Ion Engines

EarthWolf · 2004-07-24 06:28:25

Hello,

I remember hearing about how ion engines could produce better and more sustained acceleration than chemical rockets can. Though the downside is that chemical rockets can attain escape velocity, while ion engines can't. NASA has recently flown some probes with ion engines aboard in recent years. How would ion engines compare with plasma rockets and chemical rockets?

Cordially,

EarthWolf

~Eternal~ · 2004-07-24 11:14:49

Well, Chemical rockets can acheive escape velocity and surely plasma rockets could (but would it be safe?), and Ion Engines are excellent for deep space missions.

I think its fare to say if we are to use Ion engines we will need to use chemical rockets as well.

Heres some info on rockets by wikipedia:
http://en.wikipedia.org/wiki/Spacecraft … propulsion

GCNRevenger · 2004-07-24 12:53:42

Any engine can get you out of orbit after you are up there, its just a matter of how long it takes and how much fuel you have to use to do it. Chemical rocket and nuclear thermal rocket engines can leave orbit fast, but need lots of fuel to do it. Ion drives take a looong time but uses very little fuel. Plasma drives, if built somehow, are in-between... still a little slow for humans though given the exposure to zero-G and radiation belts.

Grypd · 2004-07-24 15:31:50

Smart 1 is an Esa probe that is using its ion engine to slowly power itself to a moon intercept. We could not use the Ion engine to provide the thrust for humans as it would result in too high a radiation dose to the human crew and probable starvation.

Ion engines are used as high specific impulse low weight thrusters for satelites. These allow station keeping to be reliable for long periods.

We need a more powerful engine to be able to travel to the moon and mars. This may well be nuclear engine, Solar, or a form of high thrust plasma, ie vasmir. Each has benefits and drawbacks. Nuclear is the most efficient but has the major problem it is illegal under international treaty. Also there will be major protests if used. Solar and plasma are about as efficient as each other but again have drawbacks to there use. Vasmir would need a nuclear plant, solar is the heaviest.

There are other options solar sailing, or using plasma sails but in the end the most efficient will be the use of fusion engines if we get them to work.

RobS · 2004-07-24 16:21:23

Another intriguing idea is solar-thermal. It utilizes a large solar mirror to focus heat on a graphite block, through which one runs liquid hydrogen (once the block is hot). It produces a specific impulse in the 700 or 800 range, I think; almost as good as the old NERVA nuclear thermal solid core rockets in the 1960s. Thrusts are much lower, though, because one has at most a megawatt or so of heat (I think one hundred pounds of thrust at most). But using perigee kicks--running the engine only at perigee--one can push cargo to escape velocity in a few weeks. One probably would need a chemical engine to achieve enough delta-vee for a Hohmann trajectory to Mars.

Maybe a breakthrough making much larger solar mirrors would change that. NASA was developing such an engine a few years ago; google "solar thermal rocket" and I think you'll find the references. Solar thermal are very cheap, too.

-- RobS

Grypd · 2004-07-24 16:52:52

Using plasma to create an energy sail is another interesting means of space transport.

It would by the use of large magnetic fields and plasma allow large "sails" to be created. These would allow the force of the solar wind to push a spacecraft to truly fast speeds.

Of course its theoretical as we have never created one and like all uses of high powered magnetic fields prone to damage to the machinery that makes it.

Grypd · 2004-07-24 17:09:19

http://www.islandone.org/APC/]Island one advanced propulsion info

Try this site has many advanced propulsion concepts and you should be able to get info on what could be done.

GCNRevenger · 2004-07-24 18:37:47

Since solar thermal engines still have pretty low thrusts, you might as well use Ion engines with their even higher specific impulse and high reliability (no flimsy mirror to break).

As for nuclear thermal engines, I don't know if there is a specific treaty provision that America signed which prevents the use of nuclear reactors in space. We have launched nuclear power generators for decades after all.

Furthermore, a well designed Uranium fueled nuclear thermal engine could have Isp in the region of 1000sec specific impulse and generate electric power for the craft. Oh, and don't forget that since Uranium is only a weak alpha source, for all intents a pile of dirt, it has essentially zero radiation hazard until it is activated... so a launch mishap is simply not an issue.

RobertDyck · 2004-07-24 19:18:10

Ion engines have extremely good fuel efficiency, but extremely low thrust. Numbers demonstrate this, the Space Shuttle Main Engines (SSME) have a specific impulse (Isp) of 453 seconds in vacuum. Specific Impulse is a rocket guy's term for fuel efficiency; it means that can produce 1 pound of thrust from 1 pound of fuel for 453 seconds. The NSTAR ion engine was used on Deep Space One, it had a specific impulse of 3120 seconds at throttle level 15. That's according to the In-Flight Performance of the NSTAR Ion Propulsion System on the Deep Space One Mission, by J.E. Polk, R.Y. Kakuda, J.R. Anderson, J.R. Brophy, Jet Propulsion Laboratory.

