# Most likely candidate for future spaceship propulsion

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### Poll: Most likely candidate for future spacecrafts (3 member(s) have cast votes)

#### Most likely candidate for future spacecrafts

1. Nuclear Pulse (1 votes [10.00%])

Percentage of vote: 10.00%

2. Bussard Ramjet (1 votes [10.00%])

Percentage of vote: 10.00%

3. Solar Sail (1 votes [10.00%])

Percentage of vote: 10.00%

4. Nuclear Fusion Powered (2 votes [20.00%])

Percentage of vote: 20.00%

Percentage of vote: 50.00%

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### #35 The Transhumanist

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Posted 14 September 2008 - 08:58 PM

By claiming things to be true when you know they are patently false. I've read enough of your threads to know you have been shown that objects get more massive not less massive as they get near the speed of light,

Which is what my whole argument is about, it gets so massive that it's speed decreases because it's thrust can't push it anymore.

I think you know an object does not slow down by its self, I think you know an object's mass doesn't have anything to do with it's ability to keep moving when in free fall.

Corrected.

I know you know a magnetic field doesn't affect neutral particles. I gave you link to Newtons Laws but you ignored it. If you don't want me to point out your mistakes then just say so and I'll not post in this thread anymore.

If you think you know all this about me, your gravely mistaken.

I've ALWAYS known an object's mass increases as it moves faster, which is why an object is constantly trying to stay still.

### #36 Moontanman

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Posted 14 September 2008 - 09:05 PM

Which is what my whole argument is about, it gets so massive that it's speed decreases because it's thrust can't push it anymore.

This is only true very near the speed of light and it doesn't make the object slow down just makes it harder to go faster.

Corrected.

Thank you

If you think you know all this about me, your gravely mistaken.

I've ALWAYS known an object's mass increases as it moves faster, which is why an object is constantly trying to stay still.

No, an objects mass only increases very near the speed of light, at much slower speeds this is not true and even close to the speed of light an object is only difficult to accelerate , it does not naturally slow down. Even an object very close to the speed of light will continue on at that speed until an outside force acts on it. In theory it could coast at 99.999999999% of the speed of light forever.

### #37 CraigD

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Posted 14 September 2008 - 10:33 PM

My vote is for beam-powered propulsion.

There are many possible versions of this approach. My favorite is the Robert Forward type laser-pushed lightsail, but others may well prove more feasible, including the charged particle-pushed magnetic sail Moontanman voted for. Other versions (not mentioned in the wikipedia article, but described in various other sources), commonly referred to as “fountains”, transfer momentum via bullet-like, uncharged, non-light particles.

All of these approaches, with the exception of very low-mass vehicles like Forward’s Starwisp, require the construction and operation of solar-system space based solar energy and beam generating facilities not only many times more massive than the vehicle they propel, but many times more massive than all of the vehicles launched into space in the history of mankind. Such large, in depth programs are, IMHO, the best long-term approaches to spaceflight.

PS: Gardamorg, Moontanman’s efforts to instruct you in the fundamental laws of motion are correct. I fear you’ve gotten too much of your thinking about spaceflight from popular science fiction, such as Star Trek, Star Wars, and Battlestar Galactica, in which spacecraft are depicted as behaving similarly to water surface ships, which make no headway when not under power. Real spacecraft, including all those launched to date, such as the Voyager spacecraft, the most distant manmade objects to date, are powered only briefly, when lifted from Earth, making course corrections, and inserting into orbit around, or landing, or impact with their target bodies.

### #38 The Transhumanist

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Posted 15 September 2008 - 07:25 AM

My vote is for beam-powered propulsion.

There are many possible versions of this approach. My favorite is the Robert Forward type laser-pushed lightsail, but others may well prove more feasible, including the charged particle-pushed magnetic sail Moontanman voted for. Other versions (not mentioned in the wikipedia article, but described in various other sources), commonly referred to as “fountains”, transfer momentum via bullet-like, uncharged, non-light particles.

