Author Topic: MCT Speculation and Discussion Thread 4  (Read 610489 times)

Offline meekGee

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Re: MCT Speculation and Discussion Thread 4
« Reply #840 on: 10/18/2015 05:26 PM »


Is there any concept of a reverse gravity slingshot?- e.g. Use the gravity of the moon to slow down before re-entering earth's atmosphere.

First the moon's orbit is both slow & long and worse yet inclined off plane so the chance of the moon being where you need it to be is tine.

But given that the moon's mass (1/80th Earth) is too small to shed any meaningful velocity.

I think for gravity assists, the assisting body is essentially infinite in mass, so it doesn't matter.

What matters is the speed of that body, and in the case of a reverse slingshot, a slow moon is a good thing - except you'll be using it "retrograde" so a fast moon would be even better.  However, it is what it is, and I think the answer is "good enough".

Since the vehicle is coming from Mars, you can aim anywhere you want in the Moon-Earth system, and so the vehicle-Moon-Earth system can form a single plane - so that's not a problem either.

Timing is an issue as you state, but you'll know all about that during departure from Mars.

Not saying that's it's possible or beneficial - but those specific objections I think are solvable.
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Offline Bob Shaw

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Re: MCT Speculation and Discussion Thread 4
« Reply #841 on: 10/18/2015 05:48 PM »
I'd be very wary of overly-complex orbital dynamics being used on manned missions, but for supplies - especially items where you don't mind a slower transit time (to choose some extreme examples, corrugated iron, bags of cement or water) then any amount of silly stuff would be acceptable. I think the benefits are greater on the way out than the way in, and would tend to revolve around loosely captured vehicles in halo and other orbits rather than classical fast flybys of our Moon. Coming back, you might use clever trajectories to minimise the fuel needed to get into super-high Earth orbit, saving the need for extra mass being lifted into space.

Cargo first, though!
« Last Edit: 10/18/2015 05:49 PM by Bob Shaw »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #842 on: 10/18/2015 09:06 PM »
NASA did studies for Inspiration Mars. They came to the conclusion that direct reentry from Mars at earth return would be the way to go assuming PicaX as a heatshield material. A skip reentry would increase heatshield stress.
Yes, it's true that if you're just trying to minimize heatshield mass using an ablative heatshield, you often just want to go hot and fast. But in our discussion, it's peak acceleration that we're attempting to minimize, not heatshield mass. 6 gees is a pretty good peak acceleration to use. 5 gees has also been used. Nobody that I'm aware of has used 2 gees in any serious analysis. Heck, a /regular/ reentry uses more than 2 gees. So I find that disingenuous.


Bigger spacecraft can tolerate less g forces, that is a clear pattern.  If I tried to say a small sample return capsule of under 1 m couldn't tolerate more then 6 g's then THAT would be sandbagging as such capsules and missiles routinely tolerate >20 g's.

The Shuttle was the largest re-entry vehicle ever flown and it maxed out at 3 g's for launch and 2 for landing while being half the likely entry mass of a MCT.  When I say 2 g's I am making accommodation for the vehicles size rather then just hand-waving away a WELL understood engineering relationship between size and strength and substituting the g-forces experienced by a craft of a completely different scale and claiming it as a legitimate basis for comparison.

Launch g's are experienced in one orientation, and I specified that I'm worried about the 'nose bleed' negative g's that a bi-conic would experience.  If your a fan of a 'Super Dragon' type MCT they you can claim the launch g's and aerocapture g's are oriented the same way and the same structures resist both forces and that could get you up to ~4 g's without any additional structure over what's needed for launch.  But I would then point out that your a low lift configuration with high ballistic coefficient which would increase g-forces during entry well beyond 4 g's.

This is a key point, flipping the whole vehicle over and applying forces from the opposite direction is huge.  It's the difference between designing a normal skyscraper and one that still stands when upside down.
« Last Edit: 10/18/2015 11:01 PM by Impaler »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #843 on: 10/19/2015 01:58 AM »
NASA did studies for Inspiration Mars. They came to the conclusion that direct reentry from Mars at earth return would be the way to go assuming PicaX as a heatshield material. A skip reentry would increase heatshield stress.
Yes, it's true that if you're just trying to minimize heatshield mass using an ablative heatshield, you often just want to go hot and fast. But in our discussion, it's peak acceleration that we're attempting to minimize, not heatshield mass. 6 gees is a pretty good peak acceleration to use. 5 gees has also been used. Nobody that I'm aware of has used 2 gees in any serious analysis. Heck, a /regular/ reentry uses more than 2 gees. So I find that disingenuous.


