Ion Drive

[ydoaPs]

1 mole of He=4g

c=3x10⁸

p=mv

 

p=1.2x10⁹

 

As a point of comparison, according to wikipedia, the space shuttle is approximately 10⁸kg. Our 4g of He would have about the same momentum as a space shuttle going 10m/s.

 

So, this gives us a ridiculous change in momentum for a small amount of gas(yes, I know the particles can't go at c; this is an order of magnitude thing). So, what we need is a really good ion drive. How difficult would it be to develop an ion drive capable of accelerating the gas to near light speed?

 

I'm assuming we'd need good superconductors and some sort of nuclear reactor as a power source. What else?

Edited December 7, 2010 by ydoaPs

[Sisyphus]

The reaction mass is not theoretical limit. p=mv is a Newtonian equation, and you'd be dealing with relativistic velocities. A gram (or a single atom) of He moving at C would have infinite momentum, and in theory I'm pretty sure you could give it arbitrarily large momentum.

 

Rather, the limit is the energy needed to accelerate the reaction mass... the generation of which in turn consumes mass. So I guess the limit would come from generating the energy that would be transformed into kinetic energy of the reaction mass.

[InigoMontoya]

Well, millions of dollars are being spent on that very problem every year. And yet, they're nowhere close.

 

So, you ask "How difficult would it be?" And the answer so far appears to be, "Extremely."

[ydoaPs]

The reaction mass is not theoretical limit. p=mv is a Newtonian equation, and you'd be dealing with relativistic velocities. A gram (or a single atom) of He moving at C would have infinite momentum, and in theory I'm pretty sure you could give it arbitrarily large momentum.

 

Rather, the limit is the energy needed to accelerate the reaction mass... the generation of which in turn consumes mass. So I guess the limit would come from generating the energy that would be transformed into kinetic energy of the reaction mass.

So, what we need is a fission reactor that works in zero-G?

[Moontanman]

Isn't the space probe approaching Vesta right now Ion powered?

[Mr Skeptic]

Just remember that at some point, the mass used to get the energy becomes more significant than the mass expelled. Due to the nature of kinetic energy and momentum, you're better off throwing two particles half as fast as one particle twice as fast, which would take double the energy for the same momentum.

 

How about this new guy:

The VASIMR (VAriable Specific Impulse Magnetoplasma Rocket) works by using radio waves to ionize a propellant into a plasma and then a magnetic field to accelerate the plasma out of the back of the rocket engine to generate thrust.

[D H]

Just remember that at some point, the mass used to get the energy becomes more significant than the mass expelled. Due to the nature of kinetic energy and momentum, you're better off throwing two particles half as fast as one particle twice as fast, which would take double the energy for the same momentum.

To some extent, yes. Let's take this to the extreme of expelling photons, e.g., laser propulsion. This is, simply put, a silly notion. The energy and momentum of a photon are related by E=pc, or p=E/c. Multiply this by the number of photons emitted per second and you get F=P/c, where P is the power of the photon stream. Suppose we had a laser propulsion spacecraft whose power production was equal to the entire world's electric energy production, 6.25×10¹⁹ joules per year (about 2 terawatts) for 2005. The amount of thrust produced by that laser propulsion spacecraft? 6,600 newtons. Compare that to the 66,700 newtons produced by a single RL10 rocket (which is rather old technology).

 

 

 

So, back to ion propulsion:

How about this new guy: (quote from the wiki lead paragraph on the VASIMR thruster)

First, a link to the article that Mr Skeptic quoted from: http://en.wikipedia....toplasma_Rocket.

 

This is potentially a very worthy contender. One of the biggest challenges with ion propulsion has been getting any substantial amount of thrust out of the engine. Ion propulsion engines prior to VASIMR have had a very high specific impulse but also an incredibly low thrust. Even though SMART-1 was incredibly light (367 kg launch mass), it took the vehicle over a year to get from its initial 7,035 × 42,223 km geostationary transfer orbit to past the Earth-Moon L1 point. It took another seven months to get from this point to a low lunar orbit. The reason is the incredibly low thrust produced by the ion propulsion engine used on SMART-1. The ion propulsion engines used on Deep Space 1 and Dawn (the ongoing mission to Vesta and Ceres) are also quite wimpy, producing less than 0.1 newtons of thrust max. VASIMR might just get past that problem.

