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#1 2007-04-19 17:21:23

neviden
Member
Registered: 2004-05-06
Posts: 99

Re: Solar Thermal Propulsion

I was reading some pages about solar thermal propulsion. It’s relatively simple, has moderate isp (800 – 1200 s with hydrogen) and it could be used for slow transfers. Solar thermal works like this:

beamedfig37.GIF

Proposal for solar thermal rocket: http://www.stg.srs.com/atd/STP.htm

But to be really useful it would need higher isp (around 3000 s) and preferably use anything as the propellant. That would mean basically that gasses would have to have higher temperature as they would leave rocket chamber without melting rocket chamber. (= max 1200 s) Higher temperature = higher speed = higher isp. Then I saw this two pictures for Laser Thermal Propulsion:

beamedfig39.GIF

beamedfig40.GIF

Could something like this be used only that instead of the laser we would use the sun as the source so the light?

Since sun is that far away it's light could almost be treated as the laser beam (very weak one). Use multiple big, lightweight solar concentrators that would get as much solar energy to focus one point (through the series of mirrors) that it would instantly create plasma from anything that would be there (up to 1 GW of solar thermal power?). Once we would have plasma we could control it like we control any other plasmas with magnetic fields (something like Vasimr).

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#2 2007-04-19 19:01:07

GCNRevenger
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From: Earth
Registered: 2003-10-14
Posts: 6,056

Re: Solar Thermal Propulsion

What is it with this "strong, big light weight collector" business? Has nobody ever really considers how difficult it would be to build, aim, focus, and collect gobs of sunlight? They almost always seem to be the catch-all Greenpeace solution to using the dreaded "N-word" too.

To produce 3000sec Isp with Hydrogen with rocket-like thrusts, I figure you need about 4-5GW at least (rough figure, since energy required to accelerate exhaust increases exponentially) since NERVA nuclear rockets "start" in the 1GW region. To collect that amount of sunlight energy at, say, Mars orbit you are going to need a collector on the order of 10 square kilometers.

There are practical considerations to consider too, that when you are in orbit and firing the thing you will have to continuously gimbal this massive flimsy structure to ensure that it is pointed at the Sun despite the continuous change in orientation of the engine (and the rest of the vehicle probably) And what happens if you miss? That kind of focused energy would surely wreck the ship if the beam hits anything except the engine.

Or if the supply of energy is interrupted for whatever reason, like simply going into the shadow of the body you are orbiting or from some gimbal failure, now you have ultra-cold -270C liquid Hydrogen being pumped through your formally heated engine. The temperature change surely can't be good for it. Much less the rather sudden stopping/starting of a nontrivial amount of acceleration on the rest of the vehicle.

Forget about using anything other than Hydrogen for reaction mass either, even water needs an order of magnitude more energy (100km2 aprox) to reach comparable Isp.


"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw

The glass is at 50% of capacity

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#3 2007-04-19 20:53:31

neviden
Member
Registered: 2004-05-06
Posts: 99

Re: Solar Thermal Propulsion

http://www.aoe.vt.edu/~cdhall/courses/a … s/sotv.pdf

here is proposal for modest LEO to GEO stage. 200 KW of power, 800 seconds isp, 30-60 days to GEO. It's no NERVA, but it would do it's job.

What I would like to know if there is a way to create high isp rocket by making gasses hot enough with only solar concentrators? Can you make hydrogen or even water hot enough to change it into plasma that way (without melting anything else of course)?

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#4 2007-04-20 06:11:47

neviden
Member
Registered: 2004-05-06
Posts: 99

Re: Solar Thermal Propulsion

What is it with this "strong, big light weight collector" business?

What is wrong with "strong, big, light weight collector"? I added the link to the proposed solar thermal tug, that would use such collector and I don't see any big problems with it.

Has nobody ever really considers how difficult it would be to build, aim, focus, and collect gobs of sunlight?

It's thin film with some kind of structure that holds that film in place and in the right shape to reflect lots of light. It must be strong enough to hold it's shape in weightless and airless environment. Is this really that much harder than say.. building nuclear reactor?

