VASIMR Engine (From Earth to Mars in 40 Days)

[sgt_flyer]

Yes, but for a 400kw reactor, having to manage 300kw of thermal waste for a 25% efficiency thermal cycle will still require a bigger cooling system than if you only need to manage 40kw of thermal waste for a 90% efficienct direct conversion (that's at least the efficiency figures they hope to reach for direct conversion)

That's the inherent problem with solid fuel based nuclear reactors as a whole - they can't let the reactor fuel rods go too hot - so they can't benefit from high efficiency cycles.

It's no wonder there's researches on Gas core / Molten salt reactors which can run at higher temperatures

[Robotengineer]

First thought on seeing this: 11th commandment: Thou shalt not refer to institutions such as huffingtonpost.com for space and science things, space.com is bad enough.

Anyways, the reports of theoretical missions to x from y in z number of days, are BS IMO.

[Pipcard]

What's wrong with space.com?

About VASIMR: I don't know if there will be a power source with enough power-to-mass ratio. Robert Zubrin (main advocate of the Mars Direct plan) even believes it's a "hoax".

But if it worked, it would be great to travel to Mars in 39 days.

Edited June 1, 2015 by Pipcard

[SeaDog]

Well, either nuclear reactors or several square kilometers of solar panels.

TBH, I think the solar panels option is more likely. Because "Nuclear is evil and a power reactor will instantly kill me from across the country in my sleep".

And yes, that's an entirely stupid and illogical reason for that outcome to be favored. It's an education and communication failure, not a logical argument.

Thats a strawman i often see when people are disappointed that nuclear power isn't the end all solution to various problems. Nuclear power for everything is a 50's scifi trope and in the case of space travel, there's actually not much indicating that it would be better than solar.

Solar has several strong points: While taking up much space, it can probably be made extremely light. It is extremely reliable since you can run parts of it if it gets damaged. It has already been used extensively in space. Simple forms of it could probably be constructed in situ.

So yea, the idea that we're not using nuclear reactors for stuff like this because of the opinion you quoted, that is the real education failure. It only shows you do not understand how difficult nuclear power is, especially in space.

[Rune]

That's quite an understatement. The 40 day trip to Mars requires a 200 megawatt reactor and specific power of 1, that is 1 kilogram per kilowatt. This reactor would have a mass of 200 tonnes.

How would you launch it even if it could be built?

Looking at the performance of actual reactors that have flown in space the specific powers is like 50 times worse. I did see a design proposal for a reactor called SAFE-400, which would have a specific power ratio of 13.5, still over an order of magnitude higher.

Ad Astra, the company behind VASIMIR, made a handy spreadsheet that shows what could actually be done with more reasonable performance characteristics.

https://dl.dropboxusercontent.com/u/22015656/Glover_1-19-11.pdf Page 25.

Even 10 kg/kw it's not possible to beat the Hohmann transfer, and the 'sweet spot' configuration takes over two years. So the idea that VASIMIR, or any electric propulsion, would allow quick trips to Mars doesn't seem realistic. It seems like it could make a very good cargo hauler beyond low Earth orbit though.

Hear hear! Every time I hear electric propulsion can shorten any trip, I cringe a little. Such a white-faced lie that some engineers use!

Rune. And why would you want to shorten a mere three month trip? It took longer to cross the Atlantic at a time!

[Guest]

Atlantic was not filled with deadly radiation and micro-meteorites, at least at that time.

[vger]

Atlantic was not filled with deadly radiation and micro-meteorites, at least at that time.

It came with its own set of dangers, which at the time, nobody was really equipped to account for. Famine, disease, hurricanes, etc. And wind has never been known to be a consistent reliable source energy. A boat could find itself stranded in a dead zone for weeks or months.

[basic.syntax]

NASA wants to cut travel time to Mars “in half†with new propulsion tech

Ion thrusters, nuclear rockets, and other in-space propulsion tech being looked at.

Speaking at an Aerojet Rocketdyne plant, NASA administrator Charles Bolden said the program is looking into advanced propulsion technologies that can cut the current eight-month journey to Mars "in half." Technologies such as solar-electric propulsion are definitely in the cards, but NASA may look towards more unconventional solutions such as nuclear rockets as well.

[SciMan]

Why not use an ammonia fueled Solar thermal rocket? Sure, a solar thermal rocket is an extremely old idea, but that doesn't mean it's bad.

No, not solar electric, I'm talking about using the solar energy to directly heat reaction mass, which is then expanded thru a rocket nozzle to create thrust.

