Nuclear Ion Propulsion

Antius · 2015-01-13 06:39:10

I have raised a new post to discuss nuclear ion propulsion and GW's proposal:

"OK, look. The dynamics of fast interplanetary travel demand propulsion that has at least a low-1000's of sec of Isp, and lots of vehicle acceleration capability: 0.1 to 5 gees. We have some technologies that have shortfalls, and we have those benchmarks. Not both! And we have some concepts we haven’t tried yet.

There are a bunch of electric propulsion schemes, some more mature than others. The best ones have the low-1000's of sec Isp, but all fall way short on vehicle acceleration: 0.001 gee or less. The most critical need is for a lighter-weight, more powerful electric power supply. That's what needs to be worked on. If the SLS budget were going toward that, we might have a solution in sight. But we do not.

Chemical and solid core nuclear thermal meet the thrust benchmark, but fall short on Isp. Many folks have objections to nuclear for any of a number of reasons, but we may just "have to get over it", and be careful how and where we employ it. Both technologies are essentially ready-to-use, especially the chemical. Here in the US, nobody is making LOX-kerosene engines but the newcomers like Spacex and XCOR. That needs to change. We also need to work on in-space cryogen transfers and long-term storage. The practicality for real interplanetary travel is not quite there yet, but could be.

Gas core nuclear thermal is absolutely immature, but holds the promise of meeting both the thrust and Isp benchmarks simultaneously. So, we ought to be working on it, too. That technology could enable travel if the search for a practical electric propulsion power supply fails. It's stupid to put all your eggs in one basket, as we all already know. If that word "nuclear" offends, well tough. Life ain't fair. Neither is physics. The electric power supply we seek for electric might well be nuclear. So, get over it.

BTW, my candidate power supply concept would be a nuclear thermal rocket device feeding an MHD generator. None of that heavy steam loop nonsense. Use it as a rocket for getaways and captures, use it for MHD electric on the transits between to raise midpoint speeds. Get thrust from the electrics and the released plume. Same rocket hardware does both, which saves vehicle weight. That might really start looking good if gas core nuclear was the rocket device. Then we could start looking at excess electricity to use for processing propellants while electric-thrusting on the transits. It pays to dream big. Tells you what you really need to be working on, too.

The only other technology holding the promise of meeting both benchmarks simultaneously (that I know of) is the nuclear explosion drive. We have known since 1959 that this scheme would work. The side effect to fear is not so much fallout from launches to LEO, but EMP effects, and by far! The devices make lousy blast weapons, and blast weapons make lousy explosion-propulsion devices. Trouble is, it has been ignored and forgotten since 1965. Plus, it is only efficient in very large (10,000+ ton) vessels. I suggest we ought to be working on this, too, but slower and longer term, with an eye toward giant colonization transports by next century. Could always be accelerated if we find we need it sooner.

OK, that being said, now return to electric propulsion. The problem is power supply, as I said just above. The thrusters themselves are also a bit short on basic device thrust/weight, even when you don't count the power supply, so that's another area we ought to be working on, as well.

But, a good power supply could make the simple arc jet feasible, which could use just about anything as propellant, at least in principle. To use oxygen-containing materials as electric propellants, will require solving the reactive-oxygen problem, so we should just belly up to the bar and get on with solving that issue. That solution makes in-situ electric propellants a lot more feasible-looking.

Some (few?) parts of Mars have big buried glaciers, we think, so there is massive ice we could mine, just not at every site where we'd like to base. Maybe not very many at all. Who yet knows? The atmosphere there is mostly CO2, which at those temperatures is not all that hard to freeze for easy storage. How about CO2 as an electric propellant? Could we make that work? Venus also has CO2, but it has hellish conditions and big gravity well, which make it not so attractive at this time in history.

Titan has a largely-nitrogen atmosphere and a very weak gravity well, although Saturn's is considerable. Could we use Titanian nitrogen as an electric propellant to support activities in the outer solar system? The surface "rocks" seem to be water ice, so you have that as well. Plus, methane and ethane in liquid form in the lakes. Could any of these work as electric propellants?

