Author Topic: VASIMR Engine  (Read 155382 times)

Offline Solman

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Re: VASIMR Engine
« Reply #200 on: 03/05/2015 10:42 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?
...
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).
I would think the concern would be thermally induced occilation. Parts of the array that are a few degrees off would be cooler than those that aren't. Might this not result in disaster if there is no means such as actuators to keep harmonic flexing from increasing like the Hubble arrays demonstrated?
The inference seems to be that concentrators need so much greater rigidity that their more than an order of magnitude advantage (as demonstrated by the L'Garde prototype) would somehow be eaten up by added structure. This makes no sense to me.   

Offline Burninate

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Re: VASIMR Engine
« Reply #201 on: 03/05/2015 10:49 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/

Always nice to have more industry figures around.

On the gateway: There are two main plans on this line of thinking that I've come to, which involve spiralling out, then leaving.  One relies on L2 for parking, a chemical kick to lower periapsis to Earth, and another chemical kick to escape Earth.  The second relies on some distant Earth orbit that isn't in close proximity to Luna, then a solar-electric thrust to lower periapsis to Earth, and a chemical kick to escape Earth.  This second cuts propellant usage substantially, but has departure windows that are orbital+synodic instead of monthly+synodic, and does not offer an orbital location that is reusable across synods, as far as I can tell: One departure mission, one station.
« Last Edit: 03/05/2015 10:51 PM by Burninate »

Offline Solman

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Re: VASIMR Engine
« Reply #202 on: 03/05/2015 11:06 PM »
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?

 Speaking of numbers I would say that an extraordinary claim such as >2kwe/kg for solar exceeds material or thermodynamic limits requires extraordinary proof or at the very least some numbers.
The concentrator I'm describing has been built after all.
I think people have failed to wrap their heads around the implications of having available solar cells that can exceed 40% efficiency at 900 Suns. These cells have very low mass per unit power produced and higher efficiency than any thin film cells. This efficiency means that the concentrator would be much smaller in area than a thin film array and so can have a more massive structure per unit area and still have much lower total mass per power produced. This added to the lower mass per area of reflective film as compared to thin film cells and the conducting wires they require, makes a solid case for concentrator's superiority IMO. Oh and the support structure of aluminum tubes can provide the needed radiator area at little mass penalty.
If the argument being made is that unless a tech is currently being developed it is fantasy then this tech which uses already developed parts in a new combination is no more a fantasy than any system using VASIMR is it?

Online Robotbeat

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Re: VASIMR Engine
« Reply #203 on: 03/05/2015 11:13 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?
...
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).
I would think the concern would be thermally induced occilation. Parts of the array that are a few degrees off would be cooler than those that aren't. Might this not result in disaster if there is no means such as actuators to keep harmonic flexing from increasing like the Hubble arrays demonstrated?
The inference seems to be that concentrators need so much greater rigidity that their more than an order of magnitude advantage (as demonstrated by the L'Garde prototype) would somehow be eaten up by added structure. This makes no sense to me.

Makes no sense to you or not, it's true. Stiffness costs mass. And, in fact, a LOT of aerospace structures are stiffness-limited (due to pointing requirements or launch requirements), and "specific modulus" is a useful figure-of-merit in those cases.

And yes, there are ways to damp out larger oscillations. Besides, Hubble is a space telescope so necessarily has high pointing requirements, making low-stiffness arrays a poor choice anyway.
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Offline Solman

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Re: VASIMR Engine
« Reply #204 on: 03/06/2015 12:27 AM »
 Thanks for the response Chris, but the point I was attempting to make is that the difference in the mass to achieve the structural stiffness required to prevent thermally induced movements or the mass of actuators to damp them out as they arise reduces any difference in mass per unit area that might be required for a concentrator vs. thin film - perhaps to zero.
That when added to the much smaller size required for the concentrator per unit power owing to the higher efficiency of concentrator type PV(about 4X higher at present) and the lower mass/area of reflective material vs. thin film cells should result in a much higher overall specific power for the concentrator system.

Offline Impaler

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Re: VASIMR Engine
« Reply #205 on: 03/06/2015 01:41 AM »
This second cuts propellant usage substantially, but has departure windows that are orbital+synodic instead of monthly+synodic, and does not offer an orbital location that is reusable across synods, as far as I can tell: One departure mission, one station.

While I am skeptical that SEP would remotely benefit from a periapsis drop and kick-stage burn (better to use the propellent mass in the SEP system normally).  This departure timing dose not seem too hard to overcome, the low DeltaV window for transit for a conventional rocket is nearly a month long so we should be able to consistently get a departure, though admittedly it is an instantaneous window which can be dicy.  Also depending on which side of the Earth we go around we have two possible exit trajectories which are rotating around so one should be passing any desired direction every 15 days if they are equally spaced (no clue if they are).

Offline Burninate

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Re: VASIMR Engine
« Reply #206 on: 03/06/2015 03:27 AM »
This second cuts propellant usage substantially, but has departure windows that are orbital+synodic instead of monthly+synodic, and does not offer an orbital location that is reusable across synods, as far as I can tell: One departure mission, one station.

