Space propulsion

[Major Maxillary]

I was perusing some concepts to make my spaceships go vroom in my story, and since my expertise ends just after artillery, I'd like some help.

so in this thread we talk about things like bussard ramjets and magnetoplasmadynamic thrusters.

I was planning on multiple modes of propulsion. one for interstellar, one for interplanitary, one for orbit, and one for atmosphere, the last two can be combined.

[Surlethe]

You do recall Newton's laws of motion, correct? Essentially, every type of propulsion consists of throwing stuff behind you. When you're in the atmosphere, you can take things from in front of you (air) and throw them behind you, like in a propeller or jet. In space, you're in a vacuum, so you pretty much have to take your propellant with you. The different types of engines are simply different ways of taking your propellant and throwing it behind you as fast as possible in order to accelerate yourself.

[Molyneux]

You do recall Newton's laws of motion, correct? Essentially, every type of propulsion consists of throwing stuff behind you. When you're in the atmosphere, you can take things from in front of you (air) and throw them behind you, like in a propeller or jet. In space, you're in a vacuum, so you pretty much have to take your propellant with you. The different types of engines are simply different ways of taking your propellant and throwing it behind you as fast as possible in order to accelerate yourself.

Even some of the ubertech ways work the same way; the Star Trek warpdrive is basically bunching up space in front of you and stretching it out behind, isn't it?

[Feil]

If you have the kind of power generation usually assumed for sci-fi (in other words, an ungodly powerful one) ion drives are the way to go, since you can make your supply of reaction mass go a long, long way.

For anything particularly big, you'll either need really big deployable wings or really big deployable rotors. Jet engines won't work unless you have an oxygen-rich atmosphere, so you're probably better off with propellers or rockets if you want to land anywhere but terraformed worlds.

More than likely, any spaceship worthy of the name would be purely space-born, and cary a boat or two to make landings, simply because of the immence forces that the ship would have to be able to provide in order to avoid crashing under its own weight.

[GrandMasterTerwynn]

I was perusing some concepts to make my spaceships go vroom in my story, and since my expertise ends just after artillery, I'd like some help.

so in this thread we talk about things like bussard ramjets and magnetoplasmadynamic thrusters.

I was planning on multiple modes of propulsion. one for interstellar, one for interplanitary, one for orbit, and one for atmosphere, the last two can be combined.

Depends on how realistic you want to get. Because in the universe we happen to live in, interstellar drives are confined to just four types.
1) Some sort of matter/antimatter driven rocket.
2) Some sort of fusion rocket which may or may not use antimatter for an assist. Fusion rockets can fall into two categories . . . those which carry all their fuel with them, and those which only carry enough fuel to kick them up to the 3% to 5% of the speed of light needed to deploy a Bussard ramjet.
3) Some sort of fission-based rocket used to drive a multi-generational starship.
4) Enormous sail riding a laser beam fired from your home system. Obviously this presents enormous engineering challenges and isn't terribly feasible.

Out on the fringes there may be ways to use some sort of fine-control of fundamental forces to drive your spaceship, but this is, as I mentioned, out on the fringe.

For interplanetary work, the same fusion or fission drive you might employ in interstellar space would also work in interplanetary space. Or something purely fission like a nuclear salt-water rocket. Though you might choose to use an onboard fusion/fission reactor to power an ion/plasma drive.

For orbital work, all you need are small thrusters. Gas or plasma. These are the same small thrusters you'd normally use to change the orientation of your ship, regardless of where the ship might be.

This is the point where the progression of drives for a large interplanetary/interstellar spacecraft ends. Such a beast is much too large to get into orbit from the ground, or from ground into orbit. Not without doing the equivalent of setting off a large thermonuclear weapon right where the starship is taking off, or landing at.

What you'd use, instead, would be small parasite craft. Call them shuttles or boats or pinnaces or landing craft, whatever. You'd ideally make them lifting bodies, so you can get them into the atmosphere and landed while expending a minimum of fuel to do so. They would be powered by some sort of fancy chemical rocket or jet engine/rocket hybrid with chemical/gas thrusters for orientation in space.

