Author Topic: MCT Speculation and Discussion Thread 4  (Read 502617 times)

Offline RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #680 on: 10/09/2015 02:40 PM »
I think MCT will be a system, not a single vehicle. Elon said as much. I also think that when Elon said "land the whole thing" he meant landing on Mars, not Mars and Earth.

1. The MCT would be launched into LEO by the BFR. It would be loaded with supplies, but not crew. No LAS is needed. It could be lifted by a second stage or act as its own second stage.
2. Additional BFR launches would refuel the MCT using a second stage as a tanker.
3. A maximum crew of seven would ride uphill to the MCT on a Dragon 2. The Dragon 2 returns empty.
4. MCT conducts mission to Mars and back as we have been discussing. Maximum crew of seven makes ECLSS issues simpler. Plenty of cargo capacity available for supplies if the ECLSS isn't closed loop.
5. On return, MCT does not land on Earth, but goes to LEO.
6. Dragon 2 picks up crew and samples. Depending on what the crew brings back, it might take multiple flights.
7. Another MCT is launched and both MCTs are prepared for the next synod.
8. Third mission opportunity can have three MCTs and add one each opportunity.

Reloading a returned MCT with cargo is an issue. If modular, cargo could be transferred by BFR and a tug.

A benefit is Earth planetary protection since MCTs never land on Earth.

Landing on Earth is not required for maintenance. With a launch every two years or so, a MCT will probably make no more than ten round trip flights. Even the heat shield can be designed to handle that. If engine replacement is needed, a modular engine mount system can make on orbit repairs possible.

The MCTs could be staged somewhere in cis-lunar space instead of LEO to reduce outgoing and return delta V requirements. Makes it more difficult to stage, but could be worth the effort.

Removing the requirement to land on Earth makes designing the MCT far simpler. No need for a LAS, the structure and landing legs do not have to be as strong, and less wear on the heat shield. It does require more effort to stage for the mission, but Elon mentioned orbital refueling and SpaceX will already have Dragon 2.

Sending 100 passengers and $500,000 each can wait for a later vehicle when there is a colony on Mars.

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #681 on: 10/09/2015 03:04 PM »
Why doesn't SpaceX just build the MCT with a metholox plug nozzle engine and strap 8 Falcon 9's around it to launch it off earth?  They already have the Falcon 9's.  8 would give about 12 million lbs thrust.  A plug nozzle engine would allow landing in Earth's or Mar's atmosphere.  Like the Rombus or Ithacus designs or Phillip Bono in the 1960's.  Fuel in the MCT could be stored in cylinders that are concentric circles, one inside the other and go the length of the MCT in the center.  Cargo, passengers, solar panels or whatever could be stored around the sides with bay doors and easily unloaded low.  Supporting ribs along the sides would hold the Falcon 9's and landing legs.  It would be probably 12m in diameter and slightly higher than the Falcon 9 booster.   

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #682 on: 10/09/2015 03:20 PM »
Reloading a returned MCT with cargo is an issue. If modular, cargo could be transferred by BFR and a tug.

Landing on Earth is not required for maintenance.

The MCTs could be staged somewhere in cis-lunar space instead of LEO to reduce outgoing and return delta V requirements. Makes it more difficult to stage, but could be worth the effort.

Why bring back the MCT to cis-lunar space or LEO if you do not land on Earth? Just keep it at Mars. Use it to shuttle cargo/personnel from Mars orbit to the surface.

Offline RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #683 on: 10/09/2015 03:27 PM »
Reloading a returned MCT with cargo is an issue. If modular, cargo could be transferred by BFR and a tug.

Landing on Earth is not required for maintenance.

The MCTs could be staged somewhere in cis-lunar space instead of LEO to reduce outgoing and return delta V requirements. Makes it more difficult to stage, but could be worth the effort.

Why bring back the MCT to cis-lunar space or LEO if you do not land on Earth? Just keep it at Mars. Use it to shuttle cargo/personnel from Mars orbit to the surface.

