Author Topic: MCT Speculation and Discussion Thread 3  (Read 348167 times)

Offline Lobo

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Re: MCT Speculation and Discussion Thread 3
« Reply #440 on: 04/29/2015 04:23 pm »
Lobo makes a good case for a bi-conic shape,

Thanks!  :-)

and really when were debating between a Dragon style capsule with a 15 degree wall slope and a bi-conic with perhaps 10 degrees the difference is really small.  It's more a debate between the orientation of the vehicle during entry, either bottom-forward or side-forward.  Given the similar volumes discussed the final diameter and shape of the MCT is really splitting hairs.

Agreed.  If MCT were a minimally sloped capsule shape, it would look more like a traditional biconic, rather than a biconic nosed cylinder.  Certainly a lot of side entry biconics that actually do look like that (see below).
However, I go with the biconic nosed cylinder becuase it's easier to make big tanks in a cylindrical shape, than it is to make biconic shaped fuel tanks.  That's why rocket stages are cylinders and not biconic or some other shape.  I don't think a true biconic or capsule shape really has much advantage vs. a biconic nosed cylinder during EDL (maybe someone in the know could confirm or refute that) so I'm just thinking keep it simple.  NASA's DRM 5.0 aeroshells were basically biconic nosed cylinders rather than true biconics.  They were payload only though, without having large tanks themselves, and just meant to get lander through the Martian atmosphere.  Still, I think NASA's use of the shape is a good endorsement of it's viability.


I think the disagreement is more centered on what the vehicle will be CAPABLE of largely due to disagreements over the dry mass estimate.  I simply don't find dry masses under 100 mT credible given what MCT needs to do and the volume it will have.  Comparisons to Staurn V stages don't seem credible when these were expendable rocket stages that are hold nothing but propellents and are not capable of EDL on Mars. 


I have no idea what the dry mass of MCT might really be like.  I'm using the Saturn S-II as a reference that would be an upper stage in the ball part of what I think the MCT spacrcraft/upper stage might be like.  And to show that MCT itself doesn't necessarily have to be some ginormous BFR putting near 300mt inot LEO and have 15Mlbs of thrust and 30+ Raptor engine.

So, let's say today, we could make an S-II stage that's say 35mt dry, rather than 45mt dry, with modern manufacturing techniques and materials...for example.  To that we need to add:
1)  crew cabin
2)  landing legs
3)  TPS system on the side
4)  ECLSS
5)  Sabatier reactor
6)  LH2 fed stock or equipment that can mine water vapor out of the air and then electrolyze it for H2 feed stock.  (there's some interesting research into that).
7)  Cago bay  (this is basically empty space, but the alloy housing it would add a bit of overall mass.  It could be unpressurized to reduce the housing mass required.
8  )  Deployable/retractable solar array(s) for power during transit)
9)  Anything else I can't think of right now).

If we start with 35mt base for the "stage", how much extra mass will all of this add?  I have no idea.  If about 30mt, then MCT would dry mass around 65mt, and be in our ballpark.
I don't think the ECLSS system or sabatier reactor would be all that heavy.  Ditto for a cargo area.  The crew cabin would essentially be empty space too and not mass much itself.  The crew accomodations like bathrooms, seats, controls, computers, etc would be where the mass for that comes from. 
That leaves landing legs, TPS, and the solar array.
Given the much larger surface area, I think they could use a TPS that's lighter per square ft than the Pica-X they use on Dragon.  The fragile ceramic tiles of the Shuttle should probably be avoided in favor of something a little more robust that needs less maintenance to turn around after a round trip.
What are some options that are out there to consider?  Anyone know what NASA would have used for their Aeroshells in DRM 5.0?
Can we keep this down to around 65mt, give or take?  There's the question.


I feel that only extrapolations from manned craft that carry cargo and perform atmospheric re-entry are logical (Dragon, Orion, Shuttle Orbiter all of these would make more sense), simply saying that MCT will do the ADDITIONAL task of being a second stage doesn't make it have the mass/volume ratio of a traditional 2nd stage, it just makes it a harder vehicle to engineer as it has more demands put upon it.


The problem with using those for analogs, is they are just crew (or payload) housings.  They aren't rocket ships.  MCT will need to be a rocket stage/ship.  Even if it's lofted to LEO by a 2-stage booster, it still has considerable dV requirements to provide itself that no other spacecraft has.
I think a better analog would be Rocketplane Kistler's K-1.  That's probably about as close to this as has been legitimately designed.  But it did a ballistic nose entry rather than a biconic side entry, and it landed on airbags on it's side.  So it's not a great analog, but one of the closest I think.  See below.


Also I find some flaws in the extrapolations from the Saturn 2nd Stage. 

Of course there are flaws.  It's just a rough analog to help picture what a 2-stage MCT might look like with an integrated 2nd stage/spacecraft.

