What of VASIMR - Discuss this system

Chazbro38 · 2005-04-22 23:04:29

Variable Specific Impulse Magnetoplasma Rocket (VASIMR)

A 10-month one-way trek to Mars -- the standard assumed for today's chemical rockets -- would be reduced to just four months.

The lab director is NASA astronaut Franklin Chang-Diaz, a long-time plasma rocket believer who has been hard at work on the idea since 1979. He holds a doctorate in applied plasma physics and fusion technology from the Massachusetts Institute of Technology (MIT). Chang-Diaz terms VASIMR "a power-rich, fast-propulsion architecture"

The concept first jelled at Charles Stark Draper Laboratory in Cambridge, Massachusetts, then was picked up at the MIT Plasma Fusion Center before moving to JSC.

· A VASIMR rocket system consists of three major magnetic cells denoted as "forward," "central" and "aft." To get the rocket roaring, a neutral gas, typically hydrogen, is first injected at the forward-end cell of the motor and ionized. This electrically charged gas is then heated to create a desired density in the engine's central cell. The heating is done by the action of electromagnetic waves, similar to what happens in a microwave oven, after heating, the plasma -- essentially a superheated gas -- enters a two-stage hybrid nozzle at the aft-end cell. Here the plasma detaches from the magnetic field and is exhausted to provide "modulated" thrust. This VASIMR configuration guides and controls the plasma over a wide range of temperatures and densities.

GCNRevenger · 2005-04-23 09:34:48

Well, being that Dr. Diaz's VASIMR project office has faced termination repeatedly, it sounds like he is trying to sugar-coat it a little...

A Mars trip would take SIX months one way, not ten, with conventional rockets without requiring an absurd amount of propellant.

"a power-rich"

Um no. VASIMR doesn't make any power, it uses power. The catch is, its need ALOT of power to work efficently, which means building very large nuclear reactors, easily several times bigger then what you need to power a typical Mars expedition. And, since these reactors will be heavy, it might slow down VASIMR. In order for it to REALLY cook, you would need to reduce the reactor mass alot, which means inventing a new super reactor like a Vapor-Core Fission or a Fusion reactor. $$$

Also, since VASIMR requires either superconducting or heavy-duty electromagnets to operate, I bet these would be awfully heavy and hard to keep cold... maybe you could pump your Liquid Hydrogen coolant through the superconductor magnets.

SpaceNut · 2005-04-23 19:27:58

I would hope that we would do a little searching before starting new thread. This is what returned just for vasimr alone. can we just try to keep current ones going rather than starting new threads.

*******
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What of VASIMR
Discuss this system

Ion Engines

srmeaney · 2005-04-23 20:16:19

· A VASIMR rocket system consists of three major magnetic cells denoted as "forward," "central" and "aft." To get the rocket roaring, a neutral gas, typically hydrogen, is first injected at the forward-end cell of the motor and ionized. This electrically charged gas is then heated to create a desired density in the engine's central cell. The heating is done by the action of electromagnetic waves, similar to what happens in a microwave oven, after heating, the plasma -- essentially a superheated gas -- enters a two-stage hybrid nozzle at the aft-end cell. Here the plasma detaches from the magnetic field and is exhausted to provide "modulated" thrust. This VASIMR configuration guides and controls the plasma over a wide range of temperatures and densities.

I know they like to stick to a table of elements concept but have they considered replacing the proton mass with a neutron mass. I know the difference takes you from gas to solid but the energy outputs are greater.

GCNRevenger · 2005-04-23 20:53:22

Um, what? No it won't.

Ad Astra · 2005-04-23 21:17:36

We haven't heard much from the VASIMR people in a while. It's a great idea whose time has not yet come. It's worthy of funding, but I don't know if NASA can spare the money as VSE ramps up.

We should view VASIMR as a technology that will probably not reach maturity until after the first Mars landing. Nuclear-ion or nuclear-thermal rockets will fly before VASIMR, but they will open the door to public acceptance of nuclear space propulsion and power. This element, sadly, is a big obstacle between us and exploring the solar system.