While I am skeptical that SEP would remotely benefit from a periapsis drop and kick-stage burn (better to use the propellent mass in the SEP system normally).  This departure timing dose not seem too hard to overcome, the low DeltaV window for transit for a conventional rocket is nearly a month long so we should be able to consistently get a departure, though admittedly it is an instantaneous window which can be dicy.  Also depending on which side of the Earth we go around we have two possible exit trajectories which are rotating around so one should be passing any desired direction every 15 days if they are equally spaced (no clue if they are).

The distinction is between 1) a periapsis drop from L2 using chemical and then an Oberth departure burn using chemical, and 2) A periapsis drop from high circular orbit using SEP and then an Oberth departure burn using chemical.  The distinction exists because I think the complex dynamics of the orbit around the Lagrange point may pose problems for a pure SEP periapsis drop at low thrust, while a non-EML orbit can perform this maneuver at leisure over as many orbits as required;  I'm not sure 100% this is a big deal.

Offline Nilof

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Re: VASIMR Engine
« Reply #207 on: 03/06/2015 09:46 AM »
The utillity of a kick stage really depends on how strong your SEP acceleration is. In the 1 mm/s/s range, a departure from a lunar altitude orbit would more or less look impulsive.

So the question is, is the kick stage going to provide more delta-v per reaction mass using the Oberth effect at 300s Isp than the SEP system at 3000+ seconds Isp?
« Last Edit: 03/06/2015 09:52 AM 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 #208 on: 03/06/2015 06:50 PM »
Yes that's my point Oberth is good but it's not going to close that kind of ISP gap.

To crunch the numbers we need to know C3 at EML1/2 (they are approximately the same are they not).  And then determine our target C3 for the Mars trajectory for the kick stage to achieve and then look at what it costs the SEP to match that all by itself.

When I look at this idea to provide propellent at EML1/2 from some elaborate mining operation I just don't see any practicality.  The Oberth effect from the drop is all just potential energy conversion to get your velocity up as you swing by the Earth and your going about 3 kms faster then orbital speed.  But I could simply have had a big booster starting in LEO accelerate me thouse 3 kms and then drop off the back of my vehicle.  Now my remaining vehicle is traveling 3 kms in excess of orbital velocity and is interchangeable with and gets every bit of Oberth benefit for the remaining escape burn (how ever large or small it may be) as someone coming from EML1.

The booster would need to have a mass equal to the whole rest of the vehicle it was pushing (I'm assuming HydroLox 450 ISP) yes so it is a big booster, but this one big booster just substituted for the entire in-space propellent production, the need to send my payload up to EML1 rendezvous with some station, refuel it etc etc.  The simplification is enormous and it establishes a cap on the value of EML1 propellent mass, it's worth at most twice what LEO propellent is worth even if we ignore every operational difficulty in using it.  So our in-space production process can cost no more then twice what our launch costs are. 

The higher my ISP goes the more this multiplier is reduced, say I had something like a NTR, doubling the ISP, now the throw away LEO booster is only 30% of the mass of the rest of the vehicle.  So EML1/2 propellents are now worth only 1.3 times LEO propellents.

Offline Nilof

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Re: VASIMR Engine
« Reply #209 on: 03/06/2015 07:06 PM »
I think the main value of ISRU propellant in lunar orbit/EML is for transportation in for transportation in cislunar space and lunar landings. With magnetoshell areobraking you could have a "fetcher" craft that brakes down to LEO, docks with a payload, and goes back up to lunar orbit with the payload. For lunar exploration you can similarly have a fully reusable lunar lander that runs on ISRU propellant.

This can enable very cheap access to the moon, where the only launching needed is a commercial resupply and a commercial crew equivalent.

For Mars, while a mature cislunar infrastructure can certainly help, I think a pure SEP craft is probably the more practical option.
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 Hop_David

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Re: VASIMR Engine
« Reply #210 on: 03/09/2015 03:53 PM »
A rough approximate figure from the phase I ROSA winglets is ~3 square meter per kW.

I don't know if this is the same RAPDAR stuff impaler is talking about. But I'll go with it.

So about 12000 square meters to provide 4 megawatts? If my arithmetic's right that's about 4 times the total area of the ISS Solar Array Wings (SAWs).

I seem to recall Impaler say the solar cells he envisions are about the thickness of the plastic in milk jugs. How about the supporting structure? Is the mast about the thickness of a coat hanger?

The ISS demonstrates it's doable to rotate the SAWs each orbit to keep them pointed towards the sun. And if the ion driven Mars Transfer Vehicle is in the ecliptic plane, there's no need for a beta gimbal.

To minimize aerodynamic drag the ISS SAWs will go into Night Glider mode.  Our MTV seems to have a mass less than half the I.S.S. but a cross sectional area 4 times greater. At 400 km altitude, the MTV would have aerodynamic deceleration about Eight times what the I.S.S. endures.

Delta V for low thrust spirals is about the difference in the speeds between departure and destination orbits. The difference between a 7.7 km/s LEO and 3.07 km/s GEO is about 4.6 km/s. At a millimeter per second^2 acceleration, it takes 11.6 days to accelerate 1 km/s. So I get 53 days to spiral from LEO to GEO. But 10% of that is in earth's shadow. So More like 60 days.