However, the SSME has a thrust of 2278 kN while NSTAR has a thrust of 92.4 mN at throttle level 15. Notice SSME thrust is measured in kilo-Newtons (thousands of Newtons) while NSTAR thrust is measured in milli-Newtons (thousandth's of Newtons). SSME produces 246,503,679 times as much thrust (for accuracy, round that off to 3 significant figures). The reason an ion engine is so weak is that it requires electricity as the energy source for thrust, a chemical rocket uses the chemical reaction for its energy. Any form of electric propulsion, whether ion or hall or plasma or other, all require a lot of electricity.

For now, an ion engine is great if you're not in a hurry. The small fuel tank permits a very small craft. In fact, if you're headed to Jupiter or farther the fact that you can gradually build up speed permits a faster final speed, so it'll get you there sooner. However, if you're headed to Mars the months it takes to build speed with an ion engine means it'll take longer than a chemical rocket. There have been proposals for a high thrust, high specific impulse ion engine, but it needs so much power it would require a substantial nuclear reactor.

EarthWolf · 2004-07-24 19:23:46

Hello,

Well, I remember reading in a sci-fi RPG that ion engines could be used to propel manned spacecraft. But, the downside was that ion engines couldn't produce the necessary amount of thrust in a short enough time to achieve escape velocity. About plasma rockets, how do these rockets generate thrust? Do they use some form of high energy physics to produce plasma and force the resulting plasma out of an exhaust nozzle?

Could plasma rockets be practical enough for use aboard manned spacecraft? What speeds ( Like miles per second ) can these rockets achieve? Also, I believe the Outer Space Treaty of 1967 prohibits the use of fission reactors in space. Could be wrong.

Cordially,

EarthWolf

RobertDyck · 2004-07-24 20:05:35

There are a few different kinds of plasma rockets. MagnetoPlasmaDynamic (MPD) uses an electric arc to heat gas. It directs gas through a metal nozzle, the nozzle is given a positive charge and a rod is run up the middle with a negative charge. The arc goes between the nozzle walls and the rod. The electric arc heats the gas so much it becomes plasma. The current in the cathode (rod) generates a circular magnetic field which interacts with the current in the gas to accelerate the gas. Propellant can be argon, lithium or hydrogen in increasing order of efficiency.

VAriable Specific Impulse Magnetoplasma Rocket (VASIMR) uses radio waves to heat gas the same way that microwaves heat water in a microwave oven. The gas is first ionized by radio waves then guided into a central chamber with magnetic fields. Gas molecules spiral around the magnetic field lines with a certain natural frequency. By bombarding the molecules with radio waves of the same frequency, the system heats them to 10 million degrees. A magnetic nozzle converts spiraling motion to axial motion, producing thrust. By regulating the manner of heating and adjusting the magnetic choke, the pilot can control exhuast rate. The longer gas is kept in the chamber, the hotter it gets. If you open the choke you get higher thrust and lower fuel efficiency, by closing down the choke you get lower thrust and higher fuel efficiency. Different gas temperature requires different radio frequencies to heat it further, so adjusting exhaut temperature requires regulating the radio frequencies; that's what they mean by "regulating the manner of heating". Propellant is hydrogen. This could be used for a manned Mars mission: use high thrust to get out of Earth orbit, then high specific impulse (fuel efficiency) for the cruise to Mars. It's estimated a VASIMR engine for a manned mission to Mars would require 10 megawatts.

GCNRevenger · 2004-07-24 20:44:53

TEN whole megawatts? Ouch... the ultimate culmination of the Prometheous project might be a reactor of around one megawatt... there is a type of nuclear reactor called a Gas Core nuclear reactor (not the rocket) that could, in theory, produce that kind of energy, but its a fairly far-out concept as far as nuclear engineering is concerned.

EarthWolf · 2004-07-25 16:16:35

Hello,

Cool. So, the impulse of an engine is it's seconds of thrust per pound of fuel. It seems to me that higher the impulse, lower the thrust and conversely higher the thrust, lower the impulse. How quickly would a VASIMR engine propel a manned mission to Mars?