Lobo states this stage completes the delivery to LEO, it dose not, at 2nd stage engine cut off the vehicle is only going 7 km/s and a burn of the 3rd stage is necessary to actually reach LEO, though it is a modest one of just under 1 km/s. 

So how did Skylab get into it's orbit?  There was no 3rd stage on it.
This is why I've been using Saturn INT-21 as my reference, not actually Saturn V, as that was a 3 stage LV and INT-21 was a 2-stage LV, like my MCT concept.


Lastly Lobo states the INT-21 would have delivered 140 mT to orbit, but it was actually rated at 115 mT.


No, I've said Saturn INT-21 would do almost 120mt.  115.8mt to be exact.
http://www.astronautix.com/lvs/satint21.htm

I've actually seem some people run estimates that the LEO of INT-21 would have actually been over 120mt (I believe Hyperion5 did actually, maybe I'll check back with him), but we'll stick with 116mt for now.

With the dry mass of the S-II itself which would also be in that same LEO (like it was after delivering Skylab) of 45mt, that's means about 161mt to LEO.  I would assume there was at least a couple tonnes of residual propellants in the S-II after SECO, so the total mass to LEO would probably be closer to 165mt or so.


All together this just seems a bridge too far for me to believe the MCT can be combined with it's 2nd stage, though I think it could do considerable 3rd-stage duty and thus significantly reduce the ultimate Delta-V burden on the 2nd stage.

You certainly could be right.  There might be a 2-stage booster under the MCT vehicle.  But I still think there are more advantages to having MCT as the 2nd stage.  One being you only pay the mass penalty for the upper stage(s) once, not twice.  Otherwise you are pushing the dry mass of the 2nd stage along with the dry mass of MCT to LEO.  A dedicated 2nd stage would be resuable, so you'd pay for the additional dry mass of it's reusability hardware twice.  The mass of a TPS on both the 2nd stage and MCT, landing legs on both, etc.  With just one stage, you only "pay" for all of that once.  So there's advantages to it.  And if you are refueling in LEO anyway, why not?
Not to mention it's one less development to pay for, and one less piece to have to remotely deorbit and land.  This way, you only are bringing back the actual MCT (which can be a LEO taxi configuration, or tanker configuration) back from orbit.  Rather than both an MCT and a dedicated 2nd stage.

Of course, this is all predicated on the assumption that an integrated upper stage/MCT -can- be made to work feasible.  If not, then there will be a dedicated 2nd stage. 
« Last Edit: 04/29/2015 04:29 pm by Lobo »

Offline Lobo

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Re: MCT Speculation and Discussion Thread 3
« Reply #441 on: 04/29/2015 04:28 pm »
Someone asked up thread how big the tanks have to be to return a 60 ton dry mass MCT to Earth's surface.


That was me.  Thanks!

603mt doesn't seem too prohibative.

And 603mt will weigh about 200mt on the Mars surface.  A 500klb Raptor is 226mt of thrust.  If MCT was 60mt dry, that makes a surface mass of about 220mt (equivalent).  Which would mean a single Raptor could about do the job by itself.

Not saying they'd only have a single Raptor, but we're right in this ballpark area.

And you are right, the methalox generation will have to be substantial.  I'll leave that to SpaceX to engineer.  That's above my pay grade.  ;-)


Offline hkultala

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Re: MCT Speculation and Discussion Thread 3
« Reply #442 on: 04/29/2015 04:48 pm »
Someone asked up thread how big the tanks have to be to return a 60 ton dry mass MCT to Earth's surface.


That was me.  Thanks!

603mt doesn't seem too prohibative.

And 603mt will weigh about 200mt on the Mars surface.

mars gravity is 3.711 m/s^2 , not ~3.3.

So it weights about 2238 MN.

Quote
  A 500klb Raptor is 226mt of thrust.  If MCT was 60mt dry, that makes a surface mass of about 220mt (equivalent).  Which would mean a single Raptor could about do the job by itself.

correct weight would be  ~2460 MN. So that's more than the ~2256 MN stated by elon as the raptor thrust.

So it has to weight something like <50 tonnes instead of 60 tonnes to use only one raptor for liftoff, or raptor thrust has to be greater than elon's last comment about it.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 3
« Reply #443 on: 04/29/2015 05:52 pm »
Nit, why does everyone insist the MCT will go into Mars orbit and then do a TEI burn?  I think there is a significant savings (10-20%?) in DV by going directly from Mars surface to Earth.  And with the exponential nature of the rocket equation, doesn't this translate into even bigger savings in fuel?  It just seems to me that there is absolutely no reason to loiter in Mars orbit before returning to Earth.  Sorta like me taking off on a cross country trip, but first circling my city a few times :-) 

CyberPilot, how much better do your numbers look if they do a direct launch from Mars to Earth?

