Europeans And Australians Make Space Propulsion Breakthrough

[mlorrey]

Stevehw33,<br />While I agree with you on nuclear, you are incorrect wrt electric propulsion. High Isp allows for constant acceleration that builds up over time. This isn't very important for lunar missions, but for any mission measured in tens of millions or hundreds of millions of miles, or longer, electric propulsion is far superior to chemical. It is so because with constant acceleration, your velocity is constantly building, and with extremely high Isp, you can keep it going for full missions. Here's the math:<br /><br />Lets say you have a 100 ton Mars mission, which has a choice of half its tonnage being a chemical booster, or a nuke plant and electric propulsion system. Lets assume the chemical system is 40 tons of fuel, 5 tons of structure, and 5 tons for a fuel plant to operate at Mars. If the fuel choice is LH2/LOX, the Isp is 450 secs, which means 450 lbs of thrust-seconds per lb of fuel. 40 tons is 80,000 lbs, which gives you a thrust budget of 36 million lbf-seconds of thrust. Lets assume you chose an engine that provides 100 tons of thrust (200,000 lbf), allowing you to boost at 1 g on your Hohmann transfer orbit to Mars. This allows 180 seconds of burn time with your booster, which equates to 5760 ft/sec or 2.25 km/sec, significantly less than the 5.6 km/sec needed to get to Mars from LEO. In fact, you'd need more than triple the 80,000 lbs of fuel, to 260,000 lb of LH2/LOX, to have sufficient delta-v to get from LEO to Mars orbit at Phobos altitude. <br /><br />Your return to Earth also demands 3.4 km/sec of delta-v, or 110,000 lb of fuel, (assuming you leave half your vehicle mass at Mars and come home with 50,000 lb of vehicle), which you either need to take with you, or produce on Mars from water you find there (and with what power supply for your electrolysis plant? Solar?). We'll assume that fuel for Mars descent and ascent are equally budgeted in the 100,000 lb of mission mass. If you carry that 110,000 lb of return fuel with you, you'll need an a

 

[mlorrey]

I'm sure french sub nukes are fine, but so are American sub nukes. The US Navy has never had a sub reactor accident. I'd have to say the US has more experience with building and operating this sort of nuke plant than the French do, so it should be easier to find mission candidates from among the many thousands of US Navy veterans who have served aboard our subs. I don't know of any American citizens that have served aboard French subs.

 

[JonClarke]

the french build good reactors and are world leaders in the field. Over 70% of their electricity is nuclear, I believe. In terms of space reactors the Russians are by far and away the best.<br /><br />Submarine reactors are robust, compact, and reliable. presumably why Robvinson used then in his Red-Green-Blue Mars books. However they do use a lot of water for cooling. Easy in a submarine, when you are surrounded by it. Not so easy on the moon or Mars, when you have limited water, you have to use radiators.<br /><br />10 mW is a lot of power. For a lyunar station yoyu might wat to start with something smaller, like the the US PMA's of the 50's and 60's. these were 1-2 mWs, modular and used in a number of remote locations, like Greenland and Antarctica. they weren't very relaibale though, the McMurdo one had lots of problems and was operational only 72% of the time. Another was responsible for the only fatal reactor accident in the US, although that was due to the operators rather than the instrinsic design.<br /><br />A 5/1 thermal-electrical ratio would be great. However, I think the power conversion you use is a bit optimistic. This site http://www.forecastinternational.com/archive/ws/ws5267.htm says Amytheste has a 48 mW reactor which drives 3,150 kW turbogenerators (like many modern nuclear submarines the Rubis class is turboelectric rather than direct drive). This means the electrical power is 1/15th the thermal output. So the 10-20 rule still holds, I fear.<br /><br />Jon<br /><br /> <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[mlorrey]

