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Post Info TOPIC: "Advanced Plasma Rocket Tested in Costa Rica"
10kBq Jaro


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"Advanced Plasma Rocket Tested in Costa Rica"
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http://www.aviationweek.com/publication/awst/loggedin/AvnowStoryDisplay.do?fromChannel=awst&pubKey=awst&issueDate=2007-02-19&section=In+Orbit

Advanced Plasma Rocket Tested in Costa Rica

Aviation Week & Space Technology, 02/19/2007, page 17

Edited by Frank Morring, Jr.

Ad Astra Rocket Co. expects to begin operating a 200-kw. "flight-like" engine prototype in ground test by the end of the year. Ad Astra is a Houston-based company that grew out of research into Variable Specific Impulse Magnetoplasma Rocket (Vasimr) technology conducted at Johnson Space Center by seven-time shuttle astronaut Franklin Chang-Diaz. The company has opened a facility in Costa Rica, where Chang-Diaz was born, for life-cycle testing that started at lower power levels in December 2006. Next up is a test series with a 100-kw. unit already in early checkout. Assembly of two flight variants of the engine is set for early next year, with in-space testing targeted for 2011. The company, which has an exclusive license to the original Vasimr patents under a privatization agreement with NASA (AW&ST Jan. 30, 2006, p. 12), has added new intellectual property in the past year. The Vasimr engine uses radio waves to heat propellant gas to extremely high temperatures, producing exhaust velocities in the 40-50-km./sec. range.



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John


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That is good news!  Assuming the upper exhust velocity that would give us 25 KW per newton of thrust.  This is a very good start.  And they are doing this on a shoestring budget. 



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GoogleNaut


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This is indeed good news. One of the really interesting features of the VASIMR engine is it's ability to use 'constant power throttling.' Constant power throttling is where the thrust varies with the exhaust velocity: this gives the VASIMR engine the ability to use high Isp (30,000 sec of specific impulse) low thrust (1-2 Newtons); or lower Isp (3000 seconds of specific impulse) and higher thrust (10-12 Newtons or more.)

The formula which allows relates power, thrust, and exhaust velocity is pretty easy to derive:

If K=1/2*m*v^2 (1) is the classic Newtonian kinetic energy formula, which relates the mass of an object m in kilograms, its speed v in meters per second, to it's kinetic energy in Joules. If we use a little 'calculus magic' and differentiate this equation with respect to time, holding v constant, we get:

P = 1/2*mdot*v^2 (2) where P=the kinetic power expended in Watts, to fling a stream of mass mdot in Kg/s with speed v in meters per second. This equation if applied to rockets or similar reaction devices would imply that the mechanical power of the exhaust jet would be:

Pjet=1/2*mdot*ve^2 (3) where Pjet=the actual mechanical power of the jet of gas; mdot is the propellant flow rate; ve is the exhaust velocity.

Similarly from first principles we know that Newton's Second Law of Motion states that:

F=m*a or that simply a force F in newtons, if applied to a mass m in kilograms, will impart an acceleration of a meters per second each second the force is applied.

If we apply this to rockets we will discover that if a stream of mass mdot, is ejected at constant speed ve, then the force or thrust generated will be:

F=mdot*ve

Rearranging this in a more useful form gives us:

mdot=F/ve, and then substituting this into eqn 3 gives:

Pjet = 1/2*F*ve (4) which is the constant power relation.

If a VASIMR is rated at 200KW, it is assumed that this is delivered rf power to the engine. If this is the case, and the engine is overall 85-90% efficient at converting rf power to kinetic energy of the plasma jet, then we may be able to expect that Pjet=170KW. If the specific impulse at the highest range is 30,000 seconds, then multiplying this by 9.80665 m/s^2 gives us our exhaust velocity: ve = 294,200 m/s

Substiting these values into eqn 4 and solving for the thrust gives: 0.578 Newtons. If on the other hand, we switch to a 'high gear' and lower the Isp to 3000 seconds, we can expect to get: F=5.78 Newtons of thrust. It doesn't sound particularly exciting, but keeping in mind that the purpose of the flight prototype is to determine the exact scaling laws. If the VASIMR can be scaled up to 100 MW levels, then a high efficiency engine capable of hundreds of Newtons of thrust or even thousands of newtons of thrust is possible.

