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Old 7th October 2017, 03:57 AM   #161
theprestige
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Originally Posted by smartcooky View Post
But where is the monetary return?
Where is the scientific return?
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Old 7th October 2017, 04:00 AM   #162
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Originally Posted by smartcooky View Post
I put pure science in the same category. We spend billions of dollars building a 27km long circular, underground atom smasher to try to discover what happened 13.8 billion years ago at the beginning of the Universe.

Why? Why do we need to know this? How does confirming Higgs' God Particle theory improve our everyday lives?
Tax payers fund it, and the 'return' is the joy of knowledge. A commercial venture doesn't operate that way, and SpaceX is such a venture.
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Old 7th October 2017, 04:18 AM   #163
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smartcooky, I"m not questioning whether or not it's a good idea to go to Mars. I would be very happy if SpaceX, or anyone else, pulls that off. I'm questioning whether or not it will actually happen, particularly in the time frame Musk presented.

If no one pays for it, it won't happen.

It would be awesome if someone could find a good business plan that takes them to Mars, because then we don't need outside funding, at least beyond some initial investment, to make it happen.

If we don't have that, it's still possible that we'll go. Maybe it's someone's vanity project. Maybe it's a scientific project sponsored by government. But if you think Musk is right that it's going to happen in the time frame presented can you please be clear about where you think the funding will come from?
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Old 7th October 2017, 10:22 AM   #164
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Originally Posted by smartcooky View Post
But you aren't understanding what is meant by cheaper.
Musk claims that he can build a rocket that will take someone from any point on the planet to anywhere else in 30 minutes and that it will only cost $200,000 per seat. My understanding of this is that Elon Musk is either bat-sh*t crazy or he believes his own hype. This is not a rational claim. SpaceX has done absolutely nothing, not one thing that would prove this is possible. Remember the millions spent on SCRAMjet research and vehicles like the Orient Express? This still isn't a practical idea and yet it is decades ahead of Musk.

Quote:
Also, you have to fact in that BFR is expected to be able to lift a payload of 550 tonnes to LEO.
No. It's 550,000 lbs. That's only 275 tons or 250 metric tons. And that is for an expendable vehicle. In the reusable configuration it's 330,000 lbs or 150 metric tons.

So, you lift the 187,000 lb spaceship into LEO. That seems possible but how exactly do you test it before you send it into orbit? Then assuming that you can somehow get a working unit into space you still have to lift 2.4 million lbs of fuel. Let's see:

2,400,000 / 330,000 = 7.3. So, eight launches of BFR to provision it.
If we are generous and only figure $700 million per launch that would still be $5.6 billion. That doesn't even include the cost of developing the BFR and the spaceship and the tanker. Musk's piggy bank isn't that big.
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Old 7th October 2017, 10:28 AM   #165
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Originally Posted by cjameshuff View Post
You can't replace the Merlins with Raptors. Methane is lower density than RP-1 and the Falcon 9 has already stretched about as far as it can go due to bending loads and aerodynamic issues, so the poorer mass ratio would eat up a bunch of the specific impulse advantage.
No, you only have to do it for the upper stage to increase the Isp in the vacuum engine. The first stage is working pretty well.
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Old 7th October 2017, 10:41 AM   #166
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Originally Posted by Beelzebuddy View Post
Where are you getting these requirements? Why 100 tonnes of methane - 300 tonnes of propellant? Why a year? Did someone say that was their plan? Why not 26 months, the actual length of time between launch windows? Are these just arbitrary numbers that you don't think will work?
Musk's spaceship weighs just under 3 million lbs. It takes about half of the vehicle's weight expended to achieve a lunar transfer orbit. It takes almost 3/4 of the vehicle weight to reach Mars. You would arrive with perhaps 200,000 lbs of fuel remaining out of the 2.4 million you started with. To return, you need another 2.2 million lbs of fuel plus whatever you burned getting into Mars orbit.
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Old 7th October 2017, 10:51 AM   #167
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Originally Posted by cjameshuff View Post
If it doesn't turn out to be pathologically maintenance heavy (the Raptor testing so far is promising), they should easily get the total cost per launch (including that of the vehicle, amortized across all the launches during its lifetime) below that of a partially-reusable Falcon 9 launch.
Based on what? This seems to be nothing more than a circular argument.