Chazbro38 · 2005-04-23 22:42:01

Dr. Chang-Diaz has lead the development of a high-power, electrothermal plasma rocket — the variable-specific-impulse magnetoplasma rocket (VASIMR) — that is capable of exhaust modulation at constant power. An electrodeless design enables the rocket to operate at power densities much greater than those of more conventional magnetoplasma or ion engines. An aspect of the engine design that affords a capability to achieve both high and variable specific impulse (Isp) places the VASIMR far ahead of anything available today. Inasmuch as this rocket can utilize hydrogen as its propellant, it can be operated at relatively low cost. Because the VASIMR uses plasma to produce thrust, it is related to several previously developed thrusters; namely, the ion engine, the stationary plasma thruster (SPT) (also known as the Hall thruster), and the magnetoplasmadynamic (MPD) thruster [also known as the Lorentz-force accelerator (LFA)]. However, the VASIMR differs considerably from these other thrusters in that it lacks electrodes (a lack that enables the VASIMR to operate at much greater power densities) and has an inherent capability to achieve high and variable Isp. Both the ion engine and the SPT are electrostatic in nature and can only accelerate ions present in plasmas by means of either (1) externally applied electric fields (i.e., applied by an external grid as on an ion engine) or (2) axial charge nonuniformity as in the SPT. These ion-acceleration features, in turn, result in accelerated exhaust beams that must be neutralized by electron sources strategically located at the outlets before the exhaust streams leave the engines.
In the LFA, acceleration is not electrostatic but electromagnetic. A radial electric current flowing from a central cathode interacts with a self-generated azimuthal magnetic field to produce acceleration. Although LFAs can operate at power levels higher than those of either the ion engine or the SPT and do not require charge neutralization, their performances are still limited by the erosion of their electrodes.
An MPD plasma injector includes a cathode in contact with the plasma. Although the plasma at the location of contact is relatively cold, the cathode becomes eroded and the plasma becomes contaminated with cathode material (typically tungsten). The erosion and contamination can contribute to premature failure and to increased loss of energy through radiation from the contaminants in the plasma. An equal limitation on performance is exerted by nonionized propellant in a high-power amplifier cavity that is part of the MPD; the reason for this limitation is that neutral atoms and molecules in this region lead to charge-exchange losses, which, in turn reduce the overall efficiency of the engine and increase the unwanted heat load on the first wall (the liner) of the MPD thruster.
The design of the VASIMR avoids the aforementioned limiting features. The VASIMR contains three major magnetic cells — the forward, central, and aft cells. A plasma is injected into these cells, then heated, then expanded in a magnetic nozzle. (The magnetic configuration is of a type known as an asymmetric mirror.) The forward cell handles the main injection of propellant gas and an ionization system; the central cell serves as an amplifier to further heat the plasma to desired magnetic-nozzle-input conditions; and the aft cell acts as a hybrid two-stage magnetic nozzle that converts the thermal energy of the fluid into directed flow while protecting the nozzle walls and allowing efficient detachment of the plasma from the magnetic field. During operation of the VASIMR, a neutral gas (typically, hydrogen) is injected into the forward cell, where it is ionized. The resulting plasma is then heated in the central cell, to the desired temperature and density, by use of radio-frequency excitation and ion cyclotron resonance. Once heated, the plasma is magnetically and gas-dynamically exhausted by the aft cell to provide modulated thrust. Contamination is virtually eliminated and premature failures of components are unlikely.