Bigger spacecraft can tolerate less g forces, that is a clear pattern.  If I tried to say a small sample return capsule of under 1 m couldn't tolerate more then 6 g's then THAT would be sandbagging as such capsules and missiles routinely tolerate >20 g's.

The Shuttle was the largest re-entry vehicle ever flown and it maxed out at 3 g's for launch and 2 for landing while being half the likely entry mass of a MCT.  When I say 2 g's I am making accommodation for the vehicles size rather then just hand-waving away a WELL understood engineering relationship between size and strength and substituting the g-forces experienced by a craft of a completely different scale and claiming it as a legitimate basis for comparison.

Launch g's are experienced in one orientation, and I specified that I'm worried about the 'nose bleed' negative g's that a bi-conic would experience.  If your a fan of a 'Super Dragon' type MCT they you can claim the launch g's and aerocapture g's are oriented the same way and the same structures resist both forces and that could get you up to ~4 g's without any additional structure over what's needed for launch.  But I would then point out that your a low lift configuration with high ballistic coefficient which would increase g-forces during entry well beyond 4 g's.

This is a key point, flipping the whole vehicle over and applying forces from the opposite direction is huge.  It's the difference between designing a normal skyscraper and one that still stands when upside down.
The shuttle orbiter is about the same mass as I expect a dry MCT will be, but is in a shape that I don't expect MCT to be. Those wings are very heavy structurally. If it's the side of a pressure vessel, and the whole thing is pressurized to some extent or another, then MCT could be far more structurally efficient than Shuttle was. Additionally, MCT will have access to superior materials and manufacturing techniques.

You are substituting hand-waving for actual analysis. I maintain that you are sandbagging aerocapture/entry with unrealistically pessimistic numbers.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #844 on: 10/19/2015 03:16 AM »
The shuttle dry mass was 75 mT which yes is the same dry mass as I estimate as well , but for a vehicle with STAGGERINGLY lower performance requirements then your demanding of it. 

But Shuttle did not perform entry with 100 mT of cargo plus terminal landing propellants.  So it's entry mass would be less then half of what MCT would experience, THAT eats up all the improved materials and manufacturing techniques of today as well as savings from not having wings.

No one has produced any speculative MCT that doesn't have at least 200 mT entry mass, if you have such a vehicle then present it or stop making dishonest complaints about entry mass.

You have not provided one shred of analysis here only reasoning by grasping at the performance values of other vehicles without though to scale or mass, while I just provided you with one which is admittedly incredibly simple but still demonstrates the difficulties of getting a huge heavy vehicle to withstand high g forces due to cube-square scaling.  I expect you to at least ACKNOWLEDGE THIS http://forum.nasaspaceflight.com/index.php?topic=37808.msg1437142#msg1437142 analysis and give a rebuttal before you respond again and rethink who is the one doing the hand-waving.


Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #845 on: 10/19/2015 03:47 AM »
If you're limited to just 2 gees on reentry, that's a good argument for base-first entry, same load-path.

But there are other ways to carry a buckling load. Consider, for example, if the load of 9MN (150 tons times 6 gees) was distributed over the surface of a biconic with dimensions of roughly 15m by 15m (or 10m by 22m or whatever). That is the equivalent to less than 6psi. That's less than the internal pressure of the vehicle (and likely you'd pressurize the tanks for reentry, since you'll need to do a landing burn anyway). In other words, you don't have to deal with a buckling force (like with shuttle which was mostly unpressurized) but the tensile force of a pressure vessel which you already need anyway.

It's a minimal analysis, but still far better than your hand-waving about square-cube-law making things so much worse for MCT, so impossibly worse that you'll throw out either significant aerocapture or 100 day transits.


...and heck, even at your sand-bagged 2 gees, that still doesn't rule out 100 day transfers with multi-pass aerocapture/entry at the end. With a lifting entry, you can spread out that acceleration over a long time.
« Last Edit: 10/19/2015 03:50 AM by Robotbeat »
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #846 on: 10/19/2015 04:09 AM »
...and heck, even at your sand-bagged 2 gees, that still doesn't rule out 100 day transfers with multi-pass aerocapture/entry at the end. With a lifting entry, you can spread out that acceleration over a long time.

