Author Topic: VASIMR Engine  (Read 156228 times)

Offline Hop_David

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Re: VASIMR Engine
« Reply #160 on: 03/02/2015 05:54 PM »
7 km/s Burns from EML1 are crazy high on their face but get even worse when you remember that it is 4 km/s just to get up to EML1

Ah. You didn't bother to read the link. Here it is again.

Earth isn't the only possible source of propellent. A propellent source high on the slopes of earth's gravity well changes the picture. It'd be helpful for chemical as well as ion rockets.


Offline Impaler

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Re: VASIMR Engine
« Reply #161 on: 03/02/2015 06:40 PM »
Actually I did read your link and I commented that such a source or propellent is unlikely to materialize any time soon.  And if we had it we would not want to waste it on chemical propulsion or the even more wasteful hybrid-system you described earlier.

That said I do agree that a reusable chemical Earth-departure stage is a fine idea assuming I am going full chemical departure (indeed without them the all-chemical idea is unsustainable from a cost of hardware perspective) and have said so in the MCT speculation threads, I just think it will be employed from LEO, burn from LEO return to LEO and be refueled in LEO.  EML1 is a highly desirable stopping off point for SEP vehicles because of the transit time issues and crew rendezvous necessity on their way to departure, but it makes no sense to send chemical vehicles through it, direct departure from LEO will be preferable not least of which is that I can just send the crew to the vehicle by a simple LEO capsule like Dragon or CST-100.

Offline Hop_David

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Re: VASIMR Engine
« Reply #162 on: 03/02/2015 07:20 PM »
Actually I did read your link

The link talked about the possibilities opened by other propellent sources.

So what does your 4 km/s from LEO to EML1 argument demonstrate? That you enjoy typing out a wall of text based on an irrelevant straw man.

and I commented that such a source or propellent is unlikely to materialize any time soon.

Well, I happen to believe developing extra-terrestrial propellent is more plausible than a 2kWe/kg energy source. But that is a separate argument.

If you want to argue other propellent sources are implausible that's one thing. But tossing out delta V budgets based on earth being the sole propellent source is a waste of my time.

Offline Impaler

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Re: VASIMR Engine
« Reply #163 on: 03/02/2015 08:18 PM »
Well, I happen to believe developing extra-terrestrial propellent is more plausible than a 2kWe/kg energy source. But that is a separate argument.

If you want to argue other propellent sources are implausible that's one thing. But tossing out delta V budgets based on earth being the sole propellent source is a waste of my time.

Well Earth IS the only source of propellent right now so DeltaV from LEO is what any chemical system has to work under, so it is quite relevant.  As soon as you start talking about in-space propellent sources your into a HIGHLY speculative area and were weighing two different speculative ideas.

But it needs to be a fair comparison, you've comparing the power source density that makes 39 day trips to Mars possible (and that's after the VASMIR system sand-bags the vehicle as other thrusters have better density) vs an off world propellent source and chemical architecture that's makes a 4.3 - 5.7 month transits.

This is not even remotely in the same ball-park of difficulty.  If you want a EP system to do a comparable transit 4-6 month transit it's power density can be an order of magnitude lower, making it actually quite near term.  Thus your conclusion about relative nearness/plausibility is reversed, off world propellent is much less plausible then the power-density for an EQUIVALENT PERFORMANCE electric propulsion system.

Offline Hop_David

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Re: VASIMR Engine
« Reply #164 on: 03/02/2015 10:20 PM »
Well Earth IS the only source of propellent right now

Well a 2kWe/kg power source IS a fantasy right now.

The conventional wisdom is that chemical is inadequate. NTR, NEP or SEP are needed.

Which is completely untrue if there there is a source of chemical propellent near C3=0. Bringing about such a propellent source is one of the first goals of Planetary Resources. Not to mention Jeff Greason, Bill Stone, Paul Spudis and a lot of other folks.

And given a propellent source near C3=0, a chemical EDS could indeed lend a hand to ion engines.


Offline Impaler

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Re: VASIMR Engine
« Reply #165 on: 03/03/2015 07:53 AM »
Ok Planetary Resources is going to get us to Mars, see you there in 100 year  ::)

Also did you not read the point that 2kw/kg is complete overkill and a not necessary or are you just ignoring that so you can continue making dishonest comparisons?

