Question Fusion reactors to power spaceships!! How do you convert the energy into propulsion?

Jul 27, 2020
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Having read a number of posts about fusion reactors being the ultimate means for the propulsion of spaceships, I still have not encountered the mechanism for how this would work. A recent article here on livescience.com* about small fusion reactors being ready within 5 years has renewed interest in fusion-powered spaceships, one supposes.

Clearly you get a lot of power with fusion from a small amount of mass. But energy by itself cannot drive a spaceship. You need to throw a lot of mass in the opposite direction of travel to get it moving.

Where does this mass come from, and how do you use the power from fusion to create thrust?


* https://www.livescience.com/nuclear-fusion-reactor-sparc-2025.html
 

Gringoz

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Having read a number of posts about fusion reactors being the ultimate means for the propulsion of spaceships, I still have not encountered the mechanism for how this would work. A recent article here on livescience.com* about small fusion reactors being ready within 5 years has renewed interest in fusion-powered spaceships, one supposes.

Clearly you get a lot of power with fusion from a small amount of mass. But energy by itself cannot drive a spaceship. You need to throw a lot of mass in the opposite direction of travel to get it moving.

Where does this mass come from, and how do you use the power from fusion to create thrust?


* https://www.livescience.com/nuclear-fusion-reactor-sparc-2025.html
Energy is energy no matter if it is fossil fuel, fission or fusion. For example food is cooked by heat no matter whether gas, electricity, wood, coal or microwaves. The most practical space drive currently proposed and in testing is the ion drive however that seems to require little or no fuel source at all. Also fusion could power the life spaces on the ship as well as be used for propulsion and depending on the source not be radioactive


 
Sep 23, 2020
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Having read a number of posts about fusion reactors being the ultimate means for the propulsion of spaceships, I still have not encountered the mechanism for how this would work. A recent article here on livescience.com* about small fusion reactors being ready within 5 years has renewed interest in fusion-powered spaceships, one supposes.

Clearly you get a lot of power with fusion from a small amount of mass. But energy by itself cannot drive a spaceship. You need to throw a lot of mass in the opposite direction of travel to get it moving.

Where does this mass come from, and how do you use the power from fusion to create thrust?


* https://www.livescience.com/nuclear-fusion-reactor-sparc-2025.html
 
Sep 23, 2020
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I haven't been in space but I have been under water and the problem I see with propulsion in space is like thriusting a stick under water no effort at all if done in a slow like motion but if done in the blink of an eye just about impossible another problem is not getting the craft to move its trying to stop it once your moving at unheard of speeds don't forget little to no friction up there ooh ha about that thrusting stick you move back as much as you move forward so in essence you go nowhere no ?
 

Gringoz

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I haven't been in space but I have been under water and the problem I see with propulsion in space is like thriusting a stick under water no effort at all if done in a slow like motion but if done in the blink of an eye just about impossible another problem is not getting the craft to move its trying to stop it once your moving at unheard of speeds don't forget little to no friction up there ooh ha about that thrusting stick you move back as much as you move forward so in essence you go nowhere no ?
Space is nothing like water as water provides resistance and in Earth water you have Earth gravity to repel against. Space is a vacuum with nothing to grab onto or push against.
 
Mar 6, 2020
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Energy is energy no matter if it is fossil fuel, fission or fusion. For example food is cooked by heat no matter whether gas, electricity, wood, coal or microwaves. The most practical space drive currently proposed and in testing is the ion drive however that seems to require little or no fuel source at all. Also fusion could power the life spaces on the ship as well as be used for propulsion and depending on the source not be radioactive


Only problem is, those ion engines provide little thrust as of now. Maybe in the future we can make bigger ones?
 
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Gringoz

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Only problem is, those ion engines provide little thrust as of now. Maybe in the future we can make bigger ones?
Ion propulsion does not need much thrust because there is no gravitational field in space to escape and with that the acceleration is constant. So in terms of what we have now a conventional thruster could put a ship in orbit, and the ion thruster could then take over and continue increasing speed to some as of now unknown value
 
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For example food is cooked by heat no matter whether gas, electricity, wood, coal or microwaves.
Cooking food is not going to get the spaceship moving. You need to transfer all that heat to something of substantial mass, and exhaust it to drive the ship, and almost certainly at very high temperatures.

