Author Topic: MCT Speculation and Discussion Thread 4  (Read 570988 times)

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #260 on: 06/25/2015 02:11 AM »
Would something like the joined picture be a good fuel depot?

It's essentially two BFR second stages configured for fuel storage.  The are added docking points, the same solar panels as the MCT for power, compressors for fuel cooling, radiators for the compressor heat, added gold foil insulation and shadow shields that also have solar cells on the other side to power the compressors.
There is a Bigelowe module for a maintenance crew stayover, but it should usually be automatic.
It's in LEO, and can boost itself up from time to time.  It's a bit shorter than the MCT, since it desn't need the volume for crew accomodation.  The faring protecting the compressor module has been returned to Earth  It takes about 12 tanker trips to fill, and it can service two MCT for a Mars transfer.  Dockind and fuelling is available for other types of ships as well.  The extended structure may be overkill, but seems safer somehow.

« Last Edit: 06/25/2015 02:12 AM by lamontagne »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #261 on: 06/25/2015 03:29 AM »
Some decade old sour-grapes quotes from Max Faget dose not constitute a counter argument to the fact that EVERY entry vehicle has had a back-shell with thermal protection.

Apollo imagery of people fishing out the capsules with leeward side in near pristine condition proves that Faget was right.

India's SRE-1 proves that you are wrong.


First off trying to use your EYE as a after-the-fact calorimeter on a surface that was heat-shielded is a profoundly flawed.  You have no idea what surface temperature it reached nor do you have any idea if it is truly pristine, that would take chemical analysis after-the-fact or ideally a temperature probe during the entry itself. 

Second, vehicles have to be designed for the worst possible entry profile expected, just because you may have had AN entry with low heating dose not mean you can strip the vehicle of TPS and remove all the margin because you raise the chance of a deadly burn-up.

Third this ISRO vehicle is radically different in shape (it basically looks like a low drag ICBM re-entry vehicle) then what the poster I was replying to was asking about which was the traditional blunted capsule.  Narrow conical entry vehicles have very different in thermal profile, they go much deeper in the atmosphere and are not viable for Mars entry.  Also this vehicle did an 8 kms re-entry, which is not even lunar return speed and far below any kind of direct Mars return speed.

The rear of this IRSO vehicle looks to be composed of small solar panels, solar panel are covered with glass which has a high melting point, we would not expect this to blacken or char, the forward TPS is non-ablative ceramic tile which would not deposit black char streaks.  The shuttle didn't LOOK chared when it returned from orbit but the top of it had TPS of a thinner, lower temperature type, but TPS none the less.  What is behind the panels? you have no idea.  Present some engineering documentation on this vehicle and data on what kind of heating regime it went through and you might have something.  I myself was unable to find any technical details on this vehicle.




Well, I kinda was being pedantic.  Sorry Impaler.

Sometimes a simplified explanation becomes technically incorrect, but still can be useful.  (Full disclosure: I do this all the time with non-technical listeners to get the big picture point across.)

No problem, nothing wrong with expanding a simple explanation and getting into the details so long as it's for the purpose of educating and avoiding confusion.  The actual mechanisms of heat transfer and airo-thermics around an entry vehicle are VERY complex, we could easily go on for pages so some brevity is needed, though I may have been excessive.   What I've read is that you actually get an area of secondary compression behind the vehicle and this area heats the back of the vehicle via radiation.  And I am fairly sure their is a vortex between the sides of the capsule and the shock-layers coming off the edges of the forward heat-shield this delivers convective heat to the sides.



Impaler, let me push back a bit against the premise of what you are pushing.  If I understand you correctly, you advocate lower speed reentry, such as from an orbit (both on Mars and Earth) than direct reentry for the MCT, primarily because then one could use metallic TPS rather than ablative TPS (i.e. PICA-X).  And that this is practically necessary to ensure high reusability and high flight rates.  Do I have that right?

But is avoiding ablative TPS really that important in the grand scheme?  Doesn't SpaceX intend to rapidly and frequently reuse the Dragon 2, which will surely have PICA-X.  What do we suppose is the answer here?  Is PICA-X something that can be de-ablated (reblated?) back on to the bottom of a capsule without too much hassle.  I'm imagining that there is an inch or so of the material, and half an inch ablates off during reentry (a little more some places, a little less others) and then then additional PICA-X is applied and added to what remains before the next flight - sort of like retreading a tire.  Or conversely, the entire backshell of PICA-X is designed and installed as a bolt on module, and a new one will be bolted onto the capsule for each flight (i.e. changing the tires).  I ask because I don't know.

