Solar-Powered Ion Propulsion - cheaper way to the moon?

RobS · 2004-01-20 00:19:49

A solar-powered ion propulsion system (Hall thrusters) is already possible, based on the ion engine of Deep Space 1.

Deep Space 1 had a 2.1 kilowatt solar array and generated 4.5 km/sec of delta-vee by expelling 81.5 kg of xenon over 20 months. The spacecraft had a total mass of 486 kg (a bit less than half a tonne) and the ion engine a specific impulse of 3,100 seconds (a calculation based on above numbers, however, suggests the specific impulse achieved was 2,900 seconds). The power plant (engines, solar panels, structure, voltage converters) had a power to mass ratio of 27 watts per kilogram (thus a 2.1 kilowatt engine massed 78 kilograms). The engine?s thrust was 92 millinewtons (in other words, it was capable of accelerating 1 kilogram of mass at the rate of 92 millimeters per second per second and 486 kg at the rate of 0.19 mm/sec/sec). The delta-vee of 4.5 km/sec required a continuous 4.3 kw per tonne (2.1 kw/0.486 tonnes total mass) over 20 months of engine use (but at that thrust, the engine would have been operating 274.12 days continually, so the 20 months must represent sporadic use).

The delta-vee to the Earth-moon lagrange point or to (basically) escape velocity) from low earth orbit is 3.2 km/sec. If an ion engine thrusts continuously, raising the perigee and apogee, it must achieve double that to reach L1: 6.4 km/sec. If it thrusts half the time, gravity losses are less because it does not raise the perigee, so it needs to achieve an intermediate delta-vee?perhaps 4.8 km/sec?but it needs a more powerful power plant and engines.

A 450-kilowatt power plant would mass 16.7 tonnes based on Deep Space 1 (27 watts per kilogram). If greater mass efficiency than Deep Space 1 can be achieved, it will be less. The power plant will be about 220 times larger, so great efficiency should be possible. More advanced solar cells will reduce the size of the array (Deep Space 1 used 22% efficient cells, but 30% efficient cells will soon be available). Hence for the calculations below I am assuming a power plant mass of 12 tonnes, 75% as large.

To accelerate 30 tonnes (18 tonnes payload and 12 tonnes power plant) to 4.8 km/sec the ion engine must consume 5.1 tonnes of xenon propellant. Xenon has a density 3.5 times that of water, so the tanks to hold it should mass less than 0.5 tonnes. If 5.5 tonnes of xenon are included, the 0.4 tonnes left over will be sufficient to give the empty vehicle a delta-v of 1 km/sec, enough, with use of some incremental aerobraking, to lower the tug back to low earth orbit.

A 450 kw power plant produces a thrust 450/2.1 = 214 times greater than Deep Space 1, or 92 millinewtons x 214 = 19,700 millinewtons or 19.7 newtons (4.43 pounds). This will give a 36-tonne vehicle an acceleration of 0.55 millimeters per second per second. It will require 4.8 km/sec = 4,800,000 mm/sec / 0.55 = 8,727,273 seconds = 2,424 hours = 101 days to achieve its final speed. Since it is thrusting half the time, the journey requires 202 days = 6.6 months.

This demonstrates that if a reusable solar-powered ion tug is placed in low Earth orbit, a Delta-IV heavy can launch 18 tonnes of playload and 6 tonnes of xenon fuel plus tank and the tug can push it to escape velocity (or to lagrange, or into the moon's gravitational field) in six months. By positioning stages at lagrange ahead of time, a Delta-IV can launch crew to the moon, even though it is about 1/5 the size of a Saturn-V. But you'd need:

1 launch to place the crew capsule plus stage to boost it to 3.2 km/sec

1 launch to place a lander at lagrange or in high lunar orbit to lower the capsule to the moon.

1 launch to place a boost stage on the moon to send the capsule back to Earth.

The latter two can be abolished if lunar water is found.

-- RobS

Euler · 2004-01-20 03:16:18

It has only been a few years since deep space 1, but there have already been significant improvments in ion engine design since then. NASA is currently working on a High Power Electric Propulsion (HiPEP) ion engine, which uses microwaves to ionize the zenon atoms (instead of a cathode). According to a NASA [http://www.nasa.gov/home/hqnews/2003/no … ngine.html]Press Release,

This new class of NEP thrusters will offer substantial performance advantages over the ion engine flown on Deep Space 1 in 1999. Overall improvements include up to a factor of 10 or more in power; a factor of two to three in fuel efficiency; a factor of four to five in grid voltage; a factor of five to eight in thruster lifetime; and a 30 percent improvement in overall thruster efficiency. GRC engineers will continue testing and development of this particular thruster model, culminating in performance tests at full power levels of 25 kilowatts.

sethmckiness · 2004-01-20 11:57:21

Hmm. sounds like a great way to lift infrastructure.