Myself, I agree.  I can only see MOI if SpaceX has a plan to somehow refuel in Mars Orbit.  That seems unnecessarily complicated if there's any way MCT can do a Direct ascent to Earth.  I don't know the physics behind it, if I think SpaceX won't mess with Mars orbit fueling if they don't have to.  In which case there's not need for MOI, and keep the return propellant requitements for MCT as low as possible. 

Offline Lobo

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Re: MCT Speculation and Discussion Thread 3
« Reply #444 on: 04/29/2015 06:02 pm »

As for solar energy on Mars, Having fold-down petals on the outside of the MCT can be quickly used for both in space and on Mars. If SpaceX does what I am proposing they would need to figure out all the details that you mentioned. They may need to also add ground solar. 


I would expect the in-space solar array to be different than the on-surface one.  The in-space one will be designed to deploy and retract in zero-g vacuum.  And it will have to be done automatically, unless want a series of EVA's in space to be requried to unfurl and refurl them.  X-37B has showin that a solar array can be remotely deployed and retracted again reliably, so I think that would be the case for MCT.
On the surface it's a different story.  You'll have a whole crew unloading cargo and setting up equipment.  They can simply have soft-backed sollar cell sheets that they unroll on the surface like giant tarps.  Or they could have rigid arrays that can be set up on bases to track the sun, or whatever.  But you have time and bodies on the surface to set all of that up. 

You could also have radio isotope generators.  Those would expecially be handy if you wanted to send an MCT ahead of a crew to fuel up and be waiting as a lifeboat in case there's an issue with fueling up the MCT they come in.  Deploying solar arrays and the surface remotely of the size needed seems like it'd be a pretty difficult thing to do.   So they might want a method of power generation that doesn't need to be remotely deployed. 
A full nuclear reactor is unlikely for various political reasons, but lots of isotope powered sats have been launched over the years.


Offline Lobo

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Re: MCT Speculation and Discussion Thread 3
« Reply #445 on: 04/29/2015 06:27 pm »
Why couldn't the MCT actually be a two piece lander, that the bottom half stays on Mars as habitat and/or water, methane, or oxygen storage?  Either engines could be mounted on the sides or in the center that would stay with the upper half returning to earth.  The bottom half could be around the returning center.  It could be sealed over after returning ship detaches for internal living quarters.  Outside previous methane and oxygen tanks could be refilled with ISRU made fuel and oxygen which would offer some radiation protection.  If the lander is high enough off the ground, it could also be sealed around the outside bottom for underneath living areas, vehicle storage and compression/decompression chamber for EVA's on the surface to mine for water.  Why not just bring the center or top half of the spacecraft back to earth to be reused?  Less building would be required, just modification of the bottom or outer ring of the spacecraft.  Less fuel needed for return.

It's an interesting concept, but I think it would have a few problems. 

First, if you leave the lower part of the lander back on Mars, how do you do EDL on Earth?  You'd have left your landing legs at least.
Plus, if you've seen the video of the last couple of Apollo missions, the DM of the LEM looked pretty thrashed after the AM lifted off.  I would predict at least one Raptor firing down into a lower stage to be left on Mars getting similarly thrashed.

As RobotBeat mentioned, there could be some sort of cargo module or something that could be unloaded to the surface and then moved a safe distance away and then used as a habitat, or storage shed, or whatever.  I could see that somehow being implimented.  Still, there are difficulties with any large pod.  If it's on top, how do you get it down?  If it's on the bottom, how do you get it out?  There'd be plumbing between the tanks and engines to negotiate so you'd have size constraints to it's size and shape.
But I could see something along those lines being implimented.  Maybe just multiple smaller cargo pods the size of those Pods containers you use when you move.  Or a small cargo container size, that can be pulled off the bottom of MCT with a ramp or cables, and then towed away with a rover to where they can be unloaded.  Once loaded, they could be pressurized and used as surface habs.  They could be stacked together like "apartments", and have soil pushed up around them and over them to help protect against radation.  Some sort of universal cargo/hab pod with wheels so it can be moved around.  Once unloaded, they could be interlocked together.

A company very near where I work does something kind of similar with cargo containers.  They make mobile living quarters out of them for the military.  They are shipped to wherever the Army needs them, and then power is run to them and they are instant small houses.  For Mars, they'd just be empty vessels that are sealed to hold an atmosphere.  They are loaded up on Earth with equipment and supplies, and then plugged into MCT's cargo container hold.   On Mars they are pulled off, towed away, unloaded, and then plugged into a common ECLSS with the other containers and all uses as surface habs. 

That way every MCT will bring a certain number of pre-fab surface habs with it to be left there for future use.  Perhaps they could be designed to be interlocked together and opend up to create larger hab structures, like building blocks?
Not quite like leaving part of the MCT on the surface to use, but a more feasible way to impliment that idea.