Aerocapture is hard precisely because it must come all at once, lest the craft shoot out on a modified heliocentric orbit;  Consequences for unpredictable failure usually involve loss of crew.  'Lifting entry' is primarily useful after capture has occurred, when you have plenty of time-in-atmosphere to work with.

100 day transfers don't just involve high amounts of velocity to dissipate, more and more of that velocity has to be dissipated on the first pass.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #847 on: 10/19/2015 04:39 AM »
...and heck, even at your sand-bagged 2 gees, that still doesn't rule out 100 day transfers with multi-pass aerocapture/entry at the end. With a lifting entry, you can spread out that acceleration over a long time.

Aerocapture is hard precisely because it must come all at once, lest the craft shoot out on a modified heliocentric orbit;  Consequences for unpredictable failure usually involve loss of crew.
That was also true for each and every Shuttle and Apollo reentry. The reentry corridor was narrow. But if one MCT out of 500 ships did skip out on a heliocentric trajectory, you'd just have to devote one or two stripped-down MCTs in orbit or on the surface of Mars to be ready to mount a rescue mission (or wouldn't even have to be devoted to that purpose, just ready to be fueled up when the MCTs are arriving). In the grand scheme of things, that would be fairly cheap insurance.
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'Lifting entry' is primarily useful after capture has occurred, when you have plenty of time-in-atmosphere to work with.
No, it's also useful during aerocapture because you can adjust for uncertainties in the atmosphere's density. And you can utilize negative lift to give you more time-in-atmosphere.

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100 day transfers don't just involve high amounts of velocity to dissipate, more and more of that velocity has to be dissipated on the first pass.
Sure. But not more than needs to be dissipated for lunar reentry.
« Last Edit: 10/19/2015 04:44 AM by Robotbeat »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #848 on: 10/19/2015 08:02 AM »
If you're limited to just 2 gees on reentry, that's a good argument for base-first entry, same load-path.

Yes it is, and if you want to advocate for base-first entry, though you would have other series trade-offs to justify.


That was also true for each and every Shuttle and Apollo reentry. The reentry corridor was narrow. But if one MCT out of 500 ships did skip out on a heliocentric trajectory, you'd just have to devote one or two stripped-down MCTs in orbit or on the surface of Mars to be ready to mount a rescue mission (or wouldn't even have to be devoted to that purpose, just ready to be fueled up when the MCTs are arriving). In the grand scheme of things, that would be fairly cheap insurance.

No, Shuttle did not do Aerocapture because it was not going anywhere near escape velocity on atmospheric entry and could not leave the Earth if it over-shot, it did controlled skips and banked turns along the atmosphere to dissipate energy but that is not the same as aerocapture.  When you under-shoot aerocapture you end up back on a Heliocentric orbit leaving the planet entirely.

The rescue idea is ridiculous, the lost vehicle would be heading for a aphelion well into the asteroid belt and even if another MCT on mars had the DeltaV to rendezvous with it they would have not have the DeltaV to return to mars or likely any other planet on a time scale of less then years.

No, it's also useful during aerocapture because you can adjust for uncertainties in the atmosphere's density. And you can utilize negative lift to give you more time-in-atmosphere.

Yes this is correct, the more lift you can generate the wider your aerocapture corridor is and the more adjustment can be made in real time due to atmospheric conditions.  It is an argument in favor of the bi-conic shape when doing aerocapture.

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100 day transfers don't just involve high amounts of velocity to dissipate, more and more of that velocity has to be dissipated on the first pass.
Sure. But not more than needs to be dissipated for lunar reentry.

Apollo entry was at 10.77 km/s, contrary to popular belief they could not skip off the atmosphere and be lost in space forever, they would simply have re-entered later after a large elliptical orbit during which they would run out of oxygen because they were below escape velocity.

Burnate already did the numbers, on the 102 day transit your perigee at mars is 9.17 km/s and you need to shed 4.7 km/s.  That's about half your velocity, and about 50% of the energy of the Apollo return.

The 100 day transit is even worse (it saves Earth Escape DeltaV by raising velocity at mars), cause your velocity is 13.77 km/s of which you must shed 9.2 km/s, which is 88% of your total energy.  In fact you need to shed more energy then the entire Apollo lunar return, about 60% more.