There's the ices on the surfaces of 3 of Jupiter's big moons. Water, CO2, perhaps ammonia, and some other things. Could these be used? The moons have weak gravity wells, but Jupiter's is enormous, as is its radiation hazard. Not sure how soon we might be capable of safely recovering those resources, but somebody ought to be looking at it.

Guys, the power supply issue and the what-can-we-use-as-propellant issue are the two biggest issues facing electric propulsion. Those are what we need to be working on, very big time. I just don't see that going on in anything being funded through NASA or the other space agencies, nor through DARPA or anybody else. Which is why I really don't believe any of those agencies are serious yet about sending men beyond LEO. Not even to the moon, or the SLS/Orion test schedule would look more serious about actually accomplishing something.

Solve those two issues with electric propulsion, and we can ride the ions where we want to go. Right now, the acceleration is way too low to use it for manned flight. The travel times are just too long. Vehicle acceleration at departure and capture needs to be 0.1 gee or better. Can tolerate 0.01 to 0.001 gee during transits, but higher would be better. That’s what we need.

GW"

I derived a few simple equations yesterday that help in so far as scoping the problem. Will post more later.

Quaoar · 2015-01-13 09:00:12

Antius wrote:

I have raised a new post to discuss nuclear ion propulsion and GW's proposal:
"OK, look. The dynamics of fast interplanetary travel demand propulsion that has at least a low-1000's of sec of Isp, and lots of vehicle acceleration capability: 0.1 to 5 gees. We have some technologies that have shortfalls, and we have those benchmarks. Not both! And we have some concepts we haven’t tried yet.
There are a bunch of electric propulsion schemes, some more mature than others. The best ones have the low-1000's of sec Isp, but all fall way short on vehicle acceleration: 0.001 gee or less. The most critical need is for a lighter-weight, more powerful electric power supply. That's what needs to be worked on. If the SLS budget were going toward that, we might have a solution in sight. But we do not.
Chemical and solid core nuclear thermal meet the thrust benchmark, but fall short on Isp. Many folks have objections to nuclear for any of a number of reasons, but we may just "have to get over it", and be careful how and where we employ it. Both technologies are essentially ready-to-use, especially the chemical. Here in the US, nobody is making LOX-kerosene engines but the newcomers like Spacex and XCOR. That needs to change. We also need to work on in-space cryogen transfers and long-term storage. The practicality for real interplanetary travel is not quite there yet, but could be.
Gas core nuclear thermal is absolutely immature, but holds the promise of meeting both the thrust and Isp benchmarks simultaneously. So, we ought to be working on it, too. That technology could enable travel if the search for a practical electric propulsion power supply fails. It's stupid to put all your eggs in one basket, as we all already know. If that word "nuclear" offends, well tough. Life ain't fair. Neither is physics. The electric power supply we seek for electric might well be nuclear. So, get over it.
BTW, my candidate power supply concept would be a nuclear thermal rocket device feeding an MHD generator. None of that heavy steam loop nonsense. Use it as a rocket for getaways and captures, use it for MHD electric on the transits between to raise midpoint speeds. Get thrust from the electrics and the released plume. Same rocket hardware does both, which saves vehicle weight. That might really start looking good if gas core nuclear was the rocket device. Then we could start looking at excess electricity to use for processing propellants while electric-thrusting on the transits. It pays to dream big. Tells you what you really need to be working on, too.

At the moment this engine is the best candidate for a NTR/NEP hybrid spaceship:

http://www.alternatewars.com/BBOW/Space … TRITON.pdf

When the ship is coasting, the rocket enter in idle mode, producing 25-100 KW of electrical power, using a closed cycle Brayton turbine and a little radiator, that ca be used to feed an electric thruster.
There are also very promising study about high efficiency (up to 42% termionic converter)

http://www.fkf.mpg.de/1253832/Highly-Ef … -Power.pdf

So in a future the NTR can be equipped with a very lightweight no moving parts powerful generator.

