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Vilkata
2008-Nov-14, 09:36 AM
How many ion engines (DS1, Dawn, or Hayabusa type) would it take to propel a manned mars exploration vehicle to mars and back with the vehicle having a tonnage similar to NASA estimates for such a trip and with the mission taking place over a standard Mars mission time frame?

Would this design be more weight efficient? It seems like the biggest drawback with the ion engines are weak thrust. Very efficient weak thrust however. And ion engines are very reliable, able to thrust constantly for years. It seems like a simple sollution is just cluster a hundred of them together!

---Vil.

cjameshuff
2008-Nov-14, 01:05 PM
Would this design be more weight efficient? It seems like the biggest drawback with the ion engines are weak thrust. Very efficient weak thrust however. And ion engines are very reliable, able to thrust constantly for years. It seems like a simple sollution is just cluster a hundred of them together!

You're asking the wrong question, really. It's plausible to build large ion/plasma drives. It's the power supply that's the real issue:

Momentum is m*v, kinetic energy is 0.5*m*v^2. All else being equal, doubling the exhaust velocity v lets you halve the reaction mass m, but the energy requirements to accelerate that mass double (or you can get double the momentum change from the full amount of reaction mass, at the cost of 4 times the total energy usage)...you need either a power source twice as large, or a trip twice as long. In ion engines, v is many times higher than it is in chemical engines. However, so is the mission duration. Keeping the mission time short means that energy has to be provided over a shorter period of time...you need a far larger power source, which has its own mass penalties.

Siguy
2008-Nov-14, 09:52 PM
Ion engines suck up a ton of energy. You'd need a nuclear reactor to power a large one or cluster. 400kW will only get you about 1kg thrust, and a nuclear reactor that can generate that in space would weigh a lot. Seriously. It's impossible to have lightweight shielding, since shielding needs to be heavy by design.

VASIMR looks promising, but electric propulsion may or may not be the solution. Nuclear rockets like fission-fragment rockets or nuclear thermal rockets currently have more exciting figures. The reactor in a nuclear thermal rocket could also generate enough electricity to power an auxiliary bank of ion thrusters.

cjameshuff
2008-Nov-15, 01:34 AM
It's impossible to have lightweight shielding, since shielding needs to be heavy by design.

Not necessarily. First, reaction mass can double as free shielding...it becomes less effective toward the end of the trip, but still reduces accumulated exposure. Second...distance can be very lightweight. Stick the reactor out on a end of a long compression structure or tensile tethers. Every time you double the distance, you reduce the remaining radiation you need to handle by a quarter.

The tethered option also allows you to spin the whole thing for simulated gravity, with the reactor as a counterweight, which also means the reactor is always under acceleration when operating, which may simplify its design a bit over one that must handle freefall.

A better option might be beamed power, but that takes a large amount of infrastructure in place.

Ilya
2008-Nov-15, 03:12 AM
How many ion engines (DS1, Dawn, or Hayabusa type) would it take to propel a manned mars exploration vehicle to mars and back with the vehicle having a tonnage similar to NASA estimates for such a trip and with the mission taking place over a standard Mars mission time frame?
Besides the already-mentioned weight problems, an ion-engine spacecraft takes LONGER to reach Mars than a chemical rocket. Given enough time, an ion engine can build up much more speed than a same mass chemical engine, but "enough" is quite a long time. Longer than a Hohmann trajectory to Mars.

Ion engines really shine if you want to get to outer planets (although that probably requires a nuclear power source), or to accelerate repeatedly to multiple targets (e.g. Dawn). Within inner solar system an ion engine can significantly increase payload mass, but not decrease trip time.

cjameshuff
2008-Nov-15, 03:05 PM
Besides the already-mentioned weight problems, an ion-engine spacecraft takes LONGER to reach Mars than a chemical rocket. Given enough time, an ion engine can build up much more speed than a same mass chemical engine, but "enough" is quite a long time. Longer than a Hohmann trajectory to Mars. .

