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#1 2002-01-21 18:14:18

Josh33086
Banned
From: Norwalk, CT
Registered: 2002-01-21
Posts: 11

Re: Plasma Rockets - the best propulsion system yet...

I see many people discussing the fact that it takes so long to get to Mars. With the plasma rockets which nasa is developing the time it takes to reach mars could be cut down considerable.
                   

Plasma Rockets

High exhaust velocity can be achieved by the use of a plasma, where the atoms of the gas have been stripped of some of their electrons, making it a soup of charged particles. The temperature of a plasma starts at about 11,000° C. But present day laboratory plasmas can be a thousand times hotter. Particles in such plasmas move at velocities of 300,000 m/sec. These temperatures are comparable to those in the interior of our Sun. No known material could survive direct contact with such a plasma. Fortunately, a plasma responds well to the presence of electric and magnetic fields. A magnetic channel can be constructed to both heat and guide a plasma, without ever touching material walls. Magnetized plasmas are envisioned to, some day generate abundant energy on Earth, by controlled thermonuclear fusion. Their complex physics is the subject of intense study.

Dr. Franklin Chang-Diaz, NASA

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#2 2002-01-22 12:09:53

Josh33086
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From: Norwalk, CT
Registered: 2002-01-21
Posts: 11

Re: Plasma Rockets - the best propulsion system yet...

One thing I would like to add is the fact that NASA is not giving enough funding to this project. It is up to private groups to research and develop this engine. It has always been my belief that human spaceflight should be privatized.

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#3 2002-01-22 12:29:45

RobS
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From: South Bend, IN
Registered: 2002-01-15
Posts: 1,701
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Re: Plasma Rockets - the best propulsion system yet...

Can you give a web address where more information can be found? If not, don't worry about it. I am curious how the engine works and how much energy it uses. If it could get us to Mars in three months but it takes six or eight months of energy to power the engine, it doesn't actually speed up the trip. Currently nuclear reactors weigh something like 45 tonnes per thousand kilowatts of electrical output, so if you need 10,000 kilowatts, the reactor gets extremely large (450 tonnes); or you need about 100,000 square meters of solar panels at a minimum of 4 kg per square meter, which is 400 tonnes of solar panels. This may explain why NASA doesn't spend much money on the process, too; there's no reason developing an engine that cannot currently be powered.

                  -- RobS

P.S.: I should add, though, that the figure for the weight of a reactor comes from a 1977 publication about space colonies. Maybe we can do better now (by a factor of 2 or 3?). And solar panels are getting more efficient as well.

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#4 2002-01-22 12:46:38

SteveMick
Member
Registered: 2002-01-22
Posts: 3

Re: Plasma Rockets - the best propulsion system yet...

Although it is true that plasma rockets are highly efficient, untill recently with the development of the VASMIR engine by Chaing-Diaz et al, the thrust levels were too low to reduce the transit time to Mars vs. other propulsion technologies.   Now the problem is supplying enough electricity for the high energy exhaust since power demand is at least quadrupled for a doubling of exhaust velocity: KE=1/2mv^2. Care must be taken to avoid increasing the vehicle mass too much with power production equip. as this can negate the advantage gained by the higher exhaust velocity particularly on shorter missions. For this reason, nuclear fission reactors are often seen as essential, but this may no longer be true due to recent breakthroughs in solar cell tech. Solar cells of very high specific power operating at 400 suns and 40% efficiency allow the solar option to compete with or even exceed nuclear. Because the cells use concentrated sunlight they can use lightweight concentrator mirrors which can reasonably supply power out to Jupiter and even beyond at a small fraction of equivalent nuclear power's cost.
Actually, the cheapest alternative and one of the fastest to Mars is Solar Thermal which can use the concentrated sunlight to directly heat a propellent.
I quite agree that shortening trip time is possible and desirable.
Ad Astra
Steve Mickler
Atlanta, Ga.

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#5 2002-01-22 14:13:21

Josh33086
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From: Norwalk, CT
Registered: 2002-01-21
Posts: 11

Re: Plasma Rockets - the best propulsion system yet...

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#6 2002-01-22 21:26:02

Alexander Sheppard
Member
Registered: 2001-09-23
Posts: 178

Re: Plasma Rockets - the best propulsion system yet...

Sorry I am not familiar with the unit 'suns' that you mention in your post. What exactly is a 'sun'? Furthermore, I was not aware of any solar panels currently operating at 40% efficiency. Are there such panels? Even if there aren't, that isn't to say they can't be designed, but I was just curious.

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#7 2002-01-23 11:49:44

SteveMick
Member
Registered: 2002-01-22
Posts: 3

Re: Plasma Rockets - the best propulsion system yet...

Although a "sun" is usually the solar intensity at Earth's surface: in this context it would be the solar intensity at the average distance of Earth from the Sun or about 1.3KW/m^2.
The solar cell breakthrough I was referring to was from a NASA website, but unfortunately I misplaced the location info. I'll try to find it again, but as important as the efficiency is the idea of using concentrated sunlight as this can greatly increase the power per unit mass or specific power of a photovoltaic array. In addition, the mirrors required do not need extreme optical accuracy to acheive a few hundred "suns" and therefore can be extremely low mass much like solar sails.
The big drawback of low thrust systems in general is that they cannot take advantage of the benefit of thrusting to Mars while deep in Earth's gravity well and moving fast relative to Earth. A small thrust while going fast is much more advantageous than one farther away since a small addition to a big velocity will result in much more speed. To deal with this, the VASIMIR can operate at a lower exhaust velocity and thereby greatly increase its thrust while escaping Earth or Mars.
Even so the resulting thrust is too small to provide all the necessary velocity for a fast trip in the short time a space vehicle has as it whips around in low orbit since the thrusting necessarily raises the orbit. This can be mitigated somewhat by using a series of perigee thrusts to enter a highly elliptical orbit and increase speed at perigee while lowering the amount of thrust required to escape to Mars. On the last "burn" only a couple thousand mph is needed for a fast trip. For this reason a chemical rocket boost may be just the ticket. A solar thermal rocket would be able to acheive a slowish trip without any chemical help this way and its concentrator could be used for a solar cell array for a VASIMIR to add velocity while trans Mars. The electricity could support surface operations from orbit via microwaves to a surface rectenna. Propellent can be derived by boiling off volatiles of Phobos' regolith in a solar furnace.
The real advantage of solar thermal may lie in its ability to use almost anything as propellent including the several million pounds of aluminum alloy in discarded Comsats in or near Clarke orbit. Telerobotically harvesting and processing this space "junk" could jumpstart a first Mars mission, a
"Junkyard Mars" concept if you will.
Steve Mickler

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