Author Topic: MCT Speculation and Discussion Thread 4  (Read 621475 times)

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1120 on: 11/06/2015 05:02 PM »
Since a lot more freight would have to be hauled to Mars than people.  In space could be done by SEP tugs on a continuous bases.  LEO to LMO via one or more SEP tugs.  A specialty lander for use on Mars could indeed take freight from LMO to the surface.  The lander could have ISRU equipment/solar panels to manufacture it's fuel from Martian atmosphere when not in use or there could be a separate fuel farm nearby.  Once refueled, it could go pick up another load. 

Humans could travel at a much faster rate with the MCT. 

This plan would require the BFR.
It would require a reusable second stage.
It would require a fleet of large SEP tugs.
A cargo carrier that can be transferred from the second stage to the SEP tug.
A re-usable lander at Mars.
A fuel farm at Mars or a lander large enough to have it's on ISRU equipment. 

An MCT that could be refueled in LEO and fly to Mars.  It might not need to land on Mars, just transfer the human habitation module to the lander.  MCT would fly back to earth.  Lander would take people to Mars surface.  The habitation module could be or would be about the same size as a cargo module. 

This plan might be cheaper to operate overall, but would require a lot of building and development of specialty components.  It would also require a lot of in space dockings and transfers.   However, these specialty components could be built by other companies or countries to have a stake in the colonialization process. 

On the other hand, if everything Musk wants to build is big, then It might be more simple to go directly from Earth to Mars will a couple of refueling stops for both humans and cargo an have contributors supply fuel or Martian surface cargo to have a stake in colonialization.

If humanity truly becomes interplanetary, nobody doubts that there will be a zillion different kinds of transports and spacecraft types for all kinds of purposes, filling all kinds of niches. But this is not the "MCT"... it needs to be something that SpaceX can reasonably design/build/afford mostly by themselves.

So it won't be the most optimal solution. It won't be the most efficient solution. But it will be cost-effective solution. Sound familiar? The F9 is a prime example. Sometimes a more "brute force" approach is the best one.
« Last Edit: 11/06/2015 05:03 PM by Lars-J »

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1121 on: 11/06/2015 08:31 PM »
Yep, that is what the Russians did for years in their military, etc.  Used cheap brute force, from small arms, to fighter planes that can land and take off on an unimproved runway (dirt or wheat field).  BFR should be able to launch a very heavy payload and be reused.  MCT will be also and will be needed for fast transfer of people to Mars.  Cargo or freight on the other hand might travel a different way. 

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1122 on: 11/06/2015 08:59 PM »
The lander could have ISRU equipment/solar panels to manufacture it's fuel from Martian atmosphere when not in use or there could be a separate fuel farm nearby.  Once refueled, it could go pick up another load. 

I've run the numbers and integrating ISRU into a vehicle is prohibitively massive (consuming half the cargo capacity), and would keep the vehicle on the ground for years and eliminating the ability to rapidly cycle between surface and orbit.  Propellant farms are the only viable solution.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1123 on: 11/06/2015 09:07 PM »
Cargo or freight on the other hand might travel a different way.

I think that is very unlikely. Cargo and passenger will be basically the same design, with the obvious addons for passengers. That at least in the beginning. Later on the two can develop in different directions.

An obvious difference could be that passenger MCT is faster. But Elon Musk ruled that out by making clear that speed is not for the passengers but for one synod reuse.

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #1124 on: 11/06/2015 09:13 PM »
Impaler, Lars-J, spacenut, etc.,

I wonder if the following might be an 'evolutionary' path that SpaceX might pursue:  Have an MCT that initially is used for all phases of the mission (launched to Earth orbit via BFR, is refueled there, launches to Mars, lands, drops off cargo and some/most crew, is refueled via ISRU, launches and does a direct return to Earth, lands on Earth).  This MCT would not ever transport 100 passengers, much less 100 passengers plus their cargo.  Instead it would be used for all expeditionary missions - those used to perform discovery (find easily accessible water sources), setup habitation and other infrastructure (IRSU, comms, modest chemical plants), prepare launch/landing sites, etc.  This would proceed for several synods.  Crew would be on the order of 5-15 people.  Some staying, some rotating out.

