Author Topic: MCT Speculation and Discussion Thread 4  (Read 605019 times)

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1540 on: 01/31/2016 04:30 PM »
Which argues strongly for a depot from my point of view along with the idea that in the early days when only a few BFRs exist having a depot could allow for a significant increase in the number of BFSs that can launch in one window.

An orbital depot is probably a requirement just because it would be very inefficient for BFS and/or the tankers to have to lug around the cryocooling equipment.

It will have to cryocool propellant on Mars, which is more difficult than cryocooling in heliocentric orbit.
« Last Edit: 01/31/2016 04:33 PM by Burninate »

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1541 on: 01/31/2016 04:50 PM »
In regards to the MCT Depot, I think we are in the early planning stages where general issues can be discussed before any commitments will be made. I am trying to devise a few tools to help the discussion.

Attached is an simple Excel spreadsheet that calculates some data for this purpose. The inputs are the number of berths (Line 2), the assumed diameter of MCT (line3), and the minimum acceptable gravity force at the tank location closest to the depot center (line15). Some of the data of interest to me are depot diameter, the  highest gravity force that will be recorded at the outermost edge of each tank, and the rotational acceleration required to achieve these gravity forces. These are just a few of the factors to be considered. Please check it out.

That presumes that the depot is made up of BFS elements, one suggestion that I have seen on NFS was using the BFR as the initial tank - roughly the same volume as 4 or 5 BFSs, but now we eliminate a lot of complexity and if we roll the BFR for ulage we have far fewer structural issues.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Umbrella

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Re: MCT Speculation and Discussion Thread 4
« Reply #1542 on: 01/31/2016 05:04 PM »
It will have to cryocool propellant on Mars, which is more difficult than cryocooling in heliocentric orbit.

Good point, The orbital depot is in addition to the depot on the Martian surface.  However, cryocooling on the surface might be easier since the atmosphere could be used for convection, avoiding large radiators that are necessary in a vacuum.
« Last Edit: 01/31/2016 05:18 PM by Umbrella »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1543 on: 01/31/2016 05:53 PM »
PSA: Methane and oxygen don't need cryocooling to achieve zero boil off in orbit.

Thank you.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1544 on: 01/31/2016 05:59 PM »
PSA: Methane and oxygen don't need cryocooling to achieve zero boil off in orbit.

Thank you.

No but they do in transfer operations and to avoid the need for cryocooling to achieve zero boil off in orbit you need a attitude control and reflective shielding and radiative surface area that does not get that radiation reflected back at it.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1545 on: 01/31/2016 06:05 PM »
PSA: Methane and oxygen don't need cryocooling to achieve zero boil off in orbit.

Thank you.

No but they do in transfer operations and to avoid the need for cryocooling to achieve zero boil off in orbit you need a attitude control and reflective shielding and radiative surface area that does not get that radiation reflected back at it.
They don't if both tanks are cool, which can be done passively and without a big sunshield.
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

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1546 on: 01/31/2016 06:06 PM »
PSA: Methane and oxygen don't need cryocooling to achieve zero boil off in orbit.

Thank you.

No but they do in transfer operations and to avoid the need for cryocooling to achieve zero boil off in orbit you need a attitude control and reflective shielding and radiative surface area that does not get that radiation reflected back at it.
They don't if both tanks are cool, which can be done passively and without a big sunshield.

Excuse me how does it get from one tank to the other then?
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1547 on: 01/31/2016 06:09 PM »
Pressure. Seriously, they already have to worry about hydrazine freezing in tanks. Just paint your spacecraft white and keep it out of the Sun except the skinny way. It'll naturally get that cold.
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

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1548 on: 01/31/2016 06:21 PM »
Pressure. Seriously, they already have to worry about hydrazine freezing in tanks. Just paint your spacecraft white and keep it out of the Sun except the skinny way. It'll naturally get that cold.
Hydrazine freezes at 2 C!  (not relevant)

Methane boils at -162 C and freezes at -182 C

Oxygen boils at -183 C and freezes at -219 C

Nitrogen boils at -196 C and freezes at -210 C

Oxygen could in theory with pumping and a heat exchange unit be your heat sink for the methane that was being moved around, but Nitrogen is more likely to be used just as it is here on earth.

If you use pressure then it is not zero boiloff you will be venting the destination tank, heat will still be added going through small openings, and the system still requires a lot more than just a coat of white paint and you can't be pointed away from both the sun and the earth at the same time.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1549 on: 01/31/2016 06:33 PM »
If the receiving tank is below boiling point, it'd have a lower pressure and could actually condense the propellant. No venting required.


