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What I was originally thinking was apparently based on safe levels for humans for microwave transmission. Apparently, in 1978 design studies were looking at a 10km rectenna for 750 MW, so that's about 350m for a 1MW electric thruster or VASIMR, and that almost has to be heavier than 10,000 kg. Look under microwave transmission en.wikipedia.org/wiki/Wireless_power_transmission (I know, wikipedia).
Of course, in space we could definitely do more. Plus more recently, it looks like they've been trying a lot more concentrated beams. (see this paper http://www.sspi.gatech.edu/wptshinohara.pdf). So that could work.
Maybe in a few decades, but there won't be a massive off-world electricity market for quite some time, so it won't help much in the near term. And sorry, I meant rectenna (receiver). I remember reading that they had to be surprisingly large for a sizable amount of power.
Why would you think it would end up being as heavy? For a 1MW VASIMR, we have been finding a few tons, at most, at Earth orbit, and, more importantly >200 W/kg in terms of power/weight, which is better than the small nuclear reactors we currently have (I don't remember the figures, but I remember them being very small). Even at Mars orbit, you can only divide that 2.25. It seems that shows that solar is simply better than small nuclear reactors we have, and is continuously developing already, without any additional expenditures. More importantly, it seems to me that the weight of the engines and fuel is as large as that of our power supply, so I don't think the needed breakthrough is in power system weight, I think the question of one of which is better - chemical rockets with fuel depots, or electric engines with solar - and though it seems to me that the former may be better, I don't think the power system is the deciding factor.
And I didn't mean chemical VASIMR - that's just a bad idea.
Terraformer, beaming seems like a good idea, although the infrastructure might be expensive. Although I think I remember reading that a very large (and heavy) antenna is required to beam much power, so that might defeat the purpose it is bad enough. Perhaps laser power beaming, though? Difficult, but possibly more efficient. Also, I doubt space based solar power can gain much of an advantage over Earth based power, even with launch costs as low as $100/kg, at least not with solar already at grid parity at many places on Earth.
GW Johnson, I should have time to respond to your post(s) now.
First of all, your technical arguments seem sound, my main point of disagreement would be political and cost-wise.
Nuclear engines (and chemical engines for that matter, albeit to a lesser extent) do have the advantage that they can refuel anywhere with pretty limited infrastructure, and I would agree with you, high thrust, high-Isp is ideally the way to go, but at present one of the only ways to do this is nuclear (I am sure I am missing some very important propulsion systems here). For the near future, nuclear seems like a bad choice for me. Current politicians don't seem to support manned spaceflight to begin with, and the NASA bureaucracy and even larger aerospace companies (to a lesser extent) seem frightened of nuclear. I don't really want to convince the space community that nuclear is the way to go, because even if it is, space advocates are a small group, and I doubt our collective yearnings will push policymakers. So, if we are going to push nuclear, I think it is sort of a Dorsa Brevia moment - we need to decide on the desired method of propulsion (the goal) but we really can't even think about that without addressing the means of making it happen.
The fears over NERVA are not entirely poorly founded. Just as importantly, the fears over nuclear in general are irrational in both strength and breadth. It seems everyone has qualms about it - it is not politically correct to feel otherwise. How this came to happen, I am not sure, but it is a problem that must be dealt with. If we want nuclear, we will have to have a strong, widespread push asking Congress - why not? Why is it dangerous? Is it dangerous to the budget? Why not cut some defense budget to open space to economic development, colonization, and exploration? It will take a big push, and I think any discussion of it needs to expand to include that. It may be doable, but we should not put all of our eggs in the nuclear basket.
Cost-wise, we should avoid confusing mass and delta-v with cost - they are correlated, but not absolutely related - and the key number is definitely the cost. At the end of the day, the $/kg to destination is, imho, the measure of a propulsion system. Launch window frequency, travel time, etc. are also important factors, but I think we can all agree $/kg is what makes or breaks the big-time development of space in the near term.