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martidj


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Hi all.

Have been lurking on this forum for a while but haven't signed up until now. As a mechanical engineer myself (but not aerospace) I've had lots of fun investigating all the different advanced propulsion concepts like Orion, GCNR etc. With my design engineer hat on VASIMR does sound like it meets the two most important criteria for an new advanced space drive system to succeed in the current environment i.e.

1. It can be safely tested on earth at modest cost.
2. A small scale low power prototype can be built for flight testing.

It is good news they have completed stage 1 and have sights on stage 2 in the near term. Seeing is definitely believing.

The other side of the coin is the power source to feed the drive system with the necessary Megawatts that would be needed to power a full size system. A promising solution ran for 20 years in Germany - a pebble bed reactor, which uses spherical 'pebbles' containing nuclear fuel with helium coolant gas. The gas is used to drive a closed cycle turbogenerator set directly (another mature technology), and if it overheats the graphite outer casing of the pebbles expands and quenches the chain reaction. This type of reactor isn't great for earth use due to the inflammability of graphite in air but spacecraft have no such problem.

Personally I think that the talk of direct thrust nuclear rockets is hype, whether it be Gas core - operating temp 60,000 deg C. Or perhaps Solid core - thoroughly tested under NERVA, but the reports from the time show that the system had a nasty habit of spewing out bits of reactor under full load. Plus the performance advantage wasn't great - 900 ISP over 450.

I like Nuclear electric drive systems. They mirror a current technology used on railways - the diesel electric engine. The diesel doesn't drive the wheels directly, so avoids the massive gearing and clutches that would be needed. Instead it drives a generator which supplies power to the traction motors that actually turn the wheels.

And to those worried about funding or the lack of it, think of this. From an historical perspective most of the key technologies we use now were developed by small groups of private individuals and private risk capital, not governments. The steam turbine, internal combustion engine, electric motor, telephone, television, aircraft. Indeed governments can stifle innovation, even if that is not the intent.








-- Edited by martidj at 22:20, 2007-03-03

-- Edited by martidj at 22:23, 2007-03-03

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John


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Welcome to the board.  I think you are basically right about using some sort of advanced hight tempature reactor as a power source (on the full-scale version).  One draw back to these nuclear electric systems is getting rid of the waste heat.  The only way is by radiation, i.e. Stephan-Boltzman law in which radiated energy is proportional to the fourth power of the absolute temperature.  I the bottom of the cycle isn't very hot the radiator becomes too big and you need to have a large tempature difference between then the high and low to have an efficient process.  I think the VASIMR does seem promising.

As for the solid core nuclear I think that is workable but like you say I don't see the gain as being worth the political problems of going nuclear.  In practice since these systems would be used in space a little radiation leakage would really be a problem up there.  But the NERVA type engines are heavy and with only double the Isp. It seems to me that waiting for the VASIMR technology before pushing for nuclear engergy makes sense.  Other here disagee though.

-- Edited by John at 02:18, 2007-03-04

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martidj


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The waste heat problem I think is common to many reactor designs with the turbine system being essentially Carnot cycle plant it typically has to waste over half its heat. Modern turbogenerators used for power generation have heat sink temperatures of <320K to maximise efficiency. Hardly practical for radiative heat ! Since helium is being used as the working fluid rather than steam, gas turbine temperatures can be achieved. Taking figures for existing combined cycle gas plant for the turbine only :


Combustion temperature : 1600K
Exhaust temperature : 900K


Ideal Carnot cycle efficiency = 1-(900/1600) = 43.8%. Not great - but then efficiency isn't the prime objective.  Using the Stephan-Boltzmann black body formula, for a power dissipation of 200 megawatts at 900K :

Area = Power/(SB constant*temp^4)
= 2e+8/(5.67e-8*900^4)
= 5376 square metres

A big surface area yes, but not outrageous. A 73 metre square. Further reductions can be achieved by boosting the exhaust temperature, with a loss of efficiency.


