The "Real Aerospace" Thread

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Mr.Tucker
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Re: The "Real Aerospace" Thread

Post by Mr.Tucker »

I see. I guess superconducting cables of such lengh are still some way off. Its surprising though given that the entire ISS produces around 144 kw. You still need some kind of space reactor to operate even the lower powered VASIMR. Its' sad knowing that the Phoebus II NTR produced during the 60s produced 4000MW (yes, most of it was thermal, but that doesn't mean it can't be at least partially used).

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icekatze
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Re: The "Real Aerospace" Thread

Post by icekatze »

hi hi

I hate to be a spoil sport here, but VASIMR engines are magnetoplasmadynamic thrusters. Variable Specific Impulse Magnetoplasma Rocket.

There are lots of competing designs being built by various companies, and they all suffer from the power input problem. It is my understanding that one of the main benefits of the VASIMR design over many of the competing designs is that it doesn't use electrodes to generate the plasma, which allows them to magnetically shield the whole engine chamber, which prevents corrosion over time.

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Re: The "Real Aerospace" Thread

Post by Mr.Tucker »

You're not being a spoil-sport, you're being helpful :) . So they operate on similar principles, but with a different way to create the plasma (electrodes vs microwaves). Sounds about right. I guess VASIMR would work as an interim solution until material sciences can get better at building cathodes that don't evaporate. Though I still question the idea of buiding a drive you can't power. Then again, I question a lot of things NASA does these days.

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Re: The "Real Aerospace" Thread

Post by icekatze »

hi hi

I guess you have to look at "high thrust," in a relative manner. Compared to the ion engine on the Dawn spacecraft, which gets 90 milliNewtons of thrust, the Princeton MPDT got a whopping 12.5 Newtons of thrust at 200kW. That is even high when compared to the VASIMR's 5 Newtons at 200kW in high gear, but low compared to the VASIMR's 1000+ Newtons in low gear. When you compare it to the space shuttle solid rocket booster's 12,500 kiloNewtons of thrust, none of them seem very high thrust.

There is sort of an inherent tradeoff between thrust and exhaust velocity, and getting both is really hard to do.

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Re: The "Real Aerospace" Thread

Post by Mr.Tucker »

Yeah, I agree. I was refering to the ''High thrust'' relative to other ion/drives. I still do consider the Princeton MPD as superior to the VASIMR, since it can be used in a more flexible manner while being more economical. But yes, compared to other potential tech, like nuclear thermal or fusion, they are low thrust/high Isp with high mass power requirements. The only drive which could do both ''real'' high thrust and high Isp is the nuclear salt water....which is so problematic on so many fronts that it doen't really even need to be taken into consideration.

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icekatze
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Re: The "Real Aerospace" Thread

Post by icekatze »

hi hi

I'm not sure what you mean by "more flexible." Flexibility in exhaust velocity/thrust ratios is one of the VASIMR's big selling points. I'm also not sure what you mean by more economical. The MPDT runs on Lithium, which costs $25 per 100 grams, while the VASIMR runs on argon, which costs $0.5 per 100 grams.

The Princeton 200 kW MPDT demonstrated an Isp of 4000 seconds, or about 39,226 meters per second exhaust velocity. The 200 kW VASIMR demonstrated an Isp of 4900 +/- 300 seconds, or about 48,052 meters per second exhaust velocity.

As far as I can tell, they are both very comparable to each other, with the VASIMR slightly edging out the MPDT in a few different ways.

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Cy83r
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Re: The "Real Aerospace" Thread

Post by Cy83r »

Have we covered fission fragment drives yet? Because Isps of 100,000 to 1,000,000 are amazing on this side of antimatter- Plus! the reaction mass is also the powerplant when you decelerate the ionized medium. Refueling would be an odd affair if one didn't design a revolver sort of system internally, which adds more weight to an already heavy engine.

P.S. Oh! I also realized that both the disc and vaporization chamber designs both scale up fairly well, only really being concerned with surface area which can be designed to work within a linear consumption plan, where fusion chambers seem like creating larger magnetic bottles would suffer from energy input and surface area limitations at higher volumes.

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Re: The "Real Aerospace" Thread

Post by Mr.Tucker »

What I mean is that you're able to use one engine for ''relatively'' high thrust AND also have high Isp (VASIMR has higher thrust in low gear but much lower Isp). Best of both worlds. And with an efficiency of around 80% compared to VASIMR's roughly 60%. Also worth pointing out that MPDs can work with both hydrogen and helium. But the point is it's not being investigated, and I can't grasp why.

