Author Topic: VASIMR Engine  (Read 114541 times)

Offline Robotbeat

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Re: VASIMR Engine
« Reply #180 on: 03/05/2015 02:42 AM »
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?
...
It doesn't HAVE to be perfectly rigid. If you're off by a few degrees, there's only incredibly slight reduction in power. And it is deployed, so launch harmonics isn't a concern.

This is different from a traditional antenna or a concentrating solar array which DO have to be rigid because of their high pointing requirements (phased array antenna can compensate for this).
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Offline Impaler

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Re: VASIMR Engine
« Reply #181 on: 03/05/2015 02:57 AM »
When were orbiting the Earth ever 90 minutes the sun is shifting only 4 degrees per minute, a rate that we can easily match by rotating/pivoting the boom arm that the solar array is on without significant stress.  The RAPDAR is by no means Saran wrap it looks more like a sheet of plastic at least as thick as a milk-jug, Saran wrap would be IKAROS derived system 'solar sail' stuff which would be at least 4 times higher power density and which is a good deal more speculative but still not a fantasy.  I think an advanced total system alpha of 2 is the most optimistic view that dose not fall under fantasy, aka we can see a clear design that gets us their but know that it will be very hard, and I don't see such systems coming online for decades.

I went with systems that are currently in development to give us a very near term system with numbers we could be confident about as that seems to be a big hand up in the discussion, the idea that SEP is not viable without some BIG advancement that is so speculative that it may not ever happen.

Offline Robotbeat

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Re: VASIMR Engine
« Reply #182 on: 03/05/2015 03:35 AM »
Why would you characterize IKAROS as speculative? It already flew.

A fairer suggestion would be that it is an unorthodox space structure due to its very low stiffness, and thus would require more care than usual for designing your spacecraft correctly, but I don't believe "speculative" makes sense, there.

...and I'm sorry for jumping on that word (when in all likelihood we're in violent agreement), I just think it's an important distinction to make.

People have been testing (and preparing for launch) solar sail craft for a while, now. An IKAROS-esque solar array would be no more speculative than that sort of structure. Clearly wouldn't be characterized as fantasy.
« Last Edit: 03/05/2015 03:37 AM by Robotbeat »
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Offline Robotbeat

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Re: VASIMR Engine
« Reply #183 on: 03/05/2015 03:43 AM »
I think a good proposal for a small-sat tech demo would be a "solar power sail" type craft with extremely good alpha.

IKAROS-style high voltage solar array (make it very long and skinny so that it demonstrates scalability) hooked directly up to a very high specific power thruster (built for high operating temperature so that extra heat rejection using a complicated radiator system is not required) using, say, solid-state relays. I bet you could demonstrate 1-2kW/kg OVERALL specific power (but with a system mass of, say, 100-200kg). It'd be a challenge, but should definitely be possible and would seriously change the whole calculus of SEP.

...then again, we're still waiting for even modest SEP propulsion tech on cubesats...
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Offline Robotbeat

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Re: VASIMR Engine
« Reply #184 on: 03/05/2015 03:59 AM »
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

More interesting, IMHO, is the MegaROSA concept, which can do 400-500W/kg (and hundreds of kW per wing). The smaller ROSA array is supposed to fly to ISS in 2016. (This is similar to but much, much better than Hubble's original solar arrays...)
http://www.keyt.com/news/goleta-based-company-could-power-nasa-to-mars/26754032


Or similarly, the MegaFlex array by ATK capable of 200-300W/kg and built on UltraFlex heritage:
« Last Edit: 03/05/2015 04:00 AM by Robotbeat »
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Offline Impaler

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Re: VASIMR Engine
« Reply #185 on: 03/05/2015 05:03 AM »
Why would you characterize IKAROS as speculative? It already flew.

A fairer suggestion would be that it is an unorthodox space structure due to its very low stiffness, and thus would require more care than usual for designing your spacecraft correctly, but I don't believe "speculative" makes sense, there.