I'm confident that SpaceX can reduce the per launch cost. And, based on that projection I'm sure they can reduce the cost even further. And the fact that they can make it so cheap in the future is proof that we can reduce cost now.

What you have now is $62 million. Until SpaceX announces a price below that you have nothing. Even under the best projection of saving the upper stage and fairing you only knock $15 million off the price. That doesn't even get you into fantasy projection of what you've claimed. The most optimistic projection for engine reuse is 10 launches before overhaul. The Merlins are $1.5 million apiece. The Raptor will cost more and in the near term will not be able to reach ten uses. This is reality. You aren't getting the cart before the horse. You are getting the cart before the discovery of horses.
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Old 7th October 2017, 02:01 PM   #168
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Originally Posted by Beelzebuddy View Post
Where are you getting these requirements? Why 100 tonnes of methane - 300 tonnes of propellant? Why a year? Did someone say that was their plan?
They aren't requirements, nor any part of a published plan. They're ballpark estimates based on the SpaceX plans for the ship and timescales stated. The figures I'm suggesting are designed purely to get a handle on the energy supply issues, and not to be an accurate description. It's a common technique in scientific circles.

Originally Posted by Beelzebuddy View Post
That's 25E3 kg H2. At 50 kWh per kg, it comes out to 1.25E6 kWh total.
Divided by 365, we need 3425 kWh per day.
On earth, a square meter of typical solar panel can generate 1 kwh/day. Mars is about half that, so 3.5E3 kWh turns into 7E3 m^2 of paneling.
Googling around, you can buy rollable solar panels that weigh about a kilogram per square meter.
So, to generate 300 tonnes of propellant per year, you need to land with 7 tonnes of solar paneling.
Seven metric ****tons of solar panels is an awful lot of solar panels, but it's well within reasonable weight requirements.
Thanks for entering the realm of actual maths. However, you quote an energy figure of 'typical' solar panels, then switch to thin-film panels for weight calculations. Rollable panels are significantly less efficient, so maybe you might re-evaluate the mass. Also, I don't really see those rollable panels being spread over the Martian rocks, so they'll need a structure to be attached to, i.e added mass. Then there are Martian storms - and, yes, I know they're feeble - but those structures might need anchoring to the ground.

Methinks your mass calculations need revisiting.

Originally Posted by Beelzebuddy View Post
Neither is obstinate pessimism. SpaceX does not have a history of ambitious projects which ultimately fail because of glaring logistical oversights. They do have a history of ambitious projects which ultimately succeed despite the conclusions of grumpy naysayers on the internets.
I would hesitate to describe those keen on the SpaceX Mars project as "gullible fanboys, raised on a diet of comic book sci-fi", more that they need to look at the actual science involved.
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Old 7th October 2017, 07:03 PM   #169
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Originally Posted by GlennB View Post
They aren't requirements, nor any part of a published plan. They're ballpark estimates based on the SpaceX plans for the ship and timescales stated. The figures I'm suggesting are designed purely to get a handle on the energy supply issues, and not to be an accurate description. It's a common technique in scientific circles.
"I pulled them from my ass because I thought they'd make a good hypothetical to prove my point." Also a common technique in scientific circles.

Quote:
Thanks for entering the realm of actual maths. However, you quote an energy figure of 'typical' solar panels, then switch to thin-film panels for weight calculations. Rollable panels are significantly less efficient, so maybe you might re-evaluate the mass. Also, I don't really see those rollable panels being spread over the Martian rocks, so they'll need a structure to be attached to, i.e added mass. Then there are Martian storms - and, yes, I know they're feeble - but those structures might need anchoring to the ground.

Methinks your mass calculations need revisiting.
No. I've played your little game, now you play mine: wait until there's an actual plan to criticize the plan. I'm not saying you have to fanboy or even refrain from offering a negative opinion, just stop offering arguments that rely on no one at SpaceX (or wherever) knowing what the hell they're doing. I'm tired of seeing them.

Here's a politely scientific way of saying "I think you're full of ****": They've got an ambitious plan. I think they might run into engineering trouble manufacturing fuel, but we'll see what they come up with.
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Old 7th October 2017, 07:28 PM   #170
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Originally Posted by Beelzebuddy View Post
"I pulled them from my ass because I thought they'd make a good hypothetical to prove my point." Also a common technique in scientific circles.


