Science fiction inventor with physics question

Ryan_m_b. Maybe you are right I don't like the answer, so I want it to be what I want. All right then.

Ryan_m_b
Staff Emeritus
Ryan_m_b. Maybe you are right I don't like the answer, so I want it to be what I want. All right then.
Remember as well this isn't just abstract maths, these conclusions are based on experimentally verified physical models of the world we live in.

D H
Staff Emeritus
D H I wonder if the math of all that rocket fuel required to propel my ice ship takes into consideration a certain bizzare thing about matter. For example in a room full of warm air out of the cold there are particles that have very extreme temperatures both high and low. I mean to say that not all the air particles are a cozy 76 degrees farenheit.
The answer is yes and no.

The yes part first: The term ve in the ideal rocket equation denotes the effective velocity of the exhaust. This term incorporates into a single number all kinds of things from the real world such as back pressure, variations in the velocities of molecules in the exhaust stream, etc.

Now for the no part, and it's nasty: The ideal rocket equation describes an ideal rocket, one in which the exhaust stream is at absolute zero and is perfectly collimated (all exhaust particles are moving in the same direction). That the exhaust stream in a real rocket is not at absolute zero means the rocket is not drawing on all of the energy available from whatever reaction created the exhaust stream. Some of the energy is wasted in the form of heat. That the exhaust is not perfectly collimated means that some of the momentum, and thus even more of the available energy, is lost. A real rocket will always perform worse than an ideal one. The laws of thermodynamics get in the way. The only way to have an ideal rocket is to have an infinitely long, perfectly shaped rocket nozzle. Good luck with that, and even if you did have such a device, it would have infinite mass. It would go nowhere.

As nasty as the ideal rocket equation is, things get even nastier when the spaceship's velocity starts getting even close to that of the speed of light. The ideal rocket equation is not valid if you want to make your rocket propelled spaceship go very, very fast. You need to use the relativistic rocket equation instead, and this equation makes the ideal rocket equation look like child's play.

So what if some of the particles in the exhaust of the rocket are very close to the speed of light. So that to consider the exhaust effectiveness you might multiply the exhausts over all relativity factor with the exhausts mass and velocity which would have to equal the rockets mass and velocity.
Where does this energy come from? You cannot just wave a magic wand and have energy appear from nowhere. And sorry, zero point energy doesn't work. The energy of whatever reaction coupled with the masses of the particles that comprise the exhaust sets an upper limit on the exhaust velocity.

What if you use photons, converting all of the energy from the reaction to photons? That's the best possible exhaust velocity, right? Wrong, unless your reaction is matter/antimatter annihilation, and then good luck getting/containing any sizable amount of antimatter, and good luck collimating photons that are well into the gamma range. With anything but matter/antimatter annihilation, photons are just about the worst choice. Divide the entire world's annual energy consumption, 5×1020 joules, by the speed of light and you get 53,000 newtons. That's what photons give you: Not much. Photons have very little oomph. That thrust is about 1% of the thrust produced by SpaceX's Falcon 9, 0.16% of the thrust produced by the Saturn V's first stage. To make matters worse, you are putting all of the energy out in the form of photons, so what do you do with the byproducts of your fusion? You are faced with keeping those byproducts onboard or dumping them with the exhaust. The former leaves you with a device that is even worse than a rocket, while the latter reduces the effective exhaust velocity to much less than the speed of light.

I just think that is something we should have to consider when making these rocket equations,and it is not something I can prove mathematically because I haven't gone through my college physics class yet but it is something I can feel with my gut.
Then you ought to take some physics. Your gut feelings, fed by too much bad sci fi, are wrong.

BobG
Homework Helper
It is something for the rocket to push against because I really don't believe it would take that much mass to propell something through space like that.
There is nothing for the rocket to push against. Rocket propulsion is simple conservation of momentum. There can be no net chang in momentum. If you throw stuff out the back, the forward momentum has to increase to keep the net momentum zero.

The rocket equation hides a bit of that because of the strange units - specific impulse in seconds and fuel rate in newtons/second, pounds per second. If you look at the units, you'll realize what you really have is mass times velocity of the fuel going out the back.

That does mean there's some hope. It takes energy to throw the stuff out the back at incredibly high rates of speed and that energy doesn't have to come from the fuel itself.

For example, ion thrusters use electrical energy to create acceleration of charged particles via a magnetic field. On satellites (which are moving through a near vacuum at relatively slow speeds compared to what you're looking for), the energy to generate that magnetic field comes from the solar arrays; not from the fuel. The specific impulse is around 3000 seconds, which is very efficient. Of course, the downside is a 25cm thruster, using 4500 Watts of power, can only accelerate very tiny amounts of fuel. Thrust is measured in milliNewtons (somewhere around 70 to 90 milliNewtons, but I don't really remember the exact number). It will take over a 10 to accelerate 1 kg at 1 meter/second squared. (And, in case you're wondering, these are used for attitude control and station keeping; not major changes in the orbit.)

Realistically, you won't get interstellar travel because you invent new technology. To get interstellar travel, you have to discover some new physics.

O.K. so what if I have a magical mass to energy converter converting all fuel mass to energy and could apply 100% of the energy to a photon drive. What's the best I could hope for?

Ryan_m_b
Staff Emeritus
O.K. so what if I have a magical mass to energy converter converting all fuel mass to energy and could apply 100% of the energy to a photon drive. What's the best I could hope for?
That's what D H was describing with the antimatter photon rocket above.

BobG
Homework Helper
It is science fiction, so you just do what most authors do. Just give the engine/thruster some cool sounding name and just don't explain how it's able to obtain relativistic speeds.

Or how a star fighter is able to behave like a fighter jet in outerspace.

Etc.