Edited December 7, 2010 by D H

[ydoaPs]

This may end up being a potential help to creating a good Ion drive.

 

What causes the low thrust? Is it that the power sources weren't sufficient to accelerate the ions enough?

[Mr Skeptic]

I'm almost inclined to say that it is the high specific impulse which causes the low thrust. The trouble with putting a lot of energy into each particle is that you need a lot of energy to do so.

[ydoaPs]

The trouble with putting a lot of energy into each particle is that you need a lot of energy to do so.

I was thinking at least one dedicated fission reactor.

[D H]

This may end up being a potential help to creating a good Ion drive.

Except that this is apparently bogus. No papers published in peer reviewed journals (they would be all over this if it was legit); even wikipedia steers clear (check the discussion page on wikipedia's high-temperature superconductivity page).

[ydoaPs]

Except that this is apparently bogus. No papers published in peer reviewed journals (they would be all over this if it was legit); even wikipedia steers clear (check the discussion page on wikipedia's high-temperature superconductivity page).

Darn.

[D H]

Even if it was true, how would it help? Superconductors are not power sources. All superconductors would do is eliminate resistive power loss between the generator and thruster. Given the short distances involved on a spacecraft, that loss is not going to be the big problem. Generating thrust and generating power will be the big problems.

[ydoaPs]

Even if it was true, how would it help? Superconductors are not power sources. All superconductors would do is eliminate resistive power loss between the generator and thruster. Given the short distances involved on a spacecraft, that loss is not going to be the big problem. Generating thrust and generating power will be the big problems.

With the charges needed to accelerate the particles, I thought that superconductors would be needed to make it anywhere near efficient. Is that wrong?

 

What could be used as a viable power source? All fission reactors that I've worked with require gravity to operate properly. Are there any gravity independent designs for fission reactors?

[D H]

There you go. Problems everywhere you look. Incredibly low thrust, incredibly high power consumption. Non-existant generators, at least to the extent needed to generate meaningful thrust. Plans to use VASIMR to get people to Mars predicate one kilogram per kilowatt generators. The Soviet Union's TOPAZ generator: 100 kilograms per kilowatt. The United State's NERVA generator: 65 kilograms per kilowatt. Predicating nearly two orders of magnitude in improvement is in general a no-go. About the only place that works is in the world of computing, Moore's law. There is no equivalent to Moore's law in space technology.

[the asinine cretin]

There you go. Problems everywhere you look. Incredibly low thrust, incredibly high power consumption. Non-existant generators, at least to the extent needed to generate meaningful thrust. Plans to use VASIMR to get people to Mars predicate one kilogram per kilowatt generators. The Soviet Union's TOPAZ generator: 100 kilograms per kilowatt. The United State's NERVA generator: 65 kilograms per kilowatt. Predicating nearly two orders of magnitude in improvement is in general a no-go. About the only place that works is in the world of computing, Moore's law. There is no equivalent to Moore's law in space technology.

 

Interesting. Has anything changed since 2010? As I mentioned in another thread, Ad Astra says that the VASIMR is to be tested on the ISS in the near future. I've not seen any substantial details.

 

Non-existant generators, at least to the extent needed to generate meaningful thrust. Plans to use VASIMR to get people to Mars predicate one kilogram per kilowatt generators. The Soviet Union's TOPAZ generator: 100 kilograms per kilowatt. The United State's NERVA generator: 65 kilograms per kilowatt.

 

A two inch cube of polonium-210 would emit 140kW. That could definitively get you many kilowatts per kilogram; perhaps the two orders of magnitude you desire when compared with your numbers. The half-life is less than five months but I imagine a very compact and light weight RTG could power a VASIMR on a robotic spacecraft and ramp it up to extreme speeds for outer solar system recon and the like.

 

(I haven't researched this or anything, just looked up some numbers of radioisotopes and talking out of my ass.)

