Maximum "speed" for a nuclear-based engine system?

I'm not sure if this is exactly what you're asking, but there's a good science book called Cosmos by Carl Sagan that talks about a drive consisting of "small nuclear explosions against an inerial plate" providing a sort of "cosmic putt-putt." He says that a ship with this drive could get up to as much as 10% the speed of light. I think this means you could zip back and forth throughout the solar system within a matter of hours - although it would still take 80 years to reach the nearest solar system and get back again.

The book, with loads of photos, was based on the awesome 13-part TV series.

Assuming propellant is not a problem (lets say you can scoop up hydrogen and burn it), then there is no limit. At one G constant acceleration, you will, within months, be zipping from one star to the next in minutes. (This is ship time, of course.)

Dave, you said near-limitless fuel supply. For simplicity's sake, let's say we have no limit on fuel. Theoretically then, you could accellerate to the limits of the phyiscal universe, ie: light speed (C). How long it took you to get going that fast would depend on the size of the engines.

Also, as you pointed out, stopping is as much of a problem as is getting going. In a standard chemical rocket, 90% or more of the initial mass is fuel. If you wanted to go to Neptune with you could accelerate most of the way, since the force required to stop your vessel is vastly less.

That's for a one way trip. To ensure a return ticket, you will either need a fuel source at Neptune, or take TWICE as much fuel along on the outbound trip.

Often, descriptions of the space ships of SF tend to gloss over fuel supply, capacity, distances, etc., because the stories deal with other issues. Imagine the Enterprise dead in space without fuel. I seem to recall a few episodes where they were running low...and more where the ship's engines broke down in one way or other, but I can't think if a single story where the ship needed a tanker to come by and pour dilithium crystals into the hopper.

http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

Pthom said:

If you wanted to go to Neptune with you could accelerate most of the way, since the force required to stop your vessel is vastly less.

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Bad physics.

caw

Try this:

http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

(The closing parenthesis was missing in the link.)

blackbird said:

Bad physics.

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Poor Isaac.

It sounds like you're describing the NERVA propulsion system.

But do you have any idea what a "nuclear engine" is? Briefly, to provide acceleration there must be a force acting in the opposite vector. I'm hazy, but I seem to recall that the most you could hope for would be 0.1 g.

Monstrous big high-tech engines generate a huge amount of heat. You need to get rid of it, or your engine will slag out rapidly. What you could do in atmo with a heatsink the size of a paperback would take something like a hundred times the surface, plus some sort of wafting else the heat puddles.

And, no, you can't scoop up hydrogen (interstellar or otherwise) while zooming betwixt planets. To do so (even with something like a Bussard design) requires movement to force the hydrogen into the igniter. Ever been stuck behind an overladen truck in city traffic? Even with all the upshifting, it takes two blocks to get to 10 mph, & there comes another red light.

Pthom said:

Dave, you said near-limitless fuel supply. For simplicity's sake, let's say we have no limit on fuel. Theoretically then, you could accellerate to the limits of the phyiscal universe, ie: light speed (C). How long it took you to get going that fast would depend on the size of the engines.

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You can never get to light speed, however that hardly matters to the people on the ship. For them it seems that they are going faster and faster (because of time dilation). At the speed of light (obtained only by light itself), time stops, and a photon can cross the universe in no time at all, at least by its own reckoning.

dclary said:

Well, how does a nuclear power plant work? You split an atom, and the resulting atoms around it all split and there's a big ass explosion that gets shunted somewhere and you've got electricity, right?

Couldn't you do the same thing but that energy is shunted out a big hole in the back of your ship? The explosion's force, then, exerts forward and backward, propelling the ship forward, as that's the equal and opposite reaction.

Right?

I just want something relatively low-tech (near or at current tech) that's reachable within 20 years, if we'd get off our asses and do it.

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It's conservation of momentum (mass x velocity). Momentum going out the back end equals the gain of momentum of the ship. Energy (in the form of photons) has momentum (even though it doesn't have mass), but it's much more efficient to use high velocity particles, ie, to accelerate a propellant to as high as speed as possible. So the mv of the propellant is equal the delta mv of the ship.

I'm lousy at math, but if could you calculate how long it would take to reach the midpoint of the journey accelerating at a constant 1G, then double it (for the second half of the journey spent decelerating), you'd have a believable timeline for your journey.

As Pthom pointed out upthread, the ship would be lighter when you did the flip, so it would take less 'juice' to slow you down on the second leg. But you still would want to flip over at the midpoint since going beyond it would require greater and greater g-force deceleration the further beyond the midpoint you went. Again, my math is bad, but if you accelerated at 1G for 3/4 of the trip and used up 3/4 of your fuel, sure you'd be able to slow down with the remaining fuel but you'd have to do so at 4Gs or whatever. Not too comfy for the crew. No, flipping over at the midpoint makes the most sense because then your acceleration/deceleration would be the same Gs throughout the trip.

Anyway -- as a reader, I could easily buy into a nuclear engine providing the thrust for such a trip.

Possible.

Not likely in 20 years, though.

More likely is a 3-month-long one-way trip to Mars, using mostly ordinary chemical rocketry. As I see it, you don't get to leave from a Greater Los Angeles Space Port, but from some orbital station. Then you accelerate most of the way as you use up fuel. At some point you flip the ship end for end (figuratively) and use your engines to slow down. Once there, you refill your fuel tanks and repeat the process coming home.

The bigger problem is where the planets are when you decide to make the trip. When Mars and Earth are on the same side of the sun (happens about once every two years), the trip is relatively easy. But to go next Thursday, Mars could be around on the other side of the sun and therefore twice as far away. As for Neptune, it's "year" is about 165 Earth years. So wherever it is at the time of your story, it will be pretty much in that spot the whole time: 30 times farther from the sun than is Earth.