I'm assuming a reusable interplanetary vehicle would be useful. Also direct entry to Mars instead of going into Mars orbit. I believe that is what Elon has in mind. Of course, other options are possible.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #684 on: 10/09/2015 04:14 PM »
Why doesn't SpaceX just build the MCT with a metholox plug nozzle engine and strap 8 Falcon 9's around it to launch it off earth?  They already have the Falcon 9's.

Because plug nozzles / aerospikes are not as easy to make - nor as effective - as we would all like. If they were, everyone would be doing it.

And I'm sorry, but strapping 8 F9 around a "MCT" is a terrible idea, both from launch pad complexity and engine count. 72 Merlins!!!!

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #685 on: 10/09/2015 04:40 PM »
But the point being, to say the the pieces being joined where they can separate in an emergency is "hideously complex" may be a tad over stated.  ;-)


Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #686 on: 10/09/2015 05:02 PM »
But the point being, to say the the pieces being joined where they can separate in an emergency is "hideously complex" may be a tad over stated.  ;-)



And your point with this image is...?
« Last Edit: 10/09/2015 05:06 PM by Lars-J »

Offline RocketmanUS

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Re: MCT Speculation and Discussion Thread 4
« Reply #687 on: 10/09/2015 05:12 PM »
I think that's a good point about the difference between NASA Mars and SpaceX Mars. NASA Mars is very much focused on rotation. SpaceX Mars is to create a colony.

I doubt they would be short of customers who would be willing to spend some cash (probably affordable to a lot of people via selling their home) and up sticks and become a resident of Mars.....and not return (the element of increasing the population, as opposed to several years stays and coming back).
By the time people have payed off their home or have enough equity in their home to pay for the couple to move to Mars they would have already raised their kids. Not good to start a colony with people who have already raised their kids. More likely the first colonist will be sent there by sponsors.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #688 on: 10/09/2015 05:48 PM »
By the time people have payed off their home or have enough equity in their home to pay for the couple to move to Mars they would have already raised their kids. Not good to start a colony with people who have already raised their kids. More likely the first colonist will be sent there by sponsors.

I fully agree. This always seemed a fatal flaw in Elon Musks reasoning to me.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #689 on: 10/09/2015 08:19 PM »
I think the Separate Bi-conic is attractive in this regard because rocket stages are cheaper to develop per unit of dry weight then manned capsules which are very nearly the most expensive things in aerospace development,

That could be.  I don't know how expensive a manned reusable spacecraft is vs. an unmanned reusable spacecraft.  Either way, they'll both be independent spacecraft. 
However, they -could- possibly be made to have some commonality.  unlike with the Super Capsule design.  It may be possible for them both to share essentially the same OML  one has more "tankage" inside the OML, and the other has less tankage, but additional open space for crew hab and/or cargo.  Then the two could have the same...or at least similar EDL profiles, MPS's, landing gear, landing thruster systems, TPS shapes and types, tank tooling, etc. Like two Space Shuttles with different payloads in the payload bay...kind of.  Would need an interstage adaptor to house the conical nose of the S2 though.

I estimate a bi-conic would have a mass of around 75 mT, and the 2nd stage would be 72 mT.  I expect the Integrated version would have a mass greater then 75 but probably less the the raw additive 150 of the two separate vehicles due to some savings on redundancies, but it will probably have a development cost that is per pound equal to the smaller bi-conic I'm looking at.  This will wipe out the advantage of not developing a 2nd stage and makes the total cost greater unless the integrated vehicle has an exceedingly low mass or the cost ratio between stages/capsules is extremely low perhaps due to the difficulty of 2nd stage reuse engineering.

I would really like to hear some exact mass number from Lobo about the whole vehicle stack, dry masses for the first stage, the integrated bi-conic dry mass (without abort systems if that's your preference) and propellant loads in each so I can plug them into the launch vehicle performance calculator I've been using http://www.silverbirdastronautics.com/LVperform.html and do an apples to apples comparison to see how gross take off weight differs.