And JeetsN123 beat me to the punch. Ion drives are fine for small space craft, if you don't mind spending time going somewhere. But if you want an ion-drive for a manned mission, you will need a lot more thrust than current versions.

If you have a spaceship with people and supplies on board for a long trip, it is going to have a very substantial mass. You will have to energize something, using the fusion energy, and exhaust this mass to get moving. And the more massive your ship, the more mass in 'fuel" you will need to exhaust. The weight of a Saturn V rocket standing at launch to the moon was 85% fuel.

The only advantage there is to fusion and using it for a drive mechanism is the high temperature obtained from a small mass of "fuel". An ion drive spitting out ions at relativistic speeds is going to get thing moving faster than those of today. But this would require an exhaust system that must withstand enormous heat, likely in the millions K.

Can a magnetic containment field for the exhaust structure be built side-by-side with the reactor and somehow manage this? Seems rather unlikely, but that is the reason for the question of this thread.

"Rocket thrust" of any nature involves two primary aspects - mass of the "fuel" being exhausted, and its energy. Higher energy, lower mass is clearly the optimal fuel for efficient, rapid acceleration. For a fusion reactor to power a spaceship, you will have to heat up something of significant mass, and use that for thrust.

How is that going to work?
 
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Gringoz

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Cooking food is not going to get the spaceship moving. You need to transfer all that heat to something of substantial mass, and exhaust it to drive the ship, and almost certainly at very high temperatures.

And JeetsN123 beat me to the punch. Ion drives are fine for small space craft, if you don't mind spending time going somewhere. But if you want an ion-drive for a manned mission, you will need a lot more thrust than current versions.

If you have a spaceship with people and supplies on board for a long trip, it is going to have a very substantial mass. You will have to energize something, using the fusion energy, and exhaust this mass to get moving. And the more massive your ship, the more mass in 'fuel" you will need to exhaust. The weight of a Saturn V rocket standing at launch to the moon was 85% fuel.

The only advantage there is to fusion and using it for a drive mechanism is the high temperature. An ion drive spitting out ions at relativistic speeds is going to get thing moving faster than those of today. But this would require an exhaust system that must withstand enormous heat, likely in the millions of K. Can a magnetic containment field for the exhaust structure be built side-by-side with the reactor and somehow manage this? Seems rather unlikely, but that is the reason for the question of this thread.

"Rocket thrust" of any nature involves two primary aspects - mass of the "fuel" being exhausted, and its energy. Higher energy, lower mass is clearly the optimal fuel for efficient, rapid acceleration. For a fusion reactor to power a spaceship, you will have to heat up something of significant mass, and use that for thrust.

How is that going to work?
Supplies can never be stocked for a long trip (40,000 years to the nearest star) so one must bring all of the ecosystems to grow and recycle food and Oxygen. Then when you get there you have the life forms to adapt and transplant to the new world

Think big or die small
 
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Considering the notion of ion thrust as being one of the most likely saviors of fusion-powered propulsion, it seemed a good idea to review the state of that tech. The wiki site (1) gives useful data on the results of current ion-drives for light-weight spacecraft.

From the wiki site (1), we see that :

"The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by 4.3 km/s (9,600 mph) while consuming less than 74 kg (163 lb) of xenon. The Dawn spacecraft broke the record, with a velocity change of 11.5 km/s (26,000 mph), though it was only half as efficient, requiring 425 kg (937 lb) of xenon."

end quote.

The three ion thrusters for Dawn were running for over 200 days to obtain that velocity (2). They are slow but efficient. Humans are neither, and for good reason. We do not want to spend a lot of time in space getting hammered by cosmic radiation, etc.

It appears that Xenon based thrusters is the current tech (1) :

"Ion thrusters in operational use have an input power need of 1–7 kW (1.3–9.4 hp), exhaust velocity 20–50 km/s (45,000–112,000 mph), thrust 25–250 millinewtons (0.090–0.899 ozf) and efficiency 65–80%[3][4] though experimental versions have achieved 100 kilowatts (130 hp), 5 newtons (1.1 lbf)."

end quote

It reminds us that thrust is measured in Newtons (a measurement of force), and gives some interesting details and how ion thrust may apply to our fusion-based spaceship, with so much energy from so little mass. There must be a way to make it work, right?