Basically, a solar tea-kettle, IN SPACE!

Why solar thermal instead of Nuclear thermal?

If both engines use the same propellant, bring it to the same temperature, and use the same nozzle shape and size, the specific impulse of each type of rocket should be identical, because ANY thermal rocket's performance depends only on nozzle geometry, input power (how hot does the propellant get before it goes thru the nozzle), propellant choice, and mass flow rate, not on what's putting the thermal energy into the propellant in the first place. This means that whichever thermal rocket type is lighter for the same thrust, ISP, and propellant, is objectively better.

I think Solar Thermal rockets can be lighter than Nuclear thermal rockets (at least, anywhere solar panels are practical for powering an Ion drive), when you include the mass of all subsystems (thermal management, power, control systems, etc. not including propellant tanks which are assumed to be identical)

Factors:

1. Solar Thermal doesn't need to shield the rest of the craft from its own radiation. (payload identical, shielding from solar radiation is part of payload)
   
2. Nuclear reactors are heavy, require a lot of support systems, even the fuel itself is quite heavy and the critical mass means you can't get away with less than a certain amount.
   
3. Use of inflatable plastic film spheroids with one hemi-spheroid metalized to reflect sunlight as the material for the concentrating reflectors (really low internal pressure, just enough to keep it's shape).
   
4. Graphite or Graphene used for the heat absorber/heat exchanger (light, strong, heat resistant) Graphite works with current tech, graphene possible in future.
   
5. Naturally deep-black matte color of most pure carbon materials makes carbon close to being an ideal black-body absorber material as well, so most of the energy that falls on the heat exchanger would be transferred to the propellant, instead of being reflected back into space
   

Additionally, the NTR would be more expensive simply due to the limited selection of materials to work with and tighter tolerances on the composition of those materials. Plastics and carbon composites are dirt cheap in comparison to U235 or Pu239, sunlight is free, and nuclear reactors have to have a lot of expensive inspection and simulation before they even think of starting one, let alone even considering constructing one.

Why ammonia instead of LH2, when LH2 can get better specific impulse?

As far as propellants, LH2 is a BAD CHOICE for a thermal rocket, even with the great Specific impulse it allows.

Cryogenic boil-off of LH2 due to solar heating is a bad thing when you want to keep the LH2 IN the tank for a long time, like you would need to for a VASIMR or other electric propulsion system.

And H2 is a small enough molecule that it eventually works it's way thru solid tank walls of ANY material, so even if you solve the boil-off problem it's still extremely hard to keep it where you need it.

Ammonia, on the other hand, is relatively easy to handle, especially at the low temperatures found in space.

At -30C, the tank pressure required to keep ammonia liquid is only 1.89 bar, and it's not hard to keep an even lower temperatures in space with just passive thermal control techniques and technologies (multiple layers of metalized plastic insulation, and spinning the craft about the axis of thrust to even out the heat load).

Matter of fact, ammonia is a component of many (most?) comets, and likely at least detectable in nearly all asteroids. That's a big bonus for ISRU, as well as evidence that it's easy to store ammonia in space.

[Rune]

Why not use an ammonia fueled Solar thermal rocket? Sure, a solar thermal rocket is an extremely old idea, but that doesn't mean it's bad.

No, not solar electric, I'm talking about using the solar energy to directly heat reaction mass, which is then expanded thru a rocket nozzle to create thrust.

Basically, a solar tea-kettle, IN SPACE!

Why solar thermal instead of Nuclear thermal?

If both engines use the same propellant, bring it to the same temperature, and use the same nozzle shape and size, the specific impulse of each type of rocket should be identical, because ANY thermal rocket's performance depends only on nozzle geometry, input power (how hot does the propellant get before it goes thru the nozzle), propellant choice, and mass flow rate, not on what's putting the thermal energy into the propellant in the first place. This means that whichever thermal rocket type is lighter for the same thrust, ISP, and propellant, is objectively better.

I think Solar Thermal rockets can be lighter than Nuclear thermal rockets (at least, anywhere solar panels are practical for powering an Ion drive), when you include the mass of all subsystems (thermal management, power, control systems, etc. not including propellant tanks which are assumed to be identical)

Factors:

1. Solar Thermal doesn't need to shield the rest of the craft from its own radiation. (payload identical, shielding from solar radiation is part of payload)
   
2. Nuclear reactors are heavy, require a lot of support systems, even the fuel itself is quite heavy and the critical mass means you can't get away with less than a certain amount.
   
