Author Topic: MCT Speculation and Discussion Thread 4  (Read 621688 times)

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2360 on: 06/28/2016 06:07 PM »
The return flight will not likely be in the exact optimum window. Also, it'll likely be less than 6 months.

Remember, SpaceX wants the vehicle back each time.
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Offline Elmar Moelzer

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Re: MCT Speculation and Discussion Thread 4
« Reply #2361 on: 06/28/2016 07:29 PM »
A typical 6-month return is only about 6.5 to 7 km/s from Mars surface to Earth surface (5.25 for launch to escape, 1 to 1.5 for transfer injection, 0.35 for EDL).
We had already established that the trip would be 3 months?

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2362 on: 06/28/2016 07:30 PM »
A typical 6-month return is only about 6.5 to 7 km/s from Mars surface to Earth surface (5.25 for launch to escape, 1 to 1.5 for transfer injection, 0.35 for EDL).
We had already established that the trip would be 3 months?
Who said the return trip must be the same length of time as the trip to Mars?
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2363 on: 06/28/2016 07:30 PM »
Why rush faster than 6 months on return?  Take 3-4 months Earth to Mars with humans.  One month or less on Mars. 6 months return, some little cargo and a few possible humans.  Plenty of time to get the BFS ready for the next synod which is the goal.
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Offline stoker5432

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Re: MCT Speculation and Discussion Thread 4
« Reply #2364 on: 06/28/2016 07:34 PM »
Maybe to optimize crew health for further missions if there is a crew coming back.
« Last Edit: 06/28/2016 07:42 PM by stoker5432 »

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2365 on: 06/28/2016 07:41 PM »
The return flight will not likely be in the exact optimum window. Also, it'll likely be less than 6 months.

Remember, SpaceX wants the vehicle back each time.

I don't think it's feasible or necessary for early missions to return the vehicle for launch the next synod. IMO that won't happen until LMO refueling is possible and all the kinks/upgrades are worked out of the system. Probably 2030's. That requires a lot of pre-positioned assets to support fast ground unloading/refueling and LMO refueling.

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2366 on: 06/28/2016 08:06 PM »
Why rush faster than 6 months on return?  Take 3-4 months Earth to Mars with humans.  One month or less on Mars. 6 months return, some little cargo and a few possible humans.  Plenty of time to get the BFS ready for the next synod which is the goal.

You're describing an opposition class mission. That's feasible with LMO refueling and hot reentries, but requires one leg of the trajectory dip inside the orbit of Venus which puts a lot of constraints on the spacecraft to handle different thermal and radiation environments. And 10 months in zero-gee isn't ideal for humans.

Orbital mechanics generally preclude having both short stays and fast transits.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2367 on: 06/28/2016 08:13 PM »
The return flight will not likely be in the exact optimum window. Also, it'll likely be less than 6 months.

Remember, SpaceX wants the vehicle back each time.

I don't think it's feasible or necessary for early missions to return the vehicle for launch the next synod. IMO that won't happen until LMO refueling is possible and all the kinks/upgrades are worked out of the system. Probably 2030's. That requires a lot of pre-positioned assets to support fast ground unloading/refueling and LMO refueling.
LMO refueling isn't required.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2368 on: 06/28/2016 08:32 PM »
...

Orbital mechanics generally preclude having both short stays and fast transits.
That changes if you're using massive amounts of ISRU propellants.
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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2369 on: 06/28/2016 08:36 PM »
...

Orbital mechanics generally preclude having both short stays and fast transits.
That changes if you're using massive amounts of ISRU propellants.

You are right that in orbit refuelling is not necessary. It would make using massive amounts of propellants much easier though. Especially if there were in orbit ressources for fuel available.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2370 on: 06/28/2016 08:45 PM »
...

Orbital mechanics generally preclude having both short stays and fast transits.
That changes if you're using massive amounts of ISRU propellants.

You are right that in orbit refuelling is not necessary. It would make using massive amounts of propellants much easier though. Especially if there were in orbit ressources for fuel available.
Or you just develop a lightweight vehicle with lots of room for propellant.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2371 on: 06/28/2016 08:54 PM »
...

Orbital mechanics generally preclude having both short stays and fast transits.
That changes if you're using massive amounts of ISRU propellants.

You are right that in orbit refuelling is not necessary. It would make using massive amounts of propellants much easier though. Especially if there were in orbit ressources for fuel available.

The need for orbital refueling is highly dependent on the planetary alignment. On poor alignments same-synod return takes 10 km/s just to get from the surface through TEI, and Earth reentry is over 20 km/s. On others it takes less than 7 km/s to get through injection and reentry is under 14 km/s.

