Ion Drive

[Moontanman]

I think that for realistic power in space you will need a nuclear reactor of some type, gaseous fission would be tricky and difficult. Magnetic confinement of the reacting plasma would seem to be the answer but as far as I can see there is no hope of creating magnetic fields strong enough to confine such a heavy ion plasma.

 

http://www.spacedaily.com/reports/Nuclear_Power_In_Space_999.html

 

http://en.wikipedia.org/wiki/Gaseous_fission_reactor

 

Spacecraft

 

The spacecraft variant of the gaseous fission reactor is called the gas core reactor rocket. There are two approaches: the open and closed cycle. In the open cycle, the propellant, most likely hydrogen, is fed to the reactor, heated up by the nuclear reaction in the reactor, and exits out the other end. Unfortunately, the propellant will be contaminated by fuel and fission products, and although the problem can be mitigated by engineering the hydrodynamics within the reactor, it renders the rocket design completely unsuitable for use in atmosphere.

One might attempt to circumvent the problem by confining the fission fuel magnetically, in a manner similar to the fusion fuel in a tokamak. Unfortunately it is not likely that this arrangement will actually work to contain the fuel, since the ratio of ionization to particle momentum is not favourable. Whereas a tokamak would generally work to contain singly ionized deuterium or tritium with a mass of two or three daltons, the uranium vapour would be at most triply ionized with a mass of 235 dalton (unit). Since the force imparted by a magnetic field is proportional to the charge on the particle, and the acceleration is proportional to the force divided by the mass of the particle, the magnets required to contain uranium gas would be impractically large; most such designs have focussed on fuel cycles that do not depend upon retaining the fuel in the reactor.

In the closed cycle, the reaction is entirely shielded from the propellant. The reaction is contained in a quartz vessel and the propellant merely flows outside of it, being heated in an indirect fashion. The closed cycle avoids contamination because the propellant can't enter the reactor itself, but the solution carries a significant penalty to the rocket's Isp.

 

http://en.wikipedia.org/wiki/Gas_core_reactor_rocket

 

Theory of operation

 

Nuclear gas-core-reactor rockets can provide much higher specific impulse than solid core nuclear rockets because their temperature limitations are in the nozzle and core wall structural temperatures, which are distanced from the hottest regions of the gas core. Consequently, nuclear gas core reactors can provide much higher temperatures to the propellant. Solid core nuclear thermal rockets can develop higher specific impulse than conventional chemical rockets due to the extreme power density of the reactor core, but their operating temperatures are limited by the maximum temperature of the solid core because the reactor's temperatures cannot rise above its components' lowest melting temperature.

Due to the much higher temperatures achievable by the gaseous core design, it can deliver higher specific impulse and thrust than most other conventional nuclear designs. This translates into shorter mission transit times for future astronauts or larger payload fractions. It may also be possible to use partially ionized plasma from the gas core to generate electricity magnetohydrodynamically, subsequently negating the need for an additional power supply.

[edit]

Edited May 23, 2012 by Moontanman

[the asinine cretin]

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.

I am aware of the current state of Pu-238 and the other things related to that covered in materials that I've posted in this thread. But I'd like to know how you can say that this is the main driver behind the push for ASRGs. I don't dispute that it's a driver, but you make a lot of claims like this that strike me as supposition. If you have sources for these claims please share, if not, please indicate that you're just speculating. Are you personally involved in any of these things? (I actually find it hard to believe that you don't understand the many advantages of ASRGs over RTGs.)

 

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.

Again, I'm inclined to dispute this. RTGs have their advantages, but ASRGs are significantly more efficient (even if Pu-238 production were still happening the fuel would be expensive, and radioisotopes are heavy, dangerous, et cetera), and based on what I've read you're wrong that ASRGs add more mass. Some of the sources I've provided state that ASRGs are lighter and that this is a real advantage (I'd actually like to learn more about this as these sources have not gone into detail). Again, if you have sources I'd like to see them. The basis of my impressions can be found in documents linked above.

 

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.

Where are you getting your numbers? The 2008 NASA Planetary Science Division Update indicates that the ASRG prototype has a nominal power of 140 W with 0.8 kg of Pu-238 fuel, but gives the mass as 20 kg, not 32 kg. I've also read that the 200 kW VASIMR can serve up 20 Newtons. But you're missing the key thing about my RTG/ASRG speculation, and why I brought it up at all, which was that Po-210 might be used, not Pu-238. This changes everything.