3. Use of inflatable plastic film spheroids with one hemi-spheroid metalized to reflect sunlight as the material for the concentrating reflectors (really low internal pressure, just enough to keep it's shape).
   
4. Graphite or Graphene used for the heat absorber/heat exchanger (light, strong, heat resistant) Graphite works with current tech, graphene possible in future.
   
5. Naturally deep-black matte color of most pure carbon materials makes carbon close to being an ideal black-body absorber material as well, so most of the energy that falls on the heat exchanger would be transferred to the propellant, instead of being reflected back into space
   

Additionally, the NTR would be more expensive simply due to the limited selection of materials to work with and tighter tolerances on the composition of those materials. Plastics and carbon composites are dirt cheap in comparison to U235 or Pu239, sunlight is free, and nuclear reactors have to have a lot of expensive inspection and simulation before they even think of starting one, let alone even considering constructing one.

Why ammonia instead of LH2, when LH2 can get better specific impulse?

As far as propellants, LH2 is a BAD CHOICE for a thermal rocket, even with the great Specific impulse it allows.

Cryogenic boil-off of LH2 due to solar heating is a bad thing when you want to keep the LH2 IN the tank for a long time, like you would need to for a VASIMR or other electric propulsion system.

And H2 is a small enough molecule that it eventually works it's way thru solid tank walls of ANY material, so even if you solve the boil-off problem it's still extremely hard to keep it where you need it.

Ammonia, on the other hand, is relatively easy to handle, especially at the low temperatures found in space.

At -30C, the tank pressure required to keep ammonia liquid is only 1.89 bar, and it's not hard to keep an even lower temperatures in space with just passive thermal control techniques and technologies (multiple layers of metalized plastic insulation, and spinning the craft about the axis of thrust to even out the heat load).

Matter of fact, ammonia is a component of many (most?) comets, and likely at least detectable in nearly all asteroids. That's a big bonus for ISRU, as well as evidence that it's easy to store ammonia in space.

That sounds nice, until you run the numbers and work out that a NERVA had a thermal power of 4.5 Gigawatts in a few tons (around 10 for order of magnitude, but finding figures is hard). Getting that kind of power from the sun requires mirrors that will not only be more massive, they will also create other, more serious problems like attitude control, or plain structural issues. Hell, you might even have issues with gravity gradients inducing torques.

As to VASIMIR, I wish they would stop pouting this nonsense. There is no powerplant, nuclear or else, that will provide the energy density to run a VASIMIR at the power level required to make a Mars trip in 40 days. Not even in the drawing board of the most optimistic engineers, just in a few papers Chang Diaz wrote, where it is mostly a number for power density he pulled out of his ass to show what one of his electric drives could do with a magical powerplant.

Rune. The ammonia comments, totally +1, H2 is a boop to deal with.

[SciMan]

That sounds nice, until you run the numbers and work out that a NERVA had a thermal power of 4.5 Gigawatts in a few tons (around 10 for order of magnitude, but finding figures is hard). Getting that kind of power from the sun requires mirrors that will not only be more massive, they will also create other, more serious problems like attitude control, or plain structural issues. Hell, you might even have issues with gravity gradients inducing torques.

As to VASIMIR, I wish they would stop pouting this nonsense. There is no powerplant, nuclear or else, that will provide the energy density to run a VASIMIR at the power level required to make a Mars trip in 40 days. Not even in the drawing board of the most optimistic engineers, just in a few papers Chang Diaz wrote, where it is mostly a number for power density he pulled out of his ass to show what one of his electric drives could do with a magical powerplant.

Rune. The ammonia comments, totally +1, H2 is a boop to deal with.

Sure, you need that kind of power if you want high thrust, and high thrust means you can complete burns quicker, but as the various types of electric propulsion have shown, once in orbit you don't need a lot of thrust to get anywhere fast. High thrust over a short period or low thrust over a long period, the thing that matters is the total kinetic energy change imparted to the spacecraft.

Because of critical mass, there's a definite lower mass limit for any NTR. There is no "critical sunlight" for a solar thermal rocket, so it scales down better, and if you keep the same reflectors and focus them on a smaller heat exchanger, the propellant at rocket nozzle throat is at a higher temperature, which increases specific impulse. That works up until you start melting parts of the engine, and with a carbon-composite heat exchanger/thrust chamber/nozzle, the limiting temperature is quite a bit higher than the constraints imposed on a solid-core NTR design.