My feeling is that a practical system would be very heavily based on existing commercial gas turbines and turboalternators, which have a superb reliability record in service. The Rolls Royce RB211 in static power generation mode has a shaft power output of 30 megawatts plus, half a dozen of these units would be enough. (Although as a nod to the US the GE CFM56 engine has much the same capability).



-- Edited by martidj at 17:35, 2007-03-04

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GoogleNaut


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Welcome aboard!

I too am very interested in nuclear electric propulsion systems--and I have an eye on somekind of closed Brayton Cycle gas-turbine system, possibly using a pebble bed reactor as the power source. Helium, nitrogen and CO2 have all been proposed as coolants, although helium has superior heat transfer and lower aerodynamic resistance when flowing through the turbine and compressor spools. Using nitrogen results in compressors and turbines that are more similar to conventional gas turbine technology like that used in aircraft--and this could be a real plus on the developmental side.

NEP coupled with VASIMR is an excellent nuclear electric possibility, and I am glad the Dr. Franklin Chang-Diaz and his team are working on it.

NTR (nuclear thermal rockets--NERVA) will be useful for quick jaunts to the moon and possibly mars, especially if afterburning with LOX. Still NTR is pretty reaction mass intensive.


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martidj


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There is the proposed CERMET nuclear thermal system from Pratt & Whitney which seems to address many of the problems with NERVA. My main objection to it though is the relative lack of performance for the risks and expense involved - at least for manned spaceflight . . .

However, for unmanned probes, it is very different.

Ultimately I think that NTR may well be the propulsion method of choice for long range unmanned probes where propellant expenditure, flight time and safety are not important - but longevity and ability to use any old reaction mass is. For example a probe could refuel in Saturn's rings and cruise around the outer solar system for decades. If the reactor overheats, it's an expensive space bonfire but nothing worse than that.

But for humans, Nuclear Electric has my vote.


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Interestingly enough, sometimes it is actually more 'efficient' mass wise to be slightly less 'efficient' power wise. By sacrificing a little thermodynamic efficiency and rejecting heat at a higher temperature, you can cool the power conversion system with a smaller radiator. This saves weight--sometimes quite a lot. Bumping up the power of the reactor isn't really that big a deal (within limits) since reactor power generation is so nonlinear...

I agree that NTR is technically challenging, although I like the Pratt & Whitney Triton tri-mode, LOX afterburning engine. This is a good engine for Near Earth Space and possibly as far as Mars, but I agree that if you want to move things around the solar system you must use something that creates much higher specific impulse. NTR's are going to give Isp of no more than 1000 sec, 1200 sec tops. Entry level VASIMR will give atleast 3000 sec, up to about 30,000 seconds specific impulse.

One of the really interesting ideas with VASIMR is that a single engine can process different propellant streams. By retuning an engine's Helmholtz coils with a different combination of B-field strength, and RF excitation frequency you could process Oxygen instead of Hydrogen. Why oxygen? Well, oxygen is one of the expected waste products from processing lunar regolith for metal extraction. Replacing hydrogen with oxygen results in an Isp penalty of 1/4 of the Isp with hydrogen, but a propellant tank can store 16 times more oxygen than hydrogen (after suitably venting to space first!) Anyways, a propellant tank loaded with oxygen could deliver 4 times the total impulse as the same tank loaded with hydrogen...

If the oxygen erosion problem can be solved that is...