I'd have to go look up the working numbers for the booster.  I think the IBMCT was around 86mt...about the same as the SDMCT.  And the NIBMCT was a little lighter, as it's a little shorter, with only 2 engines where the S2 and IBMCT had 3.  I think the dedicated S2 was around 70mt.  (running off memory here).
These are still in flux, as they depend on assumptions we're still undecided on.  The working method of landing the S2 would be a fair amount lighter than the LAS/landing pressure fed systems, since reliability isn't as important.  That' part of the lesser mass.

But yes, one of the hooks of the IBMCT, is that you aren't duplicating your main propulsion, landing systems, tankage and TPS.  So it's more mass efficient to LEO, although a little heavier than the NIBMCT...but much lighter than NIBMCT + Dedicated S2.

Now, if you were to assume the IBMCT had no LAS system, as I originally entertained myself, and just essentially focused on making it as simple/common/standard as possible, it may be closer in mass or even light than the NIBMCT with the whole vehicle abort engines.  Then that gets around all the issues of having it quickly separable, while designing it for compressive, tensile, and side loads.
If an LAS for Earth ascent is not deemed as absolutely necessary, but engine-out and redundant systems are deemed "safe enough", and simplicity is the focus, or just that there'd be some alternative method of getting people to LEO than riding on MCT, then IBMCT may be the favored design for that person.

If you deem an LAS for Earth Ascent as absolutely necessary, then the NIBMCT may be the favored design for that person.

If one wants to be as conservative as possible overall, then the Super Capsule concept may be favored for that person.  As it has the more simple blunt body EDL profile unlike a biconic.   No real mass balance issues like a biconic as it's always vertical, and no side loads.  But it would really have nothing in common with the S2 or the booster.  It would be 3 very unique pieces.  The S2 and booster could share the same tank tooling...but that's about it.

Those were the reasons we had 3 different concepts.  After some discussion the IBMCT with LAS concept came about as kind of a way to try to have your cake and eat it too.  The LEO efficiency of IBMCT with a means to still abort the crew.  The best of both.  It could be implausible at the end of the day...it is a stab at it only..




« Last Edit: 10/09/2015 08:31 PM by Lobo »

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #690 on: 10/09/2015 08:36 PM »
But the point being, to say the the pieces being joined where they can separate in an emergency is "hideously complex" may be a tad over stated.  ;-)



And your point with this image is...?

Lars,

Your own post here may be more appropriate, which kicked off this whole new discussion.  :-)
The 2nd and 3rd image in particular.

https://forum.nasaspaceflight.com/index.php?topic=37808.msg1431160#msg1431160


And its not just a lifeboat. That would be one thing. In the system Lobo proposes, the "lifeboat" is a major section of the re-entry system for the MCT and the primary propulsion for landing.

See Lars's post too.  It may be impractical, but I'm not the only one to propose it.  Heidmann's is more "CST-100" than "Dragon 2".  Which was our first thought on it too.  But we need a method to land on Earth which cannot be done one Vacuum Raptor as it sits.  So the idea of moving them external and landing on Earth on them as double duty was considered, vs. another set of landing engines at the base.


« Last Edit: 10/09/2015 11:00 PM by Lobo »

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #691 on: 10/09/2015 09:21 PM »
A short return trip adds few extra requirements onto the MCT as there already are requirements for a short trip to Mars. However a long return trip which goes into the orbit of Venus adds requirements that are not needed on other phases of the mission. For cargo it adds a few extra requirements, but for crew it adds far more, especially as the MCT will be limited to 25% payload on the return trip.
....
I think Elon will be satisfied if the 90% cargo missions can be reflown in 1 synod while the 10% crew missions are reflown in 2 synods. Crew MCT probably need far more refurbishment than cargo MCT so it would be a push to get them reflown the next synod anyway.

If you can do a short trip to Mars with a longer return trip, to fit into 1 synod, I would assume you could do a long trip to Mars with a short return trip.

From an Earth perspective a human-return MCT would return from Mars just a few months after the main fleet leaves for Mars. The people would offload, and it would be refurbed and leave with cargo only (having missed the short trip window) for a 1 year trip to Mars.
 