...and I'm sorry for jumping on that word (when in all likelihood we're in violent agreement), I just think it's an important distinction to make.

People have been testing (and preparing for launch) solar sail craft for a while, now. An IKAROS-esque solar array would be no more speculative than that sort of structure. Clearly wouldn't be characterized as fantasy.

I think we are generally in agreement, while IKAROS has flown the power system your extrapolating at 1 kw/kg is 'derived' from it and has some significant engineering challenges when being scaled up to the size (multiple MW) that would be the kind of total power needs that would be desired, demonstrating scale is a big part of making the specific power number relevant for SEP.  So I'm simply calling any specific power density goal which is not the design goal to a current development program a 'speculative' one. 

Essentially I assume all current development programs that are just refining current tech will meat their goals (admittedly optimistic) but anything outside of a program has some chance of not being delivered upon and thus is a 'speculation'.  Naturally their are huge ranges between very likely to be delivered upon and very unlikely to be delivered upon speculative systems.  When the speculation exceeds what even the best known materials or some very obvious thermodynamic or practicality limits then I'd call it fantasy, and I'd say that when it comes to solar power density the >2kw/kg can be called fantasy, 250W-2kw can be called speculative, and <250W/kg can be taken to the bank.

I had been trying to find the numbers on the MegaROSA and how it compared with original ROSA, I had thought the power density was much lower around 150W/kg for ROSA and couldn't find a number for MegaROSA, your link doesn't seem to have a reference to power density though, do you have one that dose?

Re: VASIMR Engine
« Reply #186 on: 03/05/2015 05:34 AM »
Maybe a bit off-topic question, but.. cout a VASIMIR engine be used to take the ISS to Mars? Not necessarily as a spacecraft.., but maybe as a support station in Mars orbit. Even if it takes 5-6 yrs to get there.. better than being decommissioned.

Offline ChrisWilson68

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Re: VASIMR Engine
« Reply #187 on: 03/05/2015 05:39 AM »
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?

It's not that you'd need much propellant for Hall thrusters, it's that you'd need lots and lots of them.  The mass of the thrusters themselves would dominate if you wanted enough thrust to get to Mars in 40 days.  Hence the idea for VASIMR -- better thrust-to-weight, at the cost of worse Isp (so more prop needed).

Offline A_M_Swallow

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Re: VASIMR Engine
« Reply #188 on: 03/05/2015 06:18 AM »
I think a good proposal for a small-sat tech demo would be a "solar power sail" type craft with extremely good alpha.

IKAROS-style high voltage solar array (make it very long and skinny so that it demonstrates scalability) hooked directly up to a very high specific power thruster (built for high operating temperature so that extra heat rejection using a complicated radiator system is not required) using, say, solid-state relays. I bet you could demonstrate 1-2kW/kg OVERALL specific power (but with a system mass of, say, 100-200kg). It'd be a challenge, but should definitely be possible and would seriously change the whole calculus of SEP.

...then again, we're still waiting for even modest SEP propulsion tech on cubesats...

The CAT thruster from the University of Michigan is a SEP for cubesats. It is due to be launched within the next year. The thruster takes 10W and fits in a 3U cubesat. The 20W solar panels on the current spacecraft could be replaced by experimental solar arrays in later launches.

http://pepl.engin.umich.edu/thrusters/CAT.html

Offline Impaler

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Re: VASIMR Engine
« Reply #189 on: 03/05/2015 06:41 AM »
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?

It's not that you'd need much propellant for Hall thrusters, it's that you'd need lots and lots of them.  The mass of the thrusters themselves would dominate if you wanted enough thrust to get to Mars in 40 days.  Hence the idea for VASIMR -- better thrust-to-weight, at the cost of worse Isp (so more prop needed).

That's backwards, Halls have higher power to mass then VASIMR.  VASIMR has higher ISP range but in the area where the ISP ranges overlap (the low end of VASIMR, the High end of Hall) the Hall has better thrust to mass ratio.  VASIMR would be the choice for a very slow trajectory with minimal propellent usage by utilizing it at near the maximum ISP it's capable of.

