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."
Which 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.
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.
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 rule of thumb 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.
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 unobtanium 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.
Several times in science fiction, a reactionless drive is invented. And then the scientist gets the bright idea that if they mount the drive inside a submarine they will have Instant Spaceship.
In reality this would not work very well. A submarine is build to resist stronger pressure outside pressing in, not stronger pressure inside pressing out. And if the submarine is nuclear powered, you had better attach some kind of heat radiator. Nuclear submarines get rid of heat by sucking in cold ocean water and spewing out hot heat sink water. This won't work in space, there isn't any ocean. Not to mention the fact that a sub nuclear reactor's coolant system requires gravity to work.
This trope seems to have been invented by John W. Campbell jr., in an article he wrote about the Dean Drive in 1960. Other novels that use this theme include The Daleth Effect by Harry Harrison (1969), Gilpin's Space by Reginald Bretnor (1983), Salvage and Destroy by Edward Llewellyn (1984), and Vorpal Blade by John Ringo (2007). There is a mention of an "inertial drive" (another name for a Dean Drive) in Randall Garrett's Anything You Can Do but there it is used as a way to make recon drones float in the air.
This also seems to have influenced a certain Matt Jeffries, designer of the original Starship Enterprise, Klingon Battle Cruiser, and related works. A couple of his designs feature a "sail" or "conning tower" which are common to submarines. Perhaps he read Campbell's Dean Drive article and was inspired. If the first few starships were actually refitted submarines, maybe purpose-built starships would retain the conning tower for tradition.
The first Matt Jeffries design with a conning tower was the Botany Bay aka DY-100 from the Star Trek episode "Space Seed." It was later re-used as Automated Ore Freighter Woden in "The Ultimate Computer".
Around 1967, the AMT plastic model company wanted to cash in on Star Trek mania. They wanted to make a line of plastic model starship kits, but of their own design. So they hired Matt Jeffries to make a starship, the Galactic Cruiser Leif Ericson. Again it had the signature submarine conning tower. Unfortunately the kit was a financial disappointment, and further starships in the line were cancelled. The kit was re-issued in 2011 due to demand from those who had the original kit when they were young.
In the early 1970's, when Larry Niven and Jerry Pournelle were writing the classic The Mote in God's Eye, they used the Leif model as the inspiration for the INSS MacArthur.
Around 1975 Matt Jefferies was hired by George Pal to work on a TV series based on THE WAR OF THE WORLDS. As you can see the Hyperspace Carrier Pegasus is an outgrowth of the Leif Ericson. Note that instead of two side engines, the Pegasus has four, two on each side. For the TV series, Jefferies actually had the Pegasus upside down in relation to the Leif Ericson, in order to make the connection less obvious. The TV series was never picked up, alas. But this is a facinating glimpse of what might have been.
Occasionally in later science fiction illustrations one again finds the submarine conning tower.
Analog December 1969. Illustration for Harry Harrison's "In Our Hands, The Stars", which was later expanded into the novel The Daleth Effect.
Harry Harrison wrote an amusing but cautionary tale called The Daleth Effect. In the novel, an Israeli scientist discover the principle for a reactionless drive. Naturally the first real test is the Submarine Spacecraft trick.
He returns to his native Denmark to develop it. He wishes to develop the idea without it falling into the hands of the military, since it also has potential as a weapon. Good luck with that.
Denmark keeps it a secret until they feel obligated to use the technology in public to rescue some cosmonauts stranded on Luna. Any fool could have told the Danes that no good deed goes unpunished.
Naturally the US, Soviet Union, and other powerful nations will stop at nothing to lay their hands on this technology. The race is on! They try all sorts of tactics to pressure the Danes but to no avail. They look on with helpless rage as the Danes establish a Lunar base and make a large ship for a visit to Mars.
Like absolute idiots the Danes invite foreign dignitaries to ride on the Mars trip. Naturally pretty much 100% of the dignitaries turn out to be secret agents. Hilarity ensues. And then the novel has a most ironic and satisfying ending.
Eccentric but brilliant scientist Saul Gilpin invents a magic hyperspace faster-than-light propulsion system / antigravity surface-to-orbit gadget which can be cobbled together from parts available from your local hardware store. He mounts it on a submarine and has instant starship. Then he and the submarine depart for parts unknown.
This makes the totalitarian government very unhappy. They want to use this technology, they do not want citizens getting their hands on it. Makes it far to easy to escape the totalitarian state. Then they find out that Gilpin has mailed blueprints of the gadget to quite a few people. Hilarity ensues.
An ancient alien interstellar empire is worried about the large US and Soviet submarine fleets. Once Earth discovered anti-grav and FTL drives, the warlike unstable Earthlings would have a ready-made fleet of combat starships. This could turn into a nasty problem.