All of these approaches, with the exception of very low-mass vehicles like Forward’s Starwisp, require the construction and operation of solar-system space based solar energy and beam generating facilities not only many times more massive than the vehicle they propel, but many times more massive than all of the vehicles launched into space in the history of mankind. Such large, in depth programs are, IMHO, the best long-term approaches to spaceflight.

PS: Gardamorg, Moontanman’s efforts to instruct you in the fundamental laws of motion are correct. I fear you’ve gotten too much of your thinking about spaceflight from popular science fiction, such as Star Trek, Star Wars, and Battlestar Galactica, in which spacecraft are depicted as behaving similarly to water surface ships, which make no headway when not under power. Real spacecraft, including all those launched to date, such as the Voyager spacecraft, the most distant manmade objects to date, are powered only briefly, when lifted from Earth, making course corrections, and inserting into orbit around, or landing, or impact with their target bodies.

Oh...

### #39 CraigD

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Posted 15 September 2008 - 08:16 AM

No, an objects mass only increases very near the speed of light, at much slower speeds this is not true and even close to the speed of light an object is only difficult to accelerate , it does not naturally slow down.

This is nearly, but not exactly correct. Bodies traveling even at very low speeds relative to an observer. For example, a 100 kg man walking at a typical speed of 1 m/s actually masses
$\frac{100 \mbox{kg}}{\sqrt{1-\left(\frac{1 \,\mbox{m/s}}{c}\right)^2}} \dot= 100.000000000625 \,\mbox{kg}$
That is, he gains about 625 nanograms - about the mass of a single human egg cell, or a small grain of sand.

Even an object very close to the speed of light will continue on at that speed until an outside force acts on it. In theory it could coast at 99.999999999% of the speed of light forever.

This is true only if the object is traveling in a perfect vacuum.

Interstellar space contains has a density $D$ of about 1 hydrogen atom per 1 to 10 cubic centimeters, or about $1 \times 10^{-22}$ to $2 \times 10^{-21} \,\mbox{kg/m}^3$ From this, you can calculate the power per unit area of friction for a spacecraft traveling in it as
$\frac{P}{A} = \frac{v^2 Dv}{2\sqrt{1- (\frac{v}{c})^2}}$.
Calculating this for the lower density for craft traveling at 0.1 c (As Janus did in post #32 of “Quickest way to get to The Super Earth”) gives a small, but troubling from a practical engineering perspective, $2.25 \,\mbox{W/m}^2$. Calculating it for 0.99999999999 c gives about 500 million!

To put this into some kind of perspective, if the spacecraft used a plug of water ice near absolute zero as frontal shielding, this shield would be vaporized at a rate of about 3.2 meters of thickness per second!

In terms of conservation of momentum, ignoring the practical engineering problem of shielding and cooling, the density of interstellar space would produce an acceleration of
$\frac{Mv}{M+\frac{DvA}{\sqrt{1- (\frac{v}{c})^2}}}-1$
Even for a tiny spacecraft massing only 1000 kg with an frontal area of $1 \,\mbox{m}^2$, traveling 0.99999999999 c, this calculates to about $0.00005 \,\mbox{m/s/s}$. In about 1 year, it would slow by about 3 m/s, to a speed of about .99999999 c. Friction would never completely stop it relative to the interstellar medium, but over the decades and centuries, would keep knocking 9s off of its speed as a fraction of c. Using numeric approximation methods, we can calculate its speed in about 1 year as 0.99999999 c, 0.9999999 c in about 100 years, 0.999999 c in about 10,000 years, and about 0,999 c in about 10 billion years, roughly the current age of the universe.

### #40 Moontanman

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Posted 15 September 2008 - 09:50 AM

This is nearly, but not exactly correct. Bodies traveling even at very low speeds relative to an observer. For example, a 100 kg man walking at a typical speed of 1 m/s actually masses
$\frac{100 \mbox{kg}}{\sqrt{1-\left(\frac{1 \,\mbox{m/s}}{c}\right)^2}} \dot= 100.000000000625 \,\mbox{kg}$
That is, he gains about 625 nanograms - about the mass of a single human egg cell, or a small grain of sand.This is true only if the object is traveling in a perfect vacuum.

