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#1 Is it hype? or a possible real answer?

Posted: Tue, 19 Apr 16, 23:14 pm
by Commander
Okay, I know. When dealing with NASA nowadays this question could be asked about almost any endeavor they undertake. However this article has way to many buzzwords in just the title alone. The article is called NASA Works to Improve Solar Electric Propulsion for Deep Space Exploration. So let's see, there's: Solar; Electric; Deep Space; and Exploration. So out of an eleven word title, nearly half are buzzwords.

As many projects in the NASA welfare programs, this one is a big budget ($67 million) with a payoff in the "not that far off" future.

The problem with this solar powered ion propulsion system concept, especially for deep space missions, is that like all solar powered machines, if you run out of light, you have no energy. I'm straining my grey matter trying to remeber all the pertinent information, and this would probably be better left to Luke, but I'll stick my neck out.

Ion engines are not like those in fiction (Star Wars T.I.E. fighters, or Twin Ion Engine Fighters for example). While great speeds are possible, they do not have great acceleration. The other problem is the solar component. the further from the star, the less light is received and able to be used. So if you're in deep space, you're not getting great amounts of power. ... xploration

#2 Re: Is it hype? or a possible real answer?

Posted: Tue, 19 Apr 16, 23:21 pm
by Commander
Follow up on the ion propulsion hype.

NASA to Discuss Latest Developments in Solar Electric Propulsion for Future Deep Space Exploration ... deep-space

Just to make it clear, I'm not against the technology being developed. just the hype.

#3 Re: Is it hype? or a possible real answer?

Posted: Wed, 20 Apr 16, 00:59 am
by Trident
Maybe the solar collectors stay near Earth, so they collect lots of solar energy. Then its just a matter of taking a really long extension cord with you ...

#4 Re: Is it hype? or a possible real answer?

Posted: Wed, 20 Apr 16, 13:57 pm
by Joe Wooten
The Juno probe to Jupiter is using HUGE solar panels for power due to the shortage of PU240 for RTG's. The greenies back here have been on NASA's case for decades about nuke power units, but solar PV is just not practical for anything past Jupiter. If you look at the orbit Juno is taking around Jupiter, it's also obvious the PV power panels are limiting the ability of the mission. It will not last as long as Galileo did or Cassini at Saturn.

The good news is that the Dept. of Energy has re-opened PU240 production again, so in a few years there will be enough for missions to the outer planets. It's time we sent new probes to Uranus and Neptune.

#5 Re: Is it hype? or a possible real answer?

Posted: Wed, 20 Apr 16, 14:29 pm
by Commander
It always amazes me how the greenies are so against nuclear power, even in space. I sometimes get the feeling that if they could find a way, they would turn off that great nuclear reactor in the sky. :lol: :P

#6 Re: Is it hype? or a possible real answer?

Posted: Wed, 20 Apr 16, 15:52 pm
by Joe Wooten
Commander wrote:It always amazes me how the greenies are so against nuclear power, even in space. I sometimes get the feeling that if they could find a way, they would turn off that great nuclear reactor in the sky. :lol: :P
They are completely irrational on the subject.

#7 Re: Is it hype? or a possible real answer?

Posted: Thu, 21 Apr 16, 04:31 am
by luke strawwalker
Thanks for the vote of confidence... On my way to Indiana so I might not be able to check in for a day or two.

Yeah, solar electric is of limited use for deep space, particularly for propulsion of manned missions. It has its place, but manned operations probably aren't it, at least not in the "standard" form for "electric propulsion" (meaning ion engines).