Antius wrote:

OK, that being said, now return to electric propulsion. The problem is power supply, as I said just above. The thrusters themselves are also a bit short on basic device thrust/weight, even when you don't count the power supply, so that's another area we ought to be working on, as well.
But, a good power supply could make the simple arc jet feasible, which could use just about anything as propellant, at least in principle. To use oxygen-containing materials as electric propellants, will require solving the reactive-oxygen problem, so we should just belly up to the bar and get on with solving that issue. That solution makes in-situ electric propellants a lot more feasible-looking.

Here is an interesting 1500s Isp hi-power arc-jet using hydrogen

http://erps.spacegrant.org/uploads/imag … 91-072.pdf

hydrogen has the highest specific impulse and may be simpler using the same propellant for NTR and for NTP. Hydrogen arc-jet may also be used for RCS.

Other promising technology is the nested channel Hall thruster

http://pepl.engin.umich.edu/pdf/IEPC-2011-246.pdf

It has an higher specific impulse (4000 s) and no grids, so theoretically it can use oxygen or any other kind of propellant (N2, CO2, NH3, H2O), but probably it has to be optimized.

Antius wrote:

Some (few?) parts of Mars have big buried glaciers, we think, so there is massive ice we could mine, just not at every site where we'd like to base. Maybe not very many at all. Who yet knows? The atmosphere there is mostly CO2, which at those temperatures is not all that hard to freeze for easy storage. How about CO2 as an electric propellant? Could we make that work? Venus also has CO2, but it has hellish conditions and big gravity well, which make it not so attractive at this time in history.
Titan has a largely-nitrogen atmosphere and a very weak gravity well, although Saturn's is considerable. Could we use Titanian nitrogen as an electric propellant to support activities in the outer solar system? The surface "rocks" seem to be water ice, so you have that as well. Plus, methane and ethane in liquid form in the lakes. Could any of these work as electric propellants?
There's the ices on the surfaces of 3 of Jupiter's big moons. Water, CO2, perhaps ammonia, and some other things. Could these be used? The moons have weak gravity wells, but Jupiter's is enormous, as is its radiation hazard. Not sure how soon we might be capable of safely recovering those resources, but somebody ought to be looking at it.

Callisto is far from radiation belt, but still inside the strong Jupiter's magnetic field and is protected from GCR. On Callisto surface radiation dose is very low, so Callisto's ice can be easy accessible.

Last edited by Quaoar (2015-01-13 09:03:10)

Antius · 2015-01-13 12:42:01

The problem with any propulsion system that combines high thrust levels with high exhaust velocity is power. This becomes especially problematic with ion engines, because electrical power systems typically require multiple energy conversion and transfer steps, which all tend to be rate limited by material properties. For this reason it is extremely difficult to produce nuclear electric power supplies with high power to weight ratio, to say nothing of shielding, which I presume will be absent in a space nuclear reactor.

For any rocket, thrust is equal to the mass flow rate of propellant into the engine, multiplied by its exhaust velocity.

F=m ̇×V_e

The output power of the engine is equal to the mass flow rate, multiplied by the kinetic energy per kg of exhaust:

P_e=m ̇/2×〖V_e〗^2

On this basis, required engine output power is:

P_e=(F×V_e)/2

Accounting for engine efficiency, input power is:

P_in=(F×V_e)/2η

Given that F=ma, the required engine input power can be expressed in terms of spacecraft mass and required acceleration:

P_in=(M_s×a×V_e)/2η

Running the numbers for the following parameters:

A spacecraft of mass 100 tonnes;
Acceleration 0.1g (1m/s2);
Exhaust velocity 40km/s (ISP = 4000);
Engine efficiency (η) = 70%

Running the numbers gives an electric power input requirement of 2857MW. That’s a lot of power, about as much as is produced by two large PWRs here on Earth. Let us make a further assumption that power supply must be no more than 30% of ship mass. That allows 50% for propellant, 10% for other structure (engines, tankage, etc.) and 10% for payload.

For a 100tonne fuelled ship the maximum mass of the nuclear power supply is then 30 tonnes. The required power to weight ratio for the power supply is then:

R(P/W)=Required Power / Maximum Weight

R(P/W) = 2,857,000kW/30,000kg = 95.2kW/kg.