They almost always seem to be the catch-all Greenpeace solution to using the dreaded "N-word" too.

It's cheaper. I have nothing against nuclear, but why build expensive nuclear reactor if sun always shines in space. My guess is big balons with plastic film would be more cheap, less prone to problems and safer (safer as in it can't explode on launch from earth and having "Another Chernobil!!" reported on every TV and newspaper). If one gets damaged, just inflate another one instead.

To produce 3000sec Isp with Hydrogen with rocket-like thrusts,

There is no need for high Isp AND high thrust. You do need high isp so that more of your spaceship is spaceship itself and not propelant, but high thrust can be replaced with longer thrusting times. The trip to Mars would take months anyway so why not use that time to change trajectory with high isp propulsion of the spaceship so that actual insertion would need minimal delta-v.

I figure you need about 4-5GW at least (rough figure, since energy required to accelerate exhaust increases exponentially) since NERVA nuclear rockets "start" in the 1GW region. To collect that amount of sunlight energy at, say, Mars orbit you are going to need a collector on the order of 10 square kilometers.

sure you do need 4-5 GW.. if you want to use your rocket few minutes, burn all your propelant and then do nothing for few months while you drift in space..

To get to Mars from LEO, you can start your GW reactor, heat hydrogen, get "rocket like thrust" and get enough delta-v (3,2 km/s for Earth Escape velocity + 0,6 km/s to get onto Mars Transfer Orbit) in few minutes, dump your "empty" reactor and then aerobrake into high Mars orbit (- 0,9 km/s), low Mars orbit (- 1,4 km/s) or even to Mars surface (- 4,1 km/s). On the way back you dock to spaceship in low Mars orbit, do another thrust (1,4 km/s + 0,9 km/s), drift for few months, dump transfer stage and aerobrake the crew back to Earth.

Or.. you can start in LEO, slowly spiral (3,2 km/s) to high earth orbit (that can be done unmanned because of van allen belt radiation), launch crews seperatly (Orion?), dock with waiting spaceship in L1, L2, HEEO or any other high earth orbit, add a little thrust (0,2 km/s) and you are on your way to Mars. Few months needed for transit is used to change orbit (0,6 km/s + 0,9 km/s) so that you get captured into High Mars orbit when you arive. After that your crew can land on Mars surface while your spaceship waits in High Mars orbit (or even spirals closer to Mars for easier access from Mars surface). When it's time to return from Mars back to Earth, your crew leaves Mars surface (4,1 km/s), docks (1,4 km/s) your waiting spaceship, leaves Mars orbit (0,2 km/s), use the rest of the months changing your orbit (0,6 km/s + 0,9 km/s) so that when you arive your spaceship enters high Earth orbit, while your crew aerobrakes back to Earth (with Orion?). Bonus: you keep your spaceship and only need to refuell it to go to Mars again.

The only things that need high thrust transfers are crew transfers and that can be done "the old way" with chemical rockets. Most of the heavy stuff can be moved with high isp and relativly low thrust. And it fits "don't mix crew and cargo" philosophy..

There are practical considerations to consider too, that when you are in orbit and firing the thing you will have to continuously gimbal this massive flimsy structure to ensure that it is pointed at the Sun despite the continuous change in orientation of the engine (and the rest of the vehicle probably) And what happens if you miss? That kind of focused energy would surely wreck the ship if the beam hits anything except the engine.

If we can point Hubble within 0.1 arcsecond of a distant star I am sure we will find a way to focus sunlight. It doesn't even need to be perfect focus (we will not use it to look at different stars)..

Or if the supply of energy is interrupted for whatever reason, like simply going into the shadow of the body you are orbiting or from some gimbal failure, now you have ultra-cold -270C liquid Hydrogen being pumped through your formally heated engine. The temperature change surely can't be good for it.

I am sure this problem could be fixed with simple swich, that would stop your supply of Hydrogen, if supply of energy is interrupted.

Much less the rather sudden stopping/starting of a nontrivial amount of acceleration on the rest of the vehicle.

Acceleration would be trivial, if the thrust would be minimal.

