Author Topic: MCT Speculation and Discussion Thread 4  (Read 505465 times)

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1840 on: 03/08/2016 04:15 PM »
'Subcooling' is actually easier on Mars because the sub part refers to temperatures below the nominal boiling point at 1atm. Or boiling points at subatmospheric pressures. On Mars the low pressure of ambient atmosphere means you can drop pressures inside your LV (during prop load) and storage tanks below 1atm without fear of imploding things. Wiki says average surface pressure on Mars is 600Pa. LOX boils at 59K in this pressure, only five Kelvins above freezing point.

Granted you must think about tank implosion, but also about explosion. The temperatures you cited are open system, but you are not going to put LOX in an open vat and allow it to evaporate/boil away. It is going into closed tanks and will have to be chilled to lower temperature as well as under some pressure in order to fit more density into the tank. Lacking the 1 atm. of pressure on the outside of the tank, the amount of interior pressure the tanks can withstand would be offset by an equal counter-pressure. So we still arrive back at the question of how cold you can get the prop when you are on Mars. You still have the limitations of a much lower electrical power supply than the North American electrical grid and you also have to dump the heat you withdraw from the prop as you chill it. With almost no atmosphere to run through a radiator from a heat pump, the heat has to be dumped into the ground or allowed to radiate directly into space or something. So I am back to wondering whether creating super-cryo prop in situ on Mars is going to be significantly harder than more typical cryo temperature prop.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1841 on: 03/08/2016 06:29 PM »
I mean what are current figures of merit for PV arrays and radiators? In 2008 80-100W/Kg for a rigid array was SoA.  The same report gave thin film systems around 2000W/Kg It lists ISS as roughly 1W/Kg, a staggeringly low number IMHO.
We have some distinctions to make:
1. Raw solar cells
2. UV-Coated/encapsulated & wired solar cells
3. Coated, wired, structurally supported solar panels
4. Solar panels mounted in an array
5. Solar panels mounted in an array mounted on a a deployment mechanism
6. A solar panel array on a deployment mechanism mounted on a large motorized rotation spindle/axle
7. The above arrangement mounted on a truss to extend away from the station
8. That truss being modular and heavy-duty enough to support indefinite amounts of length, and having additional motorized axles
9. That truss also having batteries, radiators, and substantial station infrastructure mounted on it

We launched the last one on the Shuttle for the ISS four times.  It was very heavy.  Comparing the Shuttle's payload with the power output of the truss gives a ludicrously low number even with 1980's technology.

2kw/kg probably refers to item 1.

There are two projects currently deployed from NASA to satisfy an eventual desire for a 300kw 'Government Reference Array', with minimum 100w/kg at EOL, probably at about the scope of item 6.  One firm is selling the prospect of 200w/kg, and the other is matching 200w/kg with some notation about the future possibility of 400-600w/kg.  Funding is currently focused on interim products far below this scale.  The ROSA tech has not been flown to my knowledge, while the Ultraflex tech has seen use in several missions in the last two decades - the Phoenix Lander achieved 103w/kg with 1G deployment.

The commercial sector is presently pushing the Boeing 702SP with all-SEP GTO->GSO transition.  Its solar arrays come in at 130w/kg, again probably at about the scope of item 6.
« Last Edit: 03/09/2016 12:15 AM by Burninate »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1842 on: 03/08/2016 06:33 PM »
Raw solar cells can be made as good as 40-100kW/kg.
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1843 on: 03/08/2016 06:36 PM »
Raw solar cells can be made as good as 40-100kW/kg.

Cite?

The present hype in lightweight encapsulated solar cells seems to have only demonstrated 6kw/kg and at that thickness, will float on a soap bubble:
http://news.mit.edu/2016/ultrathin-flexible-solar-cells-0226
« Last Edit: 03/08/2016 06:37 PM by Burninate »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1844 on: 03/08/2016 06:58 PM »
Thin film cells themselves, not counting contacts and substrate, are about a micron thick. At 1300kW/kg and 10-30% efficiency gives on the order of 40-100kW/kg.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1845 on: 03/08/2016 07:07 PM »
The actual absorption thickness is on the order of 50-100nm, actually. If we're talking about fundamental limits, here.