 

Just speculating here, but if polonium-210 production could be carried out on Mars you could make the trip to Mars in a few months or less and then obviously swap out the little power source for the return trip. Apparently it's been used in RTGs already so it's not that far-fetched.

 

According to the VASIMR wikipedia page the 200kW engine is serious business.

 

"Results presented to NASA and academia in January 2011 have confirmed that the design point for optimal efficiency on the VX-200 is 50 km/s exhaust velocity, or an Isp of 5000 s. Based on these data, thruster efficiency of 72 % has been achieved by Ad Astra, yielding an overall system efficiency (DC electricity to thruster power) of 60% (since the DC to RF power conversion efficiency exceeds 95%)."

 

1 kg of polonium-210 emits 140kW, and good RTG efficiency by today's standards is something like 7%. Therefore, 20 kg of radioisotope could create a worthy generator. No? Also, I tend to suppose that the 7% RTG efficiency could be improved upon in this day and age.

 

 

The wiki article mentions something about a craft with a MW solar array and five of the 200 kW thrusters. Hmm.. It might look goofy, but if it works... And maybe with the expected innovations in photovoltaic tech that I often hear about such an array wouldn't be as ridiculous as I'm imagining. I'm thinking that the ISS arrays total to something like 150-200kW (I could be wrong); extrapolating from that to 1 MW results in a rather goofy spacecraft.

 

It looks like there really isn't a Po-210 supply. Russia has one facility that produces less than 100 grams per year. That's really about it. It must have been produced in notable quantities in the past given its use for Russian lunar rovers and some U.S. experiments and things. Plutonium-238 seems to be tending toward the same fate at the moment. I wonder if Po-208 is viable for use in an RTG? The half-life of nearly 3 years seems more useful than the 5 months of Po-210. I wonder what the heat energy would be for a kg of the stuff. Oh well...

 

P.S. The prototype ASRG power system attains 30% efficiency and a fourfold reduction in fuel. With plutonium-238 as the energy source it could power a spacecraft for >14 years. If the design could be effectively scaled up for the 200 kW demands of VASIMR... The prototype seems to get 175 watts per kilogram of Pu-238, or about 6 kg per kW. It's a good step I suppose. Or maybe the uber efficient design of the ASRG with a more concentrated fuel such as Po-210 or whatever. Yeah, no doubt 30 or 40 kW per kg could be worth something. A radioisotope in between Pu-238 and Po-210 would be ideal. Oh well.

Edited May 22, 2012 by the asinine cretin

[John Cuthber]

"A two inch cube of polonium-210 would emit 140kW"

That's about 600 ml of volume. The density is about 9 so

It would weigh about 6. Kg

1 mg is about 4 curies

So you have 24 MCi of radioactive material

About 50mCi killed someone and that dose is estimated as about 200 times the lethal dose.

So the lethal dose is about 250µCi

So 24,000,000 Ci is about 100,000,000,000 times the lethal dose for a human.

 

And, in the event of the spacecraft exploding...

And don't forget that with 210 Po the generator has a half life of about 6 months

(data from wiki, but any errors in maths are my own responsibility.

Edited May 22, 2012 by John Cuthber

[the asinine cretin]

"A two inch cube of polonium-210 would emit 140kW"

That's about 600 ml of volume. The density is about 9 so

It would weigh about 6. Kg

1 mg is about 4 curies

So you have 24 MCi of radioactive material

About 50mCi killed someone and that dose is estimated as about 200 times the lethal dose.

So the lethal dose is about 250µCi

So 24,000,000 Ci is about 100,000,000,000 times the lethal dose for a human.

 

And, in the event of the spacecraft exploding...

And don't forget that with 210 Po the generator has a half life of about 6 months

(data from wiki, but any errors in maths are my own responsibility.

 

It's definitely super lethal stuff. However, so is Pu-238 and we're still launching kilos of the stuff into space. The RTGs are pretty solid. An RTG from the aborted Apollo 13 mission is at the bottom of the sea some place. Actually dispersing the material in the atmosphere like you suggest isn't really going to happen even in the worst catastrophic failure. We've put Po-210 in orbit, and the Soviets certainly launched some good chunks of the stuff with their funny looking moon carts. But anyway, if your point is that it's dangerous stuff, I wholeheartedly agree.

