Fun with: Ion Engines

[rhoenix]

Thanks to Ender pointing out a rather weighty flaw with the idea of a photon engine, that unless handwavium is used to make the power requirements anywhere near acceptable, the idea is simply impractical.

With that in mind (and reading a bit more closely at the figures this time), ion engines, though they have low thrust, are an excellent alternative. They're also not anywhere near as dramatically messy as a photon engine would be.

However, they are by necessity a low-thrust engine, simply due to how positive and negative ions interact.

From the Atomic Rocket page, here are the figures listed for an Ion drive:

Thrust (in gigawatts): 1.05
Exhaust Velocity (meters / second): 210,000
Thrust (in Newtons): 10,000
Engine Mass (in tons): 400
Power requirement (in megawatts): 800
Efficiency: 96%

Not bad at all, though without handwavium to get around the ion drive's low thrust by virtue of physics, it's much more sedate than, say, a photon engine.

Now for the question - is there any way anyone can think of to get around the ion drive's low thrust without suggesting a different method of propulsion?

[Hawkwings]

Don't use it if you want to get anywhere fast?

NASA used ion drives on a couple comet rendezvous probes. Took 'em a while to get going, but they were moving at a very respectable clip (something like 75km/s maybe) by the time they got there.

[Ender]

Now for the question - is there any way anyone can think of to get around the ion drive's low thrust without suggesting a different method of propulsion?

NSTAR style? No. It is physically impossible to scale up that kind of ion engine for high thrust. Increase mass flow and you decreaese the energy delivered to individual particles. Increase the voltage, and you get vacuum arcing across the engine.

Fortunately, the term "ion engine" is sufficiently vague that one can mimic other things like GBEs or relativistic jets or HHOs in nature to get a high thrust high velocity stream, if your tech is advanced enough.

[TheLemur]

Exhaust Velocity (meters / second): 210,000

Do you people have any idea of the scale of interstellar travel? For convenience, I've calculated a table of the mass of fuel required per kilogram of dry mass for a given speed at this exhaust velocity.

To Pluto in: Mass required:
Two years .6 kilograms
One year 1.44 kilograms
Three months 34 kilograms
One month 43 metric tons
Two weeks 12,500,000 metric tons
One week 15,700,000,000,000,000 metric tons

For reference, Proxima Centauri, the nearest star, is roughly five thousand times farther away than Pluto is. So even the last figure, to Pluto in one week, would require around ninety years to reach Proxima.

[Wyrm]

For reference, Proxima Centauri, the nearest star, is roughly five thousand times farther away than Pluto is. So even the last figure, to Pluto in one week, would require around ninety years to reach Proxima.

Mr Lemur, please note that the point of an ion engine is not to get you there fast, but to get you there efficiently. Ion engines have a low thrust but high specific impulse. That means although you have to wait around two years to get you to Pluto (ignoring for the moment the fact that, for an interplanetary mission, that's damn fast), you don't have to spend so much energy getting you and your ship (fully loaded with fuel) up into orbit, which is the most expensive part of the trip.

As for it taking years to reach Proxima Centauri... its an STL ship. It's going to spend years in transit anyway. Even with the fastest STL drives we have on the drawing board still gives you a few decades to the nearest stars.

[TheLemur]

Mr Lemur, please note that the point of an ion engine is not to get you there fast, but to get you there efficiently.

Really? So why don't we walk everywhere here on Earth? Sure, it takes several months to walk from coast to coast, but it's far more efficient than taking, say, an airplane. If energy efficiency is really such a big concern, why are we launching large spaceships in the first place?

Ion engines have a low thrust but high specific impulse.

As if I didn't already know this. My figures assume that the acceleration is instant; if it takes time to get up to speed, it'll take even longer. And I excluded the amount of propellant required to slow down; my mistake. Sorry. Please square each of the figures if you plan to stop and not go hurtling off into interstellar space.

(ignoring for the moment the fact that, for an interplanetary mission, that's damn fast)

It's fast for a modern-day interplanetary mission. For a sci-fi civilization, could you imagine paying passengers waiting a full two years to get from Earth to Pluto? And then two years back? What about military ships? Cargo ships?

you don't have to spend so much energy getting you and your ship (fully loaded with fuel) up into orbit, which is the most expensive part of the trip.

(laughs)

Modern ion engines simply cannot be used for getting into orbit. Cannot. Thrust is orders of magnitude too low.

As for it taking years to reach Proxima Centauri... its an STL ship. It's going to spend years in transit anyway.

Even for the last figure I quoted, which required a small asteroid's worth of fuel per kilo of dry mass, it would take a full hundred years to reach Proxima. A hundred years is not exactly a realistic travel time (for humans, anyway). If we use realistic fuel figures, it'll take closer to a thousand years, which is so long the ship probably won't work when you get there.

[Kuroneko]

Mr Lemur, please note that the point of an ion engine is not to get you there fast, but to get you there efficiently.

Really? So why don't we walk everywhere here on Earth?

Obviously, because we'd like to get there faster. That isn't necessarily a prime objective for certain types of ships, e.g., exploration or in a fictional universe, supply train.

If energy efficiency is really such a big concern, why are we launching large spaceships in the first place?

Maximizing specific impulse really is a big concern for long-term missions. It simply costs much less to get somewhere; yes, the trade-off is overall time, but the economic gains would be often worth it.

[fusion]

Ehh TheLemur, this is SLaM which deals which real life and things that happen in a few years (realistic).
Not bad idea for an interplantary travel, but solar sails are still much more effective. You pay nearly zero for the fuel, you just use the solar power.

[TheLemur]

Not bad idea for an interplantary travel, but solar sails are still much more effective. You pay nearly zero for the fuel, you just use the solar power.

Solar sails will not work out past the asteroid belt, not enough sunlight. We use RTGs for the same reason.

Ehh TheLemur, this is SLaM which deals which real life and things that happen in a few years (realistic).

Really? Where does it say that?

I'm also curious what orbits you used in your calculations here.

I didn't bother with orbits. Even getting to Pluto in two years would require velocities on the order of 100 km/s; an additional 7 km/s or so to break free of the Sun's gravity wouldn't have that big of an effect.

That isn't necessarily a prime objective for certain types of ships, e.g., exploration or in a fictional universe, supply train.

(laughs)

Exploration requires a lot of speed so that you can get to wherever you're exploring before the project gets canceled or runs out of funding. Supply trains also require speed because the faster they can go back and forth, the more profit is made by the owners.

Maximizing specific impulse really is a big concern for long-term missions.

Agreed, but ion engines sure as hell don't have a very high specific impulse by interstellar standards; exhaust velocity is only 0.07% of C.