To be fair, that's largely due to the mass of the power source. Station powerful lasers or very large microwave emitters at Earth and Mars, and use relatively small and lightweight photovoltaic arrays or rectennas on the craft to power the engines, and you might get something competitive with chemical rockets in trip time, and which requires lifting far less fuel, meaning either more useful payload or less to lift into orbit. But...that requires lots of infrastructure. Like a large nuclear plant and beamed-power station in orbit around Mars to power your ship during deceleration and at the start of the return trip.

Ilya
2008-Nov-15, 09:09 PM
I suppose I should have said "an ion-engine spacecraft available now or within next few decades". But OP was asking about exactly that:

How many ion engines (DS1, Dawn, or Hayabusa type)

Vilkata
2008-Nov-19, 09:25 AM
Thanks for your replies!

That clears up a lot of stuff I had been wondering about.

Are there any missions using Ion Engines + Nuclear Power for gas giant exploration? Seems like a Cassini weight ion engine + nuclear spacecraft might be able to pull off orbiting both Uranus and Neptune. Of course, it would take 20 years to get it out there.

Or maybe not? Theoretically, couldnt a space craft use aerobraking on a gas giant to slow down from very high speed? Or would deceleration be too rapid and the craft would burn up? Take the New Horizons for instance. If it were sent to Uranus, at its current top speed, would there be any possibility of it entering into orbit using neptunes atmosphere as a brake? I'm guessing it's going far too fast.

---Vil.

JonClarke
2008-Nov-19, 11:23 AM
Thanks for your replies!

That clears up a lot of stuff I had been wondering about.

Are there any missions using Ion Engines + Nuclear Power for gas giant exploration? Seems like a Cassini weight ion engine + nuclear spacecraft might be able to pull off orbiting both Uranus and Neptune. Of course, it would take 20 years to get it out there.

Or maybe not? Theoretically, couldnt a space craft use aerobraking on a gas giant to slow down from very high speed? Or would deceleration be too rapid and the craft would burn up? Take the New Horizons for instance. If it were sent to Uranus, at its current top speed, would there be any possibility of it entering into orbit using neptunes atmosphere as a brake? I'm guessing it's going far too fast.

---Vil.

JIMO was the most recent attempt to have such a mission. But it got ugly rather fast and was canned.

There have been studies of aerobraking for Neptune and Uranus, it makes good sense. For Titan also, and even Triton (the atmosphere is very thin but quite deep).

Jon

GOURDHEAD
2008-Nov-20, 03:51 PM
How many ion engines (DS1, Dawn, or Hayabusa type) would it take to propel a manned mars exploration vehicle to mars and back with the vehicle having a tonnage similar to NASA estimates for such a trip and with the mission taking place over a standard Mars mission time frame?If we can organize our thinking (in a somewhat neo-cpaitalistic way) to direct our efforts over a 300 to 500 year time interval, we can implement an ion engine transportation system much more powerful than ones with the ion engines cited that will provide round trips to Mars and Venus in approximately one week:
One-way trip times
The first column is for acceleration = 1g; the second column is for acceleration = 0.667g
Moon...................... 3.4 Hours ..................... 4.3 Hours
Mercury...................2.3 Days........................2.84 Days
Venus or Mars...........2.1 Days........................2.5 Days
Jupiter.....................6.1 Days .......................15 Days
Saturn ....................9.5 Days .......................22.5 Days
Uranus ...................13.2 Days .......................38 Days
Neptune .................16.0 Days .......................44.5 Days
Pluto ......................18.5 Days ......................45 Days
Kuiper (65AU)...........20 Days .........................58.7 Days (72AU)
Oort (52 KAU) ..........23 Months ......................2.26 Years (50.8 KAU)


This system will require the installation of a power beam generator near the sun in polar orbit. This power beam generator will be required to produce 10^(17 to 19) watts and by being located quite near the sun, the energy collection section can be much smaller than would otherwise be the case. The output of this station must be a collimated beam capable of being directed in a delicately controlled fashion to targets to which a vehicle can be directed. The vehicle will be equipped with enormous photovoltaic panels in its energy receiver which will deliver 10^17 or so watts to the propulsion system which will be ion engines of much greater capacity than are currently used. Propellant will be whatever elements are available (most likely hydrogen) expelled at the rates of 200 to 800 kg/sec at exhaust velocities of several tenths of light speed. The particle accelerators driving the ion engines will be electrostatic if grid erosion can be controled or magnetic coils if we are limited by materials characteristics.