Meanwhile, the SEP interplanetary transporter is being assembled in LEO (or other staging area).  When this is complete, and a sufficient number of expeditionary missions have transpired, the function of the MCT vehicles transforms:  At Earth, they are used to shuttle colonists and their cargo from the surface to the SEP transporter.  At Mars, they are used for the converse - they launch empty from the Mars surface, dock with the SEP transporter, offload passengers and cargo, and return to Mars surface.  Each SEP transporter will in fact transport 100 (or more) passengers, plus their cargo.

I think this minimizes the number of vehicles developed, stays consistent with SpaceX's reusability ethos, and is mostly consistent with all of the statements thus far ("land the whole thing", "100 passengers at a time", "we're looking at everything, including SEP", etc.)

Impaler, I know you doubt the technical feasibility of an MCT craft that could do all of the phases listed in the first paragraph, but if it only has to accommodate a max crew of, say, 12, does that change the outlook?

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1125 on: 11/06/2015 09:56 PM »
Gordap, that is the way I see it except SEP is slow.  It would be good for cargo, but probably not so good for people.  Now, if the SEP can accelerate for half the trip, then decelerate as needed to throw it into orbit around Mars, then it might be as fast as direct.  I do not see 100 people at a time until well after a good Martian infrastructure is established.  Surface power stations, housing, water mining, base construction, greenhouse construction etc. Any initial crew who stay will be building the colony infrastructure first, and probably rotated out until adequate living, working, farming, and recreation facilities are built and operational. 

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #1126 on: 11/06/2015 10:39 PM »
I'm one of those who in this thread adheres to Musk quotes like "land the whole thing" for the purpose of coherent speculation.  However I'll bet that most here agree that SX's concepts for MCT have likely evolved considerably from the few sometimes off the cuff statements by Elon, many several years old.  I'll state further that I believe it probable the MCT that actually flies will again have notable differences from the MCT concept that Musk reveals later this year, if he even meets that schedule.

Absolutely.

The problem is that we really need a different thread for some of these ideas - because some are good!

Basically we try to use everything we know SpaceX has said about the Mars colonial fleet. And we know that it's a moving target. And when we get BIG new news we might need a new thread. 

Then we need a thread for "contradictory Mars ideas that SpaceX might already realise work better than their early thoughts".

Bit of a mouthful. And still trying to stay true to the General SpaceX principles.

The themes and interest are distinctly different.

( also if anyone goes down the "this is how I'd do it" then the Mars HSF forum is better. )

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1127 on: 11/07/2015 06:15 AM »
Impaler, Lars-J, spacenut, etc.,

I wonder if the following might be an 'evolutionary' path that SpaceX might pursue:  Have an MCT that initially is used for all phases of the mission (launched to Earth orbit via BFR, is refueled there, launches to Mars, lands, drops off cargo and some/most crew, is refueled via ISRU, launches and does a direct return to Earth, lands on Earth).  This MCT would not ever transport 100 passengers, much less 100 passengers plus their cargo.  Instead it would be used for all expeditionary missions - those used to perform discovery (find easily accessible water sources), setup habitation and other infrastructure (IRSU, comms, modest chemical plants), prepare launch/landing sites, etc.  This would proceed for several synods.  Crew would be on the order of 5-15 people.  Some staying, some rotating out.

Meanwhile, the SEP interplanetary transporter is being assembled in LEO (or other staging area).  When this is complete, and a sufficient number of expeditionary missions have transpired, the function of the MCT vehicles transforms:  At Earth, they are used to shuttle colonists and their cargo from the surface to the SEP transporter.  At Mars, they are used for the converse - they launch empty from the Mars surface, dock with the SEP transporter, offload passengers and cargo, and return to Mars surface.  Each SEP transporter will in fact transport 100 (or more) passengers, plus their cargo.