Forget about using anything other than Hydrogen for reaction mass either, even water needs an order of magnitude more energy (100km2 aprox) to reach comparable Isp.

water is easier to handle, but if heated to high temperature it breakes into Hydrogen and Oxygen. That Oxygen reacts with anything it touches.. unless.. it's plasma and it doesn't touch anything (it can be controled with magnetic fields). If you get plasma hot enough it would have high speed (= high isp). Thrust is another matter, but I am not worried about it. More energy at the same isp = more thrust, so to increase thrust you only need to add more energy. You can't increase isp because you are limited by the temperature of your heat exchanger. Unless you don't use heat exchanger and you heat gass to the point it becomes plasma. Kind of like GCR only that the core would be heated by super concentrated sunlight and not nuclear fission.

So, Would high isp be possible?

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#5 2007-04-21 00:03:05

GCNRevenger
Member
From: Earth
Registered: 2003-10-14
Posts: 6,056

Re: Solar Thermal Propulsion

The big reason I wanted to comment on the solar-thermal concept is to dispel the intuitive notion that its fundamentally "easy" to make such a drive. In order to reach really high Isp (1000's of sec), Hydrogen must be heated to temperatures that no solid nor liquid can possibly withstand (10,000's of Kelvin). Simpler nuclear engines like NERVA use graphite or ceramic cores and can only reach ~900 sec Isp, so probably simple solar thermal "targets" like the SOTV's graphite brick can reach no higher.

Such an engine becomes quite complicated, since the "target" the solar energy is focused on must be itself a gas or suspension. You can't heat Hydrogen (or most any gas for that matter) directly since it is transparent in the visible and most of the infrared region, so basically (as illustrated in your third slide) you dump carbon black into the Hydrogen to absorb the sunlight, and hope that the Carbon doesn't totally disintegrate in the partially heated Hydrogen before it can absorb much energy. GCNR engines on the other hand, can transfer heat directly from Uranium to the Hydrogen by contact conduction and possibly X-Ray absorption at high enough temperatures.

Also given the temperatures needed to realize high Isp performance will need a great deal of energy: will the "beam window" of the engine be able to withstand it? Even a mirror can be cut by a bright enough laser, and this is much more energy and heat than the low-performance engine you originally show would have to withstand.

The superhot plasma would also have to be confined to large degree by jacketing with colder hydrogen between it and the wall; the mass of fluid/plasma flowing through the engine will be too great for any light weight magnet arrangement to contain the plasma, plus there will be non-ionized gasses plenty hot to wreck the engine. I think that it is entirely safe to say that such an engine will be not one bit less complicated or expensive than a high-energy nuclear drive like the comparable GCNR if it did work.

I contest your excuses about the viability problems of employing this technology too, the Hubble telescope is a relatively small, solidly built Aluminum tube and not a massive gossamer construct. Trying to reposition the structure must introduce significant bending forces that would make maintaining beam focus difficult. Furthermore, you have to worry about the structure wobbling back and forth due to elasticity of the material.

Also, trying to shut off the Hydrogen flow into the engine to compensate for sudden beam loss due to eclipsing a planet isn't going to work very well;  Hydrogen is typically fed to rockets by a turbopump that can't suddenly just be "run dry" all of a sudden by depriving it of propellant nor can you dump the precious stuff overboard. A pressure-fed arrangement might work, but systems for keeping the tank pumped up for long periods will add complexity and significantly decrease service life. And the temperature swings are too violent to tolerate, dropping from 1000K-10000K+ to near-zero over the span of a few moments is not something you can afford to repeat hundreds of times as you spiral out of orbit.

As far as using water as feedstock to make propellant, this isn't a very good idea: it doesn't matter what chemistry the propellant is before and after entering the engine, so long as the same average particle mass is the same, there is no performance benefit. You would need to achieve some quite extreme temperatures to ionize the majority of the propellant, and even if you did it would still weigh, on the average, six times as much as ionized Hydrogen only. Oxygen plasma is also much more destructive than Hydrogen plasma I bet due to its sheer chemically corrosive power.
__________________________________________

The viability problems aside there are some practicality ones too: the size of collector needed and coupled with a pretty complex engine to make worthwhile amounts of thrust at superior specific impulses is a tall order. Absolutely it could be hard or even harder than comparable nuclear engine concepts, plus the compromises necessary for solar thermal to compete severely limits its versatility.