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martidj


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Being a high ISP system the VASIMR drive is always going to struggle to produce really high accelerations, even at high power densities. I've heard 6 month round trip times claimed for this system. Hard to believe that a few thousand Newtons would really make a huge difference. But it does.

A bit of messing about on Excel today was illuminating. Taking a ship of mass 500 tonnes with ten 42 MW Vasimr drives, return trip distance 150 million km, constant boost mode - accelerating to halfway, then turning round and decelerating. I've assumed constant acceleration throughout for easy numbers.

Peak speed = 21 km/sec
Propellant use = 144 tonnes
Round trip time = 166 days
Thrust = 3000 N
ISP = 29500 sec

The acceleration is a mind-blowing 0.006 m/sec2. Adds up though. Could go even faster if a propellant tank was waiting at the other end to refuel from.



-- Edited by martidj at 19:42, 2007-03-05

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...now imagine sending a tanker/processor to an Near Earth Orbit Crossing Comet nucleus and supping up on a couple thousand tons of volatiles--and bringing them back to a high orbit propellant supply depot. A single tanker mission might supply the propellant demands for ten or more subsequent deep space missions.

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John


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Since VASIMR is an electromagnetic system rather than a thermodynamic system would using propellants with higher particle mass reduce specific impulse?  Now in a nuclear thermal system substituting water for LH2 would reduce specific impulse of say 900 sec to 300 sec.  In an electromagnetic system the main difference would be the energy to achieve ionization of the propellant to the same extent.  Or am I missing something?  I'm sure this is the case with electrostatic systems.

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Going with higher molecular weight species does indeed reduce the Isp performance.

Since VASIMR is a 'constant power throttling engine,' and the plasma ions are mostly thermalized in the sense that each ion has the same average kinetic energy, then we can use the relationship:

KE=const=1/2*m1*v1^2=1/2*m2*v2^2

Keeping this in mind we can algebraicly rearrange this relation solving for v2:

v2=v1*sqrt(m1/m2.)

From this we can infer that since Isp is proportional to the exhaust velocity, we can similarly suppose that:

Isp2 = Isp1*sqrt(m1/m2) Now we can compare different molecular weight species quite easily:

If a VASIMR engine at the 'low end' can produce thrust with an Isp of 3,000 seconds using hydrogen (fm. wt. = 1)

then the identical engine (all other things being equal--which may or may not hold absolutely true, but ought to be fairly close!) processing a heavier species, say Lithium (fm. wt. 6) might be able to produce:

Isp2 = (3000 sec) * sqrt(1/6)

Isp2 = 1225 seconds with Lithium as a propellant.

If we went to oxygen (fm. wt. 16) then the Isp would drop to about:

Isp2 = 3000 sec * sqrt(1/16)

Isp2 = 750 seconds.




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G R L Cowan


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martidj included:

... A bit of messing about on Excel today was illuminating. Taking a ship of mass 500 tonnes with ten 42 MW Vasimr drives, return trip distance 150 million km, constant boost mode - accelerating to halfway, then turning round and decelerating. I've assumed constant acceleration throughout for easy numbers.

Peak speed = 21 km/sec
Propellant use = 144 tonnes
Round trip time = 166 days
Thrust = 3000 N
ISP = 29500 sec

The acceleration is a mind-blowing 0.006 m/sec2. Adds up though. Could go even faster if a propellant tank was waiting at the other end to refuel from...




Can go a lot faster if a lower Isp is used. This formula for transit time 'T' assumes there is no gravity along the route and the destination is not moving with respect to the departure point, but at least it takes into account non-constant acceleration:

T = DF/J + sqrt (2 DFJ/Z) + D*(1-F) / (J/2 * ln (1 + 2 (DFZ/J^3) + sqrt ( 8 DFZ / J^3 )))

'D' is the distance, 'F' is the fraction of that distance traversed with the motor on (including the part where the rocket is throwing propellant forward so as to stop), 'J' is the momentum per unit mass of propellant (aka Isp times Earth 'g'), and 'Z' is the ratio of kinetic power in the jet to nonpropellant mass. Here 'martidj' has 420 MW over 500,000 kg, so 'Z'=840 W/kg.

