Nuclear Fusion Rockets v.s Fission

[Arian]

Question​

If I am correct, nuclear fusion rockets (if used the right way) can reach speeds of up to 10% of the speed of light, while nuclear fission rockets can reach about 5% of the speed of light.

NASA, however, is working on spacecraft that could make a one-way Earth–Mars trip in about two months, which is an improvement over the current six-month transit time.

Speed Estimates​

Light takes about 10 minutes to travel from Earth to Mars. At 10% of the speed of light, a fusion-powered spacecraft could reach Mars in about 100 minutes, or about 200 minutes when including deceleration time. That is roughly 3.3 hours.

So why is the proposed travel time still two months?

Notes​

• Nuclear fission:
• Acceleration: $10 \times 20 = 200$ minutes
• Deceleration: 400 minutes (about 6.6 hours)

 

[scott1]

Nuclear fussion reactors are very heavy and this no effinct way sustain nuclear fussion

 

[Janus]

I don't know where you got your numbers from, but any rocket's top speed is only limited by how much fuel it carries.

That being said, a fusion rocket theorectically is more efficient than a fission rocket and both are more efficient than the chemical rockets we presently use. By more efficient, I mean that in order to reach a certain speed it takes less fuel.

Now it might be possible to reach 10% c with a fusion rocket, with a reasonable fuel to payload ratio (depending on what you mean by reasonable), but I seriously doubt that you could reach 5%c with a fission rocket.

Then there's the factor of acceleration. You can't just jump in your ship and take off at 10% of c, you have to accelerate up to that speed first A a one g acceleration, it would take 38 days to reach 10% of c (and the same amount of time to slow back down.)
And in that time you would travel a distance of some 60 billion km; something like ten times the distance of Pluto.( and over shooting Mars by quite a bit.)

Now if you were to just accelerate at 1 g halfway to Mars and then decelerate the second half you could get to Mars (at its closest) in about 2 days.

Now two days beats two months, but the rub is that we don't know how to make a fusion rocket work.

The spacecraft that you speak of that NASA is developing is at least something we have a good chance of making work. (Oh, and BTW if its the system I think it is, it would be most likely powered by a fission reactor.)

 

[berkeman]

And there's the concept of a fusion (not fussion!) ramjet, where you scoop up hydrogen in space to power the reactor...less fuel to have to carry. I forget how big the scoop has to be...

Oh, and don't forget how heavy space vehicles will have to be to travel 10% of c. The frontal shielding will have to be pretty amazing...

 

[Chronos]

10% c is impossibly high. The 'scoop' concept is also impossibly optimistic. Unless you plan to convert the interplanetary 'fuel' to pure anti-matter, you can't get there from here.

 

[Astronuc]

Unfortunately, people overlook the 'real' physics and engineering and go for simple ideal equations. Ideally, if one had huge amounts of energy and little mass, then a rocket could achieve the mean speed of the exhaust.

But in reality, the higher the exhaust velocity or kinetic energy, the lower the mass expelled due to constraints on the structure of the propulsion chamber. Even a superconducting magnetic feels a back pressure from the plasma. Due to pressure limitations, the density of the exhaust is limited, and this limits the thrust.

One ultimately has to look at the specific energy available for propulsion, and high Isp's generally mean low mass flow rates and low acceleration - an inherent problem with NEP and fusion systems.

Also, one has to take the fuel along for the ride, so early one has a lot of mass to propel. Then at the other end, one has to use a lot of mass to slow down at the destination. If the mission is round trip, then propellant mass is needed for the return trip. Each phase of the mission requires less propellant, since each phase ejects propellant mass.

 

[Arian]

MY source is "Entering Space" by Robert Zubrin

 

[pervect]

If I'm correct nuclear fusion rockets, (if used the right way), can go up to 10% the speed of light. THis is while nuclear fission goes 5% the speed of light. Yet NASA is working on craft that can make a "Earth-Mars-one way in two months". This is better then our current six month.
Yet if I'm right, light takes about 10 min to get to Mars. So a craft (nuclear fussion) could go to Mars at 10% the speed of light in 100min or 200min including deacelearation time. 200min is 3.3 hours. So what's this 2 month crap?

ps: Nuclear Fission:
acceleration:10*20=200 min.
deacceleration is 400min or 6.6 hours.