No. I've played your little game, now you play mine: wait until there's an actual plan to criticize the plan. I'm not saying you have to fanboy or even refrain from offering a negative opinion, just stop offering arguments that rely on no one at SpaceX (or wherever) knowing what the hell they're doing. I'm tired of seeing them.

Here's a politely scientific way of saying "I think you're full of ****": They've got an ambitious plan. I think they might run into engineering trouble manufacturing fuel, but we'll see what they come up with.

This, and this

I'm always amazed how all those very clever, sometimes brilliant aerospace engineers and rocket scientists at SpaceX have absolutely no idea what they are doing, while a few keyboard jockeys on an internet forum always seem to have all the answers, and know more about SpaceX's plans than those at SpaceX do.
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Old 7th October 2017, 08:46 PM   #171
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Originally Posted by smartcooky View Post
I'm always amazed how all those very clever, sometimes brilliant aerospace engineers and rocket scientists at SpaceX
Name one such engineer, please. Cite their work on Musk's whateverthefrack you think they're working on, if you can.
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Old 7th October 2017, 09:29 PM   #172
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Originally Posted by theprestige View Post
Name one such engineer, please. Cite their work on Musk's whateverthefrack you think they're working on, if you can.
Tom Mueller - engine design.
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Old 7th October 2017, 09:50 PM   #173
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Originally Posted by smartcooky View Post
I'm always amazed how all those very clever, sometimes brilliant aerospace engineers and rocket scientists at SpaceX have absolutely no idea what they are doing, while a few keyboard jockeys on an internet forum always seem to have all the answers, and know more about SpaceX's plans than those at SpaceX do.
Since I didn't quote you there's a good chance you didn't see it, but could you take a look at post #163. I'd appreciate a reply.
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Old 7th October 2017, 10:53 PM   #174
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Originally Posted by Roboramma View Post
smartcooky, I"m not questioning whether or not it's a good idea to go to Mars. I would be very happy if SpaceX, or anyone else, pulls that off. I'm questioning whether or not it will actually happen, particularly in the time frame Musk presented.

If no one pays for it, it won't happen.

It would be awesome if someone could find a good business plan that takes them to Mars, because then we don't need outside funding, at least beyond some initial investment, to make it happen.

If we don't have that, it's still possible that we'll go. Maybe it's someone's vanity project. Maybe it's a scientific project sponsored by government. But if you think Musk is right that it's going to happen in the time frame presented can you please be clear about where you think the funding will come from?
Not directed at me, but I'll take a stab.

First it's important to keep in mind that is almost ideological for Musk. Profit is necessary to fund his ambitions, but it isn't his driving force.

I believe that SpaceX will privately fund as much of it themselves as possible and look to Nasa to buy seats for Astros. If not, they'll auction them to the highest bidder. How much would Anoush Ansari pay to be the first person on Mars?

Ok, maybe eccentric billionaires is a limited market, but I honestly can't see Nasa passing up the chance to land 100 curiosity rovers for the half the price of an SLS launch.
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Old 7th October 2017, 10:56 PM   #175
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Thanks, that's somewhat reasonable. I mostly just want us all to be clear about what the actual idea is here.
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Old 7th October 2017, 11:30 PM   #176
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Originally Posted by Beelzebuddy View Post
"I pulled them from my ass because I thought they'd make a good hypothetical to prove my point." Also a common technique in scientific circles.
100 tonnes is just under half the propellant capacity of the proposed rocket. 1 year is half the time available to generate the fuel, assuming the fuel will need to be ready before people arrive two years later. If not then those people are going to have a very long wait before they can relaunch, no? So, no, not "pulled from my ass", just reasonable estimates that highlight the issues.
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Old 8th October 2017, 12:04 AM   #177
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Originally Posted by GlennB View Post
100 tonnes is just under half the propellant capacity of the proposed rocket. 1 year is half the time available to generate the fuel, assuming the fuel will need to be ready before people arrive two years later. If not then those people are going to have a very long wait before they can relaunch, no? So, no, not "pulled from my ass", just reasonable estimates that highlight the issues.
It's not clear to me that such fast transits are possible. In order to avoid spending a minimum of 18 months at Mars, you'd have to leave before the start of the window, do a very fast transit. You might have as much as a week or two before you had to depart, again on a fast transit. I don't see this happening early on. Any astro going to Mars is there to stay for 18 months regardless.