But in any case, this seems to solve the problem fully, without a great deal of bother.  Surely a simpler solution than radically modifying the flight profiles to include orbital insertion on each end, which requires a great deal of additional fuel, don't you think?

Well first off their are likely to be advantages outside of the thermal protection in the vehicle being designed for low entry speed, lower g-forces which mean a lighter structural frame.  As this is integral to the vehicle any gain/loss in mass is permanent and will have to be hauled around for the whole vehicle life-span.

Second, the low-entry system is also paired with a lower DeltaV demand placed on the vehicle by offloading heliocentric transport to a second vehicle, this is in opposition to direct-Earth-return.  This reduces the tank volume needed on the lander, which then compounds to lighter structures, smaller surface areas for TPS, lower liftoff mass etc etc. 

Third the Mars surface propellent production demands are MUCH less under this proposal by something like 60-80%, we don't know exactly WHAT that process will be but it will certainly require a lot of pre-positioned equipment and power sources.  Regardless of how efficient it is (and my own estimates are that we can get 3x to 4x the equipment mass in propellents produced per synod) the less propellent we need the more usable cargo can be sent instead, or the propellents can be used locally for any number of purposes.

Lastly while I think it is reasonable to expect SpaceX to re-apply PICAX to a vehicle that uses it (reused Dragon would be great to see), the process would likely be possible only on Earth.  Where as my intent was to evolve the lander into a rapid mars-surface 2 mars orbit cargo hauler that performs ~100 such round trips per synod before returning to Earth, a PICAX shield with that many layers sounds like it would be more massive then the metallic TPS.

Offline R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #262 on: 06/25/2015 06:59 AM »
First off trying to use your EYE as a after-the-fact calorimeter on a surface that was heat-shielded is a profoundly flawed.  You have no idea what surface temperature it reached nor do you have any idea if it is truly pristine, that would take chemical analysis after-the-fact or ideally a temperature probe during the entry itself.

...


The rear of this IRSO vehicle looks to be composed of small solar panels, solar panel are covered with glass which has a high melting point, we would not expect this to blacken or char, the forward TPS is non-ablative ceramic tile which would not deposit black char streaks.  The shuttle didn't LOOK chared when it returned from orbit but the top of it had TPS of a thinner, lower temperature type, but TPS none the less.  What is behind the panels? you have no idea.


Look closer. The electrical wiring for the solar panels is still there in its bright red attachment points.

https://upload.wikimedia.org/wikipedia/commons/b/be/ISRO-SCRE-1-Spacecraft-1.jpg
AD·ASTRA·ASTRORVM·GRATIA

Online Andy Smith

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Re: MCT Speculation and Discussion Thread 4
« Reply #263 on: 06/25/2015 10:43 AM »
Use a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.

Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?

It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #264 on: 06/25/2015 11:55 AM »
Use a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.

Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?

It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?

Yes but it would mean sending a lot of expensive Raptors on a one off mission. If inflatable LOX/methane tanks are possible it seems like the more cost efficient solution to me.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #265 on: 06/25/2015 12:01 PM »
Use a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.

Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?

It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?
Very good point! And a much better idea than mine. Especially a huge first stage with no legs. Because it's big, low drag loss. Because no upper stage, low gravity losses. f9 v1.2-like mass fractions and propellant densification. Raptor's high Isp. All those engines allow you to have ability to throttle way down to prevent crushing your stage due to over acceleration of such a light stage. Just put an aerodynamic fairing on top, and yeah, it should have no problem reaching orbit.

But if it's going to be a depot, it might need better insulation than a first stage usually has. That might be a major reason why you wouldn't do this, since you might need to fly it inside a fairing.
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Online Andy Smith

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Re: MCT Speculation and Discussion Thread 4
« Reply #266 on: 06/25/2015 01:05 PM »
Use a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.

Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?

It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?

Yes but it would mean sending a lot of expensive Raptors on a one off mission. If inflatable LOX/methane tanks are possible it seems like the more cost efficient solution to me.

That's a good point. Of course if this depot had appropriate fuel levels it could later de-orbit and land for servicing - say every two years following the mars departure window. It is no longer a one off mission, just another part of the reusable infrastructure?
« Last Edit: 06/25/2015 01:06 PM by Andy Smith »

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #267 on: 06/25/2015 01:10 PM »
Use a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.

Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?

It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?

Yes but it would mean sending a lot of expensive Raptors on a one off mission. If inflatable LOX/methane tanks are possible it seems like the more cost efficient solution to me.

That's a good point. Of course if this depot had appropriate fuel levels it could later de-orbit and land for servicing - say every two years following the mars departure window. It is no longer a one off mission, just another part of the reusable infrastructure?