Something akin to this, but set up to hold cargo on the trip out, and then be used for habs.

http://www.dropboxinc.com/products/photo-gallery
http://www.energistx.com/spaces/Hurricane%20Relief%20Units.html


Offline Lobo

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Re: MCT Speculation and Discussion Thread 3
« Reply #446 on: 04/29/2015 06:32 pm »

correct weight would be  ~2460 MN. So that's more than the ~2256 MN stated by elon as the raptor thrust.

So it has to weight something like <50 tonnes instead of 60 tonnes to use only one raptor for liftoff, or raptor thrust has to be greater than elon's last comment about it.

Ahhh. thanks for the accuracy.  I was just going 1/3 for estimation.

I doubt MCT would have just one Raptor.  If it's an integrated 2nd stage, it'd need probably 3-4 to get to LEO after booster staging.  So if we're assuming that, it'd have multiple Raptors.  only 2 would be needed to get off the surface.  A cluster of 4 or 5 would result in MCT being multi-engine out scenario fault tolerant.  So I think it likely MCT will have 4-5 Raptors.  So pleanty of thrust to get 600mt of fuel and a >50mt MCT off the surface.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 3
« Reply #447 on: 04/29/2015 06:56 pm »
Musk mentioned multiple Raptors for the MCT in the AMA. This would allow much lower gravity losses and thus less propellant needed.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 3
« Reply #448 on: 04/30/2015 12:54 am »


Agreed.  If MCT were a minimally sloped capsule shape, it would look more like a traditional biconic, rather than a biconic nosed cylinder.  Certainly a lot of side entry biconics that actually do look like that (see below).
However, I go with the biconic nosed cylinder becuase it's easier to make big tanks in a cylindrical shape, than it is to make biconic shaped fuel tanks.

If the biconic has a reasonably low slope to the side wall then a cylindrical tank (or 2) can fit inside of a conic without wasting much room.  The outer form of the vehicle don't have to be the shape of the tanks inside unless it is like a rocket stage in which these are one in the same, but the thermal protection systems necessary for reentry dictates that the outer surface can't be the tank anyways.

I have no idea what the dry mass of MCT might really be like.  I'm using the Saturn S-II as a reference that would be an upper stage in the ball part of what I think the MCT spacrcraft/upper stage might be like.  And to show that MCT itself doesn't necessarily have to be some ginormous BFR putting near 300mt inot LEO and have 15Mlbs of thrust and 30+ Raptor engine.

So, let's say today, we could make an S-II stage that's say 35mt dry, rather than 45mt dry, with modern manufacturing techniques and materials...for example.  To that we need to add:
1)  crew cabin
2)  landing legs
3)  TPS system on the side
4)  ECLSS
5)  Sabatier reactor
6)  LH2 fed stock or equipment that can mine water vapor out of the air and then electrolyze it for H2 feed stock.  (there's some interesting research into that).
7)  Cago bay  (this is basically empty space, but the alloy housing it would add a bit of overall mass.  It could be unpressurized to reduce the housing mass required.
8  )  Deployable/retractable solar array(s) for power during transit)
9)  Anything else I can't think of right now).

I think we need to minimizing integration of superfluous systems into the vehicle to get as low a mass as possible.  I'd drop the Sabatier reactor and H2 equipments (atmospheric moisture extractors most likely) immediately, that is a system that makes absolutely ZERO sense to bring back to Earth as its ONLY usable on the surface of Mars and we desperately want to get as high a volume of propellent production going as possible so every bit of ISRU equipment sent to Mars needs to stay to build up production capacity.





If we start with 35mt base for the "stage", how much extra mass will all of this add?  I have no idea.  If about 30mt, then MCT would dry mass around 65mt, and be in our ballpark.
I don't think the ECLSS system or sabatier reactor would be all that heavy.  Ditto for a cargo area.  The crew cabin would essentially be empty space too and not mass much itself.  The crew accomodations like bathrooms, seats, controls, computers, etc would be where the mass for that comes from. 
That leaves landing legs, TPS, and the solar array.
Given the much larger surface area, I think they could use a TPS that's lighter per square ft than the Pica-X they use on Dragon.  The fragile ceramic tiles of the Shuttle should probably be avoided in favor of something a little more robust that needs less maintenance to turn around after a round trip.
What are some options that are out there to consider?  Anyone know what NASA would have used for their Aeroshells in DRM 5.0?
Can we keep this down to around 65mt, give or take?  There's the question.

The problem with using those for analogs, is they are just crew (or payload) housings.  They aren't rocket ships.  MCT will need to be a rocket stage/ship.  Even if it's lofted to LEO by a 2-stage booster, it still has considerable dV requirements to provide itself that no other spacecraft has.
I think a better analog would be Rocketplane Kistler's K-1.  That's probably about as close to this as has been legitimately designed.  But it did a ballistic nose entry rather than a biconic side entry, and it landed on airbags on it's side.  So it's not a great analog, but one of the closest I think.  See below.