I think you need to look at your acceleration / deacceleration times again.

It would take roughly .1 year at 1g to reach 10% of the speed of light, i.e somewhere a little over a month.

This is because 1 light year / year^2 = 9.5 m/s^2, which is slightly under 1g (call it 1 g for back-of-the envelope calculations).

IF we could continuously accelerate and deaccelerate at 1g for xxx the entire flight xxx travel time to Mars would be roughly 2 * sqrt(d / a)

(solve .5 a (t/2)^2 = d/2 to include turn-around effects to get the above).

However, with available rockets, this would be unrealistically fast. (I.e. you would find that the rocket would run out of fuel with reasonable mass ratios and exhaust velocities). You need to use more sophisticted models based on the rocket equation

http://en.wikipedia.org/wiki/Rocket_equation

the fuel/mass ratio of your rocket, and it's exhaust velocity (also sometimes specified via ISP, specific impulse) to get a better estimate of travel time.

Remember also that you have to get to mars, and back, without running out of fuel.

 

[dak246]

And there's the concept of a fusion (not fussion!) ramjet, where you scoop up hydrogen in space to power the reactor...less fuel to have to carry. I forget how big the scoop has to be...

Oh, and don't forget how heavy space vehicles will have to be to travel 10% of c. The frontal shielding will have to be pretty amazing...

The scoop has to be in the area of 100 miles in diameter I think. The scope and cost of such an engineering feat is on all whole different level than anything that has ever been done before.

 

[Astronuc]

It would seem rather impractical to build a spaceship 100 miles in diameter. Even 100 m in diameter would seem rather impractical.

 

[dak246]

I agree. Thats why I think the whole premise of collecting hydrogen ions from space as a means of propulsion is impractical...unless maybe its an endeavor 1000 years from now to send a multigenerational crew to another solar system.

 

[Danger]

If you folks are referring to the Bussard Ramjet, it uses a laser to ionize the hydrogen and the net is a funnel-shaped magnetic field rather than a physical device.

 

[Arian]

Don't you need a lot less fuel for fusion then regular chem. rockets, I mean fusion produces a lot more energy then cheichals.

 

[scott1]

Don't you need a lot less fuel for fusion then regular chem. rockets, I mean fusion produces a lot more energy then cheichals.

I don't but it might be the same. When you go faster you gain more mass so you would be heavier and the heavier you the more energy required.

 

[Arian]

Yes, but going 10%c makes rather insignifactic in mass gain. So fusion rockets are lighter then chemichal rockets even at 10% c.

 

[Intuitive]

Wouldn't it be more practical to use a giant Rail Gun on the Moon 1 mile long or so and just use the Rail Gun to launch unmanned Satellites anywhere in the Solar System.

What kind of velocities could we expect from a http://science.howstuffworks.com/rail-gun.htm" 1 mile long and Capacitors the size of Hotels to fire it off.

 

[russ_watters]

The limiting factors there are the durability of the projectile (at a hundred g's of acceleration, they have to be tough) and the construction of the gun itself (need to find an appropriately sized and angled mountain).

I'm not sure a rail gun that big is feasible, but I have heard proposals for an air rifle.

 

[MadScientist 1000]

Nuclear fussion reactors are very heavy and this no effinct way sustain nuclear fussion

What if you use a tomask reactor to create nuclear fusion, and then use MRE coils to direct the exhaust out of the other end of the ship? That should work!

 

[Astronuc]

A rocket's top speed is limited by how much propellant it carries and how much energy which can be produced and imparted to the propellant, before the propellant is exhausted. Specific energy or specific power is a good figure of merit for propulsion systems.

One must also consider the objective. If it's just a exploratory spacecraft , the bulk of the propellant can be expended to get to speed - with no deceleration at full depletion of propellant, i.e. one-way with no permanent destination. If one plans to put a spacecraft in orbit, propellant and energy are necessary to decelerate into orbit.

If one plans a round-trip, one could employ gravity assist (assuming the objective has large mass, e.g. a planet) to redirect the spacecraft , but then sufficient propellant and energy is necessary to decelerate on the return from origin (LEO or GEO perhaps).