That gives them 4 years (or two synods) to produce the return prop.
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Old 8th October 2017, 12:47 AM   #178
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Originally Posted by theprestige View Post
Name one such engineer, please. Cite their work on Musk's whateverthefrack you think they're working on, if you can.
Neel Nyak - Structures Engineer

Brian Meade - Space Operations

Samra Sangari - Senior Engineer, Senior Product Development Engineer,

Haley Hunt - Mechanical Design Engineer (previously Avionic Integration Specialist)

Jeremy Hollander - Senior Propulsion Engineer (previously Manager of Dragon Life Support Systems and Thermal Control Systems, Lead Thermal/Fluids Engineer)

Karsten Larson - Test Equipment Engineer

Would you like some more?
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Old 8th October 2017, 12:56 AM   #179
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Originally Posted by Octavo View Post
Not directed at me, but I'll take a stab.

First it's important to keep in mind that is almost ideological for Musk. Profit is necessary to fund his ambitions, but it isn't his driving force.

I believe that SpaceX will privately fund as much of it themselves as possible and look to Nasa to buy seats for Astros. If not, they'll auction them to the highest bidder. How much would Anoush Ansari pay to be the first person on Mars?

Ok, maybe eccentric billionaires is a limited market, but I honestly can't see Nasa passing up the chance to land 100 curiosity rovers for the half the price of an SLS launch.
Not to mention the chance to put dozens of satellites in Mars orbit for all manner of scientific research e.g. radar mapping, dedicated liquid water search, areological mapping and mineral prospecting.
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Old 8th October 2017, 03:37 AM   #180
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There's a very good presentation of the plans at Business Insider, including plenty of the original SpaceX diagrams and animations.

One part that got my attention was the (in orbit) 'refilling' animation. See what you think.
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Old 8th October 2017, 12:14 PM   #181
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Originally Posted by GlennB View Post
There's a very good presentation of the plans at Business Insider, including plenty of the original SpaceX diagrams and animations.

One part that got my attention was the (in orbit) 'refilling' animation. See what you think.

Have you only just seen this?
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Old 8th October 2017, 01:25 PM   #182
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Originally Posted by smartcooky View Post
Have you only just seen this?
No. Revisiting it just got me thinking about certain things.

Does it raise any questions in your mind? The logistics and their implications?
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Old 8th October 2017, 01:40 PM   #183
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Originally Posted by GlennB View Post
No. Revisiting it just got me thinking about certain things.

Does it raise any questions in your mind? The logistics and their implications?
If you want to make a point actually making it might be a good idea
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Old 8th October 2017, 02:27 PM   #184
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Originally Posted by Grashtel View Post
If you want to make a point actually making it might be a good idea
OK, but just a hefty clue. The BFR consists of two parts, the booster (which returns to Earth and soft lands) and the device that eventually heads off to Mars (in this case). Call that the Space Module (BFR-SM, or whatever).

The BFR-SM needs refuelling in order to head off to Mars, as it has burned its own original fuel (or some of it) to get to LEO. Then the tanker - an apparent duplicate of the BFR-SM, though minus the equipment/people payload - shows up in LEO to refuel the BFR-SM, having arrived via its own booster and ...

Take it from there.

or check out SpaceX's own graphic and spot the problem :
Attached Images
File Type: jpg spacex-bfr-mars-colonization-plan-schematic.jpg (25.0 KB, 17 views)
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Old 8th October 2017, 03:21 PM   #185
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Originally Posted by GlennB View Post
No. Revisiting it just got me thinking about certain things.

Does it raise any questions in your mind? The logistics and their implications?
Not really. This looks like a presentation of an outline... big on pretty pictures, short on fine detail, which is why I think it is far too soon to try doing calculations... you have nothing to base them on because Musk hasn't given us any meaningful figures. If indeed he does have a plan, this is <1% of it.