How about we try this strategy, start with your basic rocket-stage mass and fractions (and I'd recommend using F9 upper stage as an analog rather then Saturn V hardware, it's hydrocarbon based an much more modern), and then for the 'other' stuff try to derive that extra mass from vehicle that had thouse things like capsules or the shuttle orbiter, taking into account the target volumes devoted to these purposes.   That splits the difference between considering the vehicle as 'all stage' or 'all capsule'.


So how did Skylab get into it's orbit?  There was no 3rd stage on it.

According to what I've read Skylab had a mass of only 75 mT, so their was plenty of performance in the first 2 stages to reach LEO without the 3rd stage, or so I presume.


You certainly could be right.  There might be a 2-stage booster under the MCT vehicle.  But I still think there are more advantages to having MCT as the 2nd stage.  One being you only pay the mass penalty for the upper stage(s) once, not twice.  Otherwise you are pushing the dry mass of the 2nd stage along with the dry mass of MCT to LEO.  A dedicated 2nd stage would be resuable, so you'd pay for the additional dry mass of it's reusability hardware twice.  The mass of a TPS on both the 2nd stage and MCT, landing legs on both, etc.  With just one stage, you only "pay" for all of that once.  So there's advantages to it.  And if you are refueling in LEO anyway, why not?
Not to mention it's one less development to pay for, and one less piece to have to remotely deorbit and land.  This way, you only are bringing back the actual MCT (which can be a LEO taxi configuration, or tanker configuration) back from orbit.  Rather than both an MCT and a dedicated 2nd stage.

Of course, this is all predicated on the assumption that an integrated upper stage/MCT -can- be made to work feasible.  If not, then there will be a dedicated 2nd stage. 

Staging is ALWAYS going to give performance, not take it away.  Having a 2nd stage that did the full Delta-V to LEO and simply dropped the MCT there would indeed be inefficient in the sense that the empty stage mass is mass that could have been vehicle, but I'm expecting the MCT to have some propellent at launch allowing it to do 1-2 km/s after separating from the second stage, that 2nd stage has not in fact been brought to LEO.  A 2nd stage like this that is short of orbital speed like this might still go around the Earth once to allow a RTLS, otherwise downrange recovery may be needed.
« Last Edit: 04/30/2015 01:35 am by Impaler »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 3
« Reply #449 on: 04/30/2015 01:41 am »
If someone has a more accurate calculation of DeltaV from Mars surface to Earth return trajectory thous would be very useful.  Their may be some saving, but I suspect it will be minimal because a planetary surface and Low orbit are basically the same with regard to the gravity well and Oberth effect.

Of particular interest would be transit times in a direct return, how low can you go and how much dose it cost?

Offline Impaler

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Re: MCT Speculation and Discussion Thread 3
« Reply #450 on: 04/30/2015 02:06 am »

The question I was trying to answer for myself for solar power, is it enough for a unmanned ISRU MCT to generate 470MT of CH4 & LOX in 25 months fully autonomously.  Peak solar power of 40KW and working with the daily temperature variations on Mars may be enough, but I do not have the time or the knowledge to know that with any certainty.

ISRU of propellant on Mars is the pivotal technology to make the MCT system work. My guess is the first MCT to land on Mars will be a ISRU MCT.  With about 1000 m3 of volume to utilize. The first MCT will have 2-3 Sabatier reactors and possibly 4 sources of hydrogen to experiment with. 1) High pressure Hydrogen brought from Earth, 2) Water brought from Earth, 3) Water from the atmosphere, 4) Water dug from the surface.

1-2 Rovers with back-hoes will also be included for scouting the area and digging trenches to look for water to deposit in the MCT.

If all goes to plan, the first MCT will have enough fuel for the second MCT to land to return to Earth and validate the MCT for the Earth return part of the mission.

Autonomously refueling between the two MCT should be a fun. A long hose, precision landing and reliable and capable rovers.

No this would not be adequate as the chemical synthesis of hydrocarbon fuel/oxidizer mix from water and CO2 feedstock needs around 5 kW/h per kg of propellents.  In addition collection of water necessary to make this 1 kg of propellents (NOT 1 kg or water) would require an additional 5-7 kW/h if collected from the atmosphere, the most reliable means we know of (I can not even begin to estimate mass and power costs for mining ice or regolith).  Production from H2 feed stocks greatly lowers energy needs but has all kinds of other problems as I'm sure you know.

40 kW (which I assume is peak power in daylight) is only going to yield ~240 kW/h per day and produce 24 kg of propellents per day, which is about 18 mT over a Martian synod.  You need several hundred kW power systems to get the kinds of propellent mass needed for the MCTs were speculating about here even back into orbit let alone back to Earth.

Thus I conclude that a separate surface-only power system is an absolute necessity, and that it should NOT be integrated into the vehicle.  It would simply another part of the propellent production equipment that will be carried as part of the 100 mT cargo load.  Thin film solar is up to the task from all the material I have read, but if you prefer some other source such as nuclear so be it, it would still be a surface only power source.