I'm very skeptical that the cord through the martian atmosphere can be long enough to provide this much deceleration within even your g limits.  The faster your going to less time you spend in the atmosphere and your deceleration force needs to increase exponentially.   And as entry velocities increase the corridor gets narrower and narrower, according to http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930003532.pdf even with 5 g limit the entry velocity had to kept to under 9 km/s to keep the corridor to 1 degree wide with a 0.3 L/D vehicle like a capsule.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #849 on: 10/19/2015 11:27 PM »
If magnetic deceleration can be used it would help a lot with reusability. If a first pass can achieve capture then a second or third pass can achieve landing. Replacing the heatshield after every flight will cost time and money. I don't see magnetic as a requirement but i hope for it. A metallic heatshield will not do especially for EDL back on earth.
Even a PICA-X shield can take multiple entries (Musk said 100, which is an overstatement but is a lot more than MCT will need). And yes, a metallic shield would work, you just have to do it gently enough. But I suspect SpaceX is looking at PICA-X. A tanker going to LEO, most certainly, could use a metallic TPS without any hard limits on numbers of reuses (thousands are possible).

A metallic heatshield for reentry from interplanetary speeds? I doubt it. PicaX reusable yes. But from interplanetary speeds? To Mars and back, yes, thats two uses, the second very heavy. But again? Maybe two return flights, maybe not.

Edit: I hope I am wrong.

Metallic heatshield for the tankers, that would help a lot, I agree. Many tank flights without refurbishing the heatshield. But it would mean a completely different design for the tanker. The tanker coming back from LEO can probably make a significant number of flights before refurbishing with PicaX.

That would be my thought too.  Especially if the MCT was an integrated design.  Do they wan to have two different heat shields for the tanker version and the Mars version?  Or just use Pica-X on both for commonality, and just replace it after may one round trip from Mars, or like 10 [for example] trips to LEO?  Probably need to thoroughly go through a tanker after 10 round trips anyway and look for other wear items.

Now, if MCT rides empty on a dedicated S2 so that it were to have whole vehicle abort capability, then the S2 may be very different from MCT in OML and EDL profile...so there may be little commonality between them even if they both use Pica-X.   SO going with two different TPS types in that case may not be an issue.


Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #850 on: 10/20/2015 04:15 AM »
Do they wan to have two different heat shields for the tanker version and the Mars version?  Or just use Pica-X on both for commonality, and just replace it after may one round trip from Mars, or like 10 [for example] trips to LEO?  Probably need to thoroughly go through a tanker after 10 round trips anyway and look for other wear items.

Again like in other discussions we run into the situation that we need to differentiate between different phases. I would assume that initially the tanker too would have PicaX for ease of development. If and when the number of flights really takes off it can be worth it to redesign the tanker to be more reusable without changing the heatshield. Then metallic or even better magnetic would look quite attractive.

Offline Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #851 on: 10/20/2015 05:31 AM »
Again like in other discussions we run into the situation that we need to differentiate between different phases. I would assume that initially the tanker too would have PicaX for ease of development. If and when the number of flights really takes off it can be worth it to redesign the tanker to be more reusable without changing the heatshield. Then metallic or even better magnetic would look quite attractive.

Is a metallic heat shield lighter than PicaX?

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #852 on: 10/20/2015 05:53 AM »
While SpaceX clearly has experience with PICA and their own formulations of such, I don't think they would be afraid of pursuing metallic if they felt it made good engineering and cost sense.  The biggest factor in favor of metallic TPS is that is it the only way you get to a true 'gas-and-go' vehicle and we know that's ultimately what they want.  The hypothetical Tanker-2-LEO vehicle is clearly going to have a rapid launch cadence, much more so then the vehicle which actually transits to and from mars so it's the logical the first place something like metallic TPS would be useful.

The question is given all the other things that will be needed to turn a vehicle around if the process actually takes a week or two (still a huge improvement over Shuttle) dose the replacement of an ablative heat-shield really impact your turn around time significantly, especially if it is replaced only infrequently.  I suspect this will be the case, that the best vehicle that can presently be built won't really need metallic TPS for it's turn around time so it won't initially have it.  But if the turn around process is improved, perhaps after several years of doing it to the point that heat-shield replacement becomes a bottleneck then I can see this being a desirable upgrade.

Now if on the other hand metallic systems can manage to beat PICAX on weight then they have a clear path to being in the design right from the start.  It is very hard to say if this would be the case because metallics haven't been used before (but they have been well developed during the X-33 project) and we don't know how much PICA is actually used on Dragon or how much would be needed on a hypothetical Tanker.  My own analysis shows metallics being at about parity with PICA for a relatively low temperature range consistent with lifting entry vehicles, but this is based on only a speculation of the PICAX thickness and doesn't factor in any potential SpaceX improvement to the metallic system either so I can't say anything other then that it might be possible.