This may seem like overkill in terms of robustness, but this is necessary if we are to grow this system into one that supports interstellar exploration (with relative safety and comfort) and asteroid/comet protection for the Earth and other colonized sites. Also, it greatly aids terraforming and colonization efforts by having the ability to deliver the needed energy and soil and rock moving equipment to target sites. We need an integrated system with a durable infrastructure.

The sun has given us life and allowed us to develop technical competence; now we must exploit it to permeate and control the universe---in a spirit of thanksgiving of course.

Siguy
2008-Nov-20, 04:36 PM
If we can organize our thinking (in a somewhat neo-cpaitalistic way) to direct our efforts over a 300 to 500 year time interval, we can implement an ion engine transportation system much more powerful than ones with the ion engines cited that will provide round trips to Mars and Venus in approximately one week:
One-way trip times
The first column is for acceleration = 1g; the second column is for acceleration = 0.667g
Moon...................... 3.4 Hours ..................... 4.3 Hours
Mercury...................2.3 Days........................2.84 Days
Venus or Mars...........2.1 Days........................2.5 Days
Jupiter.....................6.1 Days .......................15 Days
Saturn ....................9.5 Days .......................22.5 Days
Uranus ...................13.2 Days .......................38 Days
Neptune .................16.0 Days .......................44.5 Days
Pluto ......................18.5 Days ......................45 Days
Kuiper (65AU)...........20 Days .........................58.7 Days (72AU)
Oort (52 KAU) ..........23 Months ......................2.26 Years (50.8 KAU)


This system will require the installation of a power beam generator near the sun in polar orbit. This power beam generator will be required to produce 10^(17 to 19) watts and by being located quite near the sun, the energy collection section can be much smaller than would otherwise be the case. The output of this station must be a collimated beam capable of being directed in a delicately controlled fashion to targets to which a vehicle can be directed. The vehicle will be equipped with enormous photovoltaic panels in its energy receiver which will deliver 10^17 or so watts to the propulsion system which will be ion engines of much greater capacity than are currently used. Propellant will be whatever elements are available (most likely hydrogen) expelled at the rates of 200 to 800 kg/sec at exhaust velocities of several tenths of light speed. The particle accelerators driving the ion engines will be electrostatic if grid erosion can be controled or magnetic coils if we are limited by materials characteristics.

This may seem like overkill in terms of robustness, but this is necessary if we are to grow this system into one that supports interstellar exploration (with relative safety and comfort) and asteroid/comet protection for the Earth and other colonized sites. Also, it greatly aids terraforming and colonization efforts by having the ability to deliver the needed energy and soil and rock moving equipment to target sites. We need an integrated system with a durable infrastructure.

The sun has given us life and allowed us to develop technical competence; now we must exploit it to permeate and control the universe---in a spirit of thanksgiving of course.
1g (by other means of propulsion) could also get you to Alpha Centauri in just over 4 years, but by that time you would have effectively destroyed general relativity. Which doesn't sound so bad to me...

GOURDHEAD
2008-Nov-20, 10:59 PM
1g (by other means of propulsion) could also get you to Alpha Centauri in just over 4 years, but by that time you would have effectively destroyed general relativity. Which doesn't sound so bad to me...No destruction of general relativity is either possible or desirable. Other means of propulsion become an engineering nightmare (as are some aspects of the beamed energy system). For going to Alpha Centauri we have to add a particle beam to supply propellant (which would also help travel within the solar system). The rocket equation requires between 60 and 80 trillion kilograms of propellant to move a reasonably sized ship (6 billion kg) and slow it down to achieve orbit in the AC system assuming the trip lasts 10 years of non-accelerated frame time. The trip times in the earlier entry did include slowing down to achieve orbit. I have not been able to envision a design using fission, fusion, or matter/anti-matter processes that can accommodate the shielding mass and the propulsion mass for interstellar applications. I have not done the arithmetic but my guess is that the race between adding shielding mass for the extra energy to propel the shielding mass and the added propellant mass, and slow it down to achieve orbit, will always leave the design short of sufficient energy.