[Sikon]

For hard science fiction based on real-world physics:

Most engine concepts are either high thrust or high specific impulse (efficiency/performance) but not both. For example, ion drives can be efficient for interplanetary use but operate at far too low thrust-to-weight ratio to take off from a planet. In general, heat dissipation and material limitations restrict engine performance just as much as power generation. Even today's nuclear reactors are limited not by the power generation capabilities of the fuel but rather by how much heat can be handled without melting the materials.

The most practical choice tends to be the ship having shuttles as suggested by GrandMasterTerwynn and Feil above.

However, if you do want to have planetary takeoff and good space travel capabilities in the same craft, the highest performance practical method would be a pulse fusion engine. A pulsed engine design like the Orion concept can handle a huge amount of power without its materials melting, allowing both high specific impulse and high thrust at once. It is no coincidence that the Orion concept is the only design ever proposed in the real-world capable of putting most of its mass into orbit and then traveling interplanetary distances quickly.*

Chemical rocket engines can put only a few percent of original liftoff mass into orbit. There are concepts for nuclear reactor engines, but those suitable for planetary liftoff give only moderate performance gains over chemical rockets, due to material limitations restricting the propellant temperature obtained.

In today's world, the Orion project concept is politically implausible. It would use multi-kiloton fission-based nuclear bombs as its pulse units. However, a science-fiction civilization could easily have the technology to initiate fusion without fission triggers, scalable down to any size yield. The detonations could be quite small, preventing any trouble from EMP pulses. Technology to avoid use of fission triggers does not exist today, but it is just as possible as fusion reactors.

* There is an alternative possibility which could allow a spaceship to take off from planets without using almost any reaction-mass at all: On planets with a magnetic field like earth, a civilization with superconductor performance comparable to the real world or better could make a ship able to fly off the planet by levitating at the poles. The principle could potentially also be used in the extremely weak interplanetary and interstellar magnetic fields, but probably the ship would have regular engines for space.

[Ariphaos]

I was planning on multiple modes of propulsion. one for interstellar, one for interplanitary, one for orbit, and one for atmosphere, the last two can be combined.

Any sort of FTL system is going to be a bit magicky. You can read up on Alcubierre's stuff and the people who wrote supplementary papers but in the end as long as you make something consistant you should do just fine.

Anyway, for the low-tech stuff in my setting...

Pre-FTL interstellar work was done using fusion propulsion, the first missions of which set up vast coil arrays - basically, the receiving star system set up a giant gauss cannon, aimed at a very specific point in the Solar System. Earth set up a number of these arrays, pointing to Alpha Centauri, Sirius, Procyon, 61 Cygni, Tau Ceti, Epsilon Indi and Epsilon Eridani. These accelerated their targets to - and back down from - a significant fraction of the speed of light.

For orbit and interplanetary, on the non-magitech scale my universe uses a 'fusion drive'. It's essentially a plasma thruster and the first examples of it would be lucky to get to 1% of a G with any efficiency (in fact some models had the fusion basically occur outside the ship). Combat with these is generally out of the question, except at extreme ranges.

Orbital combat vessels still use chemical propulsion in my setting, though they are not designed to be long-lived. Some lower ones are basically orbital scramjets - they raid the planet's upper atmosphere for methane, hydrogen, or oxygen.

Atmospheric combat is a combination of advanced helicopters and planes. Ionocraft and blimps are more prolific in denser atmospheres, but are generally not for combat.

For getting into orbit, my setting makes rather extensive use of space elevators. Failing that, blimps carry scramjet-types high into the atmosphere (the blimps themselves are capable of decent submach speeds).

[ThatGuyFromThatPlace]

You do recall Newton's laws of motion, correct? Essentially, every type of propulsion consists of throwing stuff behind you. When you're in the atmosphere, you can take things from in front of you (air) and throw them behind you, like in a propeller or jet. In space, you're in a vacuum, so you pretty much have to take your propellant with you. The different types of engines are simply different ways of taking your propellant and throwing it behind you as fast as possible in order to accelerate yourself.

Even some of the ubertech ways work the same way; the Star Trek warpdrive is basically bunching up space in front of you and stretching it out behind, isn't it?

That's the Alcubierre Warp Drive Actually, which has been mathematically proven but we don't know how to bend space that way yet.