It turns out the quickest trips occur when 'F' is near 0.45 -- there is free-fall piece in the middle, but not exactly in the middle, covering 55 percent of the distance where one takes advantage of the speed one has built up, but does not continue to build up more speed because the extra propellant one would have had to start with to be able to do that would slow one down, overall -- and 'J' is around 45,000 N·s/kg, aka 45,000 m/s if all the propellant particles come out at the same speed; right in the middle of the range the Aviation Week article mentioned.


--- G. R. L. Cowan, former hydrogen-energy fan
oxygen expands around boron fire, car goes



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martidj


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That's quite right - in practice you wouldn't run the engines the whole time, unless your ISP was so high that you didn't need to worry about mass fraction of propellant. I didn't really appreciate just how much power is needed to really get trip times down until doing that first crude set of calculations on it.

The sums get fun of course when planetary motion and gravity is included - never did study orbital dynamics in detail, but I believe it is a branch of mathematics on its own !

I reckon a manned Mars mission just isn't feasible unless we can get there and back within 6 months using nuclear electric drives - nuclear thermal isn't fast enough, and the reliability record with NERVA isn't the best. There are too many risk factors associated with a 3 year mission - radiation, infectious disease, cabin fever, life support failure, weightlessness. I think Nasa already knows this hence why there is no manned Mars mission schedule - just a vague statement of intentions.

And of course, no less a luminary than Sergei Korolev backed NEP in the 60's.









-- Edited by martidj at 23:35, 2007-03-13

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10kBq Jaro


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martidj wrote:
I think Nasa already knows this hence why there is no manned Mars mission schedule - just a vague statement of intentions.
FYI, see

http://www.floridatoday.com/floridatoday/blogs/spaceteam/2007/03/congress-questions-nasa-plans-for-mars.html

Tuesday, March 13, 2007

Congress questions NASA plans for Mars

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martidj


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"We can be on Mars by the mid-2030s," Griffin told members of the House Subcommittee on Commerce, Justice and Science. "We can do this."

I guess it's a start and welcome for that.

Interesting to see it was the politicians pushing for progress on this one. All to the good. No big aerospace project has ever got off the ground (pun acknowledged) without serious political backing.

30 year timescale - I guess that gives them time to develop and prove next generation propulsion systems.






-- Edited by martidj at 19:40, 2007-03-14

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10kBq Jaro


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martidj wrote:

30 year timescale - I guess that gives them time to develop and prove next generation propulsion systems.

That's what one might perhaps be led to believe...
But I'm not sure if that's what NASA's Griffin has in mind, looking at the numbers....

quote:


http://www.nasaspaceflight.com/content/?cid=5048

Griffin maps out NASA's moon and Mars plans up to 2057

<SNIP>

'The Ares V cargo vehicle will provide, with no more than a half-dozen launches, the 500 metric tons or so which is thought to be necessary for a Mars mission, based on present-day studies. As a perspective on scale, this mass is about 25 percent greater than that of the completed ISS.



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John


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RE: &amp;quot;Advanced Plasma Rocket Tested in Costa Rica&amp;quot;
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It does seem that NASA is into a major "Apollo Redux" at the present time.  They are into totally low risk techologies.  Chemical propulsion isn't really all that bad for translunar flight but while it is possible for the Mars mission we could do better than that!

Back on the specific impluse issue for VASIMR, it seems to be that the real drivers are the amount of energy it takes to achieve a given charge-to-mass ratio on the ions and the strength of the magnetic field.  I don't think it is really a kinetic theory of gases issue.