Lower thrust, higher ISP, ends up lighter than high thrust lower ISP NTR, and it runs on the same propellant. What's not to like?

Also, Fusion, Antimatter, or Antimatter catalyzed fusion would work at least as well as a nuclear fission reaction for the power source used by a VASIMR propelled spacecraft.

Those are all on the drawing boards of the most optimistic engineers, even if they don't have a complete or workable design yet (one of the reasons they are still on the drawing board).

Sure, we're probably hundreds of years away from being able to create and/or store enough antimatter in one place to make a pure antimatter based power generation system work for a useful amount of time, but antimatter catalyzed fusion requires absolutely tiny amounts of the stuff.

Of course, at that point you're probably better off directly using the energy for thrust (fusion plasma thruster/torch drive/fusion thermal rocket) instead of converting it to electrical energy to run a VASIMR. Hmm, then we're right back to looking at Ammonia as a good propellant again.

Obviously, that's firmly in the realm of "technically possible, but unlikely". And that hasn't stopped things from being made in the past.

Edited June 5, 2015 by SciMan

[78stonewobble]

Some news:

Highlights:

1. Ions and electrons in the same plume, no neutralization needed

2. Primarily using argon, 6N thrust level achieved (estimated isp 3000s)

3. More than 10000 firings of VX-200

4. 70% efficiency with argon and ISP of 5000s.

5. With krypton, they could reach 75% efficiency.

6. CDR milestone beginning 2016.

7. ISS flight milestone beginning of 2018.

8. ISS reboost currently requires 7t of fuel and 210 million USD per year

9. Smaller 80kW unit would require 1/10 of this cost

10. various application specific animations shown

The improvement in efficiency is probably the most important thing. Its improved from low-end to fairly high-end.

----------

Regarding power, people on this thread are severely underestimating solar power, which has seen huge improvements in recent years. Essentially, nuclear reactors are never going to get better than flexible blanket or thin-film arrays in the inner solar system, period. With DSS megarosa arrays or ATK's upgraded megaflex arrays, the power can readily be scaled up to multi hundred kilowatt levels, at about 4 kg/kW.

With 2 MW of these arrays and either nested halls or Vasimr, you have faster than chemical transits to Mars, in the 4-5 month range. 800 kW is enough for ~8 month transfers with one third of the mass needed using chemical. Regarding payload, this is assuming an architecture similar to the one outlined in the Nasa DRM 5.0.

40 day transits are made of pixie dust and Ad Astra is imho ill-advised to mention them, but shorter transits with far less mass needed in LEO are definitely possible, and probably the best path to Mars at this point.

So that is what?

8 tons for 2 MW?

3,2 tons for 800 kW?

Vs. eg. circa 500 kg. for 100 kW with the SAFE-400.

You also have to consider degradation of the solarpanels over time. If we wanted to reuse a transfervehicle over time.

[PB666]

Sure, you need that kind of power if you want high thrust, and high thrust means you can complete burns quicker, but as the various types of electric propulsion have shown, once in orbit you don't need a lot of thrust to get anywhere fast. High thrust over a short period or low thrust over a long period, the thing that matters is the total kinetic energy change imparted to the spacecraft.

Because of critical mass, there's a definite lower mass limit for any NTR. There is no "critical sunlight" for a solar thermal rocket, so it scales down better, and if you keep the same reflectors and focus them on a smaller heat exchanger, the propellant at rocket nozzle throat is at a higher temperature, which increases specific impulse. That works up until you start melting parts of the engine, and with a carbon-composite heat exchanger/thrust chamber/nozzle, the limiting temperature is quite a bit higher than the constraints imposed on a solid-core NTR design.

Lower thrust, higher ISP, ends up lighter than high thrust lower ISP NTR, and it runs on the same propellant. What's not to like?

Also, Fusion, Antimatter, or Antimatter catalyzed fusion would work at least as well as a nuclear fission reaction for the power source used by a VASIMR propelled spacecraft.

Those are all on the drawing boards of the most optimistic engineers, even if they don't have a complete or workable design yet (one of the reasons they are still on the drawing board).

Sure, we're probably hundreds of years away from being able to create and/or store enough antimatter in one place to make a pure antimatter based power generation system work for a useful amount of time, but antimatter catalyzed fusion requires absolutely tiny amounts of the stuff.

Of course, at that point you're probably better off directly using the energy for thrust (fusion plasma thruster/torch drive/fusion thermal rocket) instead of converting it to electrical energy to run a VASIMR. Hmm, then we're right back to looking at Ammonia as a good propellant again.