To put this number into perspective, Zubrin references a modified SP-100 reactor in his original Case for Mars. This weighed 4000kg and produced 2000kWth and 100kWe. On an electrical basis, R(P/W) is only 0.025kW/kg. If we could use an S-CO2 brayton cycle to get 50% efficiency out of the reactor without increasing mass, then the R(P/W) increases to 0. 25kW/kg, still hundreds of times too small.

If ISP is relaxed to 1000s, the R(P/W) is reduced by a factor 4 to 23.8kW/kg. This is still 100 times greater than the SP-100.

This puts the problem into perspective.

Impaler · 2015-01-13 13:51:28

I think the 0.1 g acceleration rate requirement is WAY too high, that's the kind of acceleration that gets you to planets in days when we should be targeting months. Electrical propulsion systems are not going to get anywhere near that kind of thrust level under any currently conceivable device. Much lower accelerations are perfectly viable and bring the power requirements into the realm of possible.

I think the break through that a Nuclear power system in space needs is that of a less massive heat radiator as that's the most massive part of the system.

Current radiator tech is based on fluid in tubes that run through panels, very bulky and inefficient in surface area. Maximizing the heat dissipation into a vacuum is done by increasing surface area, so having the fluid in tiny droplets in free-space will be the optimum configuration. Now obviously we must not be losing the coolant fluid so these droplets need to be a 'spray' which is being continually emitted and collected again with virtually nothing being lost. If the coolant was also the propellent then perhaps a little loss would be acceptable, but our propellents are probably gaseous and would not be usable as a coolant, some kind of ionic salt seems to be a more likely coolant.

Two possible configurations present themselves, a 'fountain' configuration that uses the vehicles acceleration to control the droplets, and 'cotton-candy-machine' which would be centripetal and independent of vehicle acceleration. Which would be better I'm not sure, but either one looks to have the potential to provide a huge reduction in thermal radiator mass.

Lastly some additional performance might be gained in a nuclear ion engine system if the thermal energy of the reactor could do the job of ionizing the propellent, ionization is a significant portion of the energy consumption and if it would be done with the low grade thermal energy then it would avoid the conversion losses of converting to electrical and then using the electricity to do the ionization. Unfortunately this would certainly entail a massive redesign of the ion engine, for one it is likely that the engine has to be immediately adjacent to the core so that the ionized gass can be expelled before it reverts back to neutral gas.

Quaoar · 2015-01-13 16:37:00

Antius wrote:

Running the numbers gives an electric power input requirement of 2857MW [...] .

It's very difficult to cope with gigawatt range electrical power in space: conversion is so inefficient that you need a very huge and massive radiator to get rid of waste heat.
Instead of powering a ludicrous low thrust reaction electric thruster, a better to use electrical power may be to inflate a mini-magnetosphere for deflecting solar wind, like a sail: Robert Winglee's mini-magnetosphere plasma propulsion (if it really works as predicted) will give 1 Newton of thrust per KW of electrical power, so a 25 KW m2p2, you can easily obtain with a P&W Triton, will give you 25 N of thrust that can be used to rise the apogee of transfer orbit, shortening transit time.
m2p2 can be generated using every kind of propellant, even oxygen.

http://www.lpi.usra.edu/publications/re … wash01.pdf

Another good use of electrical power may be to power a magnetoshell for magnetoaerocapture, saving tons of propellant.

Last edited by Quaoar (2015-01-13 16:42:25)

Antius · 2015-01-14 15:15:35

Quaoar wrote:

Antius wrote:
Running the numbers gives an electric power input requirement of 2857MW [...] .
It's very difficult to cope with gigawatt range electrical power in space: conversion is so inefficient that you need a very huge and massive radiator to get rid of waste heat.
Instead of powering a ludicrous low thrust reaction electric thruster, a better to use electrical power may be to inflate a mini-magnetosphere for deflecting solar wind, like a sail: Robert Winglee's mini-magnetosphere plasma propulsion (if it really works as predicted) will give 1 Newton of thrust per KW of electrical power, so a 25 KW m2p2, you can easily obtain with a P&W Triton, will give you 25 N of thrust that can be used to rise the apogee of transfer orbit, shortening transit time.
m2p2 can be generated using every kind of propellant, even oxygen.
http://www.lpi.usra.edu/publications/re … wash01.pdf
Another good use of electrical power may be to power a magnetoshell for magnetoaerocapture, saving tons of propellant.