And seriously the 40 day stuff is total overkill, even the VASMIR designs that did this used huge amount of propellent (400-600 MT if I recall) and pathetically small payloads to do it on top of the absurd amounts of power.  The extra crew consumables for a slower transit would be orders of magnitude less, I think crew consumables worst case are ~10 kg a day for everything (food, water, sanitation), so ~2 MT for a crew of 6 for a whole month.

SEP will certainly run up against some barrier at long transit times where savings in propellent mass are offsets by growing crew consumable mass but it will be out somewhere in the multiple months time frame and our life-support and recycling get better the cross over point gets pushed further out in time and becomes less of a factor.  Desire for fast transits has always been driven by radiation and health concerns, not consumable masses.
« Last Edit: 03/05/2015 06:45 AM by Impaler »

Offline ChrisWilson68

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Re: VASIMR Engine
« Reply #190 on: 03/05/2015 06:47 AM »
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?

It's not that you'd need much propellant for Hall thrusters, it's that you'd need lots and lots of them.  The mass of the thrusters themselves would dominate if you wanted enough thrust to get to Mars in 40 days.  Hence the idea for VASIMR -- better thrust-to-weight, at the cost of worse Isp (so more prop needed).

That's backwards, Halls have higher power to mass then VASIMR.  VASIMR has higher ISP range but in the area where the ISP ranges overlap (the low end of VASIMR, the High end of Hall) the Hall has better thrust to mass ratio.  VASIMR would be the choice for a very slow trajectory with minimal propellent usage by utilizing it at near the maximum ISP it's capable of.

No, it's not backwards.  What I said is exactly right.  Of course in the low thrust range Hall has a better thrust to mass ratio.  But we aren't talking about the low thrust range.  The question was how to get to Mars in 40 days.  That requires high thrust.  So VASIMR operating in high-thrust mode is what we are comparing to the Hall thrusters here.  And in high-thrust mode, VASIMR has better thrust to weight.

Online KelvinZero

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Re: VASIMR Engine
« Reply #191 on: 03/05/2015 07:53 AM »
Maybe a bit off-topic question, but.. cout a VASIMIR engine be used to take the ISS to Mars? Not necessarily as a spacecraft.., but maybe as a support station in Mars orbit. Even if it takes 5-6 yrs to get there.. better than being decommissioned.

Yeah that has been a subject a few times. There are a bunch of reasons against it, apparently.
This thread looks like it also lists a bunch of other threads:
http://forum.nasaspaceflight.com/index.php?topic=29003.msg906340#msg906340

Offline Impaler

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Re: VASIMR Engine
« Reply #192 on: 03/05/2015 07:59 AM »
No your still completely backwards, ISP and thrust are inversely proportional, the high ISP range is thus the low thrust range.  HALL is low to medium ISP, VASMIR is medium to high ISP.

At the medium ISP where an apples to apples comparison can be made HALL has better thrust to mass, this has been pointed out by multiple people already in this thread and in the electric propulsion thread, VASIMR is a very massive system with an impressive ISP range and high maximum ISP, but at the low end it's not competitive with a modern HALL.

Offline ChrisWilson68

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Re: VASIMR Engine
« Reply #193 on: 03/05/2015 08:09 AM »
No your still completely backwards, ISP and thrust are inversely proportional, the high ISP range is thus the low thrust range.  HALL is low to medium ISP, VASMIR is medium to high ISP.

At the medium ISP where an apples to apples comparison can be made HALL has better thrust to mass, this has been pointed out by multiple people already in this thread and in the electric propulsion thread, VASIMR is a very massive system with an impressive ISP range and high maximum ISP, but at the low end it's not competitive with a modern HALL.

Oops, you're right, thanks.

Offline Nilof

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Re: VASIMR Engine
« Reply #194 on: 03/05/2015 12:13 PM »
I had been trying to find the numbers on the MegaROSA and how it compared with original ROSA, I had thought the power density was much lower around 150W/kg for ROSA and couldn't find a number for MegaROSA, your link doesn't seem to have a reference to power density though, do you have one that dose?