It is clear from the above details one would have to ionize a lot mass of some element(s) and feed it into a "Thrust Plasma Stream" heated by our fusion reactor, which then exhausts it at relativistic speeds to get our spaceship moving quickly, to an acceptable velocity, and slow it quickly, such as for an orbital insertion.

Still, what is the "fuel" that is to be ionized and exhausted, and how would that be done considering the temperatures involved? We must form the ultra-super-heated "drive plasma" from fusion energy, and then vent these ionized particles at relativistic speeds. It would seem to present a substantial design effort, to say the least.

Alternatives are more then welcome!


1. https://en.wikipedia.org/wiki/Ion_thruster

2. https://en.wikipedia.org/wiki/Dawn_(spacecraft)
 

Gringoz

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Oct 3, 2020
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Considering the notion of ion thrust as being one of the most likely saviors of fusion-powered propulsion, it seemed a good idea to review the state of that tech. The wiki site (1) gives useful data on the results of current ion-drives for light-weight spacecraft.

From the wiki site (1), we see that :

"The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by 4.3 km/s (9,600 mph) while consuming less than 74 kg (163 lb) of xenon. The Dawn spacecraft broke the record, with a velocity change of 11.5 km/s (26,000 mph), though it was only half as efficient, requiring 425 kg (937 lb) of xenon."

end quote.

The three ion thrusters for Dawn were running for over 200 days to obtain that velocity (2). They are slow but efficient. Humans are neither, and for good reason. We do not want to spend a lot of time in space getting hammered by cosmic radiation, etc.

It appears that Xenon based thrusters is the current tech (1) :

"Ion thrusters in operational use have an input power need of 1–7 kW (1.3–9.4 hp), exhaust velocity 20–50 km/s (45,000–112,000 mph), thrust 25–250 millinewtons (0.090–0.899 ozf) and efficiency 65–80%[3][4] though experimental versions have achieved 100 kilowatts (130 hp), 5 newtons (1.1 lbf)."

end quote

It reminds us that thrust is measured in Newtons (a measurement of force), and gives some interesting details and how ion thrust may apply to our fusion-based spaceship, with so much energy from so little mass. There must be a way to make it work, right?

It is clear from the above details one would have to ionize a lot mass of some element(s) and feed it into a "Thrust Plasma Stream" heated by our fusion reactor, which then exhausts it at relativistic speeds to get our spaceship moving quickly, to an acceptable velocity, and slow it quickly, such as for an orbital insertion.

Still, what is the "fuel" that is to be ionized and exhausted, and how would that be done considering the temperatures involved? We must form the ultra-super-heated "drive plasma" from fusion energy, and then vent these ionized particles at relativistic speeds. It would seem to present a substantial design effort, to say the least.

Alternatives are more then welcome!


1. https://en.wikipedia.org/wiki/Ion_thruster

2. https://en.wikipedia.org/wiki/Dawn_(spacecraft)
Ion thrust is a new technology, examining prototype vehicles and extrapolating the results is like looking at Robert Goddards rockets and imagining riding them to another planet. My point was that ion thrusters do not need massive thrust at the start because space provides no resistance which impedes a conventional thrust engine as space provides no base or gravitational field to push off of
 
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extrapolating the results is like looking at Robert Goddards rockets and imagining riding them to another planet
This is not at all accurate. To harness the power of fusion and produce propulsion, there is a fundamental physical requirement to produce an exhaust of significant mass and velocity to propel you forward. This is fundamental Newtonian physics.

And since it is being derived from the very high temperature fusion plasma, it is only reasonable that these are the conditions which must be met regardless of engine design(s). It has nothing at all to do with old rocketry of any kind.

The extreme conditions of the exhaust heated by fusion clearly must dictate the design of the mechanism, and cannot be predicated on the designs of today, or 100 years ago. This is a whole new form of propulsion. Ultra-hot exhaust and with a low-cost energy demand - fusion.

So how this thrust could be formed by a fusion reactor is the real issue. Not by modern ion thrusters, but they gave ideas. However the energy-to-thrust conversion is accomplished, it must be by using the fusion reactor almost directly, to extract energy to power some "fuel" to enormous exhaust speeds. There seems no other way.