And I'm interested in the theoretical potential here, not merely existing prototypes. E.g., Entertaining the possibility of fuel other than Pu-238 (the key point being that Po-210 ups things by orders of magnitude). A large ASRG utilizing 10 kg of Po-210 with 30% efficiency would obviously have very different specs than a 0.8 kg Pu-238 device, or multiple 140 W Pu devices that add up to similar power output. It's a fun theoretical exercise and not something I'm interesting in bickering with you about. Your condescension and habitual naysaying anger me.

 

I think that for realistic power in space you will need a nuclear reactor of some type, gaseous fission would be tricky and difficult. Magnetic confinement of the reacting plasma would seem to be the answer but as far as I can see there is no hope of creating magnetic fields strong enough to confine such a heavy ion plasma.

 

http://www.spacedail..._Space_999.html

 

http://en.wikipedia....fission_reactor

 

 

 

http://en.wikipedia...._reactor_rocket

 

 

Thanks. That is interesting. Reading those links now. What I'm curious to know is to what extent such things are being actively developed right now. I mean, is this a possible solution for 100 years from now, 50 years from now, etc? It's encouraging to see ASRGs in the agenda-setting NASA docs, for example.

 

(Disclaimer: What follows is purely "what if" speculation for its own sake and there is no need to take a shit on it. Assume I know the obvious. If you're offended by this, just consider it sci-fi or something. Thanks.)

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) . . .

Bismuth might be obtained as a byproduct of mining and refining metals such as gold, silver, copper, cobalt, nickel, tin, etc. from asteroids. Such operations would presumably be established independent of the Polonium-210 fuel idea. Rather than having facilities on Mars it would probably make more sense to have the production capabilities on Phobos or Deimos. Perhaps one of these bodies is itself rich in relevant materials. A significant mining-refining operation on such a body would likely include a substantial nuclear reactor which could supply the neutrons. The production of short-lived but super high output Po-210 power source "refills" might then be a relatively painless byproduct of profit-driven infrastructure development. Basically, operations on Phobos receiving and processing materials from NMOs and asteroid belt objects; operations on Luna receiving and processing NEO materials; and a two-way express route powered by the Po byproducts.

 

Of course if space operations were already that advanced the current topic of powering VASIMR and the like would be obsolete. Again, this is just a fun imaginary thing. But I do suspect that Po-210 will prove to be a useful fuel with a variety of applications in the future. The theoretical potential is interesting anyway.

 

As far as the real topic of getting the VASIMR in operation in the near future, based on what I've read, the direction they seem to be going is solar power. Of course this limits the applications considerably. But I recall something about a 2 MW solar array with multijunction photovoltaics with greater than 30% efficiency. I don't know how accurate or current this information might be. And I'd be curious to know what kg per kW ratio they can achieve. Must continue Googling...

 

Although, for the lunar cargo spacecraft, perhaps solar arrays in cis-lunar space could beam power to the craft via microwave, thus radically reducing the mass required by power systems. I've researched space-based solar power and the possible efficiency is actually very good, but the receivers tend to be huge and the distances involved are of course far less than would be required for a lunar-bound spacecraft. Maybe laser power beaming from cis-lunar arrays and/or a lunar surface array.It would be interesting to explore such concepts anyway.

 

P.S. This site is fun. http://www.asterank.com/

Edited May 24, 2012 by the asinine cretin

[the asinine cretin]

In case anyone is interested.

 

Electric Propulsion: Which One For My Spacecraft?

 

Gas Core Nuclear Rocket Engines Promise and Reality

 

 

Regarding Beamed Power

 

Photovoltaic Receivers for Laser Beamed Power in Space

 

Pulsed Laser Illumination of Photovoltaic Cells

 

Beamed-Energy Propulsion Study (this doc is effin huge, be warned)

 

NASA Technical Reports Server (search results)

 

 

VASIMR Related

 

Ad Astra Publications

 

Nuclear Electric Rocket

 

SAFE-400 Fission Engine (400 kW; 1.2x10^3 kg)

 

Design and analysis of the SAFE-400 space fission reactor

 

An Ad Astra study (found here) describes a mission using 12 MW VASIMR propulsion for a < 3 month trip to Mars and allocated about 25 mT of mass budget for the reactors (including radiators). It might then take 30 of the SAFE-400 reactors to achieve 12 MW. At 1200 kg each that's about 39 mT; significantly over budget. However, I suppose a megawatt-scale reactor would have a smaller specific mass. I wonder how the SAFE design would scale? What might the mass be for a 1 WM version, or a 6 MW version? (The SAFE-400 is basically the result of some fiddling "on the side" with discretionary money; it's not unreasonable to suppose we could do better with serious funding.) The study also describes an epic 200 MW Mars mission. This very interesting paper was among the references. Multimegawatt NEP with Vapor Core Reactor MHD