One thing he has got absolutely right is that sending a few people at a time for a short stay the way NASA did with Apollo is not viable with regards to Mars. Musk's concept of a very big rocket is ultimately the most efficient and cheapest way to go - sending 100 missions with five tonne payloads would be hugely more expensive that sending five missions with 100 tonne payloads. This is especially so if the transport system you are using is not single purpose and is made to do multiple roles... Space shuttle they way it should have been.

In my lifetime, I have seen things happen that I would have thought were impossible; things that even leading scientists and engineers deemed impossible, or at least very unlikely, and that existed only in the fanciful imaginations of science fiction writers - airliners that carry 850 passengers, men landing on the moon and returning to earth, space probes leaving the solar system, rovers on Mars, personal hand-held video phones, satellite TV, computers in the home, the internet and the ability to talk to people like you without placing a long distance call or being a ham radio operator, recording devices with no moving parts, and many more, too many to list here, and so many that I have learns not to be dismissive of new ideas.

Landing rocket boosters on land, tail down on a column of rocket exhaust is the stuff of 1960's sci-fi, and was deemed either impossible, or so technically difficult that it would never be a safe or practical thing to do. SpaceX makes it look like duck soup! On that basis, the jury is out on whether his plans are viable.
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Old 8th October 2017, 03:30 PM   #186
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Originally Posted by GlennB View Post
OK, but just a hefty clue. The BFR consists of two parts, the booster (which returns to Earth and soft lands) and the device that eventually heads off to Mars (in this case). Call that the Space Module (BFR-SM, or whatever).

The BFR-SM needs refuelling in order to head off to Mars, as it has burned its own original fuel (or some of it) to get to LEO. Then the tanker - an apparent duplicate of the BFR-SM, though minus the equipment/people payload - shows up in LEO to refuel the BFR-SM, having arrived via its own booster and ...

Take it from there.

or check out SpaceX's own graphic and spot the problem :
I don't have the time or the patience to play stupid guessing games. Make your point
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Old 8th October 2017, 03:50 PM   #187
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Originally Posted by smartcooky View Post
I don't have the time or the patience to play stupid guessing games. Make your point
He's assuming that because the first ship launched with basically no fuel, the refueling ships which basically look like the first ship from the outside must also launch with no fuel. Because ships which look similar on the outside must have the same configuration on the inside, right? And GlennB is the first person ever to point that out. It really is a blessing to have him here doing the hard thinking for us. It never would have occurred to me.
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Old 8th October 2017, 04:44 PM   #188
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Originally Posted by Beelzebuddy View Post
He's assuming that because the first ship launched with basically no fuel, the refueling ships which basically look like the first ship from the outside must also launch with no fuel. Because ships which look similar on the outside must have the same configuration on the inside, right? And GlennB is the first person ever to point that out. It really is a blessing to have him here doing the hard thinking for us. It never would have occurred to me.
Of course, this is nonsense

1st BFR-SM launches with no fuel and huge payload (fuel+payload = N tonnes)

2nd BFR-SM launches with all fuel and no payload (fuel+payload = N tonnes)

In fact, they don't even have to have the same configuration. SpaceX might decide at some future time that its worthwhile having a purpose built tanker, with no payload bay. They could even build auxiliary fuel tanks that go into the payload bay.

Furthermore, this could be a possible solution to the time taken to make fuel on Mars.... send a tanker. It might not take all the fuel needed, but it could give them a healthy head start and bring the time for additional fuel manufacture down to, say 12 months
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Old 8th October 2017, 07:14 PM   #189
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Originally Posted by GlennB View Post
OK, but just a hefty clue. The BFR consists of two parts, the booster (which returns to Earth and soft lands) and the device that eventually heads off to Mars (in this case). Call that the Space Module (BFR-SM, or whatever).

The BFR-SM needs refuelling in order to head off to Mars, as it has burned its own original fuel (or some of it) to get to LEO. Then the tanker - an apparent duplicate of the BFR-SM, though minus the equipment/people payload - shows up in LEO to refuel the BFR-SM, having arrived via its own booster and ...

Take it from there.

or check out SpaceX's own graphic and spot the problem :
No problem. The first ship's payload is the Mars Mission Module. It used all its fuel to get to LEO, and needs more fuel to get to Mars.

