Nuclear Fusion Rockets v.s Fission

1. Apr 27, 2006

Arian

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 whats this 2 month crap?:surprised

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

2. Apr 27, 2006

scott1

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

3. Apr 27, 2006

Janus

Staff Emeritus
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.)

4. Apr 27, 2006

Staff: Mentor

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....

5. Apr 28, 2006

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.

6. Apr 28, 2006

Astronuc

Staff Emeritus
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.

7. May 1, 2006

Arian

MY source is "Entering Space" by Robert Zubrin

8. May 10, 2006

pervect

Staff Emeritus
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.

Last edited: May 11, 2006
9. May 10, 2006

dak246

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.

10. May 10, 2006

Astronuc

Staff Emeritus
It would seem rather impractical to build a space ship 100 miles in diameter. Even 100 m in diameter would seem rather impractical.

11. May 10, 2006

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.

12. May 16, 2006

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.

13. May 22, 2006

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.

14. May 22, 2006

scott1

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.

15. Jun 25, 2006

Arian

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

16. Jun 26, 2006

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.

Last edited by a moderator: Apr 22, 2017
17. Jun 26, 2006

Staff: Mentor

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.

18. Jun 26, 2006

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! :surprised

19. Jun 27, 2006

Astronuc

Staff Emeritus
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.

20. Jun 27, 2006