Like all rocket engines, and propulsion systems, you must provide thrust, and in this case, a lot of it. If there are other ideas for harnessing the energy of a 15 million K fusion power source, to create thrust in another way, feel free to let us know.
 
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gringoz. jeezusss. THIS IS SO WRONG, i WON'T bother explaining it, to you. !!!
you Really should have LESS CONFIDENCE in the correctness of your (wacky) opinions.!!!!!!

"... as space provides no base or gravitational field to push off of."

l m F a o
 
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"....You need to throw a lot of mass in the opposite direction of travel to get it moving."
* https://www.livescience.com/nuclear-fusion-reactor-sparc-2025.html
or (b)
throw a LITTLE mass, very, Very, fast; eg., ion thruster.

this sort of (gradual) thrust works, fine, albeit slowly, in space.
AFTER escape velocity has been reached.

reaction mass is required. at present, zenon ions are 'thrown'; ONE at a time.!!!

your question suggests that you SHOULD STUDY nuclear reactors, in order to Acquire the information that you now lack.

the pursuit of (in-person) extrasolar exploration will have to wait, for a NEW means of travel.

at least, we can use autonomous ships/vehicles, for in-the-neighborhood sorties.
d
 

Gringoz

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This is not at all accurate. To harness the power of fusion and produce propulsion, there is a fundamental physical requirement to produce an exhaust of significant mass and velocity to propel you forward. This is fundamental Newtonian physics.

And since it is being derived from the very high temperature fusion plasma, it is only reasonable that these are the conditions which must be met regardless of engine design(s). It has nothing at all to do with old rocketry of any kind.

The extreme conditions of the exhaust heated by fusion clearly must dictate the design of the mechanism, and cannot be predicated on the designs of today, or 100 years ago. This is a whole new form of propulsion. Ultra-hot exhaust and with a low-cost energy demand - fusion.

So how this thrust could be formed by a fusion reactor is the real issue. Not by modern ion thrusters, but they gave ideas. However the energy-to-thrust conversion is accomplished, it must be by using the fusion reactor almost directly, to extract energy to power some "fuel" to enormous exhaust speeds. There seems no other way.

Like all rocket engines, and propulsion systems, you must provide thrust, and in this case, a lot of it. If there are other ideas for harnessing the energy of a 15 million K fusion power source, to create thrust in another way, feel free to let us know.
There is nothing accurate in the realm of harnessing fusion power for propulsion because fusion power does not exist and may never. There is also no reason to extrapolate that space travel will be accomplished thru exhaust thrust. Thrust engines work against the force of gravity that does not exist in space. Just as rockets were new once so will be the next engine. You ever do the math as to how big a thruster would need be to arrive at Proxima Centauri. I keep saying it but you miss the point, it does not matter how much power fusion creates if it has nothing to push against. This does not mean that fusion may not be a fine power source for something not yet invented. You seem to think that fusion would be an engine, it's not just as a reactor is not an engine, just fuel for the engines
 
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your question suggests that you SHOULD STUDY nuclear reactors, in order to Acquire the information that you now lack.
This has nothing whatever to do with nuclear reactors. They are run on fission of heavy nuclei and are in no way related to fusion reactors, which operate at 15 million K or higher, with enormous pressures and containment issues with mag fields, etc. Nuclear reactors sit statically in water, heating it to steam at low temperatures. Very low tech and in no way related to fusion tech, which is very high tech. That much is certain.

Without question, they are vastly dissimilar.


throw a LITTLE mass, very, Very, fast; eg., ion thruster.
Yes, that is the very idea with the high temperature of the plasma thruster proposed so clearly in my previous post. And slow accelerations are not acceptable (or slow breaking) for humans space flight, with the shortest trips being optimal for clearly limited supply requirements, etc..

And circumventing common notions of ion-propulsion yielded the only logical solution, which is a high temperature discharge of super-heated plasma obtained from the reactor.

Perhaps a unique magnetic containment field can be made in one region of the fusing plasma so as to vent a portion into our plasma thrust chamber in order to energize it. After all, it would seem that you would need to vent the fusion plasma at some point or the build up of helium would ultimately poison the fusion efficiency. So perhaps you add fuel at the same time you are venting, providing thrust and powering the spaceship.