-- Edited by John at 17:57, 2007-03-17

-- Edited by John at 03:20, 2007-03-18

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One of the things that I've been looking at recently is the physical size of the unit. I am learning that it's a delicate balance regarding the mass of the unit, the strength of the B-field of the solenoid coils which affects not only the cyclotron resonance (and hence the rf discharge frequency) but the radius of the orbits of the ions. A properly tuned motor must have minimal impacts from various ion species as this will lead to erosion. Especially so if an engine is attempted which processes oxygen. The original VX-10 tested by Dr. Franklin Chang-Diaz's used an inner fused quartz tube which threaded through both the helicon plasma generator, and the ion-cyclotron radio frequency antenna downstream. I suspect that some kind of quartz tube will have to be engineered as an easily replaceable barrier that will increase engine life by being used up or sacrificed to protect engine interior surfaces--not unlike a sacrificial anode used on boats and ships to reduce the affects of corrosion from seawater...


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To continue with this line: is it better to create a relatively large engine that can process tens to hundreds of megawatts of electrical power--or is it better from a magnetic point of view to create a cluster of smaller units that can process the equivalent power?

Engineering wise, I tend to lean towards a few large units. It is generally simpler (simpler plumbing and electrical connections,) and more reliable (statistically overall.) But there is a delicate balace of allowable cyclotron orbits of the carged particles and the physical size of the plasma channel: the size of the orbits is dictated by the strength of the magnetic field, and the need to create helicon antennas of a certain size to couple the rf efficiently to the plasma. The strength of the solenoid B-field and that of the Helmholtz coils that act as the mirrors and magnetic nozzle is dictated primarily by the current flowing through the presumed superconducting coils--and the geometric configuration (size) and number of turns of that coil. All of these things must converge simultaneously to a solution or range of solutions that result in an 'average' orbit size in which most of the population of charged particles will follow. The outlyers, especially the statistically small population of charged particles which will have much greater energy than the average, will have much large orbits, and will collide with the walls. This is the population that will cause almost all of the erosion--so some erosion will be unavoidable.

I've learned that because of the physics of plasma, the VASIMR engine which at first seems fairly simple in principle, is in fact quite complicated because of the 'ensemble' of so many interacting physical principles. As the old saying goes: "The Devil is in the Details..."



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martidj


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Straw poll. How ready are we really for a Mars mission ?

I've rated each component out of 10 in terms of its technology 'readiness' rating. What does everyone think. To me, it looks like a good 20 years of continuous work is required before a launch.


1. Chemical heavy lift boosters - 500 tonnes to LEO. (to answer a previous post I think this should an IMLEO target even for a a nuclear powered system)
9/10 - The Ares rockets are rehashes of existing designs at 140 tonnes / launch - pretty confident these will work.

2. High power reactor - say 1GWE.
7/10 - Various designs e.g. pebble bed reactor have been proposed and look promising but require a full scale design and test facility.

3. Heat - electricity conversion
8/10 - Brayton and Rankine cycle plant has been studied in detail with working prototypes tested, also the mathematical models of turbomachinery are accurate and scaleable. Conventional gas turbine plant and high frequency alternators can be used for electrical power. As above - work to do but it should be straightforward enough.
(To answer the previous post, I think that around 6 separate power conversion assemblies consisting of turbine / alternator / NEP drive units / heat rejection system would be a good compromise between complexity and redundancy)

4. Heat rejection system and fluid control
5/10 - a weak spot. System must be light, micrometeroid proof and able to withstand 900K temperatures and thermal cycling. Also requires valve system that is reliable but precise in flow control. However, this is an engineering problem, it just requires lots of development work.

5. Electric thrust system
4/10 - Basic proof of concept only whichever system is chosen. Requires full scale test and flight trials. Concerns over long term durability of NEP systems under high power - will erosion be a problem ?

6. Life support system
8/10 - With all that power, everything can be fully recycled using high energy systems. No need to worry about hydroponics.