The mid to late 2020's are relatively poor alignments, and I doubt the infrastructure will be in place to allow rapid return and reuse. But the early 2030's are quite good, good enough to allow same-synod reuse without orbital refueling. By the late 2030's when the alignments shift again, that infrastructure might be in place and the campaign of launches with rapid reuse can continue.

MCT might have grown in capacity by then, however.... it wouldn't surprise me to see a couple versions with improved performance after the early 2020's missions.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2372 on: 06/29/2016 11:28 PM »
Your payload is too small at 100 mt, I'm thinking payload to LEO of around 200 mt even after engine loss, so that would require 4 functional engines and 6 total.

But why would you want that? Staging to LEO and LEO to Mars surface have almost exactly the same DV requirements, so the vehicle should be optimized to deliver the same payload to both. A typical fast transit and EDL only requires ~6 kms. Launching 200t payload and 100t ship into LEO requires about 1400t of propellant in the US, but transit and EDL of 100t payload and 100t ship only require about 800t. You're shipping an extra 20% to 30% of useless dry mass (engines and tankage) to Mars and back.

For LEO tanker runs the 200t payload is useful, but that can be accomplished with larger tanks that the Mars transit ship doesn't need. And it still doesn't really need 6 engines for high-thrust engine out capability, because a tanker won't be carrying people and it can always use some of it's extra fuel load for margin. An off-nominal LEO launch has a lot of options for abort, particularly if a rapid launch cadence, on-orbit refueling, and LEO rendezvous are SOP.

You misunderstand, I foresee at F9 like configuration with a rather plain 2nd stage that is just reusable (much like the original F9 video) the 200 mt is the total payload atop that 2nd stage and needs to be that large in order to carry the BFS and it's payload and small amount of propellant for emergency landing needs in case of an abort.

I do not believe the 1 synod round-trip plan is remotely viable and Musk is wishing when he expressed it, but it will not come to pass because it requires both too much propellant and has too high of a entry velocity particularly at Earth.  Finally it is a terribly inefficient cargo delivery method, as cargo will dominate by at least 10:1 over crew the optimum vehicle would be optimized for cargo and then sped-up to accommodate crew.

The whole premise of 1 synod cycles is more frequent trips and better amortization of the vehicle.  But the difference is between terrible and abysmal amortization because 1 synod is 780 days which means your looking at 7 uses of a vehicle or 14 over it's entire 30 year lifespan (and that's assuming it last that long).  A fundamentally different strategy is needed to make amortization work and it is simple, keep the earth launch vehicle and mars landing vehicle at Earth and Mars for a long as possible doing many surface to orbit shuttle flights bringing cargo up and down respectively.  A much simpler freight vehicle goes between Earth and Mars and it suffers the poor amortization hit but it carries large amounts of cargo externally to compensate.  The ideal Earth launch vehicle is thus a simple 2nd stage with a payload fairing which can launch any cargo imaginable, and the ideal mars lander is a modest size capsule that can fit on top of it as a 3rd stage.

Departure from Earth would be from a high orbit like EML1, departure from Mars will be from LMO, refueling at both locations makes for a vehicle vastly smaller with only ~4 km/s needed to perform any one leg which is the Mars assent, a leg which can not be sub-divided and is the long pole.  The constant harping on a similar DeltaV between LEO->Mars and Mars surface->Earth is a false prophet because correct strategy is to create the smallest single DeltaV leg possible between viable rendezvous points so as to defeat the rocket equation piece meal.  Vehicle Rendezvouses, fuel transfer and multiple restart engines are all baseline assumptions we have with regard to SpaceX technology, once you have thouse capabilities it's crazy not to use them throughout the whole mission architecture to get more leverage out of your IMLEO.

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2373 on: 06/30/2016 04:07 PM »
...  The ideal Earth launch vehicle is thus a simple 2nd stage with a payload fairing which can launch any cargo imaginable, and the ideal mars lander is a modest size capsule that can fit on top of it as a 3rd stage.
...

