Mass ratios are the bane of atomic rocket designers. No matter how potent the drive is, you are going to have several kilograms of propellant for each kilogram of rocket. This puts severe limits on the sorts of missions a rocket can perform before the ever-hungry engine has to be fed again. Every gram counts due to the The Tyranny of the Rocket Equation
This is because all rockets utilize Newton's Third Law of action and reaction. You throw something backwards (the propellant) and in reaction the rocket moves forward. This is why rockets are called "reaction drives."
Naturally, the thought occurs that if you can figure out how to make a spacecraft move without using propellant, all the problems with mass ratio vanish. You'd have a "reactionless drive." Unsurprisingly there is a TV Trope entry on this topic.
A reactionless drive would be great, were it not for the unfortunate fact that it would violate the law of conservation of momentum.
Now, it is true that Newton's third law has some rare occasions where it does not apply (certain situations with magnetically coupled particles and gravitational forces acting between objects moving very rapidly), but the law of conservation of momentum is a genuine iron-clad rock-solid no-exception law. In a closed system the total quantity of momentum cannot change. It has been verified to within one part in 1e15, and no exception has ever been found.
Which means in a closed system, a reactionless drive is impossible, since it would change the total quantity of momentum.
(Note that it is possible to avoid that law with an open system, with something like a solar sail, a spacecraft launched by a mass driver based on an asteroid, pellet-stream propulsion, or a Starwisp. In these cases, the propulsion system is external to the spacecraft, so the system is open and the law does not apply.)
However, a little thing like violating a law of physics isn't going to stop the crack-pots. Face it, the second law of thermodynamics hasn't stopped all the people attempting to create perpetual motion machines of the first kind.
And even if you, the science fiction author, hand-waved one into existence for your SF novel, you've still got problems.
A working reactionless drive could turn a cheap solar power array and a brick into civilization destroying weapon of ubermassive destruction.
Burnside's Advice is Friends Don't Let Friends Use Reactionless Drives In Their Universes.
Well, keep in mind that Burnside's First Law is more what you'd call "guidelines" than actual rules. You are allowed to have reactionless drives in your science fiction novel. You just have to take into account that reactionless drives will give you cheap planet-cracking weapons, and plan your science fiction universe accordingly.
The trick is making a reactionless drive that doesn't give you the ability to shatter planets with the Naval equivalent of a rowboat (which would throw a big monkey wrench into the author's carefully crafted arrangement of combat spacecraft). Reactionless drives, with no fuel/propellant constraints, will give you Dirt Cheap Planet Crackers. If you have a reactionless drive, and stellar economics where most of the common tropes exist (privately owned tramp freighters), you also have gravitic drive missiles. Unfortunately avoiding Planet Crackers Done Real Cheap is almost impossible to justify on logical grounds, so SF author is faced with quite a daunting task.
(Note that while propulsion systems like photon or tachyon drives are not reactionless, they do manufacture propellant as needed instead of carrying it. This also circumvents the Tyranny of the Rocket Equation and is subject to Burnside's Advice. The difference is that these drives are NOT forbidden by the laws of physics. Photon drives are not much of a problem because you need an outrageous three hundred megawatts for each pathetic Newton of thrust. Tachyon drives ARE a problem, since they do not.)
The quick and dirty solution is to make the reactionless drive rely upon some incredibly difficult-to-obtain material or equipment (such as negative matter). This material will become a controlled substance, with the death penalty or equivalent for possession. And if civilians can own reactionless drive spacecraft, the drive (or entire ship) will probably incorporate some kind of self-destruct mechanism which the astromilitary can remotely detonate. Plus a booby trap to trigger self-destruct if anybody tries to tamper with the the detonator.
The big problem is hostile astromilitaries will be armed with cheap planet crackers. The astromilitary will need some sort of strategy and/or defensive weapon to protect the planet.
The fun started back in 1960 when the John W. Campbell (the father of the Golden Age of Science Fiction) decided to make some excitement by giving free publicity to Norman Dean and his infamous "Dean Drive". It allegedly could convert rotary motion into linear motion, i.e., it was a reactionless drive. U.S. Patent 2,886,976. "Just think," Campbell said, "stick one of these in a submarine and you have instant spaceship!"
Another common name for the Dean Drive is the "inertial drive."
Campbell was miffed that mainstream scientists were not even interested in looking at the drive. But in this case, the scientists were acting properly. Faced with the fact that the Dean Drive obviously violated the law of conservation of momentum, well, extraordinary claims require extraordinary proof. A box vibrating on a pan balance that makes the beam scale look like it had lost an ounce or two is not anywhere near convincing enough.