The VASIMR offers numerous advantages over the prior art:
Its unique electrodeless design provides not only high thrust at maximum power but also highly efficient ion-cyclotron-resonance heating, and high efficiency of the VASIMR regarded as a helicon plasma source.
Because the VASIMR operates at relatively high voltage and low current, its mass is relatively low. This means that a one-ship human mission will not depend on a high-energy, complex rendezvous near Earth to achieve escape velocity. Instead, a rapid interplanetary transfer will be achieved with an adaptable exhaust, which will provide optimal acceleration throughout the mission.
The residual magnetic field of the engine and the hydrogen propellant will be effective as a shield against radiation.
Because of its continuous acceleration, the VASIMR will be able to produce a small amount of artificial gravitation, thereby reducing the physiological deconditioning produced by weightlessness.
The variability of thrust and Isp at constant power will afford a wide range of capabilities to abort.
Because hydrogen is the most abundant element in the universe, the supply of hydrogen could likely be regenerated in situ.

GCNRevenger · 2005-04-24 07:22:07

VASIMR does show alot of promise, just not right now. In the future, when we get good at making large (MWe) reactors and better superconductor magnets, then it would be a viable alternative to the fuel-hungry NERVA/Timberwind nuclear rockets. This is still some time down the road, easily 35 years.

RobertDyck · 2005-04-24 08:44:06

I know they like to stick to a table of elements concept but have they considered replacing the proton mass with a neutron mass. I know the difference takes you from gas to solid but the energy outputs are greater.

Protons can be stored as propellant because a single proton with an electron orbiting it is an atom of hydrogen. Rocket engineers are quite familiar with means to store hydrogen propellant. Free neutrons cannot be stored, they have a half-life of 10.3 minutes. That means half of the neutrons will spontaneously decompose within 10.3 minutes, half of the remainder will decompose in another 10.3 minutes, half of what's left will decompose in another 10.3 minutes, etc. A neutron decays into a proton, electron, and a type of neutrino called an electron antineutrino. Decay which releases an electron is called beta decay; high speed electrons are called beta radiation.

Neutrons are only stable within the nucleus of an atom. Science fiction may talk about some solid material called neutronium, but it doesn't exist. If you were able to make such a thing it would be too heavy to carry. Atoms of matter are mostly empty space. The nucleus is very small, the cloud of electrons is comparatively very large. Electrons have negative charge so it's the positive charge of protons that hold them to the nucleus. A solid material of just neutrons would not have any electrons, so the theoretical solid would consist of one giant nucleus. A neutron star consists of this matter, but a sphere of neutron star material just 48.5 metres diameter would mass as much as the Earth. Armour made of this material would be so heavy you couldn't move. Besides, without the intense gravity of a neutron star you can't store solid neutrons. Neutron stars are typically 1.4 times the mass of our Sun and about 20km diameter.

You could make free neutrons by some nuclear reaction that releases neutron radiation, but why? A tank of liquid hydrogen would weigh a lot less than a nuclear fission reactor necessary to generate enough neutrons for propellant.

srmeaney · 2005-04-24 09:40:01

I'm quite aware that neutrinos are critical in the fusion process by way of the manufacture of neutrons in the Solar fusion cycle. Mostly i was interested in system stability. Is the VASMIR stable under solar flare conditions (moreso than conventional rockets of the proton+oxygen variety)?

Basicly the process of Neutron mass manufacture is this: you build a superconducting shell, cool it to superconducting temperatures in a zero gravity condition, saturate the nuclear mass (say Oxygen) with the neurtinos which results in nuclear drift. The nuclear bonds separate and the protons with a positive charge have a desire to escape. the neutrons build in a gravitationaly attracted deposit and congeal in a solid neutron mass which must be shielded from em radiation or you get neutron radiation with fissionable products.

Fundamentaly I was considering the idea of a solid propelant of Neutrons. where "decay" on the "burning surface" could have been an electromagneticly induced process. The more EM Energy the greater the impulse.