Interstellar space contains has a density $D$ of about 1 hydrogen atom per 1 to 10 cubic centimeters, or about $1 \times 10^{-22}$ to $2 \times 10^{-21} \,\mbox{kg/m}^3$ From this, you can calculate the power per unit area of friction for a spacecraft traveling in it as
$\frac{P}{A} = \frac{v^2 Dv}{2\sqrt{1- (\frac{v}{c})^2}}$.
Calculating this for the lower density for craft traveling at 0.1 c (As Janus did in post #32 of “Quickest way to get to The Super Earth”) gives a small, but troubling from a practical engineering perspective, $2.25 \,\mbox{W/m}^2$. Calculating it for 0.99999999999 c gives about 500 million!

To put this into some kind of perspective, if the spacecraft used a plug of water ice near absolute zero as frontal shielding, this shield would be vaporized at a rate of about 3.2 meters of thickness per second!

In terms of conservation of momentum, ignoring the practical engineering problem of shielding and cooling, the density of interstellar space would produce an acceleration of
$\frac{Mv}{M+\frac{DvA}{\sqrt{1- (\frac{v}{c})^2}}}-1$
Even for a tiny spacecraft massing only 1000 kg with an frontal area of $1 \,\mbox{m}^2$, traveling 0.99999999999 c, this calculates to about $0.00005 \,\mbox{m/s/s}$. In about 1 year, it would slow by about 3 m/s, to a speed of about .99999999 c. Friction would never completely stop it relative to the interstellar medium, but over the decades and centuries, would keep knocking 9s off of its speed as a fraction of c. Using numeric approximation methods, we can calculate its speed in about 1 year as 0.99999999 c, 0.9999999 c in about 100 years, 0.999999 c in about 10,000 years, and about 0,999 c in about 10 billion years, roughly the current age of the universe.

Thanks Craig, notice I did say theoretically:doh:, the interstellar medium is as much a barrier to near speed of light travel as the problem of accelerating to near light speed. Arthur C. Clark took this into consideration in the book Songs Of Distant Earth, the space craft used zero point energy to travel very close to the speed of light but it had to have a huge shield of ice to keep from being destroyed by the interstellar gas turned into cosmic rays by the ships speed. Even grains of dust caused huge craters on the shield. I'm not sure how to figure it but I'm sure there is a point of trade off where speed becomes deadly and it is probably considerably less than .99999 c

### #41 The Transhumanist

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Posted 27 September 2008 - 08:44 AM

This post has been moved here.

### #42 Moontanman

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Posted 27 September 2008 - 10:28 AM

[quote][quote name='Gardamorg']You know what would be even better?

Laser beams that are so hot and intense that they fuse and transmute matter in the back of a space craft into energy used to propel the craft.[/quote]

On the face of it not a bad idea. But I have yet to see any sign of anyone having a laser even close to being that powerful.

[quote]The ship's fuel will will undergo a series of stages, first, the fuel is raw hydrogen, second, a remote laser beam fuses and heats this hydrogen into plasma, this laser beam is very powerful, and fired from a solar powered space station/operating laser canon.[/quote]

As long as the ship is nearby not a bad idea.

[quote]In a few minutes, this plasma will be so hot that it will burn separate hydrogen into plasma without the need of a high powered laser, this will cause a chain reaction, the plasma will become so hot that it's atoms will release energy by themselves, and the hydrogen will become so hot that it will transmute into more plasma, which in turn will release more energy, making the craft self sustained.[/quote]

This sounds like tecnobabble to me, you need to give us some idea of how this works, I know of no way to do what you are talking about. Hot atoms do not spontaneously release energy, they only have the energy the laser has pumped into them.

[quote]The problem is the fuel (hydrogen) will be burned at an abnormal rate, which is why the hydrogen will be scooped up from the interstellar medium.[/quote]

Ok so you don't intend to carry your hydrgen with you?

[quote]It won't need a magnetic field because it's already moving so fast that it's speed will allow it to cover cover so much space in so little time, that it's speed compensates for the width a magfield would have to be.[/quote]

How do you scoop up hydrgon if it's not charged?