That said the in-space solar panels are certainly needed, 40 kW sounds like a reasonable amount, though I could see arguments for it to be higher.  If it is also deployable on the surface then it would be a nice supplemental power source and a good gap-filler before the surface power system is deployed.  I favor a disk deploying radially from under neath the nose-cone of the MCT, as this would shade the rest of the vehicle in space and on the Martian surface, use an otherwise inconvenient volume of the MCT and give maximum protection of the solar panels during EDL.


Offline BobHk

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Re: MCT Speculation and Discussion Thread 3
« Reply #451 on: 04/30/2015 02:31 am »
MCT could go light on the power systems if a separate ship landed nearby to provide power, like a Red Dragon launched before main mission.  If the Red Dragon is essentially a solar station + nuclear pile, rtg, artg or whatever, you could land dozens before the first MCT arrives, reducing mass/volume needs for power generation on the MCT itself.

Land them in a cluster, then run a kilometer of power cable to them when the MCT lands.

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Re: MCT Speculation and Discussion Thread 3
« Reply #452 on: 04/30/2015 05:38 am »
If someone has a more accurate calculation of DeltaV from Mars surface to Earth return trajectory thous would be very useful.  Their may be some saving, but I suspect it will be minimal because a planetary surface and Low orbit are basically the same with regard to the gravity well and Oberth effect.

Of particular interest would be transit times in a direct return, how low can you go and how much dose it cost?

It is a bit more to it since you dont have to raise the periapsis of the escape trajectory above the horizon. Still, the spacecraft needs to achieve escape velocity. So the savings are the "shortcut" that the spacecraft takes in a velocity triangle. However, the escape trajectory from surface has to be more "up" than "sideways", indicating higher gravity losses. I cant imagine that the savings will be significant. I can be totally wrong on this one though, its just my personal, feeling based opinion. As is btw. the opinion of other NSF members that the direct return trajectory saves a lot of fuel.

Unless someone can show me the math, I dont believe that a direct return trajectory is better than going to orbit first.

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 3
« Reply #453 on: 04/30/2015 09:53 am »
If someone has a more accurate calculation of DeltaV from Mars surface to Earth return trajectory thous would be very useful.  Their may be some saving, but I suspect it will be minimal because a planetary surface and Low orbit are basically the same with regard to the gravity well and Oberth effect.

Of particular interest would be transit times in a direct return, how low can you go and how much dose it cost?

It is a bit more to it since you dont have to raise the periapsis of the escape trajectory above the horizon. Still, the spacecraft needs to achieve escape velocity. So the savings are the "shortcut" that the spacecraft takes in a velocity triangle. However, the escape trajectory from surface has to be more "up" than "sideways", indicating higher gravity losses. I cant imagine that the savings will be significant. I can be totally wrong on this one though, its just my personal, feeling based opinion. As is btw. the opinion of other NSF members that the direct return trajectory saves a lot of fuel.

Unless someone can show me the math, I dont believe that a direct return trajectory is better than going to orbit first.

But even if the savings is 2% (and I think it's quite a bit more), what purpose is served by going to orbit first?

Offline Ben the Space Brit

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Re: MCT Speculation and Discussion Thread 3
« Reply #454 on: 04/30/2015 09:54 am »
IMO, with regard to shape, I think that maximum utilisable volume will be a key. To me, that means a cylinder for the outer mould line with a biconic or even smoother aerodynamic nose fairing (either disposable or a key part of the vehicle itself).
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Re: MCT Speculation and Discussion Thread 3
« Reply #455 on: 04/30/2015 11:18 am »
But even if the savings is 2% (and I think it's quite a bit more), what purpose is served by going to orbit first?

There are several reasons. It makes targeting the correct escape trajectory easier. I assume that the colony will not be directly at the equator of Mars, so when launching, launch time and the rotation of mars will determine the orbital plane of the MCT. For TEI, the MCT orbital plane must intersect the Ecliptic plane at the correct line (two planes intersection is a line). The TEI burn must happen in the correct direction of the MCT orbit. So there are two parameters that need to be controlled, both are time dependent. With a direct launch to TEI, both time dependent parameters (1: orbital intersection of the launch orbit and the ecliptic; 2: TEI burn direction) must line up. This is very very unlikely to ever happen.

Mars equator plane is 1.8 degrees off of the ecliptic. So if the colony is exactly at the equator, the error might be small enough so that a direct TEI is physically possible. I dont know what it takes to correct for up to ~2 degrees orbital plane misalignment on a TEI. I guess a larger correction burn during the transition would be necessary. The whole operation would be more error prone.