As you pointed out, the intensity of solar power drops off RAPIDLY with distance from Earth away from the Sun (IIRC from memory, Earth gets about 7 watts per square meter of solar energy). Due to the inverse square law that states the intensity decreases on the square (or square root) as distance doubles, (light intensity-- might not be describing it right but you get the idea from basic high school science class-- inverse square law says if you cut the distance in half from the source, the energy intensity (light/heat) squares, or 4X at 0.5 the distance... or 8X at 0.25 the distance, etc.) Anyway, at the Mars orbit distance, you get about HALF the solar energy you do at the Earth orbital distance from the Sun. Jupiter is much farther out still (nearly twice as far from the Sun as Mars) so it gets like 1/16 the solar energy Earth does at 1 AU distance from the Sun (roughly speaking, off the top of my head, and I'm not taking time to look it up tonight). That's why the probe you mentioned is using THREE solar panels, each the size of a 40 foot 18 wheeler flatbed float (trailer) in order to generate a rather meager and pathetic amount of power for the probe's instruments-- I want to say on the order of like 100 watts?? (I read about the probe awhile back but it's been a long time ago, so I might be misremembering... I know it's a LOT of solar panels for a pittance of power, and they're having to be extremely miserly with even that! That's why there's really NO substitute for nuclear at those distances, period. BUT, if you can't get approval to get an RTG and without one your mission is DOA, you figure out how to make it work with solar panels, I guess, even if it means launching a shoebox of micro-instruments attached to 3 semi-trailer size solar panels...

Even for surface ops on Mars, solar just isn't a good fit... essentially, for all intents and purposes, you'd have to launch TWICE the amount of solar panels that you would actually need at Earth to get enough power at Mars. That's a lot of weight and expense.

When you're talking about propulsion, you're right-- and mass is the ABSOLUTE KILLER. Remember high school science class-- F=MA... force equals mass times acceleration. Manned missions are MASSIVE by definition-- we can shrink instruments, electronics, etc, to almost miraculously small proportions-- but you can't shrink humans-- and they need "X" amount of food per day, "Y" amount of water per day, "Z" amount of oxygen per day, and systems to prevent them from freezing to death or roasting to death and scrub the carbon dioxide out of the air so they don't suffocate on their own exhalations, radiation protection, clothing and equipment (we don't want crews to fly bare-assed) and all that equals WEIGHT that cannot easily or practically be shrunk down (much). So, by their nature, manned missions are MASSIVE. Deep space exploration CAN be done by 3 guys in a can with half the interior volume of a minivan, but only for about a two-week round trip to the Moon and back, including perhaps maybe a week-ish there in lunar orbit. Any longer and it just becomes ill-advised. So, basically for any deep space exploration missions beyond about 2, maybe 3 weeks, you need a hab module of some sort-- more mass.

Now, to 'shorten the trip' you have to accelerate that mass up to very high velocities (delta-v). You also have to do it EFFICIENTLY, not only in terms of propellant (mass) but also in terms of time (delta-t). This was done during Apollo with the S-IVB stage, powered by a 200,000 lb thrust rocket engine to accelerate the manned spacecraft from 17,000 mph in LEO to 25,000 mph on a translunar trajectory through escape velocity. A few minutes burn time was all it took to accelerate them up to speed, and then a 3 day coast out to the Moon, with most of that time being spent "out there" coasting at low speed "over the hump" from the Earth's gravity well into the lunar gravity well (at that point, the spacecraft had decelerated "coasting uphill" until it was only traveling about 2,000 mph as it moved from the Earth's sphere of influence into the Moon's sphere of influence.) Now, you COULD do it much faster-- Pluto New Horizons made the same trip in around 8 hours IIRC... BUT it becomes incredible fuel-intensive to then decelerate enough to go into lunar orbit... so it's all a tradeoff. PNH wasn't going to the Moon, so it needed every drop of speed it could get to zoom by Jupiter and pick up a gravity assist "slingshot" to Pluto...

