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If I have a spacecraft that has only so much conventional fuel for a conventional rocket and also has an ion engine, could I have it burn the rocket fuel off, and then continue in acceleration with the ion engine? I know that ion engines can accelerate until they run out of fuel or reach near the speed of light, and I don't see any reason why the above scenario couldn't happen... But I wanted to see what everyone else thought.

 

Thanks in advance.

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As usual, it's impossible to accelerate anything to the speed of light due to the increasing mass of the object as it approaches the speed of light, and thus requiring an infinitely increasing force.

 

Accelerating something to the speed of light however will take quite alot of time. I understand that Ion engines are not particularly "thrustful" compared to conventional rocket engines. Some calculations show here that, and I will assume up to half the speed of light, where Newtonian estimation still generally holds:

 

Given the ion engine produces a thrust of T Newtons, and your average bulky satellite of 3 tons.

 

acceleration = T/mass

acceleration = T/3000

 

velocity = acceleration x time

150,000,000 meters/second =(T x days)*28.8

 

Days = (5208333)/Thrust

 

Typical ion engines create a fraction of a Newton of thrust? But give it a "boost" and say it does 100 N !!!

 

Days = 52083 days = 143 years

 

:)

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Yes, not only can you do it, that's how it's actually done. The space probes which have used ion engines (Deep Space 1, SMART-1, MUSES-C) were all launched by rockets using conventional propellent. The second or third stage motors put the probes into a parking orbit or sent them on their merry way and then dropped off. The propellent burns were on the order of minutes, the ion engines then "burned" for weeks, or months or years!!

 

If you were designing a probe to go to Pluto or Alpha Centauri, you would want the biggest, most powerful conventional rocket motor you could afford to give yourself a kickstart. Once the fuel is exhausted you'd want to dump the motor and the fuel tanks and allow the ion engine to kick in. As mezerashi pointed out, the actual thrust of the ion engine is rather feeble. There's no point in carrying the excess mass of the conventional motor and depleted tanks with you. I'll leave out the 432 potential problems in mounting such a mission, but you probably get the drift. ;)

 

If you're so inclined, look up MUSES-C (aka Hayabusa). It's an ambitious JAXA project which uses an advanced ion engine to send a probe to an asteroid, deploy a hopper, collect a sample, and then return it to earth. It has been slowly sneaking up on the asteroid with it's ion engine for several months now. It actually uses small conventional motors on its final approach, but will revert back to ion power to send itself back to earth with the sample cannister. Neat stuff!!

 

http://www.isas.ac.jp/e/enterp/missions/hayabusa/index.shtml

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That's really... amazing. I had no idea JAXA was attempting anything like that. Wow. Mezarashi, I was well aware that you can't accelerate to the speed of light, and I'm assuming you were just saying that because your post was about accelerating to that speed. It makes sense, you were just covering your back. However, my question was more along the lines of what formulaterp answered. To be honest, it was so perfectly logical, that I was somewhat surprised that it's launched like that! ;-P So, it would be perfecly logical to make a "magnetic cannon" in space that shoots it on its way much like a maglev train, and further accelerate with the ion engine. Just a thought, not like it's happening any time soon.

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Here you go! Everything you ever wanted to know about ion engines, but were afraid to ask (note: .pdf file). And seriously, I mean everything.

 

http://nmp-techval-reports.jpl.nasa.gov/DS1/IPS_Integrated_Report.pdf

 

Well technically it's just Deep Space 1's ion engine. OK so I oversold it.

 

Also there's this earlier thread:

 

http://www.scienceforums.net/forums/showthread.php?t=5914

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I thought it could only approach the speed that the particles are being accelerated at, which was rouhgly 200 thousand miles an hour.

 

Oh, to put the thrust into perspective, I read somewhere that it is equivalent to the force of a peice of paper resting on your hand.

 

edit* I have another question too, about different sources of 'fuel' for the ion engine. Isn't there something out there better than Xenon? I mean xenon is heavy and will provide high amounts of momentum but I think its ionization energy is too high... meaning more energy required to ionize it. So, I was looking at the table and I saw cesium. It has an incredibly low ionization energy and is heavier than xenon (like only 1 a.m.u. but still, more is better). I know that since it is on the far left bottom corner that it is excessively corrosive, but I wouldn't think that would matter so long as it doesn't come in contact with the metal. If it has to go through a magnetic chamber anyways what does it matter? I don't think it is radioactive either. So why not the switch to Cesium?

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I thought it could only approach the speed that the particles are being accelerated at' date=' which was rouhgly 200 thousand miles an hour.

 

[/quote'] The only thing that limits the final velocity of any rocket is how much fuel it carries.

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Yes, purely based on the way that it's set up. Space is (more or less) frictionless. Conservation of momentum means that each particle emitted by the engine exerts an equal force on the craft. So every particle emitted is accelerating the craft - just, after the velocity of the particles is reached, the particles will still be moving in a forward direction even after being pushed out the back. I (kind of) understand, but I don't think I'm going to be able to put it into words.