So assuming that the 2020's missions will be entirely methalox propulsion (SEP can't be a long pole), this is basically a non-integrated large Dragon architecture? It does free up a lot of design constraints, but it adds some additional issues. As I see them:

Pros:
a) Tankage volume in the capsule is around 1/3 as large (~360 m^3 vs ~975 m^3).
b) The capsule can be probably around 40% lighter for the same landed payload.
c) Might be able to do direct return if it doesn't need to carry payload.
d) The capsule does far less deep-space propulsion (mostly EDL and launch), no gigantic, fragile vac nozzles needed.
e) The vac-optimized 2nd stage can do most of the heavy lifting in vacuum (to within 1 km/s of TMI), and with a low ballistic coefficient and good heatshield doesn't need to do hypersonic retroprop at Earth entry.
f) The constraints around LAS are much easier for a ~150t unfueled capsule than a ~1000t fueled capsule.
g) The capsule doesn't need a dispenser system to be useful for satellite launches. Multi-sat deployment from 2nd stage is more proven technology.
h) 2nd stage needs some endurance but doesn't have to fly beyond GTO.
i) 2nd stage doesn't need any abort capabilities.
j) Capsule and 2nd stage can basically be scaled-up Dragon and Falcon, reducing development costs and major risks.
k) The 2nd stage is a potential lower-cost path to a minimum viable sat-launch product, since second stage recovery experiments can still fly profitable payloads even if they end up expending the stage.

Cons:
i) Mars Orbital Rendezvous is required before trans-Earth injection if returning any payload.
ii) There always has to be 2 capsules at Mars in order to send one home, even on very early missions.
iii) Capsules have to perform 2 (or more) Mars decents and ascents on average before being serviced at Earth.
iv) Integrating 3 vehicles for every capsule launch slows launch cadence and adds expense.
v) Capsule and 2nd stage don't share a lot of development or manufacturing costs with each other, requiring 2 full dev programs and 2 manufacturing lines (and maybe a 3rd for BFR, depending how similar it is to 2nd stage).
vi) The second stage needs a unique reentry and landing architecture with a lot of potential development risks.
vii) Developing 3 codependent vehicles presents a larger potential for delays and perhaps higher bar for minimum viable Mars transport product.
viii) Doesn't reduce the number of launches required for an early Mars mission, and might increase them since 2 capsules need to be at Mars for the first crewed mission.

Edit: clarity.
« Last Edit: 06/30/2016 07:02 PM by envy887 »

Offline jpo234

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You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2375 on: 07/11/2016 01:48 PM »
Richard Heidmann (Snecma and Ariane) about the MCT.

A year ago he wrote a more in depth article on the same concept with an English translation:
http://planete-mars.com/what-could-the-mars-colonization-transport-mct-spacex-project-look-like-continued/

None of it makes any sense to me. A 10,000 tonne GLOW stack with 120 mN (27 mlbf) of total thrust? 700 tonne-thrust Raptors? A winged belly-landing super-shuttle? Integrated habitats?

Offline jpo234

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Re: MCT Speculation and Discussion Thread 4
« Reply #2376 on: 07/11/2016 01:59 PM »
A winged belly-landing super-shuttle?

In the latest article the "super-shuttle" is not really a shuttle. It's more about maximizing the surface available for aerobraking. This is at least how I interpret the renders.
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2377 on: 07/11/2016 02:11 PM »
A winged belly-landing super-shuttle?

In the latest article the "super-shuttle" is not really a shuttle. It's more about maximizing the surface available for aerobraking. This is at least how I interpret the renders.

It's maximizing the surface available for aerobraking by entering with the ventral side forward, like the shuttle. That's great for increasing ballistic drag, but not great for optimizing structural mass. And the horizontal take-off/landing is interesting.

Offline jpo234

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Re: MCT Speculation and Discussion Thread 4
« Reply #2378 on: 07/11/2016 02:22 PM »
A winged belly-landing super-shuttle?

In the latest article the "super-shuttle" is not really a shuttle. It's more about maximizing the surface available for aerobraking. This is at least how I interpret the renders.

It's maximizing the surface available for aerobraking by entering with the ventral side forward, like the shuttle. That's great for increasing ballistic drag, but not great for optimizing structural mass. And the horizontal take-off/landing is interesting.

That image is from the older article. The article from 07/09/2016 shows a different BFS: just small winglets. IMHO this one is meant to actually land vertically.

EDIT: Of course it will enter "with the ventral side forward", but later on reorient for a vertical landing. I don't know whether this is actually possible.
« Last Edit: 07/11/2016 02:28 PM by jpo234 »
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2379 on: 07/11/2016 02:34 PM »
That image is from the older article. The article from 07/09/2016 shows a different BFS: just small winglets. IMHO this one is meant to actually land vertically.

EDIT: Of course it will enter "with the ventral side forward", but later on reorient for a vertical landing. I don't know whether this is actually possible.

Where does it show a vertical landing? He seems rather stuck on the horizontal landing, and there are no engines in the rear of that thing in the illustrations.

Which makes me wonder... how does it get to orbit? Horizontally?

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