To avoid subjecting the passengers to long periods of high radiation, Impaler has suggested a chemical ferry take the astronauts to GEO where they can rendezvous with the MTV after it's passed through the Van Allen Belts.

Besides a two month trip through the Van Allen Belts this is also a 60 day spiral through the LEO and GEO orbital debris with a 12000 square meter cross section. Impaler's GEO rendezvous scheme spares the human passengers but the MTV is still subjected to some wear and tear.



Offline Hop_David

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Re: VASIMR Engine
« Reply #211 on: 03/09/2015 04:53 PM »
Why would you characterize IKAROS as speculative? It already flew.

Sunlight's power density at Venus is more than double than at 1 A.U.

And at 1.52 A.U. Mars distance, it's less than half, more like 2/5.

So I don't know how applicable Ikaros' 1 kWe/kg is to solar arrays on an MTV.

Also at perihelion from an earth to Venus Hohmann, the mast need only move at 2 degrees per day to stay aligned with the sun. In LEO a mast would need to move 4 degrees per minute.


Online Robotbeat

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Re: VASIMR Engine
« Reply #212 on: 03/10/2015 01:01 AM »
Why would you characterize IKAROS as speculative? It already flew.

Sunlight's power density at Venus is more than double than at 1 A.U.

And at 1.52 A.U. Mars distance, it's less than half, more like 2/5.

So I don't know how applicable Ikaros' 1 kWe/kg is to solar arrays on an MTV.

Also at perihelion from an earth to Venus Hohmann, the mast need only move at 2 degrees per day to stay aligned with the sun. In LEO a mast would need to move 4 degrees per minute.
Nah, the 1kWe/kg was calculated (by me, just so we're clear...) assuming 1 AU, not Venus' orbit.

And yeah, the mast for a solar array would not be a "coat hanger" thick, but the components would! Like a solar sail structure.
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Online Robotbeat

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Re: VASIMR Engine
« Reply #213 on: 03/10/2015 01:08 AM »
I.e. this sort of thing, but with very thin solar cells operating at high voltage:



(The mass of that thing is just 20kg for 1000 m^2 of area... solar cells would add to the weight as would the thruster, but that can be calculated in a relatively straightforward manner.)
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Offline Nilof

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Re: VASIMR Engine
« Reply #214 on: 03/10/2015 10:04 AM »
I.e. this sort of thing, but with very thin solar cells operating at high voltage:



(The mass of that thing is just 20kg for 1000 m^2 of area... solar cells would add to the weight as would the thruster, but that can be calculated in a relatively straightforward manner.)

There are a few issues with non-stiff structures though. For example, how do you dock with a spacecraft like this one? How do you do quickly change thrust direction during an approach maneuver?

The cost of stiffness can be reduced dramatically with in-space construction of trusses, so I'm not convinced that abandoning stiff structures is necessarily the best way to go about things.
« Last Edit: 03/10/2015 07:34 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 Hop_David

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Re: VASIMR Engine
« Reply #215 on: 03/10/2015 03:52 PM »
Nah, the 1kWe/kg was calculated (by me, just so we're clear...) assuming 1 AU, not Venus' orbit.

I'd like to see these calculations.

The solar sail material in that vid is 3 microns thick. Seran Wrap's 12 microns.

Offline cdleonard

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Re: VASIMR Engine
« Reply #216 on: 03/11/2015 12:31 PM »
Plans to test VASIMR on ISS apparently cancelled. Paywalled source: http://sen.com/blogs/irene-klotz/nasa-nixes-ad-astra-rocket-test-on-the-space-station

Offline A_M_Swallow

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Re: VASIMR Engine
« Reply #217 on: 03/11/2015 01:19 PM »
Plans to test VASIMR on ISS apparently cancelled. Paywalled source: http://sen.com/blogs/irene-klotz/nasa-nixes-ad-astra-rocket-test-on-the-space-station

Either kill the VASIMR or find a way to attach it and solar panes to a spacecraft. Since they have navigation and RCS a Dragon or Cygnus would do nicely.

Offline Nilof

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Re: VASIMR Engine
« Reply #218 on: 03/11/2015 02:55 PM »
Plans to test VASIMR on ISS apparently cancelled. Paywalled source: http://sen.com/blogs/irene-klotz/nasa-nixes-ad-astra-rocket-test-on-the-space-station

Either kill the VASIMR or find a way to attach it and solar panes to a spacecraft. Since they have navigation and RCS a Dragon or Cygnus would do nicely.

This has been discussed before. The Dragon is simply way too underpowered to test a full VASIMR system. The new Cygnus will have ultraflex arrays though, so there may be a chance that it can be equipped with ultraflex megaflex arrays for the test and later BEO servicing missions(waiting for Jim to correct me).
« Last Edit: 03/11/2015 09:23 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.

Online Robotbeat

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Re: VASIMR Engine
« Reply #219 on: 03/11/2015 03:21 PM »
Use batteries. Plenty of power, then.
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