From the Martian perspective you'd have that cargo MCT arriving a year after the passenger MCTs (which have been sent back with cargo on a slow return already). Offload the cargo, load with passengers, and launch back on a quick trajectory again.

It allows each MCT to be used once per synod - but I apologise in advance that I don't have a good enough grasp of the orbital mechanics involved. Is there any reason we hear more about a short trip to Mars, 1 month stay, with long return - and not the opposite long trip to Mars, 1 month stay, with short return?

Offline RocketmanUS

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Re: MCT Speculation and Discussion Thread 4
« Reply #692 on: 10/09/2015 10:07 PM »
A short return trip adds few extra requirements onto the MCT as there already are requirements for a short trip to Mars. However a long return trip which goes into the orbit of Venus adds requirements that are not needed on other phases of the mission. For cargo it adds a few extra requirements, but for crew it adds far more, especially as the MCT will be limited to 25% payload on the return trip.
....
I think Elon will be satisfied if the 90% cargo missions can be reflown in 1 synod while the 10% crew missions are reflown in 2 synods. Crew MCT probably need far more refurbishment than cargo MCT so it would be a push to get them reflown the next synod anyway.

If you can do a short trip to Mars with a longer return trip, to fit into 1 synod, I would assume you could do a long trip to Mars with a short return trip.

From an Earth perspective a human-return MCT would return from Mars just a few months after the main fleet leaves for Mars. The people would offload, and it would be refurbed and leave with cargo only (having missed the short trip window) for a 1 year trip to Mars.
 
From the Martian perspective you'd have that cargo MCT arriving a year after the passenger MCTs (which have been sent back with cargo on a slow return already). Offload the cargo, load with passengers, and launch back on a quick trajectory again.

It allows each MCT to be used once per synod - but I apologise in advance that I don't have a good enough grasp of the orbital mechanics involved. Is there any reason we hear more about a short trip to Mars, 1 month stay, with long return - and not the opposite long trip to Mars, 1 month stay, with short return?
An emplty MCT would have a greater delta v capability than one that is carring cargo or crew. This could reduce the travel time back to Earth and or have more return windows.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #693 on: 10/09/2015 10:08 PM »
As far as we know all MCT, both cargo and passenger, will go on trajectories that allow them to return after a short stay on Mars and return in the same synod. At least that is what Elon Musk has set as a goal for his transport system to reuse them every synod instead of every second synod.
That means the return leg will be significantly longer than the leg earth-mars. You are introducing a new requirement for shorter return flights that would mean that manned MCT could not be reused every synod. That may or may not be the case. IMO it is just a reason to reduce the number of people who return to earth to a minimum, maybe have more water as shielding for at least a part of the crew space for the return leg. But that is problematic as the mass budget available for return is much smaller.
I do not believe we have shown whether this is possible or not, in terms of how much delta V is required for what overall mission duration & entry velocities.
« Last Edit: 10/09/2015 10:10 PM by Burninate »

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #694 on: 10/09/2015 10:48 PM »
Reloading a returned MCT with cargo is an issue. If modular, cargo could be transferred by BFR and a tug.

Landing on Earth is not required for maintenance.

The MCTs could be staged somewhere in cis-lunar space instead of LEO to reduce outgoing and return delta V requirements. Makes it more difficult to stage, but could be worth the effort.

Why bring back the MCT to cis-lunar space or LEO if you do not land on Earth? Just keep it at Mars. Use it to shuttle cargo/personnel from Mars orbit to the surface.

I'm assuming a reusable interplanetary vehicle would be useful. Also direct entry to Mars instead of going into Mars orbit. I believe that is what Elon has in mind. Of course, other options are possible.