That's a roughly factor 10 improvement in energy efficiency over ion rocket thrust. To provide a 0.1g acceleration for a 100t spacecraft would require about 100MW. Still requires a high R(P/W), but an achievable one. Would you need to be outside of Earths magnetic field before you switched it on?

I did a little research. The Saturn V engine power was 180GW at takeoff. That's impressive.

Terraformer · 2015-01-14 17:38:27

Okay, so 1 kg of HydroLox will give about 4000 Ns of impulse, at ~410 s Isp. That contains about 13 MJ of energy. The M2P2, it is claimed, will give 1 Ns/kJ. Assuming we can get 10 MJ/kg of HydroLox using fuel cells, 1 kg will give us 10,000 Ns of impulse, an improvement of factor 2.5 over using a rocket engine... affected, of course, by the fact that you're retaining the water. Maybe it will come out equal in effect. But you now have a lot of radiation shielding you wouldn't have otherwise.

So maybe, use a HydroLox engine for a quick burn to reach escape, allowing you to use the Oberth effect, and then use the water to power an M2P2. Perhaps even use solar power to build up a supply of HydroLox over the journey, if you can use it to slow down at the other end?

Antius · 2015-01-16 08:42:10

If the thrust required to reach Earth departure velocity of 5.08km/s, is provided over 11 days, then the power requirement drops to more manageable 15MW for a 100tonne vessel. If power supply weighs 30tonne, then the R(P/W) is only 0.5kW/kg, a far more easily achievable value. And 11 days is only 6% of the total orbital transfer time (180 days). The acceleration in that case would be 0.0053m/s2, or 0.0005g. Coeicidentally, such an acceleration level is roughly equal to the weight of the spacecraft in Phobos gravity.

What would the additional gravity losses be in this sort of transfer scenario?

Quaoar · 2015-01-16 09:41:55

Antius wrote:

That's a roughly factor 10 improvement in energy efficiency over ion rocket thrust. To provide a 0.1g acceleration for a 100t spacecraft would require about 100MW. Still requires a high R(P/W), but an achievable one. Would you need to be outside of Earths magnetic field before you switched it on?
I did a little research. The Saturn V engine power was 180GW at takeoff. That's impressive.

Yes, you need to be outside Earth magnetosphere to switch on your m2p2.

Another interesting option may be the MITEE hybrid electrothermal rocket: a low pressure high temperature NTR, that can reach 1700 s of Isp with an arc-jet afterburner:

http://web.archive.org/web/200503171455 … /PUR-8.PDF

SpaceNut · 2016-03-20 10:35:50

This is as good of a place to post this as any nnovative Interstellar Explorer: Radioisotope Propulsion to he Interstellar Medium

An interstellar “precursor” mission has been under discussion in the scientific
community for over 25 years. Fundamental scientific questions about the interaction of the
Sun with the interstellar medium can only be answered with in situ measurements that such
a mission could provide.

Well in the timeframe that we have been wanting the voyager probes have gotten to the distance and are still not there yet....

kbd512 · 2016-03-23 16:36:01

I actually think fusion powered rockets are more feasible: MSNW LLC

NASA is actually devoting some funding to this and development hasn't encountered in insurmountable obstacles so far, so all things being equal this is the most probable candidate for a rocket that can deliver humans to planets in days or weeks instead of months. Even so, there's probably another two years worth of development before an engine is ready for trials and another two years after that before a flight ready test article can be prepared for launch.

I see fusion powered rockets as more of a "we can bring you home, no matter what" type technology than something that's actually required for a Mars mission. With AG and emerging life support, power generation, and power storage technology, there's really no hard requirement for new rocket technology. It makes the mission shorter, but the real solution always has been and always will be to figure out how to live and work in space.

I'm not opposed to development of better in-space propulsion tech, but it should not be an excuse to not do the mission until the new rocket tech is ready.