Interest in the Dean Drive faded away as Dean refused to let anybody examine the gadget, with the notable exception of John W. Campbell and G. Harry Stine. At least without forking over some money first. Even (now) SF author Jerry Pournelle tried to get permission to examine the drive on behalf of the airplane company he was employed at the time, but was turned down.
After Dean died, Stine made a brief resurgence of interest in the 1980's, but it died too, and later so did Stine. A close examination of the patent reveals that the device is actually a complicated ratchet pulling itself along a metal tape, not a reactionless drive.
Physicist Milton Rothman notes that Dean Drive apologists wave their hands and talk about the strange relationship between force and changing acceleration as a justification for the drive, but all they are doing is revealing the depths of their ignorance about basic physics.
My take is:
- Inventor Roger Shawyer's theoretical basis for his EmDrive appears to be total rubbish. It violates conservation of momentum, which would basically mean scrapping all of physics and starting over from scratch, yet still predicting the same results of every experiment in physics in the last few hundred years. This is because of the Correspondence Principle. Conservation of momentum is required and maintained in Maxwell's equations, Newtonian mechanics, special relativity, electrodynamics and quantum mechanics (and their combination, quantum electrodynamics).
- Shawyer's theoretical basis may have nothing to do with the equipment. That is, his basis may be rubbish but he accidentally stumbled onto an arrangement of equipment that actually does create anomalous thrust.
- It is a good general rule to be skeptical of positive results when the measurements are at the limit of accuracy.
- The fact that three experiments by three different researchers have shown positive results is interesting. However, there are questions about the results.
- If the EmDrive actually works, it really and truly is a reactionless drive. Which means it is a weapon of mass destruction that would make the Dinosaur-killer asteroid look like a wet fire-cracker.
John Baez points out that the NASA experiment measured a force that was one thousandth as big as from the Chinese experiment (The incredible shrinking force! In 10 years the device will be using quantum gravity and producing even less force. ). And also that there were some serious problems with the experimental setup (which Mr. Baez goes into in detail).
Corey Powell has an interesting analysis of the history of this affair in an article entitled Did NASA Validate an “Impossible” Space Drive? In a Word, No..
Ethan Siegel does further analysis, along with the red-flag warnings that should tip off careful readers that something suspicious is underway, in his article How to fool the world with bad science
A group of German scientists did an analysis of the EmDrive, building their own from the blueprints and discovered that it does not work. They actually discovered a flaw in the methodology that gave a false positive.
They knew right away that something was wrong when they got the exact same thrust value reported at 50 watts when they ran their version at two watts. In the real world if you turn down the power input it also lowers the output.
The killer finding though was when they did a "null test." They ran the test with zero power going to the microwave cavity (meaning that full power went to the entire EmDrive but the power going to the microwave cavity was intercepted and absorbed by an attenuator). And they still got the same thrust value.
What was even more weird is that you get the same amount of thrust but in the opposite direction if you turn the test rig to point in the opposite direction. This should not happen. Real propulsion systems like rockets always create thrust in the direction the combustion chamber is pointing, regardless of the direction the chamber is aimed at. If the chamber is aimed North the thrust should be in a northernly direction. If the chamber is aimed East the thrust should be easterly. Something is rotten in the state of Denmark if you aim the chamber East and find the thrust is in a southern direction.
You would be understandably surprised if you aimed a rocket at the ground but when the burn started the rocket climbed into orbit moving backwards instead of augering into the ground. Yet this is what the instruments indicated that the EmDrive was doing.
Which logically leads skeptical scientists to wonder how accurate the thrust measuring instrument is.
Yep, that was the problem. The cables that carry the current to the microwave amplifier run along the arm of the torsion bar (the thrust measuring instrument). Although the cable is shielded, it is not perfect (because the researchers did not have enough mu metal). As it turns out the Earth's magnetic field causes the current in the cable to create a force pushing the cable sideways. Since the cable was attached to the arm of the torsion bar, this pushed the torsion bar sideways as well, which made a false reading that the microwave cavity was creating thrust. And if you turned the EmDrive to point in the opposite direction, this of course changed its orientation relative to the Earth's magnetic field, reversing the direction of thrust.
Bottom line: The EmDrive does not work, it produces zero thrust. The problem was the thrust measuring instrument was lying.
Like many such drives, the heart of the gizmo is an oscillator. A sizeable mass is moved first forwards, then backwards. Due to Newton's third law, the spacecraft will move in the opposite manner: first backwards, then forwards. With a conventional oscillator the spacecraft winds up in the exact same spot it started since the motion forwards then backwards cancel each other out.