For interplanetary, I like the Orion Drive but less 'interesting' fusion methods work just as well.

[Major Maxillary]

You do recall Newton's laws of motion, correct? Essentially, every type of propulsion consists of throwing stuff behind you. When you're in the atmosphere, you can take things from in front of you (air) and throw them behind you, like in a propeller or jet. In space, you're in a vacuum, so you pretty much have to take your propellant with you. The different types of engines are simply different ways of taking your propellant and throwing it behind you as fast as possible in order to accelerate yourself.

Even some of the ubertech ways work the same way; the Star Trek warpdrive is basically bunching up space in front of you and stretching it out behind, isn't it?

That's the Alcubierre Warp Drive Actually, which has been mathematically proven but we don't know how to bend space that way yet.

For interplanetary, I like the Orion Drive but less 'interesting' fusion methods work just as well.

And then there's this thing that I saw once on one of those science shows that rather than pushing on your ship, it pulls on the nearest gravity well causing you to move towoards it... anyway, I'm compfortable with warp for quick in-system trips and hyperspace gadgetry for interstellar travel.

If you have the kind of power generation usually assumed for sci-fi (in other words, an ungodly powerful one) ion drives are the way to go, since you can make your supply of reaction mass go a long, long way.

I still haven't decided on the powerplants.

fission? fusion? antimatter? magic? baby powered steam engine? Who knows!

however, it's safe to assume that considdering these ships needlots of energy just to power their giant electromagnet guns and electrofied armor alone, they'll need to have a metric shitton of voltage.

For getting into orbit, my setting makes rather extensive use of space elevators. Failing that, blimps carry scramjet-types high into the atmosphere (the blimps themselves are capable of decent submach speeds).

I was thinking on just, you know, keeping big ships in space and giving the smaller ships magnetic lift gizmoes which counter the planet's magnetic field.

[Arrow]

I still haven't decided on the powerplants.

fission? fusion? antimatter? magic? baby powered steam engine? Who knows!

however, it's safe to assume that considdering these ships needlots of energy just to power their giant electromagnet guns and electrofied armor alone, they'll need to have a metric shitton of voltage.

Fission is a possibility, but I'd perfer fusion. You could have your reactors scale to very high power outputs, if your material tech can handle it, and if you have the fuel to sustain the output for the required time. Antimatter is expensive as hell to make, and not safe to handle in the quantities required to power a starship (or most anything else, for that matter). If you don't mind a little bit of handwavium, you could have a total conversion reactor that uses some method to directly convert matter to energy without using antimatter (say, a field to surpress the strong force). You could 'tune' the device to have characteristics you want (ie x% mass is converted to energy at y frequency, which is then absorbed by uber-photoelectric panels or heats fluids to power a turbine). Total conversion reactors will have much higher fuel efficiency than a fusion reactor.

[Admiral Valdemar]

About the Alcubierre Warp Drive, does anyone know how it addresses the problems of causality for instance? If you're going FTL, then you suddenly have a very handy time-machine.

[Arrow]

Ok, I know Wikipedia isn't a reliably source, but this article claims:

For those familiar with the effects of special relativity, such as Lorentz contraction and time dilation, the Alcubierre metric has some apparently peculiar aspects. Since a ship at the center of the moving volume of the metric is at rest with respect to locally flat space, there are no relativistic mass increase or time dilation effects. The on-board spaceship clock runs at the same speed as the clock of an external observer, and that observer will detect no increase in the mass of the moving ship, even when it travels at FTL speeds. Moreover, Alcubierre has shown that even when the ship is accelerating, it travels on a free-fall geodesic. In other words, a ship using the warp to accelerate and decelerate is always in free fall, and the crew would experience no accelerational g-forces. Enormous tidal forces would be present near the edges of the flat-space volume because of the large space curvature there, but by suitable specification of the metric, these would be made very small within the volume occupied by the ship.

Take with the approprate dosage of salt.

As for my own universe, which uses the Alcubierre warp drive and wormholes, defined the in-universe definitions such that any warp field or wormhole configuration that premits time travel is physical impossible - I don't want to have to deal with time travel in my setting.

