A Universe without Artificial Gravity...

[mike wightman]

I wonder if a plasma/ion combo drive is feasible - if so it could solve some problems as you describe.

Could an interface vessel with a fusion reactor onboard have several engines - some sort of fusion tea kettle which switches to scramjet mode then an ion/plasma drive for the final kick into orbit?

Alternatively build beanstalks to achieve orbit and then your ships can concentrate on space drives only.

Mining moons and asteroids becomes a profitable venture to provide the ion/plasma drive fuel.

Colonising a new world would begin with the construction of an orbital station and a beanstalk...

...a different sort of TU indeed. But one with potential.

 

[Uncle Bob]

Ion drives can carry a years worth of fuel at 0.00001 G or less. Of limited usefulness.

But both ion and plasma drives are 1960s concepts. Contemporary concepts like VASIMR or AIM are a lot more interesting.

Interface vessels will require either a huge mass ratio or a butch power supply. I favor VTOVL vehicles with a teakettle drive because they can land on small planets with thin air (Mars) as easily as a planet with thick atmosphere.

 

[aramis]

Bob: VASIMIR is a plasma rocket.

Abstract. An efficient plasma source producing a high-density (~1019 m-3) light gas (e.g. H, D,
or He) flowing plasma with a high degree of ionization is a critical component of the Variable
Specific Impulse Magnetoplasma Rocket (VASIMR) concept. [...]

(from NASA PDF on http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference.html

It's a specific type of plasma rocket.


Back to Ion:

If we strip all the science stuff, and look an the remainder on DS1, the Engine, Power and Fuel total less than 30KG. This includes the solar panels; the budget for the sapcecraft is a mere 2.4kW, and drive output is 94N, and fuel lasted more than 200 days.

If we allow a 50% Ion Drive, we get 60kg loading, or 0.0001G (1.5mm/s/s accell) from the extant drive (NASA loaded heavier, with loads of science instrumentality). It's an order of magnitude better than the 1960's, and well below theoretical maximum for extand ion drives; nuclear power (prohibited in space by treaty) could be far higher energy density than the solar panels by weight. The extant drives, tuned a little differently can produce up to 10 times the thrust, at (IIRC) about 100 times the power requirement.

Now, extrapolate a few tech levels, and you get a fairly long term, up to 1G drive (at about 90% spaceframe total working mass); useless in atmosphere, but then so is plasma.

 

[Uncle Bob]

I think assuming you can get a 10x increase in Vex just by throwing power at it is optimistic. Certainly grid erosian will become a serious problem. OTOH VASIMR is more than a plasma drive, more evan than a 1960s MHD drive, with a Isp of up to 10-30K (I have seen an estimate of 50k, but NASA is only going 30K).

The DS-1 thruster had a Isp of only 2-3K, so even if you get your 10x increase in Isp you will still be in the range of a TL8 VASIMR. NASA's next generation HiPEP engine will increase power levels by 10x, but Isp will only be 6K.

And that is already blurring the line between ion and MHD, since it generates ions from microwave heated Xenon gas, a plasma. "MHD" or "electric" propulsion are much safer terms than "plasma" or "ion".

And there is no problem using nuclear power: every NASA probe beyond mars orbit has used nuclear power, and will continue to. The Soviets even used them in LEO, which was not a good idea, but legal. NASA studies show a gas core reactor and MHD power system combined with VASIMR would approach 1MWe/ton. And that is what is being proposed for manned Mars missions.

There is a treaty that restricts nuclear weapons in space which eliminates Orion-style drives. It may even affect micro explosions like ACMF or AIM, but nothing restricts reactors.

 

[aramis]

Actually Bob, no they did not use fission nor fusion. They used radio-thermal (decay) generators (RTG). In which, a thermocouple system turns heat into electricity. There is no fusion nor fission occuring on the Voyagers; only radioactive decay.

They are explicitly exempt from the relevant treaties; a fission reactor is specifically banned in orbit. Don't recall the treaty, but it's from the sixties or seventies. Most of what is now the EU signed it, as did china, the USSR, the US and canada.

as for the 10x efficiency, from a different NASA Whitepaper which I can not find right now. It states that the power useages for the drive are in fact at optimal power-to-thrust, not optimal thrust; at the point where additional power will give additional thrust, but not at the same rate.

Ion drives are specifically NOT throwing superheated materials; such plasma would require massive magnetics to handle.

All the plasma designs involve elements of magnetic systems; MHD itself is a term meaning only the physics of magnetic fluids; all plasmas are by definition magnetic; it's one of the the key differences between plasma and gaseous states.

