how could ion propulsion's thrust be amplified?

[bonzelite]

vog, what is your take on the maglev technology adapted to frictionless wheel levitation? that is, a disk or wheel that "hovers" around superconducting magnets instead of using traditional axles and bearings? it could be used in a non-rail application on public roads. the tread compound around the disk would be affixed to the outer edge. <br /><br />in essence, you'd have a free-floating conductive wheel, suspended in air between the magnets. it would be like having incremental caliper-like superconductors arranged about the outer surface faces of the wheel. there would be no bearings or half-shafts to wear out.

 

[CalliArcale]

I'm not vogon, but I am an engineer. <img src="/images/icons/wink.gif" /> (Alas, I'm a software engineer, so my comments may not be all that useful to you.) That notion is sound in theory. The main practical problems will be cost, maintaining control of the wheel without a mechanical connection to it, and power supply. (Electromagnets can be very thirsty, power-wise.) <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[bonzelite]

your comment is useful, calli. it helps me realize that the idea is at least plausible, ie, believable. it could be done. <br /><br />the array of magnets could be coupled to a more traditional steering system, whilst the actual wheel floats. the power supply must not be bulky, though. that is an issue. an RTG (radioisotope thermoelectric generator) would be compact enough to be mounted within a fuselage-like vehicle body. <br /><br />because the vehicle must approach Mach 1 for a prolonged period, there can be no issue with wheel bearing failure due to friction. <br /><br />what do you think of hydrazine thrusters mounted in the centers of the wheels to aid in steering? the thrusters could be pivot-mounted so that they can act independently of the actual mechanical steering. you would have the regular contact patch of the wheel tread + the "hub-centric" thrust of the TCM burns to create grip steering. the thruster's active burns would be more like pulses in this case, giving fractions of a second corrections. it would be literally a traction control system.

 

[mikeemmert]

NUCLEAR POWER!! This would entail a gas-core nuclear reactor, which would be made from graphite or other hight temperature material. Since uranium, plutonium, and all their compounds vaporize at white heat, simply use vapor for the fuel elements. This would entail bomb-grade uranium or plutonium. Such a reactor could weigh just a few hundred pounds and produce as much horsepower as a battleship.<br /><br />The problem is obtaining the fuel. Maybe Saddam Hussein could procure some from la-la land...or Crawford...

 

[bonzelite]

i thought the RTGs on board the voyagers, galileo, and cassini are not actual reactors. (?)

 

[igorsboss]

<font color="yellow">Electromagnets can be very thirsty, power-wise.</font><br /><br />Just before the great forgetting, there was a thread discussing electric rail-guns for launching stuff like this.<br /><br />The problem with an electric rail-gun isn't just that the electromagnets want a lot of power, but also that they want it all at once. Each electromagnet needs to be energized at full power for just a split-second, with precise timing. You essentially need to detonate a series of big electric firecrackers. There was only one technology capable of handling this: huge capacitors.<br /><br />The conclusion was that the electric rail-gun technology would be inferior to a simple chemical cannon.<br /><br />The problem reminds me of punkin-chunkin... See www.punkinchunkin.com for more.<br /><br />Except that if you are on an asteroid<br />1) You can't use a compressed air cannon (because there is no atmosphere)<br />2) You can't use a gravitationally-powered trebuchet (microgravity)<br /><br />But you can:<br />3) Build a spring-powered catapult to launch buckts of rocks.<br />4) Build a centerfugal dirt-thrower to launch clouds of dust.<br /><br />Ya know, it would probably be realtively easy to build an atomic-electric-powered centerfugal asteroid-dust-thrower. It would make one hell of a dust tail! It would also get the whole asteroid spinning pretty good, too.