[GrandMasterTerwynn]

About the Alcubierre Warp Drive, does anyone know how it addresses the problems of causality for instance? If you're going FTL, then you suddenly have a very handy time-machine.

I believe, since one is warping space, it doesn't actually violate causality, since it's spacetime, and the photons and everything else being conveyed on it, that's being stretched and compressed.

The biggest problem with the Alcubierre drive is that it requires large quantities of exotic matter with imaginary mass (how one generates this is an exercise left to the reader,) and a similarly enormous energy investment (something like the energy content one would liberate by combining Jupiter-sized masses of matter and antimatter, though early estimates involved the energy content of the entire universe) just to get working.

[Admiral Valdemar]

So, in essence, it's about as possible as any other FTL idea. Guess that leaves just removing oneself from this brane and travelling via hyperspace. That just assumes hyperspace doesn't follow relativity or causality and that one can even travel into the bulk at will.

[Patrick Degan]

About the Alcubierre Warp Drive, does anyone know how it addresses the problems of causality for instance? If you're going FTL, then you suddenly have a very handy time-machine.

I believe, since one is warping space, it doesn't actually violate causality, since it's spacetime, and the photons and everything else being conveyed on it, that's being stretched and compressed.

The biggest problem with the Alcubierre drive is that it requires large quantities of exotic matter with imaginary mass (how one generates this is an exercise left to the reader,) and a similarly enormous energy investment (something like the energy content one would liberate by combining Jupiter-sized masses of matter and antimatter, though early estimates involved the energy content of the entire universe) just to get working.

The other problem with it is that the warp can only propagate at c. Sure, once you've got it established from A to B, you'll get there really really fast. But you'll be waiting an awfully long time for that transitway to open.

[Admiral Valdemar]

It would be better just travelling at one gee acceleration constantly with a drive that doesn't need to carry its own reaction mass and has a shitload of energy. You could circumnavigate the known universe with a subjective ship time of just a couple of decades, course, real-time would go by like a blur.

[Arrow]

The other problem with it is that the warp can only propagate at c. Sure, once you've got it established from A to B, you'll get there really really fast. But you'll be waiting an awfully long time for that transitway to open.

Well shit, back to the drawing board for my universe's FTL drive...

[Patrick Degan]

The other problem with it is that the warp can only propagate at c. Sure, once you've got it established from A to B, you'll get there really really fast. But you'll be waiting an awfully long time for that transitway to open.

Well shit, back to the drawing board for my universe's FTL drive...

Sort of depends. If you want to posit a galactic society which has essentially spent millenia colonising its way across the galaxy and establishing Alcubierrie transit lanes between systems, it can still work out well enough. It simply means that your society has been a few centuries to a few thousand years in the making.

[Sikon]

I was thinking on just, you know, keeping big ships in space and giving the smaller ships magnetic lift gizmoes which counter the planet's magnetic field.

That would work. Many planets would not naturally have a suitable magnetic field like earth, but any terraformed planets might have one created artificially.

If one has shuttles with large superconducting coils of the needed critical current (critical magnetic field) performance for such magnetic flight, they also could be used for artificial gravity if desired. Using sufficiently strong magnetic fields to diamagnetically levitate frogs has been possible for years as described here.

Higher performance superconductors could create a field of much more than 1g effective gravity or effective antigravity.

Effective artificial gravity can also be created just with rotating sections.

However, intense magnetic fields could be used as a form of inertia compensation. Such is common in soft science fiction, but this would differ from everything I have seen by being possible within known physics. For example, a warship could have high acceleration beyond what would otherwise be too much for the crew.

It would be better just travelling at one gee acceleration constantly with a drive that doesn't need to carry its own reaction mass and has a shitload of energy. You could circumnavigate the known universe with a subjective ship time of just a couple of decades, course, real-time would go by like a blur.

That is definitely an option in science fiction. However, one could get a similar effect with far greater scientific plausibility by an alternative method.

Just have a crew able to go into suspended animation.

One option is simply to go the soft science fiction route, having a FTL drive, with no need to explain how it works.

However, if one doesn't do that, suspended animation is known to be totally possible within known scientific laws, while reactionless drives have a limited likelihood comparable to FTL.