Now, you could probably respond, "But the trip time with VASIMR is going to be longer." Not necessarily so. Trip times with ion or other electric engines on space probes (powered by solar) that we have experience with take a long time because they are drastically underpowered, providing accelerations in the 0.00001 g range. VASIMR is a whole nother animal, because it is properly powered with 10 MW.<br /><br />As a comparison, a nuke or chemical propelled vehicle thrust into a Hohmann transfer orbit will take 259 days to get from Earth to Mars.<br /><br />An ion propulsion like VASIMR, if its acceleration rate is higher than the rate of solar gravitational acceleration, will take a more or less straight line course to Mars from an Earth escape trajectory. According to Dr. Diaz, VASIMR would require 30 days or less to achieve Earth Escape velocity. Assuming that the mission launch occurs when Mars and Earth are more or less at opposition, i.e. closest to each other (I don't know why they call it that, it sounds like they are on opposite sides of the Sun to me, I suppose Mars opposes the Sun in our sky, but that is very Earth-centric), which is about 40 million miles. As both planets will be moving during the voyage, the course is actually diagonal to the straight line distance, probably at a 90 degree angle, to maximize the velocity of the Earth to the mission. Mars moves 1.4 million miles/day at opposition, so, assuming VASIMR/MARS leaves LEO a month before opposition, and assuming it accelerates for another 30 days, at 0.076 ft/sec^2, VASIMR/MARS will reach a peak velocity of 268,000 mph and will have covered 48 million miles. This does not count the 30 km/sec (~70k mph) velocity of the Earth or the distance that boost would provide. I don't have time to do the whole calculation, but it appears that given a full time acceleration, and turn around to decelerate at the halfway mark, VASIMR would provide a trip time of about 90 days or less, compared to the 259 day Hohmann

 

[JonClarke]

Nice post about relative high thrust long duration propulsion systems.<br /><br />Some quibbles. VASIMR is not intrinsically 30 mW. It is scalable, from a few kW up to 10's of mW. At least in theory. We don't yet know whether the upper power end is actually viable. Actual acceleeration will depend on spacecraft mass.<br /><br /><br />Of course the whole operational assumptions about using a 30 mW propulsion system for early Mars mission are deeply flawed. But that is another story.<br /><br />Jon<br /> <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[mlorrey]

Thanks. I went by the full 10 MW power performance chart on one of the NASA VASIMR sites.

 

[daniko]

I think there is a way to use the benefits of the large Isp of the "Dual-Stage 4-Grid (DS4G) ion thruster". The way is to trade Time for Load:<br /><br />A transport vehicle could transfer to Mars orbit - the return fuel and mission equipment. Transportation times could range from 1 to 2 years.<br />If the speed that must be reached from LEO is V_delta=5600 m/s (mentioned above) and the exaust speed is U=210000 m/s then the mass of the fuel needed is: <br />= /> M_f = M_load * (10^(V_delta/(2,31*U))-1)<br />so we get<br />= /> M_f = M_load * 0,027<br />the mass of the fuel is 2.7% of the mass of the transported Load.<br /><br />The vehicle could be with Solar Powered Ion Engine. It could ferry loads from LEO to Mars. It could return after mission back so to be reused.

 

[mlorrey]

Keep in mind the dv to return to LEO is 3.4 km/s. Unless you are being refuelled at Mars by an indigenous fuel supply, expect to carry enough fuel to transport the req'd return fuel to Mars. However, what is your acceleration rate? How many newtons per watt? This is the real limitation. Going on 2.7% fuel fraction is fine, but at what rate is that being used? After some googling, this device consumes 171 kW per Newton. This is about the same as VASIMR at its high Isp setting, though VASIMR gets 25,000+ secs, DS4G gets only 19,000 secs. At its low Isp high thrust setting, VASIMR gets more than half the DS4G Isp, while burning only a mere 10 kW/N.<br /><br />While good for unmanned missions, this would be not as good as VASIMR. Another problem with DS4G is that it uses grids and electrodes, which are known to wear out significantly. VASIMR is designed to be electrodeless for this reason (as are Hall thrusters).<br /><br />Sure you can trade time for load, but with a manned mission, you can't do that, because crew require consumables that can't be replaced. The longer your trip time, the larger your load of consumables.

 

[dreada5]

Don't want to take this off-topic. But I just had a thought. <br /><br />I was recently reading an article on Heim Theory and Hyperspace propulsion etc.<br /><br />http://www.newscientistspace.com/article/mg18925331.200<br />http://en.wikipedia.org/wiki/Heim_theory<br /><br />And now after reading about all these recent advances in Ion thrusters with exhaust velocities of 100+ km/s, I thought if ANY of Heim's Theory is proven useful and we can change the mass of particles, imagine the effect that would have on ion drive propellants. You could accelerate a spaceship to velocities like 50km/s in just a few minutes!<br /><br />It makes you think how quickly we'd be able to revolutionised interplanetary travel so that travel between planets was measured in hours rather than months using an almost science fiction-like ion drive!<br /><br />Just a thought...