For instance, you mentioned the low T/W ratios of VASIMR and electric engines increasing delta-v, which does happen - but for a sufficiently high isp engine, a doubling of delta-v doesn't matter much. Your fuel load increases, you have to carry more life support for manned trips, but if you can reuse any of the machinery associated with that, that is all chump change. The key is - can the engine be refueled and reused. Can the same be said for any machinery you bring along? I see chemical, electric, and nuclear all being capable of this. Chemical we have the most experience with, and it can be refueled anywhere in the solar system, with somewhat more difficulty than nuclear if you want to use, for instance, CH4 instead of H2. Electric we have little experience with, so they may be expensive, but it has great Isps. Unfortunately, the noble gases are on Earth and Mars, not in space, so refueling in orbit may be difficult. However, they use very little fuel so this may not be an issue, and it may be cheaper than refueling chemical rockets (that is a whole other topic, which it would be nice to have a thread on). Nuclear is the end all, be all, because of the capability for high isp and high thrust/weight, but the political challenges are difficult. That is a war to be fought - rationalists against irrationalists - and one that with sufficient forces is hard to loose. So, I concede that electric may not be the best method of propulsion. However, what advantages does nuclear possess that refueling chemical in orbit does not, in terms of cost. Nuclear will have to be redeveloped, at great cost, and the issues of reusability from is still there from Earth to LEO. If that problem can be solved for nuclear, it can be solved (VTOL, probably) for chemical rockets. Once in orbit, their advantage is primarily in terms of Isp. The refueling locations for chemical and nuclear are mostly the same, but we have less development cost and a nearer-term implementation. Overall, the cost is the cost of development + the cost of the booster + fuel / how many times you reuse it. If interplanetary boosters could be reusable chemical or nuclear rockets, the cost would plunge, just as for launchers. For the near term though, it seems to me that the cost of developing a reusable interplanetary chemical booster would be basically 0. Nuclear could cost a lot. It seems to me that the cheapest option is to develop cheap launches and/or asteroid mining to get cheap fuel to orbit and then use a reusable chemical booster, which would not have to be much different from current boosters, it could just circle around and return home. It seems like it would just require bigger, longer fuel tanks to store the extra fuel. That shouldn't matter if you are going to reuse it, and if the fuel is cheap, both of which could be true.
As for testing propulsion on the Moon. That would be extremely expensive. Given how little money NASA has, I don't think we could do that. As for mining-for-profit, what exactly would we mine on the Moon that we couldn't get from NEAs? What is there, other than He-3, that is valuable enough to bring back to Earth? Like I said in my last post, the Moon seems a very costly diversion from developing the technologies that will allow us to begin working towards those kinds of big goals. It seems like an economic backwater - launch windows aside, as JohnNH4H brought up.
Also, just want to check - are we all agreeing now that VASIMR would not require nuclear reactors? That was the original post, though I am pleased with how this discussion has diverged from that.
And thank you JohnNH4H, by the way.
I am short on time, so this will probably be a short post.
I agree with you on the refueling in space. If it can be done, it could be much cheaper than a more expensive low thrust drive (unless you can get the fuel for the same in space as well, which might be worth looking into). You do have to consider the fuel source (which I will come back to). It's also a nice near term solution which develops some off world industry in the course of solving a problem, which is good.
On the Oberth effect - if you could explain the Oberth effect and the triangle more in the future, it would be helpful. I believe what it comes down to is the energy difference of a delta-v at a high velocity is greater than at a small velocity, because K = 0.5mV^2. But I may be wrong.
For radiation - the extra time spent in the belts is a problem, but, like you said, a small chemical burn can solve that. Even some large-retros should be enough to solve it for cargo. As for radiation concerns for humans, I thought Zubrin already killed that worry. If you're going to Mars, you're probably willing to take some risks. It is not optimal, but it's really not likely to reduce your lifespan a lot. I would be a bit more concerned about red lung disease among early colonists.
On escape velocity - I couldn't help but correct, but certainly the delta-v required should not vary if you depart from the same orbit. Escape velocity is, after all, simply an expression, in terms of kinetic energy, of the depth of the gravity well, which should not depend on using a high-thrust or low-thrust engine. I think this would bring the 8 km/s delta-v you were using down to 6.7 km/s (as you previously showed).
On your VASIMR calculations - how many engines and how much power were you using (a single 200kW). About argon, it is not present on the Moon, but it in the atmosphere on Mars, and on Earth (not sure how you could harvest the latter, but it could be doable).
