Author Topic: NEP AG transit to Mars  (Read 56850 times)

Offline MikkelR

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RE: NEP AG transit to Mars
« Reply #20 on: 01/10/2006 01:46 pm »
I do not believe that a better alternative than the Mars Direct mission architecture will be possible to come up with. Spending 20-30 days on the martian surface is just not the way to go if you want maximum science output for the cost of the mission.

Offline Andy L

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RE: NEP AG transit to Mars
« Reply #21 on: 01/10/2006 02:19 pm »
Does NTP lose out when placed against NEP, as NEP can also provide electrical power to the ship's systems? Or have I got that wrong on the Electrical subsystem of the NEP?

Offline nacnud

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RE: NEP AG transit to Mars
« Reply #22 on: 01/10/2006 02:25 pm »
That depends on the NTR. There is a vairent called a Bimodal Nuclear Thermal Rocket that can provide electrical power to the spacecraft as well as thrust.

Offline Andy L

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RE: NEP AG transit to Mars
« Reply #23 on: 01/10/2006 03:17 pm »
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nacnud - 10/1/2006  9:25 AM

That depends on the NTR. There is a vairent called a Bimodal Nuclear Thermal Rocket that can provide electrical power to the spacecraft as well as thrust.

Thanks, still learning and working that one out with the "Bi" modal.

Offline nacnud

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RE: NEP AG transit to Mars
« Reply #24 on: 01/10/2006 03:24 pm »
Same here, I'm still only halfway through Role of NTR/BNTR/NEP thread :)

Offline Avron

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RE: NEP AG transit to Mars
« Reply #25 on: 01/12/2006 02:46 am »
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simonbp - 9/1/2006  8:50 PM

 This might freak out the greens as if the reactor is not on just the right course to skip out on a heliocentric trajectory, it could end up as a radioactive crater...

Simon ;)

I have two issue, that someone could provide some insights...

1) How could one safely launch this spaceship, without the huge issues that will be raised by the greens, and its associated political fallout for other nations?
2) What would you do with the ship when the vehicles design life is exceeded, how can it be disposed of? I don't think a dumping it in the Pacific would work, or "Parking" it on a planet or moon would be acceptable?

Offline Sergi Manstov

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RE: NEP AG transit to Mars
« Reply #26 on: 01/12/2006 04:17 am »
Has Russia or the former Soviet Union worked on such propulsion. Is there any knowledge that could be shared?

Offline nacnud

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RE: NEP AG transit to Mars
« Reply #27 on: 01/12/2006 10:04 am »

Energia has plans for a solar electric manned Mars mission.


Offline Avron

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RE: NEP AG transit to Mars
« Reply #28 on: 01/12/2006 06:42 pm »
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vanilla - 11/1/2006  11:51 PM

Quote
Avron - 11/1/2006  9:46 PM
1) How could one safely launch this spaceship, without the huge issues that will be raised by the greens, and its associated political fallout for other nations?
There's nothing to "freak" out about at the launch...the reactor is ice-cold, radioactively.  You don't want it to go critical and begin fissioning fuel if it falls in the drink, but that's not that hard to do, and particularly easy if you use a molten-fluoride reactor, as I mention on the other thread.

Radioactivity is proportional to how fast things are decaying.  Uranium doesn't decay fast (otherwise it would have all decayed away by now).  Fission products decay quickly and are very radioactive.  A reactor doesn't have fission products until you operate it.  You don't operate it until it's in space and in an orbit that will not reenter for thousands of years.

I fully understand the technical angle, but how does one cover the political angle?

Offline PlanetStorm

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RE: NEP AG transit to Mars
« Reply #29 on: 02/18/2006 05:25 pm »
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simonbp - 9/1/2006  7:50 PM

But the magic is in the thrust; whereas an NTP or chemical manned Mars mission would depart directly from LEO, a practical ion-engine-based mission (solar or nuclear) would have significantly lower thrust, and thus acceleration, and thus spend a significant amount of time climbing up through the Van Allen belts before reaching escape velocity. The spindly NEP spacecraft I've seen also have the distinct disadvantage of being both hard to construct in space (meaning beyond simple docking manoevers) and being to unwieldly for aerobraking (let alone aerocapture) meaning they need a lengthly decceleration burn in order to enter Mars orbit. All this adds up to NEP generally requiring more launches, longer flights, and thus more money.

Could you get around these problems by building a more powerful electric thruster? Yes, that's VASMIR. Would such a rocket plus the nuke to power it end up smaller and more powerful than just an NTR? I doubt it...