 

[mlorrey]

One of the proposed problems with Heim Theory based propulsion is that the field generated would create a barrier between the ship and things outside it. Photons could not come in, so you couldn't see where you were going, nor could matter pass out of the barrier, so any propulsion system that is based on reaction mass could not be used while a Heim Field is operating.<br /><br />What it would likely do is allow a normal ship to first travel out to the limits of the solar system normally, then turn around, and begin a 'dive-bomb' maneuver to gain velocity from the Sun's gravity. Once you've reached perhilion, you turn on the Heim Field, which greatly decreases your inertial mass, and you slingshot out of the solar system at high speed without gravity losses of any significance.<br /><br />Another feature of Heim Theory is that it is possible to turn photons into graviphotons and back, which could theoretically allow you to use the sun's light as a means of generating a gravity field which could accelerate you toward or away from objects at high speed... It is all speculative, and there are a few issues with the theory.

 

[SteveMick]

I haven't contributed much to this forum for some time because of the outrageous critisism I received for suggesting that concentrating solar cells of the type discussed here existed and were competitive with the ridiculously overweight Prometheus reactor. Now that that program is history, it is gratifying to see that the previously unworkable technology is worthy of consideration. As I have been saying for 25 years, SOLAR THERMAL ROCKETS are a viable alternative and can be used in conjunction with solar electric to effect rapid trips to Mars for instance without the 10MW discuused in this thread. The same concentrator that supplies the solar cells can directly heat hydrogen or other propellent to produce thrust and acheive 1/100 to even 1/10 gee acceleration. A series of thrusts at perigee can raise the orbit of a vehicle untill a final lower Isp higher thrust manuver at perigee can send it on a Mars trajectory. At this point, solar electric can shorten travel time. Aerobraking can be acheived by separating the payload from the concentrator which can then use thrust to brake while the payload aerobrakes and then they can rendevous in Mars' orbit later.<br /> Whether that's a good way to aerobrake or not, its just mentioned as a possibility. As for the mass, the concentrating solar cells are barely over 1kg/KW and the concentrator can easily be a fraction of this. Add a cooling system for the cells and use the structure of the concentrator as radiator and perhaps the mass goes to 2kg/KW. For the 10MW at Mars mentioned above this would rise to maybe 4kg/KW since the concentrator only would have to be increased. This gives a mass somewhere around 40,000kg or less.<br /> The Starfire program brought solar thermal almost to the point of on-orbit demonstration. The Glenn developed equipment was extensively tested sucessfully in 2002, but the current administration doesn't like solar so we got Prometheus. <br />Steve

 

[twocanntwo]

People talk about going to mars and leaving the solar system.They still don`t have a base on the moon and the space station is a joke!why can`t Dr.Roger Walker of ESA`s Advanced concepts team work on things closer to home?Years ago WE were told there would be people living on Mars and the Moon,Well we are still waiting.

 

[JonClarke]

Let's not get carried away! You talk about VASIMR as if 25,000 secs is a done deal. It is not. VASIMR is a laboratory test rig that has reached about 10,000 sec (claims of 30,000 are pure extrapolation). It is a long way from even the most basic space test. Also VASIMR project is appallingly under funded and hasn't really gone anywhere in years.<br /><br />The DS4G unit is also a test unit, but has already shown a much higher Isp at a comparable level of development. Because the technology is basically similar to ion propulsion already in service it is much more likely to fly sooner than VASIMR. Part of the innovation of the system is major reduction of grid erosion. As a result it, unlike VASIMR is well funded.<br /><br />Not that ANU are ignoring Helicon thrusters either. They are also working on their own unit which shows comparable performance to VASIMR.<br /><br />It is too early to say which is superior to the other. Like most propulsion technologies it will depend on application.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[mlorrey]

Lots of things have shown high Isp. Big whoop. The important thing is to get reasonably low watts/ newton and high thrust density while at high Isp.<br /><br />I happen to like the DS4G design. It is relatively simple compared to VASIMR. The problem is that 'reduced grid erosion' is not the same as 'no grid erosion'. VASIMR doesn't have grids or electrodes to erode, and has evolvability (i.e. more RF heater stages up the Isp even more) and scalability. Moreover, it's laboratory "test rig" as you so disparagingly put it, is scaled at a level one would expect of a drive for an interplanetary vessel, while DS4G is the size of a vernier thruster and is operating at miniscule power and voltage levels. Show me a DS4G the size of the VASIMR "test rig" with no grid erosion and equal thrust density and power/thrust ratios, and I'll be interested.