And lastly NO, using chemical propulsion from EML1 dose not 'lend a hand' to SEP any more then say taking a Hummer and having it push a Prius around would 'lend a hand', it wastes propellent that could otherwise be going into the SEP vehicle without reducing our transit time by any appreciable amount. 

That propellent is not going to be cheap no mater how advanced or large scale our fantasy production process becomes, we are going to want to send as much stuff to Mars per unit of our precious propellent as possible.  That rules out using chemical UNLESS we absolutely must because of high acceleration needs.

Offline RanulfC

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Re: VASIMR Engine
« Reply #166 on: 03/03/2015 05:34 PM »
And lastly NO, using chemical propulsion from EML1 dose not 'lend a hand' to SEP any more then say taking a Hummer and having it push a Prius around would 'lend a hand', it wastes propellent that could otherwise be going into the SEP vehicle without reducing our transit time by any appreciable amount.

Really? 'cause just about every study I've seen if you JUST launch from EML-2 straight to an interplanetary trajectory its ok, but it you say do an Earth-flyby from EML 1/2 whatever drive you use for the injection at perigee is greatly enhance and chemical will do it better than straight SEP does. From EML-1 to interplanetary injection is a (to use your totally inadequate "cars" analogy :) ) is a Hummer pushing a Prius to the crest of a hill so the (supposedly under-powered and slow accelerating Prius doesn't take forever to get UP the hill in the first place) can then push itself over the crest and down the other side. In proper terms the chemical booster (which then remains near its source of propellants for ease of access) gives the SEP a very neat "kick-in-the-pants" to get it out of the local gravity field without the SEP having to waste propellant doing it for itself.

SEP is a low thrust system unless something changes drastically with future systems while chemical/nuclear are high thrust systems which trade propellant for time. SEP takes time to go anywhere and EVENTUALLY can reach some serious delta-v but it trades time for the higher efficiency and lower thrust.

Quote
That propellent is not going to be cheap no mater how advanced or large scale our fantasy production process becomes, we are going to want to send as much stuff to Mars per unit of our precious propellent as possible.  That rules out using chemical UNLESS we absolutely must because of high acceleration needs.

And therefore chemical by your argument, (and its one Elon Musk, Zubrin, etc have used) is superior to SEP because we in fact are limited on TIME and not propellant. Once out away from Earth, (and beyond the Van Allens) time is a bit more on our side but not that much.

if it sounds like I'm arguing AGAINST SEP then you don't understand the argument because I'm not. But it does take time and effort to get to the point where SEP is comparable with chemical for the final push to interplanetary trajectory. In the end getting the ship, personnel and all equipment to point around EML-1/2 is a prerequisite to using SEP efficiently. With just about as much effort you can also get propellant from the Moon so the two are far from mutually exclusive proposals.

Randy
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British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Impaler

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Re: VASIMR Engine
« Reply #167 on: 03/03/2015 09:51 PM »
OK lets keep going with this Hummer/Prius analogy, it is kind of amusing.

The flaw in your hill analogy is that EML1 is not the top of a hill, it is more like the lip of the grand canyon with the canyon being the Earths gravity well (it's not literally C3=0 but its very close).  Once your up on the plateau (heliocentric space) it is perfectly flat and the Prius is free to accelerate.  If you were going to use a Hummer to push a Prius you would do it going up the side of the canyon UNTIL you got to the lip, not after that point. 


Your correct that we can just depart from EML1 directly to Earth Escape without doing any swing buys of the moon or Earth.  And the DeltaV is VIRTUALLY THE SAME.  The Chemical high thrust system gets a LITTLE boost of something like 100 m/s from plunging deep into thouse gravity wells and burning with an Oberth effect.  But is in no way comes even remotely close to making them as efficient as Electric propulsion, when the ISP difference is an order of magnitude the tiny improvement in the chemical system is pathetic.

How do you claim that 'In the end getting the ship, personnel and all equipment to point around EML-1/2 is a prerequisite to using SEP efficiently' is 'With just about as much effort you can also get propellant from the Moon'.  That is absurd, the SEP vehicle simply spirals out for a few months, you know cause it is a VEHICLE designed to do exactly that, and then a capsule dose a rendezvous and transfers crew something that our capsules are very good at and do at ISS routinely, a rendezvous at EML1 will be adding a trans-lunar injection step which we did back under Apollo-8 and plan to do again with Orion.  This is simple operational and technologically.