7. Mars Lander
8/10 - Possibly N204 / H202 combination.



-- Edited by martidj at 19:42, 2007-03-18

-- Edited by martidj at 19:47, 2007-03-18

-- Edited by martidj at 19:58, 2007-03-18

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10kBq Jaro


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martidj wrote:
2. High power reactor - say 1GWE.
7/10 - Various designs e.g. pebble bed reactor have been proposed and look promising but require a full scale design and test facility.
Could you be more specific, please ?
The pebble bed reactors I'm familiar with have low power density, and are therefore completely unsuitable for space propulsion.

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martidj


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Two sources are useful pointers here.

First, a quote from page 3 of a recent US Energy dept report commissioned for Prometheus.

http://sti.srs.gov/fulltext/ms2004842/ms2004842.pdf.

A distinguishing feature of the Pebble Bed Reactor is the direct hydrogen cooling of small (400-500micrometer diameter) coated particle fuel spheres. The fuel is packed between two concentric porouscylinders, called “frits,” which confine the fuel but allow coolant flow around the particles. A number of these small annular fuel elements would be arrayed in a cylindrical moderator block to form the Pebble Bed Reactor core. Coolant flow is directed radially inward, through the packed bed and hot frit, and axially out through the inner annular channel. Because of the large heat transfer area of the Pebble Bed Reactor element, bed power densities two to ten times larger than the peak power densities demonstrated in the NERVA program may be possible.

I guess the critical phrase is 'may be possible'. The context of the report is for NTR use hence the hydrogen reference - but as a heat generation system, not sure there is much to beat it for kw/kg.


The second is so old it has to be carbon-dated but seems that these guys were thinking on the same lines.

Title: Compact, high-power nuclear reactor systems based on small diameter particulate fuel
Authors: Powell, J. R.; Botts, T. E.
Affiliation: Brookhaven National Lab., Upton, NY.
Publication: In R and D Associates Proc. of the AFOSR Spec. Conf. on Prime-Power for High Energy Space Systems, Vol. 1 5 p (SEE N83-15841 06-44)
Publication Date: 00/1982
Category: Energy Production and Conversion
Origin: STI
NASA/STI Keywords: GAS COOLED REACTORS, MAGNETOHYDRODYNAMIC GENERATORS, REACTOR TECHNOLOGY, SPACECRAFT POWER SUPPLIES, TECHNOLOGY ASSESSMENT, THERMODYNAMIC CYCLES, ELECTRIC POWER SUPPLIES, ENERGY CONVERSION, NUCLEAR REACTORS, REACTOR DESIGN
Bibliographic Code: 1982pphe....1S....P
Abstract

Two compact, high-power nuclear reactor concepts are discussed. Both are gas-cooled cavity-type reactors which utilize particulate fuel of the type now used in HTGR reactors. Unshielded reactor volumes are on the order of one cubic meter. The Fixed Bed Reactor operating temperature is limited to 2500 K and the output power to 250 MW(e). In the Rotating Bed Reactor fuel is held within a rotating porous metal drum as a rotating fluidized bed. Rotating Bed Reactor outlet temperatures up to 3000 K and output power levels up to 1000 MW(e) are achievable. Both reactors can be brought up from stand by to full power in times on the order of a few seconds, due primarily to the short thermal time constant for the fuel particles. Turbine and MHD Brayton are the power conversion cycles of choice. Open cycle operation is generally favored for applications operating at less than 1000 sec of equivalent integrated full power. At power levels above 1 MW(e), the liquid droplet radiator is the favored means of heat rejection. Power system specific power levels of 10 kW(e)/kg (not including shield) appears to be quite feasible.



-- Edited by martidj at 22:19, 2007-03-19

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10kBq Jaro


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Oh.... OK, so this is more like particle bed than pebble bed.

Makes sense.

Thanks for the info & references.