[quote]It can accelerate to this speed because it doesn't need energy at all, just hydrogen, the energy is supplied by the space station that fires the laser beam which splits atoms with heat alone, and after a certain point in time it doesn't even need the laser.[/quote]

A laser would only work at extreme close range, lasers spread out over time, by the time you were a few thousand miles away even the strongest laser would be spread to far to heat up anything to the extreme temps you going for. Also no amount of heat will split hydrogen atoms or any other atoms for that matter.

[quote]So this craft is in a sense, a ramjet, a laser beam powered space craft, and a self sustained fusion reactor.

It doesn't need charged hydrogen.[/quote]

To pick up enough hydrogen in the way a ram jet would pick up oxygen in the atmosphere would require it to be traveling a substantial fraction of C. Accelerating to that speed would require an enormous amount of hydrogen to start with and hydrogen fusion triggered by a remote laser would not be self supporting. Even if it was how would you restart it if you had to turn it off for some reason? also how would you decelerate at the end of your trip?

### #43 The Transhumanist

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Posted 27 September 2008 - 10:32 AM

[quote name='Moontanman'][quote]

On the face of it not a bad idea. But I have yet to see any sign of anyone having a laser even close to being that powerful.

As long as the ship is nearby not a bad idea.

This sounds like tecnobabble to me, you need to give us some idea of how this works, I know of no way to do what you are talking about. Hot atoms do not spontaneously release energy, they only have the energy the laser has pumped into them.

Ok so you don't intend to carry your hydrgen with you?

How do you scoop up hydrgon if it's not charged?

A laser would only work at extreme close range, lasers spread out over time, by the time you were a few thousand miles away even the strongest laser would be spread to far to heat up anything to the extreme temps you going for. Also no amount of heat will split hydrogen atoms or any other atoms for that matter.

To pick up enough hydrogen in the way a ram jet would pick up oxygen in the atmosphere would require it to be traveling a substantial fraction of C. Accelerating to that speed would require an enormous amount of hydrogen to start with and hydrogen fusion triggered by a remote laser would not be self supporting. Even if it was how would you restart it if you had to turn it off for some reason? also how would you decelerate at the end of your trip?[/quote]

Right on top of it, realizing this I edited the post, and moved it to the BEC thread, before you posted.

### #44 The Transhumanist

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Posted 27 September 2008 - 10:35 AM

My idea has been modified, see if it STILL has any flaws.

If it were possible to teleport a craft's fuel, than it wopuld be possible to travel at light speed.

Once teleported, the craft's fuel will will undergo a series of stages, the fuel starts out as raw hydrogen, a remote laser beam fuses and heats this hydrogen into plasma, this laser beam is very powerful, and fired from a solar powered space station, that is also a very large Bose Einstein Condensate Transmitter, to teleport the fuel to the depths of our galaxy, it needs to be BIG.

In a few minutes, the plasma generated from the heating hydrogen will be so hot that it will burn newly arrived hydrogen (from a BEC Receiver built into the craft) into plasma without the need of the high powered laser.

This will cause a chain reaction, the plasma will become so hot that it's atoms will release energy by themselves, and the hydrogen will become so hot that it will transmute into more plasma, which in turn will release more energy, making the whole craft self sustained.

The craft requires NO energy, the space station that fires the laser beam is what acts as an energy source, it converts the fuel into energy, and over time allows to fuel to convert itself into energy, and the fuel is teleported from the space station's BEC Transmitter.

The Craft can be as large as needed as well.

Once the Craft reaches light speed, it can coast to it's destination.

### #45 CraigD

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Posted 28 September 2008 - 11:56 AM

My idea has been modified, see if it STILL has any flaws.

Yes, the idea of accelerating any sort of machine to the speed of light in vacuum in ordinary space is fatally flawed. These flaws have been explained so many times in so many books and websites that I won’t repeat them here.