Whats transit time? A few months?

Sounds like a great way to park a base camp on the Moon. and to park some infrastructure on Mars.

Our lifter Technology needs some work, but I think with some modifications existing equipment can be used to get us into LEO, and then get some alternate means to get past that.

I think, for the time being,(30-60 year) Chemical is going to be the only way to get the Delta-V to LEO.

RobertDyck · 2004-01-20 13:53:00

Deep Space 1 used the NSTAR ion engine developed by the Glenn Research Center of NASA. Hall thrusters also use electricity to accelerate xenon or krypton gas to high exhaust velocity, but they?re based on a different principle. Russia has developed Thruster Anode Layer (TAL) Hall Thrusters which perform as well as NASA?s ion engines, but they are different technology.

Deep Space 1 had a problem with its ion engine after launch. Something shorted out the grids. The best guess was a flake broke off from the plating at the back of the engine, but that?s only a guess. They put the spacecraft in a slow spin to work it loose via thermal expansion and contraction from solar heating and the cold of space. It worked mostly, but the top 3 throttle settings would still short the grids. They must have worked it loose from one of the grids, but didn?t get rid of it; high voltage would still arc. Specific impulse (fuel efficiency) was dependant on throttle setting; it wasn?t as high at lower thrust settings.

Spectrolab produces solar panels for space. Their improved triple junction photovoltaic cells have beginning of life (BOL) 26.8% @ maximum power or 26.5% @ load voltage, and end of life (EOL) 22.5% @ maximum power or 22.3% @ load voltage. They mass 840 grams per square metre, bare (without glass coverslide, support backing, arms, hinges, wires). The mass for an entire panel is 2.06 kg/m^2 including 3 mil doped ceria coverslide, or 2.36 kg/m^2 with 6 mil doped ceria coverslide. These panels produce 316 W/m^2 for a panel area < 2.5 m^2, or 330 W/m^2 for panel area > 2.5 m^2.

Spectrolab?s new cells are ultra triple junction. They have BOL 28.0% @ maximum power or 27.7% @ load voltage, and EOL 24.1% @ maximum power. No EOL efficiency listed at load voltage. These new cells mass the same.

In space, solar panels degrade due to radiation; that?s why they have BOL and EOL. These triple junction cells are the same ones that produce 30% on Earth. You don?t have to worry about radiation on Earth. Engineers plan for end of life performance, so this means space performance has increased from 22.3% to about 23.9%.

RobS · 2004-01-20 18:03:47

This is all very useful information. Has anyone seen any data about how much an ion engine would mass? One article I saw gave a power to powerplant ratio of 27 w per kg for Deep Space 1's engine; has anyone seen better numbers than that?

-- RobS

RobertDyck · 2004-01-21 02:21:57

According to "NSTAR Ion Thrusters and Power Processors", the thruster had a mass of 8.2kg, the PPU (Power Processing Unit) cable mass was 0.95kg, the thruster power cable was 0.77kg, the PPU was 14.50 including micrometeoroid shielding, and the DCIU (Digital Control and Interface Unit) was 2.51kg. However, according to "In-Flight Performance of the NSTAR Ion Propulsion System on the Deep Space One Mission", the thruster was 8.33kg, PPU was 15.03kg, DCIU was 2.47kg, Xenon Feed System was 20.47kg, and PPU to thruster cable was 1.70kg. Actually, the propellant feed system and gimbal are the heaviest components.

Flight performance is quite variable. Throttle setting 15 had PPU input power of 2.52 kW, engine input power of 2.29 kW, calculated thrust of 92.4 mN, main flow rate of 23.43 sccm, cathode flow rate of 3.70 sccm, neutralizer flow rate of 3.59 sccm, and specific impulse of 3120 seconds. Setting 14 had a lower thrust and lower power, but specific impulse of 3157 seconds, setting 13 had 3185, and setting 12 had 3174. It isn't linear. Setting 0 used only 0.53 kW of PPU input power, produced 20.6 mN of thrust, and had specific impulse of only 1972 seconds.

The PPU converted power from the voltage supplied by solar panels to what was needed by the engine. Conversion isn't 100% but it was very close.

Palomar · 2004-01-21 13:25:19

*Please pardon the intrusion and no I don't want to take this off-topic but I really don't want to create an entirely new thread for my question (particularly since I've already posted it elsewhere and it was overlooked or ignored [not an accusation...I know it's hard to keep up with all the conversations here]...dicktice responded to that question, with a few of his own as well, and I'll leave it to him to repost it if he wants).