[Wyrm]

Mr Lemur, please note that the point of an ion engine is not to get you there fast, but to get you there efficiently.

Really? So why don't we walk everywhere here on Earth? Sure, it takes several months to walk from coast to coast, but it's far more efficient than taking, say, an airplane. If energy efficiency is really such a big concern, why are we launching large spaceships in the first place?[/qoute]

You do realize that in space you have to carry all your reaction mass with you from the get go, and for the entire trip, do you not? Unlike terrestrial travel, where you can pull most of your reaction mass from the atmosphere when you fly an airplane. Also, unlike in terestrial travel, there's very little drag in space, which means that small, sustained thrusts can build up in very little time.

Your analogy fails completely. Ion engines are practical when specific impulse is of overriding concern. In spaceflight, it is an overriding concern.

Ion engines have a low thrust but high specific impulse.

As if I didn't already know this. My figures assume that the acceleration is instant; if it takes time to get up to speed, it'll take even longer.

And any engine with a specific impulse lower than that of an ion drive is going to take more reaction mass per kilogram of payload than an ion engine. Look on Atomic Rocket's engine choice section, look for 'specific impulse'.

Presumably, rhoenix is talking about ion engines because he's feeling out a corner of hard sci-fi that his story comfortably fits in. An ion engine will get you to Pluto in two years (your figure is wrong, by the way: the mass ratio of fuel to payload is, to first order, 3.00 for the two year trip with an ion engine. But then, the same trip with chemical rockets has a mass ratio of 2.11e22. Ugh. Getting out of the solar system on ion engines will require a mass ratio of merely 18.97. Chemical rockets? Don't ask.

And I excluded the amount of propellant required to slow down; my mistake. Sorry. Please square each of the figures if you plan to stop and not go hurtling off into interstellar space.

Whatever. As you found out yourself, impatience is penalized.

(ignoring for the moment the fact that, for an interplanetary mission, that's damn fast)

It's fast for a modern-day interplanetary mission. For a sci-fi civilization, could you imagine paying passengers waiting a full two years to get from Earth to Pluto? And then two years back? What about military ships? Cargo ships?

Depends greatly on why you want to get to Pluto. Two years is a long time for a pleasure trip, but if you want to start a new life on Pluto while fleeing almost certain doom on Earth, two years is damn reasonable.

And no, if ion engines are the state of the art of rocket propulsion, then pleasure cruises to Pluto will simply not be possible. Accept it and move on.

you don't have to spend so much energy getting you and your ship (fully loaded with fuel) up into orbit, which is the most expensive part of the trip.

(laughs)

Modern ion engines simply cannot be used for getting into orbit. Cannot. Thrust is orders of magnitude too low.

I wasn't talking about liftoff, moron. I was talking about the leg of the trip after that, which is getting from just outside Earth's gravity well to Pluto. On liftoff, getting the payload into space as quickly as possible minimizes fuel costs (because of gravitational drag), thus the optimal solution is high thrust, even at the expense of low specific impulse.

In space, drag is not much of a problem. You are free to use tiny amounts of thrust over long periods of time to achieve high delta-V. You can slowly thrust into higher and higher orbits until you escape Earth's gravity, then build into higher and higher solar orbits until you reach Pluto. All on very little thrust.

As for it taking years to reach Proxima Centauri... its an STL ship. It's going to spend years in transit anyway.

Even for the last figure I quoted, which required a small asteroid's worth of fuel per kilo of dry mass, it would take a full hundred years to reach Proxima. A hundred years is not exactly a realistic travel time (for humans, anyway). If we use realistic fuel figures, it'll take closer to a thousand years, which is so long the ship probably won't work when you get there.

Nobody said that interstellar travel is easy, dearheart.

Any engine with a lower specific impulse than ion engines will by necessity take even more fuel to achieve the same delta-V. To achieve better than a small asteroid, you need an engine with higher specific impulse, period. As bad as the ion figure seems, chemical rockets would need even more fuel to make the same trip.

[Wyrm]

I'm also curious what orbits you used in your calculations here.

I didn't bother with orbits. Even getting to Pluto in two years would require velocities on the order of 100 km/s; an additional 7 km/s or so to break free of the Sun's gravity wouldn't have that big of an effect.

Hmmm, i calculated the excape velocity of the sun to be 618 km/s. But I guess that doesn't take into account the fact that we live almost on the upper lip of the sun's gravity well. Mea culpa.

My calculation of a delta-V of 231 km/s was based on constant acceleration of 0.0003 G, which is for a high-thrust ion engine with a light payload, and the final velocity was just from the fact that you needed to cover 7.3 billion km in two years.

That isn't necessarily a prime objective for certain types of ships, e.g., exploration or in a fictional universe, supply train.

(laughs)

Exploration requires a lot of speed so that you can get to wherever you're exploring before the project gets canceled or runs out of funding.

By your own crude calculations, it can take as little as two years to get to Pluto. This is about the same time it took Voyagers 1 & 2 to reach Jupiter, which is about ten times closer, and Gallileo took six years to make the same trip.

Of course, to achieve this acceleration, you need accelerations at the high end of what is possible for ion engines...

Supply trains also require speed because the faster they can go back and forth, the more profit is made by the owners.

Not necessarily. Bulk supply trains for predictible consumables require a steady flow of goods to the customer. But that can be achieved by simply launching many vehicles in succession by about as much time between as you want them to arrive. It requires a large capital investment, but once the trains start arriving, the income is steady.

Maximizing specific impulse really is a big concern for long-term missions.

Agreed, but ion engines sure as hell don't have a very high specific impulse by interstellar standards; exhaust velocity is only 0.07% of C.

If they're the best you got, then they're the best you got. If rhoenix plans to take the slow road to the stars, then he needs a better engine.

[rhoenix]

I remember Kuroneko suggesting a two-stage system before, and given what I'm reading and finding out more about in this thread, using ion engines would make the most sense for pure space travel, with a much higher power system for descending into or leaving through an atmosphere.

Currently, I really can't see a better idea, though I'm certainly not done looking.

[TheLemur]

Hmmm, i calculated the excape velocity of the sun to be 618 km/s.

(laughs)

Even if we did live on the surface of the Sun (what surface?), the faster you are going, the less extra velocity you need to add to get out of a gravity well. For example, if you want to overcome a 1 MJ/kg gravity barrier starting from scratch, you'll need to accelerate to 1,440 m/s. But if you're already going at 5,000 m/s, you only need to accelerate by 200 m/s.

My calculation of a delta-V of 231 km/s was based on constant acceleration of 0.0003 G, which is for a high-thrust ion engine with a light payload, and the final velocity was just from the fact that you needed to cover 7.3 billion km in two years.