Now, about the Moon. I don't have time to lay out everything I have to say on the moon, but let's examine the choice between NEOs and the Moon for fuel and general development. The asteroids possess most heavy metals, with rare-Earth metals in spades, even among the stony ones. Carbon compounds are also present on the asteroids. While some may be present on the Moon, we don't know how much or in what form. Though the LCROSS results did show CO and CO2 (http://lcross.arc.nasa.gov/observation.htm) which may be promising. Carbonaceous chondrites have pretty much all kinds of organics, and stony chondrites do have carbon, albeit in very low (0.3-2% by weight) concentrations (again, the Moon probably does little better, and accessibility is hard to compare). This is important because it is possible we may need to manufacture CH4 (example, Musk's Mars rocket). However, the asteroids have something to drive their development - rare Earth metals. The Moon, on the other hand, does not. Moreover, if one is to mine the moon robotically, it offers a slew of challenges. Asteroid mining, as planned by groups like planetary resources, on the other hand, is intended to be a robotic affair. The presence of organics on asteroids (and certainly on carbonaceous NEOs, which should be accessible by the same technology as the stony ones containing rare earth metals) as well as water makes them great fuel stations that can pay for their own development. The Moon will have a much harder time doing so. Also, the technological challenges are greater. On the Moon, you will have to not only develop mining equipment and processes, but also land them with precision, have them operate in -200 C cold, have them perform chemical operations to manufacture fuel, and have them export this fuel to Earth using either reusable rockets, perhaps based on something similar to Musk's Falcon 9 reusable, or have the industrial base to build rockets. Now, doing all this robotically is certainly doable, and it is aided greatly by the opportunity to use telerobotics, but it is much more difficult than mining NEOs. And doing either with humans is almost not an option. The costs would be astronomical and make the cost of fuel produced, while still advantageous, not the revolutionary change we need. The NEOs can pay for their development, the Moon cannot. That is a big deal.
Sorry GWJohnson, I will respond to your comment next chance I get.
First, it wasn't 10000 kg of solar panels for 1000 kg of cargo, it was, at the most pessimistic, 10000 kg to 25000 kg of cargo, and this included the complete power system, not the panels alone. Also, I think I can narrow it down a bit more now. This NASA project http://sbir.gsfc.nasa.gov/SBIR/abstract … SBIR_11_P1, is building 200-500W/kg panels. Assuming this, 350W/kg, is a reasonable estimate, we get 2900kg/MW of power, which is 25 N of thrust.
Also, you are right about low thrust engines requiring higher delta-vs (as I mentioned). I did not take this into account. If you can find accurate figures for this, I will use them. This page has some, but they are heavily contradicted by other numbers from throughout the internet. http://www.lr.tudelft.nl/en/organisatio … a-v-value/ But I will use 8 km/s to be conservative.
The Numbers:
(This is a long section, but please at least read the last few sentences)
So, let's say we use five ISS class 200kW VASIMR engines. The arrays weigh 3,000 kg and overall it probably weighs from 10,000-15,000 kg dry. Add 25,000 kg of cargo. Assume VASIMR has an exhaust velocity of 50 km/s and a thrust of 5.7N/200kW, which is 28.5N/MW. (http://www.adastrarocket.com/aarc/VX200). Using the Tsiolkovsky equation and a delta-v of 8 km/s, we get Mo/Mf = 1.173, so 15% of the mass is fuel. So, we have a 15,000 kg rocket producing 28.5 N of thrust. This is easily capable of carrying 25000 kg to Mars orbit with little fuel expenditure. Even assuming the 53,000 kg payload of a Falcon 9 Heavy, we can show that it takes ((53000 kg + 15000 kg * 1.17 (mass fraction)) * 8000 m/s)/28.5N = 260 days. A long trek, but acceptable for cargo. If you use a round trip delta-v of 16 km/s, you get a mass fraction (Mo/Mf) of 1.377, or 27% fuel by mass and takes ((53000kg + 15000kg * 1.377 * 8000 m/s)/28.5N = 303 days. However, now the engine and the panels can be reused, so we have a cargo cycler. The cycler can be fueled with argon in LEO and later argon from Mars at the end of it's trip and reused several times at the very least, if not more. Ideally, you would scale it up to larger scale, however, it is may be much more efficient than using chemical rockets for Martian cargo, especially for large payloads, which would require massive rockets. It is debatable which would be cheapest, but it is worth investing in. And it is definitely not the power requirements that make it problematic.