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Regarding maximum velocity, you are leaving out relativistic effects. Personally I don't think you have to worry about it too much. We won't be approaching even 0.01c anytime soon.

 

As far as the Xenon fuel being used: Early ion engines (SERT I and II missions) used Mercury and Cesium as fuel. Over the past 3 decades or so, the industry has settled on Xenon due to lack of corrosive effects and much safer handling conditions. One has to consider environmental effects and safety precautions driving up the cost of developing any type of propulsion method. Ask any rocket scientist about Project Orion, a proposed nuclear propulsion system. Orion drops actual thermonuclear devices out the back of the rocket, detonates them, and the blast effect propels the craft forward. It's incredibly efficient, and can result in impressive velocities. But you try getting that proposal past the local town council.

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That's what they call "black budget," but our current president can't even pronounce the word correctly ("nucular" :D), much less image a spaceship using it...

 

And as far as I understand, the negative byproducts of such a propulsion system are relatively few (to us) because none of the harmful particles would ever reach us. But I don't know much about ORION, so I'm not one to talk - but it's a very intriguing project.

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  • 2 weeks later...

Duh, I wasn't even thinking about storage of cesium... thats where it would be corrosive. But, how can it be environmentally unsafe? I don't think an Ion Engine would even be feasible in an atmosphere.

 

"just, after the velocity of the particles is reached, the particles will still be moving in a forward direction even after being pushed out the back. I (kind of) understand, but I don't think I'm going to be able to put it into words."

 

Ah yeah I kind of understand what you are saying. But, I think since the velocity can only be approached it would still travel backwards... just really, really slowly.

 

(sorry for the delay in response, I've been away on vacation)

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Where'd you go? Vacations are always nice. I thought I might clarify, since after reading it just now, my last post (#12) is a little vague. It's referring to nuclear propulsion systems. Anyways, in case it wasn't clear.

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I went to Oregon and Washington (thanks for asking :) ). Went to the coast and saw a few mountains (Rainier, St. Helens, Hood, Baker). We mainly went for my cousin's wedding but I had lots of fun though, got to see a lot of cousins I don't see that often.

 

Ah, yeah sorry about that, I wasn't clear enough. I was referring to post 11#, yours makes sense.

 

As far as the Xenon fuel being used: Early ion engines (SERT I and II missions) used Mercury and Cesium as fuel. Over the past 3 decades or so, the industry has settled on Xenon due to lack of corrosive effects and much safer handling conditions. One has to consider environmental effects and safety precautions driving up the cost of developing any type of propulsion method.

 

I think calbiterol might be referring to all propulsion techniques in general... I was just trying to make sure I wasn't missing anything.

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  • 9 months later...

its possible to run an ion engine on anything because it is ejected as a plasma. Xenon is picked because it won't erode the engine parts as much as other gases. the fact that is has a high atomic mass also helps. this means it will stay in the engine longer during the acceleration phase and we can get more momentum out of it. Radon in theory would be better but its radioactive. you could also use a heavy metal like lead but it can build up on the insides of the engine and cause blockages. Xenon just works out to have the most advantages over disadvantages.

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An article in Scientific American, written by two NASA scientists suggests that, within 500 years, humans will be able to travel between 10% and 20% of light speed.

 

The biggest problem is not what we do in space, but how we get into space. Once we have close to zero gravity and air pressure, acceleration via ion engines can reach enormous speeds given time.

 

There is a group of engineers working on a 'cable car' into space. If we make up a cable, 70,000 kilometres long, stretching out into space from the equator, then centrifugal force from the Earth's rotation will hold it taut, and cable cars can move up and down into space, using just a small cost in electricity. The material for the cable has to be enormously strong for its weight (tensile strength). However, we already know that carbon nanotubes have enough strength. The engineers working on this have already made a one mile long cable using nanotubes. They have lifted it into the air via a balloon. They claim they will have the space cable done by the year 2020. If so, the most important limitation on space travel will be overcome.

 

Imagine a cable 70,000 km long, going from the equator up into space. Imagine an advanced magnetic levitation transport system running along that cable. Imagine a cable train/space capsule that has climbed above 100 km along that cable. For the next 70,000 kms it will accelerate in low gravity and effective vacuum. It will accelerate at one hell of a rate. If the timing is right (and with computers we have no excuse to get it wrong) when it gets to the end of the cable, at enormous speed, it will shoot off to its destination with no need for rocket fuel. Six months later, it is at Mars, or wherever we send it. Ion rocket fuel is then used only for deceleration.

 

This technology is theoretically possible. It seems to be only a matter of time and money before the first cable into space is done. Then the sky, literally, is the limit.

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  • 2 months later...
I thought it could only approach the speed that the particles are being accelerated at' date=' which was rouhgly 200 thousand miles an hour.

 

Oh, to put the thrust into perspective, I read somewhere that it is equivalent to the force of a peice of paper resting on your hand.