Propellent production on the moon is outrageously harder to do, requiring a whole mining infrastructure operating autonomously at near absolute zero, storing it and then loading into tankers to orbit (which will burn ~80% of the propellents produced on the surface to deliver the 20% remainder to orbit and then land again).  No one in their right mind could equate these things in difficulty.

Offline Hop_David

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Re: VASIMR Engine
« Reply #168 on: 03/03/2015 10:55 PM »
The argument that we don't want to spiral out from Earth is only applicable for CREW and even then only for the first few Radii of the Earth where the Belt is.  We would send the bulk of the mission mass up to a high orbit and then after the crew arrives we just Spiral out some more for the Earth-Escape.  The initial escape velocity from Earth will be low but that is fine, we are going to be building velocity over months.  The speed that the Chemical departure from Earth would have gotten us (1 km/s which still costs a hefty 20% propellent fraction)

Incorrect.

Falling from EML2 will give you an extra 3.1 km/s at perigee. Not 1 km/s.

3.1 over the 7.7 km/s LEO speed is 10.8 km/s, a perigee velocity just a hair under escape.

At 10.8 km/s you wound have your burn enhanced by an healthy Oberth benefit. A .5 km/s burn at perigee will buy you 3 km/s Vinfinity.


will be obtained in mere 11 days at an acceleration of 1 mm/s^2.

Most proposed Mars Transfer Vehicles (MTV) I've seen mass 130 tonnes.

Spiraling from LEO to C3=0 takes about 7 km/s delta V. Then spiraling to Mars takes another 6 km/s. Assuming a 30 km/s exhaust velocity, it'd take 70 tonnes of xenon impart 13 km/s to a 130 tonne MTV.

So now we have 200 tonnes not counting the power source and ion rocket engines.

To give an acceleration of 1mm/s^2 to 200 tonnes requires a 200 newton ion engine.

Please describe the power source you have in mind for a 200 newton ion engine.

Offline Hop_David

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Re: VASIMR Engine
« Reply #169 on: 03/03/2015 11:26 PM »
Ok Planetary Resources is going to get us to Mars, see you there in 100 year

What? You want to colonize Mars in a few decades. And with the same breath you say extracting water from a Near Earth Asteroid is a bridge too far?

That's hilarious.

Also did you not read the point that 2kw/kg is complete overkill and a not necessary or are you just ignoring that so you can continue making dishonest comparisons?

To be fair you have distanced yourself from the 39 day VASIMR trips.

In an earlier post I had thought you were talking about an 11 day spiral out of earth's gravity well. So I thought you were still stuck on this magic alpha.

On a more careful reading I see you were talking about 11 days to accelerate 1 km/s.

So yes, it is wrong  for me to wave the 2kWe/kg in your face. My apologies.

And lastly NO, using chemical propulsion from EML1 dose not 'lend a hand' to SEP any more then say taking a Hummer and having it push a Prius around would 'lend a hand', it wastes propellent that could otherwise be going into the SEP vehicle without reducing our transit time by any appreciable amount.

This based on the assumption that burns totaling 1 km/s gets you 1 km/s delta V. Which is of course wrong.

The propellent and life support consumables from an asteroid would be high in earth's gravity well and have lots of potential energy. Nudges totaling .4 km/s suffice to send the ship to a low perigee where potential energy is converted to kinetic energy. At perigee the space ship would be moving 10.8 km/s. At this high speed the Oberth benefit is huge and another .5 km/s suffices for Trans Mars Insertion. Thus about 1 km/s suffices for TMI.

Spiraling from LEO to C3=0 takes about 7 km/s. If the acceleration is 1 km/s each 11 days, it'd take 77 days to spiral out of earth's gravity well. Falling from EML2, it'd take about 9 days to reach escape velocity.

And even if exhaust velocity is 30 km/s, 7 km/s to get out of earth's well is nothing to sneeze at.
exp(7/30)-1= .263.

Offline Burninate

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Re: VASIMR Engine
« Reply #170 on: 03/04/2015 12:08 AM »
My attitude is that 39 day transits are nuts.  90 day transits are unrealistic.  ~200 day transits are realistic, but not spiralling out from HEO - it takes too long for a given power.