I think this minimizes the number of vehicles developed, stays consistent with SpaceX's reusability ethos, and is mostly consistent with all of the statements thus far ("land the whole thing", "100 passengers at a time", "we're looking at everything, including SEP", etc.)

Impaler, I know you doubt the technical feasibility of an MCT craft that could do all of the phases listed in the first paragraph, but if it only has to accommodate a max crew of, say, 12, does that change the outlook?

That is very nearly exactly what I have been saying, only with the MCT being slightly less capable. 

I think the MCT will be dependent on SEP tug assistance to do initial crew missions at an acceptable speed (for crew health/GCR issues), but may be able to do cargo missions on it's own.

We have argued a lot about achievable DeltaV values, let me lay down some numbers that express the DeltaV achieved from full propellant loads with different amounts of cargo, 100 mT (the outbound cargo goal), 25 mT (the return cargo goal which would be some kind of habitat), and 0 mT (presumably the return from a cargo mission).

100 mT Cargo + 300 mT propellant + 75 mT dry mass -> 3.7 km/s
25 mT Cargo + 300 mT propellant + 75 mT dry mass -> 5.1 km/s
0 mT Cargo + 300 mT propellant + 75 mT dry mass -> 6 km/s

These numbers are a lot lower then other people want to see but I think they are realistic and if intelligently combined an evolving set of missions can be created that serve both cargo and passenger missions.  The empty 6 km/s allows direct Earth return on a hohmann transfer of an empty cargo mission, if the 100 mT cargo is ISRU equipment which deploys to the surface, pumps propellants into the MCT and then remains behind on launch, then a propellant farm is built up without abandoning any vehicles and the return capability is well validated before humans are sent. 

The 4.9 km/s assent with 25 mT cargo is enough to reach mars orbit AND have enough propellant for another decent with FULL cargo (800 m/s propulsion), this allows the vehicle to be a reusable tanker to mars orbit depositing 25 mT per trip of any desired mass, and to act as a rapid reusable downward cargo hauler AT THE SAME TIME. 

Also it means that the MCT fully fueled in mars orbit can do rapid transit back to Earth (150 day) though this would require 12 tanker-up/cargo-down flights to get the necessary propellants in orbit (with the first flight staying in orbit to act as a depot), but this is consistent with the expected 10:1 crew/cargo ratio.  The very first exploration missions wouldn't have so many MCT's or propellants available and will refuel via SEP tugs sent to mars with propellants.  Only once a high rate of propellant production is in place could this many MCT's be refueled and relaunched.

This will also allow the unloading of cargo from an in-space-only freight vessel while simultaneously refilling that vessel with propellant for Earth return.  These large freight vehicles would be a later addition to the system and would greatly increase the cargo delivery rate and it's efficiency, but are not necessary for the initial deployment as cargo can be sent in the MCT directly all be it with poor amortization.

The 3.7 km/s when fully loaded allows the vehicle to make a fast transit to mars (150 day) from a high orbit (EML1 followed by lunar and Earth slingshot burns) while retaining enough propellants to land at mars.  It would be placed in high orbit and sent propellants by a SEP tug in the 1 MW power range, if we were sending cargo by slow hohmann transfer then only 1/6th of the full propellant load (~50 mT) is needed at EML1 which may be low enough to just have it carried in the MCT from LEO eliminating propellant transfer for such cargo missions.

It also makes the fully loaded MCT ideal for making the fast (3-5 day) transfer from LEO to EML1 to quickly ferry passengers when the numbers are great enough to make use of Dragon or other small taxi craft inconvenient.  Being such a short transit the accommodations can be far more cramped then would be possible for the full transit and I expect some from of transit-hab will be employed.  The MCT in question will likely be docked to and towed by the transit vehicle which would transfer the necessary landing propellants and passengers back to the MCT which would land and disembark the passengers at mars using the same short term accommodations.