You can get the power output per square meter in the link I gave in my earlier post:
http://www.nasa.gov/offices/oct/home/feature_sas.html

where power output and exact dimensions of the wing are given. For mass, there is this official figure of 200-400W/kg BOL at the wing level depending on the particular blanket technology used.

The 2 MW KSP example ship I showed earlier has arrays based on the MEGAROSA with correct sizes and a pessimistic 180 W/kg power density at the wing sixpack level. Each of the sixteen arrays on the booms are packs of six 20 kW winglets. So sizes are correctly accounted for making it a good way to get a rough idea of the relative sizes of the solar arrays.
« Last Edit: 03/05/2015 12:20 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 jgreason

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Re: VASIMR Engine
« Reply #195 on: 03/05/2015 12:39 PM »
While I won't speak to this specific design, more generally I am quite convinced that thin film solar approaches at 1 kW/kg are definitely possible near term.  However there is very little serious work going on, and packaging such systems for launch and then deploying them without spoiling the mass is not at all trivial

But do keep thinking -- it is not crazy, at least at 1kW/kg rather than two

Thin film solar is extremely fragile, however, so the packaging is really challenging.

Offline tea monster

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Re: VASIMR Engine
« Reply #196 on: 03/05/2015 02:40 PM »
I was thinking that this would be an excellent case for a Lagrange point 'Gateway Station'. You could set up a station there with a propellent depot and a manipulator telerobot. If the Telerobot was good enough, you may not even have to have the station permanently manned.

Ships and probes to the planets could be boosted directly there from Earth on launch and could go through check out at the station before boosting to their outbound destinations. Any unpacking or assembly would take place at the station with manipulator arms, and in a worse case scenario, a ship with a fault could be repaired by humans before it boosted off to Mars. It would eliminate tom-foolery like what happened to the Gallileo probe's antenna. A large chunk of the station's cost could be recouped by saving one flagship probe project from a post-launch glitch.

Offline Chris Bergin

Re: VASIMR Engine
« Reply #197 on: 03/05/2015 02:45 PM »
Big welcome to the site's forum to Mr. Greason, two posts back!

I was thinking that this would be an excellent case for a Lagrange point 'Gateway Station'. You could set up a station there with a propellent depot and a manipulator telerobot. If the Telerobot was good enough, you may not even have to have the station permanently manned.


Gateway you say? :)

http://www.nasaspaceflight.com/tag/gateway/
« Last Edit: 03/05/2015 02:45 PM by Chris Bergin »

Offline Burninate

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Re: VASIMR Engine
« Reply #198 on: 03/05/2015 10:32 PM »
No your still completely backwards, ISP and thrust are inversely proportional, the high ISP range is thus the low thrust range.  HALL is low to medium ISP, VASMIR is medium to high ISP.
This is an insufficient statement. Isp and thrust are not inversely proportional.  They are inversely proportional given fixed propulsive power (propulsive power being the product of propulsive thermal efficiency and actual system power).  Doubling Isp while keeping mass flow constant, doubles thrust but it increases the kinetic energy of the rocket exhaust per second, the propulsive power, by a factor of 4.  With double Isp but fixed propulsive power, you need to cut mass flow by a factor of four and thus cut thrust in half.

You cannot assume across technologies, however, that propulsive thermal efficiency will be the same, and system power is an arbitrary variable set by the engineers in designing the scale of the device.

PS: For people not doused in the terminology, Isp is simply a constant (1/9.81m/s^2) multiplied by the exhaust velocity
« Last Edit: 03/05/2015 10:43 PM by Burninate »

Offline Robotbeat

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Re: VASIMR Engine
« Reply #199 on: 03/05/2015 10:38 PM »
Propulsive efficiency is roughly the same, though. You may be comparing 60% to 80% efficiency, but a 2000s vs 3500s Isp.
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