Ion designs are generally simpler, and given that simpler is more reliable, I suspect plasma will only be used when and where absolutely needed.

And the Xenon gas is not heated to a true plasma, either... if it were, the object grid would be vaporized. It's nowhere near the boundary you imagine it to be. It's agitated to ionize it; similar happens to Oxygen in laser-printers, and many other gases. hit them with the right frequency, and they shed electrons or grab electrons (an ion being any atom with an imbalance between electrons and protons). One you have an ion, even relatively non-magnetic atoms in default "neutral" state can be thrown about with magnetic fields while ionized. Ion drives take ions and use differentially charged grids to accelerate them.

The ISP numbers are far higher than you indicate. Did you do the math correctly? Or was it a wag like your guess at drive efficiency?

All Ion and Plasma drives are inherently magnetic; the term MHD drive is so bland and broad as to be meaningless in this context.

 

[Uncle Bob]

Sorry, the Cassini probe has a Brayton cycle nuclear thermal reactor. And some of the big soviet satelites had big reactors with kilos of plutonium on board. They might have been RTG, but it is hard to tell. Certainly they are as dirty as a NTR.

There is no treaty restricting nuclear power in space, at least not one the USA signed. NTR power has allways been part of NASAs deep-space program. The prevalence of RTG power up to now has been strictly an engineering decision, not political.

I took the ISP numbers from NASA sites and confirmed it on two others. Sorry to burst your bubble, but Google on DS-1, ion, and Isp

I didn't say a HiPEP was a plasma drive, but it blurred the line. A plasma, by definitiion, is a gas with free ions. Those ions may be allowed to escape down a nozzle (if there are enough of them at a high temperature, a pure plasma drive) accellerated magnetically (MHD/VASIMR) or electrostatically (ion). VASIMR, like HiPEP, uses microwaves to ionize a working fluid.

Ion drive has, IIRC, better theoretical efficiency than MHD but there are serious technical problems with operating above a few kV.

 

[aramis]

Bob, you had better check your sources for reliability.

http://saturn.jpl.nasa.gov/spacecraft/index.cfm
says:

Three Radioisotope Thermoelectric Generators -- commonly referred to as RTGs -- provide power for the spacecraft, including the instruments, computers, and radio transmitters on board, attitude thrusters, and reaction wheels.

I'll take NASA's word.

Wikipedia http://en.wikipedia.org/wiki/Cassini-Huygens cites the same power source, tho. (To be honest, i checked there first, to find the cassini mission page... searching NASA is not easy, as they use more acronyms per MB than I care to think about...)

BTW, the brayton cycle refers to Jet engines, according to Wikipedia. http://en.wikipedia.org/wiki/Brayton_cycle

It can be used sans combustion with a thermal source, and is the basis for the NTR's of which you speak, but all the linnks *I* can find from NASA indicate all of these are concepts and models, not active drives in use, and substitute a nuclear pile, either RTD or Fission, to heat a working fluid.

Nearly every unmanned spacecraft now is powered by plutonium... for it's radio-thermal decay, not for fissionability. Radio-isotopic decay releases heat and electrons. The electrons can be captured, and the the heat converted by thermocouples into electricity.

Nasa also cites a Specific impulse up to 1.9k to 3.1k s in http://gltrs.grc.nasa.gov/reports/1999/TM-1999-209439.pdf for DS1 on extant power sources.

As for plasmas, all plasmas are ionized gasses, not all ionized gasses are plasmas. The plasmas references in drives are hot-plasmas (4th state of matter); the "plasma" in ion drives is not; it's merely an ionized gas, without all the specific heat, density, and other odd requirements for True, aka Hot, plasmas.

 

[jappel]

A bit further down the speculative path, and involving either some optimistic assumptions or a bit of stretch regarding G-tolerances, is the milieu of Walter Jon Williams' "Dread Empire" series. The ships use anti-matter conversion torches and in wartime military vessels routinely pull multiple Gs. (Interstellar travel is accomplished via naturally occuring wormholes.) Naval crews wear G-suits when very high accellerations are called for, and routinely take a cocktail of medications to protect against at least some effects of prolonged high accellerations. The accelleration couches in action stations are mounted in cages and can rotate fully as needed. Still, crew do suffer casualties from strokes and other ailments as a result of the long-term debilatory effects.

One minor subplot highlights the different G-tolerances of the various races involved - one race is from avian stock complete with hollow bones, and is drastically limited in how many Gs they can pull for prolonged periods.