Cordially,

EarthWolf

Grypd · 2004-07-25 17:07:56

VASMIR if it relies on having a power generation of 10 Mw would not be as useful as nuclear. But it does not need that amount of power it works at 50kw so it is actually the only Electric engine that can compare to a nuclear power sourced rocket. Also its fuel is hydrogen this allows for lunar refuelling so again a cost saving can be made.

In mars direct it states a nuclear powered mars mission could be done in 130 days with about 45 tons of "cargo" to intercept mars. Using vasmir this speed could be reached but as vasmir has a lot higher specific impulse it could probably take time off or if engine efficiency improved from the 60% we have got it(by the use of superconductors) an increased cargo load. There are limits to how fast the mission could be done as safety must be a priority and should engine fails take place a means to return home be available. This must be studied further!

GCNRevenger · 2004-07-25 21:16:59

No, specific impulse and thrust are not fundimentaly related, which makes the concept a little bit confusing. It is possible to make an engine with high thrust and impulse, it would just require huge amounts of energy. In theory an ion engine can have as high a thrust as a chemical or nuclear one, it would just require silly amounts of electricity, and would retain its very high impulse.

Sure you can make a VASIMR run on a tiny amount of electricity, you just won't get anywhere with it. VASIMR is an unusual engine, it is a thermal AND an electromagnetic engine as I understand it. With a purely electromagnetic engine, it can be built to keep its specific impulse high even with very low amounts of energy at the expense of thrust... but in thermal engines, the less energy you put into it, the less impulse it has as well as losing thrust, because impulse is dependant on high temperatures.

It is possible to make a VASIMR engine run with a small amount of electricty if thrust is not important to you, but it will be hard to configure it to heat the Hydrogen as much inside, losing specific impulse.

The VASIMR engine, like the ion engine, still has the problem of fairly low thrust even though it is much better than ion drives. Since its thrust is low, it will still take a long time to accelerate to a high speed, the question is if it can get going very fast before it reaches Mars, or it isn't any good.

Also, a VASIMR engine is going to be heavy, especially with the very large space nuclear power reactor and its associated radiators. Since gas core reactors are a ways off, the weight of the reactor and the engine (with its supercold magnets and superhot microwave generators) will seriously cut into the mass budget of the ship, and slow you down.

A small set of nuclear thermal engines however, can be very light weight, since all they basicly are is a pipe filled with a little Uranium and its support structure with control rods on the outside and a turbopump. They don't need to be that big either, since they produce alot of thrust.

The ultimate goal for rockets for interplanetary travel is probobly the Gas Core Nuclear Rocket (GCNR, hence the namesake), my favorite, uses gasseous Uranium to heat Hydrogen to extreme (25,000-50,000K) to give very high thrusts at 2000-5000sec Isp. A month to Mars anyone?

Euler · 2004-07-25 22:53:08

No, specific impulse and thrust are not fundimentaly related, which makes the concept a little bit confusing. It is possible to make an engine with high thrust and impulse, it would just require huge amounts of energy. In theory an ion engine can have as high a thrust as a chemical or nuclear one, it would just require silly amounts of electricity, and would retain its very high impulse.

Specific impulse and thrust are both proportional to the power an engine uses. Since the limiting factor for performance of electric engines is generally the power supply, thrust and isp are related to each other, and they are inversely proportional to each other.

Sure you can make a VASIMR run on a tiny amount of electricity, you just won't get anywhere with it. VASIMR is an unusual engine, it is a thermal AND an electromagnetic engine as I understand it. With a purely electromagnetic engine, it can be built to keep its specific impulse high even with very low amounts of energy at the expense of thrust... but in thermal engines, the less energy you put into it, the less impulse it has as well as losing thrust, because impulse is dependant on high temperatures.
It is possible to make a VASIMR engine run with a small amount of electricty if thrust is not important to you, but it will be hard to configure it to heat the Hydrogen as much inside, losing specific impulse.

VASIMIR can also get a high isp at the expense of thrust. Hence the name VAriable Specific Impulse Magnetoplasma Rocket.

The VASIMR engine, like the ion engine, still has the problem of fairly low thrust even though it is much better than ion drives. Since its thrust is low, it will still take a long time to accelerate to a high speed, the question is if it can get going very fast before it reaches Mars, or it isn't any good.

Ion engines are about 60-90% efficient right now(depending on isp; higher isp=higher efficiency), and I don't think that VASIMR will be able to do much better than that. This means that if they have the same power source and they are operating at the same isp, they should produce about the same amount of thrust.

The advantage of VASIMR is that it can vary the isp over a greater range of values. It turns out that an optimized least time trip to Mars using VASIMR might only operate at 3000s isp at the start of the trip, slowly raise the isp to a peak around 50,000s, and then reduce the isp back down to 3000s at the end of the trip. This more optimized thrust profile should lead to a non-trivial reduction in the total time of the journey.