I assumed 100 km/s, because I did the conservative thing and assumed acceleration was instant. Taking several years to accelerate would obviously make fuel economy *worse*, not better. I also forgot to add in the fuel for slowing down, which again *helps* my argument because it means that the real figures will be even worse than the ones I calculated out.

By your own crude calculations, it can take as little as two years to get to Pluto. This is about the same time it took Voyagers 1 & 2 to reach Jupiter, which is about ten times closer, and Gallileo took six years to make the same trip.

Then it's not going to be a concern for exploring Pluto; it will be a concern for exploring anything farther out than the Kuiper Belt. For closer missions, you need lots of mass for equipment, electronics, science experiments, and what not, so you still need a very good rocket engine if you don't want an astronomical launch mass, and the return of the mission is still proportional to how good the engine is.

Bulk supply trains for predictible consumables require a steady flow of goods to the customer.

Consumables are not "predictable". The price of bulk corn went up 80% in the last year or so. It could go down 50% just as easily.

If they're the best you got, then they're the best you got. If rhoenix plans to take the slow road to the stars, then he needs a better engine.

But ion engines *aren't* the best we have; if you have good enough power sources for a large ion engine, you can use the power to plasmify hydrogen and then use an electric field to eject the ions. We already do this on a small scale; it's called "television". We also have photonic engines, which have insanely high specific impulse at the expense of equally insane power consumption. And laser-driven lightsails, and Orion bomb-engines, and NSWR, and so on and so forth; there's quite a long list.

You do realize that in space you have to carry all your reaction mass with you from the get go, and for the entire trip, do you not? Unlike terrestrial travel, where you can pull most of your reaction mass from the atmosphere when you fly an airplane. Also, unlike in terestrial travel, there's very little drag in space, which means that small, sustained thrusts can build up in very little time.

I already know this. What is your point? My point was that ion engines are completely impractical for interstellar travel and still take months/years for interplanetary travel even assuming instant acceleration, so therefore we need a better engine if we don't want these limitations.

And any engine with a specific impulse lower than that of an ion drive is going to take more reaction mass per kilogram of payload than an ion engine. Look on Atomic Rocket's engine choice section, look for 'specific impulse'.

*smacks head*

My point was that an ion engine's specific impulse was too low, and that if we were going to go far beyond our solar system we would need an engine with higher specific impulse.

(your figure is wrong, by the way: the mass ratio of fuel to payload is, to first order, 3.00 for the two year trip with an ion engine.

This is much worse than my figure; why are you pointing at it as a refutation?

Getting out of the solar system on ion engines will require a mass ratio of merely 18.97.

18.97? Getting out of the solar system requires a delta-V of only 18 km/s or so; ion engines have much, much higher exhaust velocities than that, so I don't see why your figure is so high. How is it that your figure for getting to Pluto is better than your figure for getting out of the solar system, even though going to Pluto in two years requires an orbit with a very high eccentricity, certainly higher than 1?

Two years is a long time for a pleasure trip,

Four years. You forgot it was a round trip.

You can slowly thrust into higher and higher orbits until you escape Earth's gravity, then build into higher and higher solar orbits until you reach Pluto. All on very little thrust.

Not in two years you sure as hell can't. A single highly elliptical orbit into the outer solar system takes decades.

Any engine with a lower specific impulse than ion engines will by necessity take even more fuel to achieve the same delta-V. To achieve better than a small asteroid, you need an engine with higher specific impulse, period. As bad as the ion figure seems, chemical rockets would need even more fuel to make the same trip.

I already know all this! Please realize that I'm arguing for an engine with a *higher* specific impulse, not a lower one.

[Wyrm]

My calculation of a delta-V of 231 km/s was based on constant acceleration of 0.0003 G, which is for a high-thrust ion engine with a light payload, and the final velocity was just from the fact that you needed to cover 7.3 billion km in two years.

I assumed 100 km/s, because I did the conservative thing and assumed acceleration was instant. Taking several years to accelerate would obviously make fuel economy *worse*, not better.

It actually doesn't matter how long the burn lasts, you get the same delta-V out regardless. The delta-V you have matters which transfer orbits you use, but not the other way around. The fuel economy is exactly the same — you get exactly the same delta-V regardless of whether it's a long, feeble burn or a short, strong burn, at exactly the same mass ratio.

I also forgot to add in the fuel for slowing down, which again *helps* my argument because it means that the real figures will be even worse than the ones I calculated out.

You would need to square the mass ratio going from a simple flyby to a slowdown and stop, regardless of your propulsion method. Stop harping on the invariants and concentrate on stuff that matters.

By your own crude calculations, it can take as little as two years to get to Pluto. This is about the same time it took Voyagers 1 & 2 to reach Jupiter, which is about ten times closer, and Gallileo took six years to make the same trip.

Then it's not going to be a concern for exploring Pluto; it will be a concern for exploring anything farther out than the Kuiper Belt. For closer missions, you need lots of mass for equipment, electronics, science experiments, and what not, so you still need a very good rocket engine if you don't want an astronomical launch mass, and the return of the mission is still proportional to how good the engine is.

And in outer space, once you get into orbit, "good" means high specific impulse, not [/i]high thrust,[/i] because there's no losses due to friction. High thrust is nice, but not required. The higher the specific impulse of your orbit-to-orbit engine, you can put more mission equipment for a given launch vehicle, because your fuel fraction is lower.

Bulk supply trains for predictible consumables require a steady flow of goods to the customer.

Consumables are not "predictable". The price of bulk corn went up 80% in the last year or so. It could go down 50% just as easily.

You've obviously never heard of a "future", have you?

If they're the best you got, then they're the best you got. If rhoenix plans to take the slow road to the stars, then he needs a better engine.

But ion engines *aren't* the best we have; if you have good enough power sources for a large ion engine, you can use the power to plasmify hydrogen and then use an electric field to eject the ions. We already do this on a small scale; it's called "television".

That is an ion engine, you boob!

We also have photonic engines, which have insanely high specific impulse at the expense of equally insane power consumption. And laser-driven lightsails, and Orion bomb-engines, and NSWR, and so on and so forth; there's quite a long list.

None of which has actually been demonstrated to work. Not laser-driven light sails (though the solar powered version has worked), not any implementation of Orion engine (due to an annoying little weapons ban treaty), and neither the photon engine nor the Nuclear Salt Water Rocket have ever been built and tested, let alone flown.

You do realize that in space you have to carry all your reaction mass with you from the get go, and for the entire trip, do you not? Unlike terrestrial travel, where you can pull most of your reaction mass from the atmosphere when you fly an airplane. Also, unlike in terestrial travel, there's very little drag in space, which means that small, sustained thrusts can build up in very little time.