An obvious hazard that could be encountered is that of colliding with a rogue object (dark hadronic matter) so the system must be able to detect and avoid collision without subjecting the vehicle's contents to excessive acceleration due to transient movement perpendicular to the direction of the vehicle's motion.

mugaliens
2008-Nov-29, 03:27 PM
Station powerful lasers or very large microwave emitters at Earth and Mars, and use relatively small and lightweight photovoltaic arrays or rectennas on the craft to power the engines...

Alternatively, one could simply use very lightweight aluminum-coated mylar to reflect and concentrate the sun's energy on the solar cells. Far less problematic, and a great solution to distant missions to the outer planets.

cjameshuff
2008-Nov-29, 05:56 PM
Alternatively, one could simply use very lightweight aluminum-coated mylar to reflect and concentrate the sun's energy on the solar cells. Far less problematic, and a great solution to distant missions to the outer planets.

You've got to deploy and point those giant mirrors, and while they can be lightweight compared to what you need on Earth, mirrors of sufficient size, precise enough shape, and sufficiently rigid to hold that shape under acceleration will constitute a fair amount of mass. Many of these problems go away if the large mirror/solar cell arrays are stationary and feed a laser or microwave emitter. It makes the vehicle lighter and simpler for a given amount of supplied power.

Also, concentrators are more sensitive to direction, and a failure that causes the craft to lose its orientation or ability to point the collectors will make it quickly lose power. Unconcentrated collectors are far less sensitive to the direction of incoming light, increasing the odds of survival and recovery.

I do think the mirror idea's feasible, and it can work in conjunction with the beamed power to extend the useful range. I just think we're closer to being able to do beamed power than we are to being able to construct probes that reliably deploy giant, ultralightweight concentrating solar arrays. Given the inefficiencies of beamed power, large collector arrays will be needed (and radiators, and more of them in the case of nuclear power), but they only need to be built once for many probes, and the beamed power station can be done incrementally, with greater engineering margins (since they are stationary and weight is less of an issue), and with the possibility for repair.

Ara Pacis
2008-Nov-29, 11:55 PM
I'm not sure I understand why it needs to be in solar polar orbit, it will still occsionally go into eclipse for certain trajectories. And why try to focus sunlight? A solar beam will still not collimate as well as a laser. If we have solar PV panels of such high efficiency available, then might it be better to use those in the solar power a laser? Or maybe we could use a solar thermal system to power a laser. Then the laser could be used in one of several ways when it arrives at the vehicle: it could be used on a sail; it could be used with another PV system optimized to the laser's wavelength; or it could use the laser energy to work propellant directly (ablation or other thermal propulsion).

This way, you don't need to station a satellite beam station so close to the sun or in a rapid orbit around the sun. You could station it much farther out (by using concentrators for the PV) where it could keep it's beams on target more easily, especially if it takes up residence at the Earth-Sol L1. Moreover, if multiple ships are moving along a similar trajectory, then a beaming station in the right location could provide power to all of them for a significant time with a wide enough beam that they could all ride as long as they stay out each other's shadow, kinda how lots of ships can ride the Gulf Stream or planes can ride the jet stream.

timb
2008-Nov-30, 12:53 AM
I'm not sure I understand why it needs to be in solar polar orbit, it will still occsionally go into eclipse for certain trajectories. And why try to focus sunlight? A solar beam will still not collimate as well as a laser. If we have solar PV panels of such high efficiency available, then might it be better to use those in the solar power a laser? Or maybe we could use a solar thermal system to power a laser. Then the laser could be used in one of several ways when it arrives at the vehicle: it could be used on a sail; it could be used with another PV system optimized to the laser's wavelength; or it could use the laser energy to work propellant directly (ablation or other thermal propulsion).