Also remember that the figures you're using are at STP. Higher boiling point at 100psi ullage pressure.

Anyway, it's well understood that methane/LOx are "space storable" as XCOR advertises. Sick of arguing with people thinking that because boiloff s big for hydrogen that it must also be for methane and oxygen.
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

Offline Umbrella

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Re: MCT Speculation and Discussion Thread 4
« Reply #1550 on: 01/31/2016 06:42 PM »
If the receiving tank is below boiling point, it'd have a lower pressure and could actually condense the propellant. No venting required.


Also remember that the figures you're using are at STP. Higher boiling point at 100psi ullage pressure.

Anyway, it's well understood that methane/LOx are "space storable" as XCOR advertises. Sick of arguing with people thinking that because boiloff s big for hydrogen that it must also be for methane and oxygen.

Why has LOX/RP-1 (or even LOX/methane) historically not been used for delayed burn operations, which have almost always, if not always, used hydrazine?

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1551 on: 01/31/2016 06:45 PM »
If the receiving tank is below boiling point, it'd have a lower pressure and could actually condense the propellant. No venting required.

So how did the pressure in the receiving tank drop so low that there is no displaced volume as you fill that tank?

So at what rate do you transfer so that the energy being added as you transfer the propellant is below the rate at which the added energy is radiated away?

How do you pressurize the source tank without heating the propellant being transferred?

Also remember that the figures you're using are at STP. Higher boiling point at 100psi ullage pressure.

100psi ulage pressure on the source tank, so now it needs to cool on the receiving side. How quickly can the heat added by transfer be radiated away? When the sun is above the horizon? or with just the radiative heat load from the earth.
Anyway, it's well understood that methane/LOx are "space storable" as XCOR advertises. Sick of arguing with people thinking that because boiloff s big for hydrogen that it must also be for methane and oxygen.

There is a difference between being storable 3 or 4 diameters out from Earth and in LEO.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1552 on: 01/31/2016 08:30 PM »
Maybe the "100 t payload" is the total land mass, a 60 t dry mass mct 25 t 100 people (+their goods and consumables) and 15t cargo or more cargo and less people. You engineering fellows, is that feasible with 3-4 refueling cargos?

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150003005.pdf offers 12 tons consumables budget for a 4-person conjunction-class (1030 day) Mars mission using closed-loop ECLSS.  You're offering 25 tons for 100 people.
How do the rest of you account for this?

Offline RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #1553 on: 01/31/2016 08:44 PM »
Maybe the "100 t payload" is the total land mass, a 60 t dry mass mct 25 t 100 people (+their goods and consumables) and 15t cargo or more cargo and less people. You engineering fellows, is that feasible with 3-4 refueling cargos?

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150003005.pdf offers 12 tons consumables budget for a 4-person conjunction-class (1030 day) Mars mission using closed-loop ECLSS.  You're offering 25 tons for 100 people.
How do the rest of you account for this?

They are basing the number on a hypothetical 100 day one way flight to a Mars colony that is already operational. No need to send 100 crew if there is nowhere to go. Other flights and ISRU will provide the supplies at the colony.

25 tons / 100 people / 100 days = 2.5 kg / person / day for the BFS one way flight to Mars.

12 tons / 4 people / 1030 days = 2.9 kg / person / day for the NASA conjunction mission.

That's only a 16% difference. Close enough for an estimate.

Early BFS missions will have a small crew and will need supplies for the entire mission. Say about 24 tons for 8 crew.

Offline BSenna

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Re: MCT Speculation and Discussion Thread 4
« Reply #1554 on: 01/31/2016 09:19 PM »
Maybe the "100 t payload" is the total land mass, a 60 t dry mass mct 25 t 100 people (+their goods and consumables) and 15t cargo or more cargo and less people. You engineering fellows, is that feasible with 3-4 refueling cargos?

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150003005.pdf offers 12 tons consumables budget for a 4-person conjunction-class (1030 day) Mars mission using closed-loop ECLSS.  You're offering 25 tons for 100 people.

Thanks for the article, I was asking because I really have no idea. Even the term consumables. I wasn't considering gases ansd water, and just on the first leg trip, about 6 months. Disposal manaement, personal pelongings, even clothes should be designed to fit in the mission imo, so it's maybe closer to 35 -40 t as I (not considering HO, O et al).

