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Clean nuclear for my starship.


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What you describe is not predicted by classical physics. According to the basic laws of motion, if no outside force acts on the body, or the body doesn’t expel mass, it’s speed and direction won’t ch

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My vote is still for some form of ion drive.

The nuclear fuel would be used only to make electricity. Lots of it.

Say, 50 Megawatts.

This would accellerate an ionized plasma out the engine at speeds in excess of 1% speed of light. Very efficient thrust. The Shuttles main engines have an I(sp) rating of about 450 seconds, a nuclear thermal hydrogen engine would have a rating around 5,000 seconds and a well designed ion engine would have a rating of over 50,000 seconds.

 

I(sp) is a standard measure of rocket engine efficiency. Think of it as: How many seconds will one pound-mass of fuel provide one pound-force of thrust?

 

The current ion engines being built use Xenon as the propellant mass. Partly because it is chemically inert even as a plasma. Expensive stuff. Helium or Argon would work but the efficiency would be a little less.

 

The ion engine can "burn" for days, weeks, months at a time. Given an accelleration of only 1/32 of a Gee (one foot per second squared), in one day you would build up a velocity of 86,400 feet per second. That's 16.4 miles per second.

 

Starting from the Earth/Moon system, and keeping the engine on continuously, you would pass Pluto/Charon in about 108 days, less than 4 months, at a whopping speed of 1768 miles per second. :doh:

 

Building up a velocity of 1/10 the speed of light (18,600 miles per second), at an accelleration of 1 foot per second squared, would take 98,208,000 seconds, or 3.11 years. Alpha Centauri, here we come!!

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AFAIK, there are essentially 4 types of “nuclear rockets”:

  • Nuclear thermal rockets. An old design, considered for the Apollo program, and featured in R.A.Heinlein’s 1947 novel “Rocket Ship Galileo” and the 1950 movie ”Destination Moon”. A fission reactor heat a reaction mass such as hydrogen or water, expelling it at a high speed. Prototypes have been built and tested. Unless extraordinarily well shielded, their exhaust is dangerously radioactive.
  • Nuclear pulse propulsion. Another old design, perhaps best known by the private Project Orion rocket design, or, here at hypography, the engine of 6950. Small fusion “bomblets” are detonated behind a “pusher plate” shield, driving the ship forward (bomblets debris is the reaction mass). Hypothetical. At best, about as dangerous to anyone near the “wrong side” of the shield as a barrage of small hydrogen bombs.
  • Nuclear-powered ion or other accelerator rockets. Energy from a nuclear-powered generator is used to power an ion thruster, or some other machine to accelerate reaction mass to a high velocity. A possible variation is
    • Nuclear-powered tether propulsion. A nuclear-powered generator supplies a electrodynamic tether with electricity. Requires no reaction mass, but can only be used near bodies with substantial magnetic fields, such as Earth or a giant planet such as Jupiter.

    Other than the usual safety and waste disposal problems associated with nuclear powered, such propulsion is clean. It has a much lower thrust-to-mass ratio than the previous, “dirty” designs.

    [*]nuclear photonic rockets. A nuclear reactor is used to heat an emitter to a very high temperature. The photons emitted are its “reaction mass”, so it is essentially a reaction mass-less drive. It has a very low thrust-to-mass ratio.

I’m familiar with many proposals for “gyroscopic drives” (I had one myself, including several prototypes, as a teenager). AFAIK, all attempt to exploit gyroscopic precession to propel a spacecraft without using any reaction mass. According to the law of conservation of momentum, such designs cannot work, regardless of their specific arrangement.

 

One of the reasons a gyroscopic drives was attractive to me, as a young person with experience with various gyroscopes (including a “monster” made of a heavy flywheel and a 1.25 HP garbage disposal motor), but lacking a detailed knowledge of the fairly complicated physics of gyroscopes, was because of the relative easy with which I could make a small gyroscope propel itself horizontally in a large radius circular path, using the force of gravity to induce a velocity 90° in the direction of its rotation. When my prototypes failed to work as I’d hoped, I wrote a letter to the Sperry Corporation, once a major maker of gyroscopes, and received a very detailed and helpful reply from one of its engineers, explaining my misconceptions, and suggesting additional reading to improve my understanding of gyroscopes.

 

Though I’m curious to see ScotKrieger’s design, I stongly suspect it suffers from the same failings as my own.

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The essence of the gyro-scopic space drive.

Imagine a giant frizbie flung into the void of deep space.

Give it a high spin rate of say 100,000 rpms.

Give it a velocity close to the spin velocity at the rim.

The result is that the frizbie will travel a path that is circular.

I.E. it will come back to you.