Also, fuel, tankage, and volatiles from the Moon? How much will it cost to develop that kind of lunar industry? Also, NEAs are better, closer targets for such resources. They have just as much water, and more heavy/useful metals, and they are not as far from Earth delta-v wise.
I don't really care about speed - the only real reason to be concerned there is radiation and time, and if you are planning on going to Mars, you have time, and radiation is not a major concern (as Curiosity recently showed). I am far more concerned with price/kg. For cargo delivery, any electric system, if the engine and power system are not too heavy or too expensive to develop, is going to be better than chemical rockets for this.
As I mentioned though, the power systems I was seeing did include the complete system. In the case of the study I posted, they included batteries, and everything you need on a Juno or Cassini like probe. The only difference I can think of would be the high voltage, which would not weigh that much. So,
Power systems: I saw several studies showing systems (structures, batteries, wiring, panels, everything, reaching 300-700 W/kg). The study I posted was looking at outer solar system, cold optimized panels, and they were showing 105W/kg at Earth orbit and definitely accounting for all the incidental needs produced by a large solar system.
Thrust: Wikipedia shows VASIMR as having 5N of thrust at 200kW. If delta-v to Mars is 3.5 km/s, 5N is enough to push 1000 kg of cargo to Mars transfer orbit in 8.1 days. Of course, multiple engines could be used, and, though some inefficiencies arise with longer acceleration periods, a 40 or 50 period is certainly acceptable, and would allow for 5-6 times as much cargo to delivered. Also, as I mentioned, the engine scaled nicely from 50kW to 200kW (10x thrust/ 4x power).
Overall: 5 200kW engines could get 25 tons of cargo to Mars. From what I was seeing, the power systems required would mass <10,000kg (based on a 100W/kg figure, which, due to the outer planet missions study I saw (http://www.lpi.usra.edu/opag/nov_2007_m … _power.pdf) seems to be a hard lower limit) and possibly as little as 1400 kg (based on 700W/kg, a figure I saw referenced for Earth orbiting satellites in a NASA paper from what I think was 2007 as well). Bottom line, the power systems are probably not the problem, and it, like all electric engines, will be more efficient at delivering cargo, which will be important for Mars colonization. I think it should definitely be looked into for cycler. Bottom line, we may not need it, but for it seems like a good investment, which means, for the little we are spending on it, we should continue, if not do more. Why are so many people bemoaning the need for light nuclear power systems to power the thing?
Did anyone else see this when they visited newmars today (if you visited through google, firefox, or chrome)?
Bob Zubrin is big critic of VASIMR, one of his main points has been that VASIMR doesn't have a power supply. I was looking at wiki the other day, among other places, and I was seeing total solar systems in the range of 300W/kg - 700W/kg. That's a wide range, but the largest VASIMR engines currently proposed, the ones they are going to try on the space station, are 200kW. That's only 670 kg of solar arrays (and associated junk), even at the unoptimistic end of that range. That's not much, so why is it I am always hearing that VASIMR needs nuclear to be effective. It seems like solar electric works fine for the inner solar system. My basic question here is, what am I missing? Why is solar electric VASIMR written off?
They are going for 5N on the 200kW engine, which is capable of accelerating 1 ton 3.5 km/s in 8.1 days. This is certainly sufficient for smaller payloads, and even for large payloads, a number of such engines could be used, or a scaled up engine. The scaling from 50kW to 200kW apparently went from 0.5N to 5.0N, so, this seems promising. I will agree with Zubrin we don't NEED a VASIMR-type engine, but it certainly could be useful, especially in making Earth-Mars travel cheaper.
Note: This was the closest I could find to what I was seeing the other day, which was a NASA study of existing panels c. around 2007, but this is a NASA study for outer solar system missions. Even here you can extrapolate >105W/kg from the figures, for a complete solar system, including all the unexpected side changes you have to make. This changes the game some, not much though.
http://www.lpi.usra.edu/opag/nov_2007_m … _power.pdf
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