Fission fragment, ah yes. That design is an atomic engineer's wet dream. The problem is...well actually there are more. While extremely promising, they are basically impossible to test anywhere inside the Earth's magnetosphere, let alone atmosphere. And while the scientific principles are well understood, actually building that thing is incredibly difficult because you have no room for error. Like all NTRs the temperatures and pressures can be catastrophic if control is lost. While the concept is actually quite doable in my opinion, it's probably more difficult than it would first seem. Call it an engineer's hunch :D . Still, an avenue worth pursuing. I'd say it's the final and best possible fission design achievable in a long roadmap of tech that starts with classic NTRs, carries on to more advanced systems like liquid-core, vapor-core, nuclear electric, followed by the safest and most reachable gas core yet designed (closed-cycle, aka ''the nuclear lightulb''), before finally ending with fission fragment. It's the final branch in this tech tree, but we need to make some more steps and gain some more experience till then just to be safe. And with the recent discovery that decay rates may not be as constant as we once though, we still need a heck of a lot more research before we get there .

EDIT: Also worth pointing out that uranium ain't cheap, or plentiful in the solar system (unlike lithium, helium or other light elements).

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Re: The "Real Aerospace" Thread

Post by icekatze »

hi hi

The thrust difference between the VASIMR and the MPD is really negligible. The MPDT is not the best of both worlds, it is one of the worlds by itself, a low thrust plasma rocket with a high specific impulse. 12.5 Newtons at 4000 Isp is not appreciably different than 5 Newtons at 4900 Isp. Especially when you consider the mass to thrust ratio of the entire craft.

If the VASIMR geared down to produce 12.5 Newtons of thrust, it would probably weigh in around 4000 Isp too, and if the Lorentz-force Accelerator style MPDT readjusted their nozzle to get 4900 Isp, they'd probably drop in thrust to around 5 Newtons. In a medium gear, the VASIMR is going to have around 100 Newtons of thrust and would still be in the mid to upper 3k Isp range. (The relationship between the two for plasma rockets is inversely proportional, but since the tradeoff is not a linear one, I can only make ballpark calculations.)

Efficiency of any magnetoplasma rocket is a function of the power output. Both the VASIMR and others can exceed 70% above 200 kW, while at 100 kW they only get around 40 - 45% efficiency.

But honestly, unless someone is working in a near Earth capacity, high Isp is probably their biggest concern. Gearing down for higher thrust is helpful if someone was trying to go to the moon, cause they'll be there before they finish burning all 500 hours of their total fuel capacity. :P

Again, the VASIMR is a type of Magnetoplasma rocket, and is largely equivalent to others in that category, like the Li-LFA from Princeton or the one the Russians are working on. They could all potentially run on Argon, Xenon, Lithium (if they can solve the deposition problem), hydrogen and helium. Lithium has proven to be more effective in a lot of different applications. The lower the atomic mass of the element, the lower its efficiency in a plasma rocket.

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Re: The "Real Aerospace" Thread

Post by Keter »

A shameless snark would say VASIMR is best of both worlds too. All the promise of theory and just enough reality to keep the $$ and enthusiasts going, its immaculate theoretical performance forever spared the ignominy of practical hazards.

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Re: The "Real Aerospace" Thread

Post by icekatze »

hi hi

A shameless snark might say those things, but it would be really odd, since the VASIMR has conducted thousands of successful, stable test firings by 2013. I'm not sure such a statement would match up well with observed reality.

The Li-LFA style MPDT from Princeton is also a real device that has been tested and observed, and I suspect that if they continue work on that design, they'll probably be able to get an overall higher exhaust velocity than the VASIMR while operating at peak performance.

Whatever problems exist for radiating waste heat and generating electricity, they are problems shared by all magnetoplasma rockets.

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Re: The "Real Aerospace" Thread

Post by Mr.Tucker »

icekatze wrote:Whatever problems exist for radiating waste heat and generating electricity, they are problems shared by all magnetoplasma rockets.
Agreed. What's worrying is that the solutions are evident and well-known, yet no one seems to be pursuing them.
Keter wrote:A shameless snark would say VASIMR is best of both worlds too. All the promise of theory and just enough reality to keep the $$ and enthusiasts going, its immaculate theoretical performance forever spared the ignominy of practical hazards.
Every drive has a potential niche. What space systems would look like in case of near-term human expansion and space exploration is a menagerie of electrical and other propulsion concepts : simple stuff like arcjets for stationkeeping, multi-use systems like VASIMR or NTR for orbital tugs, high Isp for long voyages,etc. My expectation would be that cargo craft would use different types of sail designs (being propelantless is a big advantage), and that very long range mission probes (like, say, to the solar focal point) would use a combination of such drives (launch with whatever, accelerate with electric, slow down with magsail. Just an example). Fusion is still some time away in my opinion (could be wrong).