I already know this. What is your point? My point was that ion engines are completely impractical for interstellar travel and still take months/years for interplanetary travel even assuming instant acceleration, so therefore we need a better engine if we don't want these limitations.

You seem to be fixated on this ridiculous notion that non-instant acceleration == less efficient drive. Prove it. Show me the calculation that tells you that length of time a spacecraft is accelerating translates into more waste, which is especially remarkable since there doesn't seem to be a place for this waste to go.

Of all the engines that have flown, ion engines are the highest specific impulse, bar none. Since high specific impulse translates into smaller fuel fraction for the same delta-V. Or equivalently, more delta-V for the same propellant fraction. Since delta-V dictates the types of missions you can fly, you can perform some missions with an ion engine that you cannot with any other real engine.

And any engine with a specific impulse lower than that of an ion drive is going to take more reaction mass per kilogram of payload than an ion engine. Look on Atomic Rocket's engine choice section, look for 'specific impulse'.

*smacks head*

My point was that an ion engine's specific impulse was too low, and that if we were going to go far beyond our solar system we would need an engine with higher specific impulse.

Bullshit. You've stated that the delta-V to escape the solar system is around 7 km/s. I've looked around and found the escape velocity of the solar system is somewhere around 54,000 ft/s from the earth's surface, which translates to around 16.45 km/s. From the earth's surface to LEO requires a delta-V of 10 km/s, so only 6.45 km/s has to be paid by the space engine, which is close enough to your 7 km/s for government work. The mass ratio for an ion engine to achieve escape velocity from LEO is a mere 1.0338; with a bit more than one kg of propellant per kg of dry mass, an ion engine will get you out of the solar system. It won't get you out fast, and you have to wait a while for you to get out, but you will escape the solar system with this set-up.

(your figure is wrong, by the way: the mass ratio of fuel to payload is, to first order, 3.00 for the two year trip with an ion engine.

This is much worse than my figure; why are you pointing at it as a refutation?

Just a correction. You seem to need it, especially with your silly notions of instantaneous burns being more efficient than sustained burns in space.

Getting out of the solar system on ion engines will require a mass ratio of merely 18.97.

18.97? Getting out of the solar system requires a delta-V of only 18 km/s or so; ion engines have much, much higher exhaust velocities than that, so I don't see why your figure is so high.

I was erroneously using 618 km/s as the escape velocity of the solar system. I did explain this.

How is it that your figure for getting to Pluto is better than your figure for getting out of the solar system, even though going to Pluto in two years requires an orbit with a very high eccentricity, certainly higher than 1?

Because under the simplistic assumptions of my calculation — starting from rest, starting from the sun, neglecting gravity — the rocket actually covers the required 7.3 billion km to get to Pluto in two years. This is in contrast to your idiotic notion that a shorter period of acceleration is more efficient.

Two years is a long time for a pleasure trip,

Four years. You forgot it was a round trip.

No. Two years. The squared mass ratio you get for the speed up and slow down is for a trip of two years.

You can slowly thrust into higher and higher orbits until you escape Earth's gravity, then build into higher and higher solar orbits until you reach Pluto. All on very little thrust.

Not in two years you sure as hell can't.

If you apply this feeble but sustained thrust for two years, yes the hell you can. An acceleration of 0.0003 G applied over two years carries you a distance of 7.3 billion km.

A single highly elliptical orbit into the outer solar system takes decades.

A highly elliptical transfer orbit is what you get when you apply the required delta-V in a short time, which you have to do with chemical rockets. When the thrust is sustained, even if it's relatively feeble, the required energy and velocity builds up quickly. Two years is a fuckton of seconds.

I already know all this! Please realize that I'm arguing for an engine with a *higher* specific impulse, not a lower one.

Good. At least you have that much sense. On the other hand, you're arguing that the ion engine has two low a thrust to get you about the solar system, yet it clearly can. The issue is whether ion drives can do the job fast enough, a job which rhoenix has not told us, so you cannot say that ion engines cannot do the job. You also argue instant acceleration as the best case when spaceflight doesn't work like that.

[Kuroneko]

If one takes the magnetic confinement fusion figures on atomic rocket as fairly precise (despite the fact that we don't even have a working prototype), {F = 5.00e4N, v_e = 8.00e6m/s, m_e = 6.00e2kg}, let's have 144 engines with enough fuel for 200 years of operating time: 5.68e9kg fuel + 8.64e4 kg engines + ? payload = 5.69e9kg. If ships accelerates for 192.04 years, it will cover 4.42 light-years and have a final velocity will of a respectable 0.0848c for coasting for an arbitrary amount of time; the deceleration phase will take 7.96 years, with braking distance of 0.492 light-years.

I was curious as to what could be possible with optimistic, but still somewhat realistic, technology. That's not too bad for an automated ship, but obviously we wouldn't want to transport people that way. For these calculations, I've used the v = [ 1 - R^(2u) ] / [ 1 + R^(2u) ] = tanh(-u log R) derived here.

[Edit: linked to corrected version, again with thanks to ClaysGhost.]

But ion engines *aren't* the best we have; if you have good enough power sources for a large ion engine, you can use the power to plasmify hydrogen and then use an electric field to eject the ions. We already do this on a small scale; it's called "television".

You've just described a hydrogen-based ion engine.

We also have photonic engines, which have insanely high specific impulse at the expense of equally insane power consumption.

We don't. We can imagine them for sci-fi purposes, of course, but they aren't very realistic.

And laser-driven lightsails, ...

True, but then it has the advantage of getting power from an external source.

... and Orion bomb-engines, and NSWR, and so on and so forth; ...

For very long-term missions, getting the suitable reliability out of the Orion design without immensely increasing the mass of the ship is even less believable than ion thrusters (which are bad enough by themselves).

My point was that an ion engine's specific impulse was too low, and that if we were going to go far beyond our solar system we would need an engine with higher specific impulse.

I agree, but I don't think anyone here claimed they are sufficient for interstellar missions for anything less than century of travel time.

[dragon]

Bullshit. You've stated that the delta-V to escape the solar system is around 7 km/s. I've looked around and found the escape velocity of the solar system is somewhere around 54,000 ft/s from the earth's surface, which translates to around 16.45 km/s. From the earth's surface to LEO requires a delta-V of 10 km/s, so only 6.45 km/s has to be paid by the space engine, which is close enough to your 7 km/s for government work. The mass ratio for an ion engine to achieve escape velocity from LEO is a mere 1.0338; with a bit more than one kg of propellant per kg of dry mass, an ion engine will get you out of the solar system. It won't get you out fast, and you have to wait a while for you to get out, but you will escape the solar system with this set-up.

