times faster.
Other ramjets have been developed in recent years with the idea of making the nuclear reaction more effective using the least amount of fuel, such as the RAIR (Ram Augmented Interstellar Rocket) by British engineer Alan Bond, but all these ideas have ended up colliding against the real problem: building a nuclear fusion reactor . Something that, until now and despite ITER, is one of the most expensive companies and with less hopeful results in the history of science. As the French Nobel laureate in physics Pierre-Gilles de Gennes said, “We say that we are going to put the sun in a box. A nice idea. The problem is that we don’t know how to make the box.”
And let’s not forget the other big problem: how to collect the hydrogen needed to run the ship’s engine . From superconducting solenoids, capable of generating a magnetic field strong and stable enough to capture the necessary fuel, to the use of combined electrostatic and magnetic fields, the fragility of these immense funnels is evident. Of course, you can always make refueling stops on Jupiter-type planets, practically a hydrogen service station for this type of ship. Once this technology is developed, it could very well be used at the lunar base to collect hydrogen from the solar wind, which, combined with the oxygen extracted from lunar rocks, will serve as fuel for its machines and cars.
If the ramjet seems to have a very distant horizon, the propellant that borders on science fiction is the antimatter engine , coming from the mind of one of the greatest visionaries of interplanetary travel, the American physicist Robert L. Forward. Antimatter is, to put it simply, the mirror of the matter with which we are made. When an antimatter particle, for example, an antiproton, meets its corresponding matter particle, they totally annihilate; 100% of the mass is released as energy. Thus , if we annihilate a kilo of matter with another of antimatter , as much energy is released as 40 million tons of TNT .
Bearing this in mind, the theoretical speed limit for this type of motor is that of light. But things are not so simple. 40% of that energy is released in the form of lethal gamma radiation with an energy hundreds of times greater than that released in our nuclear power plants. This implies that the ship must be strongly protected from this type of radiation, considerably increasing the dead weight to be transported.
But the first thing that should be done is to reduce the energy costs so that the engine is profitable : the amount of energy needed to obtain a kilo of antimatter as we do in our particle accelerators is a million times what would be released by annihilating this antimatter in a rocket If you give more than you get, the loss is certain.
And there are no more ideas: conventional fuels, solar sails, nuclear or antimatter rockets.
Referencias:
Brady, D.A.; White, H.G.; March, P.; Lawrence, J.T.; Davies, F.J. (2014). “Anomalous Thrust Production from an RF Test Device Measured on a Low-Thrust Torsion Pendulum”. 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference.
Bussard, R.W. (1960). “Galactic Matter and Interstellar Flight”. Astronautica Acta. 6: 179–195
Tajmar, M.; Neunzig, O.; Weikert, M. (2022). “High-accuracy thrust measurements of the EMDrive and elimination of false-positive effects”. CEAS Space Journal. 14 (1): 31–44
Winterberg, F. (2012). “Matter–antimatter gigaelectron volt gamma ray laser rocket propulsion”. Acta Astronautica. 81 (1): 34–39