RobertDyck · 2005-04-24 12:24:12

You're assuming neutrons will congeal and remain stable when shielded from ElectroMagnetic radiation. Neutron decay doesn't require EM, it occurs spontaneously with half-life of 10.3 minutes. In fact your statement that "neutrons will congeal" has no evidence. If it was that simple then experimental nuclear reactors would have seen it. Why hasn't this been observed before? Answer: it isn't that simple. Without the extreme gravity of a neutron star to squish neutrons together, neutrons require protons to form a stable nucleus, and protons require neutrons. A nucleus is nothing but a congealed mass of baryons, the positive static charge of protons pulls negatively charged particles (electrons) to them. Since electrons have the same charge they tend to push against each other, so they space them selves out around. Their fast motion usually results in the electron missing the baryon mass (nucleus) so they enter orbit. But electrons move close to the speed of light and have less than 4 dimensions of space-time, actually 3 and a bit. Those fractional dimensions make electrons act weird. All this makes an atom.

You're talking about creating an atomic nucleus with just neutrons, no protons. The nuclear reactions involved are just that, nuclear. Your statement "The nuclear bonds separate and the protons with a positive charge have a desire to escape." is over simplified. The strong and weak nuclear forces of protons and neutrons interact with each other; certain configurations are stable, others are not. Unstable configurations fall apart; sometimes by splitting (fission), sometimes by a neutron beta decaying into a proton, sometimes by a proton releasing a positron to become a neutron. The decay method depends on the isotope you start with.

You might be able to create extreme density matter using exotic forms with unusual quarks. Protons and neutrons are composed of 3 up and down quarks surrounded by a cloud of gluons, according to current physics. There are a total of 6 types of quarks: up, down, top, bottom, charm, and strange. Some physicists have speculated that extremely large subatomic particles can me made of more than 3 quarks if they include a strange quark, perhaps even thousands of quarks. Matter composed of these huge particles with strange quarks would be called strange matter. I have no idea what its properties would be.

Rather than speculating about really "out there" nuclear physics, why not work on something a little more near-term. You could improve the efficiency of a nuclear-VASIMR engine if you could transfer energy directly. Create a nuclear fission reactor that directly emits EM rather than generating heat. The conversion of heat to electricity, then electricity to EM, then EM to heat in the VASIMR engine is quite inefficient. Using nuclear heat directly isn't practical because the point of a VASIMR is to heat the exhaust so hot that it's super-heated plasma. That would melt the reactor and turn it to super-heated plasma. Let GCNR debate the virtues of a Gas Core Nuclear Rocket vs. VASIMR. What VASIMR needs is a reactor to directly emit EM from a nuclear reactor, either in the band VASIMR needs or convertible via some sort of fluorescence to the correct band.

GCNRevenger · 2005-04-24 12:48:23

There is no practical way to make macroscopic quantities of "neutronium" matter since the neutrons are not charged, they would just "boil" away through tunneling even if they didn't decay. You would need a horribly powerful gravity field to contain them, and if you have that, then why are you bothering to throw mass out the back of your rocket at all?

Unfortunatly there is no way to make a fission reactor radiate energy in only a particular frequency. In the fission reaction itself, the daughter particles will assume signifigant velocity, which will inevitibly become heat. This is not counting the EM emission during fission, which - though I am not certain - cannot be alterd by any known means.

Even if you could get a fission reaction to radiate vast quantities of microwaves, how would they get out of the fuel assembly? If they are only able to leave the fuel assembly near the surface, the the microwaves deep inside wouldn't be going anywhere... The fuel assembly would sinter from within and shatter probobly. "Whoops"

A thermal nuclear reactor isn't quite so bad for VASIMR as it sounds: a carefully built reactor could capture 33% of the heat as electricity, and converting that back into heat via microwaves is pretty efficent. The real trick is to get the reactor to generate as much heat as possible for as little mass as possible. The only way to do that is to raise the operating temperature as high as you can, which will also serve to reduce your radiator mass per-watt.

Unfortunatly, are are pretty close to an upper theoretical limit for a solid-core fission reactor and turbine combination. The secret is to not use a solid core reactor at all, but to build one where the nuclear fuel will intentionally melt and even vaporize, which is a so-called "Vapor Core" reactor. You can't use regular turbines for primary power conversion since the gasseous fuel is so hot, but you could use a MHD converter, which produces electricity.