A round-trip to Mars with Martian orbit would require energy and propellant to accelerate from Earth, decelerate into orbit, accelerate out of Martian orbit, and decelerate into Earth orbit - with the amount of acceleration and deceleration depending on how fast one wishes to travel.

 

[MadScientist 1000]

I have a nagging thought at the back of my head... Suppose we build a fusion reactors and all the systems to go with it to propel the spacecraft through space to its objective. How the heck to we "jump start" the fusion reaction? A bit of anti matter will do, but will it work?

Wouldn't it be more practical to use a giant Rail Gun on the Moon 1 mile long or so and just use the Rail Gun to launch unmanned Satellites anywhere in the Solar System.

What kind of velocities could we expect from a http://science.howstuffworks.com/rail-gun.htm" 1 mile long and Capacitors the size of Hotels to fire it off.

But suppose we need to change the direction of the projectile/satelite. How do we do that? We have a lot of (delta) v in the y-axis (straight). How much (delta) v do we need in the x-axis (horizontal) for directional change?

 

[Intuitive]

Hi Astronuc.

I need the Guru for this one.

If we could some how miniturize http://cua.mit.edu/ketterle_group/Projects_1997/atomlaser_97/atomlaser_comm.html" that beam out Atoms and each Atom LASER was Nano scaled so we could array them in a fashion that would give millions of Atom Streams coming out as a coherent Mass Beam, Would something like this work if the Technology was all there?

 

[Astronuc]

I have a nagging thought at the back of my head... Suppose we build a fusion reactors and all the systems to go with it to propel the spacecraft through space to its objective. How the heck to we "jump start" the fusion reaction? A bit of anti matter will do, but will it work?

Presumably a fusion based propulsion system would require an auxilliary power system, much like commercial aircraft have Aux Power units in their tails, use portable generators when parked on the ground. In the future, spacecraft might leave from a space station or docking facility - which could be power by solar systems.

But suppose we need to change the direction of the projectile/satelite. How do we do that? We have a lot of (delta) v in the y-axis (straight). How much (delta) v do we need in the x-axis (horizontal) for directional change?

The Space Shuttle and other satellites/ spacecraft use maneuvering rockets.

 

[Astronuc]

If we could some how miniturize http://cua.mit.edu/ketterle_group/Projects_1997/atomlaser_97/atomlaser_comm.html" that beam out Atoms and each Atom LASER was Nano scaled so we could array them in a fashion that would give millions of Atom Streams coming out as a coherent Mass Beam, Would something like this work if the Technology was all there?

One would need lots of Atom LASERs. Millions or even billions of atoms is not very much. Consider that solid matter has densities on the order of 10²² atoms/cc or gases at STP have atomic/molecular density of about 10¹⁹ (atoms or molecules)/cc, and fusion plasmas have densities on the order of 10¹⁴ ions/cc, then one sees that one would need several 10's or 100's thousands of atomic lasers to begin to approach some feasibility. Then one must devise a method (process) to power the 10's or 100's of thousands of atomic lasers.

Getting back to fusion, one must realize there are few practical aneutronic reactions, and these involve nuclei such as D (deuterons), He-3, and heavier elements. This also means losses due to brehmsstrahlung increase. Also, for nuclei with greater nuclear charge, the temperature must be increased, and for a pressure limited system, the ion density must be decreased.

Fusion based on DT, one of the easiest reactions, loses about 80% of the energy to fast (14.1 MeV) neutrons, with about 20% of the energy going to 3.5 MeV alpha particles, and some of that energy must be used to offset not on brehmstrahlung loses, but also losses due to cyclotron radiation. With neutronic reactions, massive radiation shielding is required.

Finally, fusion reactors required massive structures for the support of the superconducting magnets which provide the magnetic field which confines the plasma. The structure must withstand the pressure of the confined plasma - so it has to be high strength. Then there is the cryogenic system and the propellant storage and delivery systems. All this adds to considerable mass. And practical fusion systems for electrical power have yet to be perfected.

 

[Intuitive]

One would need lots of Atom LASERs. Millions or even billions of atoms is not very much. Consider that solid matter has densities on the order of 10²² atoms/cc or gases at STP have atomic/molecular density of about 10¹⁹ (atoms or molecules)/cc, and fusion plasmas have densities on the order of 10¹⁴ ions/cc, then one sees that one would need several 10's or 100's thousands of atomic lasers to begin to approach some feasibility. Then one must devise a method (process) to power the 10's or 100's of thousands of atomic lasers.