Metallics might be inappropriate because of stagnation point temperatures in excess of what they handle such as on a low lift blunt vehicle like Dragon and thouse temperatures depend hugely on the actual entry characteristics of the vehicle which are entirely speculative right now.  Perhaps even a combination of ablatives at the stagnation point/s and metalics on the flanks and leeward sides of the vehicle could work.  I am very sure that we will not see PICAX directly applied to propellant tankage though as some have speculated, the bonding of a brittle material to a tank wall that will become cryogenic strikes me as unfeasible as ablatives are attached with adhesives which are notoriously bad when cold, nor dose it offer any means of repair or replacement as ablation takes place.  You need a structural frame that holds the ablative and allows it to be removed, this would also be the default means of using metallic TPS in large panels, though I can conceive of a monocoque design in which the a multi-layer sandwiching of metals creates the tank wall, the insulation layer and the outer skin.
« Last Edit: 10/20/2015 06:13 AM by Impaler »

Offline aceshigh

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Re: MCT Speculation and Discussion Thread 4
« Reply #853 on: 10/20/2015 01:44 PM »
why is MCT always considered a single vehicle launched from Earth? Has Musk in any way indicated that?

isnīt there any possibility MCT could be a space only craft in permanent transit between Mars and Earth, picking people on Earth and dropping them on Mars?

that way, MCT could be faster and bigger. Because you wouldnīt need to accelerate an habitat for 100 people to live for 6 months (or less if possible) EVERY trip.

Just accelerate (to catch up with MCT and transfer people) a "small" capsule where you have 100 people crammed in a tight space with their luggage.


The idea of a big transport to Mars for people to live in for 6 months being launched EVERY trip sounds to me somewhat like launching half the space station into space every time a new astronaut goes to it.

Offline RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #854 on: 10/20/2015 01:54 PM »
why is MCT always considered a single vehicle launched from Earth? Has Musk in any way indicated that?

isnīt there any possibility MCT could be a space only craft in permanent transit between Mars and Earth, picking people on Earth and dropping them on Mars?

that way, MCT could be faster and bigger. Because you wouldnīt need to accelerate an habitat for 100 people to live for 6 months (or less if possible) EVERY trip.

Just accelerate (to catch up with MCT and transfer people) a "small" capsule where you have 100 people crammed in a tight space with their luggage.


The idea of a big transport to Mars for people to live in for 6 months being launched EVERY trip sounds to me somewhat like launching half the space station into space every time a new astronaut goes to it.

Elon wants a shorter transit time and you can't do that with a cycler.

Offline aceshigh

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Re: MCT Speculation and Discussion Thread 4
« Reply #855 on: 10/20/2015 03:51 PM »
how shorter does he want the transit times?

from here https://en.wikipedia.org/wiki/Mars_cycler it seems plenty of them are under 3 months?


but anyway, isnīt there a middle term? Cyclers hardly use any fuel at all. What about a cycler which also has fuel assist BUT still would use much less fuel (for itīs size) than launching from Earth?

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #856 on: 10/20/2015 04:08 PM »
why is MCT always considered a single vehicle launched from Earth? Has Musk in any way indicated that?


Musk made some comments about "landing the whole thing" and "using MCT as the surface habitat initially."   This has lead to people assuming there won't be multiple vehicle that land, launch, and transit separately like Mars Semi-direct or something. 

It's not much to go on, and SpaceX's eventual actual MCT concept could have completely changed since those comments.  But it's all we've had to go on so far.  :-)

Offline RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #857 on: 10/20/2015 04:20 PM »
how shorter does he want the transit times?

from here https://en.wikipedia.org/wiki/Mars_cycler it seems plenty of them are under 3 months?


but anyway, isnīt there a middle term? Cyclers hardly use any fuel at all. What about a cycler which also has fuel assist BUT still would use much less fuel (for itīs size) than launching from Earth?

Note the long return to Earth time. Elon also want MCT to get back to Earth quickly so it can be prepped for its next flight the next synod.

SpaceX has mentioned looking into SEP. A hybrid system would allow a quick TMI using chemical rockets and then SEP would allow a slow continuous acceleration to reduce flight time.

why is MCT always considered a single vehicle launched from Earth? Has Musk in any way indicated that?