But as I said, its my personal opinion until convinced otherwise.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 3
« Reply #456 on: 04/30/2015 08:10 pm »

If the biconic has a reasonably low slope to the side wall then a cylindrical tank (or 2) can fit inside of a conic without wasting much room.  The outer form of the vehicle don't have to be the shape of the tanks inside unless it is like a rocket stage in which these are one in the same, but the thermal protection systems necessary for reentry dictates that the outer surface can't be the tank anyways.


And actual biconic shape and/or capsule shape, would be inefficient for tanks.  Again, on a spacecraft volume (and mass) are at a premium.  So I don't think you'd want to have a big cylindrical central tank with a tapered shell around it.  It'd have a lot of empty space in it that would be difficult to use for anything.  At least not habitat.  Maybe cargo around the bottom, if there were doors that folded out or something.  A series of equipment storage compartments or something?
If you had multiple cylinderical tanks, you'd have something more akin to the S-1B stage.  Although Boeing Methalox concept Mars and Lunar lander did have clustered skinny tanks.
So it's not that you couldn't make it work, but is the capsule shape such an advantage that you'd want too?  Seems that a cylindrical rocket tank with cargo or crew decks on the top or bottom would be much more volume efficient.

And I don't know that the outer surface can't be the tanks.  It can be the tanks with TPS segments affixed to it.  You can't have significant heat get through the TPS system anyway or else it'll burn whatever is behind it, a tank or empty shell or otherwise. 


I think we need to minimizing integration of superfluous systems into the vehicle to get as low a mass as possible.  I'd drop the Sabatier reactor and H2 equipments (atmospheric moisture extractors most likely) immediately, that is a system that makes absolutely ZERO sense to bring back to Earth as its ONLY usable on the surface of Mars and we desperately want to get as high a volume of propellent production going as possible so every bit of ISRU equipment sent to Mars needs to stay to build up production capacity.


You can drop the Sabatier and H2 equipment on Mars, but you have to account for them on the outbound trip.  And it doesn't necessarily make sense to drop them, depending on how much they weigh.  A tank of LH2 would be empty for the trip back obviously, and the Sabatier reactor might not weigh that much.
Depends if you want to keep the MCT vehicle "whole", and how integrated those systems are. 
If you drive somewhere in your car on a hot day and use your A/C, you don't rip it out of your car if you are driving home on a cool day, just because you don't need it and you'll get a little better fuel efficiency without it.  Because the next time you drive your car you don't want to have to put another A/C system back in.
So it depends if those things are "cargo" to be dropped, or intragral parts of MCT's systems and structure.


How about we try this strategy, start with your basic rocket-stage mass and fractions (and I'd recommend using F9 upper stage as an analog rather then Saturn V hardware, it's hydrocarbon based an much more modern), and then for the 'other' stuff try to derive that extra mass from vehicle that had thouse things like capsules or the shuttle orbiter, taking into account the target volumes devoted to these purposes.   That splits the difference between considering the vehicle as 'all stage' or 'all capsule'.


The F9US is a more modern stage, but it's still a "small" upper stage in relation to the booster.  Typical of EELV class rockets.  The S-II was a very large upper stage in relation to it's booster, and much closer to the size range we'd be looking at for MCT.  The mass fractions, etc. of the F9US may not scale up to an S-II sized stage, like MCT (or larger).  The S-II is already about the size we're talking about for MCT, and we can perhaps assume a modern version of it would be may 10-20% lighter?
But for the rest of it, yea.  I think if we had a mass for the ISS ECLSS, that might be an approximate analogue for MCT.  The Shuttle's was really only designed for up to 7 people for two weeks.  Not sure if it'd be a real accurate analog. 

So how did Skylab get into it's orbit?  There was no 3rd stage on it.

According to what I've read Skylab had a mass of only 75 mT, so their was plenty of performance in the first 2 stages to reach LEO without the 3rd stage, or so I presume.

That's right.  INT-21 was calculated to have about 116mt to LEO by itself.  More like 120mt if you use the safety fuel reserve, I believe).  That's pleanty to get a 75mt Skylab into not only LEO, but the 50 deg X ~250mi orbit it was on, as opposed to at typical parking orbit.

When Saturn V was lofting Apollo, that means there was much more than 120mt on top of the S-II.
The S-IVB was 120mt gross by itself.  And the Apollo CSM/LEM stack was 45mt.  Then there's the LAS tower carried part way up.  So there was over 165mt on top of the S-II.  So the S-II couldn't get to LEO without the help of the S-IVB. 


Staging is ALWAYS going to give performance, not take it away.  Having a 2nd stage that did the full Delta-V to LEO and simply dropped the MCT there would indeed be inefficient in the sense that the empty stage mass is mass that could have been vehicle, but I'm expecting the MCT to have some propellent at launch allowing it to do 1-2 km/s after separating from the second stage, that 2nd stage has not in fact been brought to LEO.  A 2nd stage like this that is short of orbital speed like this might still go around the Earth once to allow a RTLS, otherwise downrange recovery may be needed.