If you look at the most popular science fiction books, TV shows, movies; having a good story line and good characters is more important than the science. You could take the best stories out of the science fiction realm and put them into the time of Alexander the Great, the Roman Empire, or World War II and still have a great story.

For example: The movie that best compares to the original Star Trek TV series? Master and Commander! Even though the story is set during the Napoleonic Wars, the interplay between the Captain and the Doctor feels very much like Captain Kirk's relationship with Dr McCoy and Spock.

Another example: Asimov's Foundation Series. It's based on the Fall of the Roman Empire! But with a "what if" twist to find a way to reestablish the Empire at some future date - at least until the series took on a life of its own and Asimov just took it wherever (it was originally a serial for a magazine and the most important thing was to ensure readers wanted another installment so Asimov could make another paycheck, seeing as how he wasn't a famous, successful author yet).

D H
Staff Emeritus
It is science fiction, so you just do what most authors do. Just give the engine/thruster some cool sounding name and just don't explain how it's able to obtain relativistic speeds.
Bingo. The MacGuffin Drive, or something like that. Even PhD physicists who write science fiction on the side do this. In fact, they know they have to do this because the only way to make science fiction compatible with known science is to write about spaceships that take generations to get from star A to star B, or write about races to whom a thousand years is but a blink of the eye, or just stop writing science fiction altogether. Amateurs who try to make their science fiction realistic typically end up with egg all over their faces.

If you look at the most popular science fiction books, TV shows, movies; having a good story line and good characters is more important than the science.
Exactly. The best science fiction is about people, not machines. The thingy that lets the people in the story quickly flit from place to place is just a plot device, aka a MacGuffin. Hence my name the MacGuffin Drive. Plot devices help the writer write, and help keep the reader engaged. Done right and plot devices don't need a lot of motivation or justification. In fact, an author who feels a need to provide that motivation or justification should see that as indicating that the device is being used incorrectly.

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Ryan_m_b
Staff Emeritus
Exactly. The best science fiction is about people, not machines. The thingy that lets the people in the story quickly flit from place to place is just a plot device, aka a MacGuffin. Hence my name the MacGuffin Drive. Plot devices help the writer write, and help keep the reader engaged. Done right and plot devices don't need a lot of motivation or justification. In fact, an author who feels a need to provide that motivation or justification should see that as indicating that the device is being used incorrectly.
It's a matter of taste but personally I think this is an example of the worst kind of science fiction. That doesn't mean that it's bad by any stretch, just that it's a plot made possible by science fantasy rather than an exploration of the consequences of various scientificesque plot devices.

This really falls back on how hard you like your SF boiled but personally the type of SF that proposes no or very few overly-speculative plot devices and explores the social consequences of those propositions is what I consider good SF; not a variation of the Napoleonic wars in space.

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See that's where I wanted to go with this. All I really was looking for is the possibility of using ice as a starship hull. I didn't want to have to justify how the ice ship moved through space. For that I was going to use simple physics numbers to say it has a superfusion engine with spacetime compressors that allow the ship to go through more densities of spacetime than it normally would in any given moment in uncompressed spacetime. That I already had worked out.

Ryan_m_b
Staff Emeritus
See that's where I wanted to go with this. All I really was looking for is the possibility of using ice as a starship hull. I didn't want to have to justify how the ice ship moved through space. For that I was going to use simple physics numbers to say it has a superfusion engine with spacetime compressors that allow the ship to go through more densities of spacetime than it normally would in any given moment in uncompressed spacetime. That I already had worked out.
A few tips;
• When choosing technobabble terminology be wary of the words you choose. You've got to take into account both the consternation of people educated in the field and boredom of those who aren't (in other words watch out for terms like spacetime density compression that may irk scientists for being nonsense and bore laypeople for being innacessable).

• Watch out for over-explaning. Mystery is a great asset in story telling, it brings a sense of depth to the setting and allows the reader to fill in some of the blanks themselves (as well as hook them for more).

• Be consistent with your plot devices. Science fiction and fantasy are hotbeds for imagination and innovation, readers are going to be thinking of different ways the technology presented could be used. If they come accross an obvious use that is ignored (especially if it is a solution to an obstacle the characters must overcome) they will feel the world is broken. Generic example; if the setting includes the good-guy fleet outnumbered by the bad-guy fleet but also includes self replicating machines or very advanced autonomy/robotics/AI people will wonder why the protaginists are for some reason lacking in industry/numbers.

• Be intelligent with the ramifications of your plot devices. As I mentioned in the previous point there are obvious economical and industrial considerations for advanced autonomy. Good science fiction explores the ramifications of speculative science/technology (e.g. addressing unemployment, technosocialism, post-industrial economics, Jevons paradox etc in an increasingly automated society), it does not just use it to prop up a plot.

jim hardy
Gold Member
2019 Award
Dearly Missed
Exactly. The best science fiction is about people, not machines.
agreed... my all-time favorite science fiction short story was this simple one
where the spceship is an assemblage of kurmudgeonly living organisms each with its own personality. The author poked fun at our foibles, and probably at bureaucracy. It was written aboout the same time as 'Parkinson's Law'.

http://en.wikipedia.org/wiki/Specialist_(short_story [Broken])

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Again you misinterpret me; I'm not writing a novel or choosing plot devices I'm inventing things for people who play tabletop roleplaying games, such as battletech, star trek space combat simulator, star wars the role playing game, etc... All of which can get pretty inventive if you want to have a fun game. That's why I made the Ice ship. I figure an Imperial cruiser could have a time of firing turbolaser batteries at the Ice ship before destroying it. anyways I'm done with the idea now and am moving on to turning a rotating asteroid into an electrical power generator but then that would be another discussion.

Ryan_m_b
Staff Emeritus