On this website:
http://neptune.spaceports.com/~helmut/exploration99/strategy1/2_6_2_future_technology.html

You will find the following quote:

Continuous Linearly Accelerating Systems

To make use of this technique, the spacecraft would have to be accelerated during the first half of the mission, then turned around and slowed down for the second half of the mission. In this manner, a constant force of artificial gravity can be provided.

Imagine a spacecraft accelerating at 1 g. After only 35 days you would reach relativistic speeds and the resulting mass increase would slowly decrease the apparent level of gravity. A trip to Mars aboard such a spacecraft would last only three days, considering both acceleration for the first half and deceleration for the second half. Unfortunately, existing space transportation systems do not have the technology to maintain this order of acceleration.

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Michael said:

I'm not sure if this is exactly what you're asking, but there's a good science book called Cosmos by Carl Sagan that talks about a drive consisting of "small nuclear explosions against an inerial plate" providing a sort of "cosmic putt-putt."
...

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As noted, this was Project Orion, a serious but kinda sucky idea. Imagine having to explode dozens of nuclear bombs to get into space - not nice for those left on Earth. It's less bad as a propulsion method if you're already in outer space. It makes an appearance (in a launch from Earth!) in the Niven/Pournelle's novel "Footfall." I think of Project Orion as more of a BANG-BANG than a putt-putt.

Adding to confusion is that one of the future NASA projects to go back to the Moon or to go to Mars (I forget exactly) is also named Orion. I about had a heart attack when I saw that name, I thought they were reviving the old nuke-bomb-propulsion idea and seriously considering doing it.

Julie Worth said:

Assuming propellant is not a problem (lets say you can scoop up hydrogen and burn it), then there is no limit. At one G constant acceleration, you will, within months, be zipping from one star to the next in minutes. (This is ship time, of course.)

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Giving the names for these things may be useful for those wanting to investigate further. This (an engine that sucks up interstellar hydrogen to use as fuel and/or propulsion mass) is called a Bussard Ramjet.

dclary said:

Well, how does a nuclear power plant work? You split an atom, and the resulting atoms around it all split and there's a big ass explosion that gets shunted somewhere and you've got electricity, right?

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The atoms are split at a controlled rate so it doesn't heat up to a million degrees and blow up, but the splitting atoms (and resultant neutrons hitting things) creates lots of heat, which is used to turn water into steam to run a turbine which turns the electric generatir, just as in a traditional coal-fired power plant.

dclary said:

Couldn't you do the same thing but that energy is shunted out a big hole in the back of your ship? The explosion's force, then, exerts forward and backward, propelling the ship forward, as that's the equal and opposite reaction.

Right?

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Right, and that's Project Orion.

There's also nuclear power propulsion that looks more like (part of) an electric power plant. Nuclear reactions create heat in the "bell" of a rocket motor, water is injected which the heat turns into steam which exits the rocket motor at high speed, causing the rocket to move forward. Since the nuclear material remains sealed up, this is much cleaner than Project Orion.
Here's a description:
http://www.astrodigital.org/space/nuclear.html
Regardless of how it's done, a rocket needs propellant to work, and that's going to be part of the payload (unless you do the Bussard Ramjet thing), so you always have to take that into consideration - it will limit how fast you can go (you can always go farther with the same fuel by coasting much of the way, and not going as fast).

dclary said:

I just want something relatively low-tech (near or at current tech) that's reachable within 20 years, if we'd get off our asses and do it.

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I suppose either one of these (Project Orion or more 'traditional' nuclear rocket) is possible in that timeframe. The Bussard Ramjet is much more ambitious, and much less likely to be operational in 20 years.

In a rocket, mass is expelled from an orifice at high velocity. In "ordinary" rockets (bottle rockets, the Space Shuttle, ICBMs), the mass is the fuel itself, undergoing a (hopefully) controlled explosion to impart the high velocity.

In the nuclear steam example, the water isn't exploding, but is converted from liquid to gasseous state by the heat of the nuclear "fire."

But yes, since the water (in the form of steam) is what leaves the ship, that's the propellent.

________________

It occurs to me that if you were to travel a lot using the Bussard Ramjet concept, you would ultimately "sweep up" all the free hydrogen, making travel an iffy proposition.

Roger J Carlson said:

On this website:
http://neptune.spaceports.com/~helmut/exploration99/strategy1/2_6_2_future_technology.html

You will find the following quote:

...
Imagine a spacecraft accelerating at 1 g. After only 35 days you would reach relativistic speeds and the resulting mass increase would slowly decrease the apparent level of gravity. A trip to Mars aboard such a spacecraft would last only three days, considering both acceleration for the first half and deceleration for the second half. Unfortunately, existing space transportation systems do not have the technology to maintain this order of acceleration.

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The main "technology" needed to maintain acceleration is the amount of fuel required. Imagine the space shuttle, but it needs a fuel tank to go to Mars and back in a few days, and the fuel tank needs to hold maybe 100 to 1,000 times as much fuel as the big external fuel tank as is currently used for launch into orbit. This gets really impractical for chemical rockets, which makes nuclear rockets more desirable. But even nuclear rockets still need sufficient "reaction mass" to throw out of the engine for long-term acceleration.

benbradley said:

But even nuclear rockets still need sufficient "reaction mass" to throw out of the engine for long-term acceleration.

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Not to mention equally sufficient "reaction mass" to slow the craft down to its destination.

caw

Pthom said:

Once there, you refill your fuel tanks

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With . . . ?

caw