First of all, to put things into perspective, lets look at the performance being talked about here: chemical rockets can achieve an Isp in the 450sec region. Simple solid-core nuclear and solid-target solar thermal engines can hit into the 1000sec region or so. Only the advanced gas-core/gas-target nuclear and solar thermal engines have any chance of reaching 1000's of sec Isp. Electric ion drive can probably do at least 5000sec albeit with quite low thrusts.

Selecting which technology to go with ought to be weighed against what cheap chemical rockets with aerobraking can do: <1000sec engines simply do not offer a great deal of improvement over chemical engines given their price. NERVA engines, for instance, only decrease the "fuel bill" from LEO to LMO by about 30-40%. Or if trip time is a concern, the higher Isp only shaves a month or so off the trip for the same amount of fuel. Only when we start talking Isp in the 1000's range is there a really substantial, worthwhile decrease in the fuel bill or trip time for a mission to Mars. It is worth noting that a solar-thermal vehicle would not have the option to aerobrake without retractable/replacement collector elements.

As far as low-performance nuclear or solar rockets being reused for the return trip or for multiple missions, I don't think the payoff is there either. All the fuel for the departure from Earth and from Mars has to come from Earth, and this fuel has to ride in a big insulated fuel tank right? So it makes more sense to slap a cheap chemical engine on the back of the fuel tank and skip the whole reusable drive section entirely.

A few billion to develop and build either NERVA or low-end solar thermal would buy an awful lot of J-2/RL-10, nor would it cost much more to launch one additional Ares-V to make up for that 30% fuel bill penalty for chemical. The only propellant in either engine that gives these performance numbers is Hydrogen too, so chemical would have a far reduced boiloff issue. So unless we're talking high-Isp, I don't think either solar or nuclear engines make any sense. Its just easier to bring more fuel (in smaller, simpler tanks) and cheap chemical rockets. Low-performance engines like NERVA or SOTV are not good deals.

Now if we're talking high-Isp engines, I think the case for solar thermal is pretty weak. First and foremost, is simply the scale for a massive mirror to get rocket-like thrusts is prohibitive, on the order of 10,000,000m2 give or take I figure. I don't think that any collector of this magnitude will be easy to develop, fabricate, deploy, aim, nor focus reliably. So if you extend the firing time to weeks instead of hours like a nuclear engine, this does get you down to a less-than-crazy sized collector.

Except the GCNR engine works just as well at any distance from the sun, doesn't have to spiral in/out of either orbit, nor waste time decelerating for orbital capture until actually reaching the destination. And except the GCNR engine doesn't need to develop, deploy, nor care for the large collector. And except it has the option to move a large masses efficiently or execute a fast flight with much relaxed departure windows. And finally except the engine would have other uses, like manned missions to Jupiter where the sun is too dim for solar thermal.


"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw

The glass is at 50% of capacity

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#6 2007-04-21 07:02:27

neviden
Member
Registered: 2004-05-06
Posts: 99

Re: Solar Thermal Propulsion

I agree that simple nuclear/solar thermal engines are only marginally better than chemical rockets and probably not worth the trouble. They are limited by the temperature of the core/heat exchanger to maximum isp 1200 (first picture).

To get any higher we have to deal with temperatures that will melt anything or use electricity to accelerate propellant to high speeds. Higher the speed, higher the isp, smaller the need for actual propellant.