 

At least the majority of the Po-210 will decay into Pb-206 quite quickly. If Apollo 13 had used Po-210, a much smaller amount of dangerous material would have been needed and it would probably have been basically gone before I was born.

 

 

"To minimize the risk of the radioactive material being released, the fuel is stored in individual modular units with their own heat shielding. They are surrounded by a layer of iridium metal and encased in high-strength graphite blocks. These two materials are corrosion- and heat-resistant. Surrounding the graphite blocks is an aeroshell, designed to protect the entire assembly against the heat of reentering the Earth's atmosphere. The plutonium fuel is also stored in a ceramic form that is heat-resistant, minimising the risk of vaporization and aerosolization. The ceramic is also highly insoluble." - Wikipedia: RTG

 

Here's a bit of NASA's safety analysis for the Cassini-Huygens mission, which launched 3 RTGs and 129 radioisotope heaters. http://saturn.jpl.nasa.gov/spacecraft/safety/fseisd.pdf

Edited May 22, 2012 by the asinine cretin

[D H]

Interesting. Has anything changed since 2010? As I mentioned in another thread, Ad Astra says that the VASIMR is to be tested on the ISS in the near future. I've not seen any substantial details.

No news is no news. Or in some cases, it's bad news. Lots and lots of proposed technologies start with a flourish but then just fade away.

 

 

A two inch cube of polonium-210 would emit 140kW. That could definitively get you many kilowatts per kilogram; perhaps the two orders of magnitude you desire when compared with your numbers. The half-life is less than five months but I imagine a very compact and light weight RTG could power a VASIMR on a robotic spacecraft and ramp it up to extreme speeds for outer solar system recon and the like.

What you are doing here is accounting for the mass of the fuel but failing to account for the mass of all of the infrastructure needed to make that fuel useful. The decay of polonium 210 generates heat. VASMIR runs on electricity. You are ignoring the fairly massive infrastructure needed to convert that heat into electricity plus the extremely massive infrastructure needed to ensure that the PO-210 remains safely encapsulated even in the case of an explosion. That alone changes your many kilowatts per kilogram into many kilograms per kilowatt. Then there's the problem of the efficiency of that conversion of heat into electricity. RTGs are simple devices with no moving parts. The downside: They are extremely inefficient. Typically efficiency is about 5%. Your two inch cube of polonium-210 would generate about 7 kW of electricity.

 

 

Just speculating here, but if polonium-210 production could be carried out on Mars you could make the trip to Mars in a few months or less and then obviously swap out the little power source for the return trip.

Swap it out? With what? Another PO-210 based RTG? Won't work. Polonium 210 is highly radioactive. The polonium doesn't care if the generated heat is put to productive use or is just radiated out into space.

[Moontanman]

No news is no news. Or in some cases, it's bad news. Lots and lots of proposed technologies start with a flourish but then just fade away.

 

 

 

What you are doing here is accounting for the mass of the fuel but failing to account for the mass of all of the infrastructure needed to make that fuel useful. The decay of polonium 210 generates heat. VASMIR runs on electricity. You are ignoring the fairly massive infrastructure needed to convert that heat into electricity plus the extremely massive infrastructure needed to ensure that the PO-210 remains safely encapsulated even in the case of an explosion. That alone changes your many kilowatts per kilogram into many kilograms per kilowatt. Then there's the problem of the efficiency of that conversion of heat into electricity. RTGs are simple devices with no moving parts. The downside: They are extremely inefficient. Typically efficiency is about 5%. Your two inch cube of polonium-210 would generate about 7 kW of electricity.

 

 

 

Swap it out? With what? Another PO-210 based RTG? Won't work. Polonium 210 is highly radioactive. The polonium doesn't care if the generated heat is put to productive use or is just radiated out into space.

 

 

I think the idea was to produce new polonium on Mars for the return trips which of course suggests an already existing infrastructure at or on Mars.