Tom Kalbfus · 2016-03-26 06:25:59

Well if you can get to Mars in a week, that reduces your launch requirement to orbit. You don't need an elaborate interplanetary vehicle then, you don't even need to spin it for gravity, and you don't need to stay at Mars for long either because you don't have to wait for favorable planetary alignments before returning to Earth. You can just go there, collect a few rocks and then come back!

SpaceNut · 2018-01-21 13:55:37

True you do not need artificial gravity if you can get there in a few days even in a cramped capsule but are we waiting for unobtainium or just huge sums of money?

kbd512 · 2018-01-22 00:14:44

SpaceNut,

There's nothing novel about using an ionized plasma for propulsion. Using a metal foil liner imploded at supersonic speeds to create an exponentially increasing electromagnetic field that initiates fusion is novel, although NIF essentially does the same thing with lasers instead of electromagnets, but the technology is based on principles that are relatively well understood at this point.

Any propulsion scheme that purports to drastically reduce transit times requires both high specific impulse and high thrust. That rules out chemical, nuclear fission, and ordinary ion engines. The fusion driven rocket is also solar powered, but the solar panels charge a bank of super capacitors. The super capacitors provide the massive pulse of electrical energy required to implode the foil liner.

Apart from EM Drive, I found no other promising near term propulsion concepts. Everything else is thousands of tons of propellant or impossible electrical energy requirements. If you want high thrust and specific impulse, then FDR is about as good as it gets. If EM Drive works, then most propellant-based propulsion becomes obsolete and we can go wherever we want, as fast as we dare. Just remember that hitting a grain of sand at relativistic speeds is a catastrophic event.

We're going to spend billions to develop an upper stage for SLS. That upper stage, along with all kick stages, would be completely unnecessary if FDR was developed. If FDR only works as well as what's already been demonstrated in the lab, then we can deliver humans to Mars in two months or less. The gigantic cargo ships like BFR would also benefit from staying in space and acting as cyclers that cycle between Earth and Mars, whenever they're fully loaded. The chemical propellants required are those necessary to make orbit. After that, FDR can take over. If RamGen's supersonic CO2 compressors are combined with lift fans, then there's no need for propellants to land on Mars.

SpaceNut · 2018-01-22 11:11:34

True there is nothing new about ION Drives just the methods to achieve is changing, upgrading the fuel use efficency, using possibly less expensive fuels....

The fundamental of nuclear power and its shielding from the core is that you can not eat up the mass savings by powering the ION Drive with it as you want to shift that savings to payload increase and not a decrease that is eaten up by the use of nuclear.....

Antius · 2018-02-05 06:32:07

This could be a key technology that makes the inertial confinement fusion direct rocket a reality.

http://www.sciencemag.org/news/2018/01/ … mpty-space

Unfortunately, an Earth Launch FDR would generate huge EM interference problems and would also pump the Earth's ionosphere with charged particles. There ain't no such thing as a free lunch.

clark · 2018-02-27 08:53:31

https://www.nextbigfuture.com/2018/02/n … oject.html

This is still in evaluation phase; nuclear thermal propulsion. NASA will not commit to system architecture for man to mars propulsion without a high degree of confidence that this can be developed to completion under current considered timelines. Worth tracking if you are keen.

SpaceNut · 2020-11-09 20:14:27

tahanson43206 wrote:

For SpaceNut ... (re #28) ....
Are these equations applicable to the ion engine?
Ions are accelerated by magnetic fields so they are moving far faster than any chemical reaction could drive them.
How might one measure "pressure" in an ion engine?
Mass flow rate would be knowable for an ion engine.
Exit velocity would be knowable.
Is "temperature" a meaningful value for the output of an ion engine?
Having asked those questions, I note that this topic ** is ** about a Nuclear Thermal rocket, which imparts heat to a working fluid.
In a Nuclear Thermal rocket, all the parameters you listed would seem to apply.
(th)

LOX - Ideal Solar Electric Propellent?

Photon---->Ion---->Acceleration

Forget NTR, SEP is the Future Solar electric propulsion

Ion drives is with a tiny mass that is a steady stream that is accelerated to high speed that makes it work.

tahanson43206 · 2020-11-10 06:23:28

For SpaceNut re #18

Thank you for bringing this important topic back into view.