Gardamorg, I recommend you try achieving about the same level of understanding of physics as the writers of Star Trek in the 1960s. These non-scientists knew you couldn’t accelerate to the speed of light, and that however you do it, spaceflight like they were depicting would require fantastic amounts of energy be stored in a small space, and without consulting blue ribbon panels of physicist and engineers (or even much talking among themselves), came up with a couple of not too unreasonable ideas:
• Since the highest possible energy density is found in antimatter (via annihilating it with matter), use antimatter fuel
• Since you can’t travel faster than the speed of light in normal space, don’t try, but rather change space (via “warp drive”)
Although actual physics (eg: the Alcubierre metric) suggest that many times more energy is required to do the sort of things depicted in Star Trek, we can at least give a nod to its science fiction writers’ guesswork as being roughly in the right direction.

In short, for spaceflight to be much different than it is now, we need to imagine spacecraft that are much different than they are now. Imagining larger, more massive ships with more powerful rocket motors doesn’t seem a productive line of thought.

While I’ve gone and brought science fiction into the discussion, I might as well trot out the time-worn idea that Star Wars is to blame for some “dumbing down” of the popular scifi audience. While both Star Trek and Star Wars are space opera – comedy and drama with scientific trappings, not science lessons with a supporting dramatic or comedic narrative – Star Trek, IMHO, better avoided the physically silly. The writer/producer system of 1960s TV that gave us Star Trek appears to have provided a better informal scientific peer-review process than the essentially single-handed (Steven Spielberg) writer/producer process that gave us Star Wars.

### #46 Janus

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Posted 28 September 2008 - 08:06 PM

The writer/producer system of 1960s TV that gave us Star Trek appears to have provided a better informal scientific peer-review process than the essentially single-handed (Steven Spielberg) writer/producer process that gave us Star Wars.

A lot of this had to do with the fact that in developing the series, Roddenberry sought out the opinion of scientific advisers and made use of their advice when possible. For example, they spent a great deal of time and energy designing the bridge so that it not only looked good on film, but so that it was as practical a design as possible. Roddenberry felt that it would help the audience buy into the premise better. The studio attitude was more on the lines of "why are you wasting time with this? You've got some chairs and blinking lights, that's good enough."

### #47 The Transhumanist

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Posted 29 September 2008 - 08:31 PM

after all of this debating, I think the most likely candidate for a space craft will be the amalgam of many different things;

The first thing we need to do is overcome the earth's gravity, I'm talking about assembling and launching crafts from Earth Orbit. A way to do this is by building space elevators.

One problem with space elevators is that they need to be extremely strong in order to support their own weight. They can be made this strong by giving them super nano structures, using nanotubes such as these.

A problem with making such neatly built nano structures is that we can't do it. Which is where nano=assembly comes in.

If we can get a craft into space, and launch it in a vacuum, than we have overcame one obstacle, which does reserve a lot of fuel. But another obstacle is accelerating out of our solar system. A proven way to do this is with a magnetic sail. There are many advantages to a magnetic sail, one of which is it's ability to be completely self sustained by the sun until it leaves the solar system, this is because it's magfield can be expanded as it furthers the sun, thus accelerating at great speeds.

Now there is a problem with traveling the distance in between solar systems, which is vast, but this solar sail coasting is a very great advantage, after the craft can no longer use the mag sail method, it could switch to a self sustaining fusion method of propulsion.

This is perfect because so far the craft hasn't used up any fuel for fusion, so the fusion actually starts when the craft is no longer in the solar system!!! But there is one disadvantage to creating such a fusion reactor, it has to be HUGE!!!

But thanks to the space elevator, and the mag sail, it can be huge, to no risk! This self sustaining fusion craft, will be able to travel from one solar system to the other in just a few hundred years, because it will constantly accelerate as the fusion's energy release constantly perpetuates, and it is already moving fast from the mag sail's caost.

Now all that is just for GETTING there, now getting back is even harder.

### #48 freeztar

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Posted 29 September 2008 - 09:14 PM

Space elevators are one of the coolest "maybe within reaches of possibility" sci-fi novelties to think about imho. Without delving into them too much here, I just wanted to point out Craig's excellent post on calculating mass for different materials that might possibly be used for cable in a space elevator system.