Musk made some comments about "landing the whole thing" and "using MCT as the surface habitat initially."   This has lead to people assuming there won't be multiple vehicle that land, launch, and transit separately like Mars Semi-direct or something. 

It's not much to go on, and SpaceX's eventual actual MCT concept could have completely changed since those comments.  But it's all we've had to go on so far.  :-)

Personally, I think it means landing on Mars and returning to Earth orbit (or cis-lunar), but yeah, we only got old quotes to work with.

I'm sure we will be amazed at the MCT architecture when SpaceX releases it.
« Last Edit: 10/20/2015 04:39 PM by RonM »

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #858 on: 10/20/2015 05:01 PM »
While SpaceX clearly has experience with PICA and their own formulations of such, I don't think they would be afraid of pursuing metallic if they felt it made good engineering and cost sense.  The biggest factor in favor of metallic TPS is that is it the only way you get to a true 'gas-and-go' vehicle and we know that's ultimately what they want.  The hypothetical Tanker-2-LEO vehicle is clearly going to have a rapid launch cadence, much more so then the vehicle which actually transits to and from mars so it's the logical the first place something like metallic TPS would be useful.

The question is given all the other things that will be needed to turn a vehicle around if the process actually takes a week or two (still a huge improvement over Shuttle) dose the replacement of an ablative heat-shield really impact your turn around time significantly, especially if it is replaced only infrequently.  I suspect this will be the case, that the best vehicle that can presently be built won't really need metallic TPS for it's turn around time so it won't initially have it.  But if the turn around process is improved, perhaps after several years of doing it to the point that heat-shield replacement becomes a bottleneck then I can see this being a desirable upgrade.


Agreed.  The MCT that goes to Mars will probably get quite a refurbishment between missions as it will have been gone a long time, habitated for a long time, and loitered in space and on the dusty surface of MArs for a long time, that it will probably go into a processing building for an extensive inspection and refurb when it gets back.
But the tanker will have a goal of launch, dock with depot, return, repeat.  That makes it a good candidate for metallic as it offers that potential without burning a little away each return like Pica-X.

But, there will probably be a certain limited service live for safety where it will get say 10 mission (for example) and then go into a processing facility and get thoroughly inspected and refurbished where necessary.  At that point while it's off line for a while anyway, replacing the large PICA-X tiles wouldn't be too difficult.  Especially if a nice smooth biconic shape that doesn't have a lot of complex surface area like the Shuttle.  Even if a tanker had had a metallic TPS, it'll still likely need to go into the processing facility at regular intervals to make sure it stay reliable.  So a metallic TPS probably won't prevent that.  PICA would need to be of sufficient thickness to last that service life between refurbishments.  How heavy would that be vs. the metallic?  That'd probably be a major factor in evaluation of both.

I am very sure that we will not see PICAX directly applied to propellant tankage though as some have speculated, the bonding of a brittle material to a tank wall that will become cryogenic strikes me as unfeasible as ablatives are attached with adhesives which are notoriously bad when cold, nor dose it offer any means of repair or replacement as ablation takes place.  You need a structural frame that holds the ablative and allows it to be removed, this would also be the default means of using metallic TPS in large panels, though I can conceive of a monocoque design in which the a multi-layer sandwiching of metals creates the tank wall, the insulation layer and the outer skin.

Agreed.  My thoughts too.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #859 on: 10/20/2015 07:33 PM »
why is MCT always considered a single vehicle launched from Earth? Has Musk in any way indicated that?

isnīt there any possibility MCT could be a space only craft in permanent transit between Mars and Earth, picking people on Earth and dropping them on Mars?

that way, MCT could be faster and bigger. Because you wouldnīt need to accelerate an habitat for 100 people to live for 6 months (or less if possible) EVERY trip.

Just accelerate (to catch up with MCT and transfer people) a "small" capsule where you have 100 people crammed in a tight space with their luggage.


The idea of a big transport to Mars for people to live in for 6 months being launched EVERY trip sounds to me somewhat like launching half the space station into space every time a new astronaut goes to it.

Not, always, I'm one of a few voices in the wilderness that still see semi-direct like architectures as most likely and fully compatible with Musk's goals and statements so far.  And we should NOT be taking even clear statements of intent as evidence that alternatives have been taken off the table as their are FAR too many instances of early statements of intent in the engineering minutia by SpaceX that fall through.

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