As an observation. there is NO difference between the "tiny mass" of an ion engine and the "equally tiny mass" of a chemical engine. The difference is in the total number of tiny masses!

An ion engine could be just as powerful as a chemical engine, if it were putting the same number of tiny masses through the acceleration process.

The challenge for ion engine designers is to come up with a way to accelerate mass comparable in total magnitude to that of a chemical engine.

Google found this:

Some typical values of the exhaust gas velocity ve for rocket engines burning various propellants are: 1.7 to 2.9 km/s (3800 to 6500 mi/h) for liquid monopropellants. 2.9 to 4.5 km/s (6500 to 10100 mi/h) for liquid bipropellants. 2.1 to 3.2 km/s (4700 to 7200 mi/h) for solid propellants.
Rocket engine nozzle - Wikipediaen.wikipedia.org › wiki › Rocket_engine_nozzle

Taking the highest figure, 10,100 miles per hour is the exhaust velocity that might be expected from a chemical rocket at peak performance.

Google kindly converted the mph figure to 4515.104 meters per second. That would be 4.515 km/second.

Next I asked Google for the velocity of ions exiting an ion drive .... there were a number of citations ... this one seems useful:

Search Results
Web results
Ion thruster - Wikipediaen.wikipedia.org › wiki › Ion_thruster
An ion thruster or ion drive is a form of electric propulsion used for spacecraft propulsion. ... The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by 4.3 km/s (2.7 mi/s) while consuming less than 74 kg (163 lb) of xenon. ... is not ionized to its plasma form/corrode the cathode rods until it exits the tube.
Electrostatic ion thrusters · Radioisotope thruster · Lifetime · Missions
[

We can see that the ion drive produces exit velocity is comparable to very best chemical drive, as reported by Google.

The difference (that I see at least) is the simple number of tiny masses moving through the respective systems

***
In thinking about the differences between the two propulsion methods, it occurs to me to ask ... is there a hybrid possibility?

Chemical processes might exist that can yield ionized gases. I'm unsure if the exhaust from the Space Shuttle engines is ionized, but tend to doubt it.

Ionized gases could then be accelerated by a combination of electrostatic and electromagnetic methods.

(th)

tahanson43206 · 2020-11-10 06:33:42

For SpaceNut as a follow up ...

I was curious to know what the exit velocity of a VASIMR engine might be, compared to the figures reported by Google for an ion engine.

The difference is significant ...

Per Google:

Search Results
Featured snippet from the web
50 km/s.
The thrust exit velocity for the best chemical rocket engine is about 5 km/s. For the VASIMR rocket, the exit velocity is closer to 50 km/s.Feb 22, 2017
NASA's longshot bet on a revolutionary rocket may be about to ...arstechnica.com › science › 2017/02 › nasas-longshot-bet...
Feedback

This snippet reports a value for the chemical rocket that is consistent with snippets posted earlier in this series.

Thus, the VASIMR design would appear to produce exit velocity ten times greater.

If the mass pushed through the VASIMR engine were identical to that moving through a chemical engine, then (looking for correction here) I would expect the thrust to be precisely ten times greater.

I'm making the assumption that thrust will increase linearly with velocity, but since velocity squared comes up in various equations, that assumption might be incorrect. I am looking forward to comments by more knowledgeable members of the forum.

(th)

Terraformer · 2020-11-10 08:18:21

Thrust is proportional to mass flow rate and exhaust velocity. Double exhaust velocity but keep mass flow the same, double the thrust.

Calliban · 2020-11-10 10:15:06

An electric propulsion system that can use materials like raw Phobos or Lunar regolith, water, CO2, pure oxygen, etc, as propellant would be extremely valuable, as it could go anywhere and refuel with whatever it finds. An electric propulsion system that needs pure xenon as propellant is a bit like a prize race horse that eats only gourmet fodder. What is needed is a mule, that is maybe a little less efficient but is able to work with whatever it finds.