I think NEP has a lot of room for development, way beyond the low-thrust units available today. Have a look at this...

http:/www.scientificamerican.com/article.cfm?chanID=sa006&colID=1&articleID=0004E2EA-0B15-13CC-8B1583414B7F0101

At the moment, this is is just a way of generating intense electron beams for use in laboratories, but what is an intense electron beam to some is a potential propulsion mechanism to others. Perhaps the "spindly" NEP systems we envisage today are just a shadow of what is to come. The key is the nuclear power - with continuous power to spare from a low temperature reactor, there are many options for finding new ways to generate high speed beams, beyond just accelerating particles between two charged plates as in today's EP, or by running the reactor hot as in NTR.

By the way, can anyone help the web challenged amongst us (i.e., me!) who don't know how to paste a link and make it act like one?




Offline PlanetStorm

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RE: NEP AG transit to Mars
« Reply #30 on: 02/18/2006 05:44 pm »
Sorry, the link I just provided is to Scientific American, but you will need a subscription to get the full detail. The basic idea, however, is that you use an intense laser beam to ionize a cloud of helium gas. The rapid heating causes the electrons to seperate from the He nuclei. The electrical attraction between the seperated charges causes an oscillation, with the low-mass electrons moving at high speed relative to the much higher-mass He nuclei. Meanwhile, the laser wave front is continuing to propogate through the helium cloud, and if you get the energetics right, other electrons can be "picked up" and surf the very steep electric wave. The result is a bubble of electrons that follow the propogating laser wave front, and which over small distances (few cm have been demonstrated, but apparently scalable up to metres), the electrons are accelerated to potentially enormous energies.

I don't know if this exact mechanism could ever work as a practical propulsion system for a spacecraft, but it does have the benefit that you are using the lightest possible ejection mass (electrons) accelerated to the highest speeds, so on face value the rocket equation would suggest this is ideal. Either way, it shows that there are mechanisms to heat particles to very high energies without the necessity of very hot thermal reactors or vast (and therefore massive) particle accelerator.

Offline publiusr

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RE: NEP AG transit to Mars
« Reply #31 on: 03/23/2006 06:25 pm »
Those look to be very busy designs--esp. if you had to assemble them ISS style atop cramped EELVs.

Go with HLLV and NTR. It seems a lot more straight-forward.

Offline Avron

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RE: NEP AG transit to Mars
« Reply #32 on: 03/24/2006 02:56 am »
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vanilla - 22/3/2006  11:04 PM

Here on slide 10 of the presentation...they reject continuous thrust vectoring because of the issues of transferring megawatts of power across a rotating joint, and go with the "fire baton" configuration (with body-fixed thrusters), yet that design needs to continuously reorient its thrust vector (which is pointing along the angular momentum vector).  With the Canfield joint, you could keep the vehicle spinning roughly in the orbital plane and then just continually reorient the engines to point along the instantaneous thrust vector (which is relatively fixed in inertial space) while avoiding the issues of transferring power across slip rings.  A simple fat electrical cable will do.


Ok.. go with the  Canfield joint, and "A simple fat electrical cable" ( one that can do whole lot of flexing in temps of space travel - Ref MER.. cables are becomming a problem) ... what do you think is the best design and what stands out?

Offline dmc6960

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RE: NEP AG transit to Mars
« Reply #33 on: 03/24/2006 02:01 pm »
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Avron - 23/3/2006  9:56 PM

Ok.. go with the  Canfield joint, and "A simple fat electrical cable" ( one that can do whole lot of flexing in temps of space travel - Ref MER.. cables are becomming a problem) ... what do you think is the best design and what stands out?

If the ship is manned, couldn't a "simple fat electrical cable" be made to be replaceable by an EVA?  Just carry a couple spares around and if one starts giving problems, replace it.
-Jim

Offline meiza

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RE: NEP AG transit to Mars
« Reply #34 on: 03/24/2006 04:22 pm »
I've done some analysis on electric propulsion for various delta vees. Whether it be nuclear or solar, the ISP can always be optimized, given power density, initial acceleration and delta vee requirements. At non-optimal ISP, it always makes sense (gives lower total mass) to either lower isp and power source mass and add more fuel or up isp and power source and put less fuel. Or fuel=reaction mass, as would be the proper term, if you want to differentiate from the nuclear material. :)

If I didn't do any mistakes, the equation giving the relation is
deltav=2*k/(a_i*s^2*e^(1/s))
where s = v_ex/deltav, k is power density (W/kg), and a_i is initial acceleration.
Now we see that if you double the power density, you can double the initial acceleration and the other parameters stay the same. Thus a much shorter triptime is reached. (acceleration is always slow with electric propulsion.)

So, what is the power density of those proposed nuclear reactors? Near-term and molten fluoride ones? And what about solar? The Energia (I highly doubt there's some mistakes on the page) study said a 600 ton craft and 15 MW, that gives a rough estimate, if half of the mass is solar cells, 50 W/kg. Though I don't understand what they do if they only get 300N of thrust with that. (Exhaust velocity 100 km/s? Why?)