...think like a solar sail designer!!!
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Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1846 on: 03/08/2016 10:03 PM »
The actual absorption thickness is on the order of 50-100nm, actually. If we're talking about fundamental limits, here.

...think like a solar sail designer!!!
Actually I was. IIRC KE Drexlers Masters thesis was on the idea of mfg solar sails by vacuum deposition onto a wax layer, then dissolving the wax layer.  In principle excellent performance but far too fragile to survive launch.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.
So you're going to Mars to seek a better life. What does that mean to you? Always spot a fanbois by how they react to their idols failures.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1847 on: 03/09/2016 03:09 AM »
The actual absorption thickness is on the order of 50-100nm, actually. If we're talking about fundamental limits, here.

...think like a solar sail designer!!!
Actually I was. IIRC KE Drexlers Masters thesis was on the idea of mfg solar sails by vacuum deposition onto a wax layer, then dissolving the wax layer.  In principle excellent performance but far too fragile to survive launch.
Neat idea, I was thinking of something similar (but I was just thinking regular acid etching... wax is cleverer than what I was thinking). You could launch it with some sort of waxlike material that would sublimate away once heated in space.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1848 on: 03/09/2016 06:47 AM »
The actual absorption thickness is on the order of 50-100nm, actually. If we're talking about fundamental limits, here.

...think like a solar sail designer!!!
Actually I was. IIRC KE Drexlers Masters thesis was on the idea of mfg solar sails by vacuum deposition onto a wax layer, then dissolving the wax layer.  In principle excellent performance but far too fragile to survive launch.
Neat idea, I was thinking of something similar (but I was just thinking regular acid etching... wax is cleverer than what I was thinking). You could launch it with some sort of waxlike material that would sublimate away once heated in space.
Drexlers thesis should still be available at Stamford Princeton, however it'll date from the late 70's so I've no idea if it's been scanned and available online.
« Last Edit: 03/09/2016 06:53 PM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.
So you're going to Mars to seek a better life. What does that mean to you? Always spot a fanbois by how they react to their idols failures.

Offline Jim_LAX

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Re: MCT Speculation and Discussion Thread 4
« Reply #1849 on: 03/09/2016 02:22 PM »
How about launching a 3D printer that takes advantage of the vacuum of outer space to do the vacuum deposition on wax.  And then melt and recycle the wax.  You would just need to launch fresh supplies of Silicon.  This might even work for Space Based Solar Power.
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1850 on: 03/09/2016 05:21 PM »
The actual absorption thickness is on the order of 50-100nm, actually. If we're talking about fundamental limits, here.

...think like a solar sail designer!!!
Actually I was. IIRC KE Drexlers Masters thesis was on the idea of mfg solar sails by vacuum deposition onto a wax layer, then dissolving the wax layer.  In principle excellent performance but far too fragile to survive launch.
Neat idea, I was thinking of something similar (but I was just thinking regular acid etching... wax is cleverer than what I was thinking). You could launch it with some sort of waxlike material that would sublimate away once heated in space.
Drexlers thesis should still be available at Stamford, however it'll date from the late 70's so I've no idea if it's been scanned and available online.

Most masters' theses are not published in peer-reviewed journals, today.  The overwhelming majority of papers published in the late 70's have not been scanned and made available online and transferred to a paper repository we can access.

Fortunately, this is an exception to the rule:
https://dspace.mit.edu/handle/1721.1/16234
https://dspace.mit.edu/bitstream/handle/1721.1/16234/06483741-MIT.pdf?sequence=2
« Last Edit: 03/09/2016 05:23 PM by Burninate »

Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1851 on: 03/09/2016 06:59 PM »
Most masters' theses are not published in peer-reviewed journals, today.  The overwhelming majority of papers published in the late 70's have not been scanned and made available online and transferred to a paper repository we can access.