 

With so much machinery required to produce electricity from an RTG why not just just go for a real nuclear reactor? Something like a gaseous fission reactor should have a pretty good watts per kilogram ratio, use it to produce electrical power, it would radiate in the deep UV, possibly this could be turned directly into electricity? (I know I've got gaseous fission in my brain)

[the asinine cretin]

No news is no news. Or in some cases, it's bad news. Lots and lots of proposed technologies start with a flourish but then just fade away.

In other words you don't know?

 

What you are doing here is accounting for the mass of the fuel but failing to account for the mass of all of the infrastructure needed to make that fuel useful. The decay of polonium 210 generates heat. VASMIR runs on electricity. You are ignoring the fairly massive infrastructure needed to convert that heat into electricity plus the extremely massive infrastructure needed to ensure that the PO-210 remains safely encapsulated even in the case of an explosion. That alone changes your many kilowatts per kilogram into many kilograms per kilowatt. Then there's the problem of the efficiency of that conversion of heat into electricity. RTGs are simple devices with no moving parts. The downside: They are extremely inefficient. Typically efficiency is about 5%. Your two inch cube of polonium-210 would generate about 7 kW of electricity.

Right. I was only talking about the fuel and not infrastructure. But I wouldn't call thermocouples "extremely massive infrastructure." There are things to consider beyond just the fuel? No kidding, everyone knows that. Perhaps you mistook this for a formal proposal. lol. Condescension isn't the reply I was hoping for.

 

You evidently missed my comments about efficiency. Based on modern RTGs I suggested 7% as a reasonable efficiency but later mentioned a contemporary prototype device that is said to attain 30% efficiency. Since I'm freely speculating about what's possible, and not trying to be cynical, I think it's okay to imagine what could be done with a 7-30% efficient system.

 

 

Swap it out? With what? Another PO-210 based RTG? Won't work. Polonium 210 is highly radioactive. The polonium doesn't care if the generated heat is put to productive use or is just radiated out into space.

Moontanman's interpretation is correct. Obviously the radioisotope would be manufactured in situ on Mars. Clearly the killer problem with this Martian refill idea is that it assumes some serious infrastructure on Mars (for example, the neutrons required by the production process would probably require building and maintaining a large nuclear facility, as well as mining operations, refineries, and so on), but if we could already develop Mars on that scale it's hard to see why the VASIMR discussion would be relevant. In other words, it's a circular idea. There are ways to make it more worthwhile but I wouldn't actually say that I think it is workable. Just indulging in some stream of consciousness.

Edited May 23, 2012 by the asinine cretin

[D H]

In other words you don't know?

No news is most often bad news when in comes to companies that delve into new technologies. Or it might mean they are quietly working on things where progress is measured in years. Many, many years in this case.

 

 

Right. I was only talking about the fuel and not infrastructure. But I wouldn't call thermocouples "extremely massive infrastructure." There are things to consider beyond just the fuel? No kidding, everyone knows that. Perhaps you mistook this for a formal proposal. lol. Condescension isn't the reply I was hoping for.

There's a lot more to RTGs than thermocouples, particularly an RTG that contains 100 billion lethal doses of ²¹⁰Po. The fuel is encapsulated in ceramic, surrounded by iridium, which in turn is surrounded by high-strength carbon, which in turn are placed in housings, which in turn is surrounded by an aeroshell. There is a whole lot more to an RTG than just the fuel and a couple of thermocouples.

 

 

You evidently missed my comments about efficiency. Based on modern RTGs I suggested 7% as a reasonable efficiency but later mentioned a contemporary prototype device that is said to attain 30% efficiency. Since I'm freely speculating about what's possible, and not trying to be cynical, I think it's okay to imagine what could be done with a 7-30% efficient system.

Those higher efficiency systems are not RTGs. They are heat engines. Now you have a working fluid, moving parts, a heat exchanger, a generator, and you still need massive amounts shielding to keep those 100 billion lethal doses from escaping in case of some kind of accident. There's a good reason that none of the world's space agencies have gone to these speculative devices. The four fold boost in efficiency is not worth the added mass.