Edited May 24, 2012 by the asinine cretin

[Enthalpy]

As an alternative, you can heat hydrogen with Sunlight and eject it. The specific impulse is around 800 to 1200s, which suffices for a fast Mars trip AND a short stay there (which chemical propulsion cannot both offer, only one or the other), and the concentrators and engines are lighter than a reactor - also avoiding other drawbacks.

 

It's not an existing technology neither, but I put some figures on it and it looks reasonably within grasp - simpler than developing a plasma engine or a reactor.

 

Zubrin's Mars Direct is a direct competitor to Nasa's choice of Vasimr, which "may" explain Zubrin's hard tone agasint Vasimr.

His comparison with existing ion drives does not take into account the thrust magnitude. Existing ones wouldn't accelerate a huge craft in days.

But I do agree no lightweight strong reactor exists, which is a condition for strong acceleration hence short trip.

 

Anyway, I'm no supporter of Mars Direct. Pre-placing a descent-ascent module and a return craft in Martian orbit is a safer option, as it can offer redundancy and needs no trip at Mars' surface to some hopefully working equipment. Chemical propulsion allows a short stay there (2 weeks around opposition) if aerobraking at Mars and Earth - or less interesting, a shorter trip and the usual long stay. It involves passing nearer to the Sun than Earth is.

 

Sorry, moderators here don't want links to my descriptions in an other forum, which Google doesn't list neither.

[the asinine cretin]

Interesting post, Enthalpy.

 

But I do agree no lightweight strong reactor exists, which is a condition for strong acceleration hence short trip.

What do you think of the SAFE-400 reactor?

 

Anyway, I'm no supporter of Mars Direct. Pre-placing a descent-ascent module and a return craft in Martian orbit is a safer option, as it can offer redundancy and needs no trip at Mars' surface to some hopefully working equipment. Chemical propulsion allows a short stay there (2 weeks around opposition) if aerobraking at Mars and Earth - or less interesting, a shorter trip and the usual long stay. It involves passing nearer to the Sun than Earth is.

What do you think of Mars Semi-Direct, and other mission architectures inspired by Mars Direct?

 

Sorry, moderators here don't want links to my descriptions in an other forum, which Google doesn't list neither.

I'd love more information on the first thing you mentioned; namely, hydrogen + sunlight propulsion. How would that work and how did you arrive at the Isp numbers? Thanks

 

It might then take 30 of the SAFE-400 reactors to achieve 12 MW.

The SAFE-400 operates at 100 kWe and 400 kWt, therefore, it would take 120 reactors. Sorry, self.

[Moontanman]

Here is a interesting take on interplanetary travel.

 

http://members.shaw.ca/bru_b/Liberty_ship_menupg.html

[D H]

Where are you getting your numbers? The 2008 NASA Planetary Science Division Update indicates that the ASRG prototype has a nominal power of 140 W with 0.8 kg of Pu-238 fuel, but gives the mass as 20 kg, not 32 kg.

That document is old and it excludes the mass of the controller. I got my numbers from the DOE fact sheet on the ASRG dated January 2011. http://www.ne.doe.gov/pdfFiles/factSheets/SpaceRadioisotopePowerSystemsASRG.pdf

 

That said, even if you go with your 20 kg versus the 32 kg I used, does it really matter? The mass needed to power one VX-200 is still overwhelmingly large.

 

I've also read that the 200 kW VASIMR can serve up 20 Newtons.

Reference needed. Everything I've read says 5 or 5.7 newtons, even the fluff pieces out of Ad Astra such as http://www.adastrarocket.com/aarc/VX200

 

But you're missing the key thing about my RTG/ASRG speculation, and why I brought it up at all, which was that Po-210 might be used, not Pu-238. This changes everything.

No, it doesn't. You can't just substitute one energy source for another and leave everything else unchanged. You most certainly can't use something that generates so much heat that it can vaporize itself and leave everything else unchanged. And you absolutely cannot use something that is as astoundingly deadly as polonium 210 and leave everything else unchanged. Everything needs to be redesigned. The fuel needs to be diluted and encapsulated to keep it from vaporizing itself. The highly dangerous fuel mandates even stronger safety measures than are used for RTGs or the to-be-flown ASRG. The much larger heat output requires thermal rejection well beyond just dumping the heat to the spacecraft structure. The very short half life means the system would be of very limited use.