And like many such drives, the inventor is trying to unbalance the oscillator. If this can be done, the two motions will not cancel out, and we will have our magic space drive. The unbalancing is achieved by changing the weight of the sizable mass, so it is heavy when moving forwards, then somehow lighter when moving backwards.
How the heck can that be accomplished? The weight change must be such that it does not change the momentum of the system, which is a challenge.
The report suggests replacing the solid sizable mass with a ring of ions moving in a circle. The ring is orthogonal to the oscillator motion. By making the ions move at relativistic speeds the mass of the ion ring can be altered.
As always physicists cannot prove that the law of conservation of momentum is inviolate, but that's the way to bet. The Helical Engine certainly runs afoul of it.
Chris Lee offers this critique:
I had thought that one could hand-wave a reactionless drive but control it with some kind of limit on the damage. Specifically I thought that one could figure the kilowatt equivalent of the momentum change created by such a drive, and use that as the required power.
The underlying problem is that breaking the law of conservation of momentum shatters the entire mathematical framework. The specific problem is that you will get different values for the kinetic energy expended depending upon the reference frame of the observer.
Isaac Kuo said:
Dr. John Schilling said:
Why doesn't this reference frame problem occur with an ordinary rocket? Isaac explains:
Science Fiction author (and holder of two degrees in Physics) Thomas Mays came up with a marvelous unobtainium idea that sure acts like a reactionless drive, but it isn't. He used it in his short story "Bumped".
Ordinary matter in general and rocket reaction mass in particular transfers momentum by atoms colliding with each other. In Thomas Mays' gadget, there is still momentum transfer by collision but it is non-local. Essentially they are transferring their momentum through microscopic wormholes. So you could, say, transfer some momentum from part of Luna to your spaceship. Some of orbital momentum of Lunar crust orbiting Terra and orbiting Sol is stolen and transferred to the spacecraft, as if they had collided. Only they could be millions of kilometers apart, because wormholes.
The point being that the spacecraft does not have to carry its reaction mass. Which instantly frees the spacecraft from the Tyranny of the Rocket Equation, and gives you the benefits and problems of a reactionless drive (even though it technically is not reactionless). It still violates Burnside's Advice, though.
It is similar but not quite the same as the Challenger from Tom Swift in the Race to the Moon. It uses "repellatrons" (read "tractor beams" or "repulsors") which repel Terra thus propelling the spacecraft upwards. Basically it is using Terra as reaction mass. Another related concept is doing an end run around spacecraft mass ratio by somehow "teleporting" (read "Star Trek Transporter") the reaction mass from home base to the spacecraft's propellant tanks.
Photon drives have the ultimate exhaust velocity. You can't get faster than the speed of light, if you make a rocket with an FTL exhaust the shade of Albert Einstein will rise from the grave and give you an atomic wedgie. Such high exhaust velocites make for truely awesome delta V.
Even better: since you are not expending propellant, you will never run out. So if you never run out of propellant and never run out of "fuel" it means you have something like a Bussard Ramjet on steroids. Without all the pesky fuel scooping problems.
The fly in the ointment is that you will be expending energy like crazy. You will pay in energy as if you borrowed a few gigawatts from Sparky the Loan Shark. We are talking Three! Hundred! Megawatts! for one solitary pathetic newton of thrust.
Science fiction authors, hungry for the ultimate torchship, quickly started looking for some bottomless source of torrents of energy so they can feed their thirsty photon drives. It didn't take them long to find Zero-Point energy.
The fly in that ointment is that physicists cannot figure out if the energy is at worthwhile levels and have no idea how to extract vacuum energy. Except for Dr. Robert Forward's Charged Foliated batteries, and they are more an energy storage device than an energy source. But that didn't stop the science fiction authors.
Vacuum energy was used in All the Colors of the Vacuum by Charles Sheffield, Encounter with Tiber by Buzz Aldrin John Barnes, and The Songs of Distant Earth by Sir Arthur C. Clarke.
Arguably the Grand Unified Theory (GUT) drives and GUTships in Stephen Baxter's Xeelee novels are also a species of vacuum energy power sources.
But don't forget the Jon's Law. A gamma-ray laser hooked up to a vacuum energy power source will make the Death Star's main weapon look like a damp firecracker. You'll be able to punch holes in Jupiter.
Closely related to reactionless drives, but which are not, is Inertail Dampening. This is some handwavium method to accelerate a spacecraft without the injuring the crew (or crushing them into a thin layer of bloody goo on the floor).
In order to keep things logical, this section can be found here.