 

[cuddlyrocket]

The first anticipated use of DS4G is as a replacement for the current in-service ion drives, of which it appears to be superior in all respects. Once the technology has been demonstrated, and its performance assessed, then it can be considered for other missions.<br /><br />As for DS4G v VASIMR, this isn't a sporting contest. Whichever comes out top (and I too suspect that the answer will be application and mission dependent), humanity will be the winner.<br /><br />At the present time it is probaby sensible to develop both technologies. We all know the dangers of putting all your eggs into one basket.<br /><br />"The important thing is to get reasonably low watts/ newton and high thrust density while at high Isp."<br /><br />Don't forget cost and reliability.

 

[scottb50]

I would think the same basic vehicle could work with any propulsion system. To go from LEO to the moon and back, would have no need for electric power. Mars might benefit, but then you have to slow down to reach orbit so the faster you are when you get there the more it costs to get into orbit.<br /><br />Beyond Mars this is probably the best hope of reaching further out. Though you still have to slow down if you want to stop anywhere. <div class="Discussion_UserSignature"> </div>

 

[mlorrey]

Electric propulsion concepts typically involve thrusting the same amount of time, or similar amounts of time, at the start of the flight and at the end. Depending on thrust level, a mission can thrust halfway there, flip around, and thrust the rest of the way there in deceleration.<br /><br />I clearly demonstrated that a VASIMR's Isp advantage could give it the ability to burn constantly accelerating then decelerating for orbital insertion, and still burn less fuel than an NTR of the same power level, while arriving at Mars in 90 days versus 259 days with NTR. NOTE: Those numbers INCLUDE the mass of fuel for orbital insertion.

 

[SteveMick]

From Stevehw33: "The total reactor weighed about 900-1000 tons metric (exact weight is classified), had a total output of about 50 MW with 10-11 MW of usable electrical power. It was built to last for 25-30 years without refueling and was quite, quite efficient. That's a bit less than 5:1 heat/MWe output."<br /> If my limited arithmetic is correct, doesn't this mean that the specific power of this reactor is 100kg/KWe? This compared to maybe 2(est.)kg/KWe for the concentrating solar cells including concentrator near Earth to maybe a few times that farther out?<br /> Given that, would it be fair to say that those proposing nuclear for this purpose are expressing a religious belief or is it just a bias merely based on faith?<br /> The previous discussion mentioned the Density advantage of nuclear power which is not only irrelevant, but is only true if the radiators are omitted.<br /> Solar unlike any nuclear design proposed lately can function in thermal or electric modes. Why is this important? Well you may recall that JIMO was going to take TWO YEARS to escape LEO! Solar thermal can cut this to about a month or less and since it operates at more than twice the Isp of chemical, the propellent mass and therefore the mass that must be launched to LEO is much less than chemical for escape.<br /> Solar electric can use the same electric propulsion system as nuclear electric, but since solar has such a dramatically greater advantage in specific power, the travel times to Mars or even Jupiter is shorter. The concentrator mirror(s) can double as a communication antenna for dramatically increasd data rates as propsed by L'Garde or even a radar particularly if two concentrators are used.<br /> Cost and development time for solar is ridiculously low compared to nuclear.<br /> In addition, as beamed power systems become practical, few mods would be required to the solar vehicle which could then have even better specific power numbers. Also the possibility of laser wakefe

 

[scottb50]

Depending on thrust level, a mission can thrust halfway there, flip around, and thrust the rest of the way there in deceleration...<br /><br />Which is where my concern lies. Just as a starting point and without doing a bunch of calculation I would think it would take a lot of mass, even if Hydrogen is used just to cool the reactor let alone provide usable thrust. <br />If you provide constant thrust of 1g to a point where 1g deceleration would put you in orbit at Mars would need a lot of propellant as well as a pretty large reactor. I don't know what acceleration you are using to get a 90 day transit but I would think it would have to be at least a constant 1 g.<br /><br />I would also think, you would need less propellant to accelerate and decelerate at say 2-3g's, coast the majority of the trip and decelerate at 2-3g's, using convention, existing engines. <div class="Discussion_UserSignature"> </div>

 

[najab]

><i>I don't know what acceleration you are using to get a 90 day transit but I would think it would have to be at least a constant 1 g.</i><p>Not even close - using basic Physics (s = ut + 1/2at^2) after 45 days of constant 1G acceleration you would be over 10 billion kilometers from Earth, travelling at over 38,000km/sec!!!</p>

 

[darkenfast]

I believe that the New Horizons probe is going to cross the orbit of Mars in about ninety days. This "savings" in time over a minimum transfer orbit is obtained by about a 10,000 mph addition to escape velocity (I think).