Now, as you pointed out, Ion engines are VERY good at accelerating propulsive gases to extremely high velocities, which gives great "fuel economy" or specific impulse. Where a hydrogen/oxygen engine tops out with a theoretical ISP of about 470 seconds (IIRC) and the SSME's get about 450 seconds ISP, which is pretty good! A typical Ion engine can get upwards of 800 ISP, even 1000 seconds or more ISP. IOW, it gets about DOUBLE the 'fuel economy' to do the same work as a super-advanced hydrogen/oxygen engine. The PROBLEM is, it doesn't have much thrust... In fact they're pretty feeble. Now, you CAN cluster large groups of engines to get more thrust- but it becomes self-defeating because of all the additional mass of the extra engines-- the more mass you have to accelerate, the larger the push needed to accelerate the mass quickly enough to be useful. Add more engines, that's more mass, so more thrust is needed, add more engines, more mass... it just snowballs. PLUS, the more engines you have, the more solar power you need to feed them all, since they derive their power from electricity being used to ionize and accelerate charged particles of propellant gas to extremely high velocities and expelling that gas out a nozzle, thereby producing thrust. Nothing is "burned" so the energy comes from electricity. More electricity means more mass when you're talking about solar, and the amount of power (watts) you get from a given solar array mass (kilograms) is highly dependent on, among other things, the intensity of the solar flux at the distance that panel is operating-- IOW, the further away, the less power it produces, even though it weighs the same regardless of location (mass remains the same, that is to say).

Now, NASA has tinkered with ideas for using SEP (solar electric propulsion) to push things out to the Moon, like unmanned lunar landers, unmanned space station modules or habs, unmanned resupply missions, etc. Why unmanned?? Because it takes MONTHS to accelerate the mass of the payload up to a high enough velocity to get it into a translunar trajectory. In essence, they're using a low-thrust high-ISP (high efficiency) ion engine to accelerate a large mass with a commensurately smaller amount of propellant, IOW, they're trading delta-v for delta-t. If you want to accelerate a large mass quickly, you MUST have a large thrust, so you're trading delta-t for delta-v. If you're flying a crew, you NEED that fast acceleration-- why?? Because 1) you don't want to have to supply a crew with food, water, electricity, air, waste removal, clothing, etc for a month or two while they endlessly and pointlessly orbit the Earth in ever-higher orbits being nudged a bit higher every time by a little ion engine-- the extra mass would be killer for no point (oh wait, sounds like ISS-- "looking at stars, peeing in jars" endlessly circling the Earth for little purpose). #2 is the REAL killer-- you'd fry your astronauts making all those passes through the Van Allen Radiation Belts during the slow acceleration buildup over months required by a SEP ion engine. IF, however, your resupply vehicle, lander, hab, module, whatever is UNMANNED, who cares how much radiation it takes over multiple passes through the Van Allen Belts accelerating under the puny thrust of a SEP ion engine?? A little extra care in the design and shielding of the most sensitive components and problem solved! SO, for UNMANNED portions of a manned mission (support craft), SEP COULD be useful.

For lunar surface ops, solar electric is just as viable as nuclear, basically. You have to have DOUBLE the standard daily required generating capacity (so you can store up the necessary power during the two-week lunar day for the two-week lunar NIGHT when there is no sun to generate power) so in that respect it's much like Mars (but again, on Mars you'd need FOUR TIMES the standard power requirement in solar generating capability-- enough to generate the power for the surface hab during the day (at half the solar intensity of Earth) AND enough to generate power to store for the Martian 12 hr and 10 minute night, which requires ANOTHER 2X the solar panels it would on Earth. If you operate at the lunar poles, where assuming you can find a crater rim in full daylight at all times, you can get by with only 1x the required solar generating capacity needed to supply the surface base (plus spare capacity for redundancy). I don't think that's possible on Mars.

Basically, for a MANNED hab/vehicle, you need something with greater power density and higher thrust than solar electric, that is more mass-efficient. Accelerating all those heavy solar panels is a killer. Solar thermal could work, but again, with distance it's going to become less and less practical and powerful due to less solar energy available. Basically, solar thermal would focus the sun's rays on a boiler, like the liquid sodium heat loops used in some solar thermal plants here on Earth. The solar energy boils off a fluid, like liquid hydrogen, and expels it out the nozzle as a high-pressure high velocity gas that creates thrust. Water, ammonia, any number of working fluids could be used. It creates high thrust, but it's a more massive system because it requires a LOT more propellant. Much lower ISP than SEP, too.