At most, a little genetic engineering (or post-biological existence) would be sufficient, but even that is likely to be unnecessary. For example, see the article here on introduction of hibernation in rats using a technique suspected to be applicable to other mammals including humans.

If you have suspended animation, the ship can just travel at most of lightspeed with an ordinary antimatter drive and still have the passage of subjective time be arbitrarily fast.

[Major Maxillary]

Fission is a possibility, but I'd perfer fusion. You could have your reactors scale to very high power outputs, if your material tech can handle it, and if you have the fuel to sustain the output for the required time. Antimatter is expensive as hell to make, and not safe to handle in the quantities required to power a starship (or most anything else, for that matter). If you don't mind a little bit of handwavium, you could have a total conversion reactor that uses some method to directly convert matter to energy without using antimatter (say, a field to surpress the strong force). You could 'tune' the device to have characteristics you want (ie x% mass is converted to energy at y frequency, which is then absorbed by uber-photoelectric panels or heats fluids to power a turbine). Total conversion reactors will have much higher fuel efficiency than a fusion reactor.

Is it possible to make a fusion bomb?

[RedImperator]

Is it possible to make a fusion bomb?

What do you think the hydrogen bomb is actually doing with that hydrogen?

[Major Maxillary]

Is it possible to make a fusion bomb?

What do you think the hydrogen bomb is actually doing with that hydrogen?

I completely forgot about that sucker.

[Kuroneko]

About the Alcubierre Warp Drive, does anyone know how it addresses the problems of causality for instance? If you're going FTL, then you suddenly have a very handy time-machine.

Well, that depends. If arbitrary creation and destruction of superluminal warp bubbles is allowed, I believe causality can be violated. However, superluminal bubbles also have horizons which would presumably forbid their creation. Even in the best-case scenario when one can create a superluminal bubble, there doesn't appear to be any way to make it stop because of the horizon. The lone Alcubierre-type metric, of course, doesn't have any causal problems, so in a sense there are no causal problems at all for a stable superluminal bubble, because it becomes causally disconnected from the rest of the universe.

The biggest problem with the Alcubierre drive is that it requires large quantities of exotic matter with imaginary mass .. something like the energy content one would liberate by combining Jupiter-sized masses of matter and antimatter...

There exist modifications that work on |E|<1kg, although the matter distibution is not physical for other reasons.

The other problem with it is that the warp can only propagate at c. Sure, once you've got it established from A to B, you'll get there really really fast. But you'll be waiting an awfully long time for that transitway to open.

Er, no. I'm not sure what you're referring to, but if you're thinking of gravitational waves, then it's a result of either linearized GTR or a shortwave approximation. In general, large disturbances won't have to be particularly wavelike in the first place because GTR is not linear, and there's a substantial difficulty of even defining their speed unless there is a particularly nice background. That's not something unique to gravitational waves--even if one wants to do something simple like comparing momentum four-vectors "here" vs. "there", then one runs into the problem of these vectors being in different tangent spaces, with no globally consistent (coordinate and path independent) way of transporting them.

[Kuroneko]

This is ridiculuous; it now takes me several minutes just to load a single forum page. On a cable modem.

It would be better just travelling at one gee acceleration constantly with a drive that doesn't need to carry its own reaction mass and has a shitload of energy. You could circumnavigate the known universe with a subjective ship time of just a couple of decades, course, real-time would go by like a blur.

At that point, lone hydrogen impacts will do substantial damage to the ship and the cosmic background radiation will turn into a blaze from the front. The Lorentz gamma is γ ≅ 2e4, 5e8, 1e13 for one, two, and three decades, respectively. At three decades, a hydrogen atom would impact with the kinetic energy of 2e3J, if disbelief in the fact that the ship ever made it that far is suspended.

[Crayz9000]

What you'd use, instead, would be small parasite craft. Call them shuttles or boats or pinnaces or landing craft, whatever. You'd ideally make them lifting bodies, so you can get them into the atmosphere and landed while expending a minimum of fuel to do so. They would be powered by some sort of fancy chemical rocket or jet engine/rocket hybrid with chemical/gas thrusters for orientation in space.

A good model to base such a lifting body on would probably be the "Dyna-Soar" project.