 

edit* I have another question too, about different sources of 'fuel' for the ion engine. Isn't there something out there better than Xenon? I mean xenon is heavy and will provide high amounts of momentum but I think its ionization energy is too high... meaning more energy required to ionize it. So, I was looking at the table and I saw cesium. It has an incredibly low ionization energy and is heavier than xenon (like only 1 a.m.u. but still, more is better). I know that since it is on the far left bottom corner that it is excessively corrosive, but I wouldn't think that would matter so long as it doesn't come in contact with the metal. If it has to go through a magnetic chamber anyways what does it matter? I don't think it is radioactive either. So why not the switch to Cesium?[/quote']

 

It could go maybe a million mph. You would need a large amount of xenon as fuel needs go up exponentially past exhausest speed. On regular xenon rockets, it's ionized by the sun, however, since they accelarate at about 1/4000 g, you'd be so far from the sun, you'd have to ionize the xenon before it launched, because you'd be too far from the sun for you to get even that much accelaration from your engines.

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an ion engine can accelerate steadily up to almost any velocity, mass ejected out the back will provide constant acceleration, discounting inertial dialation at high velocities (large fractions of C).

an accelerometer on board will register a constant acceleration though, acceleration is a function of time and the inertial dialation is exactly proportional to time dialation. (very probably the same thing)

 

the only limitations on an ion engine are the power supply and fuel capacity.

exceeding exhaust speed is no problem, the force is applied against already moving particles, momentum laws still apply.

 

modern ion rockets use solar power, but you can use any power source, deep space satellites (old tech) use a lump of sub critical plutonium and a thermopile cell. they deliver about 1-4 watts.

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who says an ion engine uses a constant force?

 

if the engine is limited by some power supply

then the speed that the craft can reach is somewhat limited by that fact, as the velocity v time graph becomes a square root function, and so after a period of time the acceleration on the rocket becomes neglegible. It was always my understanding that nasa doesn't use ion thrusters for speed, but instead for missions that require alot of delta v, Cassini used an ion engine because it needed to change course a number of times while it visited Jupiter's moons.

 

note: here is the math to find the acceleration and velocity that any craft will have if it uses a constant amount of power to propel itself.

 

PT=1/2MV^2

2PT=MV^2

2PT/M=V^2

(2PT/M)^(1/2)=V

 

to get the acceleration just take the derivative of velocity (V) with respect to time (T), but so you can see that an ion engine will never get a craft up to a very high velocity without bringing with it a power source able to supply several kilowatts of power, also you then have to consider the power to weight ratio as the weight could have the same effect on the crafts performance as the lack of power.

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you raise a valid point, kinetic energy does not follow a linear increase with respect to velocity, however this does not apply to rockets.

there are key differences between acceleration in rockets and cars ect.

in a car at a standstill, a small amount of kinetic energy will apply a large acceleration, the acceleration will drop off as the total energy increases.

 

in a rocket, you are applying a force against an ever replaced propellant,

the propellant is forced out the back and each particle will gain a set momentum.

 

the momentum laws will apply to the craft itself and the particle about to be fired. the particles already fired are essentially excluded from the equation since they cannot apply any force what so ever.

 

so say each particle gains 1 unit of momentum relative to the craft, the craft will gain 1 unit of momentum per particle it fires. if it fires one particle per second, the momentum and velocity will increase linearly.

 

if you really wanted to, you could take a look at the center of mass including all particles and the craft, the center of mass will only remain in fixed and constant motion if the rocket accellerates uniformly.

 

looking at your math, E=1/2 MV^2

PT does not equal kinetic energy

your math is suggesting that MVT = 1/2 MV^2

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Rocket Man, not quite linearly - don't forget that the mass of the propellant is changing, which affects velocity change that is determined from momentum laws. But this only means that the craft is going to accelerate ever so slightly more as it loses fuel via exhaust.

 

My $.02. I could be wrong, because ion engines might be different than normal rockets (and at any rate the mass expended by fuel is miniscule at best), but that's what I was led to believe.

 

Cheers.

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I don't know if this helps at all, but I'm kind of on a time budget (doen't that make me look considerate).

To accelerate to the spped of the exhaust takes approx. 1.7 times the mass of the ship, which isn't too bad. However, if we are unable to increase the velocity tof the propellant, we must increase the mass. To accelerate to twice the propellant's speed, however, takes 6.4 times the mass of the ship. With a chemical roket, this brings us up to 0.00003 the speed of light (I don't know if that includes accelerating the propellent itself). Excited yet?

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Rocket man you misunderstood my terms, P represents power (FV or E/T), and you can be quite assured that power times time equals energy. You can also be quite assured that the basic laws of physics are the same on earth as they are in space, in other words my math applies to both the car and the ion rocket (conventional rockets are more complicated).

 

the problem with your logic is that the limiting factor here is the amount of power available to the rocket and not the force applied, also be sure you don't think of the rocket's reference frame as a valid one, as it is accelerating.

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