Plus the craft doesn't have to generate all of the thrust it can use a gravity assist fly by from the larger planents to pick up speed, after all thats what most of the other probes have done.

I know an ion engine has a Isp of about 3000 and untill we get things like a VASMIR rocket or a NEP ion engine which can have up to 13000 Isp, then conventional ion engines are the best choice for deep space unmanned missions.

NEP provides higher power, thrust, and specific impulse than an SEP system, although the thrust still pales to insignificance compared to our chemical rockets.
For a comparison specific impulse of chemical rockets is about 400secs, Deep Space 1 uses SEP and produces around 3300secs, NEP on the other hand could produce specific impulse as high as 13,000secs.

link

[Ender]

Mr Lemur, please note that the point of an ion engine is not to get you there fast, but to get you there efficiently.

Really? So why don't we walk everywhere here on Earth? Sure, it takes several months to walk from coast to coast, but it's far more efficient than taking, say, an airplane. If energy efficiency is really such a big concern, why are we launching large spaceships in the first place?

You think walking is more efficient then a car, bike, or airplane? What are you, some kind of moron? Walking is one of the most inefficient means of travel out there - it's only benefit is the fact that it allows you to transverse a wide amount of terrain. And even then that fails when you scale it up, with 4 legs, wheels, and tracks becoming more versital for mass and terrain as you increase the mass being moved. But hey, lets not let the facts get in the way of your arrogance.

[Sikon]

However, they are by necessity a low-thrust engine, simply due to how positive and negative ions interact.

From the Atomic Rocket page, here are the figures listed for an Ion drive:

Thrust (in gigawatts): 1.05
Exhaust Velocity (meters / second): 210,000
Thrust (in Newtons): 10,000
Engine Mass (in tons): 400
Power requirement (in megawatts): 800
Efficiency: 96%

Atomic Rockets is wrong here. For example, 1.05 gigawatts "thrust" from 0.8 gigawatts input power for an ion engine corresponds not to the stated 96% efficiency but rather to an efficiency impossibly more than 100%. And see the following for the general picture of what fundamentally limits thrust:

Now for the question - is there any way anyone can think of to get around the ion drive's low thrust without suggesting a different method of propulsion?

Ion engines are one of a variety of thrusters (real and conceptual) that are powered by an external power supply. For example, electric space propulsion includes resistojets, MPD thrusters, pulsed plasma thrusters, etc. In the case of today's ion engines, the power available is that supplied by small solar panels, though nuclear-electric concepts have been proposed. Either way, the fundamental limiting factor on thrust is not ion engines in themselves so much as the electrical power supply.

Consider the general situation with exhaust velocity versus thrust for any thruster running on a particular limited amount of external power.

Kinetic energy is KE = 0.5 * M * V^2. Momentum is P = M * V. Using the two preceding relationships, one can calculate an upper limit on the maximum thrust of any engine running off a given amount of external power with a particular exhaust velocity, whether an ion engine or any of the variety of other types that can function in such a role.

In general, higher exhaust velocity means a lower ratio of thrust force to necessary power consumption. That's because the ratio of momentum to energy in the exhaust becomes less and less for greater exhaust velocity, a ratio proportional to P / KE, which is (M * V) / (0.5 * M * V^2) = 2 / V.

High exhaust velocity can be preferred because it allows greater eventual vehicle velocity, e.g. greater velocity being obtained eventually for a given mass of propellant (provided that the power supply continues receiving solar power or doesn't run out of nuclear fuel), but there is a thrust tradeoff.

As an initial random example, consider a craft supplied with 1 kilowatt of power, such as a small probe with solar panels.

First consider if its exhaust velocity is relatively slow, 1000 m/s. Then each kilogram of exhaust has 0.5 * 1 kg * (1000 m/s)^2 = 0.5 MJ of kinetic energy, so the upper limit on the exhaust expelled per second is 1 kJ / 0.5 MJ, which is <= 0.002 kg / sec. The momentum of the exhaust expelled each second is <= 2 kg * m/s. So a thrust of 2 newtons is the upper limit for such an engine, while inefficiencies would make the actual figure less.

At the other extreme, take the craft with 1 kilowatt of power and have it power an engine with a vastly greater exhaust velocity of 100,000,000 m/s. Now, each kilogram of exhaust has 5E15 joules of energy. As a result, the 1 kilowatt from the craft's electrical supply can accelerate no more than 2E-13 kilograms of exhaust per second. And such has a momentum corresponding to merely < 0.00002 newtons of thrust instead of the orders of magnitude greater thrust for the other 1-kilowatt engine.

As an even more extreme example of the relatively low thrust to power ratio resulting from using exhaust with an astronomically high ratio of energy to momentum, consider the 1-kilowatt powering a photon-emitting thruster. It has no more great thrust than the non-noticeable recoil of a spotlight. The momentum of photons is P = E / c, so up to 1-kilowatt of photons corresponds to no more than at most 0.000003 newtons of thrust.

When there are concepts like laser-driven interstellar lightsails, those use astronomical amounts of power relative to their mass, like concepts where billions or trillions of watts of light from giant space lasers would be directed onto a lightweight giant sail thinner than household aluminum foil, to send such up to a number of percent the speed of light, a speed impossible for a low exhaust-velocity engine to obtain without running out of fuel. Observe they potentially obtain such precisely by not depending upon onboard electrical power, unlike ion engines, the giant massive immobile lasers helping the tiny sailcraft.

Real-world ion engines aren't nearly as an extreme situation as an engine using 100,000,000 m/s particles for thrust or one using photons for thrust. However, typical ion engines have an exhaust velocity much higher than chemical rocket engines at the same time as typically far lesser power, limited by an external power supply like small solar panels. Such results in very low thrust.

A random example of a real-world ion engine is a 2.3 kilowatt thruster described here. From its propellant exhaust velocity mentioned to be 146000+ km/hr or 40600+ m/s, one can calculate in the usual manner that its exhaust is 822+ MJ per kilogram, so its 2.3 kilowatt power implies no more than 0.0028 grams of exhaust expelled per second, corresponding to no more than 0.11 newtons of thrust. That's an upper limit, neglecting inefficiencies and such. The actual figure is a little less, actually being 0.09 newtons for that ion engine. That particular ion engine uses xenon for propellant, an element very expensive per unit mass, but, when current space probes cost the equivalent of many times their mass in gold anyway, such isn't much of a concern to NASA.