 

[comga]

"Notice that the ray-trace on figure 2 shows different colors being focused on different places. So I guess that they do utilize chromatic abberation to increase efficiency by locating cells optimized for certain colors at the focal point of those colors."<br /><br />Now this is something within my own field.<br /><br />What you see in the illustration is know as axial color. The focal length is different for different wavelengths because the index of refraction changes with wavelength. Blue, with higher index, focuses closer and red with lower index focuses farther away from the lens. However, this cannot be used to put the different wavelengts on differently optimized cells without hitting the others.<br /><br />For making use of the differences, it is possible in principle to create lateral color, that is different wavelengths focusing in different locations. The basic way to do this is to include a constant wedge in each Fresnel segment. You could then put optimized cells in side-by-side rows along the line focus. It would seem that the scale, the distance between the focusing element and the solar cells, would have to be greatly increased from that of the SCARLET (sp?) arrays or those in the Boeing paper. <br /><br />Venturing outside of my field, if it were possible to create a solar cell whose peak efficielcy wavelength changes in one drirection, that could be matched to the dispersion of the colors.<br /><br />In regards to your earlier comments, Fresnel lenses will be more efficient than diffraction gratings. All diffraction elements have some "diffration efficiency" losses, which can only be eliminated at a particular wavelength, and would increase drastically over large spectral bandwidths. Gratings I have optimized near the peak of the solar spectrum have almost no effect at all at NIR wavelengths beyond one micron. Note also that the incresed thickness of the tiny elements in a Fresnel lens should not dominate the substrate, although there may be case

 

[john_316]

I haven't deeply looked deep into VASMIR but is there a sight I goto to look at this 90 day trip to Mars with VASMIR?<br /><br />I know there are studies with Nuclear Fueled Rockets that go there in 120 days but where does the 259 day study come from with NTR? If I believe correctly the GCNR and SCNR both can do a Mars trip in 120 days. Whats this 259 day travel time? Or is that for there and back again? <br /><br />Because I think the physics allow you to go there in 120 days with nuclear propulsion and another 120 days back which comes to 240 days for a round trip jaunt.<br /><br />I am very perplexed to believe ANY ION type of rocketry will get us ie (manned mission) to Mars in under 90 days. Sorry just don't think it can happen just as I don't believe solar power will achieve 70-80% efficiency anytime before 2020 either. <br /><br /><br />Whats the Delta V requirment for 150 day one way trip to Mars with VASMIR? Isn't a NCR something like 23 to 25 Miles Per Second or in that range? The longer the ride the less the Delta V correct? <br /><br />Even with constant VASMIR operation how long would it take to get upto 25 Miles Per Second to get a 150 day trip outbound to Mars? <br /><br />I am being optimistic here with a 150 day trip for any rocket type...<br /><br /><br /><img src="/images/icons/smile.gif" /><br />

 

[spacester]

259 days is the flight time for a Hohmann (minimum energy) transfer between Earth and Mars <b>assuming circular orbits.</b> (Hohmann transfers typically imply chemical propulsion.)<br /><br />There is no such thing in the real solar system. Mars' orbit is <i>very eccentric </i>and whenever I see a study that fails to account for the major differences resulting from that fact a yellow flag goes up. I have not looked into the study you're referencing enough to know if there's a red flag right behind it. I know that some of these propulsion guys play it a little fast and loose with their orbital mechanics. I even once caught one of my heroes (Winglee) on a gross exaggeration. <div class="Discussion_UserSignature"> </div>

 

[mlorrey]

I wouldn't say Mars is 'very' eccentric, but it is eccentric, however it doesn't suffer from rapid precessional frame dragging like, say, Mercury. The 259 day figure is the shortest amount of time to go by a Hohmann Transfer.<br /><br />And, no, chemical propulsion is not implied in Hohmann transfers. Hohmann Transfers are typically the shortest transfers one can make with the minimum dv between two orbits, and are followed when the vehicle propulsion is high thrust for short periods at the start and end of the journey.<br /><br />There are lower dv trajectories one can follow, typically going through Lagrange points, but these take longer transit times.<br /><br />Only electric propulsion and various sorts of solar sails can use the faster constant acceleration trajectories.