But it would need significant modifications to handle re-entry as it was designed for something like 2.5 km/s re-entry.  Another possibility though would be to retrieve the engines only or keep them as a stock of on orbit spares for replacements (or even ship them to Mars as a pool of spares there).
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #268 on: 06/25/2015 01:13 PM »
Or use end of life Raptors.
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Online Andy Smith

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Re: MCT Speculation and Discussion Thread 4
« Reply #269 on: 06/25/2015 01:33 PM »
Use a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.

Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?

It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?
Very good point! And a much better idea than mine. Especially a huge first stage with no legs. Because it's big, low drag loss. Because no upper stage, low gravity losses. f9 v1.2-like mass fractions and propellant densification. Raptor's high Isp. All those engines allow you to have ability to throttle way down to prevent crushing your stage due to over acceleration of such a light stage. Just put an aerodynamic fairing on top, and yeah, it should have no problem reaching orbit.

But if it's going to be a depot, it might need better insulation than a first stage usually has. That might be a major reason why you wouldn't do this, since you might need to fly it inside a fairing.

I can see the insulation being an issue - perhaps I was being too optimistic - would it depend on where the depot was located, raise it sufficiently far that the Earth isn't adding to the heat load, then "end on" to the sun?

If its using the methane / oxygen to power itself, run the refrigeration plant etc. Then it would need some detailed maths to balance heat loads and fuel being used vs the cost of additional insulation / infrastructure (ie one launch vs a depot requiring multiple launches for a non-returnable asset).

One other problem would be where do we install radiators? It would increase the valve and plumbing complexity - but (wild suggestion I know) how about using the engine bells?

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #270 on: 06/25/2015 01:37 PM »
  Another possibility though would be to retrieve the engines only or keep them as a stock of on orbit spares for replacements (or even ship them to Mars as a pool of spares there).

Scratch the part that suggests using them as spares on orbit or at Mars, wrong engines, though the concept works with MCT derived depots. A variant of Lamontagne's design seems most likely. Start with one, and build up to 2, 4, or even more as demand dictates. One thing I see is having liquid hydrogen at this depot as the "refrigerant" and of course potentially to sell to other customers.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Online Andy Smith

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Re: MCT Speculation and Discussion Thread 4
« Reply #271 on: 06/25/2015 01:40 PM »
Use a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.

Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?

It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?

Yes but it would mean sending a lot of expensive Raptors on a one off mission. If inflatable LOX/methane tanks are possible it seems like the more cost efficient solution to me.

That's a good point. Of course if this depot had appropriate fuel levels it could later de-orbit and land for servicing - say every two years following the mars departure window. It is no longer a one off mission, just another part of the reusable infrastructure?

But it would need significant modifications to handle re-entry as it was designed for something like 2.5 km/s re-entry.  Another possibility though would be to retrieve the engines only or keep them as a stock of on orbit spares for replacements (or even ship them to Mars as a pool of spares there).

Isn't it just fuel? If it can get to orbit then it can propulsively slow down, it doesn't need to re-enter at orbital speeds - whether fuelling the depot in order to bring it home is cost effective I don't know. But I could see the benefits of being able to inspect it before launching the next iteration as being useful?

Offline ArbitraryConstant

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Re: MCT Speculation and Discussion Thread 4
« Reply #272 on: 06/25/2015 02:11 PM »
Also because of the intensity of the black body radiation of the earth and its daytime reflection of heat, I see the depot needing far more active cooling than the MCT which will only need to keep its propellant from boiling off near Mars and between Mars and Earth but will not need to keep it cool for long in the 10 radii range of the Earth.
Not sure this is a problem. Other depot studies have noted this as an issue for hydrogen but methane is quite a mild cryogen in comparison. With solar power a methane prop depot should be able to be zero boiloff anywhere.

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #273 on: 06/25/2015 02:42 PM »
Yes boiling points are:

-252.8 degrees C for hydrogen
-182.9 degrees C for oxygen
-162 degrees C for methane. 

Hydrogen is much harder to keep from boiling off. 

I worked for a natural gas company and we liquefied natural gas in the summer for winter peeks.  Boil off was not that big of a problem on the ground, and space is colder.  Tanks on the ground were doubled like a thermos bottle, with a vacuum pulled between the inner storage tank and outer shell.  There was about 3' of space between them (1m), so keeping cold wasn't hard, and that is in the deep south. 
« Last Edit: 06/29/2015 01:14 PM by spacenut »

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #274 on: 06/25/2015 02:52 PM »
I do like the idea of using a BFR first stage for a refueling depot for the MCT's.  Seems like it wouldn't be such a problem with removal of all but 4-5 Raptor engines to get it into orbit.  Also removal of landing legs unless one wants to deorbit it.  Then installing extra insulation on the tanks, add solar panels and shading panels if necessary, and adding docking facilities like previously proposed. 