The problem with a drive system that combines high exhaust velocity with high thrust, is power requirements. Power requirements are proportional to thrust x exhaust velocity. That's just basic physics. The power of the Saturn V engines at takeoff, was equivalent to the electrical generating capacity of one of the larger European countries. It is nigh on impossible to build an electrical power system with sufficient power to weight ratio to allow an electric propulsion system to rival chemical propulsion in terms of thrust-weight ratio. Even if we could, the power supply to the engine would melt it. This is why electric propulsion is invariably low thrust.

Last edited by Calliban (2020-11-10 10:29:55)

tahanson43206 · 2020-11-10 10:46:59

For Terraformer re #21 ... thank you for confirming that my recollection was reasonably close to right!

For Calliban re #22 ... thank you for taking up this topic!

I went back over the topic to confirm this is your first (of what I hope will be many) post...

Along the way, I found this:

BTW, my candidate power supply concept would be a nuclear thermal rocket device feeding an MHD generator. None of that heavy steam loop nonsense. Use it as a rocket for getaways and captures, use it for MHD electric on the transits between to raise midpoint speeds. Get thrust from the electrics and the released plume. Same rocket hardware does both, which saves vehicle weight. That might really start looking good if gas core nuclear was the rocket device. Then we could start looking at excess electricity to use for processing propellants while electric-thrusting on the transits. It pays to dream big. Tells you what you really need to be working on, too.

I ** think ** that is the essence of VASIMR, which is (apparently) able to outperform ion drives by at least an order of magnitude.

Furthermore, I don't see any limit to number of engines that can be driven by a single nuclear fission reactor.

In space, and especially for a freight carrier, I don't see a need for a lot of shielding.

Calliban, since you are the only person currently active in the forum who has ** real ** experience designing a fission reactor, please consider what you would do if you were offered a contract to design a fission reactor to power as many VASIMR engines as are needed to produce whatever thrust is desired.

Let's say (for just one example) our need is to move a ton of Asteroid Bennu from where ever it happens to be to Lunar L1.

We'd like to move that ton is some semblance of a reasonable time, so that the vehicle can return to Bennu to take another bite.

A nuclear fission reactor would be ideal for long term service like that, and water (if it can be made useful in this context) would be an ideal throw mass.

It is ** not ** necessary to worry about the current inability of the United States to approve such a reactor. China or India are perfectly capable of building and deploying such a system.

(th)

Calliban · 2020-11-11 06:55:19

I would propose a boiling potassium reactor with uranium nitride fuel.

https://www.sciencedirect.com/science/a … 0566500256
https://archive.org/details/nasa_techdo … 9/mode/2up
https://www.osti.gov/biblio/4133861-boi … ctor-space

This idea has been kicked around since the 1950s, as it was obvious even back then that it would have unsurpassed power density and power-weight ratio.

A direct cycle reactor, so no heavy heat exchangers. Also, the very high hot leg temperature allows heat to be rejected at high temperature whilst maintaining good cycle efficiency. This is important, because it allows radiator surface area and mass to be minimised.

Another bonus: potentially very few moving parts. The coolant is liquid metal and therefore electrically conducting. That allows the use of MHD generators and pumps.

Longer term, a fission fragment reactor offers the promise of direct conversion of fission fragment kinetic energy into electricity.

Last edited by Calliban (2020-11-11 07:00:51)

tahanson43206 · 2020-11-11 08:37:00

For Calliban re #24

Thank you! I'll check the links you provided. My tentative plan is to approach Ad Astra Rocket (http://www.adastrarocket.com).

They are working on a thruster and have need of a space suitable fission reactor. While the United States is currently out of the picture, other nations are capable of mastering the technology, and ** may ** have the political wherewithal to arrange the resources and approvals for a project.

What I'm going for is a heavy duty pusher, capable of giving a Starship fully loaded with fuel and oxidizer, the impulse it needs to reach Mars on a Hohmann trajectory. The pusher can then return itself to LEO to take the next commission.

This use of a fission powered vehicle would be far more efficient than would sending the vehicle on a flight to Mars, because it would not be available for any other useful purpose while it is on a flight,

The second version of the vehicle could then be released to tackle a flight.

The pusher would also be able to send a fleet of asteroid nibblers to Bennu or any other suitable objective, thus saving those devices the need to accelerate themselves in the early part of their missions.

(th)

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