A number of reasons why I think SEP/NEP is better suited for interplanetary transfer than chemical + aerocapture:

- I imagine that after aerocapture at both Mars and Earth the heatshield would need a complete overhaul, respectively be replaced. Something you probably have to do on Earth. Entry from Mars orbit is benign in comparison.
- No volume constraints. A hab providing space and supplies for dozens of people is going to be huge (bigger than BA2100). You also need solar arrays and radiators. In the future you might even want stuff like artificial gravity.
- In order to arrive back at Earth within one synod you must refuel MCTs in Mars orbit, meaning some MCTs must fly refueling missions from Mars.
- Significantly lower IMLEO, less refueling operations. It will matter for cost.
- You need to develop a powerful SEP tug, but your Mars lander is going to be simpler. In general with a high-ISP system things are not so mass-sensitive.
- Technological progress favors SEP.
- If you want to go to Jupiter one day, good old chemicals won't do it  ;)

Anyway, old arguments.

« Last Edit: 10/09/2015 11:04 PM by Oli »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #695 on: 10/10/2015 03:14 AM »
I'm strongly in favor of SEP and see considerable use for it making VERY FAST transit possible.

How is this possible?  Wouldn't the system need to be huge and have magical power sources and such (insert anything Zubrin has ever said).  NO, you can get a Fast transit on the order of 100 days to Mars with a slow, low power SEP system.

The trick is you use your SEP to move your Mars bound vehicle with propellants up to high Earth orbit and then then drop by the Earth for a huge Oberth assisted burn.  For 2 km/s you should leave Earth with huge escape velocity and reach mars in 100 days (average). 

Now the problem is capturing at mars, the answer is Magneto-Plasma Aerocapture, this lets us avoid expensive propulsive capture and is then followed by about a week of Plasma assisted aerobraking which lets the eventual EDL be from a gentle 4 km/s.  So we get to have both fast transit and easy low speed EDL.

The SEP system has not even left Earth yet in this scenario, so you can do either one of two things, bring it back down to LEO for refueling and do it again (basically making it a Cis-lunar tug), or send it to mars by the conventional slow method of spiraling out from the Earths SOI (the SEP is too delicate to take the high thrust of the Oberth maneuver).  In the latter case your going to arrive much later then the manned capsule but if this is a conjunction mission the crew will be spending around 600 days on mars so their is plenty of time for the SEP to arrive before it is needed for departure which is what I favor.

The MCT would only need to reach low Mars orbit and would then rendezvous with the SEP and head for Earth, this return transit is made reasonably short by the fact the MCT is a completely dry shell now of only 100 mT (75 vehicle mass + 25 return cargo) and the SEP is nearly dry too so power to weight ratios are increased, also were not aiming to match Earth's orbit and capture gently, were going to simply intersect it on an elliptical orbit around the sun, that cuts the DeltaV needed.  At Earth we used the Magneto to capture again and bring both SEP and the MCT down to LEO (they probably need to separate to do this as the SEP is more delicate and would slow the process down for the MCT).  The crew can be retrieved via a Dragon capsule now, and we need to send another tanker to LEO to put landing propellants into the MCT, if we use enough the MCT can do a lot of retro-propulsion on entry and bring it's entry speed down from the 7.7 km/s of orbit down to the range of 4 km/s which matches it's mars entry speed, so all the thermal protection systems can be designed for this low performance point.

IMLEO is estimated at 570 mT of which 100 mT is the cargo load, 75 mT is the MCT dry mass, 200 mT is chemical propellant in the MCT (2 tanker loads of 100 mT each), 155 is SEP propellant, 15 mT is the SEP tank and 22 mT is the SEP hardware which has a power output of 4.5 MW which corresponds to an alpha value of 5 kg/kw.

BTW Using a braking system like Magneto Plasma is the only way I can see an Integrated Bi-conic and direct Earth return being viable, without it the entry conditions are too extreme to meet the low dry mass fractions that it's advocates are proposing.

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #696 on: 10/10/2015 03:59 AM »
I'm strongly in favor of SEP and see considerable use for it making VERY FAST transit possible.

How is this possible?  Wouldn't the system need to be huge and have magical power sources and such (insert anything Zubrin has ever said).  NO, you can get a Fast transit on the order of 100 days to Mars with a slow, low power SEP system.