Actual thrust is not that critical. We can always trade smaller thrust for longer trip times. This is not that critical, because we would have to spend time on route measured in months, time to actually do anything in months or years and time to return back in months no matter which propulsion we use. That means our spaceship would have to be big enough for crew to live in for years, with multiple redundant life support systems and everything else people would need to survive and function. And that would probably also include growing food (grains, vegetables), enough supplies to last for a long time, good protection from space radiation, maybe even artificial gravity (spinning habitat). All this would be heavy (couple 100 MT) and would require multiple launches and basic assembly. To get this thing moving anywhere you would need couple of times it’s weight in propellant (launched from Earth on expensive rockets) if using rockets or by taking more time to get anywhere by using high isp engines. High isp engines could take months to get this spaceship from LEO (where it would be assembled) to high earth orbit and nobody would care, because the crew would be launched from Earth only when this spaceship would be waiting in high earth orbit and would need only small push to get on it’s way. Once on it’s way it wouldn’t matter if it takes 2 or 6 months, since they would be protected and could use that time for final preparations and training before the actual mission. The stuff that would land on Mars could still airbrake, but the actual spacecraft could stay in high Mars orbit in case of problems on the surface. The crew could always return to spacecraft and wait there until they could return. On the return back the crew would use the same craft that brought them to high earth orbit to return back to earth and spacecraft itself would stay there ready for another mission.

Any kind of high isp engine system will be complex, since we are talking about high temperatures or high speeds. But the biggest problem in space is not lack of power but lack of cooling. Creating big mirrors in space is not that hard (think solar sails), but if you focus all that power onto solar cells with 40% efficiency (not to mention that solar cells degrade with time), that run engines at 40-80% efficiency (how much of the energy is actually spent on ejecting propellant), that means a lot of heavy cooling.

If we could skip “conversion to electricity” part and could reduce cooling needs or could use a lot more power if needed. It could get us to Mars, Asteroids, do heavy moving around Earth orbits,.. Jupiter would need a nuclear engine, but I don’t see humans around Jupiter anytime soon.

Yes, GCNR has more thrust it’s better and so on, solar collectors would be big, hard to focus, everything would be very hot, thrust would not be all that high, but from what I read it could be done? Maybe not in GW range then maybe in KW range? Could solar radiation be concentrated to the degree that it would produce gasses hot enough to get us high isp?

(I am not asking anyone to actually build it, only if this concept has some fundamental problems that couldn’t be overcome.)

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#7 2007-04-21 19:03:14

cjchandler
Member
From: canada
Registered: 2006-06-24
Posts: 138

Re: Solar Thermal Propulsion

I've always been partial to solar thermal, but only because it might be easier to build one in-situ that to enrich uranium, (other wise I'm all for NSWR) Maybe to get a better isp you could use some variant of ablative laser propulsion. Either a solar pumped laser (cooling could be a big problem) or use some fancy cooled secondary mirrors to have the focus of a constant source of light pan across the propellent block to simulate the fast pulses needed. I have no idea if that is possible, but it could give some fantastic isp (up to 20000). The paper I saw it in seems to have a dead link now, but here is a google cache without the pictures-

http://72.14.205.104/search?q=cache:brK … cd=2&gl=ca


Ad astra per aspera!

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#8 2007-04-23 11:42:48

C M Edwards
Member
From: Lake Charles LA USA
Registered: 2002-04-29
Posts: 1,011

Re: Solar Thermal Propulsion

Regarding the "sudden" transition from high temperature operation to cryogenic temperatures, the thermal mass vs. absorbtion area requirements are such that it would be unlikely to be as sudden as GCNRevenger is implying, even if the engine pumps are never shut down.  The average solar thermal rocket probably simply could not handle enough fuel to act like you'd stuck it in a bucket of ice just because the lights were turned out for a few minutes.  The thrust is bound to be small at high temperatures, and that means vanishingly small fuel consumption rates, too.  It probably could not cool off fast enough to do real damage.

However, you still would not want to douse a graphite heating element or any similarly brittle material in this fashion.  There are nickel alloys that can take that kind of quenching, fast and repeatedly (steels, too, but none of them very compatible with hydrogen), but continuous operation would require operating temperatures less than 2000 K.  With hydrogen, that limits any practical solar thermal rocket (i.e., one that you can shut down on one side of the penumbra and restart on the other without cracking the engine block) to around 890 ISP.

That's the major problem with solar thermal rocketry - the practical temperature limit is significantly less than that of chemical rockets, and the thrust is probably miniscule.