The second ship's payload is fuel for the first ship. They meet in LEO, thus solving the first ship's fuel problem.

The second ship used all its fuel getting the first ship's refueling payload to LEO, but that's not a problem, because the second ship only needed to get to LEO.
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Old 8th October 2017, 10:12 PM   #190
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Originally Posted by GlennB View Post
OK, but just a hefty clue. The BFR consists of two parts, the booster (which returns to Earth and soft lands) and the device that eventually heads off to Mars (in this case). Call that the Space Module (BFR-SM, or whatever).

The BFR-SM needs refuelling in order to head off to Mars, as it has burned its own original fuel (or some of it) to get to LEO. Then the tanker - an apparent duplicate of the BFR-SM, though minus the equipment/people payload - shows up in LEO to refuel the BFR-SM, having arrived via its own booster and ...

Take it from there.

or check out SpaceX's own graphic and spot the problem :
It never creases to impress me when armchair critics are able to easily spot fatal flaws in a complex design that a man worth 15bn dollars has bet his 20bn dollar company on! I bet Musk feels like a moron now. You should tweet him and let him know.
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Old 8th October 2017, 10:51 PM   #191
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Originally Posted by GlennB View Post
OK, but just a hefty clue. The BFR consists of two parts, the booster (which returns to Earth and soft lands) and the device that eventually heads off to Mars (in this case). Call that the Space Module (BFR-SM, or whatever).

The BFR-SM needs refuelling in order to head off to Mars, as it has burned its own original fuel (or some of it) to get to LEO. Then the tanker - an apparent duplicate of the BFR-SM, though minus the equipment/people payload - shows up in LEO to refuel the BFR-SM, having arrived via its own booster and ...

Take it from there.

or check out SpaceX's own graphic and spot the problem :
You'd think that by looking at the image, it would be clear what the plan is. It shows 4/5 tankers refilling the ship. That should give you a hint that the Ship ends up on orbit with tanks still 1/5 to 1/6 full.
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Old 8th October 2017, 11:06 PM   #192
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Originally Posted by Octavo View Post
You'd think that by looking at the image, it would be clear what the plan is. It shows 4/5 tankers refilling the ship. That should give you a hint that the Ship ends up on orbit with tanks still 1/5 to 1/6 full.
Besides, how much fuel does it actually need to get to Mars? Enough for...

1. a TMI burn
2. some MCC burns on the journey
3. landing burn at Mars (they appear to have ditched the idea retrofiring to slow down and will use direct atmospheric entry instead)

Surely this will be a lot less fuel that they would have used to get to LEO
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Old 8th October 2017, 11:35 PM   #193
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Originally Posted by smartcooky View Post
Besides, how much fuel does it actually need to get to Mars? Enough for...

1. a TMI burn
2. some MCC burns on the journey
3. landing burn at Mars (they appear to have ditched the idea retrofiring to slow down and will use direct atmospheric entry instead)

Surely this will be a lot less fuel that they would have used to get to LEO
Depends on how fast you want to get there. A cargo only transport on a slow transit might need only 3.6km/s delta-v while Earth surface to LEO requires on the order of 8.5km/s

I believe for the faster transits planned for crew, you need more like 6km/s
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Old 8th October 2017, 11:57 PM   #194
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Originally Posted by theprestige View Post
No problem. The first ship's payload is the Mars Mission Module. It used all its fuel to get to LEO, and needs more fuel to get to Mars.

The second ship's payload is fuel for the first ship. They meet in LEO, thus solving the first ship's fuel problem.

The second ship used all its fuel getting the first ship's refueling payload to LEO, but that's not a problem, because the second ship only needed to get to LEO.
But then the tanker soft lands back on Earth?
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Old 9th October 2017, 12:02 AM   #195
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Originally Posted by GlennB View Post
But then the tanker soft lands back on Earth?
I'm not sure why this is hard to understand. F9 already reserves margins for landing. The BFB and BFS will both do the same. They are deliberately being designed to be over-sized in terms of payload lofted, so that the impact of reserving prop for landing can be absorbed. The system is designed to be able to loft 150t into LEO while still RTLS the booster and having enough prop in the BFS to still deorbit and land.

So the first ship carries 150t into orbit - those 150t are all supplies and equipment. The 1st tanker now flies to orbit. It is carrying 150t of prop, 0t of cargo.