The only thing I see, is in a Boeing proposal was to have the depot spin slowly to make the fuel stay on the end being pumped from, especially when near empty.  So the depot tanks might have to be in a circle with docking in the center, then spin slowly while fueling and refilling.  Say have an x shape, 3 tanks on three legs, with the 4th leg for refueling and refilling, with the ability to spin slowly, (SEP thrusters maybe attached to create spin and to stop spin) or use boil off for thrusters. 

Offline ArbitraryConstant

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Re: MCT Speculation and Discussion Thread 4
« Reply #275 on: 06/25/2015 03:10 PM »
Your original flight cost was calculated correctly (800 passenger flights, 8000 cargo flight, 8800 total at $50M a pop yields the $440B.

The cost of the cargo is presently unknown. My gut feeling is that the answer to question "what do you have to pack  in order to live on Mars" is quite lengthy, complex and thus expensive.
I think Musk is assuming most things will be manufactured locally before flights reach that level.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #276 on: 06/25/2015 03:35 PM »
Yes boiling points are:

252.8 degrees C for hydrogen
182.9 degrees C for oxygen
162 degrees C for methane. 

Hydrogen is much harder to keep from boiling off. 

I worked for a natural gas company and we liquefied natural gas in the summer for winter peeks.  Boil off was not that big of a problem on the ground, and space is colder.  Tanks on the ground were doubled like a thermos bottle, with a vacuum pulled between the inner storage tank and outer shell.  There was about 3' of space between them (1m), so keeping cold wasn't hard, and that is in the deep south.
You forgot negative signs.  Also: Please just use kelvins.  It's much easier.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #277 on: 06/25/2015 03:52 PM »
Also because of the intensity of the black body radiation of the earth and its daytime reflection of heat, I see the depot needing far more active cooling than the MCT which will only need to keep its propellant from boiling off near Mars and between Mars and Earth but will not need to keep it cool for long in the 10 radii range of the Earth.
Not sure this is a problem. Other depot studies have noted this as an issue for hydrogen but methane is quite a mild cryogen in comparison. With solar power a methane prop depot should be able to be zero boiloff anywhere.


There was a paper I read in the last month (and I know it is linked to here on NSF and I will look for it later) that suggested LOX and Methane would be fine more than 10 radii from Earth at Earth's distance from the sun with simply passive cooling, but that near Earth and potentially Mars more cooling would be required. And remember a LEO depot will spend roughly half its time above a sunlit Earth that is radiating significantly more than its black body night time amount and that it will cover a significant fraction of the visible area around the depot.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline ArbitraryConstant

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Re: MCT Speculation and Discussion Thread 4
« Reply #278 on: 06/25/2015 04:07 PM »
And remember a LEO depot will spend roughly half its time above a sunlit Earth that is radiating significantly more than its black body night time amount and that it will cover a significant fraction of the visible area around the depot.
I could say the same of LNG depots on the ground.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #279 on: 06/25/2015 04:09 PM »
Also because of the intensity of the black body radiation of the earth and its daytime reflection of heat, I see the depot needing far more active cooling than the MCT which will only need to keep its propellant from boiling off near Mars and between Mars and Earth but will not need to keep it cool for long in the 10 radii range of the Earth.
Not sure this is a problem. Other depot studies have noted this as an issue for hydrogen but methane is quite a mild cryogen in comparison. With solar power a methane prop depot should be able to be zero boiloff anywhere.


There was a paper I read in the last month (and I know it is linked to here on NSF and I will look for it later) that suggested LOX and Methane would be fine more than 10 radii from Earth at Earth's distance from the sun with simply passive cooling, but that near Earth and potentially Mars more cooling would be required. And remember a LEO depot will spend roughly half its time above a sunlit Earth that is radiating significantly more than its black body night time amount and that it will cover a significant fraction of the visible area around the depot.
True, it is much more difficult in LEO, but an actuated passive system is very flexible, and even a static passive system can be done.  Mount a cone-shaped reflective thermal shroud around the tanks, and point it normal to the orbital plane, and so long as your choice of orbital plane isn't very far from the ecliptic, you can be mostly in radiative thermal contact with deep space rather than the Earth or the Sun.
« Last Edit: 06/25/2015 04:09 PM by Burninate »

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