Anyone who knows at what point helium would poison the reactor is requested to provide such an estimate.


So it remains a simple question. How do you convert the high temperature of the fusion reactor to thrust?
 

Gringoz

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This has nothing whatever to do with nuclear reactors. They are run on fission of heavy nuclei and are in no way related to fusion reactors, which operate at 15 million K or higher, with enormous pressures and containment issues with mag fields, etc. Nuclear reactors sit statically in water, heating it to steam at low temperatures. Very low tech and in no way related to fusion tech, which is very high tech. That much is certain.

Without question, they are vastly dissimilar.




Yes, that is the very idea with the high temperature of the plasma thruster proposed so clearly in my previous post. And slow accelerations are not acceptable (or slow breaking) for humans space flight, with the shortest trips being optimal for clearly limited supply requirements, etc..

And circumventing common notions of ion-propulsion yielded the only logical solution, which is a high temperature discharge of super-heated plasma obtained from the reactor.

Perhaps a unique magnetic containment field can be made in one region of the fusing plasma so as to vent a portion into our plasma thrust chamber in order to energize it. After all, it would seem that you would need to vent the fusion plasma at some point or the build up of helium would ultimately poison the fusion efficiency. So perhaps you add fuel at the same time you are venting, providing thrust and powering the spaceship.

Anyone who knows at what point helium would poison the reactor is requested to provide such an estimate.


So it remains a simple question. How do you convert the high temperature of the fusion reactor to thrust?
Actually there are no fusion reactors that produce more energy than goes into producing that energy, simplified to fusion power reactors do not exist so you are making pronouncements on fiction.

Also electric plasma thrusters can not power anything outside of a gravitational field. Why would you want to vent million degree plasma and what purpose would this have minus gravity

Do you understand the concept that space offers nothing to thrust against? Applying Earth propulsion in space is like swimming in rock
 
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Where does this mass come from, and how do you use the power from fusion to create thrust?

Is it possible to be more of a pessimist than this ? Here you are in possession of almost unlimited power, you can even perform transmutations of the elements. What about using that power to, either make a highly condensed fuel before lift off, or even using that phenomenal power to increase the density of matter inside the space ship till a usable level is reached? The most important thing to remember is that it does not take a lot of power to get up to light speed. So in order to reach 0.8 c at an acceleration of 1 g requires very little power, it would take about 9.5 to 10 months to reach 0.8 c. If the weight of the space ship is given it is easy to calculate the amount of force required to produce and acceleration of 1 g.
 

Gringoz

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Is it possible to be more of a pessimist than this ? Here you are in possession of almost unlimited power, you can even perform transmutations of the elements. What about using that power to, either make a highly condensed fuel before lift off, or even using that phenomenal power to increase the density of matter inside the space ship till a usable level is reached? The most important thing to remember is that it does not take a lot of power to get up to light speed. So in order to reach 0.8 c at an acceleration of 1 g requires very little power, it would take about 9.5 to 10 months to reach 0.8 c. If the weight of the space ship is given it is easy to calculate the amount of force required to produce and acceleration of 1 g.
Again power for thrust is useless in space no matter how unlimited it be. That said fusion is not unlimited because the elements to fuse are the fuel which must be available. Keeping in mind that the nearest star is 4.24 light years away or 24,887,585,664,000 miles, I believe at current speeds it would take 40,000 or so years to reach. Fusion does not effect the time or speed of travel
 