Anyway, this 100 crew mission, is further in the future, not in the 11, 12 years time frame. I've just seen this:

http://www.thespacereview.com/article/2909/1

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1555 on: 02/01/2016 02:29 AM »
In regards to the MCT Depot, I think we are in the early planning stages where general issues can be discussed before any commitments will be made. I am trying to devise a few tools to help the discussion.

Attached is an simple Excel spreadsheet that calculates some data for this purpose. The inputs are the number of berths (Line 2), the assumed diameter of MCT (line3), and the minimum acceptable gravity force at the tank location closest to the depot center (line15). Some of the data of interest to me are depot diameter, the  highest gravity force that will be recorded at the outermost edge of each tank, and the rotational acceleration required to achieve these gravity forces. These are just a few of the factors to be considered. Please check it out.
What is the safety circle in ths case?
What is the fuel volume/mass in the depot?
Are we talking about fuelling 9 ships at a time?
Is this a modification of the six shooter depot?
« Last Edit: 02/01/2016 02:30 AM by lamontagne »

Offline Stardhingy

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Re: MCT Speculation and Discussion Thread 4
« Reply #1556 on: 02/01/2016 02:58 AM »

Why has LOX/RP-1 (or even LOX/methane) historically not been used for delayed burn operations, which have almost always, if not always, used hydrazine?

The ease of lighting and reliability of hypergolic engines is probably the major consideration.

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #1557 on: 02/02/2016 12:37 AM »
Maybe the "100 t payload" is the total land mass, a 60 t dry mass mct 25 t 100 people (+their goods and consumables) and 15t cargo or more cargo and less people. You engineering fellows, is that feasible with 3-4 refueling cargos?

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150003005.pdf offers 12 tons consumables budget for a 4-person conjunction-class (1030 day) Mars mission using closed-loop ECLSS.  You're offering 25 tons for 100 people.
How do the rest of you account for this?

Depends on your assumptions for "closed loop".

3 tons per person for ~1000 days isn't a very good closed loop. A person needs about 5 kg/day of life support consumables, so 3000 kg for 1000 days means you only save 40% with the closed loop... huh?

Also, the MCT is probably carrying supplies for 100-250 days (depending on transit length) not 1000. Remember there are supposed to be ~10 cargo missions per crew mission (or something like that).


Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1558 on: 02/02/2016 02:47 AM »
Maybe the "100 t payload" is the total land mass, a 60 t dry mass mct 25 t 100 people (+their goods and consumables) and 15t cargo or more cargo and less people. You engineering fellows, is that feasible with 3-4 refueling cargos?

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150003005.pdf offers 12 tons consumables budget for a 4-person conjunction-class (1030 day) Mars mission using closed-loop ECLSS.  You're offering 25 tons for 100 people.
How do the rest of you account for this?

Depends on your assumptions for "closed loop".

3 tons per person for ~1000 days isn't a very good closed loop. A person needs about 5 kg/day of life support consumables, so 3000 kg for 1000 days means you only save 40% with the closed loop... huh?

Also, the MCT is probably carrying supplies for 100-250 days (depending on transit length) not 1000. Remember there are supposed to be ~10 cargo missions per crew mission (or something like that).
As you can see in the joined graph (Ames research center trajectory browser), most synods offer 120 to 180 day missions to Mars.  So if you do the math, you will find 52 tonnes of consumables for 180 days with 5 kg/person.  As mentionned by others, you will only be having 100 passengers when there is a base in place, so the rest of the trip time is not applicable.
To reduce to 25 tonnes you need to go to dehydrated foods, and do some fierce water and atmospheric recycling.  But that equipement will be essential on Mars, so it should be part of a colony package.



Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #1559 on: 02/02/2016 03:05 AM »
My guess is the breakdown of the 5 kg is :
0,7 kg of oxygen, turned into CO2
3 kg of water
1,3 kg of food,
So there would be no allocation for recycling at all?

The oxygen can be recycled from CO2 using the sabatier process and some hydrogen, either stock of from the water coming from the food combustion.  As long as the unit weighs less that ,7*100*180 = 14 tonnes, including solar cells to run it, then we're ahead on that.

The water should be easy, the ISS already does that.  We need to treat 300 kg per day, plus whatever is used for sanitation and cleaning.  We can allow for some of it to get dirty, since even dirty water will be a good Martian import.  Again 5x100x180 = 52 tonnes, and we should be able to have a good cleaning system for much less mass than that.

The food will become compost.  It'll be recycled on the colony.


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