 

I don't remember the formula, but it is directly related to:

1. The mass of the frizbie.

2. The rpms of the frizbie.

3. The velocity of the frizbie.

 

Now who out there can remember the formula?

A Dr Stevenson (Dr of Physics) once taught me the formula in a physics class. He said the problem was that in order to see any measureable results, that you would have to either have mass in the millions of pounds or very high speeds. I remember using the formula in class to demonstrate that he was right, but almost thirty years have dimmed my memory of the formula. The problem thirty years ago was that materials did not exist that would allow us to create anything that could spin and be controlled at the speeds we are talking about. But times have changed.

PS I am pretty good at math, are you?

This is a math Q, try it for fun.

if X cubed = -1, prove X = -1, proof IS NOT saying -1 cubed = -1.

Ans in next reply.

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Boerseun is always right* because he's "the Doctor".

 

*except when he's wrong, of course.

 

I suspect your Dr. Stevenson may have been trying to use Relativity to steer the frizbie spaceship. Assume a velocity near 1/2 c, going directly away from an observor on the Earth. Assume the spin of the frizbie at its rim is near 1/2 c. Then one could argue that the rim on one side (say, the left) is barely moving at all (almost stationary), relative to the observer on Earth, and the other side (right) is traveling at nearly c, relative to the same observer.

 

With a little bogus math, it may (MAY!) be possible to finagle some kind of steering force out of this by assuming the left rim has a "normal" mass and the right rim has a much higher relativistic mass. But again, this result would be bogus.

 

Alterations in mass are <<observed>> phenomena from the Earth. They would not occur at all to an observer on the frizbie ship itself.

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I too have heard of exploding nukes as a means of propulsion. It was dismissed because of the radiation and because the acceleration was too high to be practical on anything that was of moderate or small size. If we could build something big (massive) enough with heavy shielding then we might as well bring enough propellent for conventional rockets instead.

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The essence of the gyro-scopic space drive.

Imagine a giant frizbie flung into the void of deep space.

Give it a high spin rate of say 100,000 rpms.

Give it a velocity close to the spin velocity at the rim.

The result is that the frizbie will travel a path that is circular.

I.E. it will come back to you.

What you describe is not predicted by classical physics. According to the basic laws of motion, if no outside force acts on the body, or the body doesn’t expel mass, it’s speed and direction won’t change.

 

Perhaps you mean for the speed of the center and rim of the Frisbee to be a significant fraction of the speed of light?

 

According to special relativity, this would result in one side of it having a different mass and shape than the other. It would be difficult to describe this, as it would be necessary to consider how the forces holding the frisbee together act in relativistic terms, with distance, time, and simultaneity different for different parts of it. A simplistic analysis, however, predicts that, because one side of the frisbee’s rim masses greater than the other, the frisbee would experience a force and acceleration perpendicular to its direction of travel, and would, as you suggest, travel in a circle.

 

I suspect a more thorough analysis would not. Given that the engineering challenges of building a frisbee that can spin a significant fraction of the speed of light and an engine that could spin it that fast are vastly beyond present-day capabilities, and that, even if analysis did not contradict the simplistic analysis, such a “drive” would not have no capability of changing the vehicle’s speed, only its direction, I can’t justify going to the effort of such an analysis.

 

If you have the ability to accelerate an spacecraft to the speed necessary for the proposed drive to work, why would you need such a drive?

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Wow! dissapear for a couple of days and things really get romping!:turtle:

 

Wikipedia has great resources on nuclear rockets.

The pulse rocket is a difficult scenario to see as feasible because fission bombs can't be made smaller than a few kilos and then at low efficiency. You really want many tiny ones per second to make it smooth enough. The only way for this is an antiproton "catalysed" reaction where an AP reacts with P in a heavy nucleus blowing it apart and releasing ~30 neutrons. Those neutrons wont be much use anyway because they'll get away in any small enough bomb, so youd use at least 1 antiproton for every couple of nuclei.

Saltwater nuclear rockets are fairly elegant, basically being a solution of fissiles in water fed into a combustion chamber at a rate to maintain a critical mass. Sort of a reactor with chain reaction limited by the exhaust velocity and water providing extra pressure and reaction mass. Could be efficient if your reaction chamber and thrust cone were magnetic bottles. Dirty as buggery though and unlikely to be as useful of reaction mass as a nice clean ion or photon system.