Aside from moderate variance in efficiency, any other engine powered by the same 2.3 kilowatts of power would have similarly low thrust, whether an ion engine or another type, aside from lower exhaust velocity engines being able to have a greater thrust to power ratio. That's why one can't just improve the ion engine for much higher thrust unless one changes the whole system, like having a vastly more powerful electrical power supply or not depending on electrical power.

Some craft could have vastly greater thrust. An example is the Orion concept for a large ship with nuclear pulse propulsion delivering vastly more power than small solar panels provide a present-day ion engine. Such can be equivalent to trillions of watts rather than in the kilowatt range. A ship of thousands of tons mass can have many millions of newtons of thrust at the same time as great exhaust velocity.

***********

Incidentally, the situation with chemical rocket engines may be worth explaining. For chemical rocket engines, there is not really a thrust versus exhaust velocity tradeoff because the engine is not limited by a particular amount of electrical power being externally supplied, but, rather, the propellant combinations giving higher exhaust velocity are those which are more energetic (aside from molecular weight variance). And, during the brief operating time before running out of fuel, a chemical rocket engine can produce up to gigawatts of power per ton, like a giant blowtorch rather than the more limited power to mass ratio of electrical power plants or the small solar panels powering today's ion engine craft.

As a result of that particular situation, different from electrically powered engines, chemical rocket engine propellants are always preferred to be those that give highest specific impulse, except for separate practical factors like propellant density, storability, ease of handling, etc. (For example, engineers frequently design for kerosene/LOX or even mere solid rocket fuel instead of higher specific impulse LH2/LOX, but that's because of such practical factors, with otherwise the higher specific impulse chemical fuel always being preferred over the other chemical fuels).

By the nature of chemical rocket engines working through expansion of a chemically-heated hot gas, it is impossible for them to obtain more than a few km/s exhaust velocity. Such makes them borderline at best for interplanetary travel.

***********

Ion engines can allow a space probe to obtain greater eventual delta-v (change in velocity) for a given amount of fuel, despite the slow acceleration resulting from the limited power of ion engines operating off existing small solar panels, making them have relative suitability for small probes launched from earth today compared to having more massive tanks of chemical rocket fuel. They tend to be superior to chemical rocket engines for interplanetary travel today.

However, once there is much being done in space, after the current era of launch costs orders of magnitude above fuel costs ends, then there would be a tendency to have large massive spaceships instead of today's tiny craft. In that case, although it is technically possible to propel such with giant ion engines, a much more likely primary method of propulsion is mass drivers, since then any portion of the quadrillions of tons of asteroid dirt in bodies throughout the solar system can be used as cheap propellant. Such are limited by the electrical power supply. However, as suggested before, in such a scenario that could correspond to cargo transports with gigawatts instead of kilowatts of supplied power. The current situation is messed up by astronomical launch costs from earth, but, with sufficiently cheap propellant like the extraterrestrial rock usable by mass driver cargo transports, the optimal exhaust velocity is about 2/3rds of the total delta-v (change in velocity) involved in a given mission, in terms of maximizing the average amount that can be transported per unit time by a ship with a given number of gigawatts of electrical power.

Have excessively high exhaust velocity and the ship is propellant-efficient yet provides too little thrust relative to power consumption, a little like the photon-emitting drive illustrated earlier. Have excessively low exhaust velocity, and the rocket equation makes the ship's starting mass be too many times greater than its payload, too much of its impulse going into accelerating unused propellant rather than payload. In between, there is the optimal. Such an optimal specific impulse also applies to a second likely option for bulk shipment by a space civilization, also able to "refuel" almost anywhere with inactive former comets throughout the solar system: ice rockets.

A third particularly attractive option is nuclear pulse propulsion such as Orion variants, if politics permitted such by then.

I already know all this! Please realize that I'm arguing for an engine with a *higher* specific impulse, not a lower one.

Yes, nobody is likely to use an ion engine for interstellar travel, tending towards way too low performance compared to alternatives like nuclear pulse propulsion optimized for that mission.

Although technically ion engines might be made with high exhaust velocity (such as if one treats it as not being a strict distinction between keV electrostatic ion engines and some multi-meter-length electrostatic DC particle accelerators reaching up to the low MeV range) if that was the primary goal, the big factor is that they are limited by the onboard power plant supplying them with electricity.

Also, let's illustrate why excessively extreme exhaust velocity wouldn't help with an electric propulsion system.

Let's illustrate why a nuclear reactor powering an ion engine wouldn't work for a starship if the ion engine was an imaginary one that accelerated its exhaust to 99% the speed of light. (I'm not arguing that ion engines with 0.99c exhaust are workable but rather illustrating how such shouldn't even be a goal, that such is automatically impractical).

As a random illustration, an extraordinarily high-performance concept for a gas-core nuclear reactor for nuclear-electric space propulsion with a MHD generator is estimated to obtain 2.7 MW per metric ton of electrical power generation relative to mass. Suppose one takes that and has it accelerate its exhaust to 99% the speed of light. Sent to 99% the speed of light, the exhaust has about 5.48E17 joules of kinetic energy per kilogram (accounting for relativity, though it really doesn't matter for order-of-magnitude picture in the illustration). As a result, per ton of nuclear reactor mass providing power, an upper limit neglecting inefficiencies for the exhaust able to be expelled by the ship is 4.9E-12 kilograms per second, under 5 nanograms a second. Each kilogram of it has a momentum of 2.1E9 kg * m/s. So the upper limit on the resulting thrust is 0.01 newtons per metric ton of nuclear reactor. Such nominally takes 95000 years to reach 10% of the speed of light, except it would run out of unused nuclear fuel long before then if it didn't break down first. And the preceding is neglecting mass of the ship other than the electrical power plant, neglecting inefficiencies of the ion engine itself, etc.

Although that electrical power to weight ratio is already one of the highest among concepts for which much at all analysis has been performed, one could conceivably guess more than 2.7 MWe/ton for technology enough into the future. But any very foreseeable system wouldn't change the big picture. For this application, the hypothetical ion drive expelling exhaust at 99% lightspeed is actually worse than one with a more optimal, slower exhaust velocity. The latter would increase the momentum delivered relative to power consumption. Such would help considering the limited available electrical power and the factors discussed earlier in this post.

An exhaust velocity closer to 2/3rds of the total delta-v would be more optimal, as too high exhaust velocity is inefficient, a lesser version of the situation where it is inefficient to accelerate an object from 0 to 1 m/s with a powerful spotlight when vastly less power could do so in less time if the "exhaust" had a more appropriate momentum to energy ratio.

However, even an ion drive with more suitable exhaust velocity is still not optimal for a starship. For interstellar travel with large cargoes, what one wants is not the power to mass ratio obtained by using an ion drive, nor that of any other system using onboard electrical power for thrust, more likely instead seeking the power to mass ratio obtainable with external nuclear pulsed propulsion.