 

Your condescension and habitual naysaying anger me.

Tough. My habitual naysaying is mild compared to what happens in a real peer review. The standard line for those proposing new ideas is to "check your egos in at the door."

 

 

An Ad Astra study (found here) describes a mission using 12 MW VASIMR propulsion for a < 3 month trip to Mars and allocated about 25 mT of mass budget for the reactors (including radiators). It might then take 30 of the SAFE-400 reactors to achieve 12 MW. At 1200 kg each that's about 39 mT; significantly over budget.

Try 120 rather than 30 SAFE-400s. Read your own source. A SAFE-400 produces 400 kW of thermal power but only 100 kW of electrical power. Another point: That 1200 kg is the mass of the reactor. It does not include the mass of the thermal radiators, and 120 of those generators will require a lot of thermal radiators. Once again, read the source that you provided.

 

 

I side with Zubrin on the idea of using VASIMR to get to Mars. It's a hoax on the scale of Solyndra.

[the asinine cretin]

No, it doesn't. You can't just substitute one energy source for another and leave everything else unchanged. You most certainly can't use something that generates so much heat that it can vaporize itself and leave everything else unchanged. And you absolutely cannot use something that is as astoundingly deadly as polonium 210 and leave everything else unchanged. Everything needs to be redesigned. The fuel needs to be diluted and encapsulated to keep it from vaporizing itself. The highly dangerous fuel mandates even stronger safety measures than are used for RTGs or the to-be-flown ASRG. The much larger heat output requires thermal rejection well beyond just dumping the heat to the spacecraft structure. The very short half life means the system would be of very limited use.

Of course. But the fact that we developed Po-210 RTGs in the late '50s makes me inclined to think you're exaggerating.

 

Wikipedia mentions the half-life as the primary reason why Pu-238 was favored. And again, holy crap, no kidding the short half life means it's of limited use. Did I ever suggest otherwise? I thought I made that point pretty clear on multiple occasions.

 

But really, my scenario was completely speculative and evolved to assume extensive space infrastructure. It's just a "what if" scenario so don't worry about it.

Oh, and the more general point was not that Po-210 is a panacea, but that alternative isotopes may be worthwhile for particular applications. I picked Polonium as an example because I knew RTGs had been built using this element and BECAUSE of its short half-life; although there are nearly 30 radioisotopes considered to be candidates for RTG tech.

 

Tough. My habitual naysaying is mild compared to what happens in a real peer review. The standard line for those proposing new ideas is to "check your egos in at the door."

This isn't a peer-reviewed journal it's an internet forum. Don't be a dick or I'm telling Wil Wheaton. Also, if you're going to be so condescending and presume to "peer-review" someone's speculative internet comments at least read what they fuckin' write and do more than restate things they've already said.

 

 

Try 120 rather than 30 SAFE-400s. Read your own source. A SAFE-400 produces 400 kW of thermal power but only 100 kW of electrical power. Another point: That 1200 kg is the mass of the reactor. It does not include the mass of the thermal radiators, and 120 of those generators will require a lot of thermal radiators. Once again, read the source that you provided.

I've already made that point. I guess I won't fault you for not picking up on it. I've had a tendency in this thread to start reading a doc, post about it, finish reading it, and then edit my post and append comments.

 

I side with Zubrin on the idea of using VASIMR to get to Mars. It's a hoax on the scale of Solyndra.

That may be. I hope not. If you have evidence for this claim I'd be interested in seeing it.

Edited May 27, 2012 by the asinine cretin

[swansont]

This isn't a peer-reviewed journal it's an internet forum. Don't be a dick or I'm telling Wil Wheaton. Also, if you're going to be so condescending and presume to "peer-review" someone's speculative internet comments at least read what they fuckin' write and do more than restate things they've already said.

 

!

Moderator Note

Focus on the topic at hand. If you're right, back it up. If you aren't, attacking someone rather than their critique is not acceptable. Neither is substituting your standard of civility for the board's.

 

Being corrected on matters of technical accuracy is precisely what one should expect to happen here. Making it personal is unacceptable.

[the asinine cretin]

!

Moderator Note

Focus on the topic at hand. If you're right, back it up. If you aren't, attacking someone rather than their critique is not acceptable. Neither is substituting your standard of civility for the board's.