 

One thing on Wikipedia that I don't understand. They reckon the most energy efficient rocket varies its exhaust velocity to expel its reaction mass at equal but opposite velocity to its forward velocity. How does it get of the start line? What if you are accelerating towards something moving away from you and measuring your velocity relative to that. Obviously squirting your reaction mass at something moving away from you isn't going to help you catch up let alone efficiently. I understand their logic that exhaust with no velocity has no kinetic energy and so all the KE is the ships but it still makes no sense to me.:hyper:

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One thing on Wikipedia that I don't understand. They reckon the most energy efficient rocket varies its exhaust velocity to expel its reaction mass at equal but opposite velocity to its forward velocity. How does it get of the start line? What if you are accelerating towards something moving away from you and measuring your velocity relative to that. Obviously squirting your reaction mass at something moving away from you isn't going to help you catch up let alone efficiently. I understand their logic that exhaust with no velocity has no kinetic energy and so all the KE is the ships but it still makes no sense to me.:(
Energy efficiency – how much energy it takes to give a payload a given kinetic energy – is of lesser concern in rocketry than in the mechanics of traction motors. Usually, specific impulse – how much reaction mass it takes to give a payload a given momentum – is a bigger concern.

 

Rocket equations are basically momentum equations, with energy calculated afterwards. Both the exhaust and the ship have momentum and kinetic energy. The momentum of the exhaust-ship system is always zero, while its total energy can be anything not less than zero.

 

Consider the following (unrealistically simple) examples of the momentum ([math]P = mass \times velocity[/math]) and kinetic energy ([math]E = \frac12 mass \times velocity^2[/math]) of a 1000 kg empty mass rocket ship with a change in velocity ([math]\Delta V[/math]) of 1 m/s, and various reaction masses:

 

[math]P= 1001 \, \mbox{kg} \cdot 0 \, \mbox{m/s} = 1 \, \mbox{kg} \cdot -1000 \, \mbox{m/s} + 1000 \, \mbox{kg} \cdot 1 \, \mbox{m/s} = 0[/math]

[math]E= \frac12 1 \, \mbox{kg} (1000 \, \mbox{m/s})^2 + \frac12 1000 \, \mbox{kg} (1 \, \mbox{m/s})^2 = 500500 \, \mbox{J}[/math]

 

[math]P= 1010 \, \mbox{kg} \cdot 0 \, \mbox{m/s} = 10 \, \mbox{kg} \cdot -100 \, \mbox{m/s} + 1000 \, \mbox{kg} \cdot 1 \, \mbox{m/s} = 0[/math]

[math]E= \frac12 10 \, \mbox{kg} (100 \, \mbox{m/s})^2 + \frac12 1000 \, \mbox{kg} (1 \, \mbox{m/s})^2 = 50500 \, \mbox{J}[/math]

 

[math]P= 1100 \, \mbox{kg} \cdot 0 \, \mbox{m/s} = 100 \, \mbox{kg} \cdot -10 \, \mbox{m/s} + 1000 \, \mbox{kg} \cdot 1 \, \mbox{m/s} = 0[/math]

[math]E= \frac12 100 \, \mbox{kg} (10 \, \mbox{m/s})^2 + \frac12 1000 \, \mbox{kg} (1 \, \mbox{m/s})^2 = 5500 \, \mbox{J}[/math]

 

[math]P= 2000 \, \mbox{kg} \cdot 0 \, \mbox{m/s} = 1000 \, \mbox{kg} \cdot -1 \, \mbox{m/s} + 1000 \, \mbox{kg} \cdot 1 \, \mbox{m/s} = 0[/math]

[math]E= \frac12 1000 \, \mbox{kg} (1 \, \mbox{m/s})^2 + \frac12 1000 \, \mbox{kg} (1 \, \mbox{m/s})^2 = 1000 \, \mbox{J}[/math]

 

[math]P= 11000 \, \mbox{kg} \cdot 0 \, \mbox{m/s} = 10000 \, \mbox{kg} \cdot -.1 \, \mbox{m/s} + 1000 \, \mbox{kg} \cdot 1 \, \mbox{m/s} = 0[/math]

[math]E= \frac12 10000 \, \mbox{kg} (.1 \, \mbox{m/s})^2 + \frac12 1000 \, \mbox{kg} (1 \, \mbox{m/s})^2 = 550 \, \mbox{J}[/math]

 

[math]P= 101000 \, \mbox{kg} \cdot 0 \, \mbox{m/s} = 100000 \, \mbox{kg} \cdot -.01 \, \mbox{m/s} + 1000 \, \mbox{kg} \cdot 1 \, \mbox{m/s} = 0[/math]

[math]E= \frac12 100000 \, \mbox{kg} (.01 \, \mbox{m/s})^2 + \frac12 1000 \, \mbox{kg} (1 \, \mbox{m/s})^2 = 505 \, \mbox{J}[/math]

 

The energy efficiency can be made arbitrarily close to 100% by making the ratio of reaction to payload mass arbitrarily close to infinity. But this is far from what most people would consider a “more efficient rocket ship”. Instead, what we think of as an “efficient” rocket is one where the reaction mass to payload is ratio is low. For example, a single-stage-to-orbit rocket (if one existed), with about the same mass of propellant as payload, we think of as more efficient than Saturn V, which has about 30 times as much propellant as payload.