Fortunately, this is an exception to the rule:
https://dspace.mit.edu/handle/1721.1/16234
https://dspace.mit.edu/bitstream/handle/1721.1/16234/06483741-MIT.pdf?sequence=2
Excellent work.  That's the one I was thinking of.

MIT

Well I completely screwed up that reference  :-[
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.
So you're going to Mars to seek a better life. What does that mean to you? Always spot a fanbois by how they react to their idols failures.

Online AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #1852 on: 03/09/2016 08:11 PM »
'Subcooling' is actually easier on Mars because the sub part refers to temperatures below the nominal boiling point at 1atm. Or boiling points at subatmospheric pressures. On Mars the low pressure of ambient atmosphere means you can drop pressures inside your LV (during prop load) and storage tanks below 1atm without fear of imploding things. Wiki says average surface pressure on Mars is 600Pa. LOX boils at 59K in this pressure, only five Kelvins above freezing point.

Granted you must think about tank implosion, but also about explosion. The temperatures you cited are open system, but you are not going to put LOX in an open vat and allow it to evaporate/boil away. It is going into closed tanks and will have to be chilled to lower temperature as well as under some pressure in order to fit more density into the tank. Lacking the 1 atm. of pressure on the outside of the tank, the amount of interior pressure the tanks can withstand would be offset by an equal counter-pressure. So we still arrive back at the question of how cold you can get the prop when you are on Mars. You still have the limitations of a much lower electrical power supply than the North American electrical grid and you also have to dump the heat you withdraw from the prop as you chill it. With almost no atmosphere to run through a radiator from a heat pump, the heat has to be dumped into the ground or allowed to radiate directly into space or something. So I am back to wondering whether creating super-cryo prop in situ on Mars is going to be significantly harder than more typical cryo temperature prop.

This isn't true.
The tanks can (and probably will) be kept near Mars atmospheric pressure, but sub-cooled to slightly below saturation temperature.  Saturation temperature at that pressure is near the triple point as stated -- and LOX is an essentially incompressible fluid, so its density is set by its temperature.  Nearest analogy is keeping a pot of water on the stove just below boiling temperature... it will remain liquid unless you add energy.

On the closed tank comment, this will be a necessity because the CO2 (and a bit of water) in Martian atmosphere will freeze-out in a LOX bath, so the tanks must remain sealed to maintain purity of the LOX.  A slight pressure might be used, but nothing is sacred about or 'easier' at one Earth atmosphere.
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Offline john smith 19

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Re: MCT Speculation and Discussion Thread 4
« Reply #1853 on: 03/10/2016 06:29 AM »
Most masters' theses are not published in peer-reviewed journals, today.  The overwhelming majority of papers published in the late 70's have not been scanned and made available online and transferred to a paper repository we can access.

Fortunately, this is an exception to the rule:
https://dspace.mit.edu/handle/1721.1/16234
https://dspace.mit.edu/bitstream/handle/1721.1/16234/06483741-MIT.pdf?sequence=2
Just had a chance to skim it.

I think Drexler ran tests that showed you can mfg thin metal films by depositing them on a wax layer and then removing the layer but I don't think he ever built an actual machine.

Possibly his neatest feature is the idea of deliberately introducing cuts and waves into the film to stop tears and absorb stress without stiffeners. I was sort of reminded of the cuts made in foils to make flexure bearings.

I'll also note that such a device would scale down quite well, making it an interesting (but very challenging) project for someone to pursue. Rotary motion in UHV is tricky. With a lubricant on the bearings you run the risk of it vaporizing and contaminating the system (give the oils on a fingerprint can certainly do this). Without it you risk ceasing up.

Keep in mind that on orbit the vacuum is effectively outside the container and hence the various "chambers" in the machine would be more there to stop various vapors mixing and depositing in the wrong places, more like partitions than actual mini vacuum chambers.