Spiral out to ~1LD on 4000s xenon over several years, then lower periapsis to ~400km over several months with xenon, then send a Crew Dragon on a methane transfer stage up to rendezvous, then Oberth burn the last of its 380s methane (~400m/s) to transition to a low energy Mars transfer, then spend three months coasting and four months burning xenon (a few hundred m/s) to capture.

You just arrived at Mars orbit rendezvous with a ~2/3 IMLEO mass fraction and an intact solar propulsion system without staging or aerocapture.

Ion propulsion doesn't give you speed, it gives you prepositioned assets on very low IMLEO.  A high elliptical Earth orbit transfer vehicle & fuel, which is >80% of the dV to Mars, is a prepositioned asset that human crew can inhabit and leave orbit with in a brief amount of time.
« Last Edit: 03/04/2015 12:37 AM by Burninate »

Offline Impaler

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Re: VASIMR Engine
« Reply #171 on: 03/04/2015 06:30 AM »
Please describe the power source you have in mind for a 200 newton ion engine.

I'll describe both the thrusters and the power source.  20 Concentric Nested Hall thrusters (the X3 from University of Michigan in testing now) operating at 200 kw each and delivering 10 N each.  Total mass is 6,000 kg including power processing units.  Power would be 4MW provided by a solar array with a power density of 250W/kg (design goals of arrays in development now) and massing a total of 16,000 kg.  This gives a total propulsion system mass of 22,000 kg.  The alpha value is thus 5.5 kg/kw quite low and nothing spectacular.  And this is enough to do the job, in fact it may even be excessive, we can spiral out slower then 77 days at higher ISP to save propellents for the heliocentric phase of the journey.

Offline Hop_David

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Re: VASIMR Engine
« Reply #172 on: 03/04/2015 03:34 PM »
Please describe the power source you have in mind for a 200 newton ion engine.
I'll describe both the thrusters and the power source.  20 Concentric Nested Hall thrusters (the X3 from University of Michigan in testing now) operating at 200 kw each and delivering 10 N each.

20 kilowatt per newton Hall thrusters. Yes, that's plausible. In fact that's what I used in Catching an Asteroid. The Keck vehicle would have 4 10 kw thrusters which I figured would provide two newtons.

Total mass is 6,000 kg including power processing units.  Power would be 4MW provided by a solar array with a power density of 250W/kg (design goals of arrays in development now) and massing a total of 16,000 kg.

I'd like to know more about these solar arrays in development. Could you provide a link?

One figure I'm interested in is watts per square meter. What would be the area of this 4 mega watt solar array?

Offline Nilof

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Re: VASIMR Engine
« Reply #173 on: 03/04/2015 04:33 PM »

I'd like to know more about these solar arrays in development. Could you provide a link?

One figure I'm interested in is watts per square meter. What would be the area of this 4 mega watt solar array?

A rough approximate figure from the phase I ROSA winglets is ~3 square meter per kW.

If the design uses phase 1 winglets along a long boom which extends 15 meters each way, you get roughly ~30 square meters of arrays, or 10 kW, per meter of boom. That gives us ~400 m of trussings to support the winglets. For a cross shaped design this gives you four 100m booms.

This is big, but then again so is your 200 ton ship with a bigger payload than a BA 2100.  If you went with a ~50 ton ship pushing a Skylab sized BA-330 habitat that launches on a single SLS launch, you're looking at four ~25 meter booms, which is short enough to comfortably fit inside the largest SLS fairing.

In fact, this last ship design is something I've actually done in KSP with the realism overhaul modpack. See the attached screenshot for a an example ~60 ton ship with 2 MW of solar arrays, which was launched without a hitch with a single SLS.

It isn't a beauty since it was built out of parts that really weren't meant to be used together, but it does give you a rough idea of the relative sizes of the solar panels needed.
« Last Edit: 03/04/2015 05:04 PM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Impaler

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Re: VASIMR Engine
« Reply #174 on: 03/04/2015 05:15 PM »
Here is the solar array, it's from the Airforce and is called RAPDAR (Roll-out And Passively Deployed Array).