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Re: The "Real Aerospace" Thread

Post by Keter »

I stopped keeping up when I read... I think a year or three ago that they were still stuck on that plasma detachment issue. So I guess that's not a problem anymore?

Whatever the case, they need to put it up in space for TRL 7.
edit-
http://sen.com/blogs/irene-klotz/nasa-n ... ce-station ...

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Mr.Tucker
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Re: The "Real Aerospace" Thread

Post by Mr.Tucker »

Keter wrote:I stopped keeping up when I read... I think a year or three ago that they were still stuck on that plasma detachment issue. So I guess that's not a problem anymore?

Whatever the case, they need to put it up in space for TRL 7.
edit-
http://sen.com/blogs/irene-klotz/nasa-n ... ce-station ...
This forum discussion may be relevant: http://forum.nasaspaceflight.com/index. ... =34788.135. Seems they're orbiting it exactly because they're not sure if it's still a problem. If it works, I just hope they do something with it :( .

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Re: The "Real Aerospace" Thread

Post by Keter »

Yes and as you can see at the (current) end of that thread you have that same link brought up. Maybe they can get a ride to some other demo platform...

I now also remember that a number of people from the industry thought the whole project was dubious (for reasons I can't recall) and that it was basically Diaz exploiting his position/reputation. In itself it's not basis for anything, but it struck me because the people saying so were not the kind to say something like that just for the sake of voicing disapproval. I can't recall if it was on NSF or at one of the NASA facilities from one of the lab researchers.

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Re: The "Real Aerospace" Thread

Post by Mr.Tucker »

I believe Bob Zubrin became an adamant opponent of VASIMR, and pointed out some inconsistencies in the design, which has spurned others to look into them and point out issues. I think he doesn't have anything against the drive itself, but against the idea that it's necessary for Mars. This would be a relevant debate by him (in his unique style :) ): https://www.youtube.com/watch?v=myYs4DCCZts

El-Hazard
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Re: The "Real Aerospace" Thread

Post by El-Hazard »

After reading various discussion on Outsider posts perhaps someone can cure my curiosity concerning gravity. How in so many sci-fi flicks and books
does rotating a cylinder in space create gravity?

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Re: The "Real Aerospace" Thread

Post by icekatze »

hi hi

Technically speaking, rotating a cylinder does not create gravity. It does, however, create "Centrifugal Force", which is a close enough approximation for people. It is only a simulation of gravity though, and not quite the same. If you knew where to look on a rotating space habitat, you would probably be able to see some differences. (When dropping an object, it wont fall straight down, for example.)

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Re: The "Real Aerospace" Thread

Post by Razor One »

El-Hazard wrote:After reading various discussion on Outsider posts perhaps someone can cure my curiosity concerning gravity. How in so many sci-fi flicks and books
does rotating a cylinder in space create gravity?
You can actually test for this if you have enough space to swing a bucket of water.

Take a bucket, fill it about 1/4 of the way with water.
Hold the bucket with a firm grip. One hand ought to be fine, but two hands for safety.
Now start spinning.
You'll feel a force trying to pull the bucket away from you. It will rise up to the horizontal and the water won't spill out. If you watch the water, it will be flat to the bottom of the bucket.
If you're feeling particularly brave and don't want feel dizzy, this works just as well vertically as it does horizontally.
Swing your arm in a circle vertically with the bucket at the end. As long as you're going at a decent clip, the centrifugal force that Icekatze mentioned will keep the water flush with the bottom of the bucket. Go too slowly, and you'll get a nice cheap shower for your efforts. If you swing too swiftly, the strength of the centrifugal force may actually outpace your grip and the bucket will go flying out of your hand and give someone else a nice cheap shower for your efforts.

You can also enjoy this phenomenon at carnivals or fairs. When I was young, I used to love the ride called the Round Up, which effectively demonstrates this force in action. Once it got up to speed, it honestly felt like lying flat on the ground. The only way you could tell you weren't was the rushing wind, the spinning background, and that the Earth's gravity hadn't entirely disappeared. At the top of the arc you felt lighter (Centrifugal force - gravity), at the bottom of the arc you felt heavier (Centrifugal force + gravity). Sadly they retired it while I was still young and I haven't been to many carnivals or fairs since, but if I come across one anytime soon I'd happily ride it.

You can also see its effects on carousels. Like so:



:lol:
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Mr.Tucker
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Re: The "Real Aerospace" Thread

Post by Mr.Tucker »

An interesting presentation by Dr. Landis. Feel free to watch the other presentations in the Starship Century symposium: https://www.youtube.com/watch?v=ogKKjpQvfuM

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