It is all an extremely elegant arrangement, at each cargo loading point the vehicle has sufficient DeltaV for the critical mission legs it would actually face at that point while dove-tailing well with force-multiplier vehicles that would be added later like SEP tugs.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1128 on: 11/07/2015 09:16 AM »
Impaler, Lars-J, spacenut, etc.,

I wonder if the following might be an 'evolutionary' path that SpaceX might pursue:  Have an MCT that initially is used for all phases of the mission (launched to Earth orbit via BFR, is refueled there, launches to Mars, lands, drops off cargo and some/most crew, is refueled via ISRU, launches and does a direct return to Earth, lands on Earth).  This MCT would not ever transport 100 passengers, much less 100 passengers plus their cargo.  Instead it would be used for all expeditionary missions - those used to perform discovery (find easily accessible water sources), setup habitation and other infrastructure (IRSU, comms, modest chemical plants), prepare launch/landing sites, etc.  This would proceed for several synods.  Crew would be on the order of 5-15 people.  Some staying, some rotating out.

Meanwhile, the SEP interplanetary transporter is being assembled in LEO (or other staging area).  When this is complete, and a sufficient number of expeditionary missions have transpired, the function of the MCT vehicles transforms:  At Earth, they are used to shuttle colonists and their cargo from the surface to the SEP transporter.  At Mars, they are used for the converse - they launch empty from the Mars surface, dock with the SEP transporter, offload passengers and cargo, and return to Mars surface.  Each SEP transporter will in fact transport 100 (or more) passengers, plus their cargo.

I think this minimizes the number of vehicles developed, stays consistent with SpaceX's reusability ethos, and is mostly consistent with all of the statements thus far ("land the whole thing", "100 passengers at a time", "we're looking at everything, including SEP", etc.)

Impaler, I know you doubt the technical feasibility of an MCT craft that could do all of the phases listed in the first paragraph, but if it only has to accommodate a max crew of, say, 12, does that change the outlook?

That is very nearly exactly what I have been saying, only with the MCT being slightly less capable. 

I think the MCT will be dependent on SEP tug assistance to do initial crew missions at an acceptable speed (for crew health/GCR issues), but may be able to do cargo missions on it's own.

We have argued a lot about achievable DeltaV values, let me lay down some numbers that express the DeltaV achieved from full propellant loads with different amounts of cargo, 100 mT (the outbound cargo goal), 25 mT (the return cargo goal which would be some kind of habitat), and 0 mT (presumably the return from a cargo mission).

100 mT Cargo + 300 mT propellant + 75 mT dry mass -> 3.7 km/s
25 mT Cargo + 300 mT propellant + 75 mT dry mass -> 5.1 km/s
0 mT Cargo + 300 mT propellant + 75 mT dry mass -> 6 km/s

These numbers are a lot lower then other people want to see but I think they are realistic and if intelligently combined an evolving set of missions can be created that serve both cargo and passenger missions.  The empty 6 km/s allows direct Earth return on a hohmann transfer of an empty cargo mission, if the 100 mT cargo is ISRU equipment which deploys to the surface, pumps propellants into the MCT and then remains behind on launch, then a propellant farm is built up without abandoning any vehicles and the return capability is well validated before humans are sent. 

The 4.9 km/s assent with 25 mT cargo is enough to reach mars orbit AND have enough propellant for another decent with FULL cargo (800 m/s propulsion), this allows the vehicle to be a reusable tanker to mars orbit depositing 25 mT per trip of any desired mass, and to act as a rapid reusable downward cargo hauler AT THE SAME TIME. 

Also it means that the MCT fully fueled in mars orbit can do rapid transit back to Earth (150 day) though this would require 12 tanker-up/cargo-down flights to get the necessary propellants in orbit (with the first flight staying in orbit to act as a depot), but this is consistent with the expected 10:1 crew/cargo ratio.  The very first exploration missions wouldn't have so many MCT's or propellants available and will refuel via SEP tugs sent to mars with propellants.  Only once a high rate of propellant production is in place could this many MCT's be refueled and relaunched.

This will also allow the unloading of cargo from an in-space-only freight vessel while simultaneously refilling that vessel with propellant for Earth return.  These large freight vehicles would be a later addition to the system and would greatly increase the cargo delivery rate and it's efficiency, but are not necessary for the initial deployment as cargo can be sent in the MCT directly all be it with poor amortization.