I think staging helps performance...for traditional rockets with stages not refueled in orbit.  I think that's the new twist that is changing things.  Because staging helps by kicking off empty mass when the fuel has been burnt out of it.  However, every stage on the stack means there's duplicate tanks, plumbing and engines.  HAving just one upper stage means a heavy stage is going to LEO, but it can then be it's own 3rd stage, so it's mass isn't duplicated.
If you put MCT on a two stage rocket, then you have the dry mass of MCT and the dry mass of the 2nd stage. 

And look historically.  Most LV's that are only going to LEO are two stage.  Saturn 1B, Titan II, INT-21, Energia, STS, SLS (less DCSS or EUS), J-130 (if ever built), etc.  Some of those are 1.5 stage LV's but essentially 2-stage.  So you really don't need 3 stages to get to LEO.  You need 3 stages to get to Mars or the Moon.  And MCT would be it's own 2nd stage going to LEO, refuel, and then be it's own 3rd stage to TMI.
In that way, it can actually plausibly be more mass efficient than if it were launched on top of a 2-stage LV with a partially fueled MCT to be it's own kick stage.  Because then you are pushing up the dry mass of two stages instead of just one.

And it gets even better.  The Saturn V/Apollo stack was essentially a 6 stage LV. 
S-IC, S-II, S-IVB, LEM DM, LEM AM, and SM.  Each of those stages had it's own tanks, engines, plumbing, and electronics, etc.  With the CM essentally being the "Payload" that's returned....although it had it's own tanks, plumbing, RCS engines, electronics and systems.  Almost a 7th stage rather than a payload.
An integrated 2nd stage/MCT means you can do all of that with just two stages.  Refueling once in LEO, and refueling again on the Mars surface.  That's just two sets of tanks, engines, and systems, rather than 7.  One set of RCS thrusters rather than 4.  (The S-IVB had a type of attitude control as I understand?)
One crew area and one ECLSS rather than 3 (CM, SM, and LEM)
Etc.

And -that- is really the beauty of it, even if it were just a bit less than optimal efficient to LEO.  That irrelevant anyway, as any extra fuel burnt up to get to LEO can be replenished there.  IT doesn't have to be particularly fuel efficient to LEO.


EDIT:  Spelling

« Last Edit: 04/30/2015 11:58 pm by Lobo »

Offline TomH

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Re: MCT Speculation and Discussion Thread 3
« Reply #457 on: 04/30/2015 10:35 pm »
...even if the savings is 2% (and I think it's quite a bit more), what purpose is served by going to orbit first?

Consumables cached in parking orbit for the return journey: food, batteries, medicine, clothing, spare parts, water, anything and everything that you need for the return but that you don't want to expend the ΔV/impulse for downmass/upmass. You'd need to do a trade to measure taking all that to the surface and back up again as opposed to leaving it in parking orbit and picking it back up on the return.  Even if you've recycled or made water on the surface, its heavy to bring up from the surface; leave your water there and pick up this water going uphill. Leave old worn out clothes on the surface (except for what you're wearing); pick up fresh ones from parking orbit. Maybe the trades would say better just to take it all to the surface and back on direct return. Maybe they's show better to park that in orbit and pick it up on the way back. I dunno, but examining the trades is the best way to decide.

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Re: MCT Speculation and Discussion Thread 3
« Reply #458 on: 05/01/2015 12:00 am »
Consumables cached in parking orbit for the return journey: food, batteries, medicine, clothing, spare parts, water, anything and everything that you need for the return but that you don't want to expend the ΔV/impulse for downmass/upmass. You'd need to do a trade to measure taking all that to the surface and back up again as opposed to leaving it in parking orbit and picking it back up on the return.  Even if you've recycled or made water on the surface, its heavy to bring up from the surface; leave your water there and pick up this water going uphill. Leave old worn out clothes on the surface (except for what you're wearing); pick up fresh ones from parking orbit. Maybe the trades would say better just to take it all to the surface and back on direct return. Maybe they's show better to park that in orbit and pick it up on the way back. I dunno, but examining the trades is the best way to decide.

Except that it is probably much more fuel effective to land and use the atmosphere for most of the breaking instead of using fuel to enter an orbit. Also the departure orbit inclination will probably be totally different than the arrival orbit inclination. Additionally, making rendezvous is much more complicated than making none. Keep it simple guys. The whole operation is complicated enough. Adding more operational hurtles like this will just increase the failure probability.

Offline TomH

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Re: MCT Speculation and Discussion Thread 3
« Reply #459 on: 05/01/2015 01:50 am »
Except that it is probably much more fuel effective to land and use the atmosphere for most of the breaking instead of using fuel to enter an orbit.
This could be put into orbit ahead of time via aerobraking on a separate launch. That allows you to know its safely in place before the humans even leave Earth. I see the merit in your other points.

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