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martidj


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A pebble seems to have a very elastic definition - 0.5 mm to 100 mm diameter in the published literature ! These high power designs are plausible but do push the envelope a bit. I would prefer a better safety margin. The NERVA experience didn't exactly inspire confidence when operating at 2700K.

Much better to run in 'lazy' mode well within the known materials safe zone - (2000K ?) with higher flow rates to dissipate the heat, and also the use of an inert gas as the working fluid rather than hydrogen, which can cause steels to lose ductility. This also makes the heat exchange system easier to design. Modern CFD techniques not available in the 80's should be able to predict very accurately the flow rates, thermal gradients and material stresses.

There is also the issue of automatic feedback control and instrumentation. Flow and temperature sensors would obviously be the most critical. Very accurate readings and robust gauges are available now at 2000K - but at 2700K ?


-- Edited by martidj at 07:05, 2007-03-20

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GoogleNaut


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I don't know why it never occured to me to try using Google Patent search until very recently--but I found a plethora of interesting documents available at the USPTO. They can be browsed completely for free--and they are available as a pdf download for $3US apiece.

First one: the original Pratt & Whitney white paper on the TRITON NTR engine is available at Boeing engineering systems:

http://www.engineeringatboeing.com/dataresources/AIAA-2004-3863.pdf

This is the paper that Bruce referred to in his interview that is still posted on the front end of this website. A very interesting read!

Now for a VASIMR specific patent:

try: 6334302 B1 this is one of Dr. Franklin Chang-Diaz' patents from 2002.

Doing a google patent search for "nuclear thermal rocket engine" turned up many, many entries. Quite a few of which are pretty good:

3546069 which is for a gas core reactor rocket
3793832 which looks at several NTR core geometries
3709781 which is a patent on a space nuclear power reactor
5475722 which is an unusual 'aerospike' NTR arrangement
5410578 which is an NTR core presented by the Bab**** and Wilcox boiler company (famous for
also making nuclear power plants!)

I encourage everyone to 'get their feet' with the Google Patent search. I've known about it for a long time--just for some reason I never thought to apply to this subject! Cheers!

Ty Moore


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martidj


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A superb resource.

Ahhh . . . patents.

I've been dealing with these a lot lately at work as we go from concept studies to actually nailing down the designs for the next generation of products - it is biosensor technology, and a very crowded market.

There are some choice phrases in there that I've seen a few times, I think the patent lawyers put them in. How about 3546069 :

Although the invention has been described with reference to a particular embodiment, it will be understood by those skilled in the art that the invention is capable of a variety of embodiments such as a plurality of vortex tubes and/or vortex tubes of other configurations within the scope and spirit of the appended claims.

Since no one has built one yet, there isn't anyone skilled in the art, but that's ok - it is something new ! At least you would think so. But look at the date. This was submitted in 1968, and it references a 1959 paper.

A caution on patents though. They don't necessarily mean that the invention is a practical engineering proposition - only that it might be so. Also in practice a patent based on a full scale working device has far more legal force than one describing a concept only. Hence Vasimr with its extensive test record is on solid ground, gas core reactor concepts less so.


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GoogleNaut


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Yes indeed. Patents for working models are given preference. Patents for working products--much more so...

And I also appreciate the disclaimer--as evidence by many actual patents out there--just because something is patented does not necessarily mean it will work...

However, as an "Idea Mine" patents are superb for gaining some insights into how something works--even something like Nuclear Thermal Propulsion or washing machines!

Ty Moore


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John


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Well Googlenaut I guess on our discussion above after looking at the patent drawing I think I'm right about the propellant flow coming out of 18 but you are right about the "augmentation" thrust after 26.  The latter would be a somewhat normal thermodynamic process.  But, do you think that it could crack the H2 into monatomic hydrogen to increase the specific impulse?

I've been reading about these devices (plasma thrusters) since I was a kid.  Don't you think we could speed up this development if we could get some reasonable funding?

-- Edited by John at 13:42, 2007-03-23

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