There is also a PV cell architecture that uses the difference between the work functions of two dissimilar metals to make electricity. The metal thicknesses would be in this order of magnitude, although one of them would be thinner to let the light get to the junction area.

One thing I'm not clear on. Do solar sails open up the launch windows to other planets?

Could you set up a pipeline of sail vehicles leaving say every month to the same destination? AFAIK solar sails are the only technology that can return the core structure of the vehicle [EDIT without any infrastructure at the destination for refueling ] ready for another trip.

Ongoing support to remote settlements has to be sustainable. Such a fleet of vehicles would might be one way to do this.
« Last Edit: 03/11/2016 07:22 PM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.
So you're going to Mars to seek a better life. What does that mean to you? Always spot a fanbois by how they react to their idols failures.

Offline jsgirald

Re: MCT Speculation and Discussion Thread 4
« Reply #1854 on: 03/12/2016 01:41 PM »
I just stumbled upon an old soviet idea for a reusable space shuttle:


Although it's not designed for anything Mars related, it rather looks like an elongated Kliper, I found a couple interesting points that might be relevant re MCT.

1. I like the skids idea, it's relatively lightweight and if needed wheels can be attached to them for ground operation. These need not be carried by the vehicle after the first trip, in fact a single set of wheels can be shared by all the fleet.

2. The rounded triangle profile looks interesting, apparently was designed for hypersonic flight above 12km. Below that, it used parachutes with small rockets just before touching the ground. Remove the parachutes and add bigger rockets and it might work in Mars lower gravity. Problem is, they'd have to be used for take off too, and the main engine would be fired for the big push only.
This sort of horizontal landing makes a lot easier the unloading of payload in Mars, this is usually not mentioned, but a tall lander makes handling cargo a lot harder.

3. It was about 34 m long, and weighed 200 mt(!!). I find this quite unbelievable given that the shuttle orbiter weighed a lot less and had wings. But probably a fully loaded MCT (sans TMI propellant) would be that heavy, probably not much bigger than this either.

Obviously, the main problem would be the need for quite powerful landing/take off rockets that don't appear to be something that SpaceX may be considering. Personally I think an expander cycle variant of their Raptors (downsized perhaps) might work, but this is just my MCT not Musk's.

For more info, have a look at these:
http://forum.nasaspaceflight.com/index.php?topic=30044.0
https://falsesteps.wordpress.com/2012/10/06/mtkvp-glushkos-opening-gambit/
http://www.buran.ru
http://www.astronautix.com/craft/mtkva.htm
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Offline R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #1855 on: 03/18/2016 09:11 AM »
Can I get a factcheck / primer on this?

Is 'slush propellant' the same concept as 'subcooled propellant', or is it a subset?  Does propellant ever actually see 1atm?

NIST databooks are excellent;

Oxygen properties
Alkane (methane-butane) properties
Nitrogen properties

Slush propellant is a 2-phase mixture, fine solid particles in a liquid. The idea is to increase density because most substances are denser in solid phase. Kind of special case of subcooling, subcooled so close to melting point that has partially frozen.

AFAIK never used in flight. Would be even more problematic to handle than subcooled pure liquid, special problems like how to keep the solid particles from settling out of the mixture (agitation?) and pumping said mixture in the engine.
« Last Edit: 03/18/2016 09:14 AM by R7 »
AD·ASTRA·ASTRORVM·GRATIA

Offline kaoru

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Re: MCT Speculation and Discussion Thread 4
« Reply #1856 on: 03/19/2016 08:52 AM »
Here is my engineering module (upper stage Raptor engines & tanks) to date.  The person at the top shows scale.  The airlock/doors are dynamic which mean they actually open/close.  The engine canting mount also works, canting the Raptor 15 degrees with the engines retaining gimbal room.  The nacelles internally have ablative nozzle extension which also cants with the engine.  Next up is landing legs and filling in the engineering spaces with IVF, Draco methlox tankage, plumbing.  Also have to finish the upper deck with ECLSS, tankage (water & nitrogen), batteries, etc.  The only thing I don't like is the separated tanks; I should have made a common bulkhead/dome.  Less weight and height but would have more time to resize everything.