It appears to be is a thin-film solar membrane on a memory shape material that unrolls in the warmth of the sun.

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCoQFjAA&url=http%3A%2F%2Fwww.dtic.mil%2Fcgi-bin%2FGetTRDoc%3FAD%3Dada444956&ei=E0b3VLmZAoapogSclYGYDw&usg=AFQjCNGIl9ZWrPlnvVDUGJO1T3-RcwZOOg&sig2=NOHULiIg5knnM_pVDUIBNA

http://www.nsti.org/Nanotech2009/abs.html?i=1497


Offline Stormbringer

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Re: VASIMR Engine
« Reply #175 on: 03/04/2015 05:37 PM »
just throwing out a random thought... have you guys seen that recently deployed bowl shaped thing on SMAP:

http://www.space.com/28676-dirt-satellite-antenna-video.html

its 20 meter diameter.

and there is plans for this thing:

http://www.space.com/28650-space-telescope-tech-aragoscope.html

with a half a mile diameter. a half mile diameter.

arent there new solar cell materials that roll and unroll?
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Offline tea monster

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Re: VASIMR Engine
« Reply #176 on: 03/04/2015 07:47 PM »
As far as I'm aware, the rapdar stuff is rolable.

I'd be interested in some figures for the power to area as well. I'm thinking of doing a 3D model of one of these things. I was thinking that you could unfurl the solars in petals from the central core. You could even put a gentle spin on the ship to stabilize them.

On that subject, how much fuel would you need for an X3 hall thruster vehicle if you were to try to get to Mars in 40 days? It might be wasteful, but how does it compare to having to carry all the extra consumables for a crew for the extra months? Also, if you were an astronaut, and they told you you could get to Mars in 40 days, or you would have to spend an extra 6 months living in a balloon the size of a school bus, what would your reaction be?
« Last Edit: 03/04/2015 07:53 PM by tea monster »

Offline Hop_David

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Re: VASIMR Engine
« Reply #177 on: 03/04/2015 11:14 PM »
Here is the solar array, it's from the Airforce and is called RAPDAR (Roll-out And Passively Deployed Array).

It appears to be is a thin-film solar membrane on a memory shape material that unrolls in the warmth of the sun.

It's not rigid? How will you keep the surface perpendicular to the sun's rays?

The thrust vector would be perpendicular to the radius vector from earth's center, correct? and the thrust would have to be through the ship's cg.

So the ship and thrusters would be rotating 360 every 90 minutes when it's in low orbit and slower as the orbit rises.

But the solar arrays will need to remain pointed towards the sun.

Looking at the PDF I didn't see watts per square meter. At the moment I'm imagining this power source as a few hectares of Seran Wrap.

Offline Hop_David

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Re: VASIMR Engine
« Reply #178 on: 03/04/2015 11:18 PM »
On that subject, how much fuel would you need for an X3 hall thruster vehicle if you were to try to get to Mars in 40 days?

It's the 2kWe/kg alpha that Sorensen was objecting to. More fuel doesn't help that alpha.

Offline Solman

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Re: VASIMR Engine
« Reply #179 on: 03/05/2015 02:04 AM »
Well Earth IS the only source of propellent right now

Well a 2kWe/kg power source IS a fantasy right now.

The conventional wisdom is that chemical is inadequate. NTR, NEP or SEP are needed.

Which is completely untrue if there there is a source of chemical propellent near C3=0. Bringing about such a propellent source is one of the first goals of Planetary Resources. Not to mention Jeff Greason, Bill Stone, Paul Spudis and a lot of other folks.

And given a propellent source near C3=0, a chemical EDS could indeed lend a hand to ion engines.

I think "fantasy" is perhaps a little strong with respect to 2kWe/kg but of course YMMV  :)
I realize that the solar concentrator based system I advocate does not currently exist as a TRL 9 space system but all the elements to build it do exist and have been separately tested or used in space and the system of large concentrator with actively cooled PV array for high solar concentration is being used by an outfit in Australia currently to generate electricity. With the 17 kW(thermal)/kg prototype concentrator developed by L'Garde it is difficult to see how it could do less than 5kW/kg.
A large concentrator facilitates solar thermal for LEO to escape with the PV moved into the focus to power SEP thereafter with only the mass penalty of the bread box sized STP engine.

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