 

As more compact sources of greater energy are developed, “next generation”, “advanced” rockets can be expected to continue to have higher payload to propellant ratios, but lower energy efficiencies.

 

The energy for a rocket – its fuel - must, of course, come from somewhere. Currently, most rockets get it from the chemical energy of their propellant – reaction mass and fuel are the same. Because most chemical reactions are (very roughly) the same in energy output, this puts a bottleneck on payload to propellant ratios. To have higher PTP ratios, NG rockets will need more to have much more energy available - nuclear explosives and antimatter seem promising ways to get it.

 

The energy from any fuel comes, ultimately, from change in it mass (per [math]E=M c^2[/math]), so there’s a limit to how high a PTP ratio can be, and a point of diminishing returns – but both are far, far off in terms of practical engineering.

 

Where energy is hard to get (such as solar-powered and/or long-duration spacecraft), a niche for low payload to propellant ratio, high energy efficiency rockets will likely exist.

 

PS: We talked a lot about rocket equations in last year’s A problem with KE = ½mv². Help.

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I've always been a big fan of flywheels as energy storage devices. If one at 1000rpm has say 1 kilojoule of kinetic energy then at 1 000 000rpm(technically feasible just) it would be storing 1 gigajoule. They may have enough energy storage to enable recycling energy from orbit to ground and back again.

I've also been contemplating the possibility of using a syncrotron as a particle drive. Seems to me it would allow much higher reaction mass velocity than a linear electrostatic ion gun device.

It may even be possible to use the syncrotron to store energy in flywheel fashion. The magnetic field would transfer centripetal forces on the particles to the structure, possibly allowing far higher rotational velocity of mass than a physical flywheel could be engineered to handle. Superconductor magnets would need no energy to do this.

It may even be possible to use relativistic mass increase to increase the energy stored in a smaller number of particles.

Of course relativistic particles bleed out and fired away as rocket exhaust should be a phenominal use of reaction mass.

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I've always been a big fan of flywheels as energy storage devices. ...

relativistic particles...fired away as rocket exhaust should be a phenominal use of reaction mass.

Flywheels are great for storage of energy in a machine that can handle rotational motion, but a rocket engine is not one of those machines. There is no practical way of converting rotational momentum, however great, into propulsive thrust.

 

The use of relativistic particles as a rocket exhaust is already being researched. It's called "ion propulsion". The particles are atomic nuclei electrically accelerated to very high speed. The exhaust of the NEAR probe to the asteroid EROS was emitted at more than 1% of the speed of light, in the region where relativity begins to have significant effect.

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Flywheels are great for storage of energy in a machine that can handle rotational motion, but a rocket engine is not one of those machines. There is no practical way of converting rotational momentum, however great, into propulsive thrust.

 

The use of relativistic particles as a rocket exhaust is already being researched. It's called "ion propulsion". The particles are atomic nuclei electrically accelerated to very high speed. The exhaust of the NEAR probe to the asteroid EROS was emitted at more than 1% of the speed of light, in the region where relativity begins to have significant effect.

Thats a pretty impressive velocity at 3 000 000 m/s. Was that a linear electrostatic accelerator? I wouldn't have thought it was possible to get them that fast that way.

Still if we want to go faster than that then the most energy efficient way is seemingly even more eg/ at 0.9c our exhaust should be 0.9c velocity. I'm still not sure of what we need for good efficiency when slowing down from that sort of v. Relativity is so confusing.:turtle:

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How do you use nuclear fuel to propell rockets anyway? Heat can be produced, but for the thrust, you'll need some mass to be ejected right?

 

Power for a star ship is orders of magnitude higher than for a interplanetary space craft. What we need at this time is a reliable reusable nuclear powered space craft. It is possible to use nuclear power to raise payloads into orbit and to travel around the solar sytem. We need to develop magnetic confined plasma fission nuclear engines. This is the link to the info about this concept by the author who I am sure can explain it much better than me. Oh well I still can't post a link to the forum. Mabe you can google it at nuclear space.com Liberty ship. If you can take a look at this, I'm not sure if will do for intersllar but it nails down interplanetary for sure. A reliable, reusable nuclear rocket that can be used to do almost anything inside the solar system. Maybe the concept could be modified for use in an interstellar type space craft.

 

Michael

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