Getting back to fusion, one must realize there are few practical aneutronic reactions, and these involve nuclei such as D (deuterons), He-3, and heavier elements. This also means losses due to brehmsstrahlung increase. Also, for nuclei with greater nuclear charge, the temperature must be increased, and for a pressure limited system, the ion density must be decreased.

Fusion based on DT, one of the easiest reactions, loses about 80% of the energy to fast (14.1 MeV) neutrons, with about 20% of the energy going to 3.5 MeV alpha particles, and some of that energy must be used to offset not on brehmstrahlung loses, but also losses due to cyclotron radiation. With neutronic reactions, massive radiation shielding is required.

Finally, fusion reactors required massive structures for the support of the superconducting magnets which provide the magnetic field which confines the plasma. The structure must withstand the pressure of the confined plasma - so it has to be high strength. Then there is the cryogenic system and the propellant storage and delivery systems. All this adds to considerable mass. And practical fusion systems for electrical power have yet to be perfected.

Hi Astronuc.

Is there any data on how fast the Atoms are traveling in the Atom Beam per second of an Atom LASER?

Maybe Microtechnology can mass produce special Atom LASER Chips that can be moduled into any fashion, I have seen some amazing things done inside Microchips, it shouldn't be difficult for them using Microtechnology Equipment and Engineering and their ability to readapt their Engineering format especially in the Micro LASER Tech industries.

I can almost imagine a huge coherent Beam of Atoms that are actually Arrays working together as a Single Beam that appear to be a massive columb of coherent thrust.

Hopefully one day they could sit down and design the most sophistocated High Permormance High Energy Atom LASER Space Craft Engine on a larger scale and call it a Star Drive developed for the Mythical Pheonix Bird. ie. Space Craft of the Future.

I had to throw the Myth in there for Suspence.

 

[Astronuc]

Is there any data on how fast the Atoms are traveling in the Atom Beam per second of an Atom LASER?

Well, based on these comments -

The analogy to spontaneous emission in the optical laser is elastic scattering of atoms (collisions similar to those between billiard balls). In a laser, stimulated emission of photons causes the radiation field to build up in a single mode. In an atom laser, the presence of a Bose-Einstein condensate (atoms that occupy a "single mode" of the system, the lowest energy state) causes stimulated scattering by atoms into that mode. More precisely, the presence of a condensate with N atoms enhances the probability that an atom will be scattered into the condensate by N+1.

In a normal gas, atoms scatter among the many modes of the system. But when the critical temperature for Bose-Einstein condensation is reached, they scatter predominantly into the lowest energy state of the system, a single one of the myriad of possible quantum states. This abrupt process is closely analogous to the threshold for operating a laser, when the laser suddenly switches on as the supply of radiating atoms is increased.

In an atom laser, the "excitation" of the "active medium" is done by evaporative cooling - the evaporation process creates a cloud which is not in thermal equilibrium and relaxes towards colder temperatures. This results in growth of the condensate. After equilibration, the net "gain" of the atom laser is zero, i.e., the condensate fraction remains constant until further cooling is applied.

Unlike optical lasers, which sometimes radiate in several modes (i.e. at several nearby frequencies) the matter wave laser always operates in a single mode. The formation of the Bose condensate actually involves "mode competition": the first excited state cannot be macroscopically populated because the ground state "eats up all the pie".

The output of an atom laser
The output of an optical laser is a collimated beam of light. For an atom laser, it is a beam of atoms. Either laser can be continuous or pulsed - but so far, the atom laser has only been realized in the pulsed mode. Both light and atoms propagate according to a wave equation. Light is governed by Maxwell's equations, and matter is described by the Schroedinger equation. The diffraction limit in optics corresponds to the Heisenberg uncertainty limit for atoms. In an ideal case, the atom laser emits a Heisenberg uncertainty limited beam.

- the atom laser has low energy per atom. Quite the opposite for high energy propulsion.

 

[Arian]

Why do you need to have plasma rockets, I am only speaking o Fusion not nessacily jump starting the reactions.