The 3.7 km/s when fully loaded allows the vehicle to make a fast transit to mars (150 day) from a high orbit (EML1 followed by lunar and Earth slingshot burns) while retaining enough propellants to land at mars.  It would be placed in high orbit and sent propellants by a SEP tug in the 1 MW power range, if we were sending cargo by slow hohmann transfer then only 1/6th of the full propellant load (~50 mT) is needed at EML1 which may be low enough to just have it carried in the MCT from LEO eliminating propellant transfer for such cargo missions.

It also makes the fully loaded MCT ideal for making the fast (3-5 day) transfer from LEO to EML1 to quickly ferry passengers when the numbers are great enough to make use of Dragon or other small taxi craft inconvenient.  Being such a short transit the accommodations can be far more cramped then would be possible for the full transit and I expect some from of transit-hab will be employed.  The MCT in question will likely be docked to and towed by the transit vehicle which would transfer the necessary landing propellants and passengers back to the MCT which would land and disembark the passengers at mars using the same short term accommodations.

It is all an extremely elegant arrangement, at each cargo loading point the vehicle has sufficient DeltaV for the critical mission legs it would actually face at that point while dove-tailing well with force-multiplier vehicles that would be added later like SEP tugs.

Setting aside for the moment that you're assuming a free supply of propellant & probably a prepared pad on the surface...

Your whole architecture relies on very low dry mass, combined with very low descent delta V.  Descent propellant requirements are likely to be even higher with F9R-style SSRP than with conventional ablative heatshield.  Where are you getting 800m/s?  I've read one estimate that large sphere-cone Mars landers are likely to reach ~Mach 6 (~1500m/s) on heat shield alone, though using F9R-style SSRP instead of an ablative heatshield, gravity losses from sane acceleration rates, a lander as huge as MCT, and terminal landing requirements associated with unprepared pad, will likely increase that.  I've been using 2km/s propulsive dV until someone can show me better numbers.
« Last Edit: 11/07/2015 09:24 AM by Burninate »

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1129 on: 11/07/2015 10:07 AM »
SSRP will not be used instead of a heatshield. It will be used once the speed has dropped so much that lift plus a heat shield are no longer useful. Only the landing will be propulsive. Propulsion will set in at super sonic speed, so SSRP.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1130 on: 11/07/2015 02:08 PM »
Burninate: Terminal velocity at Mars is much, much lower than 1.5km/s for any reasonable ballistic coefficient and for any of the low altitude landing sites that are likely to be used. For Dragon-like ballistic coefficient of 300kg/m^2, you have about 350m/s terminal velocity. And terminal velocity is proportional to the square root of ballistic coefficient, so even if you think I'm wrong about ballistic coefficient, it won't make much difference.
« Last Edit: 11/07/2015 02:09 PM by Robotbeat »
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1131 on: 11/07/2015 03:57 PM »
Burninate: Terminal velocity at Mars is much, much lower than 1.5km/s for any reasonable ballistic coefficient and for any of the low altitude landing sites that are likely to be used. For Dragon-like ballistic coefficient of 300kg/m^2, you have about 350m/s terminal velocity. And terminal velocity is proportional to the square root of ballistic coefficient, so even if you think I'm wrong about ballistic coefficient, it won't make much difference.

From what I'm told, on Mars objects with that sort of BC tend to smack into the ground *long* before they reach terminal velocity.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1132 on: 11/07/2015 05:34 PM »
That is from high speed direct entry and low L:D ratio, I am proposing just the opposite.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1133 on: 11/07/2015 11:51 PM »
That is from high speed direct entry and low L:D ratio, I am proposing just the opposite.
That's like a high speed direct entry straight into the planet and zero L/D ratio, and probably a relatively high altitude.

If you're coming in from a Mars orbit (anything less than escape, it doesn't really seem to matter), then a Dragon-like ballistic coefficient gives you about 500m/s velocity at -5km coming in totally ballistic with zero L/D. I simulated it.