5 or 6 refills later and you're good to go just about anywhere you want.
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Old 9th October 2017, 12:10 AM   #196
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Originally Posted by GlennB View Post
But then the tanker soft lands back on Earth?
Yes. Presumably the SpaceX engineers will over the next several years work out a method for transferring less than 100% of the fuel off of the tanker so it has enough to land again, I am sure that with the numerous simple and effective options for doing so that already exist they can manage.

As for how the tankers will manage to have enough fuel to be able to transfer to the Mars ship and still soft land I would guess they would do it by launching either with a full fuel load and no cargo or a cargo that consists of extra fuel tanks. A BFR is able to launch 150 tons or cargo into orbit and still soft land, so launching 0 tons of cargo with the same amount of fuel loaded will result in it arriving in orbit with a lot more fuel that it needs to soft land.
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Old 9th October 2017, 01:02 AM   #197
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Originally Posted by Grashtel View Post
Yes. Presumably the SpaceX engineers will over the next several years work out a method for transferring less than 100% of the fuel off of the tanker so it has enough to land again, I am sure that with the numerous simple and effective options for doing so that already exist they can manage.
Prop transfer in space is understood and should not present problems for SpaceX. If you look at the back of the ship, you can see 4 sets of piping. A set of inlets for LOX and C4H and a set of vents for same. micro g acceleration from the RCS to settle the tanks and then pressure differential to pump the fuel & oxidiser across.

As for retaining the landing propellant, remember that the Ship carries header tanks for landing (solves a lot of the sloshing issues on a nearly empty ship), so there will be no need to worry about transferring too much - just don't allow transfer out of the header tanks.
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Old 9th October 2017, 02:38 AM   #198
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Originally Posted by GlennB View Post
But then the tanker soft lands back on Earth?
Yep. Why not?

All it has to do is reserve enough fuel for the job (a de-orbit burn and a landing burn) just like any other BFR-SM. In fact it would take less fuel than any other BFR-SM because, unlike the proposed passenger version, or one returning from a mission in LEO, or from Mars, it will have no payload.
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Old 9th October 2017, 03:28 AM   #199
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Originally Posted by Octavo View Post
5 or 6 refills later and you're good to go just about anywhere you want.
I'm wondering why 5 or 6 refills will be needed to go to Mars

Just using Apollo 17 as an example

Stage 1 - burn time 165 sec firing x5 F1 engines, consuming 2160t of propellent

Stage 2 - burn time 360 sec firing x5 J2 engines, consuming 1005t of propellant

Stage 3 - burn time 147 sec firing x1 J2 engines, consuming 33t of propellant

At this point Stage 3 has been injected into orbit - the 11 engines have fired for at total of 682 seconds, consuming 3197t of propellant.

Next comes TLI, a 347s burn of the S-IVB's J2 engine, consuming most of the remaining 67t of propellant, resulting a Δv of of about 3.1km/s.

Now given your earlier point regarding TMI needing Δv to be about 6 km/s, that is of course going to require more propellant. Also, BFR-SM with its payload will be a good deal heavier than a Saturn S-IVb with a CSM and LM attached, but I cant see the need for 5 (750t) or 6 (900t) of propellant for TMI and landing at the other end unelss they intend to take extra propellant to shorten the time taken for manufacture of return fuel on Mars.

What am I missing?
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Old 9th October 2017, 03:54 AM   #200
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Originally Posted by Octavo View Post
I'm not sure why this is hard to understand. F9 already reserves margins for landing. The BFB and BFS will both do the same. They are deliberately being designed to be over-sized in terms of payload lofted, so that the impact of reserving prop for landing can be absorbed. The system is designed to be able to loft 150t into LEO while still RTLS the booster and having enough prop in the BFS to still deorbit and land.

So the first ship carries 150t into orbit - those 150t are all supplies and equipment. The 1st tanker now flies to orbit. It is carrying 150t of prop, 0t of cargo.

5 or 6 refills later and you're good to go just about anywhere you want.
Fair enough. A mix of my own misapprehensions and a little loose reporting about 'disposable' tankers made the SpaceX presentation appear contradictory.

My apologies for mouthing off so loudly.
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