As an added bennefit, the MHD produces electricity that the VASIMR engine could use directly, no transformers or conditioners, which will save signifigantly on mass and wasted energy... Oddly enough, the neutron flux from the fuel vapor also will cause the gas to become a plasma at lower temperature then normal, increasing the efficency even more as you can harness the energy of "extra" neutrons to an extent

RobertDyck · 2005-04-24 14:26:39

Paper thin foil of fissionable material will let EM get out without thermalization. The metal of the fissionable material itself can be shaped as a wave guide to direct EM out, while facing foils can absorbe neutron radiation from each other. Highly enriched uranium with >10% U235 can operate without a moderator. Nuclear thermal rockets are usually designed with >98% U235 since U238 is just dead weight. However, most of the energy of uranium fission is released has kenetic energy of high speed neutrons. A layer of moderator over the uranium foil could slow (moderate) the neutrons in a controlled step-wise fashion, releasing EM with each step. Engineer the steps to release EM in the desired band. You can't run coolant through the wave guides, the EM band is designed to be absorbed by hydrogen in the VASIMR, so back the uranium foil with a neutron reflector, then a coolant channel. It'll generate less heat because much of the energy will be emitted directly as EM, but there will be heat. Generate electricity from that heat.

GCNRevenger · 2005-04-24 21:12:24

The volume and how delicate such a reactor would be would be serious trouble to such a reactor, but the bigger problem is this: You can't make the reactor radiate in only the microwave spectrum. If you are using a moderator to slow down fast or epithermal neutrons down so that their interactions will emiss in the microwave region, then they will inevitibly generate a VAST quantity of heat in the process, and you will lose most of your energy.

RobertDyck · 2005-04-24 23:43:17

Yes and no. Uranium fission will release 2 or 3 neutrons with about 1 MeV energy. Those neutrons need to be slowed to about 0.02 eV for an non-enriched uranium reactor. Faster neutrons can be absorbed but the chance of each collision resulting in absorption is low. Graphite is often used as a moderator because it absorbs very few neutrons. Inelastic collision of a neutron with a graphite atom will slow the neutron, and the graphite atom releases the energy as gamma radiation. Heavy water can also be used as a moderator. Each deuterium atom absorbs a little of the neutron's energy; it takes several collisions to slow the neutron enough. The energy from each collision is released as EM. That can be engineered by selecting a moderator that absorbs just the right amount of energy via inelastic collision to release a microwave photon. The result is close to 100% of the energy of high speed neutrons converted to microwaves.

However, the total energy of fission is much higher. Uranium doesn't always split neatly; most reactions are:
U235 + n -> Ba144 + Kr90 + 2 n + energy (how much?)
U235 + n -> Ba141 + Kr92 + 3 n + 170 MeV
U235 + n -> Zr94 + La139 + 3 n + 197 MeV
Notice the total energy is close to 200 MeV, with 170 MeV common. Each of the 3 neutrons will carry 1 MeV so that leaves 167 MeV carried by the fission products. That's 1.647% of the energy of fission directly converted to microwaves. But what is the efficiency of thermal to electric conversion? I believe it's only 2%.

Fragile? Compose fuel elements with solid heat sink material connecting connected to a coolant pipe, with the heat sink fin either a neutron reflector or coated with one. Then apply uranium sheet, it doesn't have to be too thin because high speed neutrons will pass through it, not microwaves. Then apply a neutron moderator that won't absorb microwaves. Shape the space between fins to be a microwave wave guide. You could make the moderator thicker by using a microwave phaseonium material. Phaseonium uses a trick of quantum mechanics that makes the emitter transparent to the same EM frequency it emits. That means the emitter doesn't reabsorb, making it much more efficient. Such a material would permit the moderator to be relatively thick.