This way, you don't need to station a satellite beam station so close to the sun or in a rapid orbit around the sun. You could station it much farther out (by using concentrators for the PV) where it could keep it's beams on target more easily, especially if it takes up residence at the Earth-Sol L1. Moreover, if multiple ships are moving along a similar trajectory, then a beaming station in the right location could provide power to all of them for a significant time with a wide enough beam that they could all ride as long as they stay out each other's shadow, kinda how lots of ships can ride the Gulf Stream or planes can ride the jet stream.

I'm sure that there are technical holes you can pick in Gourdhead's proposal, though he does seem to have thought about it a very great deal. I'm more bothered by the political implausibility of the world's devoting its entire resources for the next half millennium or so to making Gourdhead's dream reality.

Ara Pacis
2008-Nov-30, 05:57 AM
I'm sure that there are technical holes you can pick in Gourdhead's proposal, though he does seem to have thought about it a very great deal. I'm more bothered by the political implausibility of the world's devoting its entire resources for the next half millennium or so to making Gourdhead's dream reality.

Would that be based on current GDP or future GDP? Oh wait, you're referring to the interstellar plan that uses trillions of kg of propellant and billions of kg for payload? Yeah, that's pushing it. I'm more interested in a more limited version of the concept for interplanetary use.

korjik
2008-Nov-30, 06:01 AM
colliding with a rogue object (dark hadronic matter) .

commonly known as a rock. :)

GOURDHEAD
2008-Dec-01, 02:23 PM
I'm not sure I understand why it needs to be in solar polar orbit, it will still occsionally go into eclipse for certain trajectories. And why try to focus sunlight? A solar beam will still not collimate as well as a laser. If we have solar PV panels of such high efficiency available, then might it be better to use those in the solar power a laser? Or maybe we could use a solar thermal system to power a laser. Then the laser could be used in one of several ways when it arrives at the vehicle: it could be used on a sail; it could be used with another PV system optimized to the laser's wavelength; or it could use the laser energy to work propellant directly (ablation or other thermal propulsion).

This way, you don't need to station a satellite beam station so close to the sun or in a rapid orbit around the sun. You could station it much farther out (by using concentrators for the PV) where it could keep it's beams on target more easily, especially if it takes up residence at the Earth-Sol L1. Moreover, if multiple ships are moving along a similar trajectory, then a beaming station in the right location could provide power to all of them for a significant time with a wide enough beam that they could all ride as long as they stay out each other's shadow, kinda how lots of ships can ride the Gulf Stream or planes can ride the jet stream.I am using the term collimated beam in a very generic sense and expect it to be converted at the station to optimize efficiencies of energy transfer in every way possible, chief among which would be selecting the frequencies to transmit to the interstellar vehicle. The beam generators should be in polar orbit to minimize exposure of each planet and occupied site to the beam which could have catastrophic destructive power on such objects. Also, the vehicle is a beam rider and if it is out of contact with the beam for more than a few minutes after it gets a few light hours away, re-contact becomes very difficult, and knowledge that re-connection has been made will take a while to re-establish. Keeping the vehicle in the beam will be achieved by beam-vehicle protocols not by vehicle to beam controller communication.

As for the devotion of world's resources to such a project, there are currently many people doing non-essential work, some at less than optimal efficiencies, and many with no jobs at all. Such a program will require many technical jobs to produce the parts and assemblies of each of the elements of the system and they, most of them, will be produced in the participating countries reducing the problem to one of establishing equitable pay for the various tasks. Each of the interstellar system workers will need to be fed and clothed and provided the myriad of services they would otherwise need. Building this system will create the economic tide that will raise each boat.

The availability of the more rare elements that are used in highly efficient photovoltaic panel manufacture at the rates of manufacture required by the system is currently unknown, at least to me, and could be a show stopper. (More prospectors and miners to be fed, clothed, and serviced.)

For safety and reliability reasons there should be at least one beam generator for each vehicle. A standby beam generator should be available as a secondary energy source should the primary one fail.