Kaoru

Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #1857 on: 03/19/2016 10:50 AM »
Here is my engineering module (upper stage Raptor engines & tanks) to date.  The person at the top shows scale.  The airlock/doors are dynamic which mean they actually open/close.  The engine canting mount also works, canting the Raptor 15 degrees with the engines retaining gimbal room.  The nacelles internally have ablative nozzle extension which also cants with the engine.  Next up is landing legs and filling in the engineering spaces with IVF, Draco methlox tankage, plumbing.  Also have to finish the upper deck with ECLSS, tankage (water & nitrogen), batteries, etc.  The only thing I don't like is the separated tanks; I should have made a common bulkhead/dome.  Less weight and height but would have more time to resize everything.

Kaoru

What use do you envision for the airlock tunnel when the airlock itself is not in use? I mean, it is a lot of space in a space constrained design...

Edit: since it runs by the engines, wouldn't it make sense to allow engine compartment access from the tunnel?
« Last Edit: 03/19/2016 10:52 AM by mfck »

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1858 on: 03/19/2016 11:05 AM »
Here is my engineering module (upper stage Raptor engines & tanks) to date. [...]

You don't need to overdo the symmetry. Provided you don't have a shifting CoM during delicate manoeuvres (like landing), a rocket design can cope if specific elements aren't physically symmetrical.

Hence, I'd run that airlock access tunnel along the outside of the vehicle. Being a long vertical tube, it isn't aerodynamically harmful during launch/landing. (And assuming the re-entry attitude is off-centre (to add lift), the tunnel can be on the dorsal side; protected from re-entry heating.)

It removes that hole through the tanks. And it removes issues with either having warm air piped to the centre of the cryo-tanks, or allowing the tunnel to reach cryo-temps during the flight -- both of which I suspect are bad choices.

More broadly, have you been thinking about the cargo version? There will be a lot more of those than crewed ships. IIRC, Musk talked of 10:1. How does your design allow cargo to be unloaded?

What use do you envision for the airlock tunnel when the airlock itself is not in use? I mean, it is a lot of space in a space constrained design...

Storage of ground equipment not needed in flight? EVA suits, etc.

Plus supplies and parts during the trip. These would be transferred to the hab sections after landing, to allow the airlock to be used, then moved outside.

Edit: since it runs by the engines, wouldn't it make sense to allow engine compartment access from the tunnel?

{laughs} I really like the idea, but suspect that the amount of maintainable systems you could access would be limited, and number of situations where you have the vehicle safe in space but with an engine failure would be even more limited.

The only scenario that's realistic is on the Martian surface, where they landed safely but either something glitched or they just want to do a check-out and service before lift-off for Earth-return. In which case, you'd be better off designing for access to the entire engine system from outside (as you would on Earth for a first stage after a launch-abort).

Offline kaoru

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Re: MCT Speculation and Discussion Thread 4
« Reply #1859 on: 03/19/2016 03:34 PM »

What use do you envision for the airlock tunnel when the airlock itself is not in use? I mean, it is a lot of space in a space constrained design...

Edit: since it runs by the engines, wouldn't it make sense to allow engine compartment access from the tunnel?
You hit the nail on the head.  The space where the engine nacelles and tankage (gaseous methane/oxygen, nitrogen, etc.) are are unpressurized.  However, the IVF and other equipment would be pressurized and accessible.  The intent is to be able to service (everything that's serviceable) from the inside; important for long term trips where everything is in-situ.

In doing the model, it was just easier to stretch the tunnel.  I'll be updating it with a pressure vessel and bulkheads for the engines.

Kaoru
 
« Last Edit: 03/19/2016 03:39 PM by kaoru »

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