 

Being corrected on matters of technical accuracy is precisely what one should expect to happen here. Making it personal is unacceptable.

 

I don't agree with your assessment (cynically restating what I've already said or alluded to as though a new insight is correction? --I call it obnoxious) and I find your use of the mod tags in sharing your feelings to be worthy of contempt. Am I supposed to be threatened? Fortunately I have a life outside of this website.

 

EDIT:

 

Okay, in honor of Wil Wheaton and in the name of Sybok, I hereby swallow my pride and renounce my uncivilized behavior on this thread. I'd like to thank you, swansont, for your fraternal correction, and I do so from the depths of my heart.

D H, I must commend you for suffering my foolish behavior and I resolve to treat you with the utmost respect from here on out. I'd also like to thank you for your insightful commentary on my own imbecilic posts; I very much appreciate your time.

 

Best wishes, and God bless.

Edited May 27, 2012 by the asinine cretin

[D H]

Anyway, I'm no supporter of Mars Direct.

Nor am I. A bigger question: Why send people to Mars at all? For one thing, it is extremely, extremely expensive. The required technology for a human trip to Mars, fast or slow, is way beyond our current capabilities. Zubrin's approach predicates just as much, if not more, hocus pocus than does Ad Astra's.

 

Yet another big problem with sending people to Mars is that it I fear it will be Apollo all over again. We'll send people there once and then it will be over. The cost for that one trip will be much to high to bear a redo. What is needed is a permanent human presence in space, and not just low Earth orbit. Sending humans to Mars does not accomplish this goal. IMHO, it is counterproductive to accomplishing this goal.

 

Finally, why spend an enormous amount of money to get out of one fairly deep gravity well only go back down into another? If the goal is to get to outer space, well then do that. Go to an asteroid. Mine asteroids. Make human-capable structure out of asteroids. Or go back to the Moon, permanently. Or build a space station orbiting about the Earth-Moon L1 or L2 points. All of these are considerably cheaper than a round-trip to Mars, have a better payback, are scalable, and go much further to creating that permanent human presence in space.

 

 

 

Here is a interesting take on interplanetary travel.

 

http://members.shaw.ca/bru_b/Liberty_ship_menupg.html

Nice science fiction story. So-so on the science, lousy on engineering and technology, and atrociously bad when it comes to politics.

 

This is the kind of venture that can only be funded by governments. Political support for nuclear technologies was close to non-existent prior to Fukushima. It is non-existent in this post-Fukushima world. It's going to be another decade, minimum, before we put Fukushima behind us. New space technologies take 15 to 20 years to go from the beyond the paper (TRL level 2-3) stage to reality (TRL level 9). Nuclear propulsion is something for those being born now to see to fruition.

 

Another political problem are the current economic woes. There are signs that our current economic woes are about to get worse. Greece may bring down Europe and China's economy is starting to falter. Add in the current weirdness of Republican party and the apparent spinelessness of the Democratic party and you get nada for NASA.

 

 

 

Okay, in honor of Wil Wheaton and in the name of Sybok, I hereby swallow my pride and renounce my uncivilized behavior on this thread. I'd like to thank you, swansont, for your fraternal correction, and I do so from the depths of my heart.

D H, I must commend you for suffering my foolish behavior and I resolve to treat you with the utmost respect from here on out. I'd also like to thank you for your insightful commentary on my own imbecilic posts; I very much appreciate your time.

 

Best wishes, and God bless.

Thank you. That took both a good deal of courage and and a good deal humility to say that.

[the asinine cretin]

Thank you. That took both a good deal of courage and and a good deal humility to say that.

http://www.youtube.com/watch?v=r4p8qxGbpOk

[the asinine cretin]

Damn. Too bad Zubrin didn't get to actually debate the Ad Astra people.

[dragonstar57]

what if the fuel was launched in small amounts and the ship/power source constructed either in orbit or on the lunar surface?

this way no shielding would required to protect against atmospheric distribution if the ship blows up in space no big deal, plenty of radiation in space already.

[Moontanman]

This looks interesting.

 

http://www.universetoday.com/95991/new-flying-tea-kettle-could-get-us-to-mars-in-weeks-not-months/#more-95991

[dragonstar57]

what if you take the drive and scale it down? try to create a small ion drive to maneuver satellites etc

such a derive would only have to generate enough to accelerate relatively small objects at creatively small rates for relatively small amounts of time.