The trick is you use your SEP to move your Mars bound vehicle with propellants up to high Earth orbit and then then drop by the Earth for a huge Oberth assisted burn.  For 2 km/s you should leave Earth with huge escape velocity and reach mars in 100 days (average). 

Now the problem is capturing at mars, the answer is Magneto-Plasma Aerocapture, this lets us avoid expensive propulsive capture and is then followed by about a week of Plasma assisted aerobraking which lets the eventual EDL be from a gentle 4 km/s.  So we get to have both fast transit and easy low speed EDL.

The SEP system has not even left Earth yet in this scenario, so you can do either one of two things, bring it back down to LEO for refueling and do it again (basically making it a Cis-lunar tug), or send it to mars by the conventional slow method of spiraling out from the Earths SOI (the SEP is too delicate to take the high thrust of the Oberth maneuver).  In the latter case your going to arrive much later then the manned capsule but if this is a conjunction mission the crew will be spending around 600 days on mars so their is plenty of time for the SEP to arrive before it is needed for departure which is what I favor.

The MCT would only need to reach low Mars orbit and would then rendezvous with the SEP and head for Earth, this return transit is made reasonably short by the fact the MCT is a completely dry shell now of only 100 mT (75 vehicle mass + 25 return cargo) and the SEP is nearly dry too so power to weight ratios are increased, also were not aiming to match Earth's orbit and capture gently, were going to simply intersect it on an elliptical orbit around the sun, that cuts the DeltaV needed.  At Earth we used the Magneto to capture again and bring both SEP and the MCT down to LEO (they probably need to separate to do this as the SEP is more delicate and would slow the process down for the MCT).  The crew can be retrieved via a Dragon capsule now, and we need to send another tanker to LEO to put landing propellants into the MCT, if we use enough the MCT can do a lot of retro-propulsion on entry and bring it's entry speed down from the 7.7 km/s of orbit down to the range of 4 km/s which matches it's mars entry speed, so all the thermal protection systems can be designed for this low performance point.

IMLEO is estimated at 570 mT of which 100 mT is the cargo load, 75 mT is the MCT dry mass, 200 mT is chemical propellant in the MCT (2 tanker loads of 100 mT each), 155 is SEP propellant, 15 mT is the SEP tank and 22 mT is the SEP hardware which has a power output of 4.5 MW which corresponds to an alpha value of 5 kg/kw.

BTW Using a braking system like Magneto Plasma is the only way I can see an Integrated Bi-conic and direct Earth return being viable, without it the entry conditions are too extreme to meet the low dry mass fractions that it's advocates are proposing.

That sounds far too ambitious. First of all, why VERY FAST transit? What's the point? Not worth the effort IMO, not in any near future.

You probably want some chemical propulsion on your SEP stage, at least for the crew transfer, but not anything on the scale of MCT.

Magneto-Plasma Aerocapture? Are you sure that's not only for aerobraking? Either way, probably far from ready.

Why take the Lander back to LEO if you do not need it for aerocapturing? If they can't "refurbish" it on Mars at the beginning, you might as well expend it.

4.5MW is huge and 5kg/kw is far below the numbers I've seen.

The SEPs can cycle between LEO/HEO and HEO/Mars, that way they are back a lot faster and you need less power.

375t is a freaking huge payload for SEP. You can divide that into cargo/hab/lander and get 100t+ pieces. Again, less power.

My 2 cents.



Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #697 on: 10/10/2015 06:51 AM »
That sounds far too ambitious. First of all, why VERY FAST transit? What's the point? Not worth the effort IMO, not in any near future.

Musk has had time of flight goals that are very aggressive as he seems to want to basically outrun the radiation danger and have a transit time that most people would find acceptable when crammed in small spaces.  The main point is that can get such transit times using SEP in an indirect way but getting to HEO followed by a TMI on Earth swingbuy, if we find transit times don't need to be so short we can subtract chem propellants from the lander and have it make a lesser burn on Earth swing buy and we would get something as low as the traditional Hohomann transfer. 