I did look over the posts so far, for any clue or indication which -might- answer my question...but as we know, do-dads and numbers mixed with symbols tend to make my brain melt, so:

What are the chances a return to Camp Deadrock will be done via the Saturn rocket model being brought out of the mothballs or something similar to Saturn? Won't the proposed "back to Luna" plans require similarly staged craft as Saturn?

Any low tech answers would be appreciated (numbers drive me nuts...don't be cruel please).

--Cindy

RobertDyck · 2004-01-21 14:10:18

Hi Cindy,

Figures in Encyclopedea Astronautica show a Russian Proton K lift capacity to LEO, and a Proton K with an upper stage lifting a spacecraft into translunar trajectory. Using that ratio to estimate the capacity of a Delta IV Large (aka Delta IV Heavy) it should be able to inject 7.0 tonnes into a translunar trajectory. That's the heaviest launch vehicle America now has, and that's less than even a modern Soyuz spacecraft. (Soyuz-TM masses 7.15 tonnes, and Soyuz-TMA masses 7.25 tonnes). That means we will need a heavier launch vehicle to get to the Moon just for a fly-by. Any realistic implementation of what George W. Bush called for would require at least a lunar lander for the first mission, so at least double the lift capacity of Delta IV Large. Building a base will require substantial landers to deliver base modules. That sounds like it'll require something on the scale of a Saturn V. I only see two ways to accomplish that: a Russian Energia or a Shuttle Derived Vehicle.

Bottom line: SDV here we come.

Palomar · 2004-01-22 10:17:40

Hi Cindy,
Figures in Encyclopedea Astronautica show a Russian Proton K lift capacity to LEO, and a Proton K with an upper stage lifting a spacecraft into translunar trajectory. Using that ratio to estimate the capacity of a Delta IV Large (aka Delta IV Heavy) it should be able to inject 7.0 tonnes into a translunar trajectory. That's the heaviest launch vehicle America now has, and that's less than even a modern Soyuz spacecraft. (Soyuz-TM masses 7.15 tonnes, and Soyuz-TMA masses 7.25 tonnes). That means we will need a heavier launch vehicle to get to the Moon just for a fly-by. Any realistic implementation of what George W. Bush called for would require at least a lunar lander for the first mission, so at least double the lift capacity of Delta IV Large. Building a base will require substantial landers to deliver base modules. That sounds like it'll require something on the scale of a Saturn V. I only see two ways to accomplish that: a Russian Energia or a Shuttle Derived Vehicle.
Bottom line: SDV here we come.

*Okay, thanks Robert.

It's not that I'm so "hung up on" the Saturn V (for its beauty and the nostalgia factor); rather, since we haven't left LEO since Apollo and of course the Saturn V, I was simply wondering in that context.

The decision to go back to Luna is rather abrupt, and I'm not greatly familiar with any new designs in craft for that purpose...if there are many...so that's why I was wondering about Saturn V or something similar coming "out of the mothballs." Now I'm rambling...so "over and out."

--Cindy

RobertDyck · 2004-01-22 12:53:43

No problem Cindy. I also think we should go directly to Mars, and only to the Moon as a spin-off of a Mars mission. I view the Moon as "been there, done that". But George W. Bush has made his decision, so let's make the best of it. After all, a decision to build a permanent base on the Moon with preparation to go to Mars "eventually" is not too far from using the Moon as a proving ground for Mars hardware; it'll just take about twice as long to get to Mars. At least we're moving in the right direction.

The Saturn V was a nice vehicle, but it is thoroughly obsolete and decommissioned. It used then-new computers that were leading edge, high performance, and compact for the mid 1960?s. I heard that they were ?only? the size of a small room. Even the computers in Shuttle are about the size of a bar fridge. Today a PDA has more computing power; I think my DataLink wristwatch has more power than the computer that ran Saturn V stages. The factory that built S-IC, the first stage of a Saturn V, has been converted to make external tanks for Shuttle. The rocket engines for Saturn V have not been manufactured for years. The Mobile Launchers have been cut up to convert them into Mobile Launch Platforms for Shuttle. New exhaust holes have been cut in them, and the hold-down clamps have been replaced with ones for Shuttle. The service structure (tower) on two of the Mobile Launchers have been removed and converted into the static service structure on the two launch pads. The service structure tower from the third ML was cut into pieces and left rusting in the grass and weeds of Merritt Island; I read a message on this message board that it may be re-assembled at the Smithsonian.