I'll skip some discussion now since techniques for plausible interstellar travel have been discussed to some degree here and here. I get into a lot more details, but a general resulting observation is that appropriate nuclear pulsed propulsion allows interstellar colonization by a future space civilization, potentially with under a lifetime of transit time as discussed, particularly if they have hibernation and/or life extension.

For its performance, the Orion project and its variants is actually fairly unique. For example, in contrast, the questionable nuclear salt water rocket concept has not had proper engineering investigation of its heat transfer issues to my knowledge, which tend to be vastly above those of chemical rocket engines that already push the limits of regenerative cooling. Such is particularly not to be underestimated when one talks about continuous exposure to high temperature and heat transfer rather than brief periodic pulses. I have searched but never found any papers with suitable calculations for the NSWR online, nor so far evidence that such exist offline. It is an interesting idea, but can't be treated as equivalent in plausibility to Orion, the latter based on real experiments with the effect of brief high-temperature plasma exposure in nuclear weapons tests starting with the famous Eniwetok test.

A common brainbug is that any nuclear pulsed propulsion design must result in excessive shocks. This is not based on proper calculations but apparently on some sort of intuitive assumption, when the situation really is determined by quantitative factors like the distance of the pulses from the plate / magnetic nozzle, the yield of each pulse, etc. Millions of dollars were spent on real engineering investigation including real mathematical calculations with the Orion concept. While only some of the total is online, a random example in the form of a NASA server with a giant PDF file here illustrates one couple hundred page segment of publications resulting from it. It was plausible with the original Orion concept. It's still better with techniques like those in the Mini-Mag Orion concept, or the ultimate degree of yield adjustable beyond regular fission trigger limitations discussed in some of the papers linked to in one of my earlier posts.

Techniques like nuclear pulse propulsion can give a lot more raw power per unit mass than plausible electrical generation systems. And interstellar missions have far less flexibility in the acceptable power to mass ratio than interplanetary missions, practically needing the high performance of options such as Orion variants rather than ion engines.

Ion engines have their uses, but such tend to correspond to certain interplanetary applications, not interstellar.

[Howedar]

Lemur, not to be an ass, but I am a rocket scientist. You are not. You really should stop posting in this thread, because you're making a bloody great fool of yourself. Just a friendly suggestion.

[dragon]

Lemur, not to be an ass, but I am a rocket scientist. You are not. You really should stop posting in this thread, because you're making a bloody great fool of yourself. Just a friendly suggestion.

Rocket scientist cool. Whats are some good books for space exploration and technology as my I need some reference material for my Master Thesis. Which I really need to get started on.

[His Divine Shadow]

I haven't read very closely but did anyone suggest dr. saxtons solutions? He uses second-stage accelerators to increase the speed of the ions before they leave the ship, I'm not sure if thats feasible or has problems but I thought it worth mentioning.

[rhoenix]

I haven't read very closely but did anyone suggest dr. saxtons solutions? He uses second-stage accelerators to increase the speed of the ions before they leave the ship, I'm not sure if thats feasible or has problems but I thought it worth mentioning.

I haven't read them yet, but I plan to now - thank you for the suggestion. It was actually thinking of Star Wars' unobtanium ion engines, which are casually capable of entering or leaving an atmosphere without much in the way of apparent problems. However, I do know that there's a very large disparity between what we know and understand about ion engines now, versus that sci-fi universe. I'm very glad I made this thread though - I hadn't realized the disparity was even larger than I first understood.

As Sikon's extremely informative post stated, given what we know physics-wise regarding ion engines, they're far better for short-range, small-payload inter-planetary trips. I'll also be looking into the mass driver idea for propulsion (though I'm tempted to consider that a second choice, considering asteroids usually have valuable materials within them for mining), as well as the ice rocket idea, which sounds intriguing.

However, regarding ice rockets - Sikon, or someone else, could you describe this idea a bit more, or post a link where I may read about them?

Thank you to all for your responses in this thread. I'm learning quite a bit.

[Howedar]

Lemur, not to be an ass, but I am a rocket scientist. You are not. You really should stop posting in this thread, because you're making a bloody great fool of yourself. Just a friendly suggestion.

Rocket scientist cool. Whats are some good books for space exploration and technology as my I need some reference material for my Master Thesis. Which I really need to get started on.

That's an impossibly open-ended question and I'm not well suited to answer it. Go talk to a librarian, that's what they're there for.

[TheLemur]

It actually doesn't matter how long the burn lasts, you get the same delta-V out regardless.

True. But delta-V and fuel efficiency are not the same thing. If you accelerate very slowly, your average velocity will go down because for a long period of time you're not yet at full speed. So you have to burn more fuel to compensate if you expect to arrive in the same amount of time.

You would need to square the mass ratio going from a simple flyby to a slowdown and stop, regardless of your propulsion method. Stop harping on the invariants and concentrate on stuff that matters.

I haven't even given figures for an alternative engine; I am currently trying to prove that modern ion engines are not going to work for a lot of these sci-fi applications and that we need to find something better. And for the record, squaring the mass ratio is much worse if it's very large, because the magnitude of the increase is directly proportional to the magnitude of the ratio.

High thrust is nice, but not required.

That depends on how you define "high". Once you're in solar orbit, it's true that it doesn't matter much whether you increase speed over a period of a minute or a week. But thrust from ion engines and solar sails can be so low that you spend a large portion of the trip time accelerating, at which point it does matter.

You've obviously never heard of a "future", have you?

Admittedly I didn't think of futures when I typed this, but futures are investment instruments and carry a lot of risk. Most manufacturers would not want a long-term future as compared to a short-term future or an instant purchase, because every time you do it you're rolling the dice that you won't take a big hit and go broke. Kepp in mind that most businesses have small profit/revenue ratios, so if you take even a 20% hit on the price of a major commodity it could very well screw you up for the year.

That is an ion engine, you boob!

(sigh)

I meant a present-day ion engine of the type we used on Deep Space 1. I thought you understood that.

None of which has actually been demonstrated to work.

Really? So I guess we might as well tear down this whole thread, because ion engines on the scale that you're talking about haven't been demonstrated to work either. Even a small prototype doesn't guarantee that a technology is commercially viable, because there are considerations of scale, price, and marketability. I admit that ion engines are much more thoroughly tested than any of my concepts, but that does not a triumphant refutation make. That's why we have a long list, because we don't know which ones are going to work.