That said, I still favor them for a lot of low thrust applications, and I don't agree with the assertion that their performance won't be much better than chemical rockets.  It's not sensible to dismiss an entire class of rocket engines with a 1.98 times higher ISP than the best chemical rockets just because it's not 2.00 times higher (or 2.23, as the case may be).  And there is no evidence that the difference in reliability between solar thermal and conventional chemical will be significant, either.  The only difference will be that chemical engines with similar ISP will get all their malfunctions over with in the first ten minutes.


"We go big, or we don't go."  - GCNRevenger

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#9 2007-04-23 15:24:37

GCNRevenger
Member
From: Earth
Registered: 2003-10-14
Posts: 6,056

Re: Solar Thermal Propulsion

I don't agree, thrust is important for manned space travel, with both high Isp and high thrust, cutting the usual 6mo down to 4-8 weeks is attainable. Mars in a month. 6mo is near the limit of reliable human endurance in zero-gravity/confinement; so, we're presented with two choices, either a big slow ship with lots of volume, shielding, and greenhouses or a smaller faster (and cheaper) ship, zero-g only with fewer psychological comforts. The deal about "use that time for final training" is silly, they won't be any more trained for the mission than when they head to the pad, if anything their skills and prepared-ness will suffer the longer the trip is.

With high-temperature solar thermal, only the former is possible, the latter requires substantial thrust. You can still do high-efficiency cargo the slow way with a high-thrust engine, but you can't go fast with low thrusts. Low-thrust engines just aren't versatile and will wind up being more expensive. High thrust engines would also let you go to/from Mars most any time you want instead of every other year, albeit at the cost of some fuel efficiency.

Okay done preaching about the evils of low-thrust propulsion, and on to the feasibility of a high-temperature solar thermal engine:

Creating big mirrors in space is not that hard (think solar sails)

Absolutely not, this is wrong and a bad analogy. Such a mirror is not at all like a solar sail, which need not be slewed back and forth every hour or so of the orbit, nor has the benefit of spinning to hold its shape, nor has to focus with precision onto the target. They aren't the same, and even then,  nobody has really ever deployed or used much of a solar sail to date. The cooling is a secondary concern to figuring out how to make a giant mirror work well in space.

For instance as you start going into the shadow of the orbiting body, you have to reorient the mirror with respect to the rocket in one direction, but when you start coming out of the shadow some minutes later you must reorient it in the opposite direction. Just like taking a long piece of wire at one end, swing it one way and then the other, what happens? The wire starts to sway back and forth of its own accord due to the "springiness" of the wire and its inertia. A similar thing must be dealt with here, and there is no way a flimsy gossamer support structure will be completely rigid, yet the mirror will not have room for error to focus the beam.

I am not convinced it can be done yet either, the "target" will undoubtedly be some kind of nonreflective dust, but can any material survive heating to those sorts of temperatures? To get really high Isp, we're talking about tens of thousands of degrees here, which would boil most any substance known. Will a vaporized "target material" absorb light anymore? Will it damage the engines' workings? How much will you need if it can't be recycled?

I reject the nonchalant attitude about the thermal cycling of such an engine too, it is not trivial when we're talking about the engine (not the target material) likely operating at thousands of degrees only to have the heat taken away for half an hour every ninety minutes, whether Hydrogen is being pumped into the engine or not. It doesn't matter if the damage inflicted on the engine is very small, the aggregate number of thermal cycles for a multi-month climb from orbit add up. And what about the turbopumps?

Also, the idea the engine being left on for long periods during transit being no worse than for the minutes or hours of a high-thrust engine is silly, many changes that occur very slowly at high temperature won't have time to cause damage during a short firing, but will for a long one (material creep under pressure for instance).


"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw

The glass is at 50% of capacity

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#10 2007-04-23 20:07:40

GCNRevenger
Member
From: Earth
Registered: 2003-10-14
Posts: 6,056

Re: Solar Thermal Propulsion

Upon further thought...