 

 

Moontanman's interpretation is correct. Obviously the radioisotope would be manufactured in situ on Mars.

Obviously not. Where does it come from? That in situ manufacturing would require building on Mars an automated bismuth mine, an automated bismuth refining plant, an automated nuclear reactor, an automated neutron bombardment facility to bombard the mined and refined bismuth with neutrons, an automated purification system to isolate the polonium, and an automated storage system. It's ludicrous.

[the asinine cretin]

D H,

Okay, well, your post is about 5% relevant to me. Mostly stating the painfully obvious and repeating things I've said or alluded to, which is obnoxious. Not an interesting convo.

 

 

 

With so much machinery required to produce electricity from an RTG why not just just go for a real nuclear reactor? Something like a gaseous fission reactor should have a pretty good watts per kilogram ratio, use it to produce electrical power, it would radiate in the deep UV, possibly this could be turned directly into electricity? (I know I've got gaseous fission in my brain)

 

 

Moontanman,

 

The more I look into them the more intrigued I am by stirling radioisotope generators. The tech exists and has demonstrated to be both significantly more efficient and lighter than RTGs; at least according to the stuff I've been reading. There is a big push to finish its development.

 

Fission power back on NASA's agenda

Using Nuclear Fuel for Future NASA Missions...

 

Advanced Stirling Radioisotope Generator Flight Development

 

The TiME mission is completely awesome and will likely use the tech.

 

The NASA Space Technology Roadmaps and Priorities document, as of May 4th of this year, mentioned Advanced Stirling Radioisotope Generators 35 times.

 

"NASA and DOE have been developing advanced RPSs that would use Stirling engines to replace thermoelectric converters. Because the energy conversion efficiency of the Stirling engine under development is about 5 times that of thermoelectric converters, Stirling engines require significantly smaller quantities of Pu-238 to achieve similar power levels."

 

"The planetary science decadal survey committee cited its highest priority for near-term multimission technology investment was the completion and validation of the Advanced Stirling Radioisotope Generator. (NRC, 2011, p. 11-5)"

 

 

"There were two technologies, Advanced Stirling Radioisotope Generators and On-Orbit Cryogenic Storage and Transfer, that the committee considered to be at a "tipping point," meaning a relatively small increase in the research effort could produce a large advance in its technology readiness."

 

 

In other words, don't be surprised when we start seeing these on outer solar system spacecraft. Very cool.

Edited May 23, 2012 by the asinine cretin

[D H]

The more I look into them the more intrigued I am by stirling radioisotope generators. The tech exists and has demonstrated to be both significantly more efficient and lighter than RTGs; at least according to the stuff I've been reading. There is a big push to finish its development.

The main driver behind that push is the fact that domestic production plutonium 238 has been shut down since 1993. Funding to get that domestic production restarted is barely there, and it's only on the NASA side. Producing ²³⁸Pu the job of the Department of Energy, and Congress for some reason refuses to fund the DOE side. Logjam.

 

If there was an adequate supply of plutonium 238 NASA would happily continue using RTGs. They are a very simple and very trusted technology. No moving parts. They don't fail. While much more efficient, those Stirling generators are new technology and are significantly more complex than RTGs. Increased complexity means a significantly increased likelihood of failure. That it is new also increases the risk of failure. A lot.

 

The ASRGs will be used for low power consumption devices such as sensors and communications. Using them for a high power consumption device such as a VASIMR engine is not going to get a vehicle to Mars. At least not very fast. Do the math. A VX-200 sucks 200 kW of electrical power, or the output of 1400+ ASRGs. At 32 kg per ASRG, the power source needed to supply a VX-200 masses over 45,700 kg. That's just for the power source, not the housing for those 1400+ units or the electrical cable needed to connect them to the engine. All for 5 newtons of thrust.

Edited May 23, 2012 by D H

[swansont]

Okay, well, your post is about 5% relevant to me. Mostly stating the painfully obvious and repeating things I've said or alluded to, which is obnoxious. Not an interesting convo.

 

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