You probably want some chemical propulsion on your SEP stage, at least for the crew transfer, but not anything on the scale of MCT.

I don't believe it is necessary to make a hybrid drive SEP stage, the thrust level of chemical propulsion would require the SEP to have far higher rigidity they it otherwise would and it's mass would essentially be parasitic when doing the chemical burn as we intend to leave Earth with all the escape velocity needed to reach mars.  The SEP can fly slowly and efficiently to mars on it's own and brake their via the same Magneto brake tech the lander is using so separation of the two at high Earth orbit is the most efficient solution.

Magneto-Plasma Aerocapture? Are you sure that's not only for aerobraking? Either way, probably far from ready.

It is low TRL now but it is in active development, a demonstration cube sat will be launched soon by NASA.  And yes it is very much capable of Aerocapture as well as Aerobraking.  Were dealing with new technology all over the place with MCT such as an engine that is still in development and a launch vehicle that hasn't even been designed yet so I don't see why we can't use some tech that needs some development especially when it offers IMLEO reductions of near 50%.  See this paper
https://www.nasa.gov/sites/default/files/files/Kirtley_2012_PhI_PlasmaAerocapture.pdf

Why take the Lander back to LEO if you do not need it for aerocapturing? If they can't "refurbish" it on Mars at the beginning, you might as well expend it.

Nothing in the SpaceX mars transport system will be expendable, I though that was the one universal rule we could all agree on.  And it is needed here to carry the return habitat from mars surface to orbit and it carries the primary Magneto system to brake with upon return to Earth.  The SEP would have a brake too but much smaller as it's more delicate and doesn't have the urgency because it doesn't carry the crew.

4.5MW is huge and 5kg/kw is far below the numbers I've seen.

It's a conservative value consistent with Hall thrusters with power conversion units and solar at 300 W/kg.  Perhaps you misread it as kw/kg, it is the other way.

The SEPs can cycle between LEO/HEO and HEO/Mars, that way they are back a lot faster and you need less power.

That is an attractive architecture and would fit in this plan with a slight variation as a kind of triangle route, LEO->HEO, HEO->Mars, Mars->LEO.  As we need to pick up SEP at LEO it makes sense to have the returning SEP's from Mars brake fully on Earth return rather then having the ones at HEO expend propellants to spiral down.  Much depends on how much transfer of cargo/propellants are being done at HEO or at LMO, I'm assuming none initially but that it would grow as the system matures and stations (perhaps entirely robotic) are created to handle this activity as I assume you will need robotic arms for this.

375t is a freaking huge payload for SEP. You can divide that into cargo/hab/lander and get 100t+ pieces. Again, less power.

I'm avoiding any in-space loading/unloading of landers for initial missions, the habs are inside the landers at launch and all the way to mars to be unloaded only on the surface.  If you take these things out of the landers then they don't have any chemical thrust to do TMI and would have to be pushed by the SEP, which is slow and fine for cargo but not the crew, the description I've provided is just for the crew lander. 

But you are spot on when it comes to the propellants for the TMI, they could easily be separately conveyed to HEO which would split the total payload very nearly in half.  I will need to examine what this dose for Earth return transit time though as a larger SEP speeds that up considerably.  Two identical but smaller SEP's certainly reduce development costs.
« Last Edit: 10/10/2015 06:53 AM by Impaler »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #698 on: 10/10/2015 04:01 PM »
Fast transit makes a lot of sense if you have refueling by cheap reusable launch vehicles. It cuts microgavity exposure, it cuts transit radiation dose in half, it cuts food needed in half, it cuts the needed habitat volume (thus reducing dry mass and thus cost and also partly off-setting the IMLEO increase from greater delta-v), and allows you to reuse your hardware twice as often (which means you need half as much of it).
« Last Edit: 10/10/2015 04:13 PM by Robotbeat »
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Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #699 on: 10/10/2015 04:34 PM »
Refueling on both end is key to scaling up the delivered mass, too.
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