The last Saturn V rockets themselves were left outside to rust in the rain. The first and second stages of Apollo 18 (S-IC-13 and S-II-13) were used to launch Skylab. The third stage of Apollo 18 (S-IVB-513) is on display at the Johnson Space Center. The first stage of Apollo 19 (S-IC-14) is also on display at JSC. The second and third stages of Apollo 19 (S-II-14 and S-IVB-514) are on display at the Kennedy Space Center. The first stage of Apollo 20 (S-IC-15) is on display at the Michoud Assembly Facility, the factory where it was made. The second stage (S-II-15) is on display at JSC. The third stage of Apollo 20 (S-IVB-515) was converted into Skylab II, the backup Skylab, and is on display at the National Air and Space Museum. There were also a few test stages built: S-IC-T was an ?all systems? test stage, which means it was complete but never expected to launch; it?s now on display at KSC. S-IC-D dynamic test stage had 1 real F-1 engine and 4 simulated ones; it?s on display at the Alabama Space and Rocket Center. S-II-F dynamic test stage is on display as the second stage at the Alabama Space and Rocket Center. S-IVB-D dynamic test stage is on display as the third stage at the Alabama Space and Rocket Center. The S-IU instrument unit was the flight computer for the Saturn V. S-IU-513 launched Skylab; S-IU-514 and S-IU-515 location is unknown. The interstages were the connecting bits between the stages; they were used as utility buildings at the Alabama Space and Rocket Center in Huntsville, but are no longer there.

Palomar · 2004-01-22 13:39:57

The Saturn V was a nice vehicle, but it is thoroughly obsolete and decommissioned. It used then-new computers that were leading edge, high performance, and compact for the mid 1960?s. I heard that they were ?only? the size of a small room. Even the computers in Shuttle are about the size of a bar fridge. Today a PDA has more computing power; I think my DataLink wristwatch has more power than the computer that ran Saturn V stages.

*Nice but obsolete...(nostalgia can be cruel)

I read an article a few years ago that your average Furbee toy has more power in its chip than the Saturn computer you mention.

The remainder of your post (thanks!) reminds me of the old saying: "Today's news is tomorrow's fish wrap."

Sad, but true. There will never be another rocket as splendid and beautiful as the Saturn V. They are supposed to be totally utilitarian, rockets...but Saturn V was allowed to be elegant as well; a bit of art mixed in with hard science.

I'll shut up now.

--Cindy

dicktice · 2004-01-22 20:30:57

Cindy: We need nostalgia like yours. The Hindenberg was a beaut as well. But, while cutting-edge then--with cattle guts for making the hydrogen gas bags, nitrate-doped cotton covering, non-vectoring or feathering propellers, no reversing tail prop for docking, vented lift-gas to sink, released water-ballast to rise. . . . Its time had come and gone--just like the Saturn-5 launching rocket-stack. So . . . we build on our past experiences, i.e., Do what the Chinese are doing now, with such steady and reliable success: Copy whatever worked in the recent past, discard whatever didn't, and start redesigning the rest. Look at the mach-diamonds in their strap-on booster exhausts--and then look at our solid-rocket booster exhausts (delete 'em). Even the Soyuz booster plumes look dirty by comparison. Look at the added solar panels on their command module, so it remains functioning in orbit after the crew module detaches for reentry (smart, so adopt it). Look at the Shuttle's fueltank. It's huge, and still in production (keep it, put partitions inside, boost it into orbit and maintain it there, with whatever fuel remains, by a small add-on rocket engine, thrusters, etc. for use later). A bunch of these, launched uncrewed with dirty solid boosters, would make a dandy space platform and workshop, for whatever happens next. Don't try to specialize so much, because knows it all--Bush? (I know: he's not alone, but that bunch haven't earned the exclusive right!) Look at their enclosed vertical concrete gantry setup, where you live, and work on the assembled rocket-stack in comfort, protected from the elements until (?) hours before launch (copy it). And then we come to my own dream: Slow down the reentry vehicle in orbit, by tilting it up and lowering it at the same time, by rocket power, down into the atmosphere, until the angle of attack can be decreased enough to fly back (without ablative tile protection to keep it from burning up) to base, and land civililized, on wheels. That's where we stop copying (except perhaps, from the great Burt Rutan) and start to do our own new things to get off the planet, stepwise, and this time keep on going. Thanks Cindy.

Shaun Barrett · 2004-01-24 19:05:59

I like your pragmatic approach, Dicktice!

It looks like an adaptation of a favourite philosophy of mine: 'Keep It Simple Stupid !!!

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#1 2004-01-20 00:19:49

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#2 2004-01-20 03:16:18

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#3 2004-01-20 11:57:21

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#4 2004-01-20 13:53:00

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#5 2004-01-20 18:03:47

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#6 2004-01-21 02:21:57

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#7 2004-01-21 13:25:19

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#8 2004-01-21 14:10:18

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#9 2004-01-22 10:17:40

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#10 2004-01-22 12:53:43

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#11 2004-01-22 13:39:57

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#12 2004-01-22 20:30:57

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

#13 2004-01-24 19:05:59

Re: Solar-Powered Ion Propulsion - cheaper way to the moon?

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