On the issue of mirror supports wagging back and forth like a twanged antenna on a car, in fact once you set the thing into motion, you wouldn't even have to swing it back to induce the oscillation, just stopping the rotation would do it.

On the "target dust," would whatever form it is converted into upon heating continue to absorb heat until it becomes hot enough to achieve 1000's of seconds Isp? Gasses can absorb light, but how well compared to the solid form? You could throw more target dust into the engine, but would the temperature of the mixture increase? And just what kind of mass are we talking about here in addition to the LH2?
__________________________________________

Medium-efficiency (~1000sec) engines really aren't all they're cracked up to be, the tankage and boiloff penalties associated with large amounts of liquid hydrogen versus hydrogen and oxygen per-gram cuts your "fuel bill" by only about a third for low energy biannual Earth-Mars trips.

So yeah, I am dismissing them out of hand for TMI use. "Show me the money" so to speak, how can development, construction, and deployment of a complicated new technology make up for just bringing a little more plain old fuel?


"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw

The glass is at 50% of capacity

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#11 2007-05-06 14:10:25

neviden
Member
Registered: 2004-05-06
Posts: 99

Re: Solar Thermal Propulsion

I think the precise targeting could be done, but it would require more mass to make everything stiffer. The question is if carbon laced hydrogen could get isp in the range of 3000 – 5000 seconds to make everything worth it. that's the unknown.

But if not, then it could focus solar energy onto a big, well protected from radiation, cooled, high efficiency solar panels or some other heat to electricity converter. This would require a lot less precision and there are many electric propulsion methods that can be used. Electrical energy could power something like direct drive hall thruster. Something in the range of 1 - 10 MW would be quite possible and could do most of the interorbital transportation for the fraction of a propellant ($$$) needed otherwise.

0.6 MW SEP tug: http://www.entechsolar.com/SLA-SEP-WCPEC4.pdf
5-10 MW NEP mission to mars: http://gltrs.grc.nasa.gov/reports/2006/ … 214106.pdf

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#12 2007-05-07 09:06:33

GCNRevenger
Member
From: Earth
Registered: 2003-10-14
Posts: 6,056

Re: Solar Thermal Propulsion

This is the worst Mars mission ever!

It has the worst of all possible worlds, it still has just as long flight a time as chemical, long-duration soak in the Van Allen belts plus close approach to the Sun, and only spends 60 days on Phobos and Deimos!! NO! *bangs fists on desk*

Did you even read the links? Thats what the second one is proposing. Furthermore, if you read closely, they totally discount the need for heavier radiation shielding due to the Solar close approach., and basically summarize that there is no way to avoid this with their mission plan.

Their engine of choice is also an expensive MPD thruster and not a Hall or other ion engine; the ion engine will be heavier and require the extremely expensive fuels to reach usable thrusts.

And if you do use solar, you can't spin the vehicle to produce gravity! It would be impossible to keep the arrays aligned with the sun without an unreasonable fuel penalty.

Finally, they include no fuel to brake into Earth orbit at the end of mission, the vehicle is sent on a flyby trajectory for disposal while the crew comes back down in a capsule.

Edit: The SEP tug to the Moon makes little sense either, inflating the cost of an Ares-V launch to $1.4Bn! They also assume only 400sec Isp for the chemical vehicle, a full ~50sec less than J-2X should achieve. Its easy to win when you lie about your competitor.


"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw

The glass is at 50% of capacity

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#13 2007-05-07 11:24:03

neviden
Member
Registered: 2004-05-06
Posts: 99

Re: Solar Thermal Propulsion

I agree that (NEP to mars) is the worst mission ever. It's like they tried to make it as bad as possible. But, that is not why I put those links there. I would combine these two ships into one craft.

What I did like was rotating part, and if you replace (only 10 times bigger) radiators with hardened solar panels (300 W/kg) and direct drive Hall thruster you get something that would still go have to through Van Allen belts (that part could be unmanned, so it's not that big of a deal), but could get to Mars in reasonable time. If you change the mission to Conjunction class, you get something that is half descent.

I further explained this idea in separate thread in “human missions”, so it can be discussed and analyzed (or trashed if deserves to be trashed).

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