With even a modest L/D, you should be able to get to whatever your terminal velocity is, though that's harder to simulate (I plan to do it). Ballistic entry is old EDL tech. MSL used a lifting trajectory, and I'm sure SpaceX will, too (like Dragon).

Even a very slightly lifting reentry (like 0.3) gives you most of the benefit. It means you can carefully guide your aerocapture and entry.
« Last Edit: 11/07/2015 11:57 PM by Robotbeat »
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Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #1134 on: 11/10/2015 10:47 PM »
I really can't see SEP being used. I'm sure they are considering/have considered it, but I expect it to be rejected as unnecessary complexity.

I think simplicity is more important than mass-efficiency since

- SpaceX will probably have limited development funds

- the LV is already supposed to be too big for existing infrastructure, so the marginal cost of making it even larger may be relatively small

100 mT Cargo + 300 mT propellant + 75 mT dry mass -> 3.7 km/s
25 mT Cargo + 300 mT propellant + 75 mT dry mass -> 5.1 km/s
0 mT Cargo + 300 mT propellant + 75 mT dry mass -> 6 km/s

If MCT has a 75 mT dry mass, I would expect it to have a much larger propellant capacity than 300 mT. SpaceX has very good tankage mass fractions.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1135 on: 11/10/2015 11:12 PM »
SEP is not a difficult technology and I propose HALL thrusters which SpaceX is going to produce for it's own satellite swarm, the Solar panels are likewise a well established technology, the only challenge is scale and SpaceX clearly is not afraid of large scale as they will be building rocket stages with masses on the range of 200 mT of dry mass.  A large SEP tug would come out to under 20 mT.


I estimated all tanks and propellant lines at 5% of the propellant mass for a total of 15 mT, and 6 mT of engines.  That is a mass fraction perfectly consistent with SpaceX performance, but their are lots of other parasitic masses involved in such a vehicle, it is not just a big rocket stage with single digit dry mass fraction.  This is the central flaw in your and other similar comments.




Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #1136 on: 11/10/2015 11:24 PM »
SEP is not a difficult technology and I propose HALL thrusters which SpaceX is going to produce for it's own satellite swarm, the Solar panels are likewise a well established technology, the only challenge is scale

My objection is in creating/development an extra vehicle not the base technology itself needing development.

It's well withi [EDIT: well within their capabilities, certainly.]


Quote
I estimated all tanks and propellant lines at 5% of the propellant mass for a total of 15 mT,

That seems high. IIRC FH side booster is more like a mass ratio of 30 (at least that's what's been quoted on this forum) and that includes engines, etc.


Quote
and 6 mT of engines.

Assuming a TWR of 100:1 or better that seems reasonable


Quote
their are lots of other parasitic masses involved in such a vehicle, it is not just a big rocket stage with single digit dry mass fraction.

Certainly. It needs a heat shield, it needs landing legs and a cargo container, it needs better communication than a rocket stage so it can talk to Earth from Mars.

But I don't think those will force them to limit the delta-v drastically. They'll just end up with a huge vehicle.

(Passenger version needs a lot more stuff but that will come out of the cargo-version max capacity...)
« Last Edit: 11/10/2015 11:25 PM by Vultur »

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1137 on: 11/11/2015 12:36 AM »
I really can't see SEP being used. [....]
- the LV is already supposed to be too big for existing infrastructure, so the marginal cost of making it even larger may be relatively small

I'm not seeing your reasoning here. I would think that transferring much of the TMI and TEI requirements onto a separate SEP vehicle would allow the MCT (demoted to a lander) to be smaller, not larger. Ditto its Earth launcher.

It also means you only need to bring MCTs to Mars once. Afterwards they act as SSTO "orbital taxis", ferrying fuel up and cargo/people down; and that means that the SEP ships carry only payload/passengers from then on, increasing the mass between Earth and Mars.