Pulsating Fusion Rockets shoot a hyrdogen combination into a reaction chamber and blow it up with intesne FEL lasers (Which are rather weak at the moment) and shoot the resulting energy and mass left over out ans 5%c which is accelerated to 10%c with basic rocket techniques.
The rockets require no giant magnetic chamber except for expelling out the reactents.

When wecome to the deacceleration problem. Is a magnetic brake possible, (even if going away from a sun).

A mag sail is a giant magnetic ring that uses magnetic entites to make it stop quickly.

 

[Astronuc]

Why do you need to have plasma rockets, I am only speaking of Fusion not nessacily jump starting the reactions.

Fusion generally implies plasma.

Pulsating Fusion Rockets shoot a hyrdogen combination into a reaction chamber and blow it up with intesne FEL lasers (Which are rather weak at the moment) and shoot the resulting energy and mass left over out ans 5%c which is accelerated to 10%c with basic rocket techniques.
The rockets require no giant magnetic chamber except for expelling out the reactents.

One needs to rethink the physics, especially energy yields in terms of energy/nucleon available from fusion reactions.

Also, Lasers and SC Magnets are rather massive systems.

It makes no sense to use a fusion power plant to produce electrical energy which is then used in laser power systems (hugely inefficient), but rather use fusion directly.

Magnetic brake? Where? Against what magnetic field or charged particle field?

A mag sail is a giant magnetic ring that uses magnetic entites to make it stop quickly.

Maybe in science fiction - not in the real world.

 

[Arian]

Mag Sail- orginally propsosed by Mark Zubrin, who later said a Mag Orion (a magnetic sail that uses bombs).
The Mag Sail could be stopped by magnetic fields of planets. Jupiter's for heaven sakes would easily stop it. The Sun's for that matter could decrease it (but maybe not fast enough).

You are right aboyut lasers and magnetic fields being heavy, but then again, machine guns were big and we made small ones, we canm do the smae with lasers. The new moon ship is small then the old one. Things get smaller.

And fusion only implies that when wo atoms hit each other in such a hot, intense way, that they merge and realize mass in the form of energy. Plasma is only regared because the sun is plasma and is currently the only fusion producing machine in the solar system.

 

[Astronuc]

Mag Sail- orginally propsosed by Mark Zubrin, who later said a Mag Orion (a magnetic sail that uses bombs).

Actually, Robert (Bob) Zubrin proposed Mag Sail and later, with Dana Andrews, proposed an Orion-driven Mag Sail.

The Mag Sail could be stopped by magnetic fields of planets. Jupiter's for heaven sakes would easily stop it. The Sun's for that matter could decrease it (but maybe not fast enough).

I invite one to substantiate these claims.

You are right aboyut lasers and magnetic fields being heavy, but then again, machine guns were big and we made small ones, we canm do the smae with lasers. The new moon ship is small then the old one. Things get smaller.

The laser and magnets are heavy, and there is a limit on how small based on energy density, or specific energy of the system.

And fusion only implies that when wo atoms hit each other in such a hot, intense way, that they merge and realize mass in the form of energy.

For nuclei to fuse, they must effectively be in a plasma state - completely ionized (E > 13.6 eV) - and normally at energies of several keV.

Plasma is only regared because the sun is plasma and is currently the only fusion producing machine in the solar system.

We produce plasmas on the Earth all the time, but they require much more energy input than they produce. Certainly a 'practical' fusion-based energy production device has yet to be perfected.

 

[Arian]

Actually, Robert (Bob) Zubrin proposed Mag Sail and later, with Dana Andrews, proposed an Orion-driven Mag Sail.

I'm sorry, wrong name, but I didn't have me book with me.

I invite one to substantiate these claims.

Jupiter has a large magentic field, which also creates a very large particle field, infact, only a few of its moon escape this particle field, allowing them,perhaps, exsistence. (of home for life crap)
And a magnetic field is going to stop something.
A flight manuver would be to have the positive end of the craft pointing towards jupiters positive end. The two repel, decreasing the speed of the moving craft.

For nuclei to fuse, they must effectively be in a plasma state - completely ionized (E > 13.6 eV) - and normally at energies of several keV.

Hence the laser