Not laser-driven light sails (though the solar powered version has worked),

Technical feasibility of this is a foregone conclusion because the physics principles have already been very well demonstrated. There would even have been an actual, in-space demonstration on Cosmos 1 if it hadn't burned up.

not any implementation of Orion engine (due to an annoying little weapons ban treaty),

This is a political consideration, not a technical one. Political considerations change all the time. The treaty is only forty years old and you think it's still going to be a problem Eru-knows-how-many centuries from now?

and neither the photon engine nor the Nuclear Salt Water Rocket have ever been built and tested, let alone flown.

True. Do you have any evidence that this is because of unusability and not because of lack of capital, political concerns, immense conservatism at NASA and its contractors, and so on and so forth?

You seem to be fixated on this ridiculous notion that non-instant acceleration == less efficient drive. Prove it. Show me the calculation that tells you that length of time a spacecraft is accelerating translates into more waste, which is especially remarkable since there doesn't seem to be a place for this waste to go.

So be it. Suppose that a spacecraft wants to make a 1 million kilometer journey and has a rocket engine with a delta-V of 2 km/s, ignoring gravity and other considerations. If the acceleration is instant, using 1 km/s to speed up and 1 km/s to slow down, it will do the entire journey at 1 km/s, taking 1 million seconds or around 11.574 days. If the acceleration takes one full day, the average velocity during acceleration will be .5 km/s, so the spacecraft will cover 43,200 km during acceleration and 43,200 km during deceleration, for a total of 86,400 km. That leaves 913,600 km to be covered at 1 km/s, which will take 913,600 seconds or 10.574 days. Add in one day to accelerate and one day to decelerate, and you get 12.574 days, one full extra day over the instant acceleration spacecraft. If it takes a week to accelerate, you cover 302,400 km while accelerating and 302,400 km while decelerating, for a total of 604,800 km. This leaves only 395,200 km to be covered at 1 km/s, which will take 395,200 seconds or 4.57 days, for a total or 18.457 days or a full week more than the instant acceleration spacecraft.

Since delta-V dictates the types of missions you can fly, you can perform some missions with an ion engine that you cannot with any other real engine.

If you're going to stick strictly to what is "real", you have to assume that ion engines will only have a specific impulse of several thousand seconds, because those are the only ones that have been demonstrated. Oops.

Bullshit. You've stated that the delta-V to escape the solar system is around 7 km/s.

(again smacks head)

I meant that if you wanted to leave the solar system with 100 km/s of speed, you only needed 7 km/s more than if you wanted to leave a totally unaccelerated region of space with 100 km/s of speed. So if I wanted to be more accurate, I needed to change 100 to 107, which is minor enough of an adjustment I didn't bother.

It won't get you out fast, and you have to wait a while for you to get out, but you will escape the solar system with this set-up.

The ion-engine lunar orbiter SMART-1 took months and months to attain the 3 km/s delta-V necessary to go to the Moon. If you're trying to get to Pluto in two years you're going to have to do better than that.

From the earth's surface to LEO requires a delta-V of 10 km/s, so only 6.45 km/s has to be paid by the space engine, which is close enough to your 7 km/s for government work.

What? It requires 10 km/s to get to LEO, 3 km/s to get from LEO to escaping Earth's orbit, and an additional 5 km/s to get from escaping Earth's orbit to escaping the Sun's orbit. Note that, again, that figure assumes the acceleration is done at a very low perigee, which is obviously impossible for an ion engine because the minute you finish getting the 3 km/s you escape Earth and go into solar orbit. So your engine has to provide the full 13 km/s to escape into a hyperbolic trajectory all by itself.

starting from rest, starting from the sun, neglecting gravity

Okay, #1 and #3 I can buy as simplification, but #2? Starting from the sun changes the gravitational burden by two full orders of magnitude and the velocity by one full order of magnitude. That's not a minor error there, buddy, even by the standards of back-of-the-envelope math.

No. Two years. The squared mass ratio you get for the speed up and slow down is for a trip of two years.

What? I meant that if you could get there in two years, it was going to be a four-year trip because you also have to go back to Earth from Pluto. So you actually have to make four burns; one to accelerate at Earth, one to decelerate at Pluto, one to accelerate at Pluto and one to decelerate at Earth, although the first two and the last two can be in different spacecraft.

If you apply this feeble but sustained thrust for two years, yes the hell you can.

You cannot simply ignore gravity when you make burns which are years long, because they will lead you into big, looping, elliptical orbits which take a long time to get anywhere. And where did you get the 0.0003 G figure? DS1 only had a maximum acceleration of 25 micro-Gs, SMART-1 had an acceleration of 19 micro-Gs.

A highly elliptical transfer orbit is what you get when you apply the required delta-V in a short time, which you have to do with chemical rockets. When the thrust is sustained, even if it's relatively feeble, the required energy and velocity builds up quickly.

(sigh)

A long acceleration cannot possibly get you into a faster or more efficient orbit than a quick acceleration, because acceleration is best done when going at a high velocity, and with good acceleration you can concentrate on a very short period of high-velocity time, such as perigee near a planet.

You also argue instant acceleration as the best case when spaceflight doesn't work like that.

(smacks head)

In a perfect world where you're not going to get smashed against the bulkhead and rocket engines can do whatever you want, instant acceleration is the best case. It may not be as important as other factors, but it is still favorable to low acceleration with all else being equal.

[TheLemur]

Lemur, not to be an ass, but I am a rocket scientist. You are not. You really should stop posting in this thread, because you're making a bloody great fool of yourself. Just a friendly suggestion.

So, let me get this straight: You want me to stop posting in this thread because you say I'm wrong, without even bothering to explain why I'm wrong, even in a single instance? And you back up your assertion with a completely unsubstantiated claim of superior expertise? And I'm not even arguing with you; I'm arguing with other people who have presumably not osmotically absorbed your supposedly superior knowledge.

You think walking is more efficient then a car, bike, or airplane?

Maybe not a bicycle, but it's sure as hell more efficient than a car or airplane. Automobiles and airplanes use roughly 4 MJ per passenger mile. (source) Assuming you're taking your time, you can walk two miles in one hour. In one hour, at a very high power consumption of 500 W, you would consume 1.8 MJ, versus 8 MJ for automobiles and airplanes.

You've just described a hydrogen-based ion engine.

I *know*; I was referring specifically to modern xenon-based ion engines. How many times do I have to repeat that?

We don't. We can imagine them for sci-fi purposes, of course, but they aren't very realistic.

This is true, at least given modern power sources.

True, but then it has the advantage of getting power from an external source.

So what? What does that have to do with how good it is?

For very long-term missions, getting the suitable reliability out of the Orion design without immensely increasing the mass of the ship is even less believable than ion thrusters (which are bad enough by themselves).

What problem are you referring to specifically?