As GORDAP noted, it also gives you a clear upgrade path. You build BFR with a reusable first stage. While launching commercially, other people are essentially paying you to develop a reusable second stage, and then upgrade that into the more capable MCT. Then you use MCT to develop the fuel infrastructure in Earth orbit. Then you use orbitally refuelled MCTs to directly carry small payloads to Mars. Much, much less than 100t or 100 people. NASA (or an international joint mission) might fund the first manned visits. Meanwhile, using BFR/MCT you assemble the SEP ships in Earth orbit. The MCTs are demoted to taxi duty and you can carry the fully payloads to Mars via the SEPs; and the real colonisation effort begins.

It reduces each step to a bite-sized piece. It allows SpaceX to make money out of most steps, paying for the development of the next. It constantly undermines potential rivals by always being two steps ahead, reducing the chance that someone else will eat SpaceX's markets. What's not to love?

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1138 on: 11/11/2015 01:58 AM »
MCT would be a heck of a lot better propellant taxi and tanker if it had good mass fraction.
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Re: MCT Speculation and Discussion Thread 4
« Reply #1139 on: 11/11/2015 04:35 AM »
I really can't see SEP being used. [....]
- the LV is already supposed to be too big for existing infrastructure, so the marginal cost of making it even larger may be relatively small

I'm not seeing your reasoning here. I would think that transferring much of the TMI and TEI requirements onto a separate SEP vehicle would allow the MCT (demoted to a lander) to be smaller, not larger. Ditto its Earth launcher.


I think he was referring to the launch vehicle, aka make an even large BFR to launch an enormous MCT which has the propellant capacity to do a direct departure to mars.  In other words he thinks doubling the size of the launch rocket is worth it to avoid making a SEP tub because you know 'fewer vehicles' logic.  ::)

MCT would be a heck of a lot better propellant taxi and tanker if it had good mass fraction.

That's backwards reasoning, you want MCT to be an all purpose vehicle so you presume it to have that mass fraction for no other reason then that it fulfills your desire, not because is supported by evidence that such a vehicle is possible.

My objection is in creating/development an extra vehicle not the base technology itself needing development.

It's well withi [EDIT: well within their capabilities, certainly.]

Total cost is the metric we care about, if it be 1 vehicle or 10.  The performance demands on and resulting size of a single vehicle area a much greater threat to break the bank then is a series of smaller less ambitious vehicles which can be attacked one at a time.  The SEP tug I'm looking at would be around 1 MW and doesn't need to be started for a decade.  NASA proposes a SEP vehicle in this paper http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140002512.pdf which would be chem-hybrid and used for a full their and back again architecture which is different then what I'm proposing, but the solar power and propellant tanks are comparable to what I'm proposing.


That seems high. IIRC FH side booster is more like a mass ratio of 30 (at least that's what's been quoted on this forum) and that includes engines, etc.

Keep in mind that tank is 100 mT larger and lacks any insulation.  The need to keep propellants (both of them) at cryogenic temperatures in space and on Mars for very long duration is going to demand considerable insulation.  Finally I expect their to be multiple smaller tanks through the vehicle to accommodate good weight distribution and avoid the cargo-holds large volume, this will cut down on the efficiency of the tank.


Assuming a TWR of 100:1 or better that seems reasonable

Actually it was 150:1 for 4 engines at 2300 kN and the factor that I'm most uncomfortable with, a 100:1 ratio would be much more appropriate considering the fuel change and a full-flow engine, high ISP generally comes at the cost of a lower T:W ratio.  At a 100:1 ratio I would be looking at 9 mT for engines.

Certainly. It needs a heat shield, it needs landing legs and a cargo container, it needs better communication than a rocket stage so it can talk to Earth from Mars. 

But I don't think those will force them to limit the delta-v drastically. They'll just end up with a huge vehicle.

(Passenger version needs a lot more stuff but that will come out of the cargo-version max capacity...)

You can't just scale the vehicle to conquer thouse factors, the parasitic stuff all grows with overall vehicle mass (except for com/computer systems which are negligible already).  Growing the vehicle only makes the payload small by comparison but were talking about the vehicle dry mass before payload even gets considered.

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