For example, with a power density of 100 W/kg, and 12km/s of delta v, initial acceleration 1 km/s for every 12 days, a bit over 50% of the craft can be other stuff than power source or reaction mass. Namely, payload. Exhaust velocity is about 40 km/s or the ISP is around 4000. Higher ISP gives worse results, since although the craft needs less fuel, the power source has to be bigger to get same acceleration. These equations don't take into account fuel use that makes the craft lighter later, since it is such a small part of total mass anyway, and this is a rough estimator.

Offline meiza

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RE: NEP AG transit to Mars
« Reply #35 on: 03/29/2006 05:07 pm »
Now we're left with the question, what would be the power density from space nuclear reactors or solar power on a Mars trip. I mean actual numbers. I've been trying to find them on the internet with no luck.

The pdf was vague, it only said 6 megawatts and end mass 89 tons (without fuel). So if 60 tons was reactor and associated equipment, (radiators mostly?) it'd be 100 W/kg. (60 W/kg for actual thrust since the engine is only 60% efficient.) Wouldn't solar cells be in the same ballpark? They are an existing technology and very reliable. Though developing lighter ones would cost some money, it could be useful for other purposes too.

What did they think as reaction mass? Krypton or Argon? Xenon is probably too scarce and expensive for such massive amounts (over 100 t).

Offline Carl G

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Re: NEP AG transit to Mars
« Reply #36 on: 11/07/2006 12:47 am »
Whoa, old thread, but this is a good one. This would be an amazing ship to build.

Offline SteveMick

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RE: NEP AG transit to Mars
« Reply #37 on: 11/21/2006 07:32 pm »
The numbers you used for solar cell power density are way off. Triple junction PV for use with concentrated sunlight made by the "Solarex" co. currently approach 1KW/kg. The concentrator can easily have a factor of 10KW/kg. and as a result, mass goes up very little as sunlight intensity drops as Mars is approached. Since this tech is developed and at least two orders of magnitude cheaper, I am puzzled as to why NEP would ever be considered for this role.
 Also, a solar electric rocket can operate as a solar thermal rocket to acheive Earth escape from LEO much faster. It really is the best of both worlds and has other advantages besides. The concentrator mirrors can double as communication and/or radar antennas and the intense heat at the concentrator's focus can be used for direct ISRU.
 Please use this as the "competition" for NEP and not a straw man system. I think you'll come to agree that NEP is impractical or at least inferior for Mars transit.
Steve

Offline meiza

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Re: NEP AG transit to Mars
« Reply #38 on: 11/21/2006 07:43 pm »
Hi, I've indeed been in the dark about solar cell mass efficiencies. Are these Solarex cells for space use? References?

Offline Marcus

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RE: NEP AG transit to Mars
« Reply #39 on: 11/21/2006 08:16 pm »
Quote
Avron - 11/1/2006  7:29 PM

Quote
simonbp - 9/1/2006  8:50 PM

 This might freak out the greens as if the reactor is not on just the right course to skip out on a heliocentric trajectory, it could end up as a radioactive crater...

Simon ;)

I have two issue, that someone could provide some insights...

1) How could one safely launch this spaceship, without the huge issues that will be raised by the greens, and its associated political fallout for other nations?
2) What would you do with the ship when the vehicles design life is exceeded, how can it be disposed of? I don't think a dumping it in the Pacific would work, or "Parking" it on a planet or moon would be acceptable?

1) Fuel launched separately from the reactor packaged in ceramic "indestructible" containment. It still won't satisfy the wackos, but as long as the people with the money are okay with it, we'll suffer the picket signs outside the gates to Canaveral.

2) Keep it away from Earth. The big problem isn't the radioactive fuel. That will probably be in a non-ablative form and difficult to vaporize. It would land in a hot heap somewhere and be easy to clean up. What you don't want to do is accidentally re-enter and vaporize a few cubic meters of radioactive metal structure that's been contaminated by exposure to the reactor flux during the mission. That could measurably raise the ambient level of radiation in the biosphere. It wouldn't kill everyone, but you could probably record a statistical spike in cancer deaths if you did something like that. Say a 0.000167% increase of incidence in the general population or... 10,000 people. Lets not kill 10,000 people.

Crash it into the moon, or at least the radioactive parts. Absolutely safest solution and the lack of an atmosphere makes it attractive in that it won't disperse the contaminated materials. No chance of any measurable amount of radioactive material ever making its way into any biosphere. If the orbits work out better for somewhere else, dump it on the cheapest celestial body with sufficient mass to make sure all the little bits and pieces that break off in the crash don't have escape velocity. Unless you seriously entertain the thought that we might someday have the capability to terraform Mars or Venus yet lack the capacity to deal with a little radioactive contamination.
OPS!
One Percent for SPACE!

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