Most studies that investigate VASIMR also assume the use of power plants that are much more advanced and more powerful than the power plants used in investigations of ion engines. This may be due to the ion engine studies assuming current technology, while the VASIMR studies are assuming future technology. In any case this makes it appear that VASIMR's superiority over ion engines is much larger than is actually the case.

GCNRevenger · 2004-07-26 13:44:44

Ehhh kinda, the VASIMR engine is variable, but its performance isn't fixed over an infinte range of power levels. A measly 25kW wouldn't get you anywhere even if the thing were tiny. I don't think its practical for the engine to reach operating temperatures and be of practical size unless its in the 100's of kW for unmanned and in the MW range for manned.

As for Isp and thrust being inversely proportional, that is somewhat true for ion engines and other electric engines, but not so much for thermal engines where you simply can't scale up the Isp indefinatly at the expense of thrust.

Commodore · 2004-07-29 17:11:49

A small set of nuclear thermal engines however, can be very light weight, since all they basicly are is a pipe filled with a little Uranium and its support structure with control rods on the outside and a turbopump. They don't need to be that big either, since they produce alot of thrust.
The ultimate goal for rockets for interplanetary travel is probobly the Gas Core Nuclear Rocket (GCNR, hence the namesake), my favorite, uses gasseous Uranium to heat Hydrogen to extreme (25,000-50,000K) to give very high thrusts at 2000-5000sec Isp. A month to Mars anyone?

Isn't that out of the Nerva program?

How much hydrogen would have to be hauled around for that? Granted, hydrogen is useful for radiation shielding, and is an all around a handy element to have around.

But depending on the volume required, it could be an enormous pain to get into orbit, or preposition.

ANTIcarrot. · 2004-07-29 19:01:18

How much hydrogen would have to be hauled around for that? Granted, hydrogen is useful for radiation shielding, and is an all around a handy element to have around.

Well, that's kinda the point for high ISP engines, you need less fuel for any given deltaV. Say your GCNR engine weighed 100 tons, and you had another hundred tons of fuel, and you wanted to go to mars (4kmps) your ship can weigh up to 1,250tons. Alternatively you could accelerate 200tons to 14kmps or 100 tons to 20kmps.

Such a ship would fulfil the US Air Force's Real World specification for a space-fighter. In other words, it could literally turn around in orbit. A little more than that and you can have proper dog fights in space.

You just need to overcome all the little technical details first.

ANTIcarrot.

Commodore · 2004-07-29 21:06:52

How much hydrogen would have to be hauled around for that? Granted, hydrogen is useful for radiation shielding, and is an all around a handy element to have around.
Well, that's kinda the point for high ISP engines, you need less fuel for any given deltaV. Say your GCNR engine weighed 100 tons, and you had another hundred tons of fuel, and you wanted to go to mars (4kmps) your ship can weigh up to 1,250tons. Alternatively you could accelerate 200tons to 14kmps or 100 tons to 20kmps.
Such a ship would fulfil the US Air Force's Real World specification for a space-fighter. In other words, it could literally turn around in orbit. A little more than that and you can have proper dog fights in space.
You just need to overcome all the little technical details first.
ANTIcarrot.

Indeed, the devil is in the details.

Has such a system, even to scale, ever been tested on Earth?

GCNRevenger · 2004-08-02 12:12:56

The GCNR engine? On Earth? Oh no no no certainly not... thats still a little ways down the road (keeping the thing from melting), and there are some environmental issues that would drive the envirowackos absolutely stark raving mad, since there would be nothing really to prevent some of the Uranium fuel and its byproducts from escaping while the engine is running.

But the little solid-core NERVA rockets, which are a whole different animal than the GCNR, have been test fired numerous times, and would provide Isp roughly double of the best chemical engines, perhaps a little more. They don't offer enough thrust for a ground launch, but they can be built quite small and light.

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#1 2004-07-24 06:28:25

Re: Ion Engines

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Re: Ion Engines

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Re: Ion Engines

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Re: Ion Engines

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Re: Ion Engines

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Re: Ion Engines

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Re: Ion Engines

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Re: Ion Engines

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Re: Ion Engines

#12 2004-07-24 20:44:53

Re: Ion Engines

#13 2004-07-25 16:16:35

Re: Ion Engines

#14 2004-07-25 17:07:56

Re: Ion Engines

#15 2004-07-25 21:16:59

Re: Ion Engines

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Re: Ion Engines

#17 2004-07-26 13:44:44

Re: Ion Engines

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