 

[spacester]

My understanding, and I am going from memory here, is that there are three main obstacles to building high-thrust ion engines.<br /><br />First and most obvious is that you need lots of electrical power to accelerate more ions and faster. Presumably, we're talking about Nuclear Fission to get the energy density required.<br /><br />Secondly, there seems to be an upper limit to the ion density that can be controlled. When I researched this a few years back, I was not able to really pin this factoid down, so emphasis on the "seems to be".<br /><br />Thirdly is the limit placed on performance by erosion of the grid which pulls the ions thru and thru which the ions must pass. It is my understanding that this is the reason Xenon is used: it erodes the grid least. Typically you can't replace the grid in deep space.<br /><br />BTW, it is also my understanding that conventional wisdom says, counterintuitively, you actually want your ion to be as light as possible: heavier ions are bad. This is because the lighter ion will achieve higher velocities all else being equal, and the rocket equation indicates that exhaust velocity is the key parameter. I'm not convinced of this 'conventional wisdom', because high exhaust velocity means high specific impulse, but what we're worried about is thrust, and my intuition continues to tell me that heavier ions should give you higher thrust. Nevertheless, my research clearly indicated that the experts want the ion as light as possible. It may be that heavy ions are ruled out due to the erosion issue. I really am not sure . . . <br /><br />Hopefully someday someone here will finally clear these issues up. <div class="Discussion_UserSignature"> </div>

 

[bonzelite]

that's pretty useful info actually. very clear, even if some of it is somewhat unknown. <br /><br />ok assume the craft is an earth-based plane. the grids can be swapped out regularly. it needs lots of thrust to take off from a conventional runway. the nuclear hazards notwithstanding, could such an engine be made to deliver the off-the-line power of thrust? provided we use the heavy ions of some other element? <br /><br />this turns to this "upper limit" you speak of. so you are saying that perhaps you can only inject so many ions into a given space, in the context of the ion engine, before there is an instability?

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p>The problem with an electric rail-gun isn't just that the electromagnets want a lot of power, but also that they want it all at once. Each electromagnet needs to be energized at full power for just a split-second, with precise timing. You essentially need to detonate a series of big electric firecrackers. There was only one technology capable of handling this: huge capacitors. <p><hr /></p></p></blockquote><br /><br />The problem, though, is that the capacitors drain very rapidly and must be recharged. It's useful for railguns, but not for a maglev or another application where the electromagnets are needed continually, rather than in short bursts. You make a very good point about the high load, though. I'm no EE, but I know enough to know that this machine is going to have to have some pretty sturdy wiring to maintain this load. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p>i thought the RTGs on board the voyagers, galileo, and cassini are not actual reactors. (?)<p><hr /></p></p></blockquote><br /><br />You are correct!<br /><br />RTG stands for Radioisotope Thermoelectric Generator. As plutonium decays, it produces heat. A lot of heat, actually, which is one of the major problems in storage of the stuff. A thermocouple is used to convert this heat into electricity. It's not hugely efficient, but it's very reliable, has no moving parts, and lasts for decades with no maintenance. The power supply isn't breathtakingly huge, and of course plutonium is very expensive and requires expensive handling and packaging as well. You also need to isolate the power supply from the vehicle, because the radiation it gives off may confuse the computers and will definitely screw up a lot of scientific measurements if you're not careful where you put the RTG. They don't produce an enormous amount of power, but they work in darkness and are useful in deep space far from the Sun. They can also serve a dual purpose and be rigged into the spacecraft's thermoregulation system to keep it from getting too cold.<br /><br />No scientific probe has been powered by a nuclear reactor to date. However, as power demands keep going up, it is inevitable eventually. Reactors can produce enormously more power than RTGs. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[bonzelite]

and you can isolate the RTG by shielding the living h@LL out of it? insofar as the heat produced, what are we talking? thousands of degrees F? or in Kelvin temperatures? <br /><br />i saw a pic of an RTG casing next to a person. it doesn't seem that huge. are you meaning the electrical power supply that is converting the heat into energy? space probes seem to have this whole package pretty compact. but then again, they are only needing a few watts of power. <br /><br />an ion propelled launch vehicle, or something like it, would require thousands of pounds of thrust, continually.

 

[mikeemmert]

Golly, you made me reread my post! I never said anything about RTG's. They're obsolete, they operate on alpha decay, and fission is about 10 times more energetic. We need actual reactors. Unless these use HIGHLY enriched fuel, bomb grade, in fact, they will be too heavy for space use.<br /><br />Fissionable material exists. Might as well put it to good use. If it's sent to the Kuiper belt or the Oort cloud or out 300 astronomical units where the Sun's gravitational lens comes to a focus, it will not be availiable for blackmail or terrorism.<br /><br />The only way to get very high power to weight ratios is very high temperatures. Also, forget about thermocouples, they're only about 1%-2% efficient. It would be better to use tubines or magnetohydrodynamics.