The two books are The Praxis and The Sundering, with the third book, Conventions of War (I think it has a different title in the UK) coming out this fall. Highly recommended, but then I recommend WJW's work in general.

- John

 

[Malenfant]

Um, yeah. Cassini - and indeed every other US space mission launched so far - does not have a nuclear reactor on board. IIRC the old soviet Cosmos satellites did though.

RTGs are not 'nuclear reactors' - they're basically a lump of plutonium that sits in a heavily shielded box. As the plutonium decays, it emits heat. The heat is then converted to electricity via thermocouples. While they may generate a lot more power (a few hundred watts), the thermocouples have a maximum efficiency of about 10% at most. As such, you can't get an awful lot of power out of them - only a few dozen watts at most.

http://en.wikipedia.org/wiki/Radioisotope_thermal_generator

Oh, and Aramis - here's the NASA Cassini mission page.

 

[Uncle Bob]

Mmmm. I'l concede Cassini. Still, you find lots of concepts at NASA for NTRs. I have read a lot about engineering problems, never one word about any "treaty" obligation. I would be obliged if you could find one for me. If nothing else I would love to see how a diplomat defines the difference between nuclear and radiological with a 6th grade science vocabulary.

I am glad we can agree on the 2-3ks Isp for DS-1. HiPEP will be 6ks, VASIMR will be 10k-30ks. AT TL 7 ion is ahead, by TL8 it is 'way behind. While it is possible it will catch up, I know of no reason to believe it will.

A plasma is definned as an ionized gas. There are no pressure or energy requirements. The exhaust of an ion drive is neutralized, but while it is being accellerated it is a plasma. As for hot and cold plasmas heat is just moving particles. Ion engines 20-30,000 m/s exhaust is very hot. The 100-300,000 m/s exhaust from the VASIMR is nearly all linear, so if you want to define "heat" as random motion it is a very small component of the exhaust.

The earliest "plasma" rocket concepts used resistance heating to superheat a gas into a plasma, estimated Isp 1-2k. In a MHD system nearly all of the energy of the exhaust (99%) comes from the magnetic fields.

So in an ion drive 100% of the energy comes from electrostatic accelleration of nuclei (plus a trivial amount for ionization). In a MHD drive, 99%+ comes from the magnetic accelleration of nuclei. In a plasma drive 100% comes from electrothermal heating.

While a MHD drive does have a plasma exhaust, calling a MHD drive a "plasma" drive is misleading at least.

 

[Malenfant]

I don't think there's any treaty prohibiting nuclear reactors in space. There is however a treaty prohibiting nuclear WEAPONS in space.

The main reason why reactors aren't used much in space is because (a) they increase the mass of the ship, which means you need bigger boosters to get it off the ground and (b) there are a number of groups who scream blue bloody murder at the thought of sticking a nuclear reactor on top of a rocket because they're terrified there'd be an accident and it'd blow up in the atmosphere and Poison The Whole World (TM). Of course, these are the sorts of people who don't believe NASA scientists and engineers because They Know Better.

 

[Uncle Bob]

I think above a certain power level (100 KW?) nuclear-thermal power plants become lighter than RTG. Very few systems need that level of power.

And RTGs are much more reliable, being essentially solid-state devices. Even if they are a little heavy they are preferred for long duration missions.

 

[Fritz_Brown]

Unfortunately, Mal, on occasion scientists and engineers have given them cause to believe that They Know Better. :? Remember the "Oh! That was in metric?" moment with the Mars probe?

To get around this fear, we need fusion reactors. Launch them "empty", then fill with H2 and start once in space. Hey! That sounds familiar...

 

[Uncle Bob]

Nothing wrong with H2 or D2 (tritium is nasty in quantity) fully fueled. Hell, if you have an aneutronic reaction going launch it live. Won't be much worse than a gas turbine and a couple of tons of JP4.

We can't seem to get it to work, yet. It has been ten years off for forty years. Pity you can't make a breakthrough by Presidential order. Or even buy one reasonbly.

 

[Sifu Blackirish]

Fritz88 said:

2) Heavy-worlders gain an advantage with their ships. If your entire crew comes from a 1.75G world, your ship could be built to sustain 1.75G, with bursts higher than 6G possibly. How does that impact combat/recruiting/who runs things?

An old, old trilogy called Starwolf by Edmund hamilton explored this. The Starwolves were a culture of galactic raiders and pirates, and were from a heavy gravity planet. They could outlast anyone else at sustaining high-grav acceleration.

 

[Fritz_Brown]

Yes, I remember that! I don't think I have any of those, though, in my collection.