Then there's nuclear. You CAN do NEP, or nuclear electric propulsion, but it's better to just do NTP and call it good. Nuclear Thermal Propulsion (like the old NERVA nuclear rocket engine of the 60's) uses nuclear fission to heat a working fluid (hydrogen, or some other working fluid like water or ammonia, which COULD be obtained using in-situ resource utilization (ISRU), not necessarily having to be produced on Earth). This working fluid is then expelled as a high-temperature, high-velocity stream of gas through a rocket nozzle, providing high thrust at high ISP efficiency. Nuclear reactors produce MOST of their power as THERMAL energy-- and every time you change energy forms, you lose efficiency and have varying amounts of waste. That's why nuclear ELECTRIC propulsion is not very good-- you have to first convert MOST of the nuclear thermal power into ELECTRICITY, losing a lot of it in the process to inefficient conversion, and then lose more converting that electric power into high-velocity gas as inertia for propulsion, another inefficient conversion. Plus, you'd have all the extra mass of the conversion equipment, which you could have dispensed with had you simply used the thermal energy from the nuclear reaction and converted that to inertia (acceleration) via the working fluid high-velocity gas in the first place.

There's a nuclear engine I read about that could be operated in different modes-- nuclear thermal for high-thrust acceleration phase required for a massive manned spacecraft, then a low-power state where it would operate in a low-intensity nuclear-electric mode via thermocouples (IIRC) to generate power for the spacecraft and its life support systems during the 'coast" phase of the mission. It could also be put into an essentially shut-down "standby" during surface ops when low power is needed.

Later! OL J R: )

#8 Re: Is it hype? or a possible real answer?

Posted: Thu, 21 Apr 16, 16:26 pm
by Commander
Thank you Luke, I knew you would be able to give more specifics.

So, to put it in terms that others may understand, the ion engines get great gas mileage but have terrible horsepower. We have to watch and understand the techno babble these guys put out. for example...
An advanced electric propulsion system could potentially increase spaceflight transportation fuel efficiency by 10 times over current chemical propulsion technology and more than double thrust capability compared to current electric propulsion technology. The next step will be to demonstrate this new electric propulsion system in space. Development of this technology will advance future in-space transportation capability for a variety of deep space human and robotic exploration missions, such as the NASA’s Asteroid Redirect Mission (ARM), as well as private commercial space missions.
What they are saying, using the analogy I started above is

We have a small block four cylinder engine that is ten times more fuel efficient than a large block V8 and gets twice as much horsepower as our old four cylinder.

#9 Re: Is it hype? or a possible real answer?

Posted: Thu, 21 Apr 16, 16:36 pm
by Commander
I guess they really want to taut this concept as in another email, they took the image from the article and made it the Image of the Day for NASA

with the caption
A prototype 13-kilowatt Hall thruster is tested at NASA's Glenn Research Center in Cleveland. This prototype demonstrated the technology readiness needed for industry to continue the development of high-power solar electric propulsion into a flight-qualified system.
Note the term "high power solar electric propulsion" but the lack of any output specifics. It almost sounds like they are trying to imply that for less than the power it takes to light a CFL (13kw), you can go zooming around deep space like Darth Vader in his TIE fighter.

Also note the lack of any object to give this engine any sense of scale.

#10 Re: Is it hype? or a possible real answer?

Posted: Thu, 21 Apr 16, 16:47 pm
by Joe Wooten
I'll bet any amount that the dumbass who wrote that NASA blurb WAS NOT one of their engineers. it takes considerable less power to light a CFL than 13KW, especially when the power consumption is usually about 20 to 30 WATTS, not 1300 WATTS.