Obviously, that's firmly in the realm of "technically possible, but unlikely". And that hasn't stopped things from being made in the past.

The space dejunking is probably the most useful function of all, they could charge nations and companies that make space waste to have them dispose of it.

A better strategy is to elevate to a geosynchronous orbit and then transfer it to a lunar orbit were it can be used in the future in space. The stuff is useless on Earth, provided its out of harms way it could be extremely valuable. Even 2x geosynchronous is far enough out of the way. My vision of a garbage truck is a carbon nanofiber chainlinked fence cage with an opening at one end entering a cavity which is on a two prong pivot where the ship blocks the opening. DUring the off-cycle at robot assembles and balances the junk at the back of the cage, during the on cycle the ship pivots to the back of the cage and using sensors at the front allows the object to enter the opening, the sensor use small talons to close the opening, the ship returns and covers the opening, the robot places and secures the junk and the ship moves to the next target. When the cage is finally filled, the robot seals the opening leaving the cage in orbit, a new cage is delivered by a small rocket where it is picked up. Meanwhile another ship picks up the filled cages and carries them to high earth orbit.

Finally, and this is the most important part, cyborg aliens arrive and assemble the cubic cages into one giant ship that then takes over earth and assimilates everyone giving then names like 8 of 7 and 6 of 10, I think you get a free augmentation also, but not too sure about that.

Think a smiley face is needed there?

Moving on.

I would put my money on fusion before any other planned technology. Tritium is not that dangerous, there's already large amounts in the oceans, Fission devices need a dedicated and rather gutsy specialist to be safe in launch, because the fuel rods are best kept isolated during launch in the event of a disaster otherwise a launch becomes a dirty bomb. But provided the right precautions can be made a dedicated self-contained fission driven device is plausible. The only anti-matter I see being used are on-site generated anti-matter used to catalyze low-grade thermonuclear explosions, because of the hazards involved this is strictly interplanetary (the Earth is still reeling with massive increases in carbon-14 from the nuclear testing in the 50's and 60's).

Solar energy is not just solar panels, in response to another poster, solar technology can be encorperated into parts, it is substantially more efficient than when the panels were placed on the ISS, the panels are lighter and there are new stronger lighter space-age materials that can be used for structural. Solar, in part is not a bad choice for VASMIR power supply because the VASMIR RF plasma generators generate alot of heat, and thus there will need to be some sort of radiative cooling system, anyway. If you use a Fusion reactor, the transformers that feed the lasers and other equipment need to excite the core tritide and dueteride ions also generate a substantial amount of waste heat, so these are going to need a cooling system also. Unless they can come up with electronics that generate less heat and operate at a higher temperature they are going to have alot of devices competing for radiative cooling systems and this requires surface area.

My bets are on VASMIR and Solar being the ride that gets humans to the other planets, what gets them safely back to Earth, I'm not taking odds.

[SciMan]

Deuterium is the one that's relatively common in oceans. Tritium is actually radioactive enough that nuclear power plants have to keep it contained.

The "easiest" fusion reaction to start is the Deuterium-Tritium reaction.

If you put Lithium in the reaction chamber, the neutron radiation from the fusion reaction will convert that lithium into Tritium, which can then be separated from the Lithium and sent to the fusion reaction.

And you can extract Lithium from seawater as well. It's part of the salts in seawater. The reason we don't do that is that it's not yet cost-effective compared to mining for it.

Edited June 6, 2015 by SciMan

[nilof]

So that is what?

8 tons for 2 MW?

3,2 tons for 800 kW?

Vs. eg. circa 500 kg. for 100 kW with the SAFE-400.

You also have to consider degradation of the solarpanels over time. If we wanted to reuse a transfervehicle over time.

I was being fairly conservative, and thus mentioned only near-term technologies that can safely be expected to launch within the next few years. 4 kg/kW arrays is what you get with deployables, most of that is structural mass that has to survive launch. Thin film solar can have a mass as low as 0.2 kg/kW, excluding the structure that holds it. If you use something like Spiderfab to make a good truss frame in space that does not have to survive launch to orbit(which would then weigh less than the solar arrays themselves), there's at least an order of magnitude improvement over the technologies that I mentioned in my previous post.

I am a big fan of the SAFE-400 and similar reactor designs for outer solar system exploration, since it'd allow to do things like a Pluto orbiter. For manned inner solar system exploration, solar is way more reliable and higher T/W ratios are possible.