Ara Pacis
2008-Dec-02, 03:11 AM
I am using the term collimated beam in a very generic sense and expect it to be converted at the station to optimize efficiencies of energy transfer in every way possible, chief among which would be selecting the frequencies to transmit to the interstellar vehicle. The beam generators should be in polar orbit to minimize exposure of each planet and occupied site to the beam which could have catastrophic destructive power on such objects. Also, the vehicle is a beam rider and if it is out of contact with the beam for more than a few minutes after it gets a few light hours away, re-contact becomes very difficult, and knowledge that re-connection has been made will take a while to re-establish. Keeping the vehicle in the beam will be achieved by beam-vehicle protocols not by vehicle to beam controller communication.

As for the devotion of world's resources to such a project, there are currently many people doing non-essential work, some at less than optimal efficiencies, and many with no jobs at all. Such a program will require many technical jobs to produce the parts and assemblies of each of the elements of the system and they, most of them, will be produced in the participating countries reducing the problem to one of establishing equitable pay for the various tasks. Each of the interstellar system workers will need to be fed and clothed and provided the myriad of services they would otherwise need. Building this system will create the economic tide that will raise each boat.

The availability of the more rare elements that are used in highly efficient photovoltaic panel manufacture at the rates of manufacture required by the system is currently unknown, at least to me, and could be a show stopper. (More prospectors and miners to be fed, clothed, and serviced.)

For safety and reliability reasons there should be at least one beam generator for each vehicle. A standby beam generator should be available as a secondary energy source should the primary one fail.

I see, so you're primarily looking at this for interstellar use and the primary power station orbiting the sun is dedicated to a single vehicle, or several supporting a single vehicle. That makes more sense in that context.

I was interested in adapting the idea for inter-planetary use. In that Scenario I think a solar polar orbit might be less desirable because you want it closer to the ecliptic... thinking that it would be better to beam along the plane rather than intersect it along a narrow shaft, but it's probably not really important even at interplanetary distances. At most it would be a couple degrees, unless you need a large stand-off distance from the sun (~1 degree for every 3 Million km). Hmmm, maybe a Solar Polar orbit would be better, perhaps with 2 to 4 power sats for complete coverage. I think I'll try to work something similar into my solar system infrastructure plan.

BTW, you might be able to make solar panels with quantum dots (http://en.wikipedia.org/wiki/Quantum_dot_solar_cell) using more common materials. I'm not sure what the state of research is on that though.

mugaliens
2008-Dec-07, 01:25 AM
You've got to deploy and point those giant mirrors, and while they can be lightweight compared to what you need on Earth, mirrors of sufficient size, precise enough shape...

It's under acceleration. "giant mirrors?" Fooey! Just use a superthin reflective concatenary disc. Attach it to the nose of the ion rocket, spin it (this is very critical for getting it to spread out), and apply thrust. The spinning will keep it spread out. The thrust laid on top of the spin cause it to assume it's contatenary shape and reflect solar energy over the solar-cell surface of the ion rocket, providing more than enough energy for it's propulsion.

In fact, you have to be careful it doesn't provide so much energy that you burn the rocket to a crisp...

http://i35.photobucket.com/albums/d185/mugaliens/solar-ionspacecraft2.jpg

cjameshuff
2008-Dec-07, 02:48 AM
It's under acceleration. "giant mirrors?" Fooey! Just use a superthin reflective concatenary disc. Attach it to the nose of the ion rocket, spin it (this is very critical for getting it to spread out), and apply thrust. The spinning will keep it spread out. The thrust laid on top of the spin cause it to assume it's contatenary shape and reflect solar energy over the solar-cell surface of the ion rocket, providing more than enough energy for it's propulsion.

That looks like it'd work great...for acceleration vectors directly away from the source of light. For solar power, that's straight out from the sun. It would work better with beamed power stations, which could be placed in various orbits to provide power to ships moving in the desired direction, unless your destination is deep space or another star. If you want to go from point A in the solar system to point B also in the solar system, you'll need more flexibility in vector, and either a rigid array that can be kept pointed at the sun, or those beamed power stations.