Ack! I tried to look up operating frequencies for VASIMR to compare with neutron energies. Moderator material can be hydrogen, deuterium, beryllium, or carbon; the closer atomic mass is to neutron mass, the more energy it absorbs from the neutron. It's like a billiard ball hitting another billiard ball vs. a billiard ball hitting a bowling ball. However, there's a ton of technical documents to go through on VASIMR. I don't see a simple answer for microwave frequency. There is work to be done, but it can be done.

GCNRevenger · 2005-04-25 06:16:37

"But what is the efficiency of thermal to electric conversion? I believe it's only 2%."

Ummm what? Regular nuclear reactors with Brayton turbines can hit ~33% efficency, and an MHD generator today is around 20-25% with signifigant possible improvement. The reactor concieved for JIMO would have produced 300kW of thermal energy, but 100kW of electricity.

"The energy from each collision is released as EM."

Are you referring to the Cherenkov Effect that makes the neat glow in reactor pools? That doesn't have much to do with collission, only the reduction in the speed of light in the liquid inducing a little energy loss as blue light. There wouldn't be enough conversion to fool with a Cherenkov-based system even if you could come up with an extremely "slow" moderator. If "normal" moderators emiss as gamma, that is a HUGE difference between optical/near-IR, so I don't think it is the same effect.

I don't think that any moderator material/arrangement exsists to accomplish such a huge energy change for fast neutrons all the way down to epithermal with just the right degree of energy transfer, you will inevitibly lose too much energy as heat... so you might as well just optimize it for heat and convert it that way.

SpaceNut · 2020-11-08 22:13:48

topic fixed

Mars_B4_Moon · 2021-07-27 04:48:50

Less than 40 days to Mars? there may be soon extensive testing of the VASIMR engine from NASA and possibly other nations. After a career at NASA, Chang Díaz set up the Ad Astra Rocket Company, it became dedicated to the development of advanced plasma rocket propulsion technology. the company has produced the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), an electrical propulsion device for use in space. Ad Astra has an aim to achieve the NASA-set goal of firing VASIMR continuously for 100 hours at 100 kW ...almost there? Russia and Europe and China and other groups have also discussed nuke power in space, Russia may have a nuclear space-tug on the drawing board, the Ad Astra Rocket Company is aiming to offer the technology for space tug missions to help "clean up the ever-growing problem of space trash."

Rocket Completes Record 88-Hour High Power Endurance Test
http://spaceref.com/news/viewpr.html?pid=57827

The rocket engine that could transform space travel
https://www.politico.com/newsletters/po … vel-493602

GW Johnson · 2021-07-27 19:41:59

I'm no expert on this electric propulsion stuff, but I do know that VASIMR is another way to generate a high-speed stream of ions for thrust. All these electric propulsion schemes produce milliNewtons of thrust from devices that mass up to around a ton. And that does NOT include the power supply, or the vehicle to be pushed. They are microthrust/microacceleration devices.

Some of the more eye-catching claims, like 30-40 days to Mars, are based on the mass of the power supply, and the vehicle, being zero. They cannot be. These are based on oversimplified calculations that do not account for the longer and longer path lengths you have to travel as your vehicle acceleration levels trend toward microgees.

It'll take a rather sophisticated power supply to make manned travel with these devices practical. Long trip times are OK for unmanned cargo, but not anything crewed, and for a whole variety of reasons. Such power supplies are very likely to be nuclear heat engines, NOT nuclear thermionic, in order to achieve high energy conversion efficiencies in the 20-40% range. If memory serves, the Kilopower units are demonstrating 25% at design conditions, even in their small scale, which usually drives conversion efficiencies down.

Any sort of crewed vehicle needs that high conversion efficiency at the very high power/weight ratios achievable by nuclear heat engines. They also need thrusted accelerations nearer a milligee than a microgee. THAT means you put a really big power supply and electric thruster device onto a relatively small vehicle. There is no way around that. Not until the basic electric technology finds a way to get multiple Newtons out of that ton of device, instead of milliNewtons.

Chang-Diaz at Ad Astra has done extraordinarily good work, but it's still milliNewtons per ton, and many, many KW needed to get it.

GW

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