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Gringoz: Again power for thrust is useless in space no matter how unlimited it be. That said fusion is not unlimited because the elements to fuse are the fuel which must be available.
It is a pity that contributors do not make the little bit of effort necessary to maintain the integrity of this forum, and by doing so, its interest. For your information rocket engines are practically the only type of engines that will work in space. They work according to Newton’s third law: For every action there exists an equal and opposite reaction.” The rest of your objection about the force and time needed to reach 0.8 c (239,833966.4 m/s) is also not accurate. Suppose you have a drone weighing 1000 Kg, because obviously manned flights would not be immediately possible, and you want to accelerate it to 1 g (9.8 m/s^2). For ease of purpose imagine that the drone is far out in space away from any gravitational influences. Then according to the equation f = ma, a force of 9800 N is required to propel the 1000 kg craft to 9.8 m/s^2. 9800 N is equivalent to an energy of 9800 J (advantage of metric system). While 9800 N is not a small force it is not a huge force either, for instance a good hairdryer might consume about 3000 W (3000 J) so a force of 9800 N is not impossible and to maintain this amount of force for 9.5 months should be in the realm of the possible. Consider the fact that 9.5 months is considerably shorter than the 40,000 years you propose. So in order to reach a speed of 239,833,966 m/s (0.8 c) it will take an acceleration of 9.8 m/s^2 applied over a period of 9.5 months. The possibility of being hit by space minutiae and debris is very real but then with the unlimited power available with fusion it should be possible to build some sort of very accurate and powerful force field, which would blast space debris out of the way.
 
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Is it possible to be more of a pessimist than this ?
What you see as pessimism is actually a healthy dose of realism. Science is based on realistic concepts and observations. Pessimism is an opinion, regarding another's projected probabilities of an invention, etc..

Reality is the only means by which science progresses.


What about using that power to, either make a highly condensed fuel before lift off, or even using that phenomenal power to increase the density of matter inside the space ship till a usable level is reached?
All the optimism in the world will not move a spaceship without thrust. Energy alone simply does not yield thrust. The proposals of condensing fuels and increasing the density of matter are certainly optimistic. Most would say they are also not at all realistic. Which is why reality-based concepts are preferred.
 

Gringoz

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It is a pity that contributors do not make the little bit of effort necessary to maintain the integrity of this forum, and by doing so, its interest. For your information rocket engines are practically the only type of engines that will work in space. They work according to Newton’s third law: For every action there exists an equal and opposite reaction.” The rest of your objection about the force and time needed to reach 0.8 c (239,833966.4 m/s) is also not accurate. Suppose you have a drone weighing 1000 Kg, because obviously manned flights would not be immediately possible, and you want to accelerate it to 1 g (9.8 m/s^2). For ease of purpose imagine that the drone is far out in space away from any gravitational influences. Then according to the equation f = ma, a force of 9800 N is required to propel the 1000 kg craft to 9.8 m/s^2. 9800 N is equivalent to an energy of 9800 J (advantage of metric system). While 9800 N is not a small force it is not a huge force either, for instance a good hairdryer might consume about 3000 W (3000 J) so a force of 9800 N is not impossible and to maintain this amount of force for 9.5 months should be in the realm of the possible. Consider the fact that 9.5 months is considerably shorter than the 40,000 years you propose. So in order to reach a speed of 239,833,966 m/s (0.8 c) it will take an acceleration of 9.8 m/s^2 applied over a period of 9.5 months. The possibility of being hit by space minutiae and debris is very real but then with the unlimited power available with fusion it should be possible to build some sort of very accurate and powerful force field, which would blast space debris out of the way.
Again rocket engines need something to push off of. On the Earth they begin by pushing off of the Earth and against Earth gravity. However once in space, deep space not low Earth orbit where most satellites are there is nothing for the rocket to push against, there is no mass or even a gravitational field to provide a base to push off of. The Mars missions are fueled by putting the craft on the right trajectory using the Earth as the thrust base, then using a planets gravitational field to attract and whip the craft out the other side on exactly the path needed. None of this exist in interstellar space so a thrust engine fails no matter how unlimited the power and again fusion is not unlimited power because you need fuel to fuse.
 
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Again rocket engines need something to push off of.
This is not correct. In order to propel any flying object, either on earth or in space, you need only exhaust mass at sufficient speed to start moving.

A jet fighter does not have something to "push off of". It flys by exhausting super-heated gases from the combustion of fuel and oxygen. Flight is obtained by ejecting mass away from the craft, not by pushing off something.
 

Gringoz

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This is not correct. In order to propel any flying object, either on earth or in space, you need only exhaust mass at sufficient speed to start moving.

A jet fighter does not have something to "push off of". It flys by exhausting super-heated gases from the combustion of fuel and oxygen. Flight is obtained by ejecting mass away from the craft, not by pushing off something.
A jet pushes off the atmosphere and gravity as well, space has neither. Why do you claim that fusion power is unlimited in space when fusible atoms must be provided to fuse?
 
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