### #49 Moontanman

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Posted 29 September 2008 - 09:34 PM

Space elevators are one of the coolest "maybe within reaches of possibility" sci-fi novelties to think about imho. Without delving into them too much here, I just wanted to point out Craig's excellent post on calculating mass for different materials that might possibly be used for cable in a space elevator system.

I like the idea of a space elevator as well but i wonder about it's flaws. Like it's fragility, an errant aircraft or even more likely a purposeful aircraft could knock it out easily and then all the man years of construction and mega tons of materials would be lost. I've also read that a trip up the elevator would take weeks or even months, I've looked but I can't find where I read that. A space elevator has huge potential but lots of bugs to work out.

### #50 The Transhumanist

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Posted 30 September 2008 - 07:37 PM

Space elevators would be great if achieved, no, they would be monumental.

This is because a self sustaining fusion reactor is THE greatest propulsion method, and a craft can be built large enough to fit a large fusion reactor on board if it doesn't have to escape earth.

It's perfect, the only flaw is that the metal will over heat and where out over time, which is why the fusion reactor would have to be turned off every once in a great while, and it's walls would have to be replaced, which won't be much of a problem.

### #51 CraigD

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Posted 01 October 2008 - 05:59 PM

A space elevator has huge potential but lots of bugs to work out.

Even including the words huge and lots, this is likely an understatement.

I like the idea of a space elevator as well but i wonder about it's flaws. Like it's fragility, an errant aircraft or even more likely a purposeful aircraft could knock it out easily and then all the man years of construction and mega tons of materials would be lost.

I suspect that if and when the challenges of making a space elevator are met, aircraft strikes will not be counted as among the hard ones. Given the strength of the materials required to make a space elevator, aircraft – which are already pretty fragile compared to most manmade structures – would be like tissue paper against steel in a collision. Also, note that only a small fraction of the total structure of a space elevator would be at altitudes aircraft can reach.

It’s also worth noting that, although common in popular science fictional and speculative descriptions of them, it’s not a strict requirement that space elevators reach all the way to the ground. There’re numerous advantages to not reaching the ground or anchoring them, but rather having their terminals at high altitudes (10 km or more), to which passengers and cargo must be transported via fairly conventional aircraft. (Imagine what it’d be like to land a 747 on a “runway in the sky” at 12,000 m altitude! )

I've also read that a trip up the elevator would take weeks or even months, I've looked but I can't find where I read that.

You don’t really need a reference to figure this out.

geostationary orbit is 35,786 km above the Earth’s surface. Divide by your best guess at the speed some sort of climbing elevator car could safely achieve. The fastest conventional elevators have speeds of about 10 m/s, giving a travel time of about 41 days. A fast train can go 50 m/s or so, for a travel time of about a week. Since most of the trip would be in a near vacuum, much greater speeds might be possible, but so much depends on the detailed engineering of the system that it’s hard to guess if high speeds are practical, or worth their cost and risk.

Space elevators would be great if achieved, no, they would be monumental.

I think nearly everyone agrees on this

This is because a self sustaining fusion reactor is THE greatest propulsion method, and a craft can be built large enough to fit a large fusion reactor on board if it doesn't have to escape earth.

Being able to put huge masses into orbit at costs potentially as low as surface shipping would almost certainly revolutionize spacecraft construction, but we should be wary of calling any energy source THE greatest.

As the ieee.org article Gardamorg links emphasizes, despite decades of research, nobody’s really confident that self-sustaining fusion power will be possible anytime in the near future, or even if it can be accomplished, how much net power it will produce.

Also, as the wikipedia article “energy density” describes, the theoretical maximum energy/fuel mass ratio for fusion is only from 3 to 20 time that of fission, and about 1/140 to 1/300 times that to the absolutely greatest fuel, matter-antimatter.

OK, I take it back, it seem pretty safe to name the greatest potential energy source, so long as it’s antimatter.

It’s also important to note that even having a tremendous source of energy doesn’t solve all your propulsion problems. Rockets need reaction mass, even if only in the form of the relativistic mass of photons, and the less reaction mass available, the more energy is required to produce the same thrust